Sewer Overflow formula (Water Overlay)

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

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<subs * T_s )

C_height = max( 0, T_sewer + SO + WH_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.

• SO = The 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