Oxygen Transfer at Low Drop Weirs

Oxygen transfer into water at low-drop weirs was studied by using a prototype-scale, recirculating hydraulic flume with a weir width of 0.61 m, and a downstream channel width of 1.22 m. An enhanced-oxygen environment was used to maintain sufficient difference between saturation and upstream dissolved oxygen concentrations to reduce the experimental uncertainty. The variation in the deficit ratio r as a function of tailwater depth H was evaluated by using unit discharges q of 167, 334, and 502 m3/hr· m and weir heights W of 0.54, 1.04, and 1.36 m. A semitheoretical equation for predicting r as a function of jet Froude number FJ, drop height h, and H was fitted to observed values by nonlinear regression analysis. An assumed value of 0.667 for maximum H/h (found in the literature) proved acceptable. We concluded that tailwater depths are important to predict oxygen transfer at low-drop weirs, but that designing for optimum tailwater depth to maximize oxygen transfer may not be as important as was previously thought, because large optimum ranges for H/h were observed in this study.