GSSHA—Large Watershed Hydrollogy Study Galveston Island, TX Watershed 1. Modelliing Projject Overviiew Purpose This project demonstrates how the overland boundary conditions capability in GSSHA is used to model coastal storm surges after a cyclone event. The purpose of this project was to compare GSSHA results with actual inundation depths reported by the National Oceanic and Atmospheric Administration (NOAA) for the same cyclone event in Galveston Island, TX. This model can be used in future studies to show how overland boundary conditions in GSSHA accurately simulate storm surge events. Model Background The model is located in Galveston Island, Texas as shown in Figure 1. As we are not simulating overland flow, the watershed doesn’t have an outlet location where outflow will be measured; instead, it has boundary conditions. Because the northern section of the island is the most populated, this was the area used for the model, as shown in Figure 2. Figure 1: Watershed Area Figure 2: Delineated Watershed The boundary conditions were added to represent the storm surge caused by Hurricane Ike on Galveston Island in September 2008. Hurricane Ike originated from a tropical wave off the west coast of Africa. Ike made landfall across Great Inagua Island (Bahamas) as a category 4 hurricane and then into the Northeast Coast of Cuba as a category 3 hurricane. Ike crossed Cuba overnight, and emerged into the Caribbean Sea the morning of September 8 th heading to the Gulf of Mexico. By Sept 11, Ike’s tropical storm wind swath was approximately 450 miles wide with a hurricane force wind swath of 180 miles. Ike made landfall on Galveston Island at 2:10 am September 13th as a strong category 2. Ike's large wind swath, along with the fact that it piled water over the shallowest portion of the Gulf, lead to much higher than normal storm surge flooding along the Upper Texas and Louisiana Coasts. (http://www.srh.noaa.gov/lch/ike/ikemain.php ) Ike is directly responsible for 103 deaths across Hispaniola, Cuba, and parts of the United States Gulf Coast. Extensive damage from strong winds, storm surge, and rainfall occurred over Hispaniola, the Turks and Caicos Islands, the southern Bahamas, Cuba, and the U.S. Gulf Coast from Florida to Texas. Additional deaths and significant damage occurred across parts of the Ohio Valley and southeastern Canada after Ike lost tropical characteristics. The Insurance Services Office estimates that the insured damage (not including inland flooding or storm surge) from Ike in Texas, Louisiana, and Arkansas is $9.7 billion dollars. The National Flood Insurance Program estimates that insured losses from inland flooding and storm surge is $489.5 million in the same three states. Using these preliminary figures, total damage is estimated at about $19.3 billion dollars. These estimates suggest that Ike is the fourth costliest hurricane to affect the United States, after Hurricanes Katrina (2005), Andrew (1992), and Wilma (2005). (http://www.nhc.noaa.gov/pdf/TCR-AL092008_Ike.pdf ) To determine the effectiveness of GSSHA in modeling storm surges, we calibrated the model with maximum observed inundation depths on Galveston Island using Hurricane Ike storm surges. The observed depths are shown in Figure 3. This map was generated by the NOAA National Weather Service Forecast Office. Figure 3: Inundation Depths in Galveston County. http://www.srh.noaa.gov/hgx/projects/ike08/inundation.htm Data Acquisition and Model Setup We downloaded several datasets to create a GSSHA model of this watershed. The following data were required to develop the model, most of which were obtained using the web service tools in WMS: 1. Land use data: This data was obtained from webgis.com and this particular dataset was placed on the WMS Web Service Catalog site so it could be downloaded from any copy of WMS 8.2 or later. Figure 4 shows Land Use polygons for Galveston Watershed which were used to create land use and combined index maps for determining properties such as overland Manning’s roughness in the watershed. INDUSTRIAL CROPLAND AND PASTURE STREAMS AND CANALS COMMERCIAL AND NON FORESTED WETLAND SERVICES RESIDENTIAL TRANSPORTATION LAKE Figure 4: Land Use Map for Galveston Island Watershed 2. Soil type and texture data: detailed county soil survey geographic (SSURGO) data was obtained from the NRCS. The soil texture for each polygon was obtained from the soil polygons in the SSURGO database. This texture is normally used to determine GSSHA soil infiltration parameters. Even though we gathered this data, for this model we did not use Soil type Data to create index maps because in this specific scenario of extreme flood simulation in GSSHA, the infiltration is minimal. 3. Elevation data: 10-meter resolution elevation data was available in this area from the USGS seamless elevation dataset. This data was downloaded directly from WMS. These elevations were used to determine the model extent and to create 2D grid cell elevations in the GSSHA hydrologic model. Figure 5 shows the watershed elevations symbolized by different colors, with red representing the elevation closest to sea level. Figure 5: Watershed Elevations 4. Topographic and aerial photography data: Digital raster graphic (DRG) topographic maps were obtained from the Microsoft terraserver web service client in WMS. Digital orthophoto quads (DOQ) aerial photographs were also obtained from this same service. The purpose of these maps was to identify outlet locations and other key features within the watershed (Figure 6). Figure 6: View of delineated watershed over topographic map of Galveston Island 5. Precipitation data: Because we are not simulating the effect of rainfall alone on the watershed and because precipitation is minimal compared to the storm surge caused by Hurricane Ike, a uniform intensity of 10.5 mm/hr was entered for a total of 24 hours of rain. This data was obtained form NOAA National Weather Service Forecast Office, which reported it as a Rainfall Contour Map developed for Harris County, as shown in Figure 7. Galveston Island is located South East of Harris County and these rainfall values were used as a reference for Galveston Island. Figure 7: 48-Hour Rainfall for Sept. 12-14, 2008 measured by the Harris County Flood Control District, TX http://www.srh.noaa.gov/hgx/projects/ike08/HurricaneIkeRainfall.html 6. Storm Surge Data: Historical storm surge data for the state of Texas was retrieved from the NOAA Tides and Currents website: http://tidesandcurrents.noaa.gov/station_retrieve.shtml?type=Historic+Tide+Data . Figure 8 shows the observation stations along the coast of Texas. We gathered data from two tide gauges, namely: Galveston Pier 21 (29° 18.6' N, 94° 47.6' W) and Galveston Pleasure Pier (29° 17.1' N, 94° 47.3' W) located within the island and shown inside red boxes in Figure 8. In the selection of the tide gauges we had to take into account that some of them, mostly the ones located within our watershed or within Hurricane Ike radius, failed due to strong winds. In addition, some of the gauges did not have data available from Sept 12 to Sept 15, 2008. Figure 8: Meteorological Stations along the coast of Texas. http://tidesonline.nos.noaa.gov/geographic.html Figure 9 and Figure 10 show storm surge plots for the two different tide gauges. Note in Figure 9 that peak storm surge values are missing due to gauge failure. These values were estimated using Galveston Pleasure Pier gauge as a reference. Figure 9: Storm Surge Data for Galveston Pier 21 Station Figure 10: Storm Surge Data for Galveston Pleasure Pier Station After obtaining all the data described above, we developed the model using the WMS hydrologic modeling wizard. First, the basin was delineated with an outlet located between Galveston Sewage Disposal Plant and Pier 41 in the west shore of the island as shown in Figure 11. This was just a preliminary delineation to be used to delineate a greater area of the island. With the create arc tool we extended the watershed boundaries to the extent where we needed it, as shown in Figure 12. We did not define stream parameters, redistribute the vertices on the streams or smooth streams in the watershed because the model did not require a stream outlet hydrograph. Figure 11: Preliminary Delineated Watershed We created a 60 by 60 meter 2D grid that conformed to the DEM elevations. After the grid was created, we defined the simulation time and the time step, as well as the Land Use index map shown in Figure 13, entered mapping table parameters, and processed the precipitation data described above. Figure 12: Final Watershed Extent Figure 13: Land Use Index Map 7. Boundary Conditions Setup: We set two boundary conditions in the model, one representing storm surge caused by Hurricane Ike on the gulf side of the island and one boundary condition for the bay side of the island. The boundary condition on the gulf was entered from the cell south of Greens Bayou with the following coordinates: 29 °15’55.29” N and 94°49’46.58” W. The boundary conditions extend to the farthest point east in the jetties with coordinates: 29 °14’48.82” N and 94°42’58.92” W as shown in Figure 14. The boundary conditions were set to variable depth and the storm surge time series from Galveston Pleasure Pier gauge (Station ID 8771510) were used. This storm surge data can be found at http://tidesandcurrents.noaa.gov/station_retrieve.shtml?type=Historic+Tide+Data Boundary conditions on the bay side of Galveston Island started from the end point of the gulf side boundary condition (in the jetties) with coordinates 29 °14’48.82” N and 94°42’58.92” to the Seawall south with coordinates: 29 °17’23.52” N and 94°52’11.98” W as shown in Figure 14.