Sewer Overflow formula (Water Overlay)

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

The sewer base height is located at the height indicated by the sewer overflow:

B_s = B_so

The sewer overflow height is calculated as:

H_o = H_s * T_o

The water level at the sewer is then:

w_s = h_s + B_s

Water only flows out of the sewer iff:

h_s &gt H_o and

h_s + B_s &gt w_o

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

The flow is then calculated with the SEWER_OVERFLOW_SPEED attribute of the sewer overflow.

Δh = min ( h_s - H_s , w_s - w_o)

Δw = min( max( 0 , h_s ), Δt * v_o

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.

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.