The data were developed to identify and assess potential riverine flooding impacts to wastewater infrastructure situated at ground elevations within 2 or 3 feet of the existing FEMA 100-year floodplain. The Federal Flood Risk Management Standard, Option 2 amends the definition of floodplain to establish the vertical elevation and corresponding horizontal extent of flooding for federally funded projects to recommend that standard projects be built 2 feet above the existing100 year (1% annual chance) flood elevation and that critical infrastructure be built at an elevation 3 feet above the 100 year floodplain.
The riverine flood hazard assessment is based on guidance issued in Executive Order 13690 – the Federal Flood Risk Management Standard – which provides recommendations for establishing flood elevation and hazard areas that account for uncertainties associated with future climate changes. The new standard amends the definition of floodplain to include the following approaches to establish the vertical elevation and corresponding horizontal extent of flooding for federally funded projects:
- Use data and methods informed by best-available, actionable climate science;
- Build two feet above the 100-year (1%-annual-chance) flood elevation for standard projects, and three feet above for critical buildings like hospitals and evacuation centers; or
- Build to the 500-year (0.2%-annual-chance) flood elevation.
The analysis described here corresponds with option 2, and was intended to complement recent hydrologic modeling conducted for the Rhode Island Department of Health SafeWater RI study (RIDOH, 2012). SafeWater RI developed estimates of future floodplains in the Pawtuxet River Basin using downscaled climate models; the results were then extrapolated to other watersheds in the state (option 1). By contrast, this approach used a consistent, map-based calculation, to delineate floodplains based on FEMA’s Base Flood Elevation (BFE) and the underlying topography. The mapping also extended to Special Flood Hazard Areas designated by FEMA as Zone A – areas subject to inundation by the 1-percent-annual-chance flood event that are determined using approximate methodologies and do not have assigned Base Flood Elevations.
The methodology involved developing new flood areas based on higher flood elevations than those used by FEMA, and mapping the corresponding horizontal floodplain extent. The input data for the analysis were FEMA BFEs, which are provided at each riverine transect. The BFEs were taken from each county’s DFIRM database, which was downloaded from the FEMA Mapping Center in August 2015. The mapping relied on a single source of topography data – LiDAR – which was collected statewide in 2011. A digital elevation model derived from the statewide LiDAR at 1-meter resolution (RIGIS, 2013) was applied for this assessment.