This section assesses the potential impacts on and from geology and soils that could arise from disturbances and
impacts resulting from future development consistent with the General Plan as updated by GPA No. 960, the
proposed project.
4.12.1 Existing Environmental Setting – Geology and Soils
While the County of Riverside is at risk from many natural and man-made hazards, the event with the greatest
potential for loss of life or property and economic damage is an earthquake. This is true for most of Southern
California, since damaging earthquakes are frequent, affect widespread areas, trigger many secondary effects and
can overwhelm the ability of local jurisdictions to respond. In Riverside County, earthquake-triggered geologic
effects that may occur include groundshaking, fault rupture, landslides, liquefaction, subsidence and seiche, all of
which are discussed below. Earthquakes can also cause human-made hazards, such as urban fires, dam failure and
toxic chemical releases.
Earthquakes are caused by movement of rock along a break called a fault. The movement releases pent up strain
energy in the form of waves which travel outward in all directions. These seismic waves cause the earth to vibrate
and this shaking is what we feel in an earthquake. Most earthquakes occur along plate boundaries. The outer
portion of the Earth consists of enormous chunks of rock called plates, which slowly collide, separate and grind
past each other. Frictional forces resist plate movement and the plate edges lock together. Much strain energy
builds up as the plates keep trying to move. Eventually, frictional forces are exceeded, the locked edges move and
all the stored strain energy is released in seismic waves.
Earthquake risk is very high in the heavily populated western portion of Riverside County due to the presence of
three of California’s most active faults: the San Andreas, the San Jacinto and the Elsinore. Risk is moderate in the
eastern portion of the county which includes the Coachella Valley and Blythe.
In California, recent earthquakes in or near urban environments have caused relatively few casualties. This is due
more to luck than design. For example, when a portion of the Nimitz Freeway in Oakland collapsed at rush hour
during the 1989 moment magnitude (Mw) 7.1 Loma Prieta earthquake, it was unusually empty because many were
watching the World Series. Nonetheless, California’s urban earthquakes have resulted in significant economic
losses. Riverside County is at risk from larger, more damaging earthquakes than the moderate sized, Mw 6.7
Northridge earthquake, which in 1994 caused 54 deaths and $20 to $30 billion in damage.
County of Riverside Environmental Impact Report No. 521
Public Review Draft  March 2014
4.12-1
A.
Baseline Data Sources
The existing setting discussion herein is summarized from Section 5.2 of the 1999 Existing Setting Report
prepared for the 2003 RCIP Riverside County General Plan and its Appendix H, “Natural Hazard Mapping,
Analysis and Mitigation: A Technical Background Report in Support of the Safety Element of the New Riverside
County 2000 General Plan” (“Appendix H” herein). Pursuant to CEQA, the description of the physical
environmental conditions provided in this EIR is as they exist at the time the issuance of the Notice of
Preparation (NOP), that is, April 13, 2009. This environmental setting constitutes the baseline physical
conditions by which the County, as Lead Agency under CEQA, determines whether an impact is significant.
However, for geology, soils and seismicity, the 1999 Existing Setting Report and Appendix H remain relevant to
existing conditions within the county because geologic conditions change very slowly with time and no major
earthquakes have occurred within the study area since the reports were prepared. For this reason, these
documents were found to adequately represent the county baseline existing geological and seismic conditions.
The various seismic, soils and geology information presented graphically in this section are from the Riverside
County GIS Department, generally the Riverside County Land Information System (RCLIS) database, as updated
by various means, including through information provided by the State of California (see discussion under Section
4.12.4) and by direction of the Riverside County Geologist in relation to geologic and seismic studies prepared for
proposed development sites within the county and submitted to the Riverside County Geologist. Because they
are county specific, these data sources were determined to be the best-supported substantial evidence available
and were used herein. The sources for the various land use and environmental data sets used in this section are
described in their respective sections.
B.
Fault Hazard Zones
Primary ground damage due to earthquake fault rupture typically results in a relatively small percentage of the
total damage in an earthquake, but being too close to a rupturing fault can cause profound damage. It is difficult
to reduce this hazard through structural design. The primary mitigating technique is to set back from and avoid
active faults. The challenge comes in identifying all active faults. Faults throughout Southern California have
formed over millions of years. Some of these faults are generally considered inactive in terms of present geologic
conditions. Other faults are known to be active, meaning either they have generated earthquakes in historical
times (the last 200 years) or show geologic and geomorphic indications of relatively recent movement. Faults that
have moved in the relatively recent geological past are generally presumed to be the most likely candidates to
generate damaging earthquakes in the lifetimes of residents, buildings and communities.
Earthquakes in Southern California occur as a result of movement between the Pacific and North American
plates. Faults of the San Andreas system are used to mark the boundary between these plates, but the
deformation, faulting and associated earthquakes occur in a broadly distributed zone that stretches from offshore
to Nevada. Thus, the San Andreas is one of a system of plate-bounding faults. Most of the movement between
the plates occurs along the San Andreas fault, which bisects Riverside County. The rest of the motion is
distributed among northwest-trending, strike-slip faults of the San Andreas system (principally the San Jacinto,
Elsinore, Newport-Inglewood and Palos Verdes faults), several east-trending thrust faults that bound the
Transverse Ranges and the Eastern Mojave Shear Zone (a series of faults east of the San Andreas, responsible for
the 1992 Landers and the 1999 Hector Mine earthquakes).
Pursuant to state law (see Section 4.12.2), Alquist-Priolo (A-P) Earthquake Fault Zones have been designated by
the California Geologic Survey for the Elsinore, San Jacinto and San Andreas fault zones in Riverside County (see
Figure 4.12.1 (Alquist-Priolo Fault Zones)). Additionally, the County of Riverside has developed and applied
4.12-2
County of Riverside Environmental Impact Report No. 521
Public Review Draft  March 2014
special studies zone criteria for the Agua Caliente fault zone between the Elsinore and the San Jacinto faults in
southwestern Riverside County. All of these faults have high rates of displacement and are rapidly accumulating
strain energy which will be released in earthquakes. Inevitably, the A-P Zone will expand with time. As faults are
studied, more splays are discovered.
C.
Groundshaking
For design and environmental analysis purposes, a worst-case scenario earthquake (the maximum credible
earthquake [MCE]) for Riverside County is a magnitude 7.9, based on the rupture of the entire southern segment
of the San Andreas Fault from the Cajon Pass to the Salton Sea. While other scenarios would expose portions of
Riverside County to intense groundshaking that is locally as severe as the MCE, the MCE exposes most of the
county to very high-intensity groundshaking.
Groundshaking is simply the movement of the earth resulting from an earthquake. Shaking can cause lateral
movement and is the primary reason for collapse of buildings. The strength of seismic groundshaking at any
given site is a function of many factors.
Factors of primary importance in groundshaking severity include the size of the earthquake, its distance, the paths
the seismic waves take as they travel through the earth, the type of rock or soils underlying the site and
topography (particularly whether a site sits in a valley or atop a hill). The amount of resulting damage also
depends on the size, shape, age and engineering characteristics of affected structures. Interactions between
ground motion and man-made structures are complex. Governing factors include a structure’s height,
construction and stiffness; a soil’s strength and resonant period; and the period of high-amplitude seismic waves.
Waves come in different lengths and thus repeat their motions with varying frequency. Long waves are called
long-period or low-frequency. Short waves are short-period or high-frequency. In general, long-period seismic
waves, which are characteristic of large earthquakes, are most likely to damage long-period structures such as
high-rise buildings and bridges. Shorter period seismic waves, which tend to die out quickly, will most often
cause damage near the epicenter of the earthquake, damaging structures such as one-story and two-story
buildings. Very short period waves are most likely to cause nonstructural damage, such as to equipment. In
different situations, ground displacement, velocity and acceleration can all cause damage.
Estimates of several key groundshaking parameters near the fault rupture zone for the Riverside MCE, expressed
as a percentage of gravity, are presented in Table 4.12-A (Probable Earthquake Scenarios for Riverside County).
Peak ground acceleration, which is the maximum acceleration achieved at a site, often turns out to be the
earthquake effect that predicates the most damage to buildings. Wave periods of 0.3 second and 1.0 second are
the lengths of seismic waves that commonly damage structures. All of these values are well above the threshold
for heavy damage.
Table 4.12-A: Probable Earthquake Scenarios for Riverside County
Event
Maximum
Magnitude
(Mw)
Chance of
Occurring in
30 Years
Fault
Segment
San Andreas
San Bernardino
7.3
28%
San Andreas
Coachella
7.1
22%
County of Riverside Environmental Impact Report No. 521
Public Review Draft  March 2014
Comments
Very high intensity groundshaking throughout the San
Bernardino Valley, including north central Riverside
County.
Very high intensity groundshaking throughout the
Coachella Valley, affecting desert resort communities
and agriculture.
4.12-3
Event
Fault
Segment
Maximum
Magnitude
(Mw)
Chance of
Occurring in
30 Years
San Jacinto
San Jacinto Valley
6.9
43%
San Jacinto
Anza Segment
7.2
17%
Elsinore
Temecula Segment
6.8
16%
Elsinore
Glen Ivy Segment
6.8
16%
Whittier
Whittier
6.8
5%
Comments
Highest probability of occurrence of any Southern
California fault. Brought closer to failure as a result of
stress field changes caused by the 1992 Landers
earthquake.
This event would be very destructive within the
communities of Hemet and San Jacinto.
Has not produced any significant earthquakes in
historic time.
Would be very destructive in the communities of Lake
Elsinore, Murrieta and Temecula.
Has not broken in over 1,600 years (WGCEP, 1995).
Would cause significant landslide and lifeline damage
in the Chino Hills - Corona area.
Notes:
Maximum Magnitude: the magnitude of an earthquake event based on the amount of energy released. This measurement is more accurate for large
earthquake events.
Source:
Riverside County General Plan, Appendix H - Natural Hazard Mapping, Analysis and Mitigation: A Technical Background Report in Support of the
Riverside County General Plan, 2000.
A set of design parameters for the MCE are used to estimate the damage and losses that could occur for such an
earthquake (see General Plan Appendix H). With horizontal ground displacements as great as 25 feet along the
fault and intense groundshaking that could last more than 60 seconds, damage and losses in Riverside County as a
result of the MCE or other major San Andreas Fault earthquakes would be extensive. In addition, Riverside
County must consider events on several faults. Earthquakes that are likely to occur during the design life of most
buildings could be generated by segments of the Elsinore, San Jacinto or San Andreas faults. These have been
evaluated by the Working Group on California Earthquake Probabilities (1995), as illustrated in Figure 4.12.2
(Earthquake Probability).
Based on this segmentation, there are seven types of probable earthquakes that threaten Riverside County (see
Table 4.12-A). The event with the greatest probability of occurrence in 30 years (43%) is a maximum magnitude
(Mw) 6.9 rupture of the San Jacinto Valley segment of the San Jacinto fault. The San Jacinto event is considered
the maximum probable event (MPE), the scenario deemed most likely to occur for Riverside County (in contrast
to the MCE, which is the worst expected earthquake).
D.
Liquefaction
Liquefaction is a process by which water-saturated materials (including soil, sediment and certain types of volcanic
deposits) lose strength and fail during strong groundshaking. Specifically, liquefaction is defined as “the
transformation of a granular material from a solid state into a liquefied state as a consequence of increased porewater pressure.” Liquefaction occurs worldwide, commonly during moderate to great earthquakes. Four kinds of
ground failure commonly result from liquefaction: lateral spread, flow failure, ground oscillation and loss of
bearing strength. In California, liquefaction-related ground failures were major components of the following
events:

1857 Fort Tejon earthquake

1906 San Francisco earthquake

1933 Long Beach earthquake
4.12-4
County of Riverside Environmental Impact Report No. 521
Public Review Draft  March 2014
LOS
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Data Source:County Geology (2013)/California Geological Survey (2008)
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Fault Zones
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December 16, 2013
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Disclaimer: Maps and data are to be used for reference purposes only. Map features are
approximate, and are not necessarily accurate to surveying or engineering standards. The
County of Riverside makes no warranty or guarantee as to the content (the source is often third
party), accuracy, timeliness, or completeness of any of the data provided, and assumes no
legal responsibility for the information contained on this map. Any use of this product with
respect to accuracy and precision shall be the sole responsibility of the user.
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Data Source: California Geological Survey (2003)
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December 16, 2013
5
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Probability (%) of an earthquake occuring
on a fault segment in the next 30 years
Disclaimer: Maps and data are to be used for reference purposes only. Map features are
approximate, and are not necessarily accurate to surveying or engineering standards. The
County of Riverside makes no warranty or guarantee as to the content (the source is often third
party), accuracy, timeliness, or completeness of any of the data provided, and assumes no
legal responsibility for the information contained on this map. Any use of this product with
respect to accuracy and precision shall be the sole responsibility of the user.
Figure 4.12.2
EARTHQUAKE
PROBABILITY

1971 San Fernando earthquake

1973 Point Mugu earthquake

1979 and 1981 Imperial Valley earthquakes

1989 Loma Prieta earthquake

1994 Northridge earthquake.
In 1997 and 1998, the California Geological Survey (CGS) developed guidelines for delineating, evaluating and
mitigating seismic hazards in California. In 1999, a sponsored group published “Recommended Procedures for
Implementation of CGS Special Publication 117 Guidelines for Analyzing and Mitigating Liquefaction in
California.” In it, Liquefaction Hazard Zones are defined as areas meeting one or more of the following criteria:

Areas known to have experienced liquefaction during historic earthquakes.

All areas of uncompacted fills containing liquefaction-susceptible material that are saturated, nearly
saturated or may be expected to become saturated.

Areas where sufficient existing geotechnical data and analyses indicate that the soils are potentially
liquefiable.
Areas within Riverside County susceptible to liquefaction hazards are depicted in Figure 4.12.3 (Liquefaction
Zones). The characteristics of the various liquefaction hazard zones are detailed in Table 4.12-B (General
Liquefaction Potential Zones for Riverside County).
Table 4.12-B: General Liquefaction Potential Zones for Riverside County
Rank
Groundwater Depth2
General3
Sediment Type
High
< 30 feet
< 30 feet
30-50 feet
> 30 feet
30-50 feet
50-100
50-100 feet
> 100 feet
No data
Very Susceptible
Susceptible
Very Susceptible
Susceptible
Susceptible
Very Susceptible
Susceptible
Susceptible
Bedrock
Moderate
Low
Very Low
Extremely Low
None
Recommended Policies1
General
Critical4
Construction
Facilities
Study Required
Study Required
Study Required
Study Required
Study Required
Study Required
None
Study Required
None
Study Required
None
Study Required
None
Study Required
None
None
None
None
Footnotes:
1.
Groundshaking potential in easternmost Riverside County is considered below the threshold for liquefaction and site-specific investigations should not be
required for general construction projects.
2.
Groundwater depth is based on the historic high measurement.
3.
Very susceptible sediment type includes generally granular Holocene sediments; susceptible includes generally granular Pleistocene sediments.
4.
Critical facilities are facilities designed to remain functional during and immediately after an earthquake.
Source:
Riverside County General Plan, Appendix H - Natural Hazard Mapping, Analysis and Mitigation: A Technical Background Report in Support of the
Riverside County General Plan, 2000.
County of Riverside Environmental Impact Report No. 521
Public Review Draft  March 2014
4.12-9
E.
Landslides and Rock Falls
Hillsides, generally speaking, can be unstable platforms for development. Unless a landslide is already occurring, a
steep slope can generally be thought of as existing in a state of equilibrium. When this equilibrium is disturbed by
development in hillside areas, the likelihood of slope failure, soil erosion, silting of lower slopes and downstream
flooding increases.
There are predictable relationships between local geology and mass-wasting processes like landslides and rockfall.
Slope stability is dependent on many factors and their interrelationships. Rock type and poor water pressure are
possibly the most important factors, followed by slope steepness due to natural or man-made undercutting. In
addition, many existing landslides and soil slumps have been mapped within Riverside County. Where slopes
have failed before, they will fail again. Field investigation enables identification of failure-prone slopes before an
earthquake occurs. Factors controlling the stability of slopes include the following:

Slope height and inclination

Engineering characteristics of the earth materials comprising the slope

Intensity of groundshaking
Seismically induced landslides and rockfall would be expected throughout Riverside County in the event of a
major earthquake. Factors contributing to the stability of slopes include slope height and steepness, engineering
characteristics of the earth materials comprising the slope and intensity of groundshaking. It is estimated that a
ground acceleration of at least 0.10 g in steep terrain is necessary to induce earthquake-related rockfall, although
exceeding this value does not guarantee that rockfall will occur. Because there are several faults capable of generating peak ground accelerations of over 0.10 g in Riverside County, there is a high potential for seismicallyinduced rockfall and landslides to occur. Figure 4.12.4 (Steep Slopes) shows areas of steep slopes within Riverside
County.
F.
Seismically Induced Ground Settlement
Whether or not seismically induced settlement will occur depends on the intensity and duration of groundshaking,
and the relative density of the subsurface soils (i.e., the ratio between the in-place density and the maximum
density). Sediments in the alluvial valleys of Riverside County were deposited fairly rapidly, which may lead to
conditions of low density sediments that can settle in an earthquake. Therefore, many of the valley regions that
contain relatively recent sediments may be susceptible to some degree of seismic settlement. The extent of
relatively young sediments with moderate to locally high potential for settlement may be correlated with areas of
valley fill represented on subsidence susceptibility mapping.
As demonstrated by past earthquakes, seismic settlement is primarily damaging in areas subject to differential
settlement. As an example, this can include cut-and-fill transition lots built on hillsides where a portion of the
house is built over an area cut into the hillside with the remaining portion of the house on man-made fill. During
an earthquake, even slight settlement of the fill (soil) can cause a structure to raise or lower differentially, leading
to significant repair costs.
4.12-10
County of Riverside Environmental Impact Report No. 521
Public Review Draft  March 2014
G.
Subsidence and Collapsible Soils
Ground subsidence is typically a gradual settling or sinking of the ground surface with little or no horizontal
movement, although fissures (cracks and separations) are common. Subsidence can range from small or local
collapses to broad regional lowering of the earth’s surface. While subsidence typically occurs throughout a susceptible valley, additional displacement and fissures occur at or near the valley margin. Susceptible valleys are
those predominantly filled with unconsolidated sand and silty sand that includes thin layers of silt and clayey silt.
Fine-grained alluvium and organic matter often underlie the fissure areas. Two types of fissures are associated
with subsidence. The first is generally straight and corresponds to the traces of faults, while the second is more
curvilinear on the surface and appears to correspond to the alluvium-bedrock contact at valley margins.
The causes of subsidence are as diverse as the forms of failure. They include dewatering of peat or organic soils,
dissolution in limestone aquifers, first-time wetting of moisture-deficient, low-density soils (hydrocompaction),
natural compaction, liquefaction, crustal deformation, subterranean mining and withdrawal of fluids (groundwater, petroleum, geothermal, etc.). Most of the damaging types of subsidence are induced by the extraction of
oil, gas or groundwater from below the ground surface or the organic decomposition of peat deposits, with a
resultant loss in volume. Ground subsidence can also occur as a response to natural forces, such as earthquake
movements and the evolution of a sedimentary basin as it folds and subsides. And, of course, earthquakes can
cause abrupt elevation changes of several feet.
Ground subsidence can disrupt surface drainage, reduce aquifer system storage, form earth fissures and damage
wells, buildings, roads and utility infrastructure. Regional subsidence generally damages structures that are
sensitive to slight changes in elevations, such as canals, sewers and drainages. In Riverside County, risk of damage
or harm due to regional subsidence is greatest at valley margins.
Subsidence and fissuring have been caused by falling groundwater tables and by hydrocollapse when groundwater
tables rise in Riverside County. In addition, many fissures have occurred along active faults that bound the San
Jacinto Valley and Elsinore Trough. Figure 4.12.5 (Documented Subsidence Areas) depicts areas of documented
subsidence and other areas of Riverside County that may be susceptible to subsidence. Subsidence has only been
documented in three areas of the county: the Elsinore Trough, including Temecula and Murrieta; the San Jacinto
Valley from Hemet to Moreno Valley; and the southern Coachella Valley.
These areas are all potentially sensitive to the withdrawal of groundwater. Depending on the depth and
mechanical properties of the aquifer and the overlying sediments, they can subside if groundwater resources are
not managed properly. Mitigation of ground subsidence usually requires a regional approach to groundwater
conservation and recharge. Such mitigation measures are difficult to implement if the geology of the aquifer and
overlying sediment is not well understood. Furthermore, conservation efforts can be quickly offset by rapid
growth and attendant heavy water requirements (golf courses, for example, consume about 8 acre-feet of water
per acre per year). Further, it is not uncommon for several jurisdictions to utilize a continuous groundwater
aquifer. Mitigation requires regional cooperation among all agencies.
Hydroconsolidation, or soil collapse, typically occurs in recently deposited Holocene (less than 10,000 years
before present time) soils that were deposited in an arid or semi-arid environment. Soils prone to collapse are
commonly associated with man-made fill, wind-laid sands and silts, and alluvial fan and mudflow sediments
deposited during flash floods. Particles of these soils, which typically contain minute pores and voids, may be
partially supported by clay or silt, or chemically cemented with carbonates. When saturated, collapsible soils
undergo rearrangement of their grains and the water removes the cohesive (or cementing) material, and a rapid,
substantial settlement may occur. An increase in surface water infiltration (such as from irrigation) or a rise in the
County of Riverside Environmental Impact Report No. 521
Public Review Draft  March 2014
4.12-11
groundwater table, combined with the weight of a building or structure, may initiate settlement, causing
foundations and walls to crack.
In Riverside County, collapsible soils occur predominantly at the bases of the mountains, where Holocene-aged
alluvial sediments have been deposited during rapid runoff events. Additionally, some windblown sands may be
vulnerable to collapse and hydroconsolidation. Typically, differential settlement of structures occurs when lawns
or plantings are heavily irrigated in close proximity to a structure’s foundation.
H.
Expansive Soils
Expansive soils have a significant amount of clay particles that can give up water (shrink) or take on water (swell).
The change in volume exerts stress on buildings and other loads placed on these soils. The occurrence of these
soils is often associated with geologic units having marginal stability. Expansive soils can be widely dispersed and
they can occur in hillside areas as well as low-lying alluvial basins.
Although expansive soils are now routinely alleviated by following the Riverside County Building Code, problems
related to past inadequate grading or site preparation practices constantly appear. Expansive soils are not the only
cause of structural distress in existing structures. Poor compaction and construction practices, settlement and
landslides can cause similar damage, but require different mediation efforts. Once expansion has been verified as
the source of the problem, mitigation can be achieved through reinforcement of the existing foundation or
through the excavation and removal of the expansive soils in the affected area.
I.
Wind Erosion
Wind erosion damages land and natural vegetation by removing soil from one place and depositing it in another.
It mostly affects dry, sandy soils in flat, bare areas, but wind erosion may occur wherever soil is loose, dry and
finely granulated. It causes soil loss, dryness, deterioration of soil structure, nutrient and productivity losses, air
pollution and sediment transport and deposition. The presence of dust particles in the air is a source of several
major health problems. Atmospheric dust causes respiratory discomfort, may carry pathogens that cause eye
infections and skin disorders, and reduces highway and air traffic visibility. Buildings, sheds, fences, roads, crops,
trees and shrubs can all be damaged by blowing soil, which acts as an abrasive.
Wind and windblown sand are an environmentally limiting factor throughout much of Riverside County.
Approximately 20% of the land area of Riverside County is vulnerable to high and very high wind erosion
susceptibility. The Coachella Valley, the Santa Ana River channel and areas in the vicinity of the City of Hemet
have been identified as zones of high wind erosion susceptibility. Areas susceptible to wind erosion hazards
throughout Riverside County are identified in Figure 4.12.6 (Wind Erosion Susceptibility Areas).
Windblown sand is a well-recognized hazard for developments in the Coachella Valley. It has even forced
abandonment of dwellings and subdivided tracts in the central Coachella Valley. The primary source of sand in
the Coachella Valley is the Whitewater River. Increases in the amount of windblown sand are related to episodic
flooding of the Whitewater River. A 15-fold increase in wind erosion rates in the Coachella Valley has been
documented following heavy flood events. Therefore, mitigation of windblown sand is directly related to
mitigation of flood potential on the Whitewater River.
Because windblown sand from the Whitewater River floodplain provides a large component of the sand that
sustains dune fields that, in turn, are home to several endangered species, erosion intervention efforts must be
cautiously considered.
4.12-12
County of Riverside Environmental Impact Report No. 521
Public Review Draft  March 2014
J.
Soil Erosion and Loss of Topsoil
Soil erosion is the process by which soil particles are removed from a land surface by wind, water or gravity.
Most natural erosion occurs at slow rates; however, the rate of erosion increases when land is cleared or altered
and left in a disturbed condition. The primary factors that influence erosion include soil characteristics, vegetative
cover, topography and climate.
Soil characteristics that determine the erodibility of a soil are particle size and gradation, organic content, soil
structure and soil permeability. Soils with a high proportion of silt and very fine clays are generally the most
erodible. Organic matter creates a favorable soil structure, improves soil stability and permeability, which
increases the soil’s capacity for the infiltration of water, delays the start of erosion and reduces the amount of
runoff. In addition, the less permeable the soil, the higher the likelihood for erosion. Vegetative cover aids
erosion control by shielding the soil surface from the impact of falling rain or blowing wind. Vegetation slows the
velocity of runoff, permits greater infiltration, maintains the soil’s capacity to absorb water and holds soil particles
in place.
Topography and the length and steepness of slopes are crucial to determining the volume and velocity of runoff.
As slope length and/or steepness increases, the rate of runoff increases and the potential for erosion is magnified.
Climate is a fundamental factor affecting the potential for soil erosion. When and where precipitation is frequent,
intense or prolonged, the potential for soil erosion is increased.
K.
Reducing Earthquake Hazards in Riverside County
Changes in the 1997 Uniform Building Code (UBC), encapsulated in the California Building Standards Code
(California Code of Regulations [CCR] Title 24, in particular Part 2 which encompasses the California Building
Code [CBC]) represent the most significant increases in groundshaking criteria in the last 30 years. Two changes
have special significance for the County of Riverside. The first change is a revision in soil types and amplification
factors. The second change incorporates the proximity of earthquake sources in UBC seismic zone 4 (refer to
Figure 4.12.7 for the near-source zones that affect Riverside County). Zone 4 is the highest hazard zone and
includes most of the County of Riverside. The Riverside County Department of Building and Safety defines the
UBC seismic zones in the county as follows:
The townships T2SR16E, T3SR17E, T4SR18E, T5SR19E, T6SR20E, T7SR21E, T8SR22E are
inclusive to the UBC Seismic Zone-4 and the townships lying east of listed above may be considered in the
Seismic Zone-3.
Low-rise buildings with a short predominant period of groundshaking must also consider soil effects. In the past,
only long-period structures (high-rises) were influenced by UBC requirements. The groundshaking basis for code
design is more complicated, because of the wide range of soil types and the close proximity of seismic sources.
The new soil effects are based on observations made as a result of the Mexico City and Loma Prieta earthquakes,
and affect all new buildings in western and central Riverside County. Most of the western and central portions of
Riverside County are affected by the new, near-source design factors. The 1997 UBC contains detailed
descriptions of the incorporation of the new near-source and soil parameters.
As shown in Figure 4.12.7 (Near Source Zone Regions UBC Zone Boundary), most of the western and central
portions of Riverside County are subject to near-source design factors based on the proximity of three major fault
systems (Elsinore, San Jacinto and San Andreas), as well as some smaller fault systems (Chino-Central Avenue,
Burnt Mountain and Eureka Peak). These parameters, new to the 1997 UBC, address the proximity of potential
County of Riverside Environmental Impact Report No. 521
Public Review Draft  March 2014
4.12-13
earthquake sources (faults). Groundshaking that was far more intense than expected occurred near the fault
ruptures at Northridge in 1994 and at Kobe, Japan, in 1995. The 1997 UBC also includes a near-source factor
that accounts for directivity of fault rupture. The direction of fault rupture played a significant role in distribution
of groundshaking in both quakes. For Northridge, much of the earthquake energy was released into the sparsely
populated mountains north of the San Fernando Valley. While at Kobe, however, the rupture directed energy
into the city and contributed to extensive damage. Since the rupture direction of a given source cannot be
predicted, the UBC requires roughly a 20% general increase in estimated groundshaking to account for directivity.
4.12.2 Policies and Regulations Addressing Geology and Soils
A.
State and Federal Regulations
Alquist-Priolo Earthquake Fault Zoning Act (A-P Act): The major state legislation regarding earthquake
fault zones is the Alquist-Priolo Earthquake Fault Zoning Act. In 1972, the State of California began delineating
“Earthquake Fault Zones” (called “Special Studies Zones” prior to 1994) around and along faults that are
“sufficiently active” and “well defined” to reduce fault-rupture risks to structures for human occupancy (Public
Resources Code [PRC] Sections 2621–2630). The boundary of an Earthquake Fault Zone is generally 500 feet
from major active faults and 200-300 feet from well-defined minor faults. The mapping of active faults has been
completed by the State Geologist and these maps are distributed to all affected cities, counties and state agencies
for their use in developing planning policies and controlling renovation or new construction.
Before a project can be permitted within an identified earthquake fault zone, cities and counties must require a
geologic investigation to demonstrate that proposed buildings would not be constructed across active faults. A
site-specific evaluation and written site report must be prepared by a licensed geologist. If an active fault is
identified, a structure intended for human occupancy cannot be placed over the trace of the fault and must be set
back, generally at least 50 feet from the fault. The A-P Act only addresses the hazard of surface fault rupture and
is not directed toward other earthquake hazards.
Seismic Hazards Mapping Act: Passed in 1990, the Seismic Hazards Mapping Act (SHMA) addresses nonsurface fault rupture earthquake hazards, including strong groundshaking, liquefaction and seismically induced
landslides. The California Geological Survey (CGS) is the principal state agency charged with implementing the
SHMA. The law directs the CGS to provide local governments with seismic hazard zone maps that identify areas
susceptible to amplified shaking, liquefaction, earthquake-induced landslides and other ground failures. The CGSdelineated seismic hazard zones are referred to as “zones of required investigation” and per the SHMA require
site-specific geotechnical hazard investigations when construction projects fall within these areas. SHMA’s goal is
to minimize loss of life and property by identifying and mitigating seismic hazards.
Natural Hazards Disclosure Act: This 1998 Act requires sellers of real property and their agents to provide
prospective buyers with a “Natural Hazard Disclosure Statement” when the property being sold lies within one or
more state-mapped hazard areas. These hazard areas include any Seismic Hazard Zone mapping issued by the
State Geologist. The seller or the seller’s agent must also disclose the mapping to potential buyers.
4.12-14
County of Riverside Environmental Impact Report No. 521
Public Review Draft  March 2014
LOS
ANGELES
COUNTY
SAN
BERNARDINO
COUNTY
62
Ä
EASTVALE
JURUPA
VALLEY
15
§
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CALIMESA
215
§
¨
¦
NORCO
91
Ä
RIVERSIDE
DESERT HOT
SPRINGS
60
Ä
MORENO
VALLEY
62
Ä
BANNING
79
Ä
CORONA
Ä
111
PALM
SPRINGS
BEAUMONT
§
¨
¦
215
15
§
¨
¦
Ä
SAN
JACINTO
243
10
§
¨
¦
HEMET
INDIO
LA QUINTA
WILDOMAR
COACHELLA
371
Ä
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TEMECULA
15
§
¨
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79
Ä
86
Ä
Ä
111
LA PAZ
COUNTY, AZ
IMPERIAL
COUNTY
PACIFIC
OCEAN
Data Source: Riverside County Geology (2013) /California Geological Survey (2008)
Liquefaction Susceptibility
No Groundwater Data
\\agency\tlmagis\Projects\Planning\SafetyElement\General_Liquefaction.mxd
Susceptible Sediments Susceptible Sediments Susceptible Sediments
Very High
Moderate
Moderate
High
Low
Low
Moderate
Very Low
Very Low
Highways
Area Plan Boundary
City Boundary
Waterbodies
California Geological Survey
Seismic Hazard Zones
Murrieta Quad
(See detail in Elsinore, Southwest, Sun City / Menifee Valley Area Plans)
Low
Figure 4.12.3
Very Low
0
78
Ä
SALTON
SEA
SAN DIEGO
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[
BLYTHE
74
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Deep Groundwater
10
§
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10
§
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74
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MURRIETA
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95
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PALM
DESERT
INDIAN
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ELSINORE
74
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177
CATHEDRAL
CITY
RANCHO
MIRAGE
PERRIS
MENIFEE
ORANGE
COUNTY
Ä
December 16, 2013
10
Miles
20
Disclaimer: Maps and data are to be used for reference purposes only. Map features are
approximate, and are not necessarily accurate to surveying or engineering standards. The
County of Riverside makes no warranty or guarantee as to the content (the source is often third
party), accuracy, timeliness, or completeness of any of the data provided, and assumes no
legal responsibility for the information contained on this map. Any use of this product with
respect to accuracy and precision shall be the sole responsibility of the user.
LIQUEFACTION ZONES
LOS
ANGELES
COUNTY
SAN
BERNARDINO
COUNTY
62
Ä
EASTVALE
JURUPA
VALLEY
15
§
¨
¦
CALIMESA
215
§
¨
¦
NORCO
91
Ä
RIVERSIDE
DESERT HOT
SPRINGS
60
Ä
MORENO
VALLEY
BANNING
79
Ä
CORONA
62
Ä
10
§
¨
¦
Ä
111
§
¨
¦
215
Ä
SAN
JACINTO
PERRIS
74
Ä
Ä
PALM
SPRINGS
BEAUMONT
243
177
CATHEDRAL
CITY
RANCHO
MIRAGE
HEMET
LAKE
ELSINORE
ORANGE
COUNTY
74
Ä
PALM
DESERT
INDIAN
WELLS
MENIFEE
INDIO
LA QUINTA
WILDOMAR
MURRIETA
COACHELLA
Ä
371
TEMECULA
79
Ä
15
§
¨
¦
10
§
¨
¦
10
§
¨
¦
74
Ä
79
Ä
95
£
¤
Ä
86S
86
Ä
BLYTHE
Ä
111
78
Ä
SALTON
SEA
LA PAZ
COUNTY, AZ
SAN DIEGO
COUNTY
PACIFIC
OCEAN
IMPERIAL
COUNTY
Data Source: Riverside County Contours/Intermap (2007)
\\agency\tlmagis\Projects\Planning\SafetyElement\Steep_Slope.mxd
Slope Angle
Less Than 15%
Highways
15%-25%
Area Plan Boundary
25%-30%
City Boundary
30% and Greater
Waterbodies
[
0
December 16, 2013
10
Miles
20
Figure 4.12.4
Disclaimer: Maps and data are to be used for reference purposes only. Map features are
approximate, and are not necessarily accurate to surveying or engineering standards. The
County of Riverside makes no warranty or guarantee as to the content (the source is often third
party), accuracy, timeliness, or completeness of any of the data provided, and assumes no
legal responsibility for the information contained on this map. Any use of this product with
respect to accuracy and precision shall be the sole responsibility of the user.
STEEP SLOPES
LOS
ANGELES
COUNTY
SAN
BERNARDINO
COUNTY
62
Ä
EASTVALE
JURUPA
VALLEY
15
§
¨
¦
CALIMESA
215
§
¨
¦
NORCO
91
Ä
RIVERSIDE
DESERT HOT
SPRINGS
60
Ä
MORENO
VALLEY
BANNING
79
Ä
CORONA
62
Ä
10
§
¨
¦
Ä
111
§
¨
¦
215
15
§
¨
¦
Ä
SAN
JACINTO
PERRIS
74
Ä
Ä
PALM
SPRINGS
BEAUMONT
243
177
CATHEDRAL
CITY
RANCHO
MIRAGE
HEMET
LAKE
ELSINORE
ORANGE
COUNTY
74
Ä
PALM
DESERT
INDIAN
WELLS
MENIFEE
INDIO
LA QUINTA
WILDOMAR
79
COACHELLA
10
§
¨
¦
10
§
¨
¦
74
Ä
Ä
95
£
¤
BLYTHE
74
Ä
MURRIETA
Ä
371
TEMECULA
15
§
¨
¦
79
Ä
Ä
86S
86
Ä
Ä
111
78
Ä
SALTON
SEA
LA PAZ
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SAN DIEGO
COUNTY
PACIFIC
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COUNTY
Path: \\agency\tlmagis\Projects\Planning\CindysPurpleMapGallery\Fig4.12.5-Subsidence.mxd
Data Source: RBF Consultants (2003) and Riverside County (2007)
Subsidence Zones
Areas with Documented Subsidence
Highways
Susceptible Areas
Area Plan Boundary
City Boundary
Waterbodies
[
0
December 16, 2013
10
Miles
20
Disclaimer: Maps and data are to be used for reference purposes only. Map features are
approximate, and are not necessarily accurate to surveying or engineering standards. The
County of Riverside makes no warranty or guarantee as to the content (the source is often third
party), accuracy, timeliness, or completeness of any of the data provided, and assumes no
legal responsibility for the information contained on this map. Any use of this product with
respect to accuracy and precision shall be the sole responsibility of the user.
Figure 4.12.5
DOCUMENTED
SUBSIDENCE AREAS
(
!
(
!
CITY OF
JURUPA
VALLEY
15
§
¨
¦
CITY OF
NORCO
(
!
91
Ä
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CALIMESA
215
§
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CITY OF
RIVERSIDE
60
Ä
CITY OF
MORENO
VALLEY
(
!
79
Ä
CITY OF
BANNING
62
Ä
10
§
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¦
CITY OF
BEAUMONT
CITY OF
CORONA
(
!
§
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215
CITY
OF LAKE
ELSINORE
74
Ä
ORANGE
COUNTY
Ä
74
Ä
CITY OF
HEMET
62
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(
!
Ä
177
Ä
243
(
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OF PALM
SPRINGS
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CITY
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CITY OF
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86
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CITY OF
INDIO
CITY OF
INDIAN
WELLS
CITY OF
LA QUINTA
74
Ä
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Ä
95
£
¤
(
!
CITY OF
RANCHO
MIRAGE
CITY OF
MENIFEE
CITY OF
WILDOMAR
(
!
111
CITY OF SAN
JACINTO
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CITY OF
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c
c
CITY OF
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Data Source: Earth Consultants International (2003)
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Wind Erodibility Rating
Very High
Highways
High
Area Plan Boundary
Moderate
City Boundary
Low
Waterbodies
Weather Station
[
0
Figure 4.12.6
General Prevailing Wind Direction
December 16, 2013
10
Miles
20
Disclaimer: Maps and data are to be used for reference purposes only. Map features are
approximate, and are not necessarily accurate to surveying or engineering standards. The
County of Riverside makes no warranty or guarantee as to the content (the source is often third
party), accuracy, timeliness, or completeness of any of the data provided, and assumes no
legal responsibility for the information contained on this map. Any use of this product with
respect to accuracy and precision shall be the sole responsibility of the user.
WIND EROSION
SUSCEPTIBILTY AREAS
LOS
ANGELES
COUNTY
SAN
BERNARDINO
COUNTY
62
Ä
EASTVALE
JURUPA
VALLEY
15
§
¨
¦
§
¨
¦
NORCO
91
Ä
RIVERSIDE
DESERT HOT
SPRINGS
60
Ä
MORENO
VALLEY
BANNING
79
Ä
CORONA
62
Ä
10
§
¨
¦
Ä
111
243
CATHEDRAL
CITY
RANCHO
MIRAGE
HEMET
PALM
DESERT
LAKE
ELSINORE
WILDOMAR
INDIO
LA QUINTA
79
Ä
74
Ä
MURRIETA
Ä
371
TEMECULA
79
Ä
15
§
¨
¦
95
£
¤
4
INDIAN
WELLS
MENIFEE
74
Ä
3
ne
Zo
74
Ä
Zo
ne
C
PERRIS
C
B
Ä
SAN
JACINTO
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B
U
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215
Ä
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SPRINGS
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ORANGE
COUNTY
U
CALIMESA
215
COACHELLA
10
§
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¦
10
§
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¦
BLYTHE
74
Ä
Ä
86S
86
Ä
Ä
111
78
Ä
SALTON
SEA
LA PAZ
COUNTY, AZ
SAN DIEGO
COUNTY
PACIFIC
OCEAN
IMPERIAL
COUNTY
Data Source: RBF Consultants (2003) and Riverside County (2007)
\\agency\tlmagis\Projects\Planning\SafetyElement\Subsidence.mxd
Areas of Riverside County where new near-source design
parameters are required under the 1997 Uniform Building Code
Near Source Zone Regions
Highways
City Boundary
Waterbodies
[
0
December 16, 2013
10
Miles
20
Disclaimer: Maps and data are to be used for reference purposes only. Map features are
approximate, and are not necessarily accurate to surveying or engineering standards. The
County of Riverside makes no warranty or guarantee as to the content (the source is often third
party), accuracy, timeliness, or completeness of any of the data provided, and assumes no
legal responsibility for the information contained on this map. Any use of this product with
respect to accuracy and precision shall be the sole responsibility of the user.
Figure 4.12.7
NEAR SOURCE ZONE REGIONS
UBC ZONE BOUNDARY
Building Earthquake Safety Act: This 1986 Act requires all local governments to identify all potentially
hazardous buildings within their jurisdictions and to establish a program for mitigation of identified hazards. It is
the legislative basis for the inventory of hazardous unreinforced masonry buildings and unreinforced masonry
ordinances adopted by most counties and cities in California.
Recovery and Reconstruction Act: This 1986 Act authorizes local governments to prepare for expeditious and
orderly recovery before a disaster and reconstruction afterward. It enables localities to prepare pre-disaster plans
and ordinances that may include: an evaluation of the vulnerability of specific areas to damage from a potential
disaster; streamlined procedures for appropriate modification of existing general plans or zoning ordinances
affecting vulnerable areas; a contingency plan of action; organization for post-disaster, short-term and long-term
recovery and reconstruction; and a pre-disaster ordinance to provide adequate local authorization for post-disaster
activities.
B.
Riverside County Regulations
Ordinance No. 457 - Riverside County Building and Fire Codes: Every three years, Riverside County’s
Building and Fire Codes are adapted from the California Building Standards Code (CCR Title 24), which includes
both building and fire codes. These codes establish site-specific investigation requirements, construction
standards and inspection procedures to ensure that development authorized by the County of Riverside does not
pose a threat to the health, safety or welfare of the public. The California Building Standards Code contains
minimum baseline standards to guard against unsafe development. This ordinance also adopts, in some cases
with modification to a stricter standard, a number of California State’s Title 24 codes (fire, building, plumbing,
electrical, etc.). The Riverside County Department of Building and Safety provides technical expertise in
reviewing and enforcing these codes.
Ordinance No. 547 - Implementation of the Alquist-Priolo Earthquake Fault Zoning Act: This ordinance
establishes the policies and procedures used by the County of Riverside to implement the A-P Act. Among other
things, it requires all projects proposed within an “earthquake fault zone,” as shown on the maps prepared by the
State Geologist to comply with the provisions of the A-P Act. It establishes regulations for construction,
including for grading, slopes and compaction, erosion control, retaining wall design and earthquake fault zone
setbacks.
Ordinance No. 484 - Control of Blowing Dust: This ordinance establishes requirements for the control of
blowing sand within county-designated “Agricultural Dust Control Areas.” It defines activities that may
contribute to wind erosion, identifies restrictions on activities within these areas, establishes penalties for violation
of the ordinance and identifies procedures necessary to obtain a valid permit.
C.
Existing Riverside County General Plan Policies
The following existing and proposed General Plan polices address seismic issues, such as fault rupture, seismically
induced liquefaction, landslides and rock falls. They also address steep slope risks, such as landslides, rockfall and
debris flows, as well as expansive and collapsible soils, subsidence and wind erosion.
1. Safety (S) Element Policies
Policy S 2.5: Require that engineered slopes be designed to resist seismically induced failure. For lower-risk
projects, slope design could be based on pseudo-static stability analyses using soil engineering parameters that are
County of Riverside Environmental Impact Report No. 521
Public Review Draft  March 2014
4.12-25
established on a site-specific basis. For higher-risk projects, the stability analyses should factor in the intensity of
expected groundshaking, using a Newmark-type deformation analysis.
Policy S 2.6: Require that cut and fill transition lots be over-excavated to mitigate the potential of seismically
induced differential settlement.
Policy S 2.7: Require a 100% maximum variation of fill depths beneath structures to mitigate the potential of
seismically induced differential settlement.
Policy S 2.8: Encourage research into new foundation design systems that better resist the county’s climatic,
geotechnical, and geological conditions.
Policy S 3.1: Require the following in landslide potential hazard management zones, or when deemed necessary
by the California Environmental Quality Act:
a.
Preliminary geotechnical and geologic investigations.
b. Evaluations of site stability, including any possible impact on adjacent properties, before final project
design is approved.
c. Consultant reports, investigations, and design recommendations required for grading permits, building
permits, and subdivision applications be prepared by State-licensed professionals.
Policy S 3.2: Require that stabilized landslides be provided with redundant drainage systems. Provisions for the
maintenance of subdrains must be designed into the system.
Policy S 3.3: Before issuance of building permits, require certification regarding the stability of the site against
adverse effects of rain, earthquakes, and subsidence.
Policy S 3.4: Require adequate mitigation of potential impacts from erosion, slope instability, or other hazardous
slope conditions, or from loss of aesthetic resources for development occurring on slope and hillside areas.
Policy S 3.5: During permit review, identify and encourage mitigation of onsite and offsite slope instability,
debris flow, and erosion hazards on lots undergoing substantial improvements.
Policy S 3.6: Require grading plans, environmental assessments, engineering and geologic technical reports,
irrigation and landscaping plans, including ecological restoration and revegetation plans, as appropriate, in order
to assure the adequate demonstration of a project’s ability to mitigate the potential impacts of slope and erosion
hazards and loss of native vegetation.
Policy S 3.7: Support mitigation on existing public and private property located on unstable hillside areas,
especially slopes with recurring failures where county property or public right-of-way is threatened from slope
instability, or where considered appropriate and urgent by the County Engineer, Fire, or Sheriff Department.
Policy S 3.8: Require geotechnical studies within documented subsidence zones, as well as zones that may be
susceptible to subsidence, as identified in [General Plan] Figure S-7 and the Technical Background Report [i.e.,
General Plan Appendix H], prior to the issuance of development permits. Within the documented subsidence
zones of the Coachella, San Jacinto and Elsinore valleys, the studies must address the potential for reactivation of
4.12-26
County of Riverside Environmental Impact Report No. 521
Public Review Draft  March 2014
these zones, consider the potential impact on the project, and provide adequate and acceptable mitigation
measures.
Policy S 3.9: Develop a liaison program with all county water districts to prevent water extraction induced
subsidence.
Policy S 3.10: Encourage and support efforts for long-term, permanent monitoring of topographic subsidence in
all producing groundwater basins, irrespective of past subsidence.
Policy S 3.11: Require studies that address the potential of this hazard on proposed development within “High”
and “Very High” wind erosion hazard zones as shown on [General Plan] Figure S-8, Wind Erosion Susceptibility
Map.
Policy S 3.13: Require buildings to be designed to resist wind loads.
Policy S 3.14: Educate builders about the wind environment and encourage them to design projects accordingly.
Policy S 7.7: Strengthen the project permit and review process to ensure that proper actions are taken to reduce
hazard impacts and to encourage structural and nonstructural design and construction. Damage must be
minimized for critical facilities, and susceptibility to structural collapse must be minimized, if not eliminated.
a.
Ensure that special development standards, designs, and construction practices reduce risk to tolerable
levels for projects involving critical facilities, large-scale residential development, and major commercial
and industrial development through conditional use permits and the subdivision review process. If
appropriate, impact fees should be assessed to finance required actions.
b. Require mitigation measures to reduce potential damage caused by ground failure for sites determined to
have potential for liquefaction. Such measures shall apply to critical facilities, utilities, and large
commercial and industrial projects as a condition of project approval.
c. Require that planned lifeline utilities, as a condition of project approval, be designed, located, structurally
upgraded, fit with safety shutoff valves, be designed for easy maintenance, and have redundant back up
lines where unstable slopes, earth cracks, active faults, or areas of liquefaction cannot be avoided.
d. Review proposed uses of fault setback areas closely to ensure that county infrastructure (roads, utilities,
drains) are not unduly placed at risk by the developer. Insurance, bonding, or compensation plans should
be used to compensate the County for the potential costs of repair.
Policy S 7.8: Promote strengthening of planned and existing utilities and lifelines, the retrofit and rehabilitation
of existing weak structures, and the relocation of certain critical facilities.
Policy S 7.11: Coordinate with the [California] Public Utilities Commission (PUC) and/or utilize the Capital
Improvement Program, to strengthen, relocate or take other appropriate measures to safeguard high-voltage lines,
water, sewer, natural gas and petroleum pipelines, and trunk electrical and telephone conduits that:
a.
Extend through areas of high liquefaction potential.
b. Cross active faults.
County of Riverside Environmental Impact Report No. 521
Public Review Draft  March 2014
4.12-27
c. Traverse earth cracks or landslides.
Policy S 7.13: Develop a system to respond to short-term increase in hazard on the southern San Andreas fault
based on probabilities associated with foreshocks.
Policy S 7.16: During countywide earthquake drills, encourage communication and cooperation between
emergency response staff and designated contacts at hospitals, high-occupancy buildings and dependent care
facilities.
Policy S 7.19: Establish a far-ranging, creative, forward-thinking public education and outreach campaign to
inform the community about:
a.
The hazards they face.
b. The costs of doing nothing to mitigate the hazards.
c. What is known about each hazard.
d. Why jurisdictions don’t have all the answers.
e. Mitigation incentives.
f.
What the County does for them.
g. What the County cannot be expected to do for them.
D.
Proposed New or Revised Riverside County General Plan Policies
The following revisions to existing General Plan policies are proposed as part of GPA No. 960. The revisions are
intended to enhance the policies’ implementation and comprehensive use.
1. Safety (S) Element Policies
Policy S 2.1: Minimize fault rupture hazards through enforcement of Alquist-Priolo Earthquake Fault Zoning
Act provisions and the following policies:
a.
Require geologic studies or analyses for critical structures, and lifeline, high-occupancy, schools, and highrisk structures, within 0.5 miles of all Quaternary to historic faults shown on the Earthquake Fault Studies
Zones map.
b. Require geologic trenching studies within all designated Earthquake Fault Studies Zones, unless adequate
evidence, as determined and accepted by the County Engineering Geologist, is presented. The County
may require geologic trenching of non-zoned faults for especially critical or vulnerable structures or
lifelines.
c. Require that lifelines be designed to resist, without failure, their crossing of a fault, should fault rupture
occur.
4.12-28
County of Riverside Environmental Impact Report No. 521
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d. Support efforts by the California Department of Conservation, Division of Mining and Geology California
Geological Survey to develop geologic and engineering solutions in areas of disseminated ground
deformation due to faulting and seismic activity in those areas where a through-going fault cannot be reliably
located.
e. Encourage and support efforts by the geologic research community to better define the locations and
risks of county faults. Such efforts could include data sharing and database development with regional
entities, other local governments, private organizations, utility agencies or companies, and local
universities.
Policy S 2.2: Require geological and geotechnical investigations in areas with potential for earthquake-induced
liquefaction, landsliding or settlement as part of the environmental and development review process, for any
building structure proposed for human occupancy and any structure whose damage would cause harm, except for
accessory buildings.
Policy S 2.3: Require that a State-licensed professional investigate the potential for liquefaction in areas
designated as underlain by “Susceptible Sediments” and “Shallow Ground Water” for all general construction
projects, except for accessory buildings (General Plan Figure S-3).
Policy S 2.4: Require that a State-licensed professional investigate the potential for liquefaction in areas identified
as underlain by “Susceptible Sediments” for all proposed critical facilities projects (General Plan Figure S-3).
Policy S 3.12: Include a disclosure about wind erosion susceptibility on property title for those properties located within
“High” and “Very High” wind erosion hazard zones as shown on [General Plan] Figure S-8, Wind Erosion Susceptibility Map.
2. Land Use (LU) Element Policies
Policy LU 12.1 (Previously 11.1): Apply the following policies to areas where development is allowed and that
contain natural slopes, canyons, or other significant elevation changes, regardless of land use designation:
a.
Require that hillside development minimize alteration of the natural landforms and natural vegetation.
b. Allow development clustering to retain slopes in natural open space whenever possible.
c. Require that areas with slope be developed in a manner to minimize the hazards from erosion and slope
failures.
d. Restrict development on visually significant ridgelines, canyon edges and hilltops through sensitive siting
and appropriate landscaping to ensure development is visually unobtrusive.
e. Require hillside adaptive construction techniques, such as post and beam construction, and special
foundations for development when the need is identified in a soils and geology report which has been
accepted by the County.
f.
In areas at risk of flooding, limit encourage the limitation of grading, cut and fill to the amount necessary to
provide stable areas for structural foundations, street rights-of-way, parking facilities, and other intended
uses.
County of Riverside Environmental Impact Report No. 521
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4.12-29
4.12.3 Thresholds of Significance for Geology and Soils Resources
The proposed project would result in a significant impact related to geology or soils if it would:
A. Expose people or structures to potential substantial adverse effects including the risk of loss, injury or
death involving the rupture of a known earthquake fault as delineated on the most recent Alquist-Priolo
Earthquake Fault Zoning Map issued by the State Geologist for the area or based on other substantial
evidence of a known fault.
B. Expose people or structures to potential substantial adverse effects, including the risk of loss, injury or
death involving strong seismic groundshaking.
C. Expose people or structures to potential substantial adverse effects, including the risk of loss, injury or
death involving seismic-related ground failure, including liquefaction.
D. Expose people or structures to potential substantial adverse effects, including the risk of loss, injury or
death involving landslides.
E. Result in substantial soil erosion or the loss of topsoil.
F. Be located on a geological unit or soil that is unstable or that would become unstable as a result of the
project and potentially result in on- or off-site landslide, lateral spreading, subsidence, liquefaction or
collapse.
G. Be located on expansive soil, as defined in Table 18-1-B of the Uniform Building Code (1994), creating
substantial risks to life or property.
H. Have soils incapable of adequately supporting the use of septic tanks or alternative wastewater disposal
systems where sewers are not available for disposal of wastewater.
4.12.4 Effect of GPA No. 960 on the General Plan and on Geology
and Soils
The proposed project, GPA No. 960, would have spatial effects where it involves a variety of specific General
Plan Land Use Designation (LUD) corrections and changes, several Policy Area, Study Area and overlay changes,
proposals for new trail and road alignments and standards, and an incidental commercial policy for rural areas. In
addition, GPA No. 960 includes a number of updates to proposed roadway alignments and intersection locations,
as well as functional classifications (widths, number of lanes, level of service targets, etc.), where needed
throughout unincorporated Riverside County. In this section, the geology-related changes to the General Plan are
outlined and the effects of proposed changes relative to geotechnical hazards are discussed. Specific impacts and
mitigation for the project are then evaluated according to identified significance thresholds in the section
following this one.
4.12-30
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A.
Proposed Changes to the General Plan
As part of the project review process, geological and soils-related data in the General Plan was updated and
geological policies reviewed and revised where necessary. The existing General Plan addresses geological
resources in the Safety (S) Element. GPA No. 960 includes the following geology-related updates; text of the
revised General Plan policies is provided in Section 4.12.2.C.
Seismic Hazard Zones: Text was added and policies updated to accommodate State Seismic Hazard Zone
Maps as they are released by the State of California. Several related Safety policies were revised as part of this and
related exhibits were also updated: Figure S-1, Mapped Faulting in Riverside County, and Figure S-2, Earthquake
Fault Study Zones.
Geological Hazard Mapping: As part of GPA No. 960 a variety of geology-related exhibits were updated to
include the most recently available data. These include Figure S-3, Generalized Liquefaction (with a detail noting
the quad-specific updated information released by the State of California for the Murrieta Quadrangle; specific
mapping is provided in the applicable Area Plan maps); and Figure S-4, Earthquake-Induced Slope Instability
Map (also with Murrieta Quad details, highlighted in the applicable Area Plans).
In addition to these geological changes, a variety of LUD and policy area changes are proposed, as per the
descriptions in Section 3.0 (Project Description) of this EIR and associated Figure 3-1 (and corresponding maps
within each Area Plan) that may indirectly affect geological resources. Such changes would lead to either an
increase or decrease in development potential (density or intensity); the risks associated with introducing new
people and property into areas potentially subject to the various geological hazards outlined herein would be
increased correspondingly.
GPA No. 960 also includes new and revised policies which would be implemented at a future time in locations
not foreseeable at present; for example, the new incidental rural Retail-Commercial policy, Indian fee land
policies, as others as described in Section 3.0 of the EIR. Similarly, new maps for trails and Riverside County
roads (GP Figures C-7 and C-1, respectively, plus corresponding maps within each Area Plan) indicate general
road and trail alignments, but not specific locations since specific design and construction sites must be
determined based on specific site topography, existing development and timing, as well as both existing and
future levels of service to be met. Actual locations for these improvements would be determined based on site
assessment of opportunities and constraints, particularly as related to geological hazards, soils and geotechnical
suitability to determine environmentally preferred alignments to minimize adverse effects. Likewise, other
infrastructure and utilities, such as power transmission lines, water and sewer lines, and such, are also developed
based on the providing agency’s existing and future levels of service and need assessments and forecasts; typically
based on five-year capital improvement plans. Generally, however, such improvements are not proposed until
either specific new developments or overall growth within an area triggers their need.
Accordingly, specific locations and timing of future infrastructure, including power and natural gas transmission
lines, water and sewer lines and pumps, as well as roads, schools and other public services, are not presently
foreseeable beyond the master countywide level already depicted in the 2003 General Plan and addressed
previously in EIR No. 441. These improvements would require site-specific analyses and mitigation when
proposed as part of (or to serve) future development as the General Plan builds out. As such, future impacts and
mitigation would be assessed programmatically pursuant to the performance standards outlined in this EIR, as
well as EIR No. 441, with project-specific analysis and mitigation developed at the later individual project stage.
County of Riverside Environmental Impact Report No. 521
Public Review Draft  March 2014
4.12-31
B.
Analysis of GPA No. 960 Effects on Geology and Soils
The General Plan is concerned mainly with the physical build out of Riverside County; many of the changes
associated with GPA No. 960 would affect planned land usage. In particular, proposed changes affect land use
overlays, land use designations and policies that affect the conversion of rural, semi-rural, agricultural and vacant
lands to suburban or urban uses in various parts of Riverside County.
Table 4.12-C: Geology Hazard Areas within Riverside County
Geologic Hazard
Earthquake Zones
Riverside County
Area (acres)
Areas of Potentially
Affected by Project (acres)
87,490
103,640
9,610
7,320
149,020
123,570
1,559,290
569,370
57,780
9,470
14,450
54,760
10,720
2,900
267,790
2,216,440
18,290
75,190
94,690
301,650
486,220
35,540
520
4,060
9,100
240
21,680
839,950
2,894,540
860,310
270
56,160
137,840
16,890
Alquist-Priolo Earthquake Zones
County Earthquake Zones
Liquefaction Susceptibility
Very High
High
Moderate
Low
Very Low
Subsidence
Active Areas
Susceptible Areas
Landslide Hazards
Very High
High
Moderate
Low
Wind Erosion Hazard
Very High
High
Moderate
Low
Source: Riverside County GIS Dept., analysis of RCLIS GIS data, 2011.
Table 4.12-D: Potential Project Areas Affected by Geologic Hazard Areas
Project
Components
Countywide Changes
Parcel-Specific LUD Changes
Incidental Rural Commercial
Policy
County Trails
County Roads
Conservation Acquisitions (OSCH Designated Lands)
Specific Policy Areas and Overlays
Aguanga RVSA
Airport Policies7
Anza Valley Policy Area
Chiriaco Summit RVSA
El Cariso RVSA
Fish Farms
Good Hope RVLUO
Lakeland Village
4.12-32
Earthquake
Zones
Liquefaction
Susceptibility1
Subsidence
Landslide
Hazards2
Wind
Erosion
Hazard3
Comments
Poss.4
Poss.
Poss.
Poss.
Poss.
Depending on location4
Poss.4
Poss.
Poss.
Poss.
Poss.
Depending on location4
X
X
X
X
X
X
X
X
X
X
Expected5
---
---
---
---
---
X
X
X
X
------X
X
X
X
X
--X
X
---
X
X
X
X
--X
X
X
X
--X
-----------
X
X
X
X
X
X
X
X
No development
potential.
RVSA deleted6
Text revision6
RVSA deleted6
County of Riverside Environmental Impact Report No. 521
Public Review Draft  March 2014
Project
Components
Earthquake
Zones
Liquefaction
Susceptibility1
Subsidence
Landslide
Hazards2
Meadowbrook RVLUO
Northeast Business Park
San Jacinto Ag. Potential
Development Study Area
Sky Valley RVO
X
---
--X
X
X
-----
Wind
Erosion
Hazard3
X
X
X
X
X
X
X
Study Area deleted6
---
X
X
---
X
No revision6
Key:
RVO = Rural Village Overlay
OS-CH = Open Space Habitat Conservation
RVLUO = Rural Village Land Use Overlay
RVSA = Rural Village Study Area
Comments
Footnotes:
1.
Liquefaction susceptibility within area mapped as very high, high or moderate.
2.
Landslide hazards within area mapped as very high, high or moderate.
3.
Wind erosion hazard within area mapped as very high, high or moderate.
4.
“Poss.” denotes possible geologic hazard that may apply depending on future location.
5.
Depend on future locations, but due to their linearity, marked geohazards area expected.
6.
Development consistent with existing LUDs remains permitted.
7.
LUD revisions associated with areas around Blythe, Flabob and Riverside Municipal Airports.
Source: Riverside County GIS Dept., results of GIS mapping analysis, GPA No. 960 project information, 2010.
Table 4.12-C (Geology Hazards within Riverside County) provides an overview of the geologic hazards mapped
within the county and those found to exist on or near the locations of specific spatial land use changes proposed
under GPA No. 960. The table also indicates how many acres within the areas affected by GPA No. 960 would
be directly affected by those same geologic hazards. The region that would be most affected by steep slopes and
landslides is the Anza Valley. Of a total of over 74,700 acres in the Anza Valley, 3,600 acres (5%) have a high to
very high susceptibility for slope failure. The proposed trail alignments would be most affected by liquefaction
(16,300 acres), fault rupture (over 6,850 acres) and wind erosion (over 28,800 acres), but do not include habitable
structures. Other areas of principal effect would be Anza Valley (nearly 20,000 acres for wind erosion and 5,300
acres for liquefaction and the Aguanga Rural Village Overlay region, with 1,540 acres within fault rupture hazard
areas).
For land use policy changes without currently assigned locations (Indian fee lands, incidental rural commercial,
etc.), geohazards cannot be delineated at present. Likewise, the potential for future development occurring within
the proposed revised policy areas and overlays has been generalized for this EIR, but due to the large scale of
Riverside County and these policy areas, site-specific geohazards cannot be accurately assessed at this time. More
generally, future development accommodated by the updated General Plan could be affected by a variety of
geologic hazards depending upon location. These include fault rupture, groundshaking, liquefaction, lateral
spreading and landslide hazards associated with seismic activity, as well as subsidence and soil erosion from wind
or water. Where not foreseeable at this time, such affects must be addressed programmatically, as outlined in the
subsequent section.
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4.12-33
4.12.5 Geology and Soils - Impacts and Mitigation
A.
Would the project expose people or structures to potential substantial adverse
effects, including the risk of loss, injury or death involving the rupture of a
known earthquake fault, as delineated on the most recent Alquist-Priolo Earthquake Fault Zoning Map issued by the State Geologist for the area or based on
other substantial evidence of a known fault?
Impact 4.12.A – Expose People or Structures to Substantial Adverse Effects Due to Rupture of a Known
Earthquake Fault: Future development accommodated by the proposed project would increase rural, suburban
and urban uses in Riverside County. This may increase the potential for property loss, injury or death resulting
from development where it occurs on or adjacent to known or as of yet undetected earthquake fault zones.
Compliance with existing laws, regulatory programs, General Plan policies and existing (EIR No. 441) Mitigation
Measure 4.10.1A would be sufficient to ensure that fault rupture impacts to future development accommodated
by GPA No. 960’s proposed General Plan changes would be less than significant.
1. Analysis of Impact 4.12.A
Known and unknown fault zones crisscross the County of Riverside. As indicated in Table 4.12-C, within
unincorporated Riverside County as a whole, there are a total of roughly 103,700 acres of County Earthquake
Zones and 87,500 acres of Alquist-Priolo Earthquake Zones (note, these two categories are not mutually
exclusive). Where project changes affect county fault hazard areas and Alquist-Priolo earthquake fault hazard
zones, future development on or near these faults would increase the potential for property loss, injury or death.
Table 4.12-C indicates the portions of the proposed project with known spatial impacts as they relate to the
mapped seismic and geologic hazards.
As detailed in Figure 4.12.1, Alquist-Priolo earthquake fault hazard zones have been established along the traces
of the Elsinore, San Jacinto and San Andreas Faults. Some lands within existing Alquist-Priolo zones or Countydesignated fault zones would be designated “Community Development” or “Rural” in GPA No. 960. Future
development on these lands may result in the construction and occupation of structures, critical facilities and
pipelines adjacent to known and/or as yet undetected earthquake fault zones. Such development would increase
the number of people and the amount of developed property exposed to fault rupture hazards.
To lessen the potential for property loss, injury or death that could result from rupture of faults during earthquake
events, the State of California has provided strict regulations (see below) that each county and city must follow to
ensure impacts from fault rupture are reduced to less than significant levels. In addition, Riverside County
ordinances, policies and mitigation measures have been identified to reduce potential impacts associated with fault
rupture hazards to a less than significant level. With adherence to these regulatory measures, project impacts
related to fault zones would be less than significant.
2. Regulatory Compliance for Impact 4.12.A
As detailed and explained below, compliance with the following existing laws, regulatory programs, General Plan
Policies and Mitigation Measure No. 4.10.1A from EIR No. 441 are sufficient to ensure that impacts related to
fault rupturing associated with future development resulting from GPA No. 960 changes to the General Plan are
less than significant.
4.12-34
County of Riverside Environmental Impact Report No. 521
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a. Compliance with Riverside County Regulations
Local regulations that would reduce impacts related to fault rupture include, but are not limited to, the following:
Ordinance No. 547 - Implementing the Alquist-Priolo Earthquake Fault Zoning Act: This ordinance
establishes that all applicants for a project located within an earthquake fault zone, as shown on the maps
prepared by the State Geologist, shall comply with all of the provisions of the A-P Act and the adopted policies
and criteria of this ordinance. This ordinance is applicable to all grading, building and structures, and regulates
numerous aspects of design to ensure that structures and facilities are designed with the appropriate level of
seismic safety warranted by the geology of their location. Among other things, it addresses grading, slopes and
compaction, erosion control, retaining wall design and earthquake fault zones. In addition to the requirements of
this ordinance, all applicants for a building permit for a structure used for human occupancy that lies within an
earthquake fault zone delineated by the State Geologist pursuant to PRC Section 2621 et seq. and which is subject
to Ordinance No. 547, shall comply with all the provisions of this ordinance prior to issuance of a building
permit. Compliance with these regulations would ensure that no habitable structure, critical facility or
infrastructure is built on an active fault, minimizing the potential for fault rupture to affect the structure and cause
harm to occupants and possessions. Implementation of, and compliance with, these measures would ensure
potential seismic impacts, such as fault rupture risks, are reduce to less than significant.
b. Compliance with Existing and Proposed New or Revised General Plan Policies
Compliance with Policy S 2.1 (including its proposed revisions – see Section 4.12.2.C for full text of the policy)
would ensure that future development complies with the Alquist-Priolo Earthquake Fault Zoning Act through the
provisions of a geologic study for any project within one-half mile of any Quaternary through historic faults
shown on the Earthquake Fault Studies Zones map. Based on the study, development projects may be required
to adhere to specific setbacks from faults, engineer structures to specific tolerances, engineer soils, etc. This
policy ensures that no habitable structure, critical facility or infrastructure is built on an active fault. Thus, revised
Policy S 2.1 would contribute to ensuring that fault rupture impacts on future development are less than
significant.
In addition, General Plan policies that would also contribute to avoiding, reducing or minimizing seismic risks,
such as faulting hazards, include: S 2.5, 2.6, 3.1, 3.3, 3.4, 3.8, 7.7, 7.8, 7.11; as well as proposed revised Policies S
2.2, 2.3 and 2.4.
c.
Compliance with Existing Mitigation Measures from EIR No. 441
In EIR No. 441, prepared for the 2003 RCIP General Plan, Mitigation Measure 4.10.1A was imposed to reduce
impacts from fault rupture to less than significant by requiring geotechnical studies in areas that are within fault
zones and using the resultant information to ensure that no habitable structure is built on an active or potentially
active fault. Although the potential impacts of this project would be reduced to less than significant through
regulatory compliance as per above, this measure was programmatic in nature and thus remains applicable to this
project.
Existing Mitigation Measure 4.10.1A: Before a project is approved or otherwise permitted within an A-P
Zone, County Fault Zone, within 150 feet of any other active or potentially active fault mapped in a published
United States Geologic Survey (USGS) or CGS reports, or within other potential earthquake hazard area (as
determined by the County Geologist), a site-specific geologic investigation shall be prepared to assess potential
County of Riverside Environmental Impact Report No. 521
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4.12-35
seismic hazards resulting from development of the project site. The site-specific geotechnical investigation shall
incorporate up-to-date data from government and non-government sources.
Based on the site-specific geotechnical investigation, no structures intended for human occupancy shall be
constructed across active faults. This site-specific evaluation and written report shall be prepared by a licensed
geologist and shall be submitted to the County Geologist for review and approval prior to the issuance of building
permits. If an active fault is discovered, any structure intended for human occupancy shall be set back at least 50
feet from the fault. A larger or smaller setback may be established if such a setback is supported by adequate
evidence presented to and accepted by the County Geologist.
3. Finding on Significance for Impact 4.12.A
Compliance with the above regulations, General Plan policies and existing mitigation measure would ensure that
significant impacts related to fault rupture are less than significant.
B.
Would the project expose people or structures to potential substantial adverse
effects, including the risk of loss, injury or death involving strong seismic
groundshaking?
Impact 4.12.B – Expose People or Structures to Substantial Strong Seismic Groundshaking: Like all of
Southern California, Riverside County has experienced and will continue to face groundshaking resulting from
activity on local and regional faults. Future development consistent with GPA No. 960 may increase the potential
for property loss, injury or death resulting from this groundshaking hazard. Compliance with existing laws,
regulatory programs, General Plan policies and existing EIR No. 441 mitigation measures would be sufficient to
ensure that this impact is less than significant.
1. Analysis of Impact 4.12.B
The Federal Emergency Management Agency (FEMA), in association with the National Institute of Building
Sciences, utilizes a standardized methodology based on GIS for earthquake loss estimation. This methodology,
the United States Hazards system (HAZUS), is designed to produce loss estimates for use by state, regional and
local governments in planning for earthquake loss mitigation, emergency preparedness and response and recovery
operations. This methodology deals with nearly all aspects of the built environment and with a wide range of
different types of losses. HAZUS estimations are based on current scientific and engineering knowledge and may
incorporate data on soil type, liquefaction and landslide susceptibility and building inventory. A complete
accounting of HAZUS data are provided in Appendix H of the existing General Plan.
The provisions of the California Building Standards Code (CCR Title 24) regulate the design and construction of
a building’s structural, plumbing, electrical and mechanical systems to ensure seismic safety, as well as fire safety,
energy conservation and accessibility. In addition, Riverside County Municipal Code Chapter 15.60 adopted
pursuant to the requirements of the Alquist-Priolo Earthquake Fault Zoning Act and the policies of the State
Mining and Geology Board regulates all permit applications for development projects.
Increases in population and residential and non-residential development as a result of GPA No. 960’s General
Plan changes would increase the exposure of persons and property to groundshaking hazards. The measures
identified below would mitigate potential groundshaking effects to less than significant levels.
4.12-36
County of Riverside Environmental Impact Report No. 521
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2. Regulatory Compliance for Impact 4.12.B
As detailed and explained below, compliance with the following existing laws, General Plan policies and existing
EIR No. 441 mitigation measures are sufficient to ensure that seismic groundshaking impacts to future
development accommodated by the project would be less than significant.
a. Compliance with State and County Regulations
Compliance with the following State of California and County of Riverside regulations would aid in preventing
significant impacts related to seismic groundshaking.
California Building Standards Code, Section 1613: This portion of the Code addresses structural design for
earthquake loads, to ensure new structure meet necessary seismic safety and protection standards, thereby
reducing potential impacts from earthquakes.
Riverside County Municipal Code Chapter 15.60 - Earthquake Fault Area Construction Regulations: As
indicated above, the County of Riverside implements the seismic regulations via its Municipal Code Chapter 15.60
(Earthquake Fault Area Construction Regulations) and Sections 15.60.010 through 15.60.070 which are applicable
to all construction in Riverside County. These regulations ensure new construction adheres to the necessary
seismic standards to protect against groundshaking.
b. Compliance with Existing and Proposed New or Revised General Plan Policies
Compliance with Policy S 7.7 (see Section 4.12.2.C for text of policy) would ensure that development standards,
designs and construction practices are implemented to reduce groundshaking risk to tolerable levels for projects
involving critical facilities, large-scale residential development and major commercial and industrial development.
Thus, Policy S 7.7 would contribute to ensuring that groundshaking impacts on future development are less than
significant.
Proposed revised Policy S 2.2 would also contribute to reducing seismic risks for the reasons mentioned in the
prior impact discussion. Other General Plan polices that would also contribute to avoiding, reducing or
minimizing seismic risks, such as groundshaking hazards, include: S 2.5, 2.6, 3.1, 3.3, 3.4, 3.8, 7.8, 7.11; as well as
proposed revised Policies S 2.2, 2.3 and 2.4.
c.
Compliance with Existing Mitigation Measures from EIR No. 441
In EIR No. 441, which was certified for the 2003 RCIP General Plan it was determined that in order to reduce
impacts associated with seismic groundshaking, mitigation would be necessary. The mitigation measures listed
below are from EIR No. 441 and shall also apply as part of the mitigation for programmatic EIR No. 521.
Existing Mitigation Measures 4.10.2A, 4.10.2B and 4.10.2C were imposed to provide adequate mitigation for
potential groundshaking hazards and to reduce impacts due to seismically induced groundshaking to less than significant. The measures also provided flexibility for the County of Riverside when requiring site-specific seismic
assessments for developments potentially subject to groundshaking and complying with Riverside County design
standards. Although the potential impacts of this project would be reduced to less than significant through regulatory compliance as per above, this measure was programmatic in nature and thus remains applicable to this
project.
County of Riverside Environmental Impact Report No. 521
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4.12-37
Existing Mitigation Measure 4.10.2A: The design and construction of structures and facilities shall adhere to
the standards and requirement detailed in the California Building Code (California Code of Regulations, Title 24),
County Building Code, and/or professional engineering standards appropriate for the seismic zone in which such
construction may occur. Conformance with these design standards shall be enforced through building plan
review and approval by the Riverside County Department of Building and Safety prior to the issuance of building
permits for any structure or facility.
Existing Mitigation Measure 4.10.2B: As determined by the County Geologist, a site-specific assessment shall
be prepared to ascertain potential groundshaking impacts resulting from development. The site-specific
groundshaking assessment shall incorporate up-to-date data from government and non-government sources and
may be included as part of any site-specific geotechnical investigation required in [existing EIR No. 441]
Mitigation Measure 4.10.1A. The site-specific groundshaking assessment shall include specific measures to reduce
the significance of potential groundshaking hazards.
This site-specific groundshaking assessment shall be prepared by a licensed geologist and shall be submitted to the
County Geologist for review and approval prior to the issuance of building permits.
Existing Mitigation Measure 4.10.2C: The standards stated in [existing EIR No. 441] Mitigation Measures
4.10.2A and 4.10.2B shall apply to any structure of facility that undergoes expansion, remodeling, renovation,
refurbishment or other modification.
3. Finding on Significance for Impact 4.12.B
Compliance with the above regulations, General Plan policies and existing EIR No. 441 mitigation measures
would ensure that significant impacts of strong seismic groundshaking on future development accommodated by
the proposed project are reduced to less than significant.
C.
Would the project expose people or structures to potential substantial adverse
effects, including the risk of loss, injury or death involving seismic-related
ground failure, including liquefaction?
Impact 4.12.C – Expose People or Structures to Substantial Adverse Effects Due to Seismic-Related
Ground Failure, Including Liquefaction: Portions of unincorporated Riverside County are susceptible to
liquefaction, a destructive secondary effect of strong seismic shaking. Future development associated with GPA
No. 960 within Riverside County would increase the potential for the placement of structures and facilities in or
near areas susceptible to liquefaction. Impacts associated with seismic-related ground failure and liquefaction
would be reduced through compliance with existing laws, General Plan policies and existing EIR No. 441
mitigation measures. Compliance with the regulations described below would ensure that seismic-related ground
failure and liquefaction risks to future development accommodated by the project would be less than significant.
1. Analysis of Impact 4.12.C
Liquefaction is a process by which water-saturated materials (including soil, sediment and certain types of volcanic
deposits) lose strength and may fail during strong groundshaking. Liquefaction is defined as “the transformation
of a granular material from a solid state into a liquefied state as a consequence of increased pore-water pressure.”
Liquefaction occurs worldwide, commonly during moderate to great earthquakes. Four kinds of ground failure
commonly result from liquefaction: lateral spread, flow failure, ground oscillation and loss of bearing strength.
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Areas of “very high” or “high” susceptibility to liquefaction have been identified adjacent to the Santa Ana River,
in the vicinity of Hemet and San Jacinto, in the southern Coachella Valley and along the eastern boundary of
Riverside County adjacent to the Colorado River. As indicated in Table 4.12-C, within unincorporated Riverside
County as a whole, there are a total of roughly 150,000 acres of “very high” and 123,500 acres of “high”
liquefaction susceptibility. At roughly 1,559,300 acres, the largest share of Riverside County is at moderate risk
for liquefaction. Lastly, roughly 627,200 acres have a low or very low liquefaction risk. Increases in population
and the residential and non-residential development accommodated by GPA No. 960 would increase the exposure of persons and property to liquefaction hazards. Table 4.12-C indicates the portions of the proposed
project with known spatial impacts as they relate to the mapped liquefaction potential and other seismic and geologic hazards. The measures identified below would reduce the potentially significant seismic impacts to future
development to less than significant.
2. Regulatory Compliance for Impact 4.12.C
As detailed and explained below, compliance with the following existing laws, regulatory programs and General
Plan policies are sufficient to ensure that impacts to seismic-related ground failure and liquefaction as a result of
GPA No. 960 would be less than significant.
a. Compliance With County Regulations
Compliance with the following Riverside County regulation would further prevent significant impacts to seismicrelated ground failure including liquefaction.
Ordinance No. 547: As mentioned for the prior two impacts, this ordinance addresses design and construction
for development within earthquake fault zones and Alquist-Priolo Act compliance, among other things. Compliance with these regulations would ensure that no habitable structure, critical facility or infrastructure is built on an
active fault, minimizing the potential for fault rupture to affect the structure and cause harm to occupants and
possessions. Implementation of, and compliance with, these measures would ensure that potential seismic impacts, including seismically-induced ground failure and liquefaction, are reduced to less than significant.
b. Compliance with Existing General Plan Policies
Of the General Plan policies listed in Section 4.12.2.C, Policies S 2.2 through 2.7 in particular provide mitigation
for impacts associated with seismic ground failure and liquefaction. Implementation of these General Plan
policies would reduce the seismic risks posed to future growth and development within Riverside County
accommodated by the project. Specifically:
Policies S 2.5, 2.6 and 2.7: These policies directly address reducing seismic-related ground failure, including
liquefaction, by requiring specific grading standards for those development projects that involve grading.
Other General Plan polices that would also contribute to avoiding, reducing or minimizing seismic risks, such as
ground failure and liquefaction, include: S 3.1, 3.3, 3.4, 3.8, 7.7, 7.8 and 7.11.
c.
Compliance with Proposed New or Revised General Plan Policies
Policies S 2.2, 2.3 and 2.4: These policies directly address potential seismically induced ground failure and
liquefaction by requiring developments and construction proposed in those areas that have been identified as
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having a potential for liquefaction to be reviewed by State-licensed professionals in order to ensure any potential
hazards are mitigated through appropriate design and engineering.
d. Compliance with Existing Mitigation Measures from EIR No. 441
In EIR No. 441, prepared for the 2003 RCIP General Plan, Mitigation Measures 4.10.3A and 4.10.3B were
imposed to mitigate potential liquefaction hazards. The measures ensure that areas subject to liquefaction are
studied by a qualified geologist and that the resultant study recommendations are implemented as part of project
conditions of approval. Although potential impacts would be reduced to less than significant through regulatory
compliance as per above, these measures are programmatic in nature and thus remains applicable to future
development accommodated by this project as well.
Existing Mitigation Measure 4.10.3A: As determined by the County Geologist, a site-specific assessment shall
be prepared to ascertain potential liquefaction impacts resulting from development. The site-specific liquefaction
assessment shall incorporate up-to-date data from government and non-government sources and may be included
as part of any site-specific geotechnical investigation required in [existing EIR No. 441] Mitigation Measure
4.10.1A. This site-specific groundshaking assessment shall be prepared by a licensed geologist and shall be
submitted to the County Geologist for review and approval prior to the issuance of building permits.
Existing Mitigation Measure 4.10.3B: Where development is proposed within an identified or potential
liquefaction hazard area (as determined by the County Geologist), adequate and appropriate measures such as (but
not limited to) design foundations in a manner that limits the effects of liquefaction, the placement of an
engineered fill with low liquefaction potential, and the alternative siting of structures in areas with a lower
liquefaction risk, shall be implemented to reduce potential liquefaction hazards. Any such measures shall be
submitted to the Riverside County Geologist and the County Department of Building and Safety for review prior
to the approval of the building permits.
3. Finding on Significance for Impact 4.12.C
Compliance with the above regulations, General Plan policies and existing EIR No. 441 mitigation measures
would ensure that significant impacts from seismic ground failure and liquefaction are avoided or minimized to
less than significant.
D.
Would the project expose people or structures to potential substantial adverse
effects, including the risk of loss, injury or death involving landslides?
Impact 4.12.D – Expose People or Structures to Substantial Adverse Effects Due to Landslides:
Landslides and rockfall can occur throughout Riverside County as a result of seismic activity and other natural
processes, as well as resulting from human activity. Future development within Riverside County accommodated
by GPA No. 960 would increase the potential for structures and facilities in areas susceptible to landslides or
rockfall. Compliance with existing laws and General Plan policies would reduce potential landslide and rockfall
impacts to less than significant levels.
1. Analysis of Impact 4.12.D
Areas highly susceptible to landslides and rockfall occur in and adjacent to mountainous areas throughout the
county. Thus, as delineated in Table 4.12-D (Potential Project Areas Affected by Geologic Hazard Areas), some
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of the development accommodated by the project would occur in areas highly susceptible to this hazard. Table
4.12-C indicates that within unincorporated Riverside County, there are a total of roughly 94,700 acres at “very
high” landslide potential; of which, roughly 500 acres are within areas changed under GPA No. 960. Another
301,700 acres have a “high” potential; the project area includes roughly 4,100 acres of this.
Development in susceptible areas would increase the potential for injury, death or loss of property. Table 4.12-C
indicates the portions of the proposed project with known spatial impacts as they relate to mapped seismic and
geologic hazards, including landslides and rockfall. Riverside County policies have been identified to lessen the
potential for property loss, injury, or death resulting from landslides or rockfall.
2. Regulatory Compliance for Impact 4.12.D
As detailed and explained below, compliance with the following existing laws, regulatory programs and General
Plan policies would aid in avoiding or reducing potentially significant impacts to landslides and rockfall as a result
of GPA No. 960.
a. Compliance with State and County Regulations
Compliance with the following state and county regulations would aid in preventing significant landslide and
rockfall impacts.
Title 24 - California Building Standards Codes: All development in Riverside County is required to comply
with the California Building Standards Codes in Title 24. Prior to any construction of structures, including
roadways and infrastructure, in areas of steep slopes or landslide potential, a geotechnical assessment is required
by a certified geologist. This report includes recommendations as to the stability of the site and outlines requirements for grading, site preparation and building foundations. Grading regulations implemented by the County of
Riverside require that grading plans be prepared and reviewed by the County of Riverside prior to the issuance of
grading permits and are consistent with the geotechnical study; these regulations are in place to ensure that landslide hazards are reduced to less than significant.
b. Compliance with Existing General Plan Policies
The following existing policies of the Riverside County General Plan would further prevent significant impacts
related to landslides and rockfall.
Policies S 2.5 - 2.8: For future development associated with the GPA No. 960, the County of Riverside would
require implementation of Policies S 2.5 through S 2.8 to minimize the effects of landslides and rockfall on new
development and infrastructure to less than significant levels.
Policies S 3.1 - 3.7: These policies require landslide potential hazard management zones, including geotechnical
and geologic investigations, site stability evaluations and design recommendations, as well as adequate mitigation,
against potential hazardous slope conditions.
Proposed Revised Policies S 2.2 - 2.4: For future development associated with the GPA No. 960, the County
of Riverside would require implementation of Policies S 2.2 through S 2.4 to minimize the effects of landslides
and rockfall on new development and infrastructure to less than significant levels. Other General Plan polices
that would also contribute to avoiding, reducing or minimizing rockfall and landslide risks include: S 3.8, 7.7, 7.8
and 7.11.
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3. Finding on Significance for Impact 4.12.D
Compliance with the above regulations and General Plan policies would ensure that significant impacts related to
landslides and rockfall are avoided and minimized to less than significant.
E.
Would the project result in substantial soil erosion or the loss of topsoil?
Impact 4.12.E – Result in Substantial Soil Erosion or Topsoil Loss: Areas exposed during future
development activities accommodated by GPA No. 960 revisions to the General Plan would be prone to erosion
and loss of topsoil. Wind and water are the two biggest factors in soils erosion. Human activities that remove
vegetation or disturb soil are the biggest contributor to erosion potential. Compliance with existing laws, General
Plan policies and existing EIR No. 441 mitigation measures help reduce potential soil erosion impacts and ensure
that future development would have a less than significant impact on soils.
1. Analysis of Impact 4.12.E
The future growth and development accommodated by GPA No. 960 would result in an increase in both
residential and non-residential structures, as well as infrastructure, roads and facilities. Such development would
result in alteration of existing topography, removal of existing vegetation layers and exposure of topsoil. Soil
erosion potential, either by wind or water, is substantially increased upon the exposure of underlying soils during
grading activities or other landform modifications.
Erosion by wind and windblown sand is an environmentally limiting factor throughout much of Riverside
County, especially in the Coachella Valley and eastern Riverside County. Measures identified below mitigate
potentially significant erosion impacts from future development accommodated by GPA No. 960. As shown in
Table 4.12-C, a total of roughly 21,700 acres of Riverside County are at “very high” potential for wind erosion
and nearly 840,000 acres are at “high.” A large portion of Riverside County, nearly 2,900,000 acres, is at
“moderate” wind erosion risk, while roughly 900,000 acres are at “low” risk.
The table also indicates the portions of the proposed project with known spatial impacts as they relate to mapped
wind erosion and blowsand potential. Riverside County policies and regulations have been identified to lessen
potential hazards associated with blowsand and loss of topsoil due to erosion, particularly wind erosion. Implementation of these policies and regulations would ensure that future development accommodated by the project
does not result in significant erosion or impacts to topsoil.
2. Regulatory Compliance for Impact 4.12.E
As detailed and explained below, compliance with the following existing laws, regulations and General Plan
policies would aid in avoiding or reducing potentially significant impacts to soil erosion and loss of topsoil to less
than significant levels.
a. Compliance With County Regulations
Compliance with the following Riverside County regulation would aid in preventing significant impacts to soil
erosion and the loss of topsoil.
Ordinance No. 484 – Blowing Sand Control: This ordinance establishes requirements for the control of
blowing sand within County-designated Agricultural Dust Control Areas. It defines activities that may contribute
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to wind erosion, places restrictions on erosive activities within these areas, establishes penalties for violations and
identifies procedures necessary to obtain permits. In addition, it specifies that prior to grading, developments
must have an approved erosion control plan that outlines how the site would be protected from soil run-off
during rain events and erosion hazards.
b. Compliance with Existing General Plan Policies
The following existing policies of the Riverside County General Plan would further prevent significant impacts to
soil erosion and the loss of top soil. See Section 4.12.2.C for full text of each of these policies. The
implementation of these General Plan policies would also aid in reducing the effect of soil erosion to less than
significant.
Policies S 3.5 and 3.6: For any development associated with the implementation of GPA No. 960, the County
of Riverside would be required to implement Policies S 3.5 and S 3.6 to minimize the effects of soil erosion by
identifying and encouraging mitigation of onsite and offsite slope instability, debris flow and erosion hazards on
land undergoing substantial improvements.
Policies S 3.11, 3.13 and 3.14: These policies further reduce significant wind erosion impacts by requiring studies
to determine the potential of hazardous impacts from wind erosion and identify the necessary best management
practices to prevent the erosion. They also require wind erosion susceptibility to be disclosed for all parcels with
high susceptibility. Compliance with these policies would aid in reducing potential adverse impacts of wind
erosion to less than significant levels.
c.
Compliance with Proposed New or Revised General Plan Policies
Compliance with Policy S 3.12 (see Section 4.12.2.C for full text of policy) would ensure that those parcels
susceptible to wind erosion are so noted on their property titles, thereby notifying existing or future property
owners of potential development issues related to wind erosion early on. Early awareness would aid in reducing
potential adverse impacts related to wind erosion, helping less potential wind erosion effects.
d. Compliance with Existing Mitigation Measures from EIR No. 441
In EIR No. 441, prepared for the 2003 RCIP General Plan, Mitigation Measures 4.10.9A, 4.10.9B and 4.10.9C
were imposed to reduce impacts from soil erosion and loss of topsoil to less than significant. And, Mitigation
Measure 4.10.8A was proposed to address wind erosion specifically. Although potential impacts would be
reduced to less than significant through regulatory compliance as per above, EIR No. 441 was programmatic and
thus these measures remain applicable to future development accommodated by this project as well.
Existing Mitigation Measure 4.10.8A: New development within identified or potential (as determined by the
County Geologist) wind hazard areas shall adhere to applicable provisions of Riverside County Ordinance No.
484.2 or other local, State, or federal requirements established to control or limit the windborne erosion of soil.
Prior to the approval of development permits, the County Building and Safety Department shall confirm that the
design of any proposed structure, facility, or use incorporates appropriate features to control and/or limit the
windborne erosion of soil.
Existing Mitigation Measure 4.10.9A: Riverside County, where required, and in accordance with issuance of a
National Pollutant Discharge Elimination System (NPDES) permit, shall require the construction and/or grading
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contractor for individual developments to establish and implement specific Best Management Practices (BMPs) at
time of project implementation.
Existing Mitigation Measure 4.10.9B: Prior to any development within the county, a Grading Plan shall be
submitted to the Riverside County Building and Safety Department and/or Riverside County Geologist for
review and approval. As required by the County, the grading plan shall include erosion and sediment control
plans. Measures included in individual erosion control plans may include, but shall not be limited to, the
following:
a.
Grading and development plans shall be designed in a manner which minimizes the amount of terrain
modification.
b. Surface water shall be controlled and diverted around potential landslide areas to prevent erosion and
saturation of slopes.
c. Structures shall not be sited on or below identified landslides unless slides are stabilized.
d. The extent and duration of ground disturbing activities during and immediately following periods of rain
shall be limited, to avoid the potential for erosion which may be accelerated by rainfall on exposed soils.
e. To the extent possible, the amount of cut and fill shall be balanced.
f.
The amount of water entering and exiting a graded site shall be limited though the placement of
interceptor trenches or other erosion control devices.
g. Erosion and sediment control plans shall be submitted to the County for review and approval prior to
the issuance of grading permits.
Existing Mitigation Measure 4.10.9C: Where required, drainage design measures shall be incorporated into the
final design of individual projects on site. These measures shall include, but will not be limited to:
a.
Runoff entering developing areas shall be collected into surface and subsurface drains for removal to
nearby drainages.
b. Runoff generated above steep slopes or poorly vegetated areas shall be captured and conveyed to nearby
drainages.
c. Runoff generated on paved or covered areas shall be conveyed via swales and drains to natural drainage
courses.
d. Disturbed areas that have been identified as highly erosive shall be (re)vegetated.
e. Irrigation systems shall be designed, installed, and maintained in a manner which minimizes runoff.
f.
The landscape scheme for projects within the project site shall utilize drought-tolerant plants.
g. Erosion control devices such as rip-rap, gabions, small check dams, etc., may be utilized in gullies and
active stream channels to reduce erosion.
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3. Finding on Significance for Impact 4.12.E
Compliance with the above regulations, General Plan policies and existing EIR No. 441 mitigation measures
would ensure that significant impacts related to soil erosion and loss of top soil, including wind erosion, are
reduced and mitigated to less than significant.
F.
Would the project be located on a geological unit or soil that is unstable, or
that would become unstable as a result of the project, and potentially result in
on- or off-site landslide, lateral spreading, subsidence, liquefaction or collapse?
Impact 4.12.F – Result in Development on Unstable Geological Units or Soils: Unstable geological units
and soils occur throughout Riverside County. Additionally, both natural and human activities have the potential
to cause geologic instability. If improperly engineered or constructed, some types of development, particularly
those involving heavy loads (concrete dams, for example) or affecting subsurface water levels (e.g., groundwater
pumping or replenishment facilities), have an increased potential to cause ground or soil failures. These types of
failures are in addition to those triggered by seismic events, as described in earlier impacts above. Future
development accommodated by GPA No. 960 would increase the potential for landslides, liquefaction, lateral
spreading and subsidence as a result of placement on unstable geological units or soils. However, compliance
with existing laws and General Plan policies discussed below would reduce potential impacts related to
development on or affected by unstable geological units or soil. Compliance with these would ensure that future
development accommodated by the project would have a less than significant impact.
1. Analysis of Impact 4.12.F
As the result of its varied geology, areas subject to landslide hazards, subsidence and liquefaction are found
throughout Riverside County. Soils susceptible to subsidence, hydroconsolidation or soil collapse may be
affected by a variety of natural or human activities, including earthquakes and the withdrawal of subsurface fluids.
Documented subsidence has occurred in the San Jacinto Valley, the Elsinore Trough and the southern Coachella
Valley.
Table 4.12-D indicates the components of GPA No. 960 which would be subject to these particular geologic
hazards and Table 4.12-C shows the acreage of each hazard type within Riverside County, as well as the portion
of the project’s known spatial components that occur within these mapped areas. Also, liquefaction and landslide
potential were analyzed in Impacts 4.12.C and 4.12.D, respectively.
Past construction of structures and facilities in areas of unstable geologic units and soils may increase the potential
for structure damage or interruption of utility service (through disruption of the facility). As a group, the
regulatory compliance measures identified below would ensure that future development accommodated by GPA
No. 960 does not result in any potentially significant impacts.
2. Regulatory Compliance for Impact 4.12.F
As detailed below, compliance with the following existing regulations and General Plan policies would aid in
avoiding or reducing potentially significant impacts resulting from future development on unstable geological
units or soils.
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a. Compliance with State and County Regulations
Compliance with the following State of California and County of Riverside regulations would further prevent
significant impacts related to unstable geology and soils.
Title 24 of the California Building Standards Code: All development within Riverside County is required to
be compliant with Title 24 of the California Building Standards Code which, among other things, addresses
construction of structures and facilities in areas subject to subsidence and lateral spreading. Prior to any
construction in areas on unstable soils, a geotechnical assessment of the site is required by a certified geologist and
the resultant recommendations must be implemented as a condition of project approval. These measures address
site-specific geology, slopes and soil stability, as well as the requirements for grading, site preparation and building
foundations. Also, grading regulations implemented by the County of Riverside require that approved grading
plans be consistent with the geotechnical study. Compliance with these regulations would ensure that impacts
due to unstable geology or soils are reduced to less than significant.
b. Compliance with Existing General Plan Policies
Compliance with Policies S 3.8, 3.9 and 3.10 (see Section 4.12.2.C for full text of each policy) would ensure future
development neither causes unstable geology or soils, nor introduces people and property to sites at significant
risk of such. Compliance with these policies would require geotechnical studies be performed and the resultant
recommended geotechnical measures be implemented as a condition of project approval to reduce any subsidence
or soil collapse risks to less than significant levels.
Other General Plan polices that would also contribute to avoiding, reducing or minimizing risks associated with
unstable geology and soils, such as landslide, lateral spreading, subsidence, liquefaction or collapse, include: S 2.5,
2.6, 2.7, 3.1, 3.2, 3.3, 3.4, 3.5, 3.7, 3.8, 3.9, 3.10 and 7.7; as well as proposed revised Policies S 2.1, 2.2, 2.3 and 2.4.
3. Finding on Significance for Impact 4.12.F
Compliance with the above regulations and General Plan policies would ensure that significant impacts related to
future development being located on a geological unit or soil that is unstable, or that would become unstable as a
result of the project, and potentially result in on- or off-site landslide, lateral spreading, subsidence, liquefaction or
collapse are mitigated and reduced to less than significant.
G.
Would the project be located on expansive soil, as defined in Table 18-1-B of
the Uniform Building Code (1994), creating substantial risks to life or property?
Impact 4.12.G – Result in Development on Expansive Soils: Expansive soils are widely distributed throughout Riverside County. Future development associated with GPA No. 960 would increase the potential for the
placement of structures and facilities in areas susceptible to damage resulting from expansive soils. Compliance
with existing laws and mitigation measures from EIR No. 441 help reduce potential impacts related to expansive
soils and ensure that they are less than significant.
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1. Analysis of Impact 4.12.G
Expansive soils are those soils with a significant amount of clay particles that have the ability to take on water
(swell) or give up water (shrink). When these soils swell, the change in volume exerts significant pressures on
loads (such as buildings) that are placed on them. Within Riverside County, expansive soils are widely dispersed
and can be found in hillside areas as well as low-lying alluvial bases. Future development accommodated by GPA
No. 960 could result in construction and occupation of structures in areas underlain by expansive soils.
Additionally, the past construction of structures and facilities on these soils may increase the potential for
structure damage or, through the disruption of utility facilities, an interruption of utility service. Measures are
identified below to mitigate potentially significant impacts associated with future development accommodated by
GPA No. 960.
2. Regulatory Compliance for Impact 4.12.G
As detailed below, compliance with the following existing regulations and General Plan policies would reduce
potential significant impacts resulting from expansive soils.
a. Compliance with State and County Regulations
Compliance with the following state and county regulations would reduce potentially significant impacts due to
expansive soils to less-than-significant levels.
California Building Standards Code (CCR Title 24): All development within Riverside County is required to
be compliant with the California Building Standards Code in Title 24, as related to the construction of structures
and facilities on expansive soils. Prior to any construction in areas on expansive soils, a geotechnical assessment
of the site is required by a certified geologist. This report must make recommendations as to the stability of the
site and the requirements for grading, site preparation and building foundations. As a condition of approval,
prior to the issuance of any grading permit, the County of Riverside requires grading plans satisfactorily address
the geotechnical assessment’s recommendations. These measures ensure that impacts from expansive soils are
reduced to less than significant.
b. Compliance with Existing Mitigation Measures from EIR No. 441
In EIR No. 441, prepared for the 2003 RCIP General Plan, Mitigation Measure 4.10.7A was imposed to reduce
impacts associated with expansive soils to less than significant. Although potential impacts would be reduced to
less than significant through regulatory compliance as per above, EIR No. 441 was programmatic and thus this
mitigation measure remains applicable to future development accommodated by this project as well.
Existing Mitigation Measure 4.10.7A: Proponents of new development within Riverside County shall adhere
to applicable policies and standards contained in the most recent version of the [California] Building Code related
to the construction of structures and facilities on expansive soils.
3. Finding on Significance for Impact 4.12.G
Compliance with the above regulations and mitigation would ensure that significant impacts related to expansive
soils are less than significant.
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H.
Would the project have soils incapable of adequately supporting the use of
septic tanks or alternative wastewater disposal systems where sewers are not
available for disposal of wastewater?
Impact 4.12.H – Result in Development on Soils Incapable of Supporting Septic Tanks or Alternative
Wastewater Disposal Systems: Future development accommodated by GPA No. 960, particularly in areas
outside of existing water and sewer service providers, would increase the potential for placement of structures and
facilities in areas where soils are incapable of adequately supporting septic tanks or alternative wastewater disposal
systems. However, due to laws, regulations and Riverside County policies addressing sewer requirements,
potential impacts associated with these types of soils would be avoided or reduced to less than significant levels.
1. Analysis of Impact 4.12.H
Extensive areas of unincorporated Riverside County lie outside of existing special districts that provide sewer
services. As a result, development of these lands must rely on the use of septic tanks or alternative wastewater
disposal systems. Septic systems are normally designed to function onsite and their proper functioning depends
upon specific soil characteristics. For example, liquids must be able to leach (flow out of) the septic system and
into the surrounding soil in order to work properly. Also, the septic system cannot be within the groundwater
table nor located where it would flow into or otherwise affect nearby streams or other drainages. Thus, in some
areas, soils have moderate to severe limitations on the use of septic tanks or alternative wastewater disposal
systems. To the extent GPA No. 960 accommodates development in such areas, there is the potential for effects
to soils that cannot adequately support the use of septic tanks or alternative wastewater disposal systems.
All septic-using development within Riverside County, however, has to comply with the provisions of the CBC
(CCR Title 24, Part 2), Chapters 18 and 18A, which address soils and foundations; and Chapters 16 and 16A, as
well as 17 and 17A, which address structural design, structural test and inspections. The provisions of Chapter 18
and 18A apply to all building and foundation systems. All construction, including that of septic tanks is required
to have an appropriate geotechnical investigation conducted pursuant to CBC standards. Such investigation
includes an assessment as to whether or not the site’s soils are suitable for onsite wastewater disposal systems.
Pursuant to County of Riverside enforcement of the CBC, no development utilizing onsite septic disposal shall be
approved unless the geotechnical study for the site determines that onsite soil conditions are suitable for septic
disposal. Developments proposing septic systems must also comply with the Regional Water Quality Control
Board Basin Plan, the Riverside County Department of Environmental Health’s Technical Guidance Manual and
applicable Riverside County ordinances. Compliance with these existing codes, plans and ordinances would
ensure that the effects of unsuitable soils on septic systems and alternative wastewater disposal systems are
avoided or minimized to less than significant levels.
2. Regulatory Compliance for Impact 4.12.H
Compliance with the following regulations, policies and existing mitigation measures from prior EIR No. 441
would ensure this impact is less than significant.
a. Compliance with State and County Regulations
State and county regulations regarding required geotechnical investigations (studies) as outlined in the California
Building Standard Code (Part 2, Volume CBC, Chapters 18 and 18A, “Soils and Foundations;” Chapters 16 and
16A, “Structural Design;” and Chapters 17 and 17A, “Structural Tests and Special Inspections”), as implemented
by the County of Riverside, would ensure that a site’s soil type, permeability, structural loads, design and integrity,
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as well as overall acceptability for a septic or alternative wastewater system are sufficiently established and verified
prior to project approval. This process would prevent significant impacts because project’s that cannot verify
acceptable septic disposal characteristics would not be allowed to be developed.
b. Compliance With Existing County General Plan Policies
Policy S 3.3 of the Riverside County General Plan would further reduce the impact associated with wastewater
disposal systems, such as septic tanks and other onsite facilities. See Section 4.12.2.C for full text of this policy.
Specifically, this policy requires an applicant prove to Riverside County Building Officials that the proposed
building site has soils sufficient to support septic or other wastewater system onsite prior to the issuance of a
building permit.
3. Finding on Significance for Impact 4.12.H
The analysis presented above indicates that future development accommodated by the project, GPA No. 960,
would have less than significant impacts resulting from septic tanks or alternative wastewater systems placed on
unsuitable soils. State laws and Riverside County regulations bar the issuance of building permits for facilities
relying on onsite septic wastewater disposal or other alternative onsite disposal, unless geotechnical study verifies
the suitability of the soils, geology and hydrology onsite. In this way, the County of Riverside would avoid any
significant impacts due to septic placed on unsuitable soils.
4.12.6 Significance After Mitigation for Geology and Soil
Resources
Implementation of existing regulations, General Plan policies, mitigation measures and existing requirements
would reduce potential impacts associated with fault rupture hazards, groundshaking, liquefaction, landslides and
rockfall, seismically induced settlement, subsidence and collapsible soils, soil erosion and loss of topsoil are either
avoided or minimized to less-than significant levels. Compliance with existing laws would also ensure structures,
people and property are protected from geologic hazards through engineering designed according to the applicable seismic and geological risks or that development is not permitted where such risks are excessive (i.e., higher
than typical for the given area or geology) and cannot be avoided. Compliance with existing and proposed
General Plan policies, as well as the EIR-specific CEQA mitigation measures presented above, would ensure that
any future development activities accommodated by the proposed project, GPA No. 960, appropriately identify
any known geologic hazards and fully mitigate, minimize or avoid the associated impacts. As such, project impacts associated with soils and geology would be less than significant.
County of Riverside Environmental Impact Report No. 521
Public Review Draft  March 2014
4.12-49
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4.12-50
County of Riverside Environmental Impact Report No. 521
Public Review Draft  March 2014