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In music, the USTM uses a conceptual model of a wetland system that includes two cells; an inlet pond followed by the main wetland or macrophyte cell. Stormwater can bypass the whole system, when it falls below ("Low Flow Bypass") or exceeds ("High Flow Bypass") the predefined flow rates. The macrophyte cell has a permanent volume of water, with a low flow outlet pipe, whose invert is notionally set at the standing water level of this permanent pool. An overflow weir is located at an elevation equal to the extended detention depth above the standing water level of the permanent pool. A conceptual diagram of the wetland properties in music is presented below.
Conceptual diagram of wetland properties.
Wetland Properties
The initial wetland properties dialogue box contains the parameters that describe the basic physical characteristics of the wetland system:
Location
The location name (for example: "Queens Road Wetland") will be displayed under the wetland node icon on the main worksheet.
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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 Bypass Flow (less that specified in any Low Flow Bypass) will enter and be treated by the wetland. All of the stormwater flow in excess of the High Flow Bypass amount will bypass the wetland and will not be treated by the wetland.
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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: |
Inlet Pond Volume
The inlet pond volume in wetland systems provides a pre-treatment of the stormwater prior to its discharge into the macrophyte cell of the wetland. Pre-treatment of stormwater discharging into the wetland includes sedimentation of coarse to medium-sized particles and hydrologic control by facilitating a by-pass of flow around the macrophyte zone when the water level in the macrophyte zone has reached its maximum extended detention depth. music models the performance of these pre-treatment functions using the inlet pond volume, assumed to be a permanent pool and defined in m3, as the principal parameter.
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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 wetland dialogue box will display the notional detention time, based on the wetland volume, and the Equivalent Pipe Diameter.
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Use the Notional Detention Time display to check whether the designed wetland properties (volumes and Equivalent Pipe Diameter) give approximately the required detention time. For more information, refer to Wong et al. (1998). |
Overflow Weir Width
Defines the width of the overflow weir for the main wetland pond. 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.
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A custom pipe flow, weir flow and storage relationship can be specified to represent custom outlet and storage configurations for wetland nodes. The outflow relationships can either replace or add to the standard music outflows. More information on how to use the custom outflow and storage facility is available here.
There is an option to set up the initial volume of the wetland. Clicking this button will open the Estimate Storage Properties dialog box:
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Volume requirement according to sediment loading (which is the Contributing Catchment Area*Capture Efficiency*Sediment Loading Rate*Clean Out Frequencyefficiency load*flow*contributing area).
Permanent Pool Volume Requirement
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There is an option to re-use water from the wetland, by specifying a demand. Refer to Water Re-use from Treatment Nodes for more information about the Re-use button.
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- inflow rate and water quality
- outflow rate and water quality
- low and high flow bypass rate and water quality
- overflow rate and water quality
- total outflow rate (sum of outflow, bypasses and overflow) and water quality
- computed water levels and storage
- water re-use demand and actual volume supplied.
Refer to Treatment Devices for more information about fluxes.
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The overflow weir carries a discharge when the water level in the wetland exceeds the Extended Detention Depth. The overflow weir is modelled as a broad sharp crested weir whose discharge equation is given by:
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For details about CSTR cells, refer to Treatment Devices. For wetlands, an infinite number of CSTRs would replicate the effects of plug flow through the wetland system. music defaults to 4 CSTR cells for constructed wetlands, however, as the shape of the system can vary markedly dependent on design, the number of CSTR cells that is required to represent the hydraulic efficiency of the design is dependent on that shape. Also note parameter values, which are specific to wetlands:
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