# 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 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:

- The water level used is defined by either an input 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 input area, the average water level is based on the water level 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}= The water level of the entry area at time t. - w
_{e,0}= The water level of the entry area; In case of an external area: EXTERNAL_WATER_LEVEL of the breach. - A
_{e}= The size of in the entry area; In case of an external area: EXTERNAL_AREA of the breach.

## Notes

- The water level w
_{e}cannot become lower than the surface height s_{e}, 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.

## Related

The following topics are related to this formula.

- Features
- Breach
- Formulas
- Weir formula
- Breach growth formula
- Models
- Surface model