Appendix B HYDROLOGIC AND HYDRAULIC MODELING

Transcription

1 Appendix B HYDROLOGIC AND HYDRAULIC MODELING

2 Memo To: George Hudson, PE and Jeff Robinson, PE, GEC, Inc. From: Stephen Sanborn, PE, Dynamic Solutions, LLC Date: 4/25/ /2014 CC: Chris Wallen, Vice President, Dynamic Solutions, LLC. Re: Hydrologic and Hydraulic Modeling of Ravine Aux Coquilles and Little Bayou Castine, Mandeville, LA OBJECTIVE AND APPROACH The objective of this effort was to determine if adverse flooding impacts will occur from the construction and operation of flood swing gates with flap-gate openings att the termini of Little Bayou Castine (LBC) and Ravine Aux Coquilles (RAC) at Lake Pontchartrain along the Old Mandeville coastline. The approach was to create representative one-dimensional These models weree then utilized to simulate the following scenarios: hydraulic models of LBC and RAC, using the USACE HEC-RAS (USACE, 2010) software. 1. Actual events using actual data for lake stages and rainfall events for Tropical Storm (TS) Bill, TS Lee, and Hurricane Gustav events. Each of these storm events were analyzed for existing conditions and with the flood control gates in place assuming no sea-wall overtopping. 2. A series of hypothetical cases to evaluate how the timing of the rainfall relative to the lake stage time series impacts interior drainage. The combination of the TS Leee Lake Pontchartrain stage time series and runoff from a 10-yr rain event was analyzed for cases where the peak storm surge occurred between 24 hours before to 24 hours after the peak runoff. These events were run for a) existing conditions, b) with gates, and c) with gates and pumps. These scenarios were analyzed to understand the effects of the proposed gate structure under a variety of runoff and storm surge conditions. It also provides planning-level information concerning pumping rates. 1 P age

3 SITE DESCRIPTION The city of Mandeville, LA lies on the north shore of Lake Pontchartrain (Figure 1). The areas adjacent to the lake are low-lying with elevations typically below 4.0 feet and are prone to flooding. Tropical storms often induce high stages in the lake which can overtop the existing seawall and flow into Ravine Aux Coquilles and Little Bayou Castine. Each is channelized in their lower reaches and passes under bridges near their termini at Lake Pontchartrain. The basins consist of largely residential areas with some vegetated low-lying areas. Notable rainfall events are typically the result of either the strong frontal storms that occur over the winter and spring or tropical systems which may deposit over 20 inches of rain in a single event. In order to protect Mandeville s low-lying areas from flooding the threat from high stages in Lake Pontchartrain, rainfall events, and the combination thereof must be evaluated. Figure 1 - Project area location in Louisiana and the watersheds for Ravine Aux Coquilles and Little Bayou Castine. 2 P age

4 HYDROLOGIC ANALYSES Hydrologic analyses were necessary to generate runoff hydrographs for LBC and RAC resulting from the tropical storm rainfall events and the 10-yr storm event. Basin Data A digital elevation model (DEM) from the LSU Atlas LiDAR dataset provided topographic data for the region. The watersheds for LBC and RAC were delineated from the DEM using ArcHydro (ArcHydro, 2013) Tools package for ArcGIS. The watersheds are shown on Figure 1. Rainfall Data For the tropical storm events hourly rainfall data were acquired from the NOAA National Climatic Data Center (NCDC). There are stations on the north side of Lake Pontchartrain in Hammond (30 miles WNW of Mandeville) and Slidell (20 miles ESE of Mandeville). For both TS Bill and TS Lee rainfall amounts in Hammond were greater than those in Slidell and were used to provide a conservative (larger) estimate of rainfall in Mandeville. There were no data available at Hammond or Slidell for TS Gustav so rainfall data from New Orleans Airport (35 miles SSW of Mandeville) were utilized. The hyetographs for each event are shown on Figures 2 4 (below, shown with the resultant hydrographs). The 10-year event was for a storm of duration of 24 hours with rainfall intensity maximum at 12 hours. The rainfall data was derived from the NOAA Atlas 14 Precipitation-Frequency (Perica et al., 2013) dataset. HEC-HMS modeling The USACE HEC-HMS (Scharffenberg, 2013) was used to generate runoff hydrographs for each basin. The Clark unit hydrograph transformation was utilized with the Time of Concentration (TC) and Storage Coefficient (SC) parameters computed using the Espy-Huston formula. This formula also takes into account development so the percent impervious for the HEC-HMS model was set to zero. The loss set up with an initial value and a constant loss. A constant baseflow was used for both basins. A summary of the HEC-HMS parameters for each basin is listed in Table 1. The storm events were run in HEC-HMS with 15-minute time steps. The resulting hydrographs, with the input hyetographs, are shown in Figures 2 through 5. Table 1 - Watershed modeling parameters Parameter Ravine Aux Coquilles Little Bayou Castine Watershed Area (sq. mi.) Baseflow (cfs) 1 1 Initial Loss (in) Constant Loss (in/hr) Trasformation Clark Unit Hyd. Clark Unit Hyd. TC (hr) SC (hr) P age

5 Figure 2- Hyetograph and resulting hydrographs for the 10-year, 24-hour rainfall event Figure 3 - Hyetograph and resulting hydrographs for Tropical Storm Bill 4 P age

6 Figure 4- Hyetograph and resulting hydrographs for Tropical Storm Lee Figure 5- Hyetograph and resulting hydrographs for Tropical Storm Gustav 5 P age

7 HYDRAULIC ANALYSES The objective of the hydraulic analyses was to simulate existing and proposed (with flood gate) water surface elevation and flow conditions in LBC and RAC. This requires understanding the interaction of processes driving flow and water level in these channels. Potential energy, which is manifested as stage (or elevation), is the driving force in the lake-channel systems evaluated herein. If lake stages are higher than the water level in the basin due to tidal, tropical, or frontal events, water will flow into the basin until the basin fills to reach that water surface, or until the lake stage drops sufficiently. Water will flow out of the basin when interior water levels exceed those in the lake and the difference in water surface elevation between the basin and the lake is proportional to the amount of water flowing (discharge) out of the system. The lake stage serves as a baseline water surface elevation for the system meaning that for the same outflow discharge, a higher lake stage will result in a higher interior stage. Computational hydraulic analyses were performed using the USACE s one-dimensional hydraulic model HEC-RAS (Brunner, 2013). This model is capable of simulating stage and velocity in waterways and allows the user to simulate flow constrictions such as bridges and hydraulic structures, in this case the flood control gate structure. It is the standard analyses package for simple hydraulic analyses of open channel flows. For this analysis the lower reaches of each channel were modeled explicitly while the rest of the basin was simulated as a storage area. A storage area is described by a curve relating the volume to stage, which was simulated with the watershed model, and can be used to store a volume of water that may enter back into the system. This is sufficient to model events where water flows from the lake to the basin, filling it, and then draining as the lake stage subsides. Elevation Data Elevation data for the model came from a combination of channel surveys and the LiDAR data described above. GEC staff performed detailed surveys of the lower reaches of LBC and RAC. The survey data was output in coordinates of Louisiana State Plane South, vertical datum of NAVD88. The surveys included channel cross-sections, high and low chord of the bridges, bridge decks, and floodplain elevations. The RAC survey extended from the floodwall 280 feet upstream, past the Lakeshore Drive bridge. The LBC survey extended from 60 feet into Lake Pontchartrain to 170 feet upstream of the floodwall, including the series of bridges at the bottom end of the reach. Boundary Conditions The upstream boundary condition for each system is the hydrograph generated from the hydrologic analysis (see above), which is simulated as a lateral inflow into the storage area. This is then routed through the storage area and enters the upstream end of the channel reach. The downstream boundary condition is the observed stage in Lake Pontchartrain. Stage values from the Lake Pontchartrain at Mandeville USACE gage (USACE gage number 85575) were used for TS Lee and Hurricane Gustav. The stage time series oscillate so a best fit line was used in the model to aid with stability. This gage did not have data for TS Bill so the stage value from the Rigolets (USACE Gage 85700) 6 P age

8 was used. The lake stage time series for each tropical storm event are shown in the Model Results and Analyses section of this report. The proposed conditions models also require an internal boundary condition time series for gate openings. Since the flap gates are designed to only allow flow from the basin to the lake, they should be open anytime the interior stage exceeds the lake stage by some nominal amount (to overcome sticking, friction, and other losses). A Rule boundary condition was used to specify the gate opening to be 5- feet when interior stage was 0.05 feet greater than the lake stage, and zero otherwise. This allows the gate to automatically open and close depending on hydraulic conditions in the system. HEC-RAS Model Setup Model geometry for HEC-RAS was setup using the HEC-GeoRAS software (Ackerman, 2012). GeoRAS allows the user to utilize the ArcGIS (ESRI, 2011) mapping software to create and derive data for the channel centerline, cross-sections, and storage areas and easily export it to HEC-RAS. Model cross-sections were set in reference to any hydraulic controls in each channel and at a reasonable distance apart to ensure model stability. Station-elevation data at each cross-section was derived from the combination of survey and LiDAR data. Figures 6 and 7 show longitudinal profiles of RAC and LBC, including cross-sections and structures. Storage area stage-volume curves for the RAC and LBC basins were computed from the DEM using the Storage Area tools in HEC-GeoRAS and are shown in Figure 8. Manning s n coefficient, a key component to energy losses in the models, was set to value of in each of the models. RAC_ Flood Control Gate (when utilized) Legend Ground 4 Elevation (ft) Walkway Bridge Lakeshore Drive Bridge Main Channel Distance (ft) Figure 6 - Ravine Aux Coquilles bed elevation profile with cross-section locations and structures. Interpolated cross-sections are noted by an asterisk. 7 P age

9 8 6 Flood Control Gate (when utilized) LBC_0411 Legend Ground 4 2 Elevation (ft) Lakeshoree Drive Bridge and Walkway * * * * * * * * * Main Channel Distance (ft) Figure 7 Little Bayou Castine bed elevation profile with cross-section locations and structures. Interpolated cross-sections are noted by an asterisk. Figure 8 - Elevation-Volume curves for each watershed 8 P age

10 For proposed conditions model geometry at RAC, an inline structure was added 15 feet upstream of the floodwall with a swing gate with three 5 x 5-ft flap gate openings with inverts of -2.5 feet. At LBC, there is not room for a gate structure at the end of the existing channel. The proposed scenario extends the channel, and high ground, 55 feet into the lake with the inline swing gate structure with four 5 x 5-ft flap gates, invert elevation -4.0 feet, inserted at the terminus of the channel. The models were then checked for a variety of stage, flow, and gate conditions to ensure they performed reasonably. Model calibration was not performed as the necessary data does not exist. MODEL RESULTS AND ANALYSES The LBC and RAC models were run for the following events: 1. TS Bill existing and proposed conditions, 2. TS Lee existing and proposed conditions, 3. Hurricane Gustav existing and proposed conditions, 4. TS Lee stage time series and a 10-yr runoff event, where the storm surge peak was: coincident with the runoff peak 2, 4, 6, 12, 24, and 36 hours before the runoff peak, and 2, 4, 6, 12, 24, and 36 hours after the runoff peak These events were run for existing and proposed conditions. Pumping rates were estimated for the worst-case (coincident peaks) scenario. This analysis shows the impacts of gate operations subject to the timing of storm surge and runoff. 1. Tropical Storm Bill TS Bill impacted Mandeville from June 29, 2003 to July 3, The peak storm surge was at Mandeville was 4.2 feet (from the best fit curve) and it produced 9.0 inches of rainfall in Hammond. This led to a peak runoff of 476 cfs in RAC and 649 cfs in LBC. The boundary conditions for the LBC and RAC models are shown in Figure 9. The peak storm surge and runoff are nearly aligned, with a lag between the peaks of -0.5 hours for RAC and -2.0 hours for LBC (storm surge peak relative to runoff peak). 9 P age

11 Figure 9 - Lake stage and watershed runoff boundary conditions for TS Billl In RAC, TS Bill results in a maximumm interior water surface elevation of 4.23 feet under existing conditions and 4.74 feet with the proposed flood control gatee in place (Figure 10). Under proposed conditions the gates restrict conveyance causing a 0.5-foot increase in interior water levels. The maximumm flows were 592 cfs and 560 cfs under existing and proposed conditions, respectively. For LBC, the maximumm water surface elevation was 4.19 feet under existing conditions and 4.26 feet with the proposed flood gate in place (Figure 11). The gates in LBC provide enough conveyance that there is only a nominal increase in maximum interior water level. The maximum flows were 2012 cfs and 718 cfs under existing and proposed conditions, respectively. The very large flow under existing conditions is due to the rapid decline in stage from the peak, setting up a large gradient between the watershed and lake. 10 P age

12 Figure 10- Interior maximum water surface elevation in RAC for existing and proposed (gate) conditions Figure 11 - Interior maximum water surface elevation in LBC for existing and proposed (gate) conditions 11 P age

13 2. Tropical Storm Lee TS Lee impacted Mandeville from September 2 to Septemberr 6, The peak storm surge was at Mandeville was 5.0 feet (from the best fit curve) and it produced 10.3 inches of rainfall in Hammond. This led to a peak runoff of 288 cfs in RAC and 391 cfs in LBC. The boundary conditions for the LBC and RAC models are shown in Figure 12. The peak storm surge occurred 14.5 hours after the peak runoff at RAC and hours after the peak runoff at LBC. Figure 12 - Lake stage and watershed runoff boundary conditions for TS Lee In RAC, TS Lee results in a maximumm interior water surface elevation of 5.00 feet under existing conditions and 5.05 feet with the proposed flood control gatee in place (Figure 13). The maximum flows were 267 cfs and 256 cfs under existing and proposed conditions, respectively. For LBC, the maximumm water surface elevation was 5.00 feet under existing conditions and 5.10 feet with the proposed flood gate in place (Figure 14). The maximum flows were 326 cfs and 315 cfs under existing and proposed conditions, respectively. The gates provide enough conveyance for both RAC and LBC under the lower discharges generated by this event that there is only a nominal increase in maximum interior water level. 12 P age

14 Figure 13 - Interior maximum water surface elevation in RAC for existing and proposed (gate) conditions Figure 14 - Interior maximum water surface elevation in LBC for existing and proposed (gate) conditions 13 P age

15 3. Tropical Storm Gustav TS Gustav impacted Mandeville from September 1 to September 4, The peak storm surge was at Mandeville was 5.71 feet (from the best fit curve) and it produced 6.4 inches of rainfall in New Orleans. The runoff hydrographh had three peaks, two of which had very similar peak discharges. The peak discharge for RAC was 207 cfs and for LBC was 273 cfs. The boundary conditions for the LBC and RAC models are shown in Figure 15. The peak storm surge occurred 3.75 and hours before the two distinct runoff peaks at RAC and 2.5 and hours before those at LBC. Figure 15 - Lake stage and watershed runoff boundary conditions for Tropical Storm Lee In RAC, TS Gustav results in a maximum interior water surfacee elevation of 5.72 feet under existing conditions and 5.82 feet with the proposed flood control gatee in place (Figure 16). The gates provide enough conveyance that only a nominal increasee in water surface elevation, but offer no added protection under thesee specific conditions. The maximum flows were 228 cfs and 217 cfs under existing and proposed conditions, respectively. For LBC, the maximumm water surface elevation was 5.72 feet under existing conditions and 5.41 feet with the proposed flood gate in place (Figure 17). For these conditions, where the storm surge peaks first and the peak runoff discharge is modest, the gates provide additional protection. The gates initially hold back storm surge and the watershed has the volume to capture the initial peak before water levels rise to their peak due to the second runoff spike. The maximum flows were 346 cfs and 309 cfs under existing and proposed conditions, respectively. 14 P age

16 Figure 16 - Interior maximum water surface elevation in RAC for existing and proposed (gate) conditions Figure 17 - Interior maximum water surface elevation in LBC for existing and proposed (gate) conditions 15 P age

17 4. Storm surge and Runoff Timing Scenarios A series of hypothetical cases to evaluate how the timing of the rainfall relative to the lake stage time series impacts interior drainage. The combination of the TS Lee Lake Pontchartrain stage time series and runoff from a 10-yr rain event was analyzed for cases where the peak storm surge occurs: coincident with the runoff peak 2, 4, 6, 12, and 24 hours before the runoff peak, and 2, 4, 6, 12, and 24 hours after the runoff peak Examples of a -12-hour lag, +12-hour lag and coincident peaks are shown in Figure 18. The convention used is storm surge peak relative to runoff peak so negative lags indicate the storm surge peak occurs first. It is assumed that rainfall begins on 1/1/2014 at midnight. Figure 18 Lake stage and watershed runoff boundary conditions for timing lags of -12, 0, and 12 hours For RAC, the maximumm stage occurs when the runoff and storm surge peaks coincide ( Figure 19). This is also when the greatestt difference in stage, a difference of 1.24 feet, between existing and gated conditions occurs. Table 2 details the maximumm elevation and flow for different time lags. There is less differencee in maximumm stage as the absolute lag time increases. For this gate configuration, they are beneficial when the lag between the runoff and storm surge peak is more than approximately 20 hours ahead of the runoff peak, and 25 hours after. 16 P age

18 Table 2 Values of maximum interior water surface elevationn and maximum flow in RAC, existing and proposed (gate) conditions, for timing lags between -36 and hours. Maximum Elevation Lag (hrs) Existing Gated Maximumm Flow Existing Gated Improved Stage YES YES NO NO NO NO NO NO NO NO NO NO YES Figure 19 Maximum interior stages as a function of the timee lag in storm surge to runoff peak for Ravine Aux Coquilles, existing and proposed (gated) conditions. The point at which the gated curve drops below the existing curve indicates the timing lag at which thee gates improve maximum stage. 17 P age

19 For LBC, the maximum stage again occurs when the runoff and storm surge peaks coincide (Figure 20). This is also when the greatest difference in stage, a difference of 1.24 feet, between existing and gated conditions occurs. Table 3 details the maximum elevation and flow for different time lags. There is less difference in maximum stage as the absolute lag time increases. For this gate configuration, they are beneficial when the lag between the runoff and storm surge peak is more than approximately 16 hours ahead of the runoff peak, and approximately 26 hours after. Table 3 - Values of maximum interior water surface elevation and maximum flow in LBC, existing and proposed (gate) conditions, for timing lags between -36 and +36 hours. Maximum Elevation Maximum Flow Lag (hrs) Existing Gated Existing Gated Improved Stage YES YES NO NO NO NO NO NO NO NO NO NO YES 18 P age

20 Figure 20 - Maximum interior stages as a function of the timee lag in storm surge to runoff peak for Little Bayou Castine, existing and proposed (gated) conditions. Thee point at which the gated curve drops below the existing curve indicates the timing lag at which thee gates improve maximum stage. PUMPING REQUIREMENTS The zero lag timing scenario was run with the addition of pumps for RAC and LBC to evaluate a planning- below 3.0 feet. level understanding of the pumping requirements necessary to hold interior water surface elevations For each channel, a pumping station was added just upstream of the flood control structure and discharged to Lake Pontchartrain. It was assumed the pumpss have zero startup and shutdown times and that the capacity is not affected by the head difference (a vertical pump efficiency curve). A series of three pumps was used to avoid constant cycling. The pumping requirements are summarized in Table 4. The first two pumps were made constant and the third determined by increasing the capacity in 25 cfs increments until a maximum interior stage below 3.0 feet was achieved. The maximumm interior stage for RAC was 2.89 feet, and was 2.94 feet for LBC for these conditions. Figures 21 and 22 show the headwater, tailwater, and pumping rate for the pumping scenarios in RAC and LBC. Table 4 Pumping requirements for RAC and LBC given the zero time lag scenario. Pump #1 Pump #2 Pump #3 RAC Capacity (cfs) LBC Capacity (cfs) On/Off Stage (ft) P age

21 Figure 21 Stage and pumping rate informationn for the RAC pumping scenario Figure 22 - Stage and pumping rate information for the LBC pumping scenario 20 P age

22 DISCUSSION OF MODEL RESULTS The critical factors in determining whether the flood control gates provided benefit are the peak runoff discharge and its timing relative to the peak storm surge. The TS Lee and TS Bill events lead to nominal increases in, and in one case a decrease, interior stage occur due to the implementation of the flood control gates. The peak runoff discharge for these storms is moderate, considerably less than TS Bill or the 10-year runoff event. TS Bill results in a significant interior stage increase in RAC because of the higher runoff volume and coincident timing of the runoff and storm surge peaks. The runoff peak in LBC is 36% higher while the increase in conveyance through the gates is 33%. The increase is less in LBC because, as seen in Figure 8, the watershed is able to store considerably more volume than RAC. This is evident examining the results from TS Gustav. RAC has a nominal increase in stage under proposed conditions (Figure 16) while the stage in LBC decreases by 0.31 feet (Figure 17). The gates repel the peak storm surge and the watershed is able to store much of the first runoff peak. The second runoff peak further drives up the interior stage but the lake (downstream) stage has decreased enough to allow gravity flow through the flood control structure. Analysis of the timing scenarios (bullet point 4, above) shows at what time lag of peak runoff and storm surge the gates begin to positively affect interior water levels (Figures 19 and 20). The critical lag time is shorter when the storm surge precedes the runoff peak as the gates can repel the peak and gravity flow can allow runoff to flow out of the system as the downstream stage decreases. The relative increase in stage for the zero lag scenario is greater for RAC because of lower watershed storage capacity. Increases in interior stage due to the addition of flood control gates could be averted if the channel cross-section were modified to support an increase in conveyance through the gate structure. CONCLUSIONS Ravine Aux Coquilles and Little Bayou Castine, draining the city of Mandeville, LA along the north shore of Lake Pontchartrain, were modeled to understand the effects of adding flood control gates at the terminus of each channel. The flood control gates are intended to restrain storm surge events from Lake Pontchartrain while allowing gravity flow of watershed runoff to the lake. The results indicate that the gates, constructed as proposed and studied in this report, caused increased interior water surface elevations in cases where there is greater peak runoff and when the runoff peak is closer in time to the storm surge peak. These increases in stage could be ameliorated by increasing the conveyance of the flap gates. The gates do, however, show benefits when there is a greater lag between the peak storm surge and peak runoff. A planning-level analysis of the pumping rates required to keep interior water surface elevations below 3.0 feet was performed with a hypothetical scenario of the TS Lee storm surge combined with a 10-year, 24-hour rainfall event peaking simultaneously. The models indicated that essentially the peak volume of the hydrograph would need to be pumped, with a requirement of 500 cfs for Ravine Aux Coquilles and 550 cfs for Little Bayou Castine. It is recommended that a stochastic analysis of rainfall and storm surge 21 P age

23 events be performed to construct a design event used for pumping analysis and a more detailed analysis of pump station design be performed to achieve better estimates of pumping requirements. References Ackerman, Cameron (2012) HEC-GeoRAS (Version 10) [Software], US Army Corps of Engineers. Software available at: ArcHydro (Version 2.1) [Software] (2013), ESRI, Software available at: Brunner, Gary (2010) HEC-RAS (Version 4.1) [Software], US Army Corps of Engineers. Software available at: ESRI (2011) ArcGIS Desktop: Release 10. Redlands, CA, Environmental Systems Research Institute. Sanja Perica, Deborah Martin, Sandra Pavlovic, Ishani Roy, Michael St. Laurent, Carl Trypaluk, Dale Unruh, Michael Yekta, Geoffrey Bonnin (2013). NOAA Atlas 14 Volume 8 Version 2, Precipitation- Frequency Atlas of the United States, Midwestern States. NOAA, National Weather Service, Silver Spring, MD. Scharffenberg, William (2013) HEC-HMS (Version 4.0) [Software], US Army Corps of Engineers. Software available at: 22 P age