Ch. 3— Wetland Values and the Importance of Wetlands to Man • 45 



a natural levee, stored 80,131 cubic meters (m') 

 of water. If this amount of storage were extrapolated 

 to the entire area of swampland in the watershed, 

 total wetland storage would equal 8.4 percent of 

 the total flood runoff as measured at a downstream 

 gage (52). 



Bernot found that flow was about 5,000 cubic 

 feet per second (ft'/s) into the Thief Run Wildlife 

 Management Area and the Agassiz National Wild- 

 life Refuge, while outflow was approximately 1,400 

 ft^/s. He calculated that the flood storage capacity 

 and losses due to the other factors of these two wet- 

 land areas reduced the floodpeak at Grand Forks, 

 by about 0.5 foot and at Crookston by about 1.5 

 feet (8). 



Comparison of Floodpeaks From Wetland 

 and Nonwetland Watersheds 



By studying floodpeaks in 15 watersheds, No- 

 vitzki found that floodpeaks may be as much as 80 

 percent lower in watersheds with large lake and 

 wetland areas than in similar basins with little or 

 none. Watersheds with 40-percent lake and wedand 

 area have floodpeaks only 20 percent as large as 

 those with little or no wetland area. While flood- 

 peaks were found to be lower in watersheds with 

 a large percentage of wetlands, total streamflow in 

 the spring was higher in basins with large lake and 

 wetland areas (63). 



Analysis of Flood Hydrographs 



Flood hydrographs — graphs of the time distribu- 

 tion of runoff from a drainage basin — of perched 

 peat bogs and peadands indicate that these wedands 

 temporarily store and slowly release storm waters 

 (5,9). Long-term hydrographs from the Passaic 

 River, N.J., and the Ipswich River, Mass., showed 

 that the wetlands adjacent to the rivers play an im- 

 portant role in delaying runoff (31). Synthetic hy- 

 drographs (not calculated on historical data) for 

 eight wetland areas also showed reductions in peak 

 flows (94). 



Actual flood-storage capacity often will depend 

 on environmental conditions prior to flooding or 

 on the relationship of a particular wetland to the 

 regional hydrology. For example, when evapo- 

 transpiration rates are low and water is ponded in 

 wetlands, runoff during periods of heavy precipita- 



tion may be greater from wetlands than from up- 

 land areas (because the soil is saturated and the sur- 

 face storage capacity quickly is exceeded) (51,77, 

 92). On the other hand, high rates of evapotran- 

 spiration and low water tables favor storage of flood- 

 waters. In some cases, wetlands provide no stor- 

 age capacity for floodwaters. For example, a hy- 

 drographic analysis of two Massachusetts swamps 

 indicated that both wetlands contributed signifi- 

 candy to floodpeaks because of their rapid discharge 

 of ground water (64). 



The Role of Vegetation in Flooding 



There have been a few attempts to isolate the ef- 

 fect of vegetation on flooding. The frictional drag 

 on runoff flowing through wedand vegetation is rep- 

 resented by a roughness coefficient called "Man- 

 ning's 'n.' " The higher the value of "n," the 

 greater the drag and the slower the flow velocity 

 of floodwaters. Values of "n" vary widely and are 

 highly dependent on the type and amount of vege- 

 tative cover. In general, the value of "n" for a river 

 wetlands in or adjacent to it can be approximately 

 twice the value of channels without associated wet- 

 lands (15). 



Impact of Wetland Filling and 

 Development on Flooding 



The Corps has used model-generated hydro- 

 graphs to estimate the volume of storm water that 

 could be stored in the basin wetlands of the Charles 

 River, Mass., and to determine the reduction in 

 storage, assuming future encroachment (89). Fol- 

 lowing a storm in 1955, approximately 50,000 acre- 

 ft of storm water flushed past the Charles River 

 Village gaging station with a peak flow of 3,220 

 ft^/s. This amount is equivalent to 5 inches of runoff 

 from the 184-square-mile drainage basin. On the 

 adjacent Blackstone River, which has few, if any, 

 wedands, the storm discharge peaked at 16,900 ft'/s 

 and the bulk of the storm water was discharged in 

 a much shorter time period than on the Charles. 

 Based on this analysis, it was predicted that a 40- 

 percent reduction in wetland area along the river 

 would result in a 2- to 4-foot increase in floodpeaks 

 and would increase flood damages by at least $3 

 million annually. 



Hydrographs of the Neponset River Basin, 

 Mass., were used to determine the impact of en- 



