Appendix A STORM SURGE AND WAVE HEIGHT ANALYSIS

Transcription

1 Appendix A STORM SURGE AND WAVE HEIGHT ANALYSIS

2 Memo To: Jeff Robinson, P.E., GEC, Inc. From: Silong Lu, Ph.D., P.E., D.WRE, Dynamic Solutions, LLC. Date: 1/9/2014 CC: Re: Chris Wallen, Vice President, Dynamic Solutions, LLC. Storm surge elevation and wave height analysis for City of Mandeville Shoreline Protection Study The purpose of this memorandum is to provide a summary for each task including methodologies, data sources, and existing study results that are used, and assumptions that are made for storm surge elevation and wave height analysis for City of Mandeville Shoreline Protection Study. Qualitative assessment of East Land Bridge (ELB) project on storm surge in the vicinity of Mandeville is also included. 1. Storm Surge Elevation Frequency Curve US Army Corps of Engineers (USACE) (USACE 2007 and 2009) has conducted a series of storm surge and wave height studies with a suite of modeling tools including Planetary Boundary Layer model (PBL) for wind and pressure, ADvanced CIRCulation model (ADCIRC) for surge level, the global ocean WAve prediction Model (WAM) for deep water wave, and STeady state spectral WAVE model (STWAVE) for shallow water wave for the coastal area of Louisiana after hurricane Katrina in In order to establish the frequency curves for surge and waves, 304 storms as shown Figure 1.1 were modeled for the base conditions. The base conditions are the no action or without project conditions assuming none of the Louisiana coastal protection and restoration (LACPR) alternatives are implemented. In general, the base conditions assume completion of Federally-authorized navigation, flood risk management, hurricane risk reduction, and environmental restoration projects in the planning area. The base conditions also include non-federal levees at existing design levels (USACE 2009). 1 P age

3 Figure 1.1 Simulated storm paths (USACE, 2009) The method adopted for the frequency analysis is the Joint Probability Method with Optimal Sampling (JPM-OS) that takes into account the joint probability of forward speed, size, minimum pressure, angle of approach, and geographic distribution of the hurricanes. The JPM-OS method has been used to derive the still water elevation, wave height, and wave period frequency curves at specific points using output from ADCIRC and STWAVE (USACE 2007). Figure 1.2 shows the different components and their interaction in the JPM- OS process. 2 P age

4 Figure 1.2 The different components and their interaction in the JPM-OS Process (USACE, 2007) After running all 304 storms, over 3 million data points were analyzed to derive the surge and wave heights across the Louisiana coast. The maximum stage at each of the ADCIRC grid points was used to compute the stage frequency at each of the grid points. For the Lake Pontchartrain Basin (LPB), the storm surge levels at each ADCIRC grid point for the 10-, 50-, 100-, and 500-year return periods were computed using the ADCIRC results of 152-storm simulations and were provided by USACE in ArcGIS shape file format. The storm surge levels at the selected ADCIRC grid points in the seaward vicinity of the shoreline of City of Mandeville shown in Figure 1.3 were used to compute average surge levels for the 10-, 50-, 100-, and 500-year return periods for this study. It is worthy to note that wind wave height was not included in the storm surge levels calculation. However, wave radiation stress from the STWAVE model was input to the ADCIRC model to calculate radiation stress contribution to elevated water levels (wave setup). It should be noted that for future conditions, the statistical water surfaces developed from the ADCIRC modeling of surge levels need to include the added effects of relative sea level rise (RSLR), i.e., eustatic sea level rise plus local subsidence. 3 P age

5 Figure 1.3 Selected ADCIRC grid points along the shoreline of City of Mandeville The storm surge levels for the 2-year return period, which is needed to develop a storm surge elevation frequency curve for the 2-, 10-, 50-, 100-, 500-year return periods, were not reported by USACE (USACE 2009). To estimate the storm surge level for the 2-year return period at Mandeville, the daily water level data collected at USACE station at Mandeville were utilized. The stage station was located on the west side of the harbor at Mandeville as shown in Figure 1.4 and the data collected at this location are free of shortperiod wind wave interference because they are filtered out by the narrow channel connection with the lake. Figure 1.4 Location of USACE Gage at Mandeville 4 P age

6 To obtain storm surge level for the 2-year return period using the observed stage data for the period of 1959 to 2013 as shown in Figure 1.5, peaks over threshold (POT) method of extreme value analysis (Goda, 2000) was applied. Typically, the following three steps were involved using the POT method: Choose a threshold value u; Study the statistical properties of the exceedances Yi over the threshold value u; and Fit those exceedances to a particular distribution function. It is critical to choose a proper threshold value u of the observed stages in order to obtain an accurate and reasonable result of interest, the storm surge level for the 2-year return period in this case. This was achieved through good engineering judgment and several trials of the threshold values and goodness of fitting distribution functions to the exceedances Yi As shown in Figure 1.5, a linear trendline (red line) of the observed stages was developed first using MS Excel trendline function. Several linear lines that are parallel to and deviate 1.0, 1.5, 2.0, and 2.5 ft from the trendline were then inserted and any observed stages higher than each inserted line at any given time were extracted for exceedance analysis. Based on the results of statistical analysis of the exceedances for each inserted line with different fitting distribution functions, that is, Weibull Distribution, Lognormal Distribution, and Gumbel Distribution, the Weibull Distribution as shown in Figure 1.6 was found to be the best fit distribution function for the exceedances that are higher than the inserted line with a deviation of 2.0 ft as shown in Figure 1.5. The fitting equation developed with the least square method for the exceedances is included in Figure 1.6 with the R-squared equal to The data used for the exceedance analysis in Figure 1.6 are presented in Table 1.1. With the fitting equation, the storm surge level for the 2-year return period, that is 4.09 ft NAVD88, was calculated. Because the data used for the exceedance analysis was very limited the fitting equation should be not used to calculate the surge heights for any long return periods. Combining the average storm surge levels for the 10-, 50-, 100-, 500-year return periods along the shoreline of City of Mandeville as discussed earlier, a storm surge level frequency cure was developed and presented in Figure 1.7. Table 1.2 presents the numeric storm surge levels for each return period. Note that these storm surge data include wind stress, barometric pressure, and astronomical tide for all return periods and wave setup for all except the 2-year return period. The logarithmic regression equation of the data shown in Figure 1.7 can be used to reasonably estimate the storm surge level near the shoreline of Mandeville at any given year between 2 and 500 years. Table 1.1 Observed stage used for exceedance analysis 5 P age

7 Date Observed stage (ft NAVD88) Date Observed stage (ft NAVD88) Date Observed stage (ft NAVD88) 5/31/ /9/ /5/ /21/ /22/ /26/ /6/ /23/ /27/ /18/ /24/ /16/ /9/ /25/ /2/ /10/ /26/ /29/ /11/ /8/ /30/ /12/ /15/ /31/ /13/ /31/ /5/ /4/ /27/ /6/ /10/ /28/ /7/ /11/ /29/ /8/ /23/ /30/ /17/ /29/ /31/ /5/ /30/ /1/ /11/ /7/ /2/ /12/ /15/ /23/ /13/ /18/ /28/ /16/ /3/ /18/ /8/ /22/ /24/ /14/ /25/ /2/ /15/ /17/ /3/ /18/ /18/ /9/ /20/ /19/ /10/ /8/ /20/ /10/ /9/ /21/ /16/ /3/ /22/ /17/ /2/ /23/ /26/ /3/ /24/ /30/ /11/ /25/ /22/ /12/ /3/ /3/ /13/ /4/ /26/ /14/ /5/ /27/ /10/ /6/ /30/ /3/ /7/ /21/ /4/ /9/ /29/ /5/ /12/ /30/ /29/ /13/ /31/ /30/ /22/ /20/ /31/ /8/ /21/ /1/ Table 1.2 Storm surge levels for each return period 6 P age

8 ReturnYears SurgeHeight (ft,navd88) P age

9 Figure 1.5 Observed lake stage at USACE station at Mandeville 8 P age

10 Figure 1.6 Execeedance analysis of the observed surge heights with a Weibull distribution fitting (Note the derived fitting equation is only used to calculate the surge height for the 2-year return period) 9 P age

11 Figure 1.7 Storm surge level frequency curve at Mandeville 10 P age

12 2. Wave Height Analysis Existing Flood Insurance Study (FIS) for City of Mandeville, LA (FEMA, 2012) and FIS for St. Tammany Parish, LA, and incorporated area (FEMA, 2012 have been carefully reviewed and it was concluded that no useful wave data can be used to develop wave height frequency cure near the shoreline of City of Mandeville although both studies have discussed coastal wave height analysis and provided general wave height information. Further review of other recent studies for Southeast Louisiana Flood Protection Authority- East (Ben C. Gerwick, 2012) found the color-coded map of significant wave heights, corresponding to the maximum surge levels for the 100- year event simulated with SWAN coupled with ADCIRC, as shown in Figure 2.1. Based on the color scale, the estimated significant wave height using Figure 2.1 for the 100- year event near the shoreline of Mandeville (as indicated with a red arrow in Figure 2.1) is approximately equal to 8.5 ft. The corresponding peak wave period for the 100-year event at the same location is about 6.5 seconds as shown in Figure 2.2. The corresponding water depth for the 100-year event at this location is about 13 ft (Ben C. Gerwick, 2012). Mandevill Figure 2.1 Significant wave heights corresponding to the maximum surge levels for the 100- year event 11 P age

13 Mandevill Figure 2.2 Corresponding peak periods for the 100-year event It should be noted that bottom friction was not considered in the wave height simulation with SWAN (Ben C. Gerwick, 2012); therefore, the wave heights presented in Figure 2.1 may be overestimated. The 2002 Lake Pontchartrain water depth survey by USGS indicates the average water depth near the seawall is approximately equal to 3.0 ft as shown in Figure 2.3. Because the mean water level at Mandeville is about 2.0 ft NAVD88, the average lake bottom elevation near the seawall is about -1.0 ft NAVD88. Combing the storm surge levels in Table 1.2 and the average bottom elevation, the resulting water depths near the seawall at Mandeville for different return periods are presented in Table P age

14 Figure 2.3 Water depth contours near the seawall/shoreline at City of Mandeville by USGS 2002 Table 2.1 Estimated significant wave height for each return period ReturnYears WaterDepthneartheSeawall(ft) EstimatedWaveHeightHwithDepth LimitedWaveBreaking(ft) When a wind generated wave approaches a shoreline, depth-limited wave breaking in shallow water occurs. Therefore, no matter how high the deep water wind generated waves are, the highest wave that can reach the seawall is dependent primarily on the water depth in front of the seawall. The ratio of wave height (H) to water depth (d) for wave breaking in shallow water is commonly set to 0.78 although the ratio may vary from 0.73 to 0.83 (Sorensen, 2006). For the 100-year event, with the water depth of 10.5 ft near the seawall, the wave height should be no more than 8.2 ft, that is, 10.5 ft*0.78, as wave will break when it approaches 13 P age

15 the seawall. However, as discussed above, the model simulated wave height shown in Figure 2.1 for the 100-year event is about 8.5 ft which is slightly larger than the depth-limited wave height of 8.2 ft. This may be partly due to the fact that no bottom friction was considered in the wave height simulation model (Ben C. Gerwick, 2012). The fact that the model simulated wave height is slightly larger than the wave height estimated with the breaking wave ratio near the seawall may indicate the deep water wind generated wave did break when it approached the seawall for the 100-year event. To estimate the wave heights for the other return periods near the seawall, the deep water wind generated waves were assumed to be high enough so that the wave heights at the seawall were limited by the water depth. Thus, the wave height, as shown in Table 2.1, can be estimated based on the water depth for the 2-, 10-, 50- and 500-year events, respectively. The estimated wave heights and their corresponding return years are plotted in Figure 2.4 with the regression equation. The results of this analysis can be treated as the best wave height estimation near the seawall of Mandeville when no other observed and/or simulated wave data at this location are available. Wave runup was not considered in the wave height estimation. 14 P age

16 Figure 2.4 Wave height frequency curve near the seawall at Mandeville 15 P age

17 3. Impact of East Land Bridge Project on Storm Surge in the Vicinity of Mandeville The primary objective of the ELB project (Ben C. Gerwick, 2012) is to use the ELB as a physical flood barrier to suppress storm surge in the Lake Pontchartrain Basin (LPB). Two alignments as shown in Figure 3.1 for installation of surge reduction structure along the mouth of Lake Pontchartrain were proposed: Alignment 1 follows US-90 along the land bridge. The LACPR (Louisiana Coastal Protection and Restoration) considered it due to good soil foundation present along the alignment. Alignment 2 follows the GIWW (Gulf Intracoastal Waterway)/railroad and is essentially the same as that presented in the State Master Plan (CPRA 2007). Figure 3.1 Schematic of storm surge reduction alignment options at the ELB (Note: 400- YRP is 400-year return periods and the vertical datum for the crest elevation is NAVD88 ft). To evaluate impact of the ELB on surge levels near the ELB including Lake Pontchartrain, Lake Borgne and the Gulf of Mexico with the coupled ADCIRC and SWAN modeling system, five scenarios with Alignment #2 were considered (Ben C. Gerwick, 2012): 16 P age

18 Base: FWOA-ELB-intact. FWOA-ELB-intact is a Future Without Action (FWOA) scenario representing a future situation where sea level rise (SLR) (2.3 feet) and subsidence (0.5 foot) have occurred throughout the Louisiana coastal region, but the ELB remains intact and vegetated. Scenario#1: FWOA-ELB-degraded. FWOA-ELB-degraded is an FWOA scenario similar to the base except that the ELB has been allowed to erode and disappear. This scenario is designed with the ELB degraded up to 2.5 feet below NAVD88. Scenario#2: Levee-Gate-Closed-ELB-intact. This scenario includes a proposed levee across the ELB with no openings at the Chef Menteur and Rigolets passes. This configuration will hydraulically isolate Lake Pontchartrain from Lake Borne and the Gulf of Mexico. This scenario still assumes the elevations and vegetation of the ELB are maintained. Scenario#3: Levee-Gate-Open-ELB-intact. This one is similar to the Levee-Gate-Closed- ELB-intact scenario but with free-flowing openings at the Chef Menteur (700 feet wide) and Rigolets passes (1,700 feet wide). The sill elevation at both openings is 30 feet below NAVD88. Scenario#4: Levee-Gate-Open-ELB-degraded. This scenario is designed based upon the Levee-Gate-Open-ELB-intact scenario, which allows the ELB area to be degraded as in the FWOA-ELB-degraded scenario. The results of forty (40) model simulations with different tracking paths including maximum wind speed, forward speed and minimum pressure for each scenario were used to develop statistical storm surge and wave heights within the ELB area. Compared to the Base condition, reduction in surge levels at Lake Pontchartrain was observed in Scenarios #2, #3, and #4, as shown in Figures 3.2, 3.3, and 3.4 for 100-year return period as an example, respectively, in which a proposed levee across the ELB is constructed with Chef Menteur and Rigloets passes either closed or opened (Ben C. Gerwick, 2012, USACE 2009). In Scenario #1, additional water volume entering Lake Pontchartrain due to the degraded ELB does not result in a measurable increase in surge levels in the lake. The same conclusion can be drawn if the ELB is maintained /constructed along the Alignment #1 as shown in Figure P age

19 Figure 3.2 Difference in Maximum Surge Envelope between Scenario #2 and the base scenario. Positive values indicate higher surge in the Gate-Closed scenario. Figure 3.3 Difference in Maximum Surge Envelope between Scenario #3 and the base scenario. Positive values indicate higher surge in the Gate-Closed-ELB-intact scenario. 18 P age

20 Figure 3.4 Difference in Maximum Surge Envelope between Scenario #4 and the base scenario. Positive values indicate higher surge in the Gate-Closed-ELB-subsided scenario. In summary, surge levels in the vicinity of Mandeville in Lake Pontchartrain due to various hurricane events will be reduced when a proposed levee structure across the ELB is constructed or will remain near the same when the ELB is degraded as in Scenario #1. 19 P age

21 Reference CPRA, 2007, Integrated Ecosystem Restoration and Hurricane protection: Louisiana s Comprehensive Master Plan for a Sustainable Coast. FEMA, 2012, Flood Insurance Study, City of Mandeville, LA. FEMA, 2012, Food Insurance Study, St. Tammany Parish, LA, and incorporated areas, Vol. 1 of 1. Goda, Y., 2000, Random Seas and Design of Maritime Structures, World Scientific Publishing Co. Pte. Ltd., Singapore. Ben C. Gerwick, Inc. a COWI company, 2012, New Orleans East Land Bridge Study, LPV 111 to Chef Menteur, Chef Menteur to Rigolets for Southeast Louisiana Flood Protection Authority- East (SLFPA-E) Sorensen, R. M., 2006, Basic Coastal Engineering, Third Edition, Springer. USACE, 2007, New Orleans District, Elevations for Design of Hurricane Protection Levees and Structures, Lake Pontchartrain, Louisiana and Vicinity Hurricane Protection Project, West Bank and Vicinity Hurricane Protection Project. USACE, 2009, New Orleans District, Louisiana Coastal Protection and Restoration, Final Technical Report. 20 P age