When the water depth at the weir junction exceeds the surcharge level (RL 1) the weir functions as an orifice i.e. the exponent is 0.5. The discharge coefficient for the orifice flow conditions is computed internally in the Hydraulics layer. An equivalent pipe automatically replaces the weir for the duration of surcharge.
Stability problems can be encountered at weir junctions. If this happens or is suspected of happening, the weir may be represented as an equivalent pipe. To do this, equate the pipe and weir discharge equations, e.g.,
(m/n)AR(2/3)S(l/2) = CwWH(3/2)
where m = 1.486 for units of feet and 1.0 for units of metres.
n = Manning's n for the pipe
A = Cross-sectional area
R = Hydraulic radius
S = Hydraulic grade line for the pipe
H = Head across the weir
Cw = Weir discharge coefficient
W = Weir length