Transitional Flow at Low-Head Ogee Spillway

Abstract

Low-head gated spillways provide controlled flow releases for defense against floods and a reliable supply of water for beneficial uses. In addition, a system of spillways in a large water resources system may be used for flow monitoring to assess compliance with water quality regulations and evaluate performance measures of ecological restoration projects. The flow discharge through spillways is typically calculated by rating equations applicable to free (uncontrolled) or gate-controlled flows under submerged and unsubmerged conditions. Depending on the spillway geometry, headwater and tailwater levels, and gate opening, four types of flow, each requiring a unique rating equation, can occur. However, the flow type for the given conditions must be classified first to ensure that the appropriate rating equation is used to compute discharge. Spillway flow classification requires identifying the transition thresholds between the various flow types. Results from a study conducted at the Georgia Institute of Technology to identify these transitions experimentally in a hydraulic model study of a South Florida Water Management District spillway are presented here. Results are summarized in a unified set of graphical transition curves for the four flow types in terms of the depth at the spillway crest. The depth at the spillway crest is also used to develop a head-discharge relationship for uncontrolled, submerged flow, the flow type that is the most difficult to predict.