Ponds and Sedimentation Basins are simply basins or waterbodies (eg. ponds, retarding basins) reliant on the physical settling of suspended solids as the principal treatment mechanism and can reduce peak flows through detaining a volume of the runoff and allowing for slower release during and after the event. Other mechanisms associated with phytoplankton assimilation of soluble nutrients and ultra-violet disinfection are currently not included in the modelling algorithm.
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All of the stormwater that approaches the pond below the user-defined Low Flow Bypass amount (in units of m3/s) will bypass the pond. Any flow above the Low Flow Bypass (subject to the presence of a High Flow Bypass) will enter and be treated by the pond.
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When the stormwater inflow rate exceeds the user-defined High Flow Bypass amount (in units of m3/s), only a flow rate equal to the High Flow Bypass (less that specified in any Low Flow Bypass) will enter and be treated by the pond. All of the stormwater flow in excess of the High Flow Bypass amount will bypass the pond and will not be treated.
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The Low and High Flow Bypasses are assumed to occur simultaneously. So for a Low Flow Bypass of 2m3/s, a High Flow Bypass of 8m3/s, and inflow of 10m3/s:
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Storage Properties
The Storage Properties define the physical characteristics of the main pond, detention basin or sedimentation basin.
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Defines the surface area of the pond, detention basin or sedimentation basin in m2. The hydrologic routing analysis calculates the volume of water in storage during a storm event by multiplying the depth of water above the permanent pool in the pond or sedimentation basin by this surface area.
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Defines the depth of water above the permanent pool that must be reached in the pond, detention basin or sedimentation basin before flow starts to discharge over the outflow weir. The Extended Detention Depth is defined as a depth in metres.
Permanent Pool Volume
The pond, detention basin or sedimentation basin has a permanent volume of water which does not affect the hydrologic routing of a storm event through the system. However, this permanent pool does affect the hydraulic detention time during a storm event, and so affects the treatment of contaminants that flow into the pond, detention basin or sedimentation basin. The Permanent Pool Volume is defined in m3 and is available for stormwater reuse abstraction.
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Exfiltration from the permanent pool of a pond, detention basin or sedimentation basin into the underlying soil can be modelled by defining the exfiltration rate (mm/hr). Representative exfiltration rates for different soil types are provided in the table below. The water that seeps from the permanent pool of the pond, detention basin or sedimentation basin is lost from the catchment, and cannot re-enter the system downstream. Contaminants in the water that is lost to exfiltration are removed from permanent pool, along with the exfiltrated water and are also lost from the catchment. Representative exfiltration rates for different soil types are shown in the following table.
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Defines the equivalent diameter of the outlet pipe (this may not be the diameter of the pipe itself, but the equivalent diameter of its orifices). The outlet pipe is notionally set with an invert at the standing water level of the permanent pool. The Equivalent Pipe Diameter is defined in mm. The pond, detention basin or sedimentation basin dialogue box will display the notional detention time, based on the pond or basin volume, and the Equivalent Pipe Diameter.
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Defines the width of the overflow weir for the pond, detention basin or sedimentatin basin. The overflow weir will only start to carry a discharge of water once the depth in the pond reaches the Extended Detention Depth defined above. The Overflow Weir Width is defined as a length in metres.
Custom Storage-Discharge-Height Relationship
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Note that for a sedimentation basin, the default values for k, C* and C** for each of the constituents are as shown in the diagram above.
Threshold Hydraulic Loading for C**
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