Breach flow formula (Water Overlay)

Main section: Weir formula

Flow through breaches is calculated based on the weir formula, including the consideration between free flow and submerged flow situations. However, the following also applies.

For the virtual side of the breach, the water level used is defined by the external water level of the breach. The terrain height used for the virtual side of the breach is equal to the external surface level, limiting how far the external water level can be lowered.

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

EL_new = EL_old - ( (Δw * cell) / EA )

Where:

• Δw = The water flow which takes place (out of the virtual external water source)
• cell = Cell size

• EL = EXTERNAL_WATER_LEVEL attribute of the breach.
• EA = EXTERNAL_AREA attribute of the breach.