The Need and Rationale for a Coastal Flood Risk Index

There is need for an index for estimating and communicating the potential storm surge and inundation caused by land-falling hurricanes. In 2003, the U.S. Congress passed a law which was signed by the Administration that funded the creation of such an index which would be in keeping with the well-known Saffir-Simpson Hurricane Wind Index. We propose a formula for such an index by simplifying the Navier-Stokes momentum and conservation equations configured for a hurricane about to make landfall onto a coast. Two mechanisms, one by the alongshore, and the other by the cross-shore wind stress are formulated and their relative role are discussed. The formula used the information of, besides the main driving force of hurricane wind stress, hurricane translation speed, water depth, hurricane size (fetch), and topographic slope. Other factors that affect the surge height, such as the Sea Level Rise, tidal, surface air pressure and others, can be easily added to the estimated surge height and therefore to the estimate of the inundation potential. An example is given in which the surge height calculated using the proposed formula matched the observed values for Hurricane Andrew. The impact of Sea Level Rise on the inundated area estimated using the proposed formula are compared against the ones obtained with full dynamic models.