View source for Breach flow formula (Water Overlay)

Flow through [[Breach (Water Overlay)|breach]]es is calculated based on the [[Weir formula (Water Overlay)|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 (Water Overlay)|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 [[Breach_input_area_(Water_Overlay)|input area]] or the [[External water level (Water Overlay)|external water level]]. 
* In case of an external area, the surface height used is defined by [[External surface level (Water Overlay)|external surface level]]. This height limits how far the water level can be lowered.
* In case of an [[Breach input area (Water Overlay)|input area]], the average water level is based on the [[Surface water level formula (Water Overlay)|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<sub>e,t</sub> = w<sub>e,0</sub>
|-
| otherwise  
|''w<sub>e,t</sub> = w<sub>e,t-1</sub> - ( (Δw * cell) / A<sub>e</sub> )''
|}
Where:
* Δw = The water flow which takes place (out of the virtual external water source)
* cell = Cell size
* w<sub>e,t</sub> = The [[Surface water level formula (Water Overlay)|water level]] of the entry area at time t.
* w<sub>e,0</sub> = The [[Surface water level formula (Water Overlay)|water level]] of the entry area; In case of an external area: [[External water level (Water Overlay)|EXTERNAL_WATER_LEVEL]] of the breach.
* A<sub>e</sub> = The size of in the entry area; In case of an external area: [[External area (Water Overlay)|EXTERNAL_AREA]] of the breach.

==Notes==
*The water level w<sub>e</sub> cannot become lower than the surface height s<sub>e</sub>, defined by [[External_surface_level_(Water_Overlay)|EXTERNAL_SURFACE_LEVEL]] in case of an external entry area. In case of an input area within the project, the [[terrain height (Water Overlay)|surface height]] is determined per grid cell.

==Related==
The following topics are related to this formula.
; Features
: [[Breach (Water Overlay)|Breach]]
; Formulas
: [[Weir formula (Water Overlay)|Weir formula]]
: [[Breach growth formula (Water Overlay)|Breach growth formula]]
; Models
: [[Surface model (Water Overlay)|Surface model]]

{{Template:WaterOverlay formula nav}}