Sewer Overflow formula (Water Overlay)
The sewer base height is located at the height indicated by the sewer overflow:
- Bs = Bo
The water level at the sewer is calculated based on that height:
- ws = hs + Bs
The sewer overflow threshold height Ho within the sewer is calculated as:
- Ho = ( Hs * To ) + max ( 0 , Bo - Ba )
Water only flows out of the sewer if:
- hs > Ho and
- hs + Bs > wo
The amount that can overflow relies on both the water level at the sewer overflow itself as well as the sewer overflow threshold and the stored sewer water:
- Δh = min ( hs - Ho , ws - wo)
The flow is then calculated with the SEWER_OVERFLOW_SPEED attribute of the sewer overflow and the maximum speed with which water can flow through the sewer overflow:
- Δw = min( Δh, Δt * qo / As)
The amount of water added to the exit cell(s) of the sewer overflow:
- Qt = Δw * As
- Bo = The surface height at the centerpoint of the sewer overflow, relative to datum.
- Ba = The height of the sewer relative to datum, as defined by the SEWER_OVERFLOW of the sewer overflow.
- Bs = The assumed surface height of the sewer, relative to datum.
- wo = The water level at the sewer overflow's exit, relative to datum.
- ws = The water level in the sewer, relative to datum.
- Ho = The height of the sewer overflow threshold in the sewer.
- hs = The sewer water height.
- Hs = The SEWER_STORAGE attribute of the sewer area, the maximum water height allowed in the sewer.
- Δh = The amount of water that can flow out of sewer based on the threshold and the water levels inside and outside the sewer.
- Δw = Water level change in the sewer, due to sewer overflow taking place.
- Δt = Computational timestep.
- As = The surface area of the sewer.
- qo = The SEWER_OVERFLOW_SPEED attribute of the sewer overflow.
- To = The SEWER_OVERFLOW_THRESHOLD, the threshold factor indicating the fraction of the sewer's storage height that is blocked by the sewer overflow's threshold.
- Qt = The amount of water which flows out of the sewer through the sewer overflow.
- The sewer's surface area is computed as the amount of sewered grid cells times the size of a grid cell.
- The sewer's height relative to datum is not defined in the sewer itself. It is relative to the sewer overflow and determined by the SEWER_OVERFLOW attribute of the overflow.
The following topics are related to this formula.