HYDRODYNAMIC MODELING, WAVE ANALYSIS
AND SEDIMENTATION EVALUATION
FOR THE YOSEMITE CANAL WETLAND RESTORATION PROJECT
SAN FRANCISCO, CA
Prepared for:
California State Parks Foundation
800 College Avenue
P.O. Box 548
Kentfield, CA 94914
Prepared by:
Noble Consultants, Inc
359 Bel Marin Keys Blvd., Suite 9
Novato, CA 94949
September 2005
TABLE OF CONTENTS
1.0
INTRODUCTION.............................................................................................................1-1
2.0
FIELD DATA COLLECTION ...........................................................................................2-1
2.1
BATHYMETRIC SURVEY .............................................................................................................2-1
2.2
HYDROLOGIC DATA MEASUREMENT ........................................................................................2-1
2.3
TIDAL CHARACTERISTICS AT HUNTERS POINT ........................................................................2-2
2.4
SOIL SAMPLING .........................................................................................................................2-2
3.0
TIDAL CIRCULATION AND SEDIMENT TRANSPORT SIMULATION TECHNIQUES..3-1
3.1
MODEL DESCRIPTION................................................................................................................3-1
3.1.1
RMA2 Model ....................................................................................................................3-1
3.1.2
SED2D Model ..................................................................................................................3-2
3.2
MODELED AREA AND BATHYMETRY .........................................................................................3-2
3.3
RMA2 BOUNDARY CONDITIONS ..............................................................................................3-4
3.4
RMA2 MODEL CALIBRATION ....................................................................................................3-4
3.5
SED2D BOUNDARY CONDITIONS ............................................................................................3-4
3.6
SED2D MODEL PARAMETERS .................................................................................................3-5
3.7
SIMULATED PROCEDURES ........................................................................................................3-6
4.0
ASSESSMENT OF TIDAL HYDRODYNAMICS..............................................................4-1
4.1
EXISTING CONDITIONS ..............................................................................................................4-1
4.1.1
Water Depth .....................................................................................................................4-1
4.1.2
Tidal Currents ..................................................................................................................4-2
4.2
PROJECT CONDITIONS ..............................................................................................................4-2
4.2.1
Water Depth .....................................................................................................................4-2
4.2.2
Tidal Currents ..................................................................................................................4-3
5.0
ASSESSMENT OF SEDIMENT TRANSPORT UNDER TIDAL FLOW CONDITIONS ...5-4
5.1
EXISTING CONDITIONS ..............................................................................................................5-4
5.1.1
Bed Change .....................................................................................................................5-4
5.1.2
Bottom Shear Stress.......................................................................................................5-5
5.2
PROJECT CONDITIONS ..............................................................................................................5-5
i
5.2.1
Bed Change .....................................................................................................................5-5
5.2.2
Bottom Shear Stress.......................................................................................................5-5
6.0
STORM WAVE CLIMATES AND WAVE-INDUCED EROSION .....................................6-1
6.1
6.1.1
Wind-Wave Hindcasting .................................................................................................6-1
6.1.2
Storm Waves versus Return Periods ...........................................................................6-1
6.2
STORM WAVE CLIMATES IN SOUTH BASIN AND PROJECT AREA............................................6-2
6.2.1
STWAVE Model Setup ...................................................................................................6-2
6.2.2
Existing Conditions..........................................................................................................6-3
6.2.3
Project Conditions ...........................................................................................................6-3
6.3
7.0
OFFSHORE STORM WAVES OF SOUTH BASIN .........................................................................6-1
ASSESSMENT OF WAVE-INDUCED BED EROSION FOR PROJECT CONDITIONS .....................6-4
REFERENCES................................................................................................................7-7
ii
LIST OF FIGURES
Figure 1-1
Project Site
Figure 2-1
Surveyed Bathymetry of Yosemite Canal and South Basin
Figure 2-2
Surveyed Bathymetry of the Project Area
Figure 2-3
Measured Water Levels Compared to Hunters Point
Figure 2-4
Measured Tidal Current Velocities
Figure 3-1
Finite Element Mesh (Existing Condition, Whole Domain)
Figure 3-2
Finite Element Mesh (Existing Condition, Project Area)
Figure 3-3
Finite Element Mesh (Project Condition, Project Area)
Figure 3-4
Modeled Bathymetry (Project Condition, Project Area)
Figure 3-5
Simulated Tidal Stage and Current Compared to Measurement
Figure 4-1
Simulated Water Depth During a Low Tide (Existing Condition, Whole Domain)
Figure 4-2
Simulated Water Depth During a High Tide (Existing Condition, Whole Domain)
Figure 4-3
Simulated Water Depth During a Low Tide (Existing Condition, Project Area)
Figure 4-4
Simulated Water Depth During a High Tide (Existing Condition, Project Area)
Figure 4-5
Simulated Time Series of Water Depth for Existing Condition
Figure 4-6
Simulated Inundation Frequency (Existing Condition, Project Area)
Figure 4-7
Simulated Flood Currents (Existing Condition, Whole Domain)
Figure 4-8
Simulated Ebb Currents (Existing Condition, Whole Domain)
Figure 4-9
Simulated Flood Currents (Existing Condition, Project Area)
Figure 4-10
Simulated Ebb Currents (Existing Condition, Project Area)
Figure 4-11
Simulated Time Series of Current Speed for Existing Condition
Figure 4-12
Predicted Water Depth During a Low Tide (Project Condition, Project Area)
Figure 4-13
Predicted Water Depth During a High Tide (Project Condition, Project Area)
Figure 4-14
Predicted Time Series of Water Depth for Project Condition
Figure 4-15
Predicted Inundation Frequency (Project Condition, Project Area)
Figure 4-16
Predicted Flood Currents (Project Condition, Project Area)
Figure 4-17
Predicted Ebb Currents (Project Condition, Project Area)
Figure 4-18
Predicted Time Series of Current Speed for Project Condition
Figure 5-1
Simulated Bed Change (Existing Condition, Whole Domain)
Figure 5-2
Simulated Bed Change (Existing Condition, Project Area)
Figure 5-3
Simulated Bottom Shear Stress Induced by Flood Currents
(Existing Condition, Project Area)
iii
Figure 5-4
Simulated Bottom Shear Stress Induced by Ebb Currents
(Existing Condition, Project Area)
Figure 5-5
Predicted Bed Change (Project Condition, Project Area)
Figure 5-6
Predicted Bottom Shear Stress Induced by Flood Currents
(Project Condition, Project Area)
Figure 5-7
Predicted Bottom Shear Stress Induced by Ebb Currents
(Project Condition, Project Area)
Figure 6-1
Occurrence Frequency of Wind Directions
Figure 6-2
Wind Fetches at Bayside of South Basin
Figure 6-3
Hindcasted Wave height at Bayside of South Basin
Figure 6-4
Modeled Cartesian Grid and Water Depth (Existing Condition, Whole Domain)
Figure 6-5
Simulated Wave Height for 50-Year Offshore Wave
(Existing Condition, Whole Domain)
Figure 6-6
Simulated Wave Height for 10-Year Offshore Wave
(Existing Condition, Whole Domain)
Figure 6-7
Simulated Wave Height for 1-Year Offshore Wave
(Existing Condition, Whole Domain)
Figure 6-8
Modeled Cartesian Grid and Water Depth (Project Condition, Project Area)
Figure 6-9
Predicted Wave Height for 50-Year Offshore Wave
(Project Condition, Project Area)
Figure 6-10
Predicted Wave Height for 10-Year Offshore Wave
(Project Condition, Project Area)
Figure 6-11
Predicted Wave Height for 1-Year Offshore Wave
(Project Condition, Project Area)
Figure 6-12
Wave-Induced Erosion at Location E6 During 50-Year Wave Event
Figure 6-13
Wave-Induced Erosion at Location E9 During 50-Year Wave Event
iv
LIST OF TABLES
Table 2-1
Tidal Characteristics at Hunters Point, San Francisco Bay, CA
Table 3-1
Model Mesh Elements and Nodes
Table 6-1
Hindcasted Offshore Waves at the Bayside Boundary of South Basin
Table 6-2
Potential for bed Erosion Induced by the 10-Year to 50-Year Wave Events
v
HYDRODYNAMIC MODELING, WAVE ANALYSIS AND SEDIMENTATION
EVALUATION FOR THE YOSEMITE CANAL WETLAND RESTORATION PROJECT
SAN FRANCISCO, CA
1.0
INTRODUCTION
This technical report was prepared to document the results of the numerical modeling study
together with field data collection that was been conducted by Noble Consultants Inc. (NCI) for
the Yosemite Wetland Restoration Project in San Francisco, CA. The site location is shown in
Figure 1-1.
The purpose of the study was to assist in the evaluation and design of the Yosemite Canal
Wetlands Restoration Project. The main objectives of the study were to investigate (1) the
typical tidal hydrodynamic condition, (2) the potential for sedimentation or scouring under the
typical tidal flow condition, (3) wave conditions during storm events, and (4) the potential for bed
erosion from wave hydrodynamics during storm events. Both the existing condition and the
future condition associated with the proposed project plan were investigated in the study.
The field data collection was conducted to provide the basis for establishing the existing
bathymetry and for model calibration. The field data collection efforts included a hydrographic
bathymetric survey within Yosemite Canal and the South Basin, topographic mapping using
aerial photographic techniques, field hydrologic measurements of water surface elevation and
tidal current velocity, and soil sampling.
The modeling study includes both the hydrodynamic simulation and sediment transport
simulation for the Yosemite Canal and South Basin.
The RMA2 model was used for the
simulation of typical tidal circulation, the STWAVE model was used for the prediction of wave
climates during storm events, the SED2D model was applied for the estimate of sediment
suspension and bed change under typical tidal flow condition, and potential bed erosion under
extreme storm condition was estimated using empirical relations derived from Sedflume tests on
field data.
1-1
2.0
FIELD DATA COLLECTION
2.1
Bathymetric Survey
Field data collection was required to establish the baseline bathymetry and topography within
the Yosemite Canal and South Basin in order to perform the numerical modeling study. NCI
hydrographic survey crew conducted a hydrographic survey of the project area between
Hunters Point on the north, Candlestick Park on the south and Yosemite Canal on the west.
The survey was conducted during high tide periods between September 22, 2003 and
September 25, 2003.
A tide gage that was deployed in the marina at Oyster Point was
referenced to a National Oceanographic and Atmospheric Administration (NOAA) tidal
monument at the marina. In addition to the hydrographic survey, aerial topographic mapping
techniques were used to create a topographic map of the land based on aerial photography
taken in November 2003 at a 1-foot contour interval accuracy. Figure 2-1 shows the derived
bottom elevation contour of Yosemite Canal and South Basin generated based on the collected
bathymetric and topographic data. The detailed view of the existing bottom elevation contour in
the project area is shown in Figure 2-2. The water depth was found to be shallower than -1.8
meters, North American Vertical Datum (NAVD88) for the South Basin, and shallower than +0.4
meter, NAVD88 for Yosemite Canal.
2.2
Hydrologic Data Measurement
Hydrologic data were collected in order to provide bayside (offshore) boundary conditions for
the RMA2 model simulation and to calibrate the RMA2 model parameters. Two water level
gages were deployed within the survey area from September 24, 2003 to October 13, 2003.
One gage was installed in the inner basin at a location of E1,834,451, N636,888 (meters,
California State Plane Zone 3), and the other was in the outer basin at a location of E1,834,899,
N636,558, as shown in Figure 2-3. The measured water surface elevations at the inner and
outer gages, as compared to the NOAA predicted data at Hunters Point, are shown in Figure 24. A tidal fluctuation ranging from –0.14 meters to 2.19 meters was measured in the South
Basin for the data collection period. The measurements agreed with the NOAA predicted tidal
level at Hunters Point.
During the same period an Aquadopp Current Meter was installed adjacent to the inner water
level gage, as shown in Figure 2-3, to measure the flow current velocity components (velocity in
2-1
the x, y, and z directions). Figure 2-5 shows the horizontal components of the current velocity
(x and y directions) and the resultant horizontal magnitude.
Weak tidal currents with
magnitudes less than 0.15 meter per second (m/s) were measured in the South Basin.
2.3
Tidal Characteristics at Hunters Point
The NOAA water level station closest to Yosemite Canal is located at Hunters Point
(Station ID: 9414358) at North 37o43.8’, West 122o21.4’, within San Francisco Bay. The tidal
datum epoch of this station can be used as the reference for Yosemite Canal and South Basin.
The tidal characteristics established by the NOAA at Hunters Point station for both the old
epoch (1960-1978) and new epoch (1983-2001) are presented in Table 2-1.
Table 2-1 Tidal Characteristics at Hunters Point, San Francisco Bay, CA
(NOAA Station ID: 9414358)
Elevation, meters
Elevation, meters
(Epoch: 1983-2001)
(Epoch: 1960-1978)
-
2.49
Mean Higher High Water (MHHW)
2.07
2.05
Mean High Water (MHW)
1.88
1.86
Mean Tide Level (MTL)
1.11
1.10
Mean Sea Level (MSL)
1.08
-
-
0.95
0.34
0.34
-
0.13
0.00
0.00
-
-0.57
Datum Plane
Highest Observed Water Level (12/27/74)
NGVD29
Mean Low Water (MLW)
NAVD 88
Mean Lower Low Water (MLLW)
Lowest Observed Water (12/01/1975)
2.4
Soil Sampling
Sediment surface grab samples to determine grain size characteristics were taken at three
locations as shown in Figure 2-3: in the outer South Basin (S1), in the Inner South Basin (S2),
and in Yosemite Canal (S3). Hydrometer tests were conducted on the soil samples in order to
2-2
determine the grain size of the bed material. The sediment grain size distributions at the three
locations were similar. The average median grain size at the three locations was approximately
0.005 millimeters.
2-3
3.0
TIDAL CIRCULATION AND SEDIMENT TRANSPORT SIMULATION TECHNIQUES
3.1
Model Description
Two models within the Surfacewater Modeling System (SMS) software package, RMA2 and
SED2D, were used to simulate the two-dimensional tidal circulation, sediment transport and
resulting sedimentation or erosion within Yosemite Canal and the South Basin.
3.1.1
RMA2 Model
The RMA2 modeling program computes the water surface elevation and horizontal velocity for
sub-critical, free-surface flow in two-dimensional flow fields. This particular model module is well
suited for and has been extensively applied to the simulation of complex riverine and tidal
hydrodynamics of rivers, bays and estuaries.
RMA2 computes a finite-element solution of the Reynolds form of the Navier-Stokes equations
for turbulent flows (Norton and King, 1977). The bottom friction is defined from the Manning's or
Chezy equation. Turbulent energy is represented by an eddy viscosity analogy. Forces
generated from wind and Coriolis effects can also be included. The formulation, including the
depth-integrated equations of fluid mass and momentum conservation, is presented as follows:
⎛ ∂u ∂v ⎞
∂h
∂h
∂h
+ h⎜⎜ + ⎟⎟ + u
+v
=0
∂t
∂x
∂y
⎝ ∂x ∂y ⎠
h
∂u
∂u
∂u h ⎛
∂ 2u
∂ 2u ⎞
⎛ ∂a ∂h ⎞
+ hu
+ hv
− ⎜⎜ E xx 2 + E xy 2 ⎟⎟ + gh⎜ + ⎟ + τ bx + τ sx + Ω x = 0
∂t
∂x
∂y ρ ⎝
∂x
∂y ⎠
⎝ ∂x ∂x ⎠
h
⎛ ∂a ∂h ⎞
∂v
∂v
∂v h ⎛
∂ 2v
∂ 2v ⎞
+ hu + hv − ⎜⎜ E yx 2 + E yy 2 ⎟⎟ + gh⎜⎜ + ⎟⎟ + τ by + τ sy + Ω y = 0
∂t
∂x
∂y ρ ⎝
∂x
∂y ⎠
⎝ ∂y ∂y ⎠
where h is the water depth, u and v are the horizontal flow velocity components, x, y and t are
the Cartesian coordinates and time, ρ is the water density, g is the acceleration of gravity, Exx,
Eyy are the eddy viscosity coefficients in the normal directions on x and y axis surface, Exy and
Eyx are the eddy viscosity in the shear direction on each surface, τbx and τby are the bottom
friction components, τsx and τsy are the surface wind stress components, and Ωx and Ωy are the
Coriolis stress components.
3-1
3.1.2
SED2D Model
The SED2D modeling program is a generalized finite-element computer code for twodimensional, vertically averaged suspended sediment transport in open channel flows. The
SED2D uses input of the hydrodynamic parameters such as the water surface elevation and
flow velocity that are computed from the RMA2 or another equivalent hydrodynamics model.
The formulation for sediment transport used in SED2D is the convection-diffusion equation that
can be derived from the mass conservation of sediment (Ariathurai et. al, 1977), and the
equation is:
∂C
∂C
∂C ∂ ⎛
∂C ⎞ ∂ ⎛
∂C ⎞
⎟+S
+u
+v
= ⎜ Dx
⎟ + ⎜⎜ D y
∂t
∂x
∂y ∂x ⎝
∂x ⎠ ∂y ⎝
∂y ⎟⎠
where C is the sediment concentration, Dx and Dy are the effective sediment diffusion
coefficients in x- and y-directions, respectively, the flow velocity components u and v are
provided by RMA2, and S is the bed source term that quantifies the net sediment exchange at
the bottom between the flow and the bed.
It is assumed in SED2D that clay in transport will remain in suspension as long as the bed shear
stress exceeds the critical value for deposition, and simultaneous deposition and erosion of clay
do not occur. When the shear stress is less than the critical shear value for deposition, the
source term S is computed based on Krone’s (1962) equation for deposition rate of clay beds.
When the shear stress exceeds the critical shear stress for erosion, the source term S is
computed by a simplification of Partheniades (1962) results for erosion rate of clay beds. When
the shear stress exceeds the bulk shear strength of the layer, the erosion source term is
estimated by assuming mass failure occurs over a whole bed layer.
3.2
Modeled Area and Bathymetry
The primary area of interest is Yosemite Canal. To minimize the boundary-induced error, the
modeled domain covers an expanded area including both Yosemite Canal, and a major part
(approximately 1 kilometer in length) of the South Basin.
3-2
A finite element mesh consisting of 1,798 elements and 5,543 nodes was developed to
characterize the entire modeled area for the existing without-project condition, as illustrated in
Figure 3-1. The mesh configuration in the project area has approximately 800 elements and
2,500 nodes, as shown in Figure 3-2.
The finite element mesh for the project condition was developed by modifying the mesh system
for the existing condition in accordance with the proposed project plan. The major modification
was to create elements for the proposed tidal embayments.
The modeled domain for the
project condition was represented by a mesh system of 2,295 elements and 7,020 nodes,
among which approximately 1,300 elements and 4,000 nodes are located in the project area.
The mesh system at the project site is shown in Figure 3-3 for the project condition. The mesh
systems for the existing and project conditions are summarized in Table 3-1.
Table 3-1: Model Mesh Elements and Nodes
Modeled Scenarios
Number of Elements
Number of Nodes
1,798
5,543
800
2,500
Existing
Entire Domain
Condition
Project Area
Project
Entire Domain
2,295
7,020
Condition
Project Area
1,300
4,000
The initial bathymetry used in the model simulation for the existing condition was constructed
based on the bathymetric and topographic surveys and on the supplementary data derived
using the aerial photo mapping techniques, as shown in Figure 2-1. The bathymetry for the
project condition was developed based on the proposed grading plan. Figure 3-4 shows the
bathymetry of the project area for the proposed plan. Three embayments (NW, NE & SE) exist
in the plan. The southeast (SE) area is the shallowest, with a typical bottom elevation at Mean
Higher High Water (MHHW), approximately 2.0 meters NAVD88. The bottom elevations in the
northwest (NW) and northeast (NE) areas range from about Mean Tide Level (MTL) of
approximately 1.0 meter NAVD88 to MHHW of 2.0 meters NAVD88.
Islands, with crest
elevations of approximately 2.4 meters NAVD88 are located in the NE and SE areas.
3-3
3.3
RMA2 Boundary Conditions
The boundary conditions required in the RMA2 hydrodynamic simulation includes the upstream
flow rate and downstream water surface elevation. Since the hydrodynamic simulation focuses
on tidally dominated hydrologic conditions, it is assumed that no flow discharges through the
upstream boundary of the canal. The water surface elevation measured by the outer gage, as
shown in Figure 2-4, was used as the downstream water level condition at the bayside
(offshore) boundary of the RMA2 simulation.
3.4
RMA2 Model Calibration
The model parameters such as the Manning’s roughness coefficient (n) and the turbulent eddy
viscosity (E) required in the RMA2 simulations were calibrated by matching the model
simulation with the water surface elevation data measured by the inner gage and the tidal
current velocity data collected by the current meter. The calibrated values were found to be
0.023 for the Manning’s roughness coefficient (n) and 5,000 Pascal-second for the turbulent
eddy viscosity (E). Both assigned coefficients are within the range of values recommended by
the RMA2 User’s Manual. The comparisons of the water surface elevation and current velocity
at the inner gage location between the model prediction and measurements are shown in
Figure 3-5. This figure shows the model simulations agree withmeasured data.
Sensitivity analysis was also conducted to investigate the sensitivity of simulated results to the
model parameters of Manning’s roughness coefficient and the turbulent eddy viscosity. It was
found that model results are not sensitive to the two model parameters. This is because the
tidal circulation in the South Basin and in Yosemite Canal is essentially driven by the temporal
fluctuation of water level in the Bay. In addition, the dimension of the modeled domain is
relatively small, which also limits the effect of the model parameters on simulated results.
3.5
SED2D Boundary Conditions
The boundary conditions required in the SED2D simulation include the suspended sediment
concentration (SSC) at model boundaries.
Since no flow passes through the upstream
boundary of the canal, no boundary condition was assigned at this location. The SSC at the
bayside boundary was specified based on the SSC data that was collected by USGS
(Buchanan, et al, 1995 to 2004) in San Francisco Bay from 1993 to 2002. The two stations of
3-4
USGS SSC data collection that are close to the project site are at Pier 24 in Central Bay and at
the San Mateo Bridge in the South Bay. The mean SSC measured at the middle depth of water
body averaged between 1993 to 2002 was approximately 0.0291 kg/m3 at Pier 24, and was
0.0515 kg/m3 at San Mateo Bridge. The average value of the two stations, or 0.0403 kg/m3,
was specified as the effective SSC at the bayside boundary.
It is noted that the SSC in San Francisco Bay depends on tidal currents and sediment source of
the Bay, and significant temporal variation may occur. However, detailed information about the
temporally varying SSC is not generally available and uncertainty may also be present in the
data because of the state of art for SSC measurement.
Therefore, a single value of the
effective SSC was specified at the bayside boundary. Using a single value of SSC at the
boundary to represent temporally varying SSC may affect the accuracy in estimating short-term
sediment transport. However, the effect should be limited for a long-term estimate, of interest in
this study, and particularly for the project area that is away from the boundary.
3.6
SED2D Model Parameters
Accuracy of SED2D model results significantly depends on the appropriate selection of
sediment properties that are used as model input parameters. The bed material of the project
site consists of clay and silt. The major parameters required by the SED2D model for clay
sediment transport simulation include settling velocity of sediment particles, critical shear
stresses for deposition and for erosion, erosion rate coefficient and dry density of bed material.
These parameters were specified based on the collected field data and on the supplementary
information about the model parameters obtained from references.
Based on the hydrometer tests on the soil samples on the project site, a median sediment grain
size of 0.005 millimeters was used in the model. A settling velocity of 0.141 cm/s for a sediment
particle of a median grain size of 0.005 millimeters (Cheng, 1997) was specified in the model.
The dry density of the bed material was not directly measured in this study. Instead, it was
estimated based on a Sedflume analysis that was conducted on the field sediment samples
obtained from the South Basin by Battelle et al (2005). Based on analysis, the dry density of the
bed material was specified as 480 kg/m5 for the top bed layer with a depth up to 5 centimeters,
3-5
590 kg/m5 for the second layer with a depth between 5 and 10 centimeters, and 660 kg/m5 for
the deeper layers.
When the shear stress exceeds the critical value for erosion, the source term is computed in
SED2D using the simplified linear relation between the erosion rate and the shear stress
(Partheniades, 1962). The critical shear stress for noticable erosion is typically larger than 0.5
Pa for the San Francosco Bay mud, above which erosion rates increases very rapidly with the
shear stress.
The critical shear stress for suspended mud to deposit on the bed is
approximately 0.06 Pa. Based on Partheniades (1965), only minor erosion occurs when the
shear stress is between the critical value for mud deposition and that for noticeable mud
erosion. The erosion rate coefficient, which is defined as the erosion rate of bed material per
unit area per unit increase of flow shear stress normalized by the critical shear stress for
erosion, is only on the order of 10-4 g/m2/sec for the minor erosion regime, or approximately 100
times smaller than that for the noticeable erosion regime.
The shear stress generated by tidal currents in the project area is generally less than 0.5
Pascals (Pa).
This range of shear stress mainly causes minor erosion when the shear stress
exceeds the critical shear stress of 0.06 Pa. Therefore, the linear relation between the erosion
rate and the shear stress for the minor erosion regime, instead of that for noticable erosion
regime, was used in the simulation. Based on Partheniades’s (1965) experiments, a critical
shear stress for erosion of 0.06 Pa with an erosion rate coefficient of 8×10-5 g/m2/sec were
specified in the SED2D simulation to determine the source term when erosion occurs.
3.7
Simulated Procedures
In the model simulation, RMA2 was first executed to compute the flow conditions using the
water level measured by the outer tidal gage in the South Basin for 15 days starting from
September 25, 2003. SED2D was subsequently run for the same 15-day period using the flow
conditions computed by RMA2. The 15 days of simulation period roughly covers a spring and a
neap tide cycle. The time step was 0.2 hours in the RMA2 simulation and 0.1 hours in the
SED2D simulation.
3-6
The simulated results of the 15 days simulation period were then used to assess the existing
tidal circulation and sediment transport in the South Basin and in the Yosemite Canal, as well as
the potential change that will be caused by the wetland restoration project.
3-7
4.0
ASSESSMENT OF TIDAL HYDRODYNAMICS
4.1
Existing Conditions
4.1.1
Water Depth
Figures 4-1 and 4-2 show the simulated water depths for the whole modeled area during a low
tide and a high tide, respectively. Detailed views of water depth for the project area are shown
in Figures 4-3 and 4-4. The model results indicate that periodic wet and dry processes (wet at
high tide and dry at low tide) occur within the entire Yosemite Canal and in a small section of the
South Basin as tidal elevation varies. The whole area will be inundated during a high tide. The
water depth was estimated to be approximately two to four meters in the South Basin, and
approximately 0.8 to 1.6 meters in the canal. However, all of Yosemite Canal dries out (has no
water) at low tide.
Figure 4-5 shows the time series of the simulated water depth at six reference stations along
the main flow path from the Yosemite Canal to the outer South Basin. Locations of the six
reference stations are shown in Figure 2-3. The results for the simulation period indicate some
wetting and drying at Locations 1 to 4, although minimal at Location 4, and no drying at
Locations 5-6. Temporal variations can be found in the water depth from the canal to the South
Basin in response to the tidal fluctuations.
Figure 4-6 shows the water inundation frequency in the project area that was calculated based
on the simulated water depth for 15 days. The inundation frequency represents the percent of
time that the site is inundated with water. An inundation frequency of 1.0 indicates the site is
always inundated, and zero indicates the site is always dry.
Most of the South Basin is
inundated with an inundation frequency of 1.0. The inundation frequency was found to be
approximately 40 to 50 percent in the inner segment of the canal, 60 to 70 percent in the middle
segment, and 70 to 80 percent in the lower segment.
4-1
4.1.2
Tidal Currents
A snapshot of the simulated tidal current velocity vectors together with water depth contours for
a flooding tide is demonstrated in Figure 4-7. Figure 4-8 shows the flow field for an ebbing
tide.
Figures 4-9 and 4-10 show detail views in the project area. The time series of the
simulated current velocity magnitude at the six reference stations (see Figure 2-3, L1 to L6 ) are
shown in Figure 4-11.
Circulation in the South Basin was found to be very restricted and the tidal currents are weak.
The maximum current velocity was calculated to be approximately 0.1 meters per second, which
occurs in the narrow part of the South Basin during the strongest flood and ebb tides. Tidal
currents in Yosemite Canal were slightly stronger, particularly in the middle and lower segment
of the canal. The maximum current velocity is approximately 0.25 meters per second, which
occurs in the middle and lower canal and lower segment of the canal during the strongest flood
and ebb tides. These velocities are considered low, and not likely to induce noticeable resuspension of bed material or bed scouring.
4.2
Project Conditions
The hydrodynamic conditions within Yosemite Canal and the South Basin for the proposed
project were simulated based on the bathymetry shown in Figure 3-3. The same time series of
water level measured for 15 days was specified as the bayside boundary condition. Since the
change to the hydrodynamic conditions in the outer South Basin is negligible, the analysis
focuses on the change that will occur in the project area.
4.2.1
Water Depth
Figures 4-12 and 4-13 show the predicted water depths in the project area during a low tide
and a high tide, respectively. Figure 4-14 shows the time series of the predicted water depth at
the six reference stations. The open water surface area during high tides will be significantly
increased over the existing condition as shown in Figure 4-13.
Since the elevation of the
embayments is relatively high, the open water area during low tide will be similar to the existing
condition. The water depth during high tides will range between 0.2 and 0.4 meters in the NW
embayment, between 0.2 and 1.2 meters in the NE embayment, and less than 0.2 meters in the
SE embayment. These areas will dry out during low tides.
4-2
Figure 4-15 shows the predicted inundation frequency contours. The model results indicate
that the proposed project will not significantly alter the water depth in Yosemite Canal or in the
South Basin. Periodic wet and dry processes will still occur within the Canal and in the inner
South Basin as tidal elevation varies. Figure 4-15 suggests that the inundation frequency will
range between 10-20 percent in the NW embayment, will range between 10-50 percent in the
NE area, and will be approximately 10 percent in the SE area.
4.2.2
Tidal Currents
Figures 4-16 shows a snapshot of the predicted tidal current velocity vectors at maximum
currents velocity in the canal, together with water depth contours for a flooding tide, and Figure
4-17 shows the flow field for an ebbing tide. Because of the relatively high bottom elevations,
the three proposed embayments are dry when strong flood and ebb currents occur in the canal,
as shown in Figures 4-16 and 4-17. Tidal circulation during high tides when these areas are
inundated is essentially weak. Therefore, tidal circulation in the proposed embayments will be
generally much weaker than the canal, and the chance for the bed material in those areas to be
re-suspended by tidal currents is even less than in the canal.
The time series of the predicted current velocity magnitude at the six reference stations are
shown in Figure 4-18. By comparing Figure 4-11 (existing conditions) with Figure 4-18 it is
seen that the proposed project will not significantly alter the tidal circulation in Yosemite Canal
or in the South Basin. Circulation in the South Basin will still be very weak, and the maximum
current velocity will still be approximately 0.25 meters per second within the canal during the
strongest flood and ebb tides.
These low flow velocities will not likely be able to induce
noticeable re-suspension of bed material or bed scouring in the project area.
4-3
5.0
ASSESSMENT OF SEDIMENT TRANSPORT UNDER TIDAL FLOW CONDITIONS
The sediment transport induced by tidal circulation was simulated using the SED2D model,
which used the tidal circulation parameters computed by RMA2 for 15 days. The simulated
results, including the bed change and bottom shear stress, were then analyzed in order to
assess the long-tern bed deposition or erosion, and the likelihood of sediment re-suspension in
the project area. Both the existing and the project conditions were assessed.
5.1
Existing Conditions
5.1.1
Bed Change
Figure 5-1 shows the annual bed change converted from the predicted bed deposition or
erosion extrapolated using the typical tidal conditions for 15 days. The results indicate that the
sediment bed in the South Basin and in Yosemite Canal appears to be relatively stable and
undisturbed. The annual erosion rate or deposition rate does not exceed 2 centimeters per year
under typical tidal flow conditions.
Except for the main flow path, where negligible erosion occurs, insignificant sediment deposition
occurs in most of the South Basin under typical tidal flow conditions. The erosion rate along the
main flow path was found to be less than 0.5 centimeter per year, and the estimated sediment
deposition rate ranges from 1.0 to 1.5 centimeters per year in the outer South Basin, and is less
than 1 centimeter per year for the inner basin.
Figure 5-2 shows the detailed view of the predicted annual bed change in the project area. As
a result of the weak currents, sedimentation generally occurs in the inner South Basin next to
the project site. However, minor scouring occurs in most of Yosemite Canal, and in the mouth
of the canal because of the elevated current velocities in these areas. However, the erosion
rate is minor, estimated to be less than 0.5 centimeters per year in the mouth, less than 1
centimeters per year in the middle and lower segment of the canal, and less than 0.5
centimeters per year in the upper portion. Negligible sediment deposition occurs at the furthest
end of the canal because of the weak current in this dead-end area. It is noted that the channel
is relatively deeper in the segments where erosion occurs. This is also found in the surveyed
bathymetry as shown in Figure 3-4.
5-4
5.1.2
Bottom Shear Stress
Bottom shear stress exerted by the flow on the bed is responsible for re-suspending bed
material. Figure 5-3 shows a snapshot of the predicted bottom shear stress induced by flood
currents, and Figure 5-4 shows that induced by ebb currents. The results indicate that the
maximum bottom shear stress is approximately 0.2 Pa (or N/m2) during flood currents, and 0.6
Pa during ebb currents.
Based on sediment properties in San Francisco Bay (Partheniades, 1962, 1965, and Battelle,
2005), noticable clay sediment erosion would occur only when the bed shear stress generally
exceeds 0.5 Pa or more, below which only minor erosion occurs.
Although model results
indicate that the maximum bed shear stress can reach 0.6 Pa during ebb currents, this shear
stress only exists for a very short period of time (in an order of minutes) when the local water
depth is very minimum.
Therefore, tidal currents are not likely to induce significant re-
suspension of local bed material in the Yosemite Canal under the typical tidal flow conditions.
5.2
Project Conditions
5.2.1
Bed Change
The predicted annual bed change under typical tidal flow conditions for the project condition is
shown in Figure 5-5.
Compared to the existing condition, the proposed project will not
significantly alter the shoaling or scouring pattern within Yosemite Canal. Similar to that for the
existing condition, minor scouring occurs in most of Yosemite Canal, with an erosion rate less
than 1 centimeters per year for the middle and lower segment of the canal, and less than 0.5
centimeters per year for the upper portion. A similar shoaling pattern was predicted for the
upper end of the canal.
Sediment accumulation was predicted in the three proposed embayments because of the weak
currents that will exist in these areas. However, the annual deposition rate will be less than 0.5
centimeters per year, which is considered negligible.
5.2.2
Bottom Shear Stress
5-5
The bottom shear stress for the project condition is shown in Figure 5-6 for flood currents, and
in Figure 5-4 for ebb currents. The maximum bottom shear stresses predicted for the project
condition during the flood and the ebb currents have similar magnitudes to the existing
condition. This is consistent to the negligible alternation to the peak flood and ebb current
condition in the project area that will be caused by the proposed plan. Similar to the existing
condition, tidal currents will not likely induce significant re-suspension of local bed material in
Yosemite Canal under the typical tidal flow conditions.
5-6
6.0
STORM WAVE CLIMATES AND WAVE-INDUCED EROSION
6.1
Offshore Storm Waves of South Basin
Waves propagating from the bayside boundary (offshore) of the South Basin to the project area
are generated by the winds blowing over the water surface of the South San Francisco Bay.
The wave climates at the offshore of South Basin are determined by the wind conditions in this
area.
6.1.1
Wind-Wave Hindcasting
A 57-year record of continuous wind measurements at the San Francisco International Airport
were used for hindcasting the waves at the offshore of the South Basin. The wind rose derived
from the hourly wind directions for the 57 years of record from 1948 to 2004 is shown in Figure
6-1. The 16 azimuth directions are shown along with the percentage of time winds are from that
direction. This figure shows that the prevailing winds in the area are westerly, blowing from the
west (W) and north-west-west (NWW). Since the South Basin is open to the southeast, the
storm waves that can propagate to the project area are generated by the southeast storm winds
blowing over the South San Francisco Bay from approximately the south-east-east (SEE) to
almost south-south-east (SSE). The wind fetches for these directions are shown in Figure 6-2.
The wind-generated waves at the offshore of the South Basin were hincasted using the wave
prediction model in the Automated Coastal Engineering System (ACES) that was developed by
the U.S. Army Corps of Engineers. ACES is a comprehensive set of software programs for
applying a broad spectrum of coastal engineering design and analysis technologies, including
wave prediction. The shallow water restricted wind fetch option within the wave prediction
model was used in this analysis.
6.1.2
Storm Waves versus Return Periods
The hourly offshore wave condition at the offshore of South Basin was estimated based on the
hourly wind data. The annual maximum wave condition was then derived for each of the 57
years, from which the wave heights for various return periods (years) were formulated, as
shown in Figure 6-3. Also shown in this figure is the Weibull distribution that best fits the data.
6-1
The 50-year, 10-year and 1-year offshore waves estimated based on this return frequency
analysis are sown in Table 6-1.
Table 6-1 Hindcasted Offshore Waves at the Bayside Boundary of South Basin
6.2
Return period (year)
Wave height (m)
Wave period (sec)
50
1.42
4.4
10
1.26
4.2
1
0.70
3.2
Storm Wave Climates in South Basin and Project Area
The offshore waves during extreme storm events were propagated to the South Basin and the
project area using the nearshore wave transformation model STWAVE. The wave climates
associated with the 50-year, 10-year, and 1-year offshore wave conditions were predicted.
6.2.1
STWAVE Model Setup
The STWAVE (STeady-state spectral WAVE) model was developed by U.S. Army Corps of
Engineers for nearshore wave transformation (Smith et. al, 2001). STWAVE can be applied to
quantify the change in wave parameters (wave height, period, direction and spectral shape)
from offshore to the nearshore zone, where waves are strongly influenced by variations in
bathymetry, water level, and current.
It is capable of simulating wave shoaling, refraction,
diffraction and breaking, wind-wave growth due to local sea breeze, and wave-wave interaction
and whitecapping that redistribute and dissipate energy in a growing wave field. STWAVE
solves the steady-state conservation of spectral wave action along backward traced wave rays
with source/sink terms, and the governing equations are numerically solved using finitedifference methods on a Cartesian grid.
The modeled domain in the STWAVE simulation covers a rectangular area of 2000 meters
cross-shore and 1300 meters alongshore, with a cell size of 10 meters by 10 meters. The
Cartesian grid used in the simulation is shown in Figure 6-4 for the existing condition. The
dark-green part indicates an area of (wet) ocean cells, and the light-white part indicates an area
of (dry) land cells. Also shown in this figure is the water depth contour associated with the 106-2
year tidal stage. The water depth under the 10-year tidal stage was used in the STWAVE
simulation for wave propagation in the South Basin and the project area. A 10-year tidal stage
of 6.1 feet NGVD (approximately 2.68 meters NAVD88) that was estimated for Hunters Point
(USACE, 1984) was used in this analysis.
Part of the Cartesian grid and the associated water depth contour at the project area is shown in
Figure 6-5 for the project condition. It is noted that the proposed two islands will be inundated
under the 10-year tidal stage. Figure 6-5 also shows the nine locations at which potential
erosion during extreme storm events would be estimated in Section 6.3.
6.2.2
Existing Conditions
The simulated wave heights in the South Basin and Yosemite Canal is shown in Figures 6-6 to
6-8 for the 50-year, 10-year and 1-year offshore wave conditions, respectively. It seen that the
wave height generally decreases as propagating from offshore to the basin. For the 50-year
wave, the wave height ranges from 1.1 to 1.4 meters in the outer South Basin, 0.5 to 1.0 meter
sin the wave-exposure zone and less than 0.3 meters in the wave-shadow zone at inner South
Basin, and decreases to lower than 0.4 meters in the Yosemite Canal. It is also noted that the
wave height in the inner South Basin and Yosemite Canal does not show apparent difference
between the 50-year and the 10-year offshore wave conditions. This is because waves in the
South Basin and Yosemite Canal are already broken when big waves propagating from offshore
during extreme storm events. The wave climate of broken waves mainly depends on the local
water depth and exposure condition instead of offshore wave height. Compared to the 50-year
and 10-year waves, the 1-year wave is generally lower. However, the difference is small in the
Yosemite Canal. The relatively milder wave climate in the inner South Basin and Yosemite
Canal partially attributes to the wave breaking process, and partially attributes to the contracted
cross-section in the middle of the South Basin.
6.2.3
Project Conditions
Figures 6-9 to 6-11 show the predicted wave heights in the project area for 50-year, 10-year
and 1-year offshore wave events under the 10-year tidal stage. Negligible difference is shown
in the predicted wave heights between the 50-year and 10-year offshore wave events. This
suggests that the predicted wave climate for the 50-year or for the 10-year offshore wave event
may also represents the worst wave condition that would occur in the project area under the 106-3
year tide stage. The wave height that will occur in the project area during extreme storm events
was estimated to range from 0.6 meters in the canal mouth area to less than 0.1 meters in the
upper canal and in the NW embayment. Higher waves were predicted in the canal mouth
because this area is directly exposed to the approaching path of incoming offshore waves and
has relatively deeper water depth.
Because the proposed island will be inundated under the 10-year tidal stage, the SE
embayment will also be directly exposed to the incoming waves. As a result, 0.3-meter waves
will generally exist in the tidal channel with a water depth of approximately 0.7 meters under the
10-year tidal stage.
However, the island will be exposed above the water as water level
decreases, and a much milder wave climates will exist behind the island in the SE embayment
because of the sheltering effect of the island. While the wave height was estimated to be 0.3 to
0.4 meters in the canal next to the NE embayment, small waves of 0.1 to 0.2 meters high were
estimated for the shallow inner part of the NE embayment as a result of the worse waveexposure condition. Small waves were also predicted in the upper port of the canal and in the
entire NW embayment. Waves in these areas will be lower than 0.1 meters.
6.3
Assessment of Wave-Induced Bed Erosion for Project Conditions
The potential for bed erosion that will be induced by wave particle velocities during extreme
storm events were estimated at nine representative locations as shown in Figure 6-5. A mean
duration of 11.4 hours was estimated by Battelle et al (2005) for the storm events in the South
Basin, and the storm duration used in this analysis was 12 hours. Wave conditions in the
project area depend on local water depth for given offshore wave condition, and the local water
depth will fluctuate with oscillating tide levels during a storm event. Therefore, a series of wave
climates were predicted for every half hour within the 12-hour storm duration for both the 50year and the 10-year storm events. The water level fluctuations within the 12 hours were
represented by a synthetic series of tidal levels as shown in Figures 6-12 and 6-13. The
highest tide equals to the 10-year tidal stage (approximately 2.68 meters NAVD88), and the
lowest tide equals to the lowest observed water level at Hunters Point (-0.57 meters NAVD88).
The potential for wave-induced bed erosion was calculated based on an empirical relation
determined from a Sedflume analysis for the bed material of the South Basin (Battelle et al,
2005), which links the bed erosion rate to the bottom shear stress. The instantaneous bottom
6-4
shear stress is a function of the instantaneous wave particle velocity at the bottom, which was
determined based on the linear wave theory for given local water depth, wave height and wave
period. The local wave condition was predicted using STWAVE.
Figure 6-12 shows the synthetic tidal level every half hour within the 12-hour storm duration, the
wave-induced bed erosion potential predicted for every half hour, and the cumulative erosion
potential during the 12-hour duration for the 50-year wave event at location E6. Figure 6-13
shows the results at Location E9. The results indicate that the wave-induced erosion depth
depends on water depth (or tidal level) during a storm. Because location E6 is approximately 2
meters lower than location E9, E6 will subject to a longer duration of wave motion and resulting
wave-induced erosion.
The predicted total bed erosion potentials for the nine representative locations in the project
area are summarized in Table 6-2 for the 10-year to 50-year offshore wave events. As a result
of the mild wave condition, erosion will not likely be induced by wave motions in the NW (E1)
and inner NE (S2) embayments, or in the middle (E5) and upper canal (E4). The mouth of the
canal (E6) will suffer the most serious erosion. The outer NE embayment (E3) will also be
eroded as much as 19 centimeters during storm events. Wave-induced erosion will generally
occur in the SE embayment (E7 to E9). Although the erosion duration in the relatively high NE
embayment is only 4 hours (see Figure 6-13) during the 12-hour storm event, the erosion depth
ranges from 5 centimeters to 16 centimeters.
It should point out that the estimated erosion potential induced by waves only considered the
erosion caused by the wave particle velocity that is high enough to induce a bottom shear stress
exceeding the critical shear stress for erosion. However, the wave particle velocity oscillates
with time as water surface elevation fluctuates in each wave period. As a result, sediment
deposition will occur when the wave particle velocity is low and the resulting bottom shear stress
is less than the critical shear stress for deposition. The sediment deposition will compensate
part of the erosion that occurs during the high wave particle velocities. Therefore, the actual
erosion during the extreme storm events will be less than the estimated erosion potential.
6-5
Table 6-2 Potential for bed Erosion Induced by the 10-Year to 50-Year Wave Events
Location
Erosion depth (cm)
E1
0
E2
0
E3
10 - 19
E4
0
E5
0
E6
16 –27
E7
5 –8
E8
7-12
E9
10-16
6-6
7.0
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7-7
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7-8