Name: Rule of Thumb Inundation Areas

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Description: The Mississippi Department of Environmental Quality Dam Safety Division (DSD) contracted with FTN Associates, Ltd. (FTN) to help update the hazard classifications of dams currently not in the State’s Inventory of Dams. Using a rule of thumb method (i.e., not a rigorous engineering dam break analysis) to produce approximate inundations maps from a dam failure, FTN provided suggested classifications for 1,223 dams based on hazards identified within the approximate inundation areas. Although FTN provided suggested hazard classifications, final verification of hazards and potential flooding extents for every dam and its subsequent classification is the responsibility of DSD.  FTN developed an ArcGIS (version 10.1) model that only requires a water height (or height of water behind a dam) and elevation data and applies this to a rule of thumb method to produce approximate inundation maps from a dam failure. The rule of thumb1 assumes the peak height of the breach floodwave at the dam is roughly half of the height of the water behind the dam and that the wave height is halved every 10 miles downstream (Table 1). This rule of thumb method does not take into consideration the volume of the impoundment, inflows or outflows from the dam, type of dam, condition of the dam, or the storage capacity of the floodplain downstream of the dam. FTN produced two sets of inundation areas, the first was based on a water height equal to the dam height (referred to as full inundation) and a secondary set of approximate inundation areas based on a water height that was 1/3 the height of the dam (referred to as partial inundation). The full inundation scenario was intended to be a conservative approximation for a barely overtopping dam breach and suggested dam classifications were based on hazards found in inundation areas generated from this scenario. The second scenario (partial inundation) was performed so that DSD staff could discriminate between dams which may potentially be high hazard under worst case conditions (overtopping breach) and dams which will easily be high hazard under more realistic conditions (water level of 1/3 height of the dam). To assist in the classification process FTN assigned each dam a unique ID, NOIXXXX, where NOI stands for Not On Inventory and XXXX was sequential from 0001 through 1223, e.g. NOI0123.

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