7789 - Laserfiche WebLink

Home Browse Search

4 - ~ ~ As a result of the meander cutoff, some local incision may have occurred. In addition, some slight channel incision may be attributed to the change in river flow hydrology and sediment load related to upstream flow regulation. This remains to be verified. For alluvial channels, overbank flow generally flows for discharges exceeding the 1.5 to 3 yr return period event. Although a frequency analysis has not been performed for this reach, bankfull discharge of 30,000 cfs (at E-14.5) has been exceeded only 3 times in the last 50 years. Historically, the bankfull dischazge at this location may have substantially less than 30,000 cfs. As a result of the meander cutoff and possible channel incision, some excavation of the inlet will be required to establish flow to the wetlands during seasonal peak discharges in the Green River. Depending on the preferred wetland water surface elevation, estimates of excavation cost and desired return period for inundating the wetlands, a river discharge can be specified for flooding the wetlands on a prescribed interval of years (say one in every three yeazs). A hydrologic frequency analysis should be performed for the Green River flows to determine the approximate return period wetland inundation potential. The potential for creating a connection between the river and the wetlands exists at several locations. Two preferred locations are neaz cross sections E-14.5 and E-15. Near E-15, an old meander channel presents an opportunity for a short flow path to the wetlands. This old channel thalweg or depression does not reach the wetlands area and several floodplain mounds would require excavation to complete the link between the river and the wetlands. This potential channel is labeled as the alternate channel to the wetlands in Exhibit C. More hydraulic head and better inflow control to the wetlands exists if the inlet to the wetlands would be constructed near E-14.5. This inlet would require a longer channel (as shown in Exhibit C, potential channel to wetlands), but would require less excavation because the proposed channel would follow an old river thalweg along the bluff line. Bank excavation would be required to create the inlet. The inlet could be a simple trapezoidal channel design with gate controls. The existing bank elevation is 4837. Bed elevations surveyed along the old meander thalweg line range from 4831.6 near the wetlands to 4833.3 near E-14.5 where the inlet would be located. By excavating an inlet with an invert elevation of 4833.5, inflow to the wetlands would occur at a river discharge of approximately 13,500 cfs. At this discharge, a return drain located at cross section E-17.5 would be discharging to a river water surface elevation of 4830.0. Assuming a proposed wetlands water surface elevation of 4830.5, there would be 3.5 ft of head to insure flow through the wetlands. 34