APPLICATION OF FEMA GUIDELINES FOR COASTAL FLOOD
HAZARD MAPPING TO A SITE IN NORTHERN CALIFORNIA
Dale Kerper1, Leslie Sakumoto2, Julio Zyserman1 and Seungjin Baek1
Guidance for coastal flood hazard analyses and mapping in the form of FEMA guidelines
has been available for the Atlantic Coast and the Great Lakes of the US for a number of
years. Guidelines and Specifications (G&S) applicable to the particular conditions of the
Pacific Coast of the US only became available in 2004, following the assembly of a
Technical Working Group (TWG) by FEMA. The study described herein constitutes the
first application of the new G&S for a site on the Pacific Coast of the US, and was
intended as a real test of their applicability. The physical settings existing at the selected
site provided an excellent example for testing the newly developed G&S.
INTRODUCTION
Guidance for coastal flood hazard analyses and mapping in the form of
FEMA guidelines has been available for the Atlantic Coast and the Great Lakes
of the US for a number of years. Guidelines and Specifications (G&S),
applicable to the particular conditions of the Pacific Coast of the US, only
became available in 2004, following the assembly of a Technical Working
Group (TWG) by FEMA. The Pacific Coast G&S allow for different approaches
to be followed based on coastal settings, data availability and other site-specific
environmental factors.
In order for the G&S to be tested for usability and practicality, a test site
including multiple coastal settings (classic open coast, river inlet, sand spit, and
lagoon) was selected in Northern California. It was also coincident with an
ongoing Flood Insurance Study and had topographic and bathymetric data
available.
The study described herein constitutes the first application of the new G&S
for a site on the Pacific Coast of the US, and was intended as a real test of their
applicability. The physical settings existing at the selected site provided an
excellent framework for the application of the newly developed G&S, and
allowed for the testing of several methodologies applied to multiple hazard zone
settings (riverine, coastal, tidal estuary).
Various alternative methodologies were used, the results were compared and
their applicability to the new G&S discussed. The different study approaches that
were followed and the results obtained from the analyses are presented and
discussed in the following sections.
1
2
DHI Water & Environment, Inc., 577 Second Street, Encinitas, CA 92024, USA
FEMA, Region IX, 1111 Broadway, 12th floor, Oakland, CA 94607, USA
1
2
STUDY SITE
The study site is located in Northern California where Pacific Ocean storms
are both strong and frequent, and bring large waves and heavy rains to the
region. The entire study site consists of about 12 miles of open coastline with
one lake/lagoon feature, and a river inlet with coastal floodplain. The tidally
influenced reach of the river and lagoon are also included in the analysis. The
study site is shown schematically in Fig. 1.
Figure 1. Overview of study area and key features.
3
As shown in Fig. 1, the study site settings include the following key features:
Approximately 12 miles of open beach backed by dunes.
A river inlet at the north end of the study site contributing significantly to
coastal flooding.
• A lagoon/lake at the south end of study area which can contribute
significantly to flooding, depending on the inlet configuration.
In agreement with the methodology set forth in the G&S, the following analyses
were performed to cover the various features present in the study area:
• 2D wave transformation modeling.
• Storm surge analysis based on tide gages.
• Transect based coastal hazard analysis.
• 2D wave setup modeling.
• 2D riverine analysis.
• Event based erosion.
• Combined riverine and coastal probability analysis.
•
•
APPLICATION OF THE GUIDELINES
The FEMA G&S provides general guidance for a variety of coastal settings.
The coastline was identified as a dune backed beach. Table 1 (Table D.4.2-2
from the FEMA G&S) illustrates the steps to be taken in the analysis of an open,
dune backed beach. The analysis procedure proceeds from the top of the table to
the bottom. Generally the G&S provided a very clear, efficient and flexible
methodology, but at times the details between the lines leave room for
interpretation and engineering judgment. This paper focuses on sharing the
interpretations applied whenever it was found that the G&S do not fully describe
all steps of a particular analysis. In the course of applying these procedures for
the first time, a number of issues were encountered in the following areas:
• WIS versus GROW wave data.
• Most Likely Winter Profile (MLWP).
• Storm duration for erosion computations.
• Storm duration associated with 1% Total Water Level (TWL).
• Beach erosion volumes.
• Wave setup (various issues).
• Dune with multiple berms.
• Overtopping.
• Splashdown.
• Overland wave propagation – WHAFIS.
• Combined coastal and riverine analysis and boundary conditions.
4
Table 1. Coastal analysis steps for dune backed beach.
WIS versus GROW Wave Hindcast Data
The G&S recommends the use of GROW (Global Reanalysis of Ocean
Waves) hindcast data from Oceanweather, Inc. to describe the offshore boundary
conditions for nearshore wave transformation modeling. At the time of execution
of this study, the three-hourly time series of GROW wind and wave parameters
covered the period January 1, 1970 12:00 a.m. to December 31, 2003 9:00 p.m.
In addition to the GROW data, three hourly Wave Information System
(WIS) data developed by the Waterways Experiment Station of the USACE were
available for the period January 1, 1956 12:00 a.m. to December 31, 1975 9:00
p.m. from WIS II Station 2033. This station is the closest in proximity to GROW
point 40256.
The two data sets are compared here for the six year long overlapping period
1970-1975 for the sake of highlighting their differences. Only the GROW data
were used in the analyses discussed in the following sections.
Fig. 2 shows a comparison of hindcasted significant wave heights at the two
stations. As the figure shows, there is significant scatter in the data set. Even so,
a clear tendency for WIS wave heights to be generally larger than those for
GROW data can be observed, which is in agreement with the remarks in section
D.4.4.1.3.2 of the G&S. The best linear fit (shown by the dashed line in Fig. 2)
between the two datasets has a slope of 0.749.
5
Figure 2 Comparison of significant wave heights from GROW station 40256 and WIS
statipn 2033 for the period 1970-1975.
At the time of writing, 10 years of the new Pacific Coast WIS hindcast data
have been released, but have not been evaluated under this study.
Most Likely Winter Profile – MLWP
The G&S include two approaches to determine the MLWP (most likely
winter profile) to be used in connection with the Kriebel and Dean (K&D) beach
erosion method:
1. A procedure for a study site without previously surveyed historical profiles
based on the combination of a survey from the crest of the berm or dune and
information about median sand diameter, D50, from sediment samples
collected at the site.
2. A procedure for a study site with historical profiles surveyed at the end of
the winter storm season.
With regards to the determination of the MLWP to be used as input to the K&D
erosion model, neither of the two methods that are described in the G&S could
be applied to the present study, because of the lack of suitable field data due to
rough winter conditions prevailing at the site during the period when the data
needed to be collected. Therefore, an alternative approach was suggested and
adopted for this study following consultations with the TWG.
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The adopted procedure consists of eroding the initial beach profile
(surveyed profile with an idealized dune) using the K&D geometric method
together with storm parameters (H0’, Tp, Gamma, SWL and duration D) with an
associated return period of one year as forcing. The 1-yearly parameters were
determined from an extreme value analysis (EVA) of the wave parameters and
SWL associated with each of the 34 storms leading to the largest TWL each
year, when calculated on the initial beach profile.
The procedure to define the MLWP using storm parameters with a return
period of one year was adopted because it was considered to have some physical
support: the underlying idea behind it is that the MLWP is shaped by storms
occurring on a yearly basis in average. Typical recession distances calculated
according to this procedure were in the range of 10 to 20m.
Storm Duration for Erosion Computation
The storm duration, D, is an important parameter for the computation of
profile recession and dune erosion associated with storm events. However, the
G&S do not include guidance regarding the definition of storm duration.
Therefore, a methodology was developed in consultation with the TWG.
The storm duration D was obtained by fitting a sin2σt curve (where σ = π/D
and t = time) to the time series of wave heights while keeping the maximum
wave height during the selected storm Hmax unchanged, as shown in Fig. 3. The
duration D was then calculated in such a way that the area under the Hmax.sin2σt
curve equaled that under the time series of wave height, i.e.
t1
D
H max ∫ sin 2 σ t = ∫ H (t ) dt
(1)
0
t0
or, after integration of the left-hand side of (1) above,
t1
D=
2 ∫ H (t ) dt
t0
(2)
H max
where the limits of integration t0 and t1 were selected to correspond with local
minima in wave height occurring no more than 3.5 days before and after the peak
of the storm (when H = Hmax), respectively. This limit was selected for the
present study based on the characteristics of storms in Northern California, and
should not be adopted for other studies without further considerations.
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Hmax
t0
t1
Duration D
Figure 3. Definition of storm duration D
Storm Duration Associated with 1% Total Water Level
The G&S specify that the beach profile be adjusted (eroded) in response to
the 1% total water level (TWL). Since a definition for the duration associated
with the 1% TWL is missing in the G&S, a value was adopted after consultations
with the TWG. The adopted value equals the average plus twice the standard
deviation of the duration associated with the storm leading to the yearly
maximum TWL for each of the 34 years covered by the GROW wave data.
Accurate determination of the actual value of the duration associated with
the 1% TWL is far from trivial, as the adjusted profile is used to compute
overtopping, splashdown and overland wave propagation as required. Different
values of storm duration would lead to quite different results, so clear guidance
for the determination of this parameter should be included in the G&S.
Beach Erosion Volumes
Once the 1% TWL has been determined, the G&S require that the final 1%
TWL and, eventually, overtopping rate q and volume V be computed on the
beach profile eroded by the storm associated with the 1% TWL. In the present
study, erosion volumes were calculated using both surveyed beach profiles and
corresponding MLWP as initial profile, and the profile adjusted according to the
1% TWL as final one. The results obtained are summarized in Table 2 below for
some selected transects. An overview of transect locations is shown if Fig. 4.
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Figure 4. Locations of transects for coastal analysis
It can be seen that the calculated erosion volumes are much larger than the
540 ft3/ft that are customarily used in FEMA flood studies for the Atlantic Coast
of the US.
Table 2. Computed erosion volume for selected transects
Transect
Respect to surveyed
profile
m3/m
ft3/ft
Respect to MLWP
02
3790
40796
250
2691
06
490
5274
475
5113
09
640
6889
635
6835
11
785
8450
920
9903
13
610
6566
695
7481
19
550
5920
590
6351
25
530
5705
530
5705
31
555
5974
400
4306
37
530
5705
490
5274
m3/m
ft3/ft
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Wave Setup – Tide Records
The G&S strongly urge the use of long term tide gage records in lieu of
performing detailed storm surge hindcast modeling, arguing that storm surge is a
small component of the total water level on the Pacific West Coast, and that
spatial variations between tide gages is small, implying the water level recorded
at a tide gage is representative of conditions over a wide area. Tidal levels
recorded by NOAA at Station 9419750, Crescent City, California were used in
the present analyses. The station is located approximately 15 miles away from
the study site. The time series of hourly water levels used in this study covers the
period January 1, 1940 9:00 a.m. to November 30, 2003 10:00 p.m. (∼ 64 years).
The recorded water levels include the effects of astronomical tide and surge.
The tide gage station is located inside the Crescent City harbor. It was not certain
whether wave setup was present at the tide gage location or not. Other “nearby”
gages are also in relatively shallow water and could not be used conclusively to
isolate wave setup from storm surge from one gage to the next.
Given the general uncertainty associated with what is contained in the
measurements, it was assumed that wave setup is not present in the gage
recordings. Wave modeling could have been used to help determine the
conditions under which the wave setup is present or not, but given the effort
required and the general level of uncertainty and judgment required to make this
determination, it is recommended to assume the gage does not include wave
setup.
The selected approach may lead to a conservative estimate of water levels if
wave setup is present in the gage recording. The G&S do not include
recommendations regarding how to treat the measured water levels in situations
like the one described here; such guidance would be most useful for future
studies facing similar questions.
Wave Setup - Combined Riverine and Coastal Analysis
In this study, the riverine and coastal water levels needed to be combined in
a probabilistic analysis where the two settings overlap, such as inside rivers,
estuaries and lagoons. The procedure is to complete the riverine and coastal
analysis independently and then combine the probabilities from each at the
overlap. The general procedure is illustrated in Figure 5 where the riverine and
coastal probabilities are added at a given water level to provide a combined
probability.
The G&S do not specify which boundary condition should be used for the
independent coastal and river models. For the river model it was assumed to
simply apply a static downstream coastal boundary condition set to Mean High
Water (MHW). This was determined in consultation with the TWG. The
difficulty arose when trying to determine which coastal water level to use when
combining with the riverine in the overlapping region inside the inlets of the
river and lagoon breach. It was initially assumed that the wave setup from the
transect-based coastal analysis should be excluded because it would not
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propagate into these areas. A literature search identified case studies in which
wave setup does indeed penetrate into inlets (Dunn 2000 and Tanaka 2000)
under special conditions.
Figure 5. Combined Coastal and Riverine Probability from FEMA G&S. (from FEMA
G&S, Figure D.4.4-15.)
The G&S does not provide guidance on this subject. A combined 2D
hydrodynamic and wave model was applied as a decision making tool to help
determine if the wave setup should be included in the combined analysis. The
model domains for the 5 cases tested are shown in Figure 6. The model domain
contains a short coastal shelf and an inlet into an idealized lagoon. The
dimensions and geometries are idealized but are made similar to the study site.
Case 1 is with a shallow and narrow inlet, Case 2 is with a shallow narrow inlet
with jetties, Case 3 is with a deep narrow inlet, Case 4 a deep narrow inlet with
jetties, and Case 5 is with a shallow wide inlet.
Figure 7 shows the model results of water level variation along a profile into
the lagoon. Case 1 is most similar to conditions at the study site, and it is evident
that static setup does enter into the lagoon. Based on these results, it was decided
to include the static setup in the boundary condition to the combined analysis.
The dynamic setup was not included in the river and inlet as it was believed that
the dynamic component would be damped and attenuated over a very short
distance.
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Case 1:
Shallow inlet
Case 3:
Deep inlet
Model Conditions:
Initial surface elevation = 2.6 m
Hs = 8.4 m
Tp = 13.8 s
Wave direction = shore normal
Breach width = 150 meters
Channel depth = +1 and -10 meters
Case 2: Shallow
inlet, jetties
Case 4: Deep
inlet, jetties
Case 5: Wide,
deep inlet
Figure 6. Wave and current model geometries for wave setup tests.
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Figure 7. Profile of water surface elevation rise due to static wave setup from
offshore through the centerline of the inlet, for 5 cases. (Elevation is in meters).
Overtopping, Splashdown, Overland Propagation
The G&S require that the mean overtopping rate q and volume V be
computed for those situations in which the computed total water level TWL
exceeds the elevation of the dune of the eroded beach profile. However, the
G&S do not provide guidance regarding which equation should be selected to
carry out these computations, from the many ones is included in the guidelines.
Furthermore, all the equations included in the G&S were derived for hard coastal
structures; their applicability to compute overtopping of the eroded dune is
unknown.
The equation for the calculation of mean overtopping discharge for shallow
foreshore slopes was selected following consultations with the TWG as such a
structure most closely resembles the geometry of a beach profile with an eroded
dune.
Use of the methodology included in the G&S to calculate splashdown limits
landward of the eroded dune resulted in very small horizontal distances (< 10m)
measured from the position of the eroded dune crest. The main reasons for this
are the mild slope of the eroded dune shoreface, together with low initial
velocities of the splashdown. In all cases considered, the range of the parameters
required as input for the computation of splashdown trajectories fell well beyond
the limits of the figure provided by the G&S to carry out these computations.
Consultations with the TWG confirmed the validity of extending the range of
applicability of the figure, which was achieved by using its analytical expression.
For those transects for which the crest of the eroded dune lies below the
static water level, the G&S require that overland wave propagation and
transformation be computed, and hazard zones be mapped accordingly. To this
purpose, the code called P-WHAFIS has been developed by FEMA for the
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Pacific Coast of the US. P-WHAFIS is a modified version of FEMA’s Wave
Height Analysis for Flood Insurance Studies (WHAFIS) computer program.
WHAFIS was developed for the Atlantic and Gulf coasts and includes hardcoded wind speeds. P-WHAFIS, on the contrary, allows for variable wind
speeds. The G&S require the mapping partner to obtain approval from FEMA
before using P-WHAFIS. Following consultations with the TWG, it was decided
to apply P-WHAFIS to compute wave dissipation and overland wave
propagation for present study.
CONCLUSIONS
The FEMA Guidelines and Specifications for the Pacific Coast generally
were found to be logical, flexible and well written, at least within the confines of
the coastal settings associated with this study. This study only tested a single
coastal setting and it should be expected that new questions will arise while
attempting to apply the G&S at other sites with different coastal settings.
One very key factor to the success of this study was the technical backup and
support of the TWG. The TWG provided invaluable feedback, insight and
eventual consensus that weighed heavily in the eventual success of this study.
The findings of this evaluation will be incorporated as an Appendix to the
G&S to provide additional guidance to future studies in similar coastal settings.
REFERENCES
Dunn, S., P. Nielsen, P.A. Madsen, and P. Evans. 2000. Wave setup in river
entrances. Proceedings of the 27th International Conference on Coastal
Engineering, ASCE, 3432-3445.
FEMA. 2004. Guidelines and Specifications for Flood Hazard Mapping
Partners, Section D.4 – Coastal Flooding Analyses and Mapping: Pacific
Coast.
Tanaka H., H. Nagabayashi, and K. Yamauchi. 2000. Observation of wave setup height in a river mouth. Proceedings of the 27th International Conference
on Coastal Engineering, ASCE, 3458-3471.
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KEYWORDS – ICCE 2006
APPLICATION OF FEMA GUIDELINES FOR COASTAL FLOOD HAZARD
MAPPING TO A SITE IN NORTHERN CALIFORNIA
Dale Kerper, Leslie Sakumoto, Julio Zyserman and Seungjin Baek
Abstract number 1796
FEMA
Gudelines & Specifications
Coastal Flood Hazard Mapping
Wave propagation and transformation
Storm duration
Wave setup
Beach erosion
Overtopping