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In Runoff, a weir is a control structure at the bottom of a conduit which affects the outflow characteristics of the conduit. Data must be entered for both the weir and the conduit.
Runoff mode supports two different types of weir diversions: Rectangular, and Vnotched weirs.
Rectangular Weir
This option uses a broadcrested weir control structure at the bottom of the conduit. A weir equation is used to control the outflow from the conduit instead of Manning's equation.
The weir is defined in terms of its:
Weir Length (G2  SPILL). Length of rectangular broadcrested weir (eg. width of spillway), ft [m].
Weir Height (G2  WELEV). Height of weir crest from bottom of conduit, ft [m]. The weir height may be used to store water in a conduit.
Weir Discharge Coefficient (G2  WDIS). Discharge coefficient of the weir. Note that this parameter is shared for both rectangular and triangular weir options.
Conduit Data must also be entered.
The broadcrested equation used is:
Q = C . L . (H  Hc)^1.5
where:
Q = outflow, cfs [cms]
C = weir coefficient, ft^0.5/sec [m^0.5/sec]
L = weir length, ft [m]
H = hydraulic head, ft [m]
Hc = weir crest depth, ft [m]
Vnotched Weir
This option uses a Vnotch weir control structure at the bottom of the conduit. A weir equation is used to control the outflow from the conduit instead of Manning's equation. The triangular opening of a Vnotch weir is assumed to have no upper limit.
The weir is defined in terms of its:
Angle (G2  SPILL). Vnotch angle, in degrees, of the notch opening in a Vnotch weir.
Height. Weir Crest height of the weir before flow over the weir, ft [m].
Weir Discharge Coefficient (G2  WDIS). Discharge coefficient of the weir. Note that this parameter is shared for both rectangular and triangular weir options.
Conduit Data must also be entered.
The equation used for Vnotch weirs is:
Q = C . TAN(A/2) . (H  Hc)^2.5
where
Q = outflow, cfs [cms]
C = weir coefficient, ft^0.5/sec [m^0.5/sec]
A = angle of notch (angle of opening), degrees
H = hydraulic head, ft [m]
Hc = weir crest depth, ft [m]
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