6 NOVATO BAYLANDS MODEL 6.1 Model Domain and Configuration 6.1.1 Domain Extent, Grid Configuration and Time step The Novato Creek existing conditions Bayland model domain extends from the marsh fringe in San Pablo Bay inland to approximately a 16 ft. elevation contour (Figure 6.1). The inland limit of the domain crosses the City of Novato just upstream of DeLong/Diablo Ave. and includes Warner Creek and Arroyo Avichi up to South Novato Blvd. The Rush Creek drainage and the State of CA BMK Restoration site (CABMK) are outside the model domain, however model analysis considers the volume of water discharging to these drainages. Rush Creek Drainage Novato Bayland Mode Domain
CA BMK Restoration Site Figure 6.1: Novato Bayland Model Domain The EFDC Existing Conditions (EC) Bayland model utilizes a single layer (2‐dimensional), orthogonal curvilinear grid with 22,700 cells (Figure 6.2) with cell sizes ranging from 10x15 feet in the narrow channels of the Confluence Reach up to 150x250 feet in the Deer Island and North Basins. Originally proposed as a 1‐D model, KHE configured the EC model as a 2D 6‐1 “channel focused” domain to expedite the transition to a larger Bayland domain model to be developed for the Phase 2 Alternatives Analysis. Gridding of the entire Upper Bayland reach was relatively straightforward and therefore was incorporated in EC model development. The Phase 2 model will expand this domain to include the entire Figure 6.1 domain. Figure 6.2: Novato Bayland Model Grid Model time steps range from 0.2 to 1.0‐seconds. High velocity flood flows across small cells in the Confluence Reach, and wetting and drying front propagation in the Bayland basins both contribute to the stability requirement for a small model time step9. As noted above in model strengths, EFDC dynamically allocates the model time step, automatically increasing and decreasing the simulation time increment with flow velocities to maximize computational efficiency and shorten the duration of run times. The EC model is typically run for a 96 hour simulation period, and requires 3 to 4 hours of computational time. The simulation incorporates a day of base flow tidal oscillations run‐up to the 48 hour storm hydrograph to illustrate typical tidal gradients across the domain. Model simulations also extend a day after the storm passes to simulate flood attenuation in the corridor. 6.1.2 Bathymetry, Topography and Roughness Bathymetric and topographic elevations across the solution domain for both Bay and Creek 9
For computational accuracy, the model time step must be small enough to insure that in any time step mass transport in each cell does not propagate beyond the limit of that cell. 6‐2 models were defined through a synthesis of existing data sources and project specific surveys. KHE reviewed available data from MDPW, and data generously provided by project stakeholders including BMKCSD, USACE, City of Novato (City), Novato Sanitary District (NSD), and North Marin Water District (NMWD). Elevation data gathered included levee crest elevations; bridge, culvert and other facilities point data; and general ground surface data characterizing basins and drainages. KHE rectified all information gathered to a single projection (CA State Plane Zone 3, NAD83, US feet) and vertical datum (NAVD88). Information presented in this report utilizes this coordinate system. Ground survey included establishment of a vertical control network in the Bayland (Oberkamper, 2012) and throughout the Novato Creek corridor (Ciquini & Passarino, 2012). Control surveys utilized available HPGN control points in the Bayland and at Stafford Lake, and set new points throughout the watershed as basis for project and future design surveys. To the extent possible, individual data sets were compared to known elevations in the control network to verify the vertical datum and evaluate data accuracy. Verifiable data were added to the project data base, and incorporated in the project digital elevation model (DEM). The project DEM defines a single continuous ground surface for the study area. Model accuracy is strongly dependent on the accuracy and consistency of the DEM. Golden Gate Lidar (2010) served as the starting point for the model DEM. To improve the accuracy of the LIDAR based terrain for this hydraulic study, MDPW contracted Kruse Imaging to post process the Golden Gate LIDAR data and generate “bare earth” coverage of the entire Novato Creek watershed. A metatdata summary for the Kruse analysis is provided in Appendix C.1. Generation of bare earth terrain utilizes a series of filters to remove above ground pixels associated with vegetation, structure and other non‐terrain features, and re‐interpolate the data surface utilizing only those pixels representative of ground surface elevations. Overall DEM accuracy as reported by Kruse (2012) is +/‐10 cm. The bare earth DEM maintains this accuracy under dense vegetative cover found throughout the Novato Creek riparian corridor, in hillside drainages and Bay marshland. KHE undertook a rigorous comparison between topographic point data (focused on levee crests and Bayland basins) and the LIDAR DEM to determine if vegetative cover biased the terrain surface (Appendix C.2). Although the accuracy of individual survey data sets varied widely, on average, the DEM over‐estimated Bayland elevations by +4 to 6 inches. However, KHE does not recommend downward adjustment of the DEM since the offset is within recognized error limits for the data. KHE supplemented the LIDAR data with 2011/2012 bathymetric surveys in the Novato Creek channel (provided by BMKCSD and MCFC), point elevations on levee crests (MCFC and NSD) and as built survey maps of Lynwood Basin and Pacheco Pond (MCFC). The Novato Creek (FLO‐2D) and rainfall runoff (HEC‐HMS) models previously described utilized the watershed this DEM for out of bank flow areas, and ground survey data to characterize channel 6‐3 geometry and grade10. Figure 6.3 presents the full Bayland DEM used to define the elevations of each of the points within the EFDC model domain. EFDC defines model grid cell elevations by averaging corner point values extracted from the DEM. Cell averaging tends to smooth the DEM along levee crests and channel thalweg in the Bay model because these features are frequently only 1 to 2 cells in width. To preserve the accuracy of levee crest and channel thalweg elevations, KHE applied the average of surveyed levee crest and minimum channel thalweg point values to the respective grid cells. 10
Channel cross sections and bridge surveys conducted throughout the watershed and used in model development are tied to the project vertical control network. The watershed DEM (upstream of Diablo Blvd.) does not extend into the area of newly surveyed channel cross sections. This effort was not requisite to watershed modeling or the project scope. 6‐4 Figure 6.3: Bayland DEM (NAVD88) and Associated Tidal Marsh Ecological Units 6‐5 To preserve the point elevations across cell‐centered grid calculations, these features are specified at a minimum two cell width in the domain. The resulting gridded Bay model bathymetry is presented in Figure 6.4. Figure 6.4: Bay Model Bathymetry Chezy Roughness The EFDC model utilizes a log boundary layer formulation for bed roughness with spatially variable values specified, by cell, across the solution domain. This “Chezy‐type” approach provides stable flow dynamics in intertidal cells by incorporating a viscous‐type roughness that increases with the small flow depths associated with wetting and drying cells. Bayland hydrodynamics are momentum dominated and therefore roughness is typically influential only at shallow water depths. The Novato Bayland model utilizes a roughness values ranging from 0.01 to 0.04 in the channel, and a single overbank and floodplain roughness value of 0.05 (Figure 6.5). 6.1.3 Bridges, Culverts and Levees One of the most significant limitations in the EFDC model is the lack of internal formulation of bridge and culvert exchange within the code. In order to incorporate Bridge energy losses, KHE/WRECO utilized HEC‐RAS to externally generate stage discharge relationships through the SMART and Hwy 37 crossings. The HEC‐RAS configurations included the structures and upstream and downstream reaches of Novato Creek necessary for flood wave propagation. EC 6‐6 (steady state) stage discharge curves for these structures are provided in Appendix D.411. Chezy Roughness Figure 6.5: Bay Model ‐ Chezy Roughness Coefficients KHE also used HEC‐RAS to estimate tidal exchange through the six (6) 4’ by 4’ box culverts connecting Pacheco Pond Pacheco Pond to Novato Creek . A description KHE’s HEC‐RAS modeling to predict Pacheco Pond inflow as a function of Novato Creek stage is presented in Appendix F. 6.1.4 Starting Assumptions: Basin Storage and Hydrodynamic Parameters The Bayland model domain includes ponded water levels in four ponds ‐ 3 in Lynwood Basin (Central Basin, Duckbill Pond and Heron’s Beak Pond) and Pacheco Pond. The starting water surface elevation in each pond, presented in Table 6.1, is based on MDPW operational guidelines for the start of the wet season. This assumption holds well for Pacheco Pond which drains to the culvert invert elevation within a few days after each storm. The Lynwood ponds 11
Simulating tidally driven backwater at bridges required the addition of a new hydraulic boundary condition to the EFDC code. The low chord height is used as a trigger elevation for implementation of the HEC‐RAS generated stage discharge curve. The “bridge” boundary condition remains active until the water drops below the low chord elevation. When this occurs, the boundary condition turns off, and EFDC’s solution returns to full hydrodynamics. The bridge rating curve applies to the flows under and over the bridge deck. As noted above, the input rating curves in the EC model are developed outside of EFDC using HEC‐RAS. This is standard in initial studies. However, to support design, KHE recommends bridge hydraulic data collection to characterize conveyance losses and water surface elevation change across these structures. 6‐7 accumulate storm water over the rainy season and would likely have starting water surface elevations that are higher than the assumed start of season values. Utilizing minimum values provides a reasonable indication of the volume of available design storage and permits comparison between simulation results. BMKCSD lagoons also store water throughout the year. Design values for these features (CLE, 2007) are included in Table 6.1. Table 6.1: Starting Water Surface Elevations in Bayland Ponds Initial Water Surface Elevation
Pond (ft., NAVD88) Lynwood Basin Central Basin 4.5 Duckbill Pond 3.3 Heron’s Beak Pond 2.0 Pacheco Pond 2.63 BMKCSD North Lagoon 4.6 South Lagoon 3.6 6.1.5 Sediment Loading and Transport Characteristics Lower Novato Creek sediments are a mixture of coarser alluvial non‐cohesive sediments, and fine organic cohesive bay mud. Deposition of both alluvial and bay sediments is distributed broadly across the Confluence Reach, as a function of continuously varying tidal gradients and seasonal flow regimes. Critical velocities for mobilization and shear stress for sediment scour, transport and deposition varies throughout the reach as a function of grain size, density and organic content. Characteristics of Novato Bayland sediment quantities and composition (presented in Chapter 3) indicate that upstream of Arroyo Avichi, sediments are primarily alluvial with increasing percentage of fines and bay mud with downstream distance. Bay mud is present in the bed upstream of the Arroyo Avichi confluence, and is indicative of limit upstream (flood tide) conveyance of suspended sediments. The transition to bay muds is largely complete upstream of the SMART Bridge. Current channel dredge operations remove 35,000 to 70,000 yd3 of sediment in four‐year 6‐8 maintenance cycles (pers. Comm. P. Balderama, March 2013) excavating a rectangular channel with bed elevations ranging from 1.0‐feet at SMART to 6.0 feet at the Creek confluence. At a 10 foot contour elevation, the dredged channel has a typical width of 60‐70 feet downstream of the Warner Creek confluence, approximately twice the width of the tributary channels. Dredged channel width increases to approximately 90 feet downstream of Arroyo Avichi, 120 feet downstream of Redwood Blvd., and 130 feet downstream of Hwy 101. A typical width range between 60‐ to 80 feet resumes in the undisturbed channel downstream of SMART. Assuming an average channel width of 120 feet over the 4150 foot distance between SMART and the Warner Creek confluence, dredging creates approximately 55,000 yd3 of available volume12. Alluvial sediment deposition dominates the upper portion of dredge reach (Warner Creek confluence to Redwood Blvd.), filling this glory hole to and above the grade of the downstream transition to non‐dredged channel. The high frequency dredging precludes observation of event driven mobilization and bed sorting under at‐grade conditions. Dredge records and prior studies (including core and flume testing data), summarized in Chapter 3, provide valuable sediment size and composition information used to define the Table 3.5 velocity thresholds for transport which correlates to predicted critical bed shear stress. In the Novato Bayland EC modeling, KHE evaluated sediment transport capacity based on comparison to depth averaged velocity thresholds for coarse sediments, and mixed sand (non‐
cohesive) in the fluvially dominated Confluence Reach, and cohesive bay mud in the tidally dominant Upper and Lower Bayland Reaches. Table 3.5 presents the velocity and bed shear threshold values assumed over the observed range of grain sizes found in Novato Baylands. In discussion of EC modeling results, cohesive suspended sediment transported capacity is evaluated by looking at variation in tidal velocity magnitude and asymmetry. For non‐cohesive sediments (silt, sand and gravel) sediment transport capacity is evaluated as a function of depth average velocity. 6.2 Boundary Conditions Numerical models require boundary conditions (water surface elevation, flow, or concentration) to be specified at all open domain cells across which water or energy propagates for all time steps in a simulation periods. These boundary values define the mass and energy gradients that drive flow magnitude and direction. The inflows, tidal conditions and sea level rise parameters used to characterize existing conditions are described below. Figure 6.6 illustrates the model domain and location of inflow boundary conditions. 6.2.1 Inflows Primary inflows to the Bayland model are defined at 4 locations based on model results generated with FLO2D/SWMM (WRECO), HEC‐HMS (MCFC) and HEC‐RAS (KHE) models (Figure 4.5). Novato Creek and Warner Creek inflows are FLO2D based. Arroyo Avichi inflows are 12
Available capacity is assumed to be the volume in excess of the volume available in a 60 foot wide channel. 6‐9 based on SWMM model results. SWMM was necessitated by the Arroyo Avichi storm drain outfall, which conveys creek discharges under South Novato Blvd. to Novato Creek. These storm drains flow at full capacity during design storm events. A weir at the storm drain inlet diverts Arroyo Avichi flows in excess of the 220 cfs culvert capacity to Baccaglio Basin and the West Side drainage (See Chapter 5). East of Hwy 37 at Pacheco Pond, KHE utilized HEC‐RAS to route HMS (MDPW) derived inflows through the pond and culverts to Novato Creek (See Appendix F). HMS inflows were also developed for smaller inflow points, including Cheda Creek and Simmons Slough, which were not incorporated in the EC model, but should be incorporated in future design phases. The Q10, Q50 and Q100 inflow hydrographs at these locations are plotted for design storm events in Figures 6.6a through 6.6c. Peak Inflows for Design Storms are presented in Table 6.2 Table 6.2 Novato Bayland Design Storm Peak Inflows Novato Bayland Design Storm Inflow s (cfs)
Inflow Location
Novato Creek @
Diablo Ave
Warner Creek @
So. Novato Blvd.
Arroyo Avichi @
So. Novato Blvd.
Arroyo San Jose and
Pacheco Creek @
Pacheco Pond
Design Storm
Q10
2235
960
360
1730
Q50
2645
1817
360
3485
Q100
2683
2022
345
3850
FLO-2D
FLO-2D
SWMM
HMS
Data Source
In Novato Creek, Warner Creek and Arroyo Avichi, Bayland design storms inflow peaks for Q100 and Q50 events are limited by the upstream Novato Creek channel capacity which is exceeded in the Q50 event. As a result, the peak magnitudes for the Q50 and Q100 events are comparable. The peak direction and 48‐hour storm volume is larger for the Q100 event. Arroyo Avichi inflows are limited by the capacity of the tributary storm drains. When inflows exceed this capacity, excess water is bypassed to Baccaglio basin and becomes part of the west side flows to Scottsdale Marsh. Design storm inflows from Pacheco Pond Pacheco Pond, which include both Arroyo San Jose and Pacheco Creek flows, only flow to Novato Creek when creek stage is below pond stage. As a result, inflows from Pacheco Pond are delayed until the peak of the Novato Creek hydrograph passes and typically limited to low tide periods on Novato Creek (Figure 6.7). To predict Pacheco Pond inflows to the Novato Bayland EFDC model, KHE extracted a portion of the HEC‐
RAS model of Lower Novato Bayland developed to support restoration design at CABMK (Noble, 2005). Appendix F presents the Pacheco Pond HEC‐RAS analysis. The Q50 simulation results presented in Figure 6.7 illustrates HEC‐RAS model outputs which include inflows and outflows to Pacheco Pond, pond and adjacent basin water levels, and mass movement across levees. The dashed 6‐10 lines represent stage elevations in PP, and Novato Creek downstream of Hwy 37 and near the inlet to San Pablo Bay. Solid lines represent inflows and outflows from PP to NC and the adjacent BMK and private basins. Pacheco Pond discharges (purple line) are prevented during high flows due to backwater from main‐ stem Novato Creek, and resume during period of low tide during flood recession. In the absence of a NC outfall during large storm events, Pacheco Pond overtops the adjacent BMK and Leveroni levees. As a result, Pacheco Pond discharges to Novato Creek are larger in Q10 events then during larger design storms. Sea Level rise will decrease the volume and duration of PP discharges. Figure 6.6 Novato Bayland Model Domain and Boundary Conditions 6‐11 Pacheco Pond
Figure 6.6a Q10 Bayland Model Inflow Boundary Conditions Figure 6.6b Q50 Bayland Model Inflow Boundary Conditions 6‐12 Figure 6.6c Q100 Bayland Model Inflow Boundary Conditions Figure 6.7: Pacheco Pond Q50 Simulation Results 6‐13 6.2.2 San Pablo Bay Tidal Boundary Conditions Continuous verified water level measurements from the NOS Richmond tide station (NOAA Station ID: 8417208) are used as the downstream boundary condition for the Bayland model. To evaluate changes in tidal forcing between San Pablo Bay and lower Novato Creek and support modeling calibration and validation, KHE conducted continuous water level monitoring in Novato Creek (Nov. 2011 – June 2012) adjacent to the proposed CABMK breach site and at the Redwood Blvd Bridge (Figure 6.8). Redwood Blvd. Bridge CABMK Breach Site (CABMK) Figure 6.8: KHE 2012‐2013 Monitoring Locations on Novato Creek. Design storm analysis utilizes a four day period of seasonal maximum Richmond spring tides as the representative time series (Figure 6.9). The June water level record represents tidal variations with minimal influence (low flow) from tributary inflows. The more downstream monitoring location (BMK) characterizes tides in the lower Bayland, interior of the mudflat shoal which raises Novato Creek’s thalweg elevation to between 0.0 and 1.0‐feet NAVD88. The impact of this shoal on Bayland drainage is evident in the pronounced difference between the lower limit of San Pablo Bay and BMK time series. The Redwood Blvd. (RWD) monitoring location characterizes the difference between upper and lower Bayland tides, and provides observations at the downstream boundary of the FLO‐2D model utilized in the Creek model calibration. The 2.0 (+/‐ 0.5) hour travel time required for the tidal flood wave to propagate from San Pablo Bay to downtown Novato is captured in time lag between the RWD and BMK curves. As the tide propagates up Novato Creek channel, higher high waters attenuate 0.3‐ to 0.4‐feet and the drainage gradients are further reduced ‐ particularly during lower low water periods. The instantaneous difference between upstream and downstream water levels drives energy gradients, flow and sediment transport in the Novato Creek channel. The reduction in 6‐14 amplitude in the water level curves with upstream distance from the San Pablo Bay is indicative of the reduction in flow velocity and sediment transport potential in the reach. Calibration of the EC model focuses on recreating observed maximum stage at each location, as well as the amplitude of water level fluctuation across the Bayland. 8.0
Observed WSE (NAVD88) (ft)
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
-1.0
26-Jun-13
25-Jun-13
Date
24-Jun-13
Novato Creek @ BMK Breach Site
23-Jun-13
22-Jun-13
21-Jun-13
-2.0
Novato Creek @ Redwood Blvd
San Pablo Bay at Richmond
Figure 6.9: Novato Creek Water Level Monitoring 2012/13: Representative Spring Tide 6.2.3 San Pablo Bay Tidal Boundary Conditions with Sea Level Rise (SLR) MDPW (personal communication, R. Leventhal, 2013) defined two sea level rise (SLR) scenarios to be evaluated for design storm events:  16 inches SLR: State of CA adopted 50‐year assessment criteria (BCDC, 2009);  36 inches SLR: The mean of the NRC (2012) year 2100 sea level rise projection. Model modification assumed a still‐water elevation increase in the assumed Sea Level Rise (SLR) amount (Figure 6.10). This study does not consider potential boundary condition variation associated with storm tides or wind waves. 6‐15 FIGURE 6.10: Spring Tide with 16” and 36” of Sea Level Rise (SLR) 6‐16 6.3 Model Calibration and Validation 6.3.1
Model Calibration: Low Flow (June 2012) As described above, KHE selected the spring tide low flow period of June 21‐26, 2012 for calibration of the EC model in the absence of storm flows; model calibration with storm flows is discussed in Section 6.3.2. This low flow condition poses the most strenuous test of intertidal dynamics in that the period drives the strongest likely annual tidal gradients across the domain. In addition, higher high water elevations are above typical marsh plain elevations, testing model performance in predicting the timing of water movement across this important elevation threshold. KHE monitoring is the basis for model calibration data. Novato Creek inflow data (USGS Novato Creek Gage: 11459500) during the observation period is used to define tributary inflows to Novato Baylands domain. Inflows for ungaged creeks were defined as a percentage of Novato Creek flow based on tributary drainage area. This assumption introduces some uncertainty in calibration because Novato Creek has a dam and regulated outflows during the calibration period, which may not reflect full natural runoff potential. KHE calibrated the Bayland EC model by varying input parameters and comparing simulation results (predicted) against water levels measurements (observed) during the same period. Parameters varied in model calibration included: channel/marsh plain roughness; regional/localized bed elevation; tributary inflow; wetting/drying thresholds; horizontal kinematic viscosity; and momentum coefficients. Model calibration was successful and overall good agreement was developed between predicted and observed water levels at the Redwood Hwy Bridges (RWD) and proposed BMKV Breach (BMK) monitoring sites. Model low flow calibration results are presented in Figures 6.11a and 6.11b. This effort also informed KHE model sensitivity analysis (Appendix A.2) At the CABMK breach site, the calibrated model captures the overall form and timing of tidal exchange. Peak water levels are within 0.5 feet of observed values. The overall amplitude of predicted tidal oscillation is 10 to 15% less than that observed in the creek. Intermediate and low water extremes are underestimated by 0.5 to 1.0 feet. Two potential sources of deviation are shoals upstream of both Pacheco Pond and the BMKCSD locks, and the assumed channel width and depth across the San Pablo Bay mudflats. 6‐17 Figure 6.11 a: EC Bay Model: Low Flow Calibration Results at Redwood Blvd. (RWD) 3109, Novato Creek - Low Flow Calibration
Calibration Results: Time Series Summary
3.00
Legend
Bel Marin-Model
Bel Marin-Data
Water Surface Elevation (m)
2.50
2.00
1.50
1.00
0.50
0.00
170.00
170.50
171.00
171.50
172.00
172.50
173.00
173.50
174.00
174.50
Time (days)
Figure 6.11 b: EC Bay Model: Low Flow Calibration Results at CABMK Breach Site 6‐18 175.00
175.50
176.00
The EC Bayland model predicts tidally driven water levels changes from the Bay upstream. As a result, any deviations between predicted and observed values at the CABMK location are propagated upstream to the Redwood Blvd. calibration point. Time series plots of predicted and observed values at the RWD location ( Figure 6.11a) have a comparable magnitude of deviation between tidal maxima, which likely can be attributed in part to deviations further downstream. The slope of the falling limb of the water level curve in the RWD predicted times series is less than observed, which indicates that predicted ebb flow (drainage) is slower than observed rates. 6.3.2 Storm Propagation: Storm Hydrograph (Nov./Dec. 2012) In addition to the low flow calibration, KHE attempted to perform a second round of model calibration focused on wet season flows. KHE monitoring data captured two discrete storm events – one in late‐November/early‐December 2012 (Storm A) and the second in late‐
December 2012 (Storm B). Both peak flows were close to bank full (Figure 6.12). The peak storm magnitudes during the monitoring periods were estimated to have a 2 to 5‐year return period, and were not sufficient to generate overbanks flows13. Unfortunately, the input rainfall data set was incomplete for Storm Event B (USGS, 2012), and available data was not sufficient to generate inflows to HMS model for Storm B. Storm A data is utilized in model validation and discussed below. The monitoring data collected does however provide insight into lower Novato Creek response to a bank full (Q2 to Q5) flow event. Careful inspection of the Storm B record (Figure 6.12) shows:  At this intermediate storm magnitude, the flood wave is clearly visible at the RWD site;  Flood driven water surface variations are significantly attenuated at the BMK site, where a flood wave looks like an elevated tidal signal over the storm duration;  There is a multiday period during which low tide water levels remain elevated in the creek corridor reducing the capacity for drainage; and,  There is a 2 to 3 day recession period following the end of the storm during which water levels remain above pre‐storm levels. 13
Future model calibration based on monitoring during an over bank event data is recommended to support project design. 6‐19 Figure 6.12: Novato Creek Water Level Monitoring: November‐December 2012 Storms 6.3.3 Model Validation: Nov. 28 – Dec. 6, 2012 (Storm A) Following calibration, KHE configured the model with boundary conditions (predicted creek inflows and observed San Pablo Bay tides) describing the Storm A period (Figure 6.13). Comparison between predicted model results and observed water level data for Storm A (which were not incorporated in model calibration) is the basis for validating the model as a predictive tool. Observed water levels for Storm A at RDW and BMK are presented in Figure 6.14. In the Novato EC modeling effort this test is rigorous because no storm data was used in model calibration and validation considers predicted values that are outside the model calibration range. In addition, the effort supports validation of the Watershed Models (HEC‐HMS, FLO‐2D/ SWMM and HEC‐RAS) which were used to convert precipitation data for the period of record to the inflows specified at the Bay model boundaries (Figure 6.13). EC Bayland model validation results (Figures: 6.15 and 6.16) show good general agreement between predicted and observed values at both BMK and RWD observation points At both observation sites, the model captures flood peaks well, but generally under predicts water surface elevations by up to a foot across the solution domain. Departures between predicted and observed values are of the same magnitude as those observed in model calibration. Based on these simulation results, we conclude that the model can serve as a predictive tool for study purposes. . 6‐20 Figure 6.13: Predicted Inflow Hydrographs: November‐December 2012 (Storm A) Figure 6.14 Observed Water Level Data for Model Validation: November‐December 2012: Storm A 6‐21 Figure 6.15: Bayland EC Model Validation: Storm A Water Levels ‐ CABMK Breach Site Figure 6.16: Bayland EC Model Validation: Storm A Water Levels Redwood Blvd. (RWD) Site 6‐22 6.4 Sensitivity Analysis After calibration, KHE conducted a sensitivity analysis of the Q50 simulation results. Four (4) model parameters were modified to test the variability in predicted model response to the range of natural hydraulic variation and expected site conditions. In addition to the general model configuration, there are numerous parameters defined within the code to predict open water hydrodynamics and constituent transport. Appendix A: Table A‐1 summarizes hydrodynamic input parameters specified in the calibrated EFCD model. Parameters selected for sensitivity analysis and the rational for selection are as follows: Channel roughness: Increase and decrease channel roughness (100x, 0.1x) Rationale: Changes in channel roughness may influence the volume and velocity of predicted water movement. Channel roughness is varied over two orders of magnitude to evaluate the impact of changes is surface resistance on predicted flow dynamics. Spring Low Tide Rationale: To establish the range of variability in tidal hydraulics in the study area, KHE ran a Q50 simulation assuming that a SPRING LOW TIDE was synchronized with the upstream flood wave. This assumption maximizes the discharge gradients in the system, and in turn, the indicators of sediment transport capacity (velocity and bed shear stress). Neap Tidal Amplitude: Rationale: Baseline simulations assume spring tide conditions with high tide and flood peak coincident in the upper bayland. To evaluate the sensitivity of model results to tidal amplitude, KHE will simulate tidal dynamics with a comparable neap tide conditions at the San Pablo Bay boundary. Pre‐Dredge Bathymetric Profile in the Confluence Reach: Rationale: Baseline simulations assume a bathymetric profile that reflects the 2013 post excavation bathymetric profile in the Dredge reach from Diablo Ave to the SMART Bridge. To test model sensitivity to changes in bed elevation over the range of expected conditions, KHE conducted a sensitivity analysis simulation assuming a bathymetric profile based on pre‐dredge survey data (MCFC, 2012). A longitudinal profile of water surface elevations and bed shear stress during peak flood provides a basis for comparison of simulation runs. Appendix A.2 (Figure A.2 ‐1 and A.2‐2) illustrates the predicted change in flood peaks and bed shear stress at flood peak with variation of model parameters. In the fluvially dominant conveyance reach Predicted water surface elevations predicted water surface elevations vary by as much as a foot as roughness is varied over three orders of magnitude. In tidally influenced reaches changes in roughness have no significant effect. As seen in figures A.2‐2, shear stress varies directly with changes in channel roughness. Plots at locations (pg A.2‐5) indicate that water surface variations are significantly less during a non‐storm period. Spring low tide generates a large variation in water surface elevation profile in the baylands (as is expected) since the simulation shifts the tidal boundary condition from high to low tide at the profile time. Time series plots at selected locations (A.2‐
8) indicate that the change is a results of a phase shift, but the tidal amplitudes are comparable. 6‐23 Neap tidal amplitude variations were not significant in the confluence reach but drove approximately 30% higher values of shear stress through the lower bayland reach. Variation associated with bathymetric changes also generated interesting results. Water surface elevations were less than 0.5 ft higher in the Confluence reach. This suggests that dredging does little to lower peak water surface elevations. Interestingly the un‐dredged reach produced lower shear stress values in the confluence reach, but the inverse was true in the baylands were shear stress values increased on the order of 15‐30%. In general, maximum simulation results were stable, and predicted values were reasonable over the range of parameters evaluated. 6.5 Simulation Results (Q10, Q50 and Q100) KHE configured the Novato Bayland EC model to predict water levels and flow velocities for Q10‐, Q50‐ and Q100 design storms for current, 16” SLR and 36” SLR conditions. Time series results are presented for locations immediately downstream of three Novato Creek locations: Nave Gardens (Nave); Hwy 37; and CABMK breach site (Figure 6.17). In addition, color scaled plan maps of predicted design storm peak water surface elevation (WSE) and velocity (VEL) values are used to illustrate spatial variation in the Bayland corridor. Additional simulation results comparing these sites with data for SMART, Middle Deer Island Bain (Mid. DIB) and BMKCSD North Lock are presented in full page plots in Appendix A.1. 6.5.1 Peak Flow WSEs and Velocities Maximum water surface elevations and flow velocities for design storm scenarios are presented in Tables 6.4 and 6.5 below. Peak water levels are highest at the most upstream limit of the model and dissipate with distance downstream. The highest water surface elevations are generated by the Q100 storm event, though these values are not significantly different that the Q50 +SLR maxima. Velocity maxima vary similarly across model runs. Table 6.4: Maximum Simulated Water Surface Elevations Novato Bayland Peak Water Surface Elevations (ft NAVD88)
Location
Distance US
of Mouth (ft)
Q10
Q50
Q50 + SLR16 Q50 + SLR36
Q100
CABMK Breach
3390.4
8.0
8.0
8.9
10.4
8.0
BMKCSD No. Lock
9339.2
8.2
9.1
9.6
10.4
9.2
DS Hw y 37
19105.0
10.2
10.8
10.9
11.0
11.0
Mid DIB
24894.2
11.8
12.8
12.9
12.9
13.1
NWPRR
28687.4
13.1
14.0
14.0
14.1
14.3
Nave Gardens
31725.7
13.7
14.7
14.8
14.8
15.1
6‐24 Novato Bayland Maxim um Velocity (ft/s)
Distance US
of Mouth (ft)
Q10
Q50
3390.4
4.1
4.5
4.2
4.9
4.7
BMKCSD No. Lock
9339.2
3.0
3.1
3.0
3.5
3.2
DS Hw y 37
19105.0
2.8
2.8
2.7
2.6
3.1
Location
CABMK Breach
Q50 + SLR16 Q50 + SLR36
Q100
Mid DIB
24894.2
2.8
3.1
3.1
3.1
3.5
NWPRR
28687.4
2.5
2.9
2.9
2.9
3.3
Nave Gardens
31725.7
4.4
5.0
5.0
5.0
5.6
Table 6.5 Maximum Simulated Velocities Predicted water surface elevations for each of the design storms events for the selected locations are presented in Figures 6.17. In each simulation, 48‐hour design storms are initiated one day into the simulation period. The simulations continue for 24 hours after the storm passes to characterize changes in flood wave attenuation. In all of the design storm simulations the flood crest elevation is greatest at the most upstream station and attenuates with distance downstream. During low flow periods (T=0 to T=1.25 days), the tidal amplitude is greatest at the most downstream location (CABMK) and both the high and low water maxima attenuate with upstream distance from San Pablo Bay. This shift can be attributed to channel tidal energy losses, increasing bed elevations and freshwater flows. The inverse is true of flood wave characteristics, which attenuate from upstream to down as the flood energy and volume are dissipated through increasing channel geometry, levee overtopping and tidal damping. There is less than 1.0‐foot of difference in the flood crest elevation between the Q100 and Q50 simulations. The similarity in these peak flow water levels can be attributed to both the limited upstream conveyance (Q100 and Q50 peak inflows do not differ greatly because the Novato Creek channel is capacity limited) and levee crest overtopping, which spills water into adjacent basins at the same stage for all design storm events. Though the crests of the Q50 and Q100 flood hydrographs are comparable, both the volume of the storm hydrograph and the time required for flood recession increase with design storm magnitude. 6‐25 Figure 6.17: Predicted Water Surface Elevations (WSEs) for Design Storms: Q10, Q50 and Q100 Time series of depth averaged velocity profiles for design storm events are presented in Figure 6.18. The black line on the secondary (right) axis indicates the changing flow direction, where positive direction corresponds to outflow to San Pablo Bay. During low flow periods (T=0 to T=1.25 days) predicted tidally driven velocities are less than 1.5 ft/s and oscillate across a zero velocity (slack water) value during transition between flood and ebb tide. The predicted velocities of less than 2.0 ft/s are unlikely to be sufficient to convey non‐cohesive sand/silt or larger grain size fractions. These low flow velocities are sufficient to keep recently deposited tidal sediments (flocculated clay) and suspended cohesive bay muds mobilized, but not scour the underlying consolidated clay14. (See Sections 3.3.4 and 3.3.5 for discussion of Novato Creek sediment transport and erosion thresholds). The post‐dredge channel is most clearly depositional for both bay and fluvial sediments in the vicinity of Nave Gardens (the Confluence Reach), where low flow velocity peaks are less than 0.5 ft/s.15 Predicted ebb and flood velocity magnitudes are comparable, and as such do not 14
Flume studies of Novato Creek channel sediments define a shear stress threshold for sediment mobility of 0.6 pascalsm(Pa), and a, associated critical velocity threshold for Novato Creek tidal channels of 2.0‐ft/s (Noble, 2005). 15
The magnitude of peak low flow velocities may be underestimated by the model which uses depth average values across the long and shallow intertidal channel. Additional field data collection is recommended to confirm predictive targets for low flow calibration. 6‐26 support any conclusions with respect to the relative dominance of flood or ebb tides in the system. However, the duration of flood (landward flows) periods is typically shorter than the duration of ebb (bayward flow) periods. Given comparable flood and ebb channel velocities, this suggests ebb dominance in the confluence reach. This tidal asymmetry is not predicted at the more downstream locations. The selected time series profile locations also demonstrate the longitudinal variation in velocity magnitude during flood wave propagation and recession (T=1.5 to T=4.0 days). During Q100/Q50 events, peak velocities at Nave Gardens rise rapidly to between 5.0 to 5.5 ft/s and recede with minimal tidal influence. At the peak of the storm, the velocities greater than 5.0 ft/s are likely sufficient to mobilize and transport sand to coarse (>2”) gravels (USACE‐ERDC, 2001). Downstream of Hwy 37, peak velocities reach almost 3.5 ft/s, which is sufficient to mobilize sand and fine gravels. It is interesting to note the duration of the water level/velocity peaks, the clear limit on the maximum value and the absence of tidal influence in the record. These results suggest that during extreme storm events, fluvial influences are dominant as far downstream as Hwy 37, and that bridge/levee interactions control local WSE maxima and velocity structure. The 3.75 ft/s peak discharge velocities at CABMK are comparable to Hwy 37, but are subject to stronger tidal influence. During flood recession in the Lower Bayland, the tidally driven cyclical reduction in ebb velocities shortens the duration of high velocities and capacity for sediment transport as compared to those upstream. Phase II alternatives analysis should consider domain modifications to increase flow gradients across the Upper Bayland Reach to increase the potential for sediment transport through and downstream of the Confluence Reach during large storm events. Flood recession is considered over when slack‐water conditions return to the lower Bayland approximately 48 hours after the passage of the storm peak. 6‐27 Figure 6.18: Predicted Depth Average Velocity (VELs) for Design Storms: Q10, Q50 and Q100 Q10 velocity magnitudes are markedly less than those generated in the Q50/100 storm events, with maximum values less than 1.5 ft/s. These low velocities are indicative of have limited transport capacity for grain sizes greater than sand, and are consistent with the high observed sedimentation rates in the dredge channel. These results suggest that the enlarged dredge channel geometry increases deposition rates in the reach; and that alternatives analysis should consider a multistage dredge channel geometry to increase sediment conveyance and direct deposition. The characteristics of the Q10 curves are comparable to those produced by the Q50/100 design storms. Near Nave Gardens, ebb flow periods (outflows) appear longer in duration then flood periods (the Q10 velocity curves have been segregated from the Q50 and Q100 results and plotted at a larger scale in Figure 6.19 to better illustrate tidally induced trends). This tidal period asymmetry is evidence of the fluvial influences in the Confluence Reach. Further downstream, velocities and water surface profiles at Hwy 37 lag those at CABMK on the rising limb, and fall more quickly than at CABMK, but reach a comparable peak velocity. Recession velocity peaks are also comparable, though the stronger tidal oscillation at CAMBMK creates slack‐water periods during flood recession likely to drive deposition both at CABMK and upstream. In general, Q10 velocity peaks do not exceed 1.5 ft/s, which correspond to a capacity for transport of silt and fine sands (Table 3.5). 6‐28 Figure 6.19: Predicted Depth Average Velocity (VELs) for Q10 Design Storm Plan views of design storm peak velocities in the Upper and Lower Bayland Reaches are presented in Figures 6.20 a‐c, and 6.21 a‐c. As expected, peak velocities increase with storm magnitude. 6.5.2 Confluence Reach and Upper Bayland Velocities In the Confluence reach, there is a notable drop in velocity at the Warner Creek confluence which is consistent with observed sedimentation patterns. Predicted velocities drop rapidly upstream of Rowland Ave. and continue to decrease as the dredged channel cross section expands. In all design storms, a local velocity minimum occurs at the SMART Bridge and velocities rise again downstream through the smaller un‐dredged cross section. At Q10, the relatively high velocity continues around the apex of the Duckbill pond meander. This is not the case at Q50/Q100 storms which overtop right and left bank levees reducing local channel velocities. In all three design storm simulations, upper Bayland velocity decreases occur in the shoaled mid‐section of the reach between the ponds, and again at Hwy 37. In between, velocities in the open channel peak at between 2.0 and 3.0 ft/s. 6‐29 6.5.3 Lower Bayland Velocities Simulation results In the Lower Baylands also show an increase in channel velocity with storm magnitude. A narrower and relatively higher velocity reach extends from downstream of Hwy 37 to the Pacheco Pond Confluence. Downstream of the confluence with Pacheco Pond, the peak channel velocities decreases through a rapidly widening meander and remain relative low (less than 2.5 ft/s) until the North Lock confluence. Downstream of the North Lock confluence the corridor and channel thalweg increase. The predicted peak flood velocities are the highest at this location in the Lower Bayland reach, and range from 3.25 to 4 ft/s. In comparing Q10 with Q50/100 figures, channel velocities increase the most in the narrow channel downstream of Hwy 37 where Novato Creek overtops the left and right banks. Figure 6.18 illustrates the peak velocity increases from 0.25 to 0.5 ft/sec between Q50 and Q100 simulations; which are difficult to see in plan‐view graphics. Figure 6.20a: Upper Bayland: Q10 Peak Velocities 6‐30 Figure 6.20b: Upper Bayland: Q50 Peak Velocities Figure 6.20c: Upper Bayland: Q100 Peak Velocities 6‐31 Figure 6.21a: Lower Bayland: Q10 Peak Velocities Figure 6.21b: Lower Bayland: Q50 Peak Velocities 6‐32 Figure 6.21c: Lower Bayland: Q100 Peak Velocities 6.5.4 Peak Water Levels and Velocities with SLR of 16” and 36” To evaluate the potential impact of Sea Level Rise (SLR) in Novato Baylands, KHE repeated design storm simulations increasing tidal boundary conditions by 16’’ and 36”. Selected time series plots at Nave Gardens, Hwy 37 and CABMK breach site presented in Figure 6.22 provide a good illustration of predicted SLR impacts on velocity. The time series and plan view plots illustrating changes in predicted water surface elevations and velocity for each scenario discussed below are presented in Appendix A. Changes at six points of interest can be seen by scrolling through the plots in Appendix A. Plan maps of peak water surface and velocity results for the Q50 design storms illustrate typical characteristics and variation with 16” and 36” SLR scenarios (Figures 6.23a‐c and 6.23d‐f, respectively). The increases in Q50 water surface profiles with SLR occur primarily below 11.0‐feet NAVD88 elevation, with spring higher high water reaching 8.5‐ and 10.25‐feet under SLR at 16’ and 36” scenarios, respectively (Figure 6.22). Under non‐storm conditions, SLR increases in WSEs are propagated upstream uniformly within the Bayland. However, the SLR response differs across the Bayland under peak flood conditions. With increasing storm magnitude, peak flood water surface elevations are not significantly altered by SLR. Predicted storm WSE maxima changed by less that 1.0‐foot in the Confluence and Upper Bayland Reaches . This is not the case in the more tidally dominated Lower Bayland Reach where peak water surface elevation increases correlate directly with the rise in tide. The model predicts more significant increases in the elevation of the tidal amplitude in the Upper Bayland and Confluence reaches during non‐storm 6‐33 intertidal conditions. Rising tides reduce the storm water outflow capacity for both downtown Novato and Pacheco Pond. More interesting is the shift in predicted channel velocities with SLR (Figures 6.22d‐f and Appendix A). Under low flow condition (T=0 to T=1.5 days), SLR increases the potential diurnal tidal prism, depth and channel velocities in the Lower Bayland Reach creating more potential sediment transport, erosion and channel capacity. In contrast, peak flood velocities in the Lower Bayland Reach decrease with rising tides and maintain a stronger tidal variation throughout the flood period. The increased tidal elevations associated with SLR reduce high tide flood recession velocities and in turn flood stage bedload transport capacity in the Lower Bayland Reach. This suggests that rising tides associated with SLR will create a trend of increasing low flow tidal prism and channel velocity in the Upper Bayland Reach, while decreasing flood stage conveyance and sediment transport in the Lower Bayland Reach. Alternatives working in concert with SLR should therefore seek to increase flow gradients to the extent possible in the Upper Bayland Reach to sustain conveyance; and seek to increase tidal prism in the Lower Bayland Reach to sustain channel geometry under more strongly depositional conditions. The trends observed in the Q50 simulation are also present in the Q10 and Q100 scenarios. Rising tides have a stronger influence at the lower flows (Q10 and below) and SLR has a greater impact on flood recessions. In Q100 scenarios, the higher water levels associated with peak flows lessen SLR influences in the Upper Bayland Reach because peak flood stages remain above 11.0‐feet for the majority of the storm event. As in the Q50 simulations, Q100 flood recession is notably delayed by high tide backwater which extends flood recession beyond the end of the simulation period. Both Q10 and Q100 velocity decreases are comparable in magnitude to those observed in the Q50 design storm. 6‐34 Figure 6.22: Changes in WSE with SLR16” and SLR36”
at Nave Gardens (upper), Highway 37 (middle) and CABMK (lower)
6‐35 Figure 6.23a: Novato Bayland Q50 Peak Water Surface Elevations Figure 6.23b: Novato Bayland Q50 + 16” SLR Peak Water Surface Elevations 6‐36 Figure 6.23c: Novato Bayland Q50+ 36” SLR Peak Water Surface Elevations Figure 6.23d: Upper Novato Bayland – Q50 Peak Velocity 6‐37 Figure 6.23e: Upper Novato Bayland ‐ Q50+16” SLR Peak Velocity Figure 6.23f: Novato Bayland Q50+ 36” SLR Peak Water Surface Elevations 6‐38