Wetland Assessment Criteria 21 



(iii) Flood, Erosion and Shoreline Damage Reduction 



Flood Reduction-Wetlands temporarily store flood water, slow water 

 velocities, reduce bank and shoreline erosion, and slowly release stored 

 water downstream, thereby saving lives and property. This function is 

 especially important in areas with developed floodplains, where the 

 potential for flood damage is high. Inland wetlands located along major 

 streams and around lakes stabilize shorelines and channel banks and 

 buffer developed uplands from storm, wave or erosion damage. Coastal 

 wetlands serve these functions as well as providing a buffer to reduce 

 potentially devastating effects of storm surges. 



Flood conveyance and reduction functions of wetlands relate to their 

 capacity to store and slow flood water, thereby increasing the duration of 

 the flow and reducing downstream flood peaks (Sather and Smith, 1984). 

 Many authors cite the Corps of Engineers' 1972 study of the Charles and 

 Neponset River watersheds in Massachusetts as a prime example of the 

 socioeconomic values associated with protecting wetlands to maximize 

 flood control benefits. In this study, the Corps estimated that loss of the 

 8,423 acres of wetlands within the basin would result in annual flood 

 damages of over $17,000,000 (Sather and Smith, 1984). 



Important factors influencing the flood reduction role of wetlands 

 include: size (larger wetlands provide more flood storage and flow 

 reduction); location within the basin (wetlands in the upper watershed 

 often are more effective for flood retention); texture of substrate; structure 

 of the vegetation; and connection with other wetlands (isolated wetlands 

 are generally less effective for flood control). 



The data base continues to improve regarding capability to identify 

 wetlands having high potential for flood reduction. For example, Ogawa 

 and Male (1986) have developed a methodology for assessing the flood 

 control role of individual wetlands for certain kinds of streams. 



The flood control functional value of a wetland site could be measured by 

 its potential to store floodwater and prevent future flood damage that could 

 result in substantial public costs each year. Among different wetland 

 types, riverine wetlands with adjacent open or relatively open (non- 

 developed) flood plains often have relatively high flood storage and 

 conveyance values. 



Erosion and Shoreline Damage Reduction-Wetland vegetation plays an 

 important role in reducing damages from shoreline erosion by binding 

 (i.e., plant roots hold soil) and stabilizing substrate, trapping sediments 

 and reducing wave or current energy (Reppert et al., 1979). The 

 effectiveness of shoreline vegetation in reducing erosion depends on 

 particular species, width of shoreline vegetation (e.g., the wider the 

 wetland area, the higher the value), substrate (e.g., sandy substrate is less 



National Wetlands Priority Conservation Plan 



