FIGURE 5.1 Onshore features that affect the propagation of waves, flood insurance rate zones (V and A zones), and base flood elevations. The 100-year stillwater elevation is the water level with a 1 percent annual chance of being exceeded in a given year. SOURCE: FEMA (2003).

FIGURE 5.1 Onshore features that affect the propagation of waves, flood insurance rate zones (V and A zones), and base flood elevations. The 100-year stillwater elevation is the water level with a 1 percent annual chance of being exceeded in a given year. SOURCE: FEMA (2003).

or episodically changes the ground surface and complicates flood hazard mapping, especially along the Atlantic coast, which has dunes that are reshaped by storms, and, to a lesser degree, the Gulf coast.

Storm surge, tides, and waves are the greatest contributors to coastal flooding. Storm surge is the pulse of water that washes onto shore during a storm, measured as the difference between the height of the storm tide and the predicted astronomical tide. It is driven by wind and the inverse barometric effect of low atmospheric pressure, and is influenced by tides and by uneven bathymetric and topographic surfaces. Faster wind speeds and larger storms create a greater storm surge potential. Storm surge alters topographic features that might otherwise dampen the effects of surge and wave forces. For example, sand dunes that normally prevent storm water progress onto a barrier island may be reshaped or even removed during a severe storm.

Water surface elevations at the shoreline are a combination of the average water level determined by wind setup (due to the direct action of wind stresses at the air-sea interface) and wave setup (due to breaking waves, Figure 5.2) and a fluctuating water level caused by wave runup (the maximum extent of high-velocity uprush of individual waves above the average water level). All of these factors are included in coastal flood models to estimate the BFE.

FEMA COASTAL FLOOD MODELING METHODOLOGY

The Basic Structure of Current Coastal Flood Models

Coastal flood models estimate BFEs using empirical and probabilistic input data and two modeling steps (Figure 5.3 and Table 5.1):

  1. Storm surge models are often loosely coupled with wave models to calculate the 1 percent annual chance stillwater elevation (SWEL) and the wave dynamics associated with a coastal flooding event. Recent flood studies in Mississippi and Louisiana used loosely coupled two-dimensional (2-D) surge and wave models to calculate the SWEL and wave setup.

  2. The SWEL value (with or without wave setup) from the wave and surge models is used to calculate wave crest values using erosion and wave calculations through the Coastal Hazards Analysis and Modeling Program (CHAMP) and the Wave Height Analysis



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