Assuming that you have you model already built, go the the RUN / STEADY FLOW / OPTIONS / FLOW OPTIMIZATIONS / click the lateral weir tab then click the YES box. This should do it. Additionally, if the only flow in the river down stream of the lateral weir is from the lateral weir you may try...
Is any one using hec ras 3.0 for flood routing through a subdivision. Need help with river junctions, splits and lateral wiers. Having trouble getting multiple rivers to run through junctions. My goal is to establish building pad heights 1 foot above water surface.