Sewer Overflow formula (Water Overlay)

Overflow from the sewer is calculated for the entirety of the sewer, and the single attached sewer overflow.

File:Sewer overflow model.png

File:Sewer overflow model2.png

The amount that can overflow relies on both the sewer overflow itself as well as the sewer overflow threshold:

C_threshold = max( 0 , h_s - H_s * T_s )

C_height = max( 0, T_sewer + H>sub>o + h_sewer - T_overflow )

C = min ( C_threshold, C_height )

Actual overflow is then calculated:

Δw = min( 0 , C * Σ_sewer , Δt * SOS )

where:

• h_s = The height of the water column in the sewer.
• H_S = The SEWER_STORAGE attribute of the sewer area, the maximum water height allowed in the sewer.
• Δw = The amount of sewer overflow which takes place.
• Δt = Computational timestep.

• C = The amount of water that can overflow out of the sewer
• C_threshold = The amount of water that could overflow, based on the sewer overflow threshold.
• C_height = The amount of water that could overflow, based on the properties of the sewer overflow.

• Σ_sewer = The surface area of the sewer.

• T_sewer = The average height of the terrain where the sewer is present, relative to datum.
• T_overflow = The height of the terrain at the centerpoint of the sewer overflow, relative to datum.

• H_{o = The surface height of the sewer, determined by SEWER_OVERFLOW attribute of the sewer overflow.}
• SOS = The SEWER_OVERFLOW_SPEED attribute of the sewer overflow.
• SOT = The SEWER_OVERFLOW_THRESHOLD.

Notes

• 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.

See also

• Sewer