APPENDIX C: MITIGATION ALTERNATIVES
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).
Retention pond
Flood control reservoirs are most commonly built for 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 /CRS Credit The City of Raleigh may not be eligible to receive credit for Activity 530 – Flood Protection. 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 be accepted by offering a 25-year flood protection. Table C.5 – Structural Projects Mitigation Options and Recommended Projects Action # Mitigation Action Reason for Pursuing / Not Pursuing Funding Structural Project Measures Considered by HMPC and Not Recommended
The City of Raleigh already requires developers to hold back water on site through either detention or retention. The City of Raleigh already has a program for improving stormwater conveyance systems in areas where drainage is currently inadequate will reduce stormwater flooding and prevent losses.
Promote detention and retention facilities to provide flood protection.
-
n/a
Develop stormwater conveyance systems to alleviate flooding for existing and new development.
-
n/a
Structural Project Measures and Funding Recommended for Implementation
Install cameras in flood prone areas throughout the City of Raleigh to allow us to view these locations and make informed decisions as it relates to flooding
This project will enable the City to identify locations for stormwater improvement projects.
Not yet identified
SP-1
Wake County Multi-Jurisdictional Hazard Mitigation Plan 2019
C.18
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