also flood plain (flŭd′plān′)
n. A plain bordering a river and subject to flooding at times of high water
Flood Plain Protection
The Walnut Creek Watershed Management Plan recommends that local policies and ordinances should be adopted or amended to protect flood plains in the following ways:
- Reduce structural and property losses during major flood events by preventing construction of new structures within the limits of the 100-year flood plain (1% annual exceedance probability).
- Maintain flood storage capacity by limiting grading or placement of fill materials within the 100-year flood plain.
- To the greatest extent possible, locate public infrastructure outside of the limits of the 100-year floodplain. When it can't be avoided, provide additional flood plain storage nearby through excavation or make other improvements to maintain projected highwater elevations.
- Identify areas of active stream movement and reserve areas as open space where future stream movement or flood inundation is expected.
- When establishing flood protection elevations, provide three feet of vertical separation between regulatory 100-year flood elevations and require building protection elevations to account for anticipated flood inundation levels.
The plan recommends implementing ordinances and policies to apply these standards to all new developments and where new land subdivisions are planned to occur adjacent to streams where flood risk has been defined by FEMA Flood Insurance Rate Maps. Redevelopment within existing built parcels within the floodplain should be done in a manner to cause no net increase in flood elevations. The potential for recurring losses on such properties or need for flood protection techniques should be evaluated by local jurisdictions when site plans for redevelopment are considered. Existing structures which fall within these protection zones should be identified. Past known damages to such structures may be reason to pursue opportunities to acquire and remove such structures to avoid recurrent damages.
Expected Impacts (Where Applied)
- Reduced potential for damages to buildings, property and other infrastructure during flood events.
- Maximized capacity for storage and conveyance of large flood events.
- Reduced risk of higher velocity flows or reduced travel times being caused by narrowing of the flood plain.
- Larger factor of safety above projected flood elevations.