Description

1) Application of a two-dimensional hydraulic model in a highway expansion design in Grand Cayman Island
Mostafa Razzaghmanesh, Ph.D. P.E., CFM, Remington and Vernick Engineering Inc., mostafa.razzaghmanesh@rve.com
Co-presenters: Stuart Gause, Start.Gause@rve.com; Joseph Pegnetter, Joseph.Pegnetter@rve.com; Steven Bolt, Steven.Bolt@rve.com; Denis Thibeault, denis.thibeault@nra.ky, Edward Howard, edward.howard@nra.ky

Abstract: Remington & Vernick Engineers (RVE) was retained to prepare a Hydraulic and Hydrologic (H&H) study for the proposed East-West Arterial Highway Extension (about 9 miles) by the National Roads Authority on Grand Cayman, Cayman Islands. Various rainfall data with different time steps were collected from the site. Data was analyzed and used to study the rainfall characteristics and preparing Intensity -Duration – Frequency (IDF) curves. The results of the Grand Cayman Island rainfall analysis and several land data were used to perform a two-dimensional hydraulic model. Three software packages including QGIS, HEC-HMS and HEC-RAS were used to perform the H&H study. The Hydraulic model was used to prepare inundation flood maps under various rainfall events or scenarios. The diffusion wave equation was selected for the two-dimensional (2D) studies in HEC-RAS. The 2D model was run for various 24-hours scenarios. A 2D area of almost 12 sq miles was defined over the study area. The 2D area mesh contained almost 152,000 computational cells. Additional information to run the HEC-RAS 2D model including land use, infiltration, and manning’s coefficients. Inundation flood maps for a 2-year, 10-year, 25-year, 50-year, 100-year, and the Hurricane Ivan of September 2004 were prepared. In general, the results showed that, for the studied scenarios, the depth of the floods from a 2-year event to a 100- year event would be between 0.30 ft and 6 ft along the proposed highway. The simulated Hurricane Ivan event also showed a consistency with the Grand Cayman Hurricane Ivan Flooding map of September 2004. In order to design the EW arterial highway, the simulated flood elevations shall be considered. The locations for the proposed highway drainage improvement can be identified. Also, the results of the coastal and surge analysis, performed by others, shall be taken into consideration for the design of the highway.

2) Storm surge and wave modeling of Guam/CNMI with evaluation of offshore swells
Michael Salisbury, PE, D.CE, Atkins North America, Inc., michael.salisbury@atkinsglobal.com
Co-presenters: Paul Carroll, PE, PMP, Stantec

Abstract: STARR II is supporting FEMA Region 9 in a Flood Insurance Study (FIS) to update the flood hazard information for the territories of Guam and the Commonwealth of the Northern Marianna Islands (CNMI). The storm surge and wave modeling for the region around Guam and The CNMI is unique, as compared with Atlantic Ocean and Gulf of Mexico, in that this area is affected not only by tropical storms, but also strong offshore swells originating from Northern Pacific, and the presence of fringing reefs surrounding most of the islands denoting a stark transition from deep water to shallow water in a relatively short distance. This presentation will describe a coupled two-dimensional (2D) water level and wave model (ADCIRC+SWAN) to simulate historical swells as well as typhoon events. The model was calibrated/validated for both water levels and waves using a total of four historical storm events. The model is sufficient for the purposes of updating coastal flood maps for Guam and CNMI. The methodology used to understand the relative importance of tropical storms and offshores swells to the flood hazard curve near Guam and CNMI will also be presented, which demonstrates that tropical storms dominate the frequency band of flood risk despite the existence of strong offshore swells near Guam and CNMI.

3) Simulating Hurricanes Irma and Ian with a 2-D Coastal Flood Model
Noemi Gonzalez Ramirez, FLO-2D Software Inc / Riada Engineering Corp, noemi@flo-2d.com
Co-presenters: Karen O’Brien, karen@flo-2d.com

Abstract: Hurricane Ian in September, 2022 once again demonstrated how poorly prepared for storm surge flooding our coastal areas are. Sea level rise (SLR) will exacerbate hurricane flood hazards until mitigation measures are implemented on a city or county wide scale. This trend is already posing enormous challenges with storm drain systems. The importance of having detailed coastal urban flood routing models to design flood mitigation, ensure emergency access and create evacuation routes was forcefully illustrated by Hurricane Ian. Coastal flood prediction based on existing conditions is no longer valid. SLR will exacerbate flooding with a loss of storm drain capacity and rising groundwater levels. Comprehensive flood hazard prediction with a two-dimensional storm surge model combined with spatially hurricane rainfall runoff in a coastal urban area is discussed. Buildings, walls, street flow, drainage channels, and storm drain systems are simulated that add to the complexity of unconfined coastal flooding. The replication of Hurricane Irma and Hurricane Ian flooding in Naples, Florida will be presented. The model has more than 6.5 million cells divided into 4 subdomains. A total of 503 storm drain lines consisting of 6,500 inlets, 1,400 manholes and more than 6,600 conduits constituted the storm drain system. Main canals are simulated as 1-D channels in the model. Some areas are at risk solely by the storm surge while other areas are impacted by the combined rainfall runoff and storm surge. The modeling will support of flood mitigation plans in Naples coastal communities. The FLO-2D model is a two-dimensional flood routing and storm drain model that predicts spatially variable water surface elevations to capture complex flooding in urban areas. The hurricane storm surge is simulated with a time-stage relationship at the shoreline. Spatially varied rainfall and infiltration losses are simulated with hurricane NEXRAD rainfall data.