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Flow in a vegetated swale is modelled in the USTM as uniform flow in a trapezoidal channel (although a triangular channel can be simulated by setting the base width to 0m).
Location
The location name will be displayed under the vegetated swale node icon on the main worksheet.
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All of the stormwater that approaches the swale below the user-defined Low Flow Bypass amount (in units of m3/s) will bypass the swale. Any flow above the Low Flow Bypass (up to the defined capacity of the swale) will enter and be treated by the swale.
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Storage Properties
The storage properties describe the physical characteristics of the vegetated swale and are used to determine the water depth versus discharge relationship, which is used in the hydrologic routing of stormwater runoff through the swale.
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Defines the total length of the vegetated swale in metres.
Bed Slope
Defines the longitudinal slope of the swale as a percentage.
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Defines the width of the base of the trapezoidal channel in metres.
Top Width
Defines the width of the top of the trapezoidal channel in metres.
Depth
Defines the depth of flow to the top of the channel in metres. When the stormwater flow reaches a depth that exceeds this value, flow begins to bypass the vegetated swale, and only a flow rate equal to this flow will be treated by the swale. All of the stormwater flow in excess of this flow rate will bypass the swale and will not be treated by the swale.
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Defines the height of the vegetation growing in the swale in metres. The vegetation height is used with a set of empirical relationships to determine the Manning’s n roughness of the trapezoidal channel (refer to Appendix V).
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Exfiltration from the vegetated swale 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 vegetated swale is lost from the catchment, and cannot re-enter the system downstream. Contaminants in the water that is lost to exfiltration are removed from the vegetated swale, 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.
| Soil Type | Median particle size (mm) | Saturated Hydraulic Conductivity | |
|---|---|---|---|
| (mm/hr) | (m/s) | ||
| Gravel | 2 | 36000 | 1x10-2 |
| Coarse sand | 1 | 3600 | 1x10-3 |
| Sand | 0.7 | 360 | 1x10-4 |
| Sandy loam | 0.45 | 180 | 5x10-5 |
| Sandy clay | 0.01 | 36 | 1x10-5 |
Calculated Swale Properties
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