Model Study of Spillway and Stilling Basin, Harlan County Dam, Republican River, Nebraska: Fid-Bau Portal

Model Study of Spillway and Stilling Basin, Harlan County Dam, Republican River, Nebraska

The model study of the spillway and stilling basin for Harlan County Dam, proposed for construction on Republican River, Nebraska) was conducted primarily to ensure the adequacy of the structures as originally designed. Model tests verified the sufficiency of the initial design to pass all spillway flows; however, subsequent analysis by engineers of the Kansas City District, combined with the results of model tests and recommendations by the Office, Chief of Engineers, led to development of a revised design which performed satisfactorily and was more economical to construct. Although the spillway crest shape as originally designed was found to be satisfactory, a reduction in gate width, necessitating an increased number of crest piers and a corresponding increase in over-all crest width, was made to provide better structural conditions. No alterations to the original spillway section were made during the course of the model study. However in several tests the discharge per foot of crest width was adjusted to simulate flow over the slightly wider crest. The performance of the original stilling basin, incorporating a concrete apron, subdam, two rows of baffle piers, and an end sill, was very satisfactory in the dissipation of all spillway discharges. However, six alternate designs were investigated in an attempt to effect economies in construction costs. Although it was found that satisfactory stilling basin conditions would exist with a standard hydraulic-jump type basin at elevation 1850*, the rock excavation required was considered too costly. A basin design (type 6) was finally developed in which the exit channel downstream from the end sill was paved for a distance of about 105 ft, thus forming a control sill which artificially regulated the tailwater in the stilling basin and permitted the basin elevation to be raised.