This is done by modifying the channel section geometry for both sections, upstream and downstream, from bridge or culvert structures or any other channel section. These inactive flow areas need to be eliminated from the calculations in order to get the most accurate results. Once this elevation is reached, the area becomes effective again. The velocity of the water within these ineffective areas is basically zero and does not contribute to the conveyance of that section until a certain water surface or “Trigger Elevation” is reached. So in these circumstances we need to remove the abruptness. Water prefers to flow in gradual patterns, not abrupt. This is a common occurrence at channel sections in close proximity to bridge and culvert sections with large embankment constrictions as well as channel sections with wide overbank areas. Ineffective flow areas are portions of a channel cross section that contain water but are not actively being conveyed.
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