Transitional Flow Analysis at Prototype Gated Spillways in South Florida

Water control operations in South Florida often involve the estimation of transition flow at low-head gated spillways for flood control, water supply, and water quality compliance purposes. Approaches to nontransitional flow computation have been extensively investigated, and their accuracy and applicability have been rigorously verified. However, computational algorithms for transitional flows have not generally been accepted by engineering practitioners; arguments even exist with regard to the criteria used to delineate different transitional flow regions. In this study, a generalized discharge rating equation for spillways is proposed that, in contrast to the classical flow-type-separation approaches, can estimate both transitional and nontransitional flow rates. Several verification tests were performed, including a consistency analysis of the proposed equation compared to classical flow equations for different flow conditions and its verification with field and laboratory data. The performance of the proposed equation in transitional flow regions was further evaluated using computational fluid dynamics (CFD) simulations. The discharge estimates agree well with measurements and classical rating equations, with overall computation absolute relative errors of less than 8%. Finally, the inherent limitations of traditional spillway flow-type-separation approaches were analyzed, and the analysis suggested that the generalized equation is a better alternative to overcome the hard-to-define transition thresholds and resolve the discharge discontinuity issue.