Hydraulic Structures - Rating Table Discharge vs. Available Discharge
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What does FLO-2D do if the rating table for a hydraulic structure (e.g., culvert) indicates a structure discharge which is greater than the availabe incoming supply of flow from grids surrounding the inlet node? For example, the structure rating table specifies 100 cfs flow in the structure @ 4 foot depth at the inlet node, but the available inflow that corresponds to a depth of 4 feet at the inlet node is only 50 cfs.
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This situation would most likely cause an instability problem because as the structure tries to take 100 cfs from an inflow of 50 cfs the water surface at the inlet would go way down. Then the water surface in the structure would be higher and so it would cause a backwater effect and the system would oscillate back and forth causing numerical instability. You can see this with wildly oscillating hydrographs that occur in regions of channels where the channel cross sectional area varies significantly.
There are instances where box culverts are wider than the channel and the channel will get wide and then narrow again in the culvert transition zone. In this case, it would be necessary to take the cross section upstream and downstream of that transitional area in order to make the rating table. This would help ensure that the rating table matched the physical system.
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