Breach flow formula (Water Overlay)

Flow through breaches is calculated based on the Weir formula, including the consideration between free flow and submerged flow situations. The weir formula is supplied with Breach width, which is always calculated with the Breach growth formula before calculating the weir flow.

For the entry area, the area where the water originates from, the following also applies:
The water level used is defined by either an internal area or the external water level. In case of an external area, the surface height used is defined by external surface level. This height limits how far the water level can be lowered. In case of an internal area, the average water level is determined based on the individual grid cells of the input area.

Each timestep, the external water level is changed based on the amount of water flowing in or out.

t=0 w_e,t = w_e,0

otherwise w_e,t = w_e,t-1 - ( (Δw * cell) / A_e )

Where:

• Δw = The water flow which takes place (out of the virtual external water source)
• cell = Cell size
• w_e,t = water level of entry area at time t.
• w_e,0 = EXTERNAL_WATER_LEVEL attribute of the breach.
• A_e = EXTERNAL_AREA attribute of the breach.

Notes

• The water level w_e cannot become lower than the surface height s_e, defined by SURFACE_LEVEL.

See also

• Breach