Little Sioux Control Structure, Little Sioux River, Iowa; Hydraulic Model Investigation

摘要

Since the construction of the original control structure, the channel has degraded 11 ft and flows exceeding 10,000 cfs have occurred regularly. Flows exceeding the berm height scoured the side slopes causing the riprap to fail, and convergence of the concentrated flows from the right and left bank berm sections caused the development of a severe scour hole downstream of the stilling basin. High flows during the spring of 1983 caused the structure to fail so another model investigation was necessary to develop a design for the replacement structure and to determine methods to stabilize the area downstream of the structure and the channel side slopes. Tests on a 1:25-scale hydraulic model of the replacements structure were conducted to develop the design. The model reproduced about 650 ft of topography upstream from the structure, the control structure, and 1,150 ft of topography downstream from the structure. Modifications to the original design were made to produce a structure that provided an acceptable headwater rating curve, and one with adequate energy dissipation in the stilling basin. A notched weir was developed that provided an acceptable headwater rating curve, and one with adequate energy dissipation in the stilling basin. A notched weir was developed that provided a desired range of headwater elevations for the expected discharges. The weir also produced velocities upstream and downstream from the low-flow notch for discharges less than 1,000 cfs that were considered appropriate for upstream fish migration. Stable riprap designs were determined for the channel bottom downstream from the stilling basin and the channel side slopes.