Weir Flow Calculation of Spillway Outflow Limited by Volume for Monthly Time Steps
This example model illustrates the use of a weir flow calculation to generate outflows from a reservoir via a spillway. The weir flow calculation depends on the height of water above the weir crest and so changes dynamically with change in stage in the reservoir. The dynamic change may occur across a timescale of seconds to hours.
Sometimes, Reservoir elements are used to track the volume in a reservoir in a model employing relatively long duration time steps, like monthly time steps. In this case, a check based on the water volume above the weir crest can be used on the reservoir outflow, calculated using the weir equation, so that calculated spillway outflow is limited to the available outflow volume (i.e. volume of water above the spillway invert). This model provides an example of using a limiting discharge check on calculated spillway flow. This check is useful when a model employs a long "Basic Step" duration (i.e. monthly) relative to the expected time scale of weir flow evolution (i.e. seconds to minutes), and it prevents over estimation of spillway outflow from a reservoir when using a monthly "Basic Step" duration.