Lots of cities and states have specifications or design minima in their drainage ordinances.
Level spreaders work best on slopes flatter than 10% with flows less than 30 cfs. Level spreaders should not be constructed in fill slopes. Some agencies will let you get away with a non-erosive velocity passing over the level spreader lip. Others take a more conservative view. (Briefly, flow reconcentrates within a distance, L, of the level spreader, where L is a function of the slope. Some agencies require a flow depth over the level spreader that results in a non-erosive velocity at a distance L/3(?) downstream of the level spreader where the flow has started to reconcentrate.)
