Using Numerical Modeling to Correct Flow Rates for Submerged Montana Flumes

A Parshall flume is a common device for measuring open-channel flows. Some Parshall flumes, however, have been modified or constructed without a downstream diverging section. A Parshall flume that does not have its divergent section is known as a Montana flume. Both physical modeling and computational fluid dynamics (CFD), or numerical modeling, have been utilized to assess the accuracy of and calibrate Montana flumes. Montana flumes under free-flow conditions operate similarly to Parshall flumes and, therefore, share the same rating curves. However, Montana flumes that operate under submerged conditions can deviate by as much as 15% from equations used to correct the submerged Parshall flume flow rate. The research summarized in this paper illustrates comparable results between numerical and physical modeling tests performed on a 6-in. (15.2-cm) submerged Montana flume. In addition, with the numerical model calibrated to the physical model, the same numerical procedure is used to expand testing to other sized Montana flumes under submergence. Accordingly, nine sizes of Montana flumes are numerically simulated between 6 in. (15.2 cm) and 8 ft (2.44 m). Subsequently, correction factors are created in each case for each Montana flume under submerged conditions and the application of the factors is demonstrated.