47.5 MB - The Whole Building Design Guide

terior drainage (on the land side); and long-term commitment formaintenance, inspection, and repairs. It is important to rememberthat areas behind levees are protected only up to a certain designflood level—once overtopped or breeched, most levees failand catastrophic flooding results. Levees that protect critical facilitiesusually are designed for at least the 0.2-percent-annual-chanceflood (500-year) and have freeboard to increase the factor of safety.Figure 2-23:Schematic of typicalearthen levee andpermanent floodwallFloodwall: Floodwalls are similar to levees in that they provide protectionto certain areas (see Figure 2-23). Failure or overtoppingof a floodwall can result in catastrophic flooding. A floodwall is asignificant structure that is designed to hold back water of a certaindepth based on the design flood for the site. Generally, due todesign factors, floodwalls are most effective in areas with relativelyshallow flooding and minimal wave action. As with levees, designsmust accommodate interior drainage on the land side, and maintenanceand operations are critical for adequate performance.MAKING CRITICAL FACILITIES SAFE FROM Flooding2-57