A wetland is a treatment measure that is generally used for the removal of to remove fine suspended solids and associated contaminants, as well as soluble contaminants. They can also provide significant storage for a range of storm events.
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:
Inlet Properties
The Inlet Properties define the physical characteristics of the inlet pond of the wetland system. It is assumed in the model configuration for a wetland that the macrophyte cell is preceded by a separate inlet pond. Flow is hydrologically routed through the wetland based on the characteristics defined by the user.
Low Flow Bypass
All of the stormwater that approaches the wetland below the user-defined Low Flow Bypass amount will bypass the wetland. Any flow above the Low Flow Bypass (subject to the presence of a High Flow Bypass) will enter and be treated by the wetland.
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When the stormwater inflow rate exceeds the user-defined High Flow Bypass amount, only a flow rate equal to the High Bypass Flow (less that than 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 Music models the performance of these pre-treatment functions using the inlet pond volume, assumed to be a permanent pool and defined, as the principal parameter. Estimate inlet pond volume feature has not been offered at this stage
The button
allows you to specify various parameters for the inlet volume. This is described in more details further below.
Storage Properties
The Storage Properties define the physical characteristics of the macrophyte cell of the wetland system.
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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 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.
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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.
Estimate Inlet Volume
There is an option to set up the initial pond volume of the wetland as shown in figure below:
Clicking will open the Estimate Inlet Volume dialog box as below:
Input Parameters
This defines the input parameters of the inlet.
Particle size
The average diameter of the targeted sediments. Units are in microns (1e-6m).
Settling velocity
Velocity of the particle when settling, during the sedimentation process. In other words, the velocity of the particle into water under gravity and Archimedes constraints.
Contributing Catchment Area
Area of the catchment contributing to pollutant generation.
Design Flow
The ideal flow that can be handled by the wetland.
Sediment Loading Rate
Volume of sediments expected to be received (in ha/yr).
Capture Efficiency
The proportion of total volume entering the basin that is targeted to settle.
Clean Out Frequency
The frequency of wetland maintenance (in years). In other words, how often is the basin emptied and cleaned out.
Turbulence
This is the number of CSTR cells.
Detention Depth
The depth from the top of the permanent pool to the top of the overflow weir. The inlet pond of a wetland has a detention depth of 0.
Estimates
The required area (A) of a sedimentation basin should be defined using the equation shown below:
where
R fraction of target sediment removed
Vs settling velocity of target sediment (see the table below)
Q/A applied flow rate divided by basin surface area
n turbulence or short-circuiting parameter
de extended detention depth (m) above permanent pool level
dp depth (m) of the permanent pool
d* depth below the permanent pool level that is sufficient to retain the target sediment (m) - adopt 1.0m or dp, which is lower
The table below lists the typical settling velocities (Vs) of sediments under 'ideal conditions' (velocity in standing water)
(Source: Maryland Dept. of Environment 1987 in Engineers Australia 2003)
Classification of particle size | Particle diameter (µm) | Settling velocities (mm/s) |
Very coarse sand | 2000 | 200 |
Coarse sand | 1000 | 100 |
Medium sand | 500 | 53 |
Fine sand | 250 | 26 |
Very fine sand | 125 | 11 |
Coarse silt | 62 | 2.3 |
Medium silt | 31 | 0.66 |
Fine silt | 16 | 0.18 |
Very fine silt | 8 | 0.04 |
Clay | 4 | 0.011 |
Surface Area Requirement
Surface area required by the previously fixed depth.
Sediment Storage Volume Requirement
The required volume of sediment storage (S) can be estimated using the Equation below:
S = Ca x R x Lo x Fr
Where, S = volume of storage required (m3)
Ca = contributing catchment area (ha)
R = capture efficiency (%)
Lo = sediment loading rate (m3/ha/year)
Fr = desired cleanout frequency (years)
Permanent Pool Volume Requirement
Twice the sediment storage volume.
Re-use
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 tab.
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The advanced properties section (opened using the Advanced Tab under Wetland) displays the parameters that describe the hydraulic characteristics for the outflow structures, and the parameters that describe the treatment process in the wetland.
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Orifice Discharge Coefficient
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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 sharp broad crested weir whose discharge equation is given by:
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