SURFACE WATER DETENTION


This function is important for reducing downstream flooding and lowering flood heights, both of which aid in minimizing property damage and personal injury from such events. In a landmark study on the relationships between wetlands and flooding at the watershed scale, Novitzki (1979) found that watersheds with 40 percent coverage by lakes and wetlands had significantly reduced flood flows -- lowered by as much as 80 percent -- compared to similar watersheds with no or

few lakes and wetlands in Wisconsin. The same principles apply to Georgia where studies have shown that watersheds with an abundance of wetlands moderate flood flows more than those with less wetland (Wharton 1970). After heavy rains, the former watersheds take longer to reach peak water levels and have less fluctuation than the latter watersheds which reach their peaks more quickly, produce higher peaks, and tend to have more swift flows.


For purposes of landscape-level functional assessment following W-PAWF, this function will be restricted to surface water storage of nontidal waters. Floodplain wetlands and other lotic wetlands (basin and flat types) provide this function at significant levels. While tidal wetlands along rivers serve at times to attenuate freshwater flood flows from upstream watersheds, they are excluded from this function because they are subjected to frequent tidal flooding. The water

storage function of tidal wetlands for detaining storm surges is evaluated separately via the coastal storm surge detention function. Stormwater detention ponds are designed for temporary storage of surface water and are recognized as having a high level of performance for this function.


Wetlands dominated by trees and/or dense stands of shrubs could be deemed to provide a higher level of this function than emergent wetlands, since woody vegetation (with higher frictional resistance) may further aid in flood desynchronization. However, emergent wetlands along waterways provide significant flood storage, so no distinction is made regarding the type of vegetative cover. Floodplain width could also be an important factor in evaluating the significance of performance of this function by individual wetlands (e.g., for acquisition or strengthened protection), but there is no scientifically based criterion for establishing a significance threshold based on size. Drier-end wetlands (e.g., flats), and isolated basins are rated as having moderate potential.


For this function, the following relationships are used:

High

Lentic Basin, Lentic Fringe, Lentic Island (basin and fringe), Lentic Flat associated with reservoirs and flood control dams, Lotic Stream Basin, Lotic Floodplain-basin, Lotic River Fringe, Lotic Stream Fringe (not “A” water regime), Lotic River Island- basin, Ponds Throughflow (in-stream) and associated Fringe and Basin wetlands, Terrene Throughflow Basin, Stormwater Treatment Ponds






Moderate

Lotic River Floodplain-flat, Lotic River Fringe (other than above), Lotic Stream Fringe (other than above), Lotic Stream Flat, Lotic River Island-flat, Lentic Flat, Other Terrene Basins, Other Ponds and associated wetlands (excluding sewage treatment ponds and isolated impoundments), Terrene wetland associated with ponds (TE pd , excluding isolated diked ponds), Terrene Flat








Note: Exclude the following: 1) artificially flooded wetlands (“K” water regime, unless they are in a reservoir or dammed lake), 2) isolated impounded ponds and associated wetlands, 3) any freshwater tidal wetlands that are in the Lotic landscape position, and 4) any seasonally saturated wetlands (“B” water regime) from this function.




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