Issues with Culvert Placed flat designs
Placement: you must get the culvert installed at 0.5% gradient or less which requires precision in installation. A precision that has not been common to other forest applications.
Outlet drop - If there is any initial outlet drop and sediment does not move through the culvert and there is momentum carrying through the culvert an outlet drop can be enlarged.
Depth – Because no bedforms need to backwater into culvert - At low flows the culvert will become shallow - too shallow for fish passage
Filling - If forced in flat on too steep of streams the culvert may fill in. Guidelines call for installations on slopes less than 2.5%.