Federal Guidelines for Dam Safety: Emergency Action Planning for ...

Suggested EAP Outline and Contentsdownstream inundation at higher elevations and will include additional affected populations. Afailure at the dam’s Inflow Design Flood (IDF) is considered to show the upper limit ofinundation.A sensitivity analysis (i.e., varying the breach parameters such as breach width and time tofailure for the various flood inflow conditions) is recommended in order to fully investigate theeffect of a failure on downstream areas. A sensitivity analysis allows the reviewer to identify theeffect of various failure scenarios in order to select the most appropriate failure mode fordeveloping the EAP.If the assumed failure of a dam would cause the failure of any downstream dams, the analysisshould consider the domino effect in routing the flood wave downstream. For example, if adownstream dam has an earthen embankment that would be significantly overtopped due to theupstream dam failure, then it may be necessary that the inundation zone reflect the additionalflooding from failure of the downstream dam. Many factors should be considered for these cases,such as the expected performance of the downstream dams during high flows, the lag timebetween dams, and possible operation actions at downstream dams (e.g., drawdowns) that couldalleviate the flood wave. Coordination of such studies with other downstream dam ownersshould be undertaken when feasible. The flood wave should be routed to a point where it nolonger presents a hazard to downstream life or property.b. Preparing Inundation MapsInundation maps should clearly show inundation zones, cross section information, dams, streets,buildings, railroads, bridges, campgrounds, and any other significant features. At the request ofemergency management authorities, additional features, such as highlighted evacuation routesand emergency shelters may be included on the maps. All features should be shown using localnames or terms. Printed inundation maps should be at a scale that is sufficient to clearly show thedownstream inhabited areas within the inundation zones.To assist emergency management authorities with potential evacuations, the maps should showareas inundated from a dam failure during “fair weather” and IDF conditions. The maps alsotypically show normal water levels. If inundated areas for the “fair weather” breach and the IDFbreach are essentially the same or too close to be shown separately on the inundation maps, asingle inundation area for the two breach conditions may be shown.The lines delineating the inundated area should be drawn in such thickness or form (solid line,dashed line, dotted line) as to readily identify the inundation limits as the main features of themap but not bold enough to block houses, roads or other features which are inundated by theflood waters. The area between the inundation lines representing the water level may be shadedor colored to distinguish the area of inundation. Care should be taken to select shading or colorsthat will not block important features on the map. Additionally, critical features or inundatedstructures can be highlighted to ensure visibility.Federal Guidelines for Emergency Action Planning for DamsII-21