Displaceable covers

A manhole or basin can have a cover that is displaced by pressure. Before displacement, the cover functions like a sealed cover. After displacement, the cover is like a spilling cover in that water flowing out of the cover is assumed to exit the system.

Displacement is controlled by a pressure-fraction table that determines how much the cover will be displaced by pressure in the node. The displacement fraction, f, must be defined in the interval [0;1] where f=0 means no displacement/sealed cover, and f=1 means cover is completely removed and spilling occurs at full orifice area. In the MIKE+ GUI, the fraction is defined using a percentage scale. Displacement cannot decrease with increasing pressure.

Discharge through a displaced cover may resemble orifice or weir flow, depending on the extent of displacement. A partially displaced cover is more likely to exhibit orifice flow, while flow out of a fully displaced cover is better simulated using a weir assumption. During each time step, MIKE 1D estimates flow out of the node using both assumptions and then applies the lesser of the two estimates.

The weir flow estimate is made using an approach like that used for a spilling cover.

\[ Q_{weir} = 0.63BC_{w}\Delta H\sqrt{2g\Delta H} \]

Equation: Discharge over a spilling cover, weir formula.

where

• Q_weir — discharge estimated with weir assumption [m³/s]
• C_w — linear scaling coefficient (spill coefficient) DH = H - H_gl, where H is the water level in the node and H_gl is the ground level.

The conceptual spill width, B, is related to the displacement fraction f as follows:

\[ B = 1.5e^{3.5\Delta H}d_{f} \]

Equation: Width of the spilling cover crest, growing exponentially with head above the crest.

where the orifice diameter, d_f, is given as

\[ d_{f} = \sqrt{Af4/\pi} \]

Equation: Diameter-equivalent width factor used in the spilling cover geometry.

with A being the area of the manhole or basin when it is fully open.

The orifice flow estimate assumes that the pressure head is converted to flow out of the orifice after accounting for a contraction loss coefficient.

\[ Q_{orifice} = C_{orifice}Af\sqrt{2g\Delta H} \]

Equation: Discharge over a spilling cover, orifice formula.

where

Q_orifice = discharge estimated using orifice assumption [m³/s]

C_orifice=1 - q (contraction coefficient)

q = 0.5 (1 - Af)

For stability reasons, MIKE 1D constrains the rate at which a cover can be displaced. The parameter "Maximum manhole displacement rate" controls the rate at which the displacement fraction changes per second. The parameter can be modified in the MIKE 1D engine configuration window.