D

depression storage
Natural depression on the surface of the ground that need to be filled by rainfall before runoff can take place.

design criteria
A set of standards agreed by the developer, planners and regulators that the proposed system should satisfy.

design storm
A synthetic rainfall event of a given duration and return period, which is derived by statistically analysing an historical series of rainfall events for a specific location.

detention basin
A vegetated depression which is normally dry, excepting post storm events, constructed to store water temporarily to attenuate flows. May allow infiltration of water in to the ground.

detention tanks (balancing tanks)
Tanks constructed in a sewerage system to store a volume of water temporarily during peak flows (see off-line and on-line tanks).

discharge
The volume of liquid flowing through a cross-section of conduit per unit of time.

discharge coefficient
A coefficient, derived by experiment, applied in a formula, by which the theoretical discharge of a fluid through an oriface, weir or nozzle, can be correctly calculated.

domestic (foul) wastewater
Wastewater from household services including overflows from sinks, toilets, washing machines, etc.

drain
A pipeline, usually underground, designed to carry wastewater, and/or surface water from a source to a sewer, a pipeline carrying land drainage flows or surface water from a highway.

drainage
A collection of pipes, channels and other engineering works designed to convey stormwater way from a built up environment.

dry weather flow
All flow within a drainage system excepting that caused directly by rainfall