Breach flow formula (Water Overlay)

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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 the breach width (as stand-in for the weir 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:

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

t=0 we,t = we,0
otherwise we,t = we,t-1 - ( (Δw * cell) / Ae )


  • Δw = The water flow which takes place (out of the virtual external water source)
  • cell = Cell size
  • we,t = The water level of the entry area at time t.
  • we,0 = The water level of the entry area; In case of an external area: EXTERNAL_WATER_LEVEL of the breach.
  • Ae = The size of in the entry area; In case of an external area: EXTERNAL_AREA of the breach.


  • The water level we cannot become lower than the surface height se, defined by EXTERNAL_SURFACE_LEVEL in case of an external entry area. In case of an input area within the project, the surface height is determined per grid cell.


The following topics are related to this formula.

Weir formula
Breach growth formula
Surface model