Elimination of Exposure D along Hurricane Coastline in ASCE 7

Abstract

The definition of Exposure D in ASCE 7 along the hurricane coastline does not take into account the increase in the roughness of the ocean with increasing wind speeds, the age of waves, or the changes in the wave characteristics as they approach the coast. This note reviews some of the more recent research associated with the roughness of wind-driven waves. One specific observation is that waves produced near the center of a hurricane are young waves, which exhibit a higher aerodynamic roughness than do the mature waves often associated with “straight-line” winds. A second observation is that, as waves approach the shore, they become steeper and thus exhibit a higher aerodynamic roughness. These observations suggest that, in design events, the ocean becomes aerodynamically very rough, with the value of the surface roughness approaching that of Exposure C. Based on this analysis, it is recommended that the use of Exposure D along the hurricane coastline in ASCE 7 be discontinued.