47.5 MB - The Whole Building Design Guide

2.4.2 SITE MODIFICATIONSA plan to modify the site of an existing facility that is subject toflooding requires careful examination by an experienced professionalengineer. Determining the suitability of a specific measurerequires a complex evaluation of many factors, including the natureof flooding and the nature of the site. The first part of Table2-3 in Section 2.5 identifies elements that influence the choiceof mitigation measures applicable to existing sites. Some floodcharacteristics may make it infeasible to apply site modificationmeasures to existing facilities (e.g., depths greater than 3 to 4 feet,very high velocities, insufficient warning because of flash floodingor rapid rate of rise, and very long duration).A common problem with all site modifications is the matter ofsite access. Depending on the topography of the site, constructionof barriers to floodwaters may require special access points.Access points may be protected with manually installed stop-logsor designed gates that drop in, slide, or float into place. Whetheractivated by automatic systems or manually operated, access protectionrequires sufficient warning time.Other significant constraining factors include poor soils and insufficientland area which can make site modifications eitherinfeasible or very costly. A critical facility may be among severalbuildings and properties that can be protected, increasing thebenefits. For any type of barrier, rainfall that collects on the landside must be accounted for in the design, whether through adequatelysized stormwater storage basins constructed on land setaside for this purpose, or by providing large-capacity pumps tomove collected water to the water side of the barrier.Each of these site modification measures described below has limitations,including the fact that floods larger than the design floodwill exceed the level of protection.Regrading the site (berm): Where a facility is exposed to relativelyshallow flooding and sufficient land area is available, regradingthe site or constructing an earthen berm may provide adequateprotection.MAKING CRITICAL FACILITIES SAFE FROM Flooding2-79