Updated STA-2 Hydraulic Analyses. Science and Technology Service (STS) Contract No. ST WO05. South Florida Water Management District

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1 Updated STA-2 Hydraulic Analyses Science and Technology Service (STS) Contract No. ST WO05 South Florida Water Management District August 31, 2007 Prepared by: Prepared for: Sutron Corporation South Florida Water Management District Hydrologic Services Division (HSD) Attn: Tracey Piccone, Project Manager 6903 Vista Parkway N, Suite 5 B-2 Building, 3 rd Floor West Palm Beach, FL Gun Club Road Tel: (561) West Palm Beach, FL 33406

2 Executive Summary This report summarizes the performance of updated STA-2 hydraulic analyses in support of The Long-Term Plan for Achieving Water Quality Goals. Stormwater Treatment Area 2 (STA-2) is a primary component of the Everglades Construction Project mandated by the 1994 Everglades Forever Act (section , Florida Statutes). The original STA-2 consisted of Cells 1, 2 and 3 with 6,430 acres of effective treatment area and began operation in A 1,901-acre expansion of the STA was recently completed and is scheduled to begin start-up operation as soon as the severe drought condition passes. The additional treatment area will be Cell 4 of STA-2 and will be operated in parallel with the existing three cells. Previous STA-2 two-dimensional (2-D) hydraulic models were reviewed, revised and integrated into a new STA-2 linked Cells D hydraulic model. The new model was then used in the steady flow simulations of Design Peak Flow and Low Flow and transient simulations of High Flow (Standard Project Storm) and two historical storm events. The addition of Cell 4 shows beneficial effect on STA-2 hydraulic performance: reduced hydraulic loadings for Cell 1, Cell 2 and Cell 3 and reduced water depth in these cells by up to 1.0 ft in most flow simulations. Modeling results demonstrate that Cell 4 will always receive greater hydraulic loading than the other cells and Cell 1 receives the least volume, when there is no modulation of inflow gates. Without structure gate regulations, the computed inflow distribution of Design Flow condition (3,370 cfs) is 381 cfs, 760 cfs, 880 cfs and 1,349 cfs for Cell 1, 2, 3 and 4, respectively. At least one culvert of G-367 A-F has to be closed to reduce Cell 4 inflow. Without gate modulation, Cell 4 will receive more than 50% of the total inflow under Low Flow (800 cfs). This is partially attributed to difference in land surface elevations among cells. After Cell 4 is in flow-through operation, G-337A and one or more of the G-367 culverts need to be modulated to achieve the appropriate flow distribution among the treatment cells. STA-2 Cell 4 stage-volume and stage-area relationships were built by using STA-2 Cell 4 topographic data. ii

3 Table of Contents Executive Summary...ii Table of Contents..iii List of Figures....iv List of Tables... vi 1. Introduction Previous Modeling Work Model Set-up Modeling tool Hydraulics of STA-2 System Topographic data Vegetation Representation of Water Control Structures Model Parameters Modeling Results Performance criteria for evaluation of simulation results Modeling Design Peak Flow, Low Flow and Standard Project Storm Flow Historical Storm Events Sensitivity Analysis Sensitivity Model Runs Model Limitation Discussion on Gate Operations and Structure Flows Summary and Recommendations References Appendix: STA-2 Cell 4 stage volume and state area relationships Introduction Topographic Data Methodology Stage-Volume and Stage-Area Relationships for STA-2 Cell iii

4 List of Figures Figure 1: Location of STA Figure 2: Schematic of STA Figure 3: STA-2 Cells 1-4 Land Surface Elevation (ft NGVD 29)... 6 Figure 4: STA Vegetation Map... 7 Figure 5: STA-2 linked Cells 1-4 Model Finite Element Mesh and Assigned Material Types... 8 Figure 6: Computed Water Surface Elevations (ft NGVD) for Design Peak Flow, Cell 4 Offline Figure 7: Computed Water Surface Elevations (ft NGVD) for Design Peak Flow, Cell 4 Online Figure 8: Differences in Computed Water Levels under Design Peak Flow (Cell 4 Online vs. Cell 4 Offline) Figure 9: Computed Water Depth (ft) for Design Peak Flow, all four cells online Figure 10: Velocity Magnitudes (ft/s) under Design Peak Flow, all Four Cells online Figure 11: Unit Flows (ft 2 /s) under Design Peak Flow, all Four Cells online Figure 12: Comparison of Computed and Historical G-329 Water Levels and Flows Figure 13: Comparison of Computed and Historical G-331 Water Levels and Flows Figure 14: Comparison of Computed and Historical G-333 Water Levels and Flows Figure 15: Computed Water Surface Elevations (ft NGVD) for Low Flow, Single Cells (200 cfs each) Figure 16: Computed Water Depth (ft) for Low Flow, Single Cells (200 cfs each) Figure 17: Computed Velocity Magnitudes (ft/s) for Low Flow, Single Cells (200 cfs each) Figure 18: Difference in Computed Water Surface Elevations (ft) for Low Flow, Linked Cells Model (Total Inflow: 800 cfs) vs. Single Cells (200 cfs each) Figure 19: Difference in Computed Water Surface Elevations (ft) for Low Flow, Linked Cells Model (Total Inflow: 600 cfs for Cells 2, 3 and 4) vs. Single Cells Model (200 cfs each, four cells) Figure 20: Selected Stage Hydrograph during High Flow (Standard Project Storm) Figure 21: Computed Water Depth at Time = 0.0 (High Flow) Figure 22: Computed Water Depth at Time = 24.0 (High Flow) Figure 23: Computed Water Depth at Time = 72.0 (High Flow) Figure 24: Total Inflow Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004).. 32 Figure 25: 329B_H Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004)33 Figure 26: G329B_T Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) Figure 27: G330D_H Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) Figure 28: G331D_H Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) Figure 29: G331D_T Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) Figure 30: G333C_H Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) iv

5 Figure 31: G333C_T Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) Figure 32: G332_H Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) Figure 33: G334_H Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) Figure 34: G367_H Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) Figure 35: G367_T Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004)38 Figure 36: Computed Velocity Magnitudes during Storm Event 1 (At time =72.0 hours) Figure 37: Computed Water Depth during Storm Event 1 (At time =72.0 hours) Figure 38: Total Inflow Hydrograph for Storm Event 1 (From 8/20/2006 to 8/30/2006) 41 Figure 39: S6_T Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006). 42 Figure 40: G-329B_H Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) Figure 41: G-329B_T Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) Figure 42: G-330D_H Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) Figure 43: G-331D_T Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) Figure 44: G-332_H Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) Figure 45: G-333C_H Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) Figure 46: G-333C_T Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) Figure 47: G-334_H Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) Figure 48: G-367_H Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) Figure 49: G-367_T Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) Figure 50: Comparison of Computed Stage Values (G-333C_T, Event 1) due to Different Manning s n Values Figure 51: Changes in Computed Water Levels due to Different Manning s n Values (n=0.8 vs. 0.5 for Emergent Vegetation, Design Peak Flow) Figure 52: Impact of Explicit Representation of G-337A on Stages (Design Peak Flow)50 Figure 53: Comparison of G-329A Culvert Flow Computation (FESWMS vs. FLOW Program) Figure 54: Derived Flow Rating Curves and Location for G-329A Low Flow Figure 55: Comparison of G-329A Flow Rating Curves Figure 56: Comparison of G-332 Flows (FESWMS computed vs. FLOW Program) v

6 List of Tables Table 1: Manning s n Values for Different Materials...8 Table 2: Flow Distribution under Design Peak Flow 10 Table 3: Computed Water Levels under Design Peak Flow..10 Table 4: Downstream Boundary Conditions for Low Flow Simulations.18 Table 5: Effect of Manning s n values on Flow Distribution...25 Table 6: Computed Maximum Stages for High Flow..27 Table 7: Effect of Manning s n values on Flow Distribution 48 Table A-1: STA-2 Cell 4 Stage-Volume and Stage-Area Relationship.. 58 vi

7 Updated STA-2 Hydraulic Analyses Final Report 1. Introduction Stormwater Treatment Area 2 (STA-2) is a primary component of the Everglades Construction Project mandated by the 1994 Everglades Forever Act (section , Florida Statutes). It is situated generally on and surrounding the former Brown's Farm Wildlife Management Area and is located immediately west of Water Conservation Area 2A. STA-2 provides a total effective treatment area of 8,331 acres to treat stormwater runoff originating from the Hillsboro Canal and Ocean Canal drainage basins upstream of the S-6 Pump Station. The location of STA-2 is shown in Figure 1. The original STA-2 consisted of Cells 1, 2 and 3 with 6,430 acres of effective treatment area and began operation in A 1,901-acre expansion of the STA was recently completed and is scheduled to begin start-up operation as soon as the severe drought conditions pass. The additional treatment area will be Cell 4 of STA-2 and once it demonstrates a net improvement of phosphorus and mercury and begins flow-through operations, it will be operated in parallel with the existing three cells (Goforth 2006). Two-Dimensional (2-D) linked cells hydraulic models have already been developed for Cells 1-3 of STA-2 (Sutron Corporation 2005). A single Cell 4 model was developed for the design of STA-2/Cell 4 expansion (Taylor Engineering Inc. 2005). The objective of the current modeling effort as stated in the Statement of Work is to perform updated hydraulic analyses in support of the Long-Term Plan for Achieving Water Quality Goals. The existing STA-2 linked cells model was revised to include the new Cell 4 expansion. The revised linked Cells 1-4 model was then used to analyze steady flow simulation scenarios of STA-2 for Low, Design and High Flow conditions and two historical storm events. The stage-volume and stage-area relationships for Cell 4 were also developed from Cell 4 topographic data. This final report (Deliverable 1.3) summarizes major results obtained in the modeling work for Task 1.1: Updated STA-2 Hydraulic Analyses and Task 1.2: STA-2 Draft report and model, and comments from District staff have been incorporated. 1

8 STA-2 Not to Scale S-6 Cell 4 Cell 3 Cell 2 Cell 1 WCA- 2A Figure 1: Location of STA-2 2. Previous Modeling Work Recent two-dimensional hydraulic modeling studies of STA-2 consist of an existing STA-2 linked Cells 1-3 model (Sutron Corporation 2005) and a single Cell 4 model (Taylor Engineering Inc. 2005). The Cell 4 model study was part of the STA-2/Cell 4 expansion project Basis of Design Report (Brown and Caldwell 2005). The STA-2 Cell 4 2-D hydraulic model and report (Taylor Engineering Inc. 2005) were reviewed and Cell 4 basic data and information were extracted and incorporated into the new linked Cells 1-4 model. The existing STA-2 linked cells 1-3 model (Sutron Corporation 2005) is the basis for model updating and revision. 2

9 3. Model Set-up 3.1 Modeling tool The FESWMS/FST2DH computer program was selected by the District as the modeling tool for the current hydrodynamic modeling of STAs. The computation engine FLO2DH (renamed to FST2DH in SMS 9.0) is part of the Federal Highway Administration s Finite Element Surface-water Modeling System (FESWMS). It is a public domain model code. However, the Graphic User Interface (GUI) through the Surface water Modeling System (SMS) is commercial software. SMS is needed to generate finite element mesh and perform pre- and post- processing. FST2DH simulates two-dimensional depth averaged hydrodynamic flows of surface water bodies using the finite element method. Additional information about the theoretical background of the model, its numerical method, its input and output data can be found in the User s Manual referenced in the report (Froehlich 2002). The governing equations that apply to current STA modeling are as follows: z t q1 q + + = qm (1) x y w 2 q t q t 2 q 1 1 ( ) ( ) q q z H b Hτ τ xx xy + β + gh β gh τ bx (2) x H y H x ρ x y 1 = 2 q 1 1 ( ) ( ) 1q2 q z Hτ yx Hτ b yy + β + β gh gh τ by (3) x H y H y ρ x y 2 = where z w =H + z b is water surface elevation, H = water depth, z b = bottom elevation above a certain datum, q 1 = U*H = unit flow rate in the x direction, q 2 = V*H = unit flow rate in the y direction, U and V are velocity in the x and y directions, q m = mass inflow rate (positive) or outflow rate (negative) per unit area, β= isotropic momentum flux correction coefficient that accounts for the variation of velocity in the vertical direction, g = gravitational acceleration, ρ = water mass density, τ bx and τ by = bed shear stresses acting in the x and y directions, respectively and τ xx, τ xy, τ yx, and τ yy = shear stresses caused by turbulence where, for example, τ xy is the shear stress acting in the x direction on a plane that is perpendicular to the y direction, and n= the Manning s n roughness coefficient. The bed shear stresses are computed as follows. 3

10 / = y z x z H q q q gn b b bx ρ τ (4) / = y z x z H q q q gn b b by ρ τ (5) Current STA-2 modeling work was completed with FST2DH (Release version 3.3.2, Build date: 11/15/2005) and SMS Hydraulics of STA-2 System A schematic map of STA-2 layout and flow patterns is shown in Figure 2. The model domain of STA-2 water flow consists of the Supply/Inflow Canals, treatment cells 1, 2, 3 and 4, and the Discharge Canal. These components are hydraulically connected by various water control structures (e.g. gated culverts and spillways). Figure 2: Schematic of STA-2 Stormwater runoff enters the system through pump stations S-6 and G-328. S-6 provides the primary source of stormwater flows to STA-2, with a total pumping capacity of 2,925 cfs. The agriculture pumping station G-328 provides drainage and irrigation for tributary

11 farmlands and the total outflow (drainage to STA-2) capacity is 445 cfs. Water is conveyed through the Supply Canal and then the Inflow Canal and flows into the four cells for treatment. Flow through Cell 1 is controlled by inflow culverts G-329 A-D and outflow structures G-330 A-E. Inflow culverts G-331 A-G convey water into Cell 2 and gated spillway G- 332 discharges treated flows from Cell 2. Cell 3 has gated culverts G-333 A-E as inflow control structures and outflow structure G-334 is located at the southeastern end of Cell 3, next to the Discharge Canal. The gated structure G-337A is located in the Inflow Canal adjacent to G-333E. This structure can modulate inflow into Cell 4. Flow through Cell 4 is controlled by inflow culverts G-367 A-F and outflow culverts G-368 A-D. The Discharge Canal receives treated water from the four treatment cells and the STA-2 outflow pump G-335 discharges flows from STA-2. Treated water eventually will be delivered into WCA 2A via structures G-336A-F which directs flow to the north portion of WCA-2A and G-336 G which directs flow to the southwest portion of the L-6 Borrow Canal and on to WCA-2A. During extreme storm events, diversion structures G-338 and G-339 can be opened to facilitate flow diversion from STA-2 Supply Canal. Pumping stations S-6/G-328 and G-335 are not represented as structures and they are treated as boundary conditions. The seepage collection canal is not part of the model domain since seepage losses cannot be explicitly simulated by FESWMS/FST2DH. For short-term event-based flow simulations, seepage losses, rainfall (except for standard project storm) and evapotranspiration are considered negligible compared to structure inflow. Gated culverts G-329 A-D, G-331 A-G, G-333 A-E, G-367 A-F and G-368 A-D are simulated with FESWMS/FST2DH s culvert option that has no gate operation or gateopening feature. Gated spillways G-332 and G-334 are simulated as weirs. Stormwater flow into Cell 4 can be regulated by G-337A. G-337A is a gated structure located in the Inflow Canal at the northwest corner of Cell 3. FESWMS/FST2DH has a gate structure option that is very unstable and not fully functional; therefore, G-337A was not explicitly represented in the model. Restricted cross section was used at G-337A location to consider the possible head loss. The case of partially opened gates for culverts was investigated by adjusting the entrance loss coefficients and reduced barrel geometry. 3.3 Topographic data Cell 4 topographic data were directly obtained from the single Cell 4 model (Taylor Engineering Inc. 2005). Cells 1-3 topographic data were based on previous topographic survey data and interpolated to the finite element mesh in SMS 9.2 (Sutron Corporation 2005). The average land-surface elevations in the marsh of the four treatment cells are Cell 1: ft NGVD; Cell2: ft NGVD; Cell 3: 9.92 ft NGVD and Cell 4: 8.80 ft NGVD. 5

12 These values are based on results of Cells 1-4 stage-volume and stage area relationships. The difference in land surface elevations among the four cells (Figure 3) is an important factor in stormwater inflow distribution. Cell 3 and Cell 4 are more likely to receive more inflows than Cell 1 and Cell 2 when inflow is not regulated. Local topographic features in Cell 4 include a former fish farm (minimum elevation at ft NGVD) and several transverse remnant farm canals and emergent cattail strips. Figure 3: STA-2 Cells 1-4 Land Surface Elevation (ft NGVD 29) 3.2 Vegetation Vegetation within the existing treatment cells (Cells 1, 2 and 3) consists mostly of emergent vegetation (e.g., sawgrass and cattail) in Cells 1 and 2, and submerged aquatic vegetation (SAV) and periphyton communities in Cell 3. SAV vegetation is anticipated in Cell 4. The spatial extents of major vegetation types were determined from the 2006 STA-2 vegetation map shown in Figure 4. The emergent sawgrass and cattail are 6

13 identified as different material types in the model (Figure 5). The finite element mesh was newly designed for the linked Cells 1-4 model (Figure 5). The perimeter levee was not part of the mesh, and was considered as a no-flow boundary. In the model setup, Water exchange among inflow and discharge canals and treatment cells was only possible through defined water control structures. Figure 4: STA Vegetation Map 7

14 Figure 5: STA-2 linked Cells 1-4 Model Finite Element Mesh and Assigned Material Types 3.3 Representation of Water Control Structures The STA-2 system consists of several types of water control structures. They are inflow/outflow pump stations, gated culverts, gated spillways and weir-box controlled culverts. Pump stations S-6, G-328 and G-335 were considered as model boundary conditions and pumps were not simulated in the 2-D model. FESWMS/FST2DH can only model uncontrolled culverts and weirs; therefore, a culvert or spillway in the model is either fully opened or totally closed. Partially opened culverts and spillways are the norm during real STA-2 operations. This cannot be directly represented in the model due to the model code limitation. Therefore, adjusted entrance coefficient or reduced physical geometry of culvert cross sections were used to represent partial gate openings. The static structure information in DBHYDRO, design drawings and operation plan was used in defining culvert/weir geometrical dimensions (e.g. culvert barrel diameter, length, inverts, weir length and crest). 3.4 Model Parameters Manning s n roughness coefficient is the main model parameter. The calibrated values (Sutron Corporation 2005) for emergent vegetation and SAV were used. A constant 8

15 Manning s n value of was applied to the fish farm area; and (Brown and Caldwell 2005) was applied to canals (Table 1). Table 1: Manning s n Values for Different Materials Depth (ft) Cattail/Sawgrass SAV Canals > to 1.0 Varies linearly Varies linearly to <= The default eddy turbulent coefficient value was used and model runs have shown that simulation results were insensitive to this parameter. There are still no observed stages or flow data for Cell 4 flow-through operation, therefore, no new model calibrations were attempted. 4. Modeling Results 4.1 Performance criteria for evaluation of simulation results From the STA-2 operation plan (Goforth 2006) and previous design documents, the performance criteria for STA-2 hydraulic analyses are summarized as follows. No significant zones of flow stagnation (zero flow velocity or closed circulation) or flow velocity magnitude greater than 0.1 ft/sec in the marsh area. No significant areas with water depth greater than 4.5 ft. Minimum Depth. To the maximum extent practicable, a minimum static water level of 0.5 feet above the average ground elevation of the treatment cells will be maintained to avoid dry-out of the treatment cells, subject to available water from the upstream watershed or other sources. Because of the elevated risk of methyl mercury upon dry-out, the minimum depth for Cell 1 is 12 inches. Maximum Depth. To the maximum extent practicable, a maximum static water level of 4.5 feet above the average ground elevation of the treatment cells will be observed to avoid damage to the levees and marsh vegetation. Balance water flows and phosphorus loading rates appropriately among treatment cells. Hydraulic and nutrient loading should be properly distributed among treatment cells to optimize treatment performance. 9

16 4.2 Modeling Design Peak Flow, Low Flow and Standard Project Storm Flow Steady state simulations using the 2-D linked Cells 1-4 hydraulic model were performed for Design Peak Flow (3,370 cfs), Low Flow (800 cfs) and transient flow of High Flow (Standard Project Storm). The total inflow values used were from the STA-2 operation plan (Goforth 2006) Design Peak Flow Relevant previous Design Peak Flow simulations are: Linked Cells 1-3 model (Sutron Corporation 2005) Single Cell 4 model (Taylor Engineering Inc. 2005) Two scenarios were simulated for Design Peak Flow: Cells 1-3 online with Cell 4 offline, or all four cells online; these correspond to Scenarios 4 and 3, respectively, of the Brown and Caldwell Cell 4 BODR and the March 2006 STA-2 Operation Plan. The inflow distribution among the four cells was summarized in Table 2. When Cell 4 was offline, the computed inflow distribution was slightly different from previous results (Sutron Corporation 2005). The difference can be explained by changes in model setup (e.g. new finite element mesh, updated culvert parameters and newly interpolated land surface elevations and vegetations). The entrance loss coefficient (Ke) was 0.50 for Cells 1-3 culverts in previous models, and is 0.85 in the current linked Cells 1-4 model based on DBHYDRO updated information. Cell 4 culverts have not been registered in DBHYDRO and the default value of 0.50 was used for the entrance loss coefficient (ke). The water mass errors were very small for each model run (cell-by-cell absolute mass error is at least less than 3%). The original design value of Cell structure inflow predicted a larger portion of total inflow for Cell 2. When all four cells were fully opened, Cell 4 received the largest inflow value (1,349 cfs). Modulation of G-3337A or G-367 A-F gates will be required to achieve the appropriate flow distribution. Computed water surface elevations responded to the addition of Cell 4 with decrease in water levels ranging from 0.5 ft to 1.0 ft in Cells 1, 2 and 3 (Figures 6, 7 and 8). A decrease in stages in treatment Cells 1, 2 and 3 is thought to be good for treatment performance, since water depth is reduced. There is a concern however that too much inflow will be delivered to Cell 4. Modulation of Cell 4 inflow gates will be necessary to avoid excessive hydraulic loading of Cell 4. Computed water depths, velocity magnitudes and unit flows were within performance criteria in all four cells (Figures 9, 10 and 11). 10

17 Table 2: Flow Distribution under Design Peak Flow Items Cell 1 Cell 2 Cell 3 Cell 4 total Cfs Cfs cfs cfs Cfs Cell 4 remains offline Previous Cells 1-3 simulation Design value All four cells operational In comparison to the single Cell 4 model result for the Design Peak Flow (Taylor Engineering Inc. 2005), the major difference is the total inflow into Cell 4. The single Cell 4 model applied a constant value of 1,011 cfs. On the other hand, the linked cells model let the model compute the inflow rates internally, which turned out to be 1,349 cfs. The G-368 A-D headwater level was specified as ft NGVD in the single Cell 4 model; G-368 A-D headwater levels were part of the solution in the linked cells model (11.50 ft NGVD). Therefore, computed water surface elevations were about 0.60 ft higher at G-368 in the linked cells model. G-337A or G-367 A-F can be regulated to reduce flows into Cell 4. Computed stages were summarized in Table 3. Additional simulations of two flow scenarios under Design Peak Flow (Cell 4 online) were performed to try to reduce Cell 4 inflow for a more appropriate flow distribution. In the first simulation, G-367A was closed, only G-367 B through F were fully opened, simulation results show that flow distribution was changed: Cell 1: 414 cfs; Cell 2: 808 cfs; Cell 3: 923 cfs and Cell 4: 1,229 cfs. Cell 4 inflow was reduced by 120 cfs. However, Cell 4 inflow is still too high (>1,011 cfs, the design flow value). In a second simulation, both G-367 A and G-367 F were closed, and the computed Cell 4 inflow is 1,101 cfs, with Cell 1: 448 cfs; Cell 2: 856 cfs; Cell 3: 965 cfs. These results indicate that G-337A or G-367A-F must be regulated to restrict excessive Cell 4 inflow. Optimized operations can be implemented as partially opened gates at G-337A or G-367A-F totally closing selected culverts at these structures. Table 3: Computed Water Levels under Design Peak Flow Items Cell 1 Cell 2 Cell 3 Cell 4 Inflow Canal Ft NGVD29 Ft NGVD29 Ft NGVD29 Ft NGVD29 Ft NGVD29 All four cells operational flow scenario Upstream end* Downstream end* All Cells 1, 2 and 3 operational, Cell 4 offline flow scenario Upstream end N/A Downstream end N/A *Inflow Canal upstream at G-337 tailwater; downstream at G-337A.Cells upstream at inflow structure tailwater; downstream at outflow structure headwater. Historical stage and flow daily data of STA-2 operation were obtained from DBHYDRO and the computed Cells 1-3 stages under Design Peak Flow (with Cell 4 offline) were 11

18 compared with these hydrographs as plotted in Figures It can be concluded that the computed water surface elevations under Design Peak Flow are close to or exceed the maximum stages in historical STA-2 operation. Figure 6: Computed Water Surface Elevations (ft NGVD) for Design Peak Flow, Cell 4 Offline 12

19 Figure 7: Computed Water Surface Elevations (ft NGVD) for Design Peak Flow, Cell 4 Online 13

20 Figure 8: Differences in Computed Water Levels under Design Peak Flow (Cell 4 Online vs. Cell 4 Offline) 14

21 Figure 9: Computed Water Depth (ft) for Design Peak Flow, all four cells online 15

22 Figure 10: Velocity Magnitudes (ft/s) under Design Peak Flow, all Four Cells online 16

23 Figure 11: Unit Flows (ft 2 /s) under Design Peak Flow, all Four Cells online 17

24 Figure 12: Comparison of Computed and Historical G-329 Water Levels and Flows Figure 13: Comparison of Computed and Historical G-331 Water Levels and Flows 18

25 Figure 14: Comparison of Computed and Historical G-333 Water Levels and Flows Low Flow The Low Flow condition is defined as a maximum inflow of 200 cfs for each treatment cell. In a linked cells model, the total inflow (S-6 + G-328) is 800 cfs for STA-2 under Low Flow condition. Unlike single cell models, the linked Cells 1-4 model will compute the flow distribution as part of the model solution. The downstream boundary conditions are the specified headwater levels at the outflow structures listed in Table 4. This was obtained from the desired minimum stages in the STA-2 operation plan. Table 4: Downstream Boundary Conditions for Low Flow Simulations Items Downstream stage (ft NGVD) Note Cell 1 Varied with G330 A-E weir flow Controlled by the weir crest elevations (about 13.0) Cell Desired Minimum stage Cell Desired Minimum stage Cell Desired Minimum stage One of the key findings from the Low Flow simulations is that achieving the appropriate inflow distribution among the four cells is the most important issue to be addressed. The 19

26 distribution of water depth (Figure 16) and velocity magnitude (Figure 17) under Low Flow are within the range of performance measures. A model run was made for the independent operation of each cell with an exact inflow of 200 cfs. This was done by removing all inflow culverts and outflow structures in the linked cells model, the inflow was assigned to each cell as a nodal source term of 200 cfs, and specified stage boundary conditions were applied as downstream boundary. This is equivalent to several single cell model simulations. The computed water surface elevations are shown in Figure 15 for this case. Other cases were compared to this model run to determine how flow distribution in a linked cells model under Low Flow condition affect computed water levels/depth. Figure 15: Computed Water Surface Elevations (ft NGVD) for Low Flow, Single Cells (200 cfs each) 20

27 Figure 16: Computed Water Depth (ft) for Low Flow, Single Cells (200 cfs each) 21

28 Figure 17: Computed Velocity Magnitudes (ft/s) for Low Flow, Single Cells (200 cfs each) In the second simulation, a total inflow of 800 cfs was applied to the upstream end of the Supply/Inflow Canal with all inflow structures being fully opened. Under this condition, model results demonstrated that the system has a great tendency for water to flow toward Cell 4. This is because Cell 4 has the lowest land surface elevations among the four treatment cells. The computed water surface elevations were compared to that shown in Figure 15 and the difference is plotted in Figure 18. Cell 1 inflow was zero since water levels in the Inflow Canal were lower than the crest elevations of G-330A-E weir-box (about 13.0 ft NGVD). Cell 4 received more than 70% of the total inflow and Cell 3 received most of the remaining inflow. The water levels increased in Cell 3 and Cell 4, and decreased in Cell 1 and Cell 2. 22

29 Figure 18: Difference in Computed Water Surface Elevations (ft) for Low Flow, Linked Cells Model (Total Inflow: 800 cfs) vs. Single Cells (200 cfs each) The model results from the second simulation confirmed that modulation of the gates of inflow structures is necessary to obtain a more appropriate inflow distribution for total inflow of 800 cfs or less. In a third model run, only one culvert gate of G-333 A-E and one culvert gate of G-367 A-F was opened, respectively, for Cell 3 and Cell 4. Because Cell 1 is close to static zero flow due to the physical constraint of the G-330 A-E weir crest elevations, only 600 cfs was applied as total inflow (S6 + G328). If 800 cfs were applied as total inflow, Cell 1 inflow would still have been zero. Computed stages in the Inflow Canal were less than 13.0 ft NGVD (lower than G-330 A-E weir crest elevations). Computed flows showed reduced inflow into Cell 4 (about 195 cfs) and Cell 2 inflow was about 205 cfs. The changes in water levels in comparison to the single cell model runs (see Figure 15) were only significant in Cell 1 (Figure 19). For the final Low Flow simulation (total inflow is 800 cfs), an attempt was made to optimize the inflow distribution among the four cells. Since direct gate operation is not possible in the model, artificially adjusted entrance loss coefficients or reduced culvert 23

30 diameters were used as a proxy for partial gate openings. Since the adjusted culvert parameters are not linearly related to the physical gate openings, this is an indirect approach to adjusting cell inflow in the model. There is a limitation when adjusted entrance loss coefficients (Ke) are used to represent partial gate openings, since the physical range of Ke is from 0.0 to 1.0, this approach can only apply for the minor effect of large gate openings. When gate openings are rather small, the flow regimes may have changed and reduced culvert geometry (e.g., cross sections or length of the barrels) can be used to represent partial gate openings. It was found that this is a good way to overcome the limitation of FESWMS on gate operation; during steady state flow simulation, the same head difference and discharge value correspond to a specific gate opening value on the flow rating curves for inlet culverts. For example, the barrel diameter of G-367 A-F was reduced from 6.0 ft to 2.0 ft to restrict Cell 4 inflow; model simulation results show computed G-367 culvert flow is 167 cfs and G-367 head difference (HW-TW) is 3.71 ft, then the constant gate opening to reach this specific discharge value can be deduced from G-367 rating curves. The true gate opening value will not be equal to 2.0 ft, the reduced culvert diameter. The latter is only a surrogate for tuning the Cell 4 flow rate for a more appropriate flow distribution. The effectiveness of this approach can be seen from a summary of the improved flow distribution for the Low Flow condition in Table 5. This is better than using just open/closed gate assumptions (Scenario No. 2 in Table 5). 24

31 Table 5: Summary of Low Flow Simulation Results Inflow structures Flow Scenario No.1: partial gate openings Culvert diameter adjusted Culvert diameter HW TW Ft NGVD HW- TW Computed Flow Ft NGVD ft Cfs (ft) (ft) Cell 1 G-329 A-D Cell 2 G-331 A-G Cell 3 G-333 A-F Cell 4 G-367 A-F total Inflow structures Flow Scenario No.2: full gate openings or closed Culvert diameter (ft) Gate Opening HW TW Ft (ft) NGVD HW- TW Computed Flow Ft NGVD ft Cfs Cell 1 G-329 A-D Cell 2 G-331 A-G Cell 3 G-333 A-F* or Cell 4 G-367 A-F* or total Note: *G-333 C and G-367 C culverts were closed. 25

32 Figure 19: Difference in Computed Water Surface Elevations (ft) for Low Flow, Linked Cells Model (Total Inflow: 600 cfs for Cells 2, 3 and 4) vs. Single Cells Model (200 cfs each, four cells) High Flow The High Flow condition is defined as the extreme flow condition of Standard Project Storm. The Standard Project Storm event (SPS) for STA-2 was defined as 23.6 inches of direct rainfall in a 24-hour period over the treatment cells. In addition, the structure inflow is 2,570 cfs (Goforth 2006). The direct SPS rainfall was considered as uniformly distributed over treatment cell surface area and input into the model as nodal/point source (volumetric rate). The current version of FESWMS/FST2DH has no distributed (e.g. element-based) source/sink feature. This will only slightly affect the simulation results (spatial uniformity). The total volume of rainfall is the same as applied over surface areas as usual. A transient 72-hour model run was performed to simulate the High Flow condition. The initial condition was the model results of Design Peak Flow discussed in a previous 26

33 section. This means that the SPS event began with computed stages/velocities under Design Peak steady flow condition (3,370 cfs); structure inflow was then reduced to 2,570 cfs by diverting 800 cfs via G-339 at the beginning of the storm rainfall. The SPS rainfall lasted 24 hours. The G-335 headwater level under Design Peak Flow is 8.90 ft NGVD and this was used as downstream boundary condition. It should be noted that current simulation has different initial stages and G-335 headwater level from those stated in the Operation Plan (water depth of 4.5 ft over average ground elevations; G-335 headwater at ft NGVD that can be higher than stages in the cells). Two flow scenarios were simulated: (1) all four Cells operational; (2) Cells 1-3 operational, Cell 4 closed. The rises in water surface elevations at the inflow/outflow structures in the four Cells were summarized in Table 6 and plotted in Figure 20. The maximum increases in stages were 1.0 ft, 1.3 ft, 1.7 ft and 2.1 ft, respectively, for Cell 1, 2, 3 and 4. Water depths exceeded 4.5 ft in Cell 4 and parts of Cell 2 and Cell 3 at the peak level. However, water levels retreated to that of the Design Flow condition at the 72 hours (Figures 21, 22 and 23). Computed maximum water levels were well below the crest elevation of the perimeter levees. The potential for over loading of Cell 4 is even more evident under High Flow conditions (Figure 22). Table 6: Computed Maximum Stages for High Flow Structure Stages Flow Scenarios (ft NGVD) All four Cells operational Cells 1-3 operational, Cell 4 closed G-329 HW TW G-330 HW TW G-331 HW TW G-332 HW TW G-333 HW TW G-334 HW TW G-367 HW N/A TW G-368 HW TW N/A G-335 HW

34 Figure 20: Selected Stage Hydrograph during High Flow (Standard Project Storm) 28

35 Figure 21: Computed Water Depth at Time = 0.0 (High Flow) 29

36 Figure 22: Computed Water Depth at Time = 24.0 (High Flow) 30

37 Figure 23: Computed Water Depth at Time = 72.0 (High Flow) 4.3 Historical Storm Events Two historical storm events were simulated with the new linked Cells 1-4 model. Since there are still no flows and stage data for Cell 4 flow through operation, new model calibrations were not performed. These two selected storm events were historical flow conditions before Cell 4 was constructed. Therefore, the storm events were simulated with the new linked Cells D model and simulation results were compared for the two cases of Cell 4 being offline and all four cells being online Storm Event 1 This historical storm event has been used in previous model calibration (Sutron Corporation 2005). It started from 8/25/2004 and ended at 9/5/2004. The total simulation time is 240 hours. One important reason for selecting this specific period is the constant gate openings at the inflow and outflow structures. The 15-minute averaged total inflow hydrograph (S-6 + G-328) was applied at the upstream boundary of the Supply/Inflow Canal (Figure 12). The downstream boundary conditions were specified stages at G- 335_H, G-332_H and G-334_H. Specified stages were applied at Cell 2 and Cell 3 outflow structures (G-332 and G-334) to obtain better numerical stability. When computed water levels at these gated spillways were changing rapidly and/or were of 31

38 very low values, the computation encountered numerical instabilities. The steep slopes in surface ground elevations near the outflow structures could be adjusted in the model to alleviate this numerical problem. The first simulation was made with Cell 4 offline. The computed stages at several cell inflow/outflow structure locations were compared with historical observed stage values. The second model run assumed that all four Cells were operational. This is a hypothetical case because Cell 4 was not yet in operation when this historical storm occurred. The history matching for stages during the storm event is generally quite good. The parameter values were essentially those obtained from previous calibration of the Cells 1-3 linked model. It can be seen from Figures that computed water levels were lower by as much as 1.0 ft in Cell 1 and 0.5 ft in Cell 3 when Cell 4 was added to the system. Velocity and water depth were within performance limits (Figures 36 and 37). Computed G-367 tailwater levels ranged from ft NGVD to ft NGVD. G332_H and G334_H were also plotted but not the target for history matching. Figure 24: Total Inflow Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) 32

39 Figure 25: 329B_H Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) Figure 26: G329B_T Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) 33

40 Figure 27: G330D_H Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) Figure 28: G331D_H Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) 34

41 Figure 29: G331D_T Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) Figure 30: G333C_H Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) 35

42 Figure 31: G333C_T Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) Figure 32: G332_H Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) 36

43 Figure 33: G334_H Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) Figure 34: G367_H Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) 37

44 Figure 35: G367_T Stage Hydrograph for Storm Event 1 (From 8/25/2004 to 9/5/2004) 38

45 Figure 36: Computed Velocity Magnitudes during Storm Event 1 (At time =72.0 hours) 39

46 Figure 37: Computed Water Depth during Storm Event 1 (At time =72.0 hours) Storm Event 2 A 10-day storm event starting on 8/20/2006 was simulated. The historical flow data series (S-6 + G-328) were applied as the upstream inflow boundary condition (Figure 38). In addition, historical G-335 headwater levels were used as the downstream boundary condition. The gate openings of all inflow structures were nearly constant during the storm event. Two scenarios were simulated. First, Cell 4 was offline and only water flows in Cells 1, 2 and 3 were modeled; computed stages were compared to observed historical values. Second, a model run was made by assuming all Cells 1-4 structures were fully opened. From Figures 39-45, it is observed that computed water levels were also lower by as much as 1.0 ft when Cell 4 operation was added to the system. This storm event has a peak total inflow exceeding 3,000 cfs but it is still less than the Design Peak value of 3,370 cfs. 40

47 It can be seen that the history matching of water levels between observed values and computed values (Cell 4 being offline) was not as good as the calibration period for the linked Cells 1-3 model (Storm Event 1). This simulation of Storm Event 2 can be considered as a model validation for the previous Cells 1-3 linked cells model. Flow configuration and conditions such as gate operation and vegetation changed between 2004 to For example, G-333E was closed during Storm Event 2. G-329 A-D gate openings were increased to 5.5 ft. The simulation results show that G-331 A-F tailwater levels (Cell 2) were overestimated, while G-329 A-D tailwater levels (Cell 1) were underestimated. Cell 2 outflows (G-332) did not drain well and stage drawdown occurred near G-332. This is one of the reasons that G-331D_T was overestimated. With G-333E being closed, the fitting was better for G333C_T. Water levels in the Supply/Inflow Canal (S6_T, G329B_H, and G333C_H) were a little overestimated. G332_H and G334_H were also plotted but were not the target for history matching. Figure 38: Total Inflow Hydrograph for Storm Event 1 (From 8/20/2006 to 8/30/2006) 41

48 Figure 39: S6_T Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) Figure 40: G-329B_H Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) 42

49 Figure 41: G-329B_T Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) Figure 42: G-330D_H Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) 43

50 Figure 43: G-331D_T Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) Figure 44: G-332_H Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) 44

51 Figure 45: G-333C_H Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) Figure 46: G-333C_T Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) 45

52 Figure 47: G-334_H Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) Figure 48: G-367_H Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) 46

53 Figure 49: G-367_T Stage Hydrograph for Storm Event 2 (From 8/20/2006 to 8/30/2006) 5. Sensitivity Analysis Some sensitivity model runs were made to observe how sensitive the simulation results were in response to the possible range of variation in model parameters. The major sources and factors of model errors are: Manning s n values Eddy viscosity coefficient values Topography (land surface elevations and local topographic features) Resolution of finite element mesh Time steps and numerical parameters (e.g. maximum iteration number and relaxation factor) Water control structure representation Adequacy of the governing equations and model assumptions 5.1 Sensitivity Model Runs Manning s n values: Storm Event 1 47

54 The Manning s n values are the most sensitive model parameter. The first sensitivity model run was performed for the Storm Event 1. The base Manning s n value for emergent vegetation was adjusted from 0.80 to 0.50 while other parameters and model set-up were held constant. When Cell 4 was offline, the computed stages in the Inflow Canal and treatment cells were further deviated from the observed stage values when n = 0.50 than n = 0.80 (e.g. G-333C_T in Figure 50). Obviously, this means that n = 0.80 is more appropriate for STA-2 emergent vegetation. Figure 50: Comparison of Computed Stage Values (G-333C_T, Event 1) due to Different Manning s n Values Manning s n values: Design Peak Flow Another sensitivity model run was made for Design Peak Flow. The adjustment of Manning s n values was identical to the first sensitivity model run. The impact on computed stages was only significant in Cell 1 and Cell 2, where emergent vegetation is dominant. The increases in water levels were less than 0.20 ft in Cell 1 and Cell 2 and less than 0.10 ft in Cell 3 and Cell 4 (Figure 51). The flow rates at Cell 1 and Cell 2 inflow culverts were reduced by about 3% and increased at Cell 3 and Cell 4 by about 3% (see Table 7). 48

55 Figure 51: Changes in Computed Water Levels due to Different Manning s n Values (n=0.8 vs. 0.5 for Emergent Vegetation, Design Peak Flow) Table 7: Effect of Manning s n values on Flow Distribution Emergent n values Cell 1 Cell 2 Cell 3 Cell 4 n=(0.5, 1.1) n=(0.8, 1.1) Model results were found to be relatively insensitive to eddy turbulent coefficients, mesh size, time steps and nonlinear iteration parameters. 49

56 5.1.3 G-337A: Design Peak Flow The G-337A gated structure was not explicitly represented in the original model setup. A sensitivity model run was made for the Design Peak Flow scenario with G-337A as a 4- barrel box culvert (60 inch x 80 inch) of short barrel length. The simulation result is compared to the case of representing G-337A as a reduced cross-section (Design Peak Flow). G-337A was fully opened and in the submerged orifice flow regime. The head loss of about 1.5 ft was larger than that of the reduced cross-section (about 0.5 ft). Inflows into Cell 4 were reduced to about 1,100 cfs. However, changes in computed water surface elevations for the four cells are within ± 0.2 ft (Figure 52). The difference for Low Flow can be even smaller. Figure 52: Impact of Explicit Representation of G-337A on Stages (Design Peak Flow) Model representation of water control structures As for model representation of water control structures, the simplified culvert equations in FESWMS/FST2DH are surprisingly accurate comparing to the G-329A flow rates computed by more complex algorithm in the District s FLOW program for the historical storm event 1 flow/stage data (Figure 53). This was computed outside of the model code with observed tailwater and headwater and gate opening information. FESWMS classified the culvert flow as outlet control flow and the gate information was not used in the flow equation; one explanation for this close match is that during this period, G-329A culvert flow was Type-3 Flow in the Flow Program; this form of flow equation is very 50

57 similar to that of outlet control equation in FESWMS. Therefore, the computed flow values were more likely controlled by the head difference rather than gate opening values. It should be noted that the entrance loss coefficient value (Ke) was artificially adjusted to better the match. The use of Ke = 0.85 underestimates discharge in FESWMS. A value of 0.50 gives the best fit; this can be explained by the lack of partial gate openings in FESWMS. The gate openings were varied from 0.0 to 5.0 ft; however, no restriction of culvert cross-section (diameter = 6.0 ft) was applied in the model. Figure 53: Comparison of G-329A Culvert Flow Computation (FESWMS vs. FLOW Program) 5.2 Model Limitation The limitation of current STA-2 linked Cells D hydraulic models is discussed and summarized as follows. New model calibration cannot be done for the linked Cells 1-4 model since Cell 4 flow-through operation has not yet started; model parameters applied were not optimized or validated by flows/stages history matching, although previous calibration results for linked Cells 1-3 model (Sutron Corporation 2005) were used. Representation of water control structures in the model is very limited. Gate operations can only be indirectly considered. Flow computation algorithms are simplistic when compared to those implemented in the District s FLOW program 51

58 for STA-2 water control structures. See the following sections for further discussion. Seepage, evaporation and rainfall (except for extreme design storm) were neglected in model simulations. 5.3 Discussion on Gate Operations and Structure Flows To the knowledge of the writers, FESWMS has been the preferred hydrodynamic modeling tool for STA design and planning since the beginning of STA design and construction. During the design phase, single cell steady flow models are adequate for project purpose and water control structures are not explicitly simulated in the 2-D hydraulic models. Treatment cells and flow-ways are linked together to optimize hydraulic performance after STAs are in flow-through operation. Canals, water control structures and cells must be hydraulically linked together to build the so-called 2-D linked cells model. Achieving appropriate flow distribution among treatment cells and flow-ways through gate operations is then one of the modeling objectives. Since the original FESWMS model code is designed for uncontrolled flow culverts, weirs, etc., gate opening time series cannot be input into FESWMS. However, as can be seen in Figure 53, reduced cross-sections or adjusted entrance loss coefficient can be used to consider the effect of partial gate openings and results on flow distribution were improved by this approach. In Figure 54, partial flow rating curves for G-329A were derived from historical stage and flow data of the two storm events. Only two gate openings of 5.02 ft and 5.52 were plotted and these values are the constant maximum gate openings for different periods in past G-329A operations. When trying to determine the G-329A gate opening for Low Flow condition (see Table 5), the assumed discharge at G-329A is 272/4=68.0 cfs, and the headwater and tailwater difference is 0.50 ft. The desired gate opening value will then be smaller than 5.0 ft and can be determined from the flow rating curves (Figure 54). The corresponding adjusted entrance coefficient (Ke) in FESWMS to the actual gate openings in SFWMD flow equations were demonstrated in Figure 55. When calibrated to fit SFWMD flow-rating curves, the (Ke) value were adjusted in FESWMS. For example, the (Ke) value of 0.42 and 0.50 corresponded to gate openings of 5.02 and 5.52 for G-329A. As mentioned earlier, the use of an adjusted (Ke) to represent partial gate openings is limited due to the physical range of this coefficient ( ). Very small gate openings cannot be adequately represented by this. The use of the weir option to represent gated spillways (G-332 and G-334) is another simplistic approximation of gated spillways. In Figure 56, the computed G-332 flow rates are generally close to flow data provided by the District s FLOW program. The overestimated flow rates during the first 48 hours were due to small gate openings not reflected in the weir equations. In conclusion, FESWMS can indirectly consider gate operations by model simulations with different constant gate openings for various total inflow values (e.g., ranging from Low Flow to Design Peak Flow) to determine how to achieve appropriate cell inflow 52

59 distributions. Past STA-2 operations show that the G-329 (the barrel diameter is 6.0 ft) maximum gate opening was 5.0 ft during storm event 1 (2004) and increased to 5.50 ft in storm event 2 (2006). This can be represented in the current model. Figure 54: Derived Flow Rating Curves and Location for G-329A Low Flow 53

60 Figure 55: Comparison of G-329A Flow Rating Curves Figure 56: Comparison of G-332 Flows (FESWMS computed vs. FLOW Program) 54

61 6. Summary and Recommendations Previous STA-2 hydraulic models were reviewed and integrated into a new STA-2 linked Cells D hydraulic model. The new model was then applied for the steady flow simulations of Design Peak Flow and Low Flow and transient simulations of High Flow (Standard Project Storm) and two historical storm events. Since STA-2 Cell 4 has the lowest average land-surface elevations among the four cells, water tends to flow toward Cell 4. This was confirmed by computed flow distributions in Design Peak Flow, Low Flow and High Flow. Modulation of inflow gates is necessary to achieve the appropriate flow distribution. The addition of Cell 4 shows beneficial effect on STA-2 hydraulic performance including reduced hydraulic loadings for Cell 1, Cell 2 and Cell 3 and reduced water depth in these cells by up to 1.0 foot in most flow simulations. Modeling results demonstrate that when there is no modulation of inflow gates, Cell 4 will always receive greater hydraulic loading than the other cells and Cell 1 receives the least volume. Without structure gate regulations, the computed inflow distribution of the Design Flow condition (3,370 cfs) is 381 cfs, 760 cfs, 880 cfs and 1,349 cfs for Cell 1, 2, 3 and 4, respectively; one or more culverts of G-367 A-F must be closed to reduce Cell 4 inflow to the design value of 1,011 cfs. Cell 4 alone will receive about 70% of the total inflow under the Low Flow condition (800 cfs). This is partially attributed to the difference in land surface elevations among cells. After Cell 4 is in flow-through operation, G-337A and one or more of the G-367 culverts will need to be modulated to try to achieve appropriate flow distribution among the treatment cells. Simulations show that small gate openings are needed to raise water levels in the Inflow Canal to maintain positive Cell 1 inflow when total inflow is 800 cfs or less. New model calibration and validation should be conducted when flow and stage data on Cell 4 flow-through operations are available. STA-2 Cell 4 stage-volume and stage-area relationships were built by using topographic data. 55

62 References Brown and Caldwell. May 12, STA 2/Cell 4 Expansion Project Basis of Design Report (BODR). Burns & McDonnell, Long Term Plan for Achieving Water Quality Goals. October 27, Froehlich David C User s Manual for FESWMS FST2DH, Publication No. FHWA-RD , September 2002 (Revised October 2003). U.S. Department of Transportation Federal Highway Administration. Goforth, G., March Updated Operation Plan Stormwater Treatment Area 2, prepared for the South Florida Water Management District. Sutron Corporation, April STA-2 2-D Hydraulic Modeling (Linked Cells Model) Task 1.8 Final Report. Taylor Engineering Inc., April STA 2/Cell 4 2-Dimensional Hydraulic Modeling, STA-2 Cell 4 BODR Appendix F. 56

63 Appendix: STA-2 Cell 4 stage volume and state area relationships 1. Introduction As stated in the Long-Term Plan for Achieving Water Quality Goals (Burn & McDonnell 2003), accurate stage-volume/stage-area relationships need to be developed for every cell of each STA [Bc82(4)]: A stage-volume relationship for each cell of each STA. A principal use of the new survey data described in section is to develop the water storage potential in those systems. This aids directly in management of water volumes and indirectly in the correct determination of hydraulic detention times in the STAs. Additionally, the wetted fraction of area may be developed, and used to forecast the loss of effectiveness at low stages (see section 5.2.1). An accurate stage-volume relationship becomes more critical as shallower depth operations come under consideration, because issues of short-circuiting and areal water coverage become more important at low stages. This appendix summarizes the development of stage-volume and stage-area relationships of STA-2 Cell 4 as specified in Task 1.1 of the Contract ST WO Topographic Data The STA-2 Cell 4 topographical data (Taylor Engineering, April 2005) provided the ground surface elevations for stage-volume and stage-area estimations. 3. Methodology The topographic data were part of a finite element mesh of STA-2 Cell 4. The Surface Water Modeling System (SMS 9.2) calculation tool was used to obtain the storage volume under a specified static water level (SWL) (Figure A-1). This is repeated to obtain the relationships for a series of SWL values ranging from minimum to maximum surveyed ground surface elevations. Similarly, the wetted portion of surface area under a certain static water level was determined from the contour lines of specific ground surface elevations. By assuming that the effect of the slope of the perimeter levees on storage is negligible, when the static water level reaches or exceeds the maximum surveyed ground surface 57

64 elevations, the wetted surface area is a constant value and the stage-volume relationship becomes linear. Figure A-1: 3-D View of Storage Volume and Wetted Surface Area at a Specific Static Water Level Stage-volume and stage-area relationships are provided in the form of look-up tables and curve plots for convenient future use. 4. Stage-Volume and Stage-Area Relationships for STA-2 Cell 4 Local topographic features, such as remnant farm ditches and spreader/collection canals and a fish farm were considered in the storage volume and surface area calculation. The land surface elevations in the marsh range from 7.50 ft NGVD to 9.70 ft NGVD (Figure A-2). 58

65 Figure A-2: Truncated Land Surface Elevations of STA-2 Cell 4 (7.50 to 9.7 ft NGVD) The surveyed ground elevations in Cell 4 range from ft NGVD in the fish farm to 9.79 ft NGVD in the marsh area. The spatially averaged ground surface elevation is 8.49 ft NGVD when loca depressions such as the fish farm and ditches are considered. If only the elevations in the marsh area are included, the spatially averaged ground surface elevation is 8.80 ft NGVD. Storage volume and wetted surface area were sampled at various water levels in an increment of 0.25 ~ 0.5 ft. The results are summarized in Table 5 and Figure 44. These storage volume and area data were directly obtained from SMS volume/area estimation. From Table A-1 and Figure A-3, it can be seen that the marsh area of Cell 4 is very flat. The storage volume below land surface elevation of 8.0 ft NGVD is attributed to the local topographic features (fish farm and local canals). 59