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 V-notched weirs.
This option uses a broad-crested 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 broad-crested 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 broad-crested 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]
This option uses a V-notch 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 V-notch weir is assumed to have no upper limit.
The weir is defined in terms of its:
Angle (G2 - SPILL). V-notch angle, in degrees, of the notch opening in a V-notch 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 V-notch 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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