Sewer model (Water Overlay)

Sewers allow retention of excess (rain) water which would otherwise remain or flow on the surface.

Conceptually, sewers are defined as Sewer areas. If no sewer areas are present in the project, the sewer layer will not be used in the Water Module.

Sewer areas define the areas in which the sewer system exist. The capacity of those sewers is based on the sewer area's SEWER_STORAGE attribute. The actual locations at which sewer pipes (conceptually) exist are the grid cells where sewered constructions are covered by a sewer area. The total surface area of the actual sewer is therefore dependent on the grid cell size and the sewered constructions. The total sewer storage volume is therefore determined by the sum of all grid cells marked as sewered times the grid cell size times the Sewer Storage amount.

Sewer influx

If and only if there is water on the surface, in a cell with a sewered construction, and there is a sewer present in the same location, the water can flow into the sewer. Water can flow in until the sewer is filled to its capacity. It is not possible for water to flow from a sewer back to the surface via a sewered construction, unless that construction is also a sewer overflow.

Sewer Overflow

Rain water can flow from a sewer back onto the surface via a Sewer overflow. The location of the sewer overflow marks the exit at which water flows out of the sewer. The (maximum) speed at which this water flows is determined by the SEWER_OVERFLOW_SPEED.

For sewer overflow to occur, two criteria have to be met. Firstly, the amount of water in the sewer relative to the sewer's storage capacity must exceed the SEWER_OVERFLOW_THRESHOLD. Secondly, the water level in the sewer must exceed the water level at the exit of the server overflow.

The SEWER_OVERFLOW_THRESHOLD should always be a fraction of the sewer's storage capacity.

The water level of the sewer is sum of the water height in the sewer and the height at which the sewer overflow is situated. This height is stored in the SEWER_OVERFLOW attribute of the sewer overflow construction. Conceptually, the height is the sewer is therefore directly related to the sewer overflow height.

External area

Sewer water can also be pumped to a region outside the project area. This is controlled through the sewer area's SEWER_PUMP_SPEED. Sewer water removed this way is removed from the hydrological model and cannot return back into the project area. The sewer pump speed cannot be negative; it is not allowed to pump water from outside the project area into the sewer.