Landforms and Wind Speed

Good locational decisions require a variety of data be evaluated.  A report was issued recently by the Mitigation Assistance Team (MAT) that the Federal Government (FEMA) sent to the US Virgin Islands.  The MAT evaluated the effects of recent hurricanes and developed new guidelines for building standards to build on the lessons learned.  Among them was that many building codes don’t consider the effect of landforms on wind speed.  The diagram below (from this web page) shows how wind speeds are enhanced where hills are introduced in their path, in the same way that an airplane wing enhances the flow of air.  

 

Buildings constructed to standard local codes in areas where topography can enhance the speed of storm-driven winds, therefore, are not adequately prepared and many were needlessly damaged.  The MAT prepared new wind speed maps for the Islands.  The report includes the following: 

The wind speed-up maps developed for St. Thomas, St. John, and St. Croix aid in the understanding of how topographic effects may increase the wind speed in areas of higher elevation, such as the mountainous areas of St.  Thomas.  When  this  speed-up  occurs  during  a  hurricane  or  tropical  storm  event,  the  wind  speeds  can  increase  by  more  than  20 percent  resulting  in  a  significant  increase  in  the  wind  pressures  acting on the surfaces of buildings. If these higher loads are not considered in the design and construction of a building, partial or total failure of the building may occur, as seen in Figure 2-3. This can help design professionals better account for the local conditions resulting from the USVI’s unique topography.