Implementation of Algorithms for the Simulation of Weirs in Three Dimensional Computational Models

A model of the Sacramento-San Joaquin River Delta (The Delta) in California is in development to better understand its hydrodynamics and salinity transport, a phenomenon best described in three dimensions. The University of California, Davis (UC Davis) and Resource Management Associates (RMA) are employing the three-dimensional UnTRIM (Unstructured Tidal, Residual, Intertidal, and Mudflat) solver. The Delta has several temporary barriers that significantly affect hydrodynamics and sediment transport. These temporary barriers are best described as broad-crested weirs geometrically and hydraulically. However, weir implementation in three-dimensional models is not well documented in the literature. Only a handful of reports and papers describe methods of approach, and each is different. Furthermore, no studies compare the approaches.;Based on the options in the literature and the restrictions of the UnTRIM solver, three methods of implementing broad-crested weirs were tested: (1) raising the height of a polygon edge to the height of the actual weir; (2) stopping flow through the channel and specify a flow-value with the broad-crested weir equation; and (3) adding a friction term in the momentum equation.;A series of tests in a canonical grid were completed. These tests vary in time-step duration, spatial resolution in the horizontal and vertical directions, and boundary conditions. Each method is compared in terms of stability, computational efficiency and robustness. After several tests, the weir-equation approach (2) was found to be most robust and the least computationally efficient. The friction (3) and raised-edge (1) approaches proved much more efficient, but unstable in some conditions. Though the weir-equation method (2) is considered the most robust and stable, it suffers from instabilities for specific boundary conditions and time-step durations.