Application of State of Art Modeling Techniques to Predict Flooding and Waves for an Exposed Coastal Area

Abstract

Flood Insurance Rate Maps (FIRMs) are developed by the Federal Emergency Management Agency (FEMA) to provide guidance in establishing the risk to structures and infrastructure in the coastal zone from storm surge and coincidental waves. The maps are used by state agencies and municipalities to help guide coastal planning and establish the minimum elevation standard for new or substantially improved structures. A summary of the methods used and results of 2012 FIRM mapping are presented for Charlestown, RI; a coastal community located along the exposed, southern shoreline of the state. Concerns with the methods used in the 2012 analysis are put in context with the National Research Council's (NRC) 2009 review of the FEMA coastal mapping program. New mapping is then performed using state of the art, fully coupled surge and wave modeling and data analysis methods to address the concerns in the NRC review. The new maps and methodologies are in compliance with FEMA regulations and guidelines. The approach makes extensive use of the numerical modeling results from the recent US Army Corp of Engineers (USACE), North Atlantic Coast Comprehensive Study (NACCS 2015). Revised flood maps are presented and compared with the 2012 FIRM map to provide insight into the differences. The new maps highlight the importance of developing better estimates of offshore surge dynamics and its coupling to waves, dune erosion based on local observations, and the advancement in nearshore mapping of waves in flood inundated areas by the use of state of the art, two-dimensional wave transformation models.

Application of State of the Art Modeling Techniques to Predict Flooding and Waves for a Coastal Area within a Protected Bay

“…The results of the simulations are referred to as the NAST method in the following, referring to a combination of NACCS and STWAVE. [7]. Warwick study area is shown in the red circle.…”

“…Manning roughness was specified based on a 30 m resolution map of ground cover, provided by the RI Geographic [7]. Warwick study area is shown in the red circle.…”

“…Surge elevation (meters, relative to NAVD88) vs. return period (years) for Newport, RI (Station ID #8452660). Estimates are provided from NOAA for the mean, and lower and upper 95% confidence intervals, and for NACCS for the mean and upper 95% confidence interval, for the surge only and surge plus tidal cases [7]. .…”

“…Model predictions, in another evaluation [7,10], were compared to a wide array of point observations collected during the 1938, 1954, and 2012 storms, and reported by FEMA in their Kent County FIS [2]. In this case, the slope of the linear regression line from Newport to the head of the bay in Providence, was based on the storm of interest.…”

“…It is located inside Narragansett Bay and represents a protected coastal area. A companion paper, Spaulding et al (2017) [7], provides an application of the same methods used here, but for a fully exposed coastal shoreline in Charlestown, RI. It should be noted that there are a variety of other models available in the international community that could potentially be used to address coastal flooding and the associated wave climate, including, but not limited to, FVCOM, FUNWAVE, TELEMAC, CH3D, Delft3D, SWASH, XBeach, and CMS Flow and Wave.…”