High early discharge (HED) pit
For on-site detention of stormwater, in the 1990s the Upper Parramatta River Catchment Trust in Sydney developed a storage arrangement that allows a control pit to fill quickly. This provides a relatively high head to make water flow out at a peak rate controlled by an orifice plate. It reduces storage requirements by releasing water at the permissible peak rate early in a storm event. A typical arrangement and the different states of operation of a HED system are shown below:
DRAINS outputs for the type of HED pit display levels in both the HED pit and the main storage, as shown below.
The high-level outflow from a HED pit is assumed to be located in the main storage, so it is possible that the HED pit level will exceed the overflow weir level, but no outflow occurs.
The hydraulics of these systems are complex, with the orifice plate, the weir and the flap valve all acting as controls for flows out of the system and between the pit and the main storage. An additional complication occurs when the orifice is submerged by high downstream water levels. In DRAINS a simplified system is used, with the following assumptions:
A small nominal storage is provided at the pit. If you have significant storage in the pit you can add it to the main storage value.
The flap valve is assumed to be the same size as the outlet pipe.
Outflows through the orifice are insensitive to height, so that you only need to insert the weir level and width in addition to the usual detention basin factors. The weir factor in the equation:
Q = C. w . hb
where C is a weir coefficient assumed to be 1.6,
w is the width (m) of the weir section over which flow occurs,
h is the depth (m),
and the exponent b is 1.5.
If you wish to use a different value of C you can modify the crest length to account for this. For example, If you wished to use a weir factor of 2, the length should be multiplied by 2/1.6 = 1.25.
In 2005, the Trust defined a new type of system that directed water to the main storage, rather than the control pit, and eliminated the short pipe with the flap valve. This type of system, shown below, can only be modelled accurately using the weir tool available with the DRAINS Full unsteady hydraulic model. Refer to the last example provided with DRAINS.
- Upper Parramatta River Catchment Trust (2005) On-Site Stormwater Detention Handbook, 4th Edition, Parramatta
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