SUPPLEMENTAL LEVEE FAILURE ANALYSIS USING NWS BREACH AND HEC-RAS EXISTING RD 799 LEVEE AT SANDMOUND SLOUGH

Transcription

1 SUPPLEMENTAL LEVEE FAILURE ANALYSIS USING NWS BREACH AND HEC-RAS EXISTING RD 799 LEVEE AT SANDMOUND SLOUGH FOR THE CYPRESS LAKES PROJECT CONTRA COSTA COUNTY, CALIFORNIA September 2, 2003 Prepared By: CIVIL SOLUTIONS 1325 Howe Avenue Suite 202 Sacramento, CA (916) JOB # This document prepared under the supervision of: Signature dated : The status of this report is PRELIMINARY unless the appropriate signature is provided to the left. Signature will not be provided until review is complete Civil Solutions

2 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough -i- TABLE OF CONTENTS 1 SUMMARY AND CONCLUSIONS INTRODUCTION PURPOSE OF THE STUDY SITE DESCRIPTION Location Existing Conditions Proposed Cypress Lakes Project STUDY APPROACH FLOOD HAZARD ASSUMPTIONS Model Datum Tide Hydrograph TWELVE BREACH SCENARIOS LEVEE FAILURE COMPUTER MODELS THE BREACH MODEL Input Data Description Levee Geometry and Soils Reservoir Variables Calculation of Breach Geometry and Dynamics Breach Model Results HEC-RAS Input Data Description River Reaches Cross-sections Inline and Lateral Structures Flow Data and Boundary Conditions HEC-RAS Geospatial Referencing MODELING THE LEVEE BREACH IN HEC-RAS RESULTS TIME-TO-FILL MAXIMUM FLOW VELOCITY Average Channel Velocity Velocity Distribution RECOMMENDATIONS REPAIR THE LEVEE BREACH AS QUICKLY AS POSSIBLE INTERIOR LEVEE DESIGN...29

3 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough -ii- 7.3 GOLF COURSE CONSTRUCTION...29

4 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough -iii- LIST OF TABLES Table 1 - Breach Location and Development Scenario Matrix Table 2 - Breach Model Input Summary Table 3 - Breach Model Output Data Summary Table 4 - Downstream Boundary Hydrograph Computation Rule Table 5 - Summary of Breach Data Used in Hec-ras Table 6 - Time-to-fill Summary

5 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough -iv- LIST OF FIGURES Figure 1- Vicinity Map Figure 2 - Site and Levee Boring Location Map Figure 3 - Typical Levee Cross-section Figure 4 - Breach Locations Figure 5- Comparison of 100-year High Tide Elevations and BREACH Model Reservoir Stage Figure 6 - Typical HEC-RAS With-Project Stream System Schematic Figure 7 - Typical HEC-RAS Cross-section Figure 8 - Lateral Diversion Rating Curve Figure 9 - Typical HEC-RAS Lateral Structure Figure 10a, 10b - Levee Failure Geometry and Flow Progression from BREACH Model Figure 11 - HEC-RAS Levee Breach Progression Curve Figure 12 - Typical HEC-RAS Inline Structure and Breach Figure 13 - Breach 1: Rate of Fill Analysis Figure 14 - Breach 2: Rate of Fill Analysis Figure 15 - Breach 3: Rate of Fill Analysis Figure 16 - Breach 1: Velocity Profiles Figure 17 - Breach 2: Velocity Profiles Figure 18 - Breach 3: Velocity Profiles Figure 19 - Figure 20 - Figure 21 - Figure 22 -

6 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough -v- LIST OF APPENDICES APPENDIX A YEAR TIDE DATA APPENDIX B - BREACH INPUT AND OUTPUT FILES APPENDIX C - HEC-RAS VELOCITY PLOTS APPENDIX D - OVERSIZED EXHIBITS APPENDIX E - CD

7 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough -vi- FIGURE 1 VICINITY MAP

8 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page -1-1 SUMMARY AND CONCLUSIONS This report presents the results of a study of potential flooding of the Hotchkiss Tract. The study uses two computer models (BREACH and HEC-RAS) to simulate a breach in the Sandmound Slough Levee, and to compute hydraulic and hydrologic conditions of the resulting flood. The study was undertaken to determine: Would the construction of an interior levee by the Cypress Lakes Project have a significant effect on flooding conditions, and if so, Can those changes in conditions be associated with differences in the risks to life and property created by the flooding. We identified two conditions that we believe can be related to the relative risks associated with flooding; maximum flow velocity and time-to-fill. The study evaluates these conditions for 4 levels of development; Existing conditions, Phase 1 development, Ultimate level of development and Ultimate development with golf course plus 3 potential breach locations, resulting in 12 modeling scenarios. The results of the model study indicate that, while the overall magnitude of the changes are modest, there are local increases in maximum flow velocity due to construction of the Cypress Lakes levee. This occurs mostly in the zone between the existing and proposed levees. The timeto-fill is reduced from approximately 60 hours to 57.5 hours, or by less than 5%.

9 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page -2-1 INTRODUCTION 1.1 PURPOSE OF THE STUDY This study analyzes the hydraulic and hydrologic conditions on the Hotchkiss Tract that could result from the failure of the levee on Sandmound Slough, in the vicinity of the proposed Cypress Lakes Project. It focuses on two conditions that can be associated with the risks to life and property as a consequence of such a failure. These are maximum flow velocity and time-tofill. Maximum flow velocity is directly related to the forces that endanger life and damage structures, cause erosion, etc. Time-to-fill is an indicator of the available emergency response time. The study considers three potential levee breach locations and four development scenarios, resulting in 12 site conditions with differing physical characteristics. The study is designed to allow the investigators to evaluate the effect these physical differences may have on the resulting hydraulic and hydrologic conditions of the site, and therefore the associated relative risks. 1.2 STUDY LIMITATIONS AND DISCLAIMER The study is designed to establish a basis for comparing the relative risk to life and property from an assumed levee failure. We made no attempt to determine the probability of a levee failure, and the levee failure is assumed to be equally likely at any location. The 3 failure locations were chosen to be representative of the northerly, central and southerly regions of the site, not for any conditions specific to that location. The direct impacts of the levee failure on structures within the failure zone itself was not a factor in the choice of breach locations. Due to limited data and limited model sophistication, the study is only intended as a comparison of the relative risks, not as an absolute or objective establishment of the magnitude of these risks. No attempt is made to quantify the magnitude of the losses, in lives or dollars, that would result from a levee failure. The time-to-fill factor is a relative measure of the duration of the initial flood event. It is defined as the time when the interior water surface elevation reaches the exterior tide elevation, so the initial flow of water into the site from Sandmound Slough ceases. It does not consider the elevation of existing roadways or structures, does not imply or assume an elevation below which there is no risk to life or damage to property, etc., and is not to be taken as an indication of the time available for evacuation or emergency response. It is simply a relative measure of the effect that changes in site volume due to variation in levee geometry could have on the rate at which the water surface within the Hotchkiss tract would rise in response to the driving tidal elevation. 1.3 SITE DESCRIPTION Location The study site is located on the Hotchkiss Tract, Contra Costa County, California (Figure 1). It occupies approximately 3,230 acres between Dutch Slough on the North, Sandmound Slough on the East, Rock Slough on the South, the Contra Costa Canal and Little Dutch Slough on the West

10 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page -3- (Figure 2) Existing Conditions Most of the site is below the diurnal tidal elevation in the adjacent sloughs of the Sacramento/San Joaquin Delta. Elevations range from -9 ft NGVD to over 15 ft NGVD, with most of the land surface between 0.0 and 1.0 ft Levees along Dutch Slough and Sandmound Slough protect the Hotchkiss Tract from flooding. A typical levee cross-section is shown in Figure 3. Several residences occupy the area along Sandmound Slough, including several that are on the levee interior bank Proposed Cypress Lakes Project The Cypress Lakes Project proposes to develop approximately 681 acres of the site with residential development. Current development proposal includes constructing a levee approximately 400 feet inboard of the existing Sandmound Slough Levee (RD 799 Levee). There are three potential levee configurations; Phase 1", Ultimate Development without golf course fill between the levees, and Ultimate Development with golf course fill between the levees. These levee configurations are shown on Figures A, B and C.

11 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page -4- FIGURE 2 SITE LEVE BORI LOCA MAP AND E NG TION

12 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page -5-2 STUDY APPROACH The study was developed to evaluate the hydraulic and hydrologic conditions on the Hotchkiss Tract at the location of the Cypress Lakes Development in the event of a breach in the Sandmound Slough levee. Two computer models were required for this study. The National Weather Service BREACH model, and the U.S. Army Corps of Engineers, Hydrologic Engineering Center s River Analysis System (HEC-RAS v Unsteady State). The BREACH model was used to determine the geometry of the levee breach, and HEC-RAS was used to model unsteady flow hydraulics. More details on the models is provided below. 2.1 FLOOD HAZARD ASSUMPTIONS Model Datum The BREACH model cannot work with negative elevations. The model datum was adjusted to ft NGVD for this analysis. For example, the elevation of the typical levee toe on the inboard side is -4.0 ft NGVD, represented as 96.0 ft in BREACH. All elevations were adjusted back to NGVD for the HEC-RAS modeling Tide Hydrograph The BREACH model study assumes the levee failure occurs at the 100-year high tide elevation of 7.0 ft NGVD at the beginning of a 10-day period of high tides and high river flows. This is considered to be the worst case condition for flood hazards. The tidal elevations were proportionately adjusted from the February 21, 1986 to March 3, 1986 Rock Slough gage records to correspond to the 100-year high tide elevation 1. The tide vs time hydrograph is presented in Appendix A, Table A-1 and Figure A TWELVE BREACH SCENARIOS There are 12 levee breach scenarios resulting from 3 assumed breach locations and 4 levels of development. Breach 1 is near the northern end and opposite of the proposed Phase 1 Cypress Lakes Levee. Breach 2 is centrally located, directly across from the area of the widest setback between the existing and proposed levees, and the Breach 3 is at the southern end of the Cypress Lakes project. The 3 levee breach locations are shown on Figure 4. The 4 development scenarios are: 1. Existing Conditions - no Cypress Lakes Project and no new levee construction. 2. Phase 1 - Includes the Phase 1 Cypress Lakes Levee, approximately 400 feet inboard of the existing levee. 1 The measured high tide was 9.28 ft Mean Lower Low Water (MLLW). This value was adjusted to NGVD by subtracting 3.0 ft. All values in the hydrograph were then adjusted by the ratio 7/6.28.

13 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page Ultimate Development - No Golf Course - This levee encloses the Cypress Lakes project at build-out, but without the proposed golf course fill between the levees. 4. Ultimate Development - With Golf Course. The Ultimate Cypress Lakes Levee with golf course fill between the levees. For the purpose of this study, the modeling of the development scenarios focuses on the geometry of the Cypress Lakes Levee, and how it effects the hydraulics of the flow pathway(s) downstream of the breach sites, and the total available volume of storage as it relates to the timeto-fill. The 12 scenarios are shown as a matrix in Table 1. TABLE 1 BREACH LOCATION AND DEVELOPMENT SCENARIO MATRIX Breach Location Development Scenario Breach 1 - North Breach 2 - Central Breach 3 - South Existing Conditions B1EX B2EX B3EX Phase 1 B1P1 B2P1 B3P1 Ultimate Development: No Golf Course Ultimate Development: With Golf Course B1NG B2NG B3NG B1WG B2WG B3WG 3.3 LEVEE FAILURE The study assumes the same levee failure conditions for all 12 scenarios. Typical conditions representative of the entire levee were used in the BREACH model, and the results of the BREACH model were used in all subsequent HEC-RAS model runs.

14 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page -7-

15 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page -8-4 COMPUTER MODELS The study required two computer models, BREACH and HEC-RAS The BREACH model is deterministic with regard to the levee failure. It uses the laws of physics applied to the processes of erosion and sediment transport mechanics to compute the dynamics of the levee failure, that is, how the breach geometry develops and changes over time. The model calculates flow through the breach based on a weir flow equation. It is limited to calculating flow in one direction (through the breach from the reservoir ), and does not have the ability to calculate complex hydraulics of flow in the channel downstream of the levee failure. HEC-RAS is a one-dimensional hydraulic model with recently added capabilities to calculate unsteady flow and mixed flow conditions. However, the solution procedure can become unstable and fail to converge on a rational solution when flow is near critical depth, the channel is dry, or the rate of change of flow or hydraulic properties of the channel is too rapid. HEC-RAS also calculates flow through the breach as weir flow and channel flow when the weir is submerged. It can calculate flow in both directions, depending upon the hydraulic conditions, and reports flow from the downstream side of the levee back through the breach as negative values. The levee failure routine in HEC-RAS is parametric. It does not calculate breach geometry or breach dynamics in the same manner as the BREACH model. HEC-RAS applies simple algebraic algorithms to data, supplied by the modeler, that defines the breach geometry and the breach progression. So despite more sophisticated hydraulic computation capabilities, HEC- RAS requires the modeler to specify the initial and final breach geometry, the timeline and pathway of the breach dynamics. Therefore, the results of the BREACH model analysis were used to specify the properties of the levee failure, and HEC-RAS was used to model the hydraulic conditions downstream of the failure. 4.1 THE BREACH MODEL Input Data Description Input to the BREACH model includes data describing: The levee (or dam) geometry, engineering properties of the levee soils and the grass cover, if it exists; The reservoir surface area/volume relationship, and the magnitude and timing of inflow to the reservoir; Whether the failure begins as an overtopping or piping failure, and the elevation at which the failure begins; Data to define a single tailwater cross-section and downstream channel slope; Some information on initial conditions and constraints on the breach. BREACH model input data is summarized in Table 2. The complete BREACH model input file,

16 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page -9- output file, and variable definitions can be found in Appendix B. TABLE 2 BREACH MODEL INPUT SUMMARY Variable Description Value HI Initial Tide elevation (ft) HU Elevation of top of levee HL Elevation of the bottom of the levee on the inboard side 96.0 HPI Elevation at which piping failure begins ZU Slope of the outboard face of the levee 2.25 ZD Slope of the inboard face of the levee 2.50 GL Average length of grass on the levee (inches) 2.0 GS Condition of the grass on the levee (poor) 0.0 VMP Maximum permissible velocity for grass lined channel before grass is eroded away (fps) D50S D50 (mm) of outer material on the levee.20 PORS Porosity ratio of the outer material.38 UWS Unit weight (lbs/cu ft) of outer material AFRS Internal friction angle (degrees) of outer material 31.0 COHS Cohesive strength of outer material (lb/sq ft) 6.6 UNFCC Ratio of D90 to D30 grain size of outer material Levee Geometry and Soils The typical levee cross-section in Figure 3 was provided by Barbara Burns, KSN, Inc. Soils data on grain size is taken from boring logs and analysis by Klienfelder (1999), and other soils properties are from data provided by Foott (1994). Soil cohesion was adjusted upward in response to comments in a memo from Kevin Coulton (HDR, 3/17/2003) to account for reported pore pressure cohesion in otherwise cohesionless soils.

17 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page -10- FIGURE 3 TYPICAL LEVEE CROSS-SECTION

18 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page Reservoir Variables The BREACH model uses reservoir stage as the upstream hydraulic grade line for calculating flow through the levee breach. So for this analysis, the 100-year tide cycle would have to be represented in the BREACH model as a varying water level in the reservoir. However, reservoir stage is not a direct input variable. It is calculated in the model from input variables that define the reservoir stage/volume relationship, a time series of reservoir inflows, and computed outflow through the breach. Since only 8 pairs of values can be specified for the time and magnitude of reservoir inflow, and 8 pairs of values define the reservoir stage-volume relationship, the modeler does not have sufficient control of the reservoir stage to completely model the tidal regime for the 10-day study period. However, after a number of iterations, we were able to create a time series of reservoir water surface elevations that envelope the daily maximum tide elevations for the ten day study period. The resulting reservoir stage is generally higher than the tide elevations over the study period. This should somewhat compensate for the inability of the BREACH model to account for the erosive forces of the ebb tide on breach formation and expansion. We believe this is the best that can be done given the limitations of the BREACH model. The reservoir stage is plotted with the maximum daily tide elevations in Figure 5.

19 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page Calculation of Breach Geometry and Dynamics The BREACH model calculates failures that initiate in piping mode or overtopping mode. Piping mode was chosen for this study. The BREACH model calculates orifice flow and erosion of the failure pipe until the pipe collapses, and then converts to weir flow calculations. The expansion and collapse of the pipe, and the expansion of the resulting trapezoidal breach are based on calculations of erosion rates, sediment transport capacity of the flow, and the strength of the soil and grass cover given the engineering properties of the soils specified in the input data. The elevation at which the failure pipe initiates was originally set to ft (model datum). This elevation was chosen as representative of elevations related to rodent damage, and piping failures on similar levees in the region. In response to comments from reviewers, we changed this elevation to determine the effect this variable has on the failure geometry and dynamics. The results demonstrate that if the initial pipe elevation is too high, above the phreatic zone, the pipe does not collapse during the simulation. If the pipe initiates too low, the collapse occurs within the first time step, provides no information on the flow transition from piping to weir flow, and does not result in a significantly different final weir geometry Breach Model Results The BREACH Model provides detailed output at each time step on flow conditions, reservoir state, erosion of the levee face, breach geometry, and weir hydraulics, etc. A summary of the BREACH model results is presented in Table 3, and output variable definitions and the complete output table are presented in Appendix B. TABLE 3 BREACH MODEL OUTPUT DATA SUMMARY Variable Description Value QBP MAX OUTFLOW THRU BREACH (CFS) TP TIME AT WHICH PEAK OUTFLOW OCCURS (HR) TRS DURATION OF RISING LIMB OF HYDROGRAPH (HR) TB TIME AT WHICH SIGNIFICANT RISE IN OUTFLOW STARTS (HR) 0.0 BRD FINAL DEPTH OF BREACH (FT) 13.0 BRW TOP WIDTH OF BREACH AT PEAK BREACH FLOW (FT) 188 HU ELEVATION OF TOP OF DAM (FT) HC FINAL ELEVATION OF BOTTOM OF BREACH (FT) 96.0 Z SIDE SLOPE OF BREACH AT PEAK BREACH FLOW (FT/FT) 1.26 BO BOTTOM WIDTH OF BREACH AT PEAK BREACH FLOW (FT) 155

20 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page HEC-RAS Input Data Description Input to the HEC-RAS model includes data describing the stream system layout, the channel cross-sections, bridges, culverts, levees and other hydraulic structures, and the flow conditions River Reaches HEC-RAS is organized by River and Stream Reaches. For all runs, the river is identified as Cypress Lakes. For the three Existing Conditions runs, there is only one stream reach ( Cypress Lakes ) perpendicular to the levee. For the 9 runs of the 3 with-project conditions, there are two stream reaches, Breach X North and Breach X South, where X represents the number assigned to the breach location (See Table 1). A typical stream system layout for the with-project conditions is shown in Figure 6. FIGURE 6 TYPICAL HEC-RAS STREAM SYSTEM LAYOUT

21 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page Cross-sections Channel cross-sections are identified by River Station, numbered from downstream to upstream. The cross-section name typically corresponds to the River Station of its location (the cumulative distance upstream from the beginning of the stream reach, in hundreds of feet or in miles), but not necessarily. The program assigns cross-section order based on the numeric order of the cross-section names, so no matter how they are identified, the names must increase in numeric value from downstream to upstream. Channel cross-sections are described by channel station/elevation pairs, from left to right, looking downstream. Channel station is the horizontal distance along the line of the crosssection. Channel stations can be negative, but must increase from left to right. We have defined the channel stations so that the channel center is at 0.0, and channel stations become more negative to the left and positive to the right. A typical cross-section is presented in Figure 7. FIGURE 7 TYPICAL HEC-RAS CROSS-SECTION

22 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page -15- HEC-RAS is a one-dimensional flow model, while flow through the levee breach expanding into the interior zone is clearly a two-dimensional flow condition. We made an adjustment in our modeling approach to try to account for this. For the Existing Conditions model, elliptical cross-sections were drawn out from the assumed breach centerline to the interior of the site. For the developed condition runs, the section immediately downstream of the breach were also elliptical in shape. The elliptical shape was chosen so that the expanding flow lines will be more nearly perpendicular to the cross-sections. Cross-section locations as shown on the oversize exhibits attached to this report. The detailed information for each cross-section is available in the accompanying HEC-RAS files on CD in APPENDIX E Inline and Lateral Structures Flow through a levee in HEC-RAS can be modeled as a breach in an inline structure or in a lateral structure. Inline structures are parallel to the bounding cross-sections, and lateral structures run between cross-sections, connecting them at the corresponding bank stations or overbank areas. Both are described by weir/embankment data that defines the levee profile, weir crest width, weir coefficient, upstream and downstream embankment sideslopes, weir shape, etc. Outflow from a breached lateral structure becomes lateral inflow between two specified crosssections of an adjacent stream reach. Outflow from a breached inline structure continues downstream to the next cross-section of the same reach. While the Sandmound Slough levee is a lateral structure relative to the project site, we found that it was more appropriate, for the purposes of this analysis, to model it as an inline structure. HEC-RAS requires 2 cross-sections upstream of the inline structure. These cross-sections were constructed to minimize unintended effects on the breach hydraulics, and are not intended to be accurate representations of Sandmound Slough. The cross-section immediately downstream of the inline structure is the inboard face of the levee. For the 9 With Project models, the levee breach is on the Breach X North stream reach. For the 9 with project scenarios, the connection between the North and South stream reaches is defined by a lateral structure. A weir section is specified in the lateral structure. Flow across the weir is controlled by a rating curve, and is independent of the upstream and downstream water surface elevations, weir hydraulics, etc. The rating curve assures that at any flow, one half of the flow upstream of the weir continues downstream in the North reach, and one half of the flow is over the weir into the South reach. The rating curve is presented in Figure 8. Outflow across the lateral weir from the North reach is specified to occur between the left bank stations of the bounding cross-sections, and inflow to the South reach is between the right bank stations of the bounding cross-sections. The upstream section of the South reach was duplicated to create the upstream inflow section. Without this additional cross-section, the upstream cross-section on the South reach only saw the specified upstream boundary condition inflow. With the added crosssection, the hydraulics of the upstream section of the South reach shown on the maps and graphics, and included in the output tables more accurately represents the flow conditions at the

23 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page -16- upstream boundary of the reach. A typical lateral structure is shown in Figure 9.

24 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page -17- FIGURE 8 LATERAL DIVERSION RATING CURVE TY PI CA L HE C- RA S LA TE RA L ST RU CT UR E FIGURE 9

25 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page Flow Data and Boundary Conditions The data entry interval can be specified from 1 minute to 1 year. We chose an interval of 0.25 hours (15 minutes) in an attempt to obtain sufficient data to describe the hydraulic characteristics of the levee failure as it transitions from piping failure to weir flow, and to model the initial breach flood wave. The 240 hour duration at 4 time steps per hour yields 960 time steps. Both upstream and downstream boundary conditions must be specified for each data input interval. Boundary conditions are typically specified as a flow hydrograph, stage hydrograph, stage/flow hydrograph, lateral inflow hydrograph, rating curve, or normal depth. An initial flow is also required at the upstream boundary for each stream reach. The upstream boundary condition of the North reach for all model runs is a stage hydrograph of the tidal regime for the 10-day, 100-year event. Upstream boundary condition on the South reach for all runs is a flow hydrograph of a constant 2 cfs. The downstream boundary condition is a stage hydrograph of the water surface elevation on the study site. It represents the state-of-fill of the site. That is, as flooding from the levee breach progresses, the rising water surface elevation in the study area should appear in the model as an increase in the elevation of the downstream boundary stage hydrograph. At some time - the time-to-fill - the interior water surface and the tide elevation in Sandmound Slough are equalized, and flow into the site ceases. As the tide elevation drops below the interior flood stage, flow would occur across the levee breach from the study site to Sandmound Slough and the interior flood stage would drop. Again, this drop in interior water surface elevation would be represented in the model as a drop in the downstream boundary stage hydrograph. Flow would shift from a positive value, flow from Sandmound Slough into the site, to negative, flow from the site back to Sandmound Slough, several times during the model simulation. Ideally, the stage at each time step would be computed by HEC-RAS from stage/volume data describing the site as a reservoir, a reservoir routing routine, and flow data computed during the model run. Unfortunately, HEC-RAS does not have this capability, and the downstream boundary condition stage hydrograph must be specified in the input data. Civil Solutions devised a spreadsheet program to estimate the reservoir routing computations outside of the HEC-RAS model. The initial stage hydrograph was computed from the flow data obtained from the BREACH model. The spreadsheet program computes the stage at each time step and compares it the with the tide elevation at that time step, until the computed interior stage exceeds the tide elevation. At this point the BREACH model flow data is no longer relevant, as the model can only calculate flow one way. The spreadsheet converts to a simple routine that reduces the interior stage at each time step by an amount that approximates the rate of change associated with the approximate average flow rate. The spreadsheet compares this adjusted interior stage to the tide stage, and when the next rising tide again exceeds the interior stage, the spreadsheet increases the interior stage by the same amount, etc. This produced a useable downstream stage hydrograph for the initial HEC-RAS run. We revised the spreadsheet program to do iterative runs with HEC-RAS, making use of the flow data reported by HEC-RAS at each time step, and the ability of HEC-RAS to report outflows as

26 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page -19- negative values. As before, the spreadsheet program compares the computed interior stage with the tide elevation. If the interior stage equals or exceeds the tide stage, and the next timestep flow is negative (indicating outflow), a new stage is computed and used. However, if the next timestep flow is still positive, the new stage is computed using ½ the reported inflow as outflow. In the same way, if the computed interior stage is less than the tide stage, and the reported next timestep flow is negative, ½ the reported outflow is used as inflow to compute the next timestep stage. While this is an arbitrary adjustment for the unpredictable change in flow direction, it does provide a reasonable level of convergence in 3 to 5 iterations. The computation rule is summarized in Table 4. The modeling results indicate that the unsteady flow routine is extremely sensitive to the downstream stage at the initial time steps. For some runs, the difference in a stable and an unstable model run was as little as 0.01 ft for the first value in the time series. This appears to be associated with problems the HEC-RAS solution procedure has with flows near critical depth. Therefore, the initial value in the downstream boundary stage hydrograph was chosen to achieve a stable model run, and the hydrograph was adjusted to smooth the transition using an internal interpretive routine in HEC-RAS. We attempted to maintain a similar initial boundary condition value for all runs to adhere to the comparative nature of the study. One additional adjustment was made to achieve stable model runs. A pilot channel was included in the model cross-sections to smooth out the apparent channel bed slope. Adverse channel slope, rapid changes in channel slope, and extremely steep channel slopes all tested the HEC- RAS model s computational limits HEC-RAS Geospatial Referencing HEC-RAS stream system geometry can be referenced to a coordinate system using geographic information system (GIS) technology. By overlaying the stream lines and cross-sections on a known coordinate grid, the model results can also be referenced to the same grid. The model output includes velocity and flow distribution information that Civil Solutions was able to assign to points on the model grid, creating a two dimensional picture of conditions on the site at any point in the simulation time. By stringing these images together, Civil Solutions created a video presentation of the velocity field on the site as it progressed through the 10-day simulation period. This presentation can be viewed at the website. Tom - Reference website. 5 MODELING THE LEVEE BREACH IN HEC-RAS HEC-RAS can model a breach in either a lateral structure or an inline structure. The breach can be specified to initiate as a piping failure or an overtopping failure. As indicated earlier, HEC- RAS relies on final breach geometry and a breach progression relationship provided by the modeler. The breach input data includes final bottom width, final bottom elevation, final side slopes for the left and right side of the breach and the time for full breach formation. The Breach Progression is defined by a table of Time Fraction vs Breach Fraction values.

27 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page -20- As noted above, the BREACH model and HEC-RAS approach the levee breach analysis in very different way. This led to significant differences in the flow rates across the breach that each program calculated during the crucial initial stages of the levee failure. Specifically: The BREACH model calculates the initial piping failure will expand and collapse by the end of the first hour. When the same piping failure initial conditions were used in HEC- RAS, it took several hours for the pipe to collapse. It is not clear exactly what criteria HEC-RAS uses to determine when the pipe collapses, but it appears to occur as soon as the water surface elevation touches the pipe soffit. Therefore in HEC-RAS the collapse is dependent upon the tide elevation, not the strength of the levee soils. The BREACH model calculates the levee failure will rapidly incise to its final bottom elevation, while the bottom width and top width remain relatively constant. The top width then increases rapidly while the bottom width increases slowly, and finally the bottom width increases until the side slopes achieve their final angle. So in the BREACH model, the shape of the breach that effects how it behaves as a weir varies throughout the simulation. The change in levee failure geometry and flow over time calculated by the BREACH model are graphed in Figures 10a and 10b. In HEC-RAS, the breach fraction appears to be applied uniformly to the breach geometry so that the breach reaches its final bottom elevation, final bottom width and final top width (based on the fixed side slopes provided) at the same time. This is clearly quite different from the breach progression calculated by the BREACH model, and results in much smaller initial flows through the failed levee than the BREACH model calculates. We varied the breach characteristics used in HEC-RAS to achieve initial flow rates that were consistent with those calculated by the BREACH model. These adjustments include: Changing the failure mechanism from piping mode to overtopping mode, Reducing the initial breach elevation from 4.0 ft to 1.0 ft by specifying a notch in the levee that extends to this elevation (See Figure 12), and Calculate the breach progression fraction based on the change in breach top width. The breach geometry used in HEC-RAS is summarized in Table 5, the breach progression relationship is in Figure 11, and a typical inline structure is shown in Figure 12.

28 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page -21- TABLE 5 SUMMARY OF BREACH DATA USED IN HEC-RAS Variable Value Center Station 0 Final Bottom Width 152 ft Final Bottom Elevation -4.0 ft Left Side Slope 1.25 h:v Right Side Slope 1.25 h:v Full Formation Time (hours) 240 Failure Mode Overtopping Trigger Failure at Water Surface Elevation Starting Water Surface Elevation 6.5 Ft

29 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page -22- FIGURES 10-A, 10-B LEVEE FAILURE GEOMETRY AND FLOW PROGRESSION FROM BREACH MODEL

30 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page -23- FIGURE 11 BREACH PROGRESSION FOR HEC-RAS TYPICAL HEC- RAS INLINE STRUCTURE AND BREACH FIGURE 12

31 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page RESULTS 6.1 TIME-TO-FILL The time-to-fill is the time required for the water surface elevation within the Hotchkiss Tract to equalize with the tide elevation in Sandmound Slough. Again, this is not an absolute measure of the flooding conditions or risks, and is very dependent upon the timing of the breach and the tides, etc. Table 6 presents the time-to-fill, water surface elevation at the time-to-fill and the volume of water on the Hotchkiss Tract at the time-to-fill. Figures 13, 14 and 15 show the rate of filling with the tide elevation curve for each breach location. The time-to-fill varies no more than 2¾ hours, or less than 5 percent. The differing water surface elevations is a reflection of the influence the tidal cycle has on this indicator. The water surface is higher when the time-to-fill is shorter because the tide is falling during this time interval. TABLE 6 Time-to-Fill Comparison Breach 1 Breach 2 Breach 3 Time Stage Volume Time Stage Volume Time Stage Volume Scenario (Hours) (ft) (ac-ft) (Hours) (ft) (ac-ft) (Hours) (ft) (ac-ft) Existing Conditions Phase I Ultimate: No Golf Ultimate: W/ Golf MAXIMUM FLOW VELOCITY Average Channel Velocity Figures 16, 17 and 18 show the average channel velocity at the levee breach face for the 4 levels of development at the 3 breach locations. These figure indicate that breach location has far more influence on the maximum flow velocity - at the breach - than does the differences in levee development. The figures also show that the time of the initial breach may not be the time of highest velocity. The high flow velocities toward the end of the simulation are a consequence of the greater difference in high tide and low tide elevations, combined with the large breach opening (see figure A-1), resulting in greater differences in hydraulic head across the breach, and therefore flow rates and velocities.

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35 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page Velocity Distribution As noted above in the HEC-RAS model description, it was necessary to adjust the model geometry to achieve successful model runs, as recommended by the HEC-RAS User s Manual. While these adjustments would not likely influence the average channel velocity data, they do appear in the velocity distribution results. Figures 21 through 24 show typical profiles and crosssections for the 3 developed condition scenarios. These figures compare the elevations on the ground with those used in the HEC-RAS model. The pilot channel shown on the model crosssections increases the apparent depth and width of the channel along the proposed levee toe, and significantly reduces the abruptness and steepness of local changes in the channel bed profile. During low flows, when the hydraulics properties of the channel near the bed are dominant, these two conditions will have opposite effects. That is, the smoother profile will tend to reduce the flow velocity - locally - and the greater capacity of the pilot channel will tend to increase the local flow velocity, as well as the time over which the hydraulic of the small channel dominates. When the flow exceeds the capacity of the pilot channel, the capacity of the channel crossincreases dramatically, and flow velocity is reduced accordingly. As this analysis is intended to be comparative and not absolute, the velocity distribution data does provide some insight into potential issues resulting from construction of the Cypress Lakes levee. The Velocity Distribution figures in Appendix C are composite snapshots of instantaneous maximum velocity by spacial location for each of the 12 modeling scenarios. That is, for each model run, the highest velocity for each point on the spacial grid during the simulation was extracted and included in the plot. These plots are absolute worst case conditions (for the modeled scenario), since the maximum velocities shown at various grid points may have occurred hours or days apart. There are 3 plots for each of the 12 scenarios, the maximum velocity for the first 12 hour period (at the time of the breach), the maximum ebb tide velocity - that is maximum velocity of flow leaving the site, and maximum flood tide velocity - maximum velocity of flow entering the site during the entire 10-day simulation period. These velocity distribution plots indicate that the area between sections J and G in the southern reach, and between sections S and U to the north, consistently experience the highest flow velocities outside of the immediate breach location. Between Sections J and G the construction of the proposed levee would leave a small channel along the outboard toe. Between Sections J and G, the channel is narrower, and the golf course development reduces flow area even further, leading to higher velocities in this reach.

36 Cypress Lakes - Levee Failure and Hydraulic Analysis - Sandmound Slough Page RECOMMENDATIONS Based on the analysis conducted for this study, Civil Solutions presents the following recommendations: 7.1 REPAIR THE LEVEE BREACH AS QUICKLY AS POSSIBLE This study assumed the breach would occur during a period when the daily low tides remained abnormally high. The modeling indicates that the velocities are generally highest on the ebb tide, when the tide elevations in Sandmound Slough are low and the water surface in Sandmound Slough is high. This occurred toward the end of the modeling period in this study. High velocities in the study occurred in the first two days as the breach formed, but the highest velocities occurred near the end of the 10-day model when the breach was at it s maximum size, and tidal fluctuations were returning to normal values. Therefore, the longer the breach remains open, the greater the potential for the breach to increase in size, and for erosive velocities to occur in the study area. 7.2 INTERIOR LEVEE DESIGN The design and construction of the interior levee should address the potential for erosion of the outboard toe. The creation of a channel along the toe due to the existing site topography interface with the levee fill, creation of a borrow pit for levee fill, or to promote drainage of the region between the levees should be avoided. If a channel is created by or results from levee construction, the levee toe along the channel should be armored. 7.3 GOLF COURSE CONSTRUCTION The construction of portions of the golf course between the levees reduces the cross-sectional area, and increases local flow velocity for the areas between the two levees near the proposed golf course. This may result in a sufficient change in flow regime such that structures and land which may not have been damaged (as a result of flow velocities)in the Post-project without golf scenario, may realize velocities of damaging potential in the post-project with golf scenario (i.e. velocities may exceed the 3 to 6 fps threshold where erosion and structural damage may occur).

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