APPENDIX C: MITIGATION ALTERNATIVES
o They can incorporate other benefits into structural project design, such as water supply and recreational uses o Regional detention may be more cost-efficient and effective than requiring numerous small detention basins • Disadvantages o They can disturb the land and disrupt the natural water flows, often destroying wildlife habitat o They require regular maintenance o They are built to a certain flood protection level that can be exceeded by larger floods o They can create a false sense of security o They promote more intensive land use and development in the floodplain LEVEES AND FLOODWALLS Probably the best-known flood control measure is a barrier of earth (levee) or concrete (floodwall) erected between the watercourse and the property to be protected. Levees and floodwalls confine water to the stream channel by raising its banks. They must be well designed to account for large floods, underground seepage, pumping of internal drainage, and erosion and scour. RESERVOIRS AND DETENTION
Reservoirs reduce flooding by temporarily storing flood waters behind dams or in storage or detention basins. Reservoirs lower flood heights by holding back, or detaining, runoff before it can flow downstream. Flood waters are detained until the flood has subsided, and then the water in the reservoir or detention basin is released or pumped out slowly at a rate that the river can accommodate downstream. Reservoirs can be dry and remain idle until a large rain event occurs. Or they may be designed so that a lake or pond is created. The lake may provide recreational benefits or water supply (which could also help mitigate a drought). Flood control reservoirs are most commonly built for
Retention pond
one of two purposes. Large reservoirs are constructed to protect property from existing flood problems. Smaller reservoirs, or detention basins, are built to protect property from the stormwater runoff impacts of new development. DIVERSION A diversion is a new channel that sends floodwaters to a different location, thereby reducing flooding along an existing watercourse. Diversions can be surface channels, overflow weirs, or tunnels. During normal flows, the water stays in the old channel. During floods, the floodwaters spill over to the diversion channel or tunnel, which carries the excess water to a receiving lake or river. LOCAL IMPLEMENTATION RECOMMENDATIONS Structural flood control projects that provide at least 100-year flood protection and that result in revisions to the Flood Insurance Rate Map are not credited by the CRS so as not to duplicate the larger premium reduction provided by removing properties from the mapped floodplain. Other flood control projects can
WSP June 2024 Page C.16
Wake County, NC Multi-Jurisdictional Hazard Mitigation Plan
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