Metro Bay SAMP Issues

Floodplan Hazards Issues

For entire document (pdf), click here.

For view presentation on Policy & Management Implications of Sea Level Rise (pdf), click here.

Issue 1: Expansion of the floodplain due to fill, infill development, sea level rise, runoff, and shoreline change

The combination of continued fill, infill development, historic and accelerated sea-level rise, increased runoff from impervious watershed surfaces, and shoreline change has resulted in an expanded floodplain that reduces the coastline’s ability to absorb floodwaters and increases the area likely to be flooded. The result is that storm surges will likely rise higher than in the past, though there are no updated models to quantify the height and extent of possible storm surges and flooding.

Significant filling of shoreline areas since the hurricane of 1938 has expanded the floodplain, so that some existing A-zone structures may have increased flood elevations and may need to be recategorized as V-zone structures (Boothroyd pers comm.). As a result, property owners would pay higher flood insurance premiums and need to build to higher standards when undertaking substantial remodeling. Existing commercial buildings will have to increase the height of their flood-proofing to remain above the base flood elevation. Another implication is A-zone “creep,” where flooding will expand landward into areas that are currently not in the high-hazard zone.

While the FEMA flood maps were completed in the early 1970s after the hurricane barrier was built and most of the fill occurred, these studies rely on historical data and generalized modeling. While they were updated for certain flood-zone designations in 1980s and 1990s, these modifications do not incorporate changes in sea level rise or cumulative build-out analyses for the floodplain; therefore, the maps may underestimate potential flood elevations. Regardless, property owners are allowed to request changes to floodplain zone categories with a FEMA–approved map amendment, which can be costly on a site-by-site basis and may not adequately address cumulative impacts from these site-based decisions. The Johnson & Wales dormitory project in Fields Point was granted an amendment for fill by FEMA, thereby allowing the university to elevate the structure out of (or above) the floodplain. The studies reviewed by FEMA included an analysis of impact on adjacent properties, and in this case, the consultant indicated that there was no such impact (Greenwood, pers. comm.). While one property may benefit from an amendment, it is not known what the cumulative impact would be if each property in the coastal floodplain applied for this exclusion and built in a similar manner. It should be noted that regulations and the hydrodynamics are different for riverine areas, where floodwaters can be displaced and compensatory storage of water is required for proposed fill.

Neither the municipalities nor the state educates property owners about the map amendment processes or discourages them from pursuing them.

While the hurricane barrier is designed to protect downtown Providence, the fill behind it may cause localized flooding along the Woonasquatucket and Moshassuck rivers .

The pumping capacity of the hurricane barrier exceeds the recorded peak flow of these two rivers by 2,900 cubic feet per second, according to figures from the U.S. Geological Survey and USACE.