Hydraulics of side-channel weirs for regional detention basins

Side-channel weirs can be used for flood control to divert part of a channel's flow into detention basins, thereby reducing peak discharges and stages. After passage of the flood peak, the stored water can be released back to the channel. The Harris County (Texas) Flood Control District is planning the use of several side-channel weirs which will discharge into regional detention basins. The weirs are being designed to divert approximately 50% of peak flows of up to 25,000 cfs, with accompanying depths of up to 25 feet in the channels. A 1:25 scale model was built to investigate the hydraulic characteristics of embankment-shaped side-channel weirs. This thesis describes the model, its use, and the results obtained from the model study. Particular attention has been given to 1) quantification of the discharge which flows over a side-channel weir for given weir geometries and channel flow conditions, 2) the changes which occur in the channel stage along the length of the side-channel weir as water flows past and over the weir, 3) the effects of submergence on the side-channel weir discharge and channel stages, and 4) the effect of tapering the channel (i.e., progressively reducing its width) along the length of the weir. The standard equation for discharge over a normal broad-crested weir was modified slightly for side-weir applications and a bulk discharge coefficient (C[subscript e]) was used. C[subscript e] was correlated with channel flow parameters and weir geometric parameters. The side-weir discharge causes a head change along the weir in addition to the frictional head loss. This additional head change (h[subscript c]) is needed to determine the change in stage between the ends of the weir. The model results were used to calculate h[subscript c] using the energy equation; h[subscript c] was then correlated with channel flow conditions and weir geometric parameters. Submergence effects were characterized by determining the decrease in the bulk discharge coefficient and in the additional head change along the weir for varying values of submergence. Similar analyses were made to determine the side-weir discharge and stage change for weirs placed in tapered channels