An quasi-analytic method is presented for the calculation of free-surf
ace flow over sills (e.g. darns under construction). Starting point is
the set of shallow-water equations (SWE) valid for quasi-uniform flow
, To make these equations also valid for flow over sills, adaptations
to the SWE are proposed, introducing coefficients in the convection, p
ressure and friction terms. These terms are derived from analytical an
d numerical calculations with an experimental check.