HR: 09:45h
AN: H51H-08    [Abstracts]
TI: Quantifying Wetland Functions: A Case Study
AU: * Potter, K W
EM: kwpotter@wisc.edu
AF: Department of Civil and Environmental Engineering, University of Wisconsin, 1415 Engineering Drive, Madison, WI 53706, United States
AU: Rogers, J S
EM: rogersjs77@yahoo.com
AF: Department of Civil and Environmental Engineering, University of Wisconsin, 1415 Engineering Drive, Madison, WI 53706, United States
AU: Hoffman, A R
EM: adamhoffman@wisc.edu
AF: Environmental Chemistry and Technology Program, University of Wisconsin, Water Science and Engineering Bldg., 660 N. Park St., Madison, WI 53706, United States
AU: Wu, C
EM: chinwu@engr.wisc.edu
AF: Department of Civil and Environmental Engineering, University of Wisconsin, 1415 Engineering Drive, Madison, WI 53706, United States
AU: Hoopes, J A
EM: hoopes@engr.wisc.edu
AF: Department of Civil and Environmental Engineering, University of Wisconsin, 1415 Engineering Drive, Madison, WI 53706, United States
AU: Armstrong, D E
EM: dearmstrong@facstaff.wisc.edu
AF: Environmental Chemistry and Technology Program, University of Wisconsin, Water Science and Engineering Bldg., 660 N. Park St., Madison, WI 53706, United States
AB: Wetlands are reputed to reduce peak flows and improve water quality by trapping sediment and phosphorus. However, there are relatively few studies that quantify these wetland functions. This paper reports on a study of a 45-hectare wetland in southern Wisconsin. The wetland is traversed by a stream channel that drains a predominantly agricultural 17.4 km2 watershed. During the spring and summer of 2006, we collected stage data and water samples at stations upstream and downstream of the wetland, with the former accounting for 82% of the contributing area. Continuous measurements of water stage at these stations were used to construct a streamflow record. During storm events water samples were taken automatically at 2-hour intervals for the first 12 samples and 8-hour intervals for the next 12 samples. Samples were analyzed for total suspended solids, total phosphorus, and dissolved reactive phosphorus. Ten events were observed during the observation period; the two largest events were 1 to 2-year storms. One-dimensional unsteady flow routing was used to estimate the maximum extent of wetland inundation for each event. When normalized for flow volume, all peak flows were attenuated by the wetland, with the maximum attenuation corresponding to the intermediate events. The reduced attenuation of the larger events appears to be due to filling of storage, either due to antecedent conditions or the event itself. In the case of sediment, the amount leaving the wetland in the two largest storms, which accounted for 96% of the exported sediment during the period of observation, was twice the amount entering the wetland. The failure of the wetland to trap sediment is apparently due to the role of drainage ditches, which trap sediment during the wetland-filling phase and release it during drainage. The export of sediment during the largest events appears to result from remobilization of sediment deposited in the low-gradient stream channel during smaller events. This hypothesis was supported by the finding that the estimated bed shear during large events exceeded laboratory measurements of the critical shear stress of bed sediment samples. In the case of total phosphorus, the inflow to the wetland about equaled the outflow, although the wetland sequestered 40% of the incoming dissolved reactive phosphorus. The discrepancy is almost certainly due to net export of sediment. Wetlands such as this are very common in the glaciated portion of the U.S., and many contain channels and ditches. The region is dominantly agricultural, and sediment and phosphorus are the primary causes of impaired surface-water quality. Our results suggest that these wetlands are not very effective in mitigating this impairment when flow is concentrated in channels.
DE: 0483 Riparian systems (0744, 1856)
DE: 1820 Floodplain dynamics
DE: 1862 Sediment transport (4558)
DE: 1871 Surface water quality
DE: 1890 Wetlands (0497)
SC: Hydrology [H]
MN: 2007 Joint Assembly