This is an example SWMM 5 model that can be imported into InfoSWMM or H2OMap SWMM using the Exchange/Import Command.   The low flow falls over the berm of the leaping weir into a rectangular open channel but the the "falls" is governed by an OULET Depth/Discharge Type in SWMM 5.  The flow increases in the OUTLET until a depth of 1 feet is reached where the weir starts to operate.  The OUTLET increases in flow from zero to 1 feet but still flows at a reduced rate when the weir starts to operate.  The weir stops flowing when the depth goes below 1 foot on the berm.

SWMM 5 Input File Link
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Leaping Weir With Low Flow Depth/Discharge OUTLET