Metro Bay SAMP Issues

Floodplan Hazards Issues

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Issue 5: Lack of a regional hydrodynamic model to predict the extent of flooding and potential impacts

The Metro Bay region is at the confluence of several rivers and the Bay. Regional models and research are lacking on the cumulative impacts of local flooding dynamics between the riverine and coastal systems, so it is not possible to accurately project the horizontal and vertical expansion of the floodplain. Existing flood, storm surge, and hurricane models were developed for different needs and users, so they are not integrated and they use different parameters to determine flooding dynamics. The FEMA flood studies are based on models at least 25 years old. A lack of detailed topographic and bathymetric data is a major limiting factor to improving the prediction of riverine and coastal flooding. In addition, riverine systems and coastal areas are modeled differently and have difference flooding dynamics, so regulations for filling in the floodplain or floodway differ.

Developing accurate models is only one step towards an effective regional hydrodynamic model. Users of the model must understand the output and must act based on their levels of confidence in the scenarios. There is inadequate knowledge by decision-makers and the public of how to interpret the model results or understand the risks. Two-dimensional maps do not fully convey what a 10- to 20-foot storm surge would do to neighborhoods and property. Coastal projects and topographic features are often measured with different tools and vertical datums, resulting in different elevation numbers. Applying their data to a site plan can result in unforeseen risks. For example, the inadvertent use of one datum, such as NGVD, rather than another, such as mean low water or mean high water, can result in a difference of 1 to 3 feet in the height above grade for a building foundation. Pawtucket’s 1-foot freeboard requirement, discussed above, is one way to address uncertainty and provide a margin of safety.