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- Traditional unvegetated infiltration systems (e.g soakaways, rubble trenches, french drains etc); use the Infiltration node.
- Vegetated infiltration systems: In MUSIC v5 and the versions afterwards, these are now to be modelled using the Bioretention node, selecting appropriate parameters such as no collection pipe and unlined base, because this will take into account the effect of vegetation on both water quality and also water losses due to evapotranspiration.
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MUSIC models the infiltration system by taking into account infiltration through both the base and sides of the system (unlike MUSIC v3 which assumed that infiltration occurred only through the base). It is important to note that there is no option to include lining of the base of the infiltration system. However, you can take into account lined walls, by subtracting them from the specified Infiltration Media Perimeter specified in MUSIC.Treatment in the infiltration system is assumed to occur only in the ponding (extended detention) zone; no treatment is modelled in the infiltration media. Whilst this is not strictly true, it must be remembered that MUSIC ‘loses’ water that is infiltrated and so there is no benefit in modelling the treatment performance further.
A conceptual diagram of the infiltration system properties in MUSIC is presented below:
Conceptual diagram of infiltration system properties.
Infiltration System Properties
The infiltration system properties dialogue box contains the parameters that describe the basic physical characteristics of the infiltration system. A description of each of the parameters is given below.
A conceptual diagram of the infiltration system properties in MUSIC is presented below:
Conceptual diagram of infiltration system properties.
Location
The location name will be displayed under the infiltration system node icon on the main worksheet.
Inlet Properties
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All of the stormwater that approaches the infiltration system below the user-defined Low Flow Bypass amount (in units of m3/s) will bypass the system. Any flow above the Low Flow Bypass (subject to the presence of a High Flow Bypass) will enter and be treated by the infiltration system.
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Tip Box
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:
Storage
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Properties
The Storage and Infiltration Storage Properties define the physical characteristics of the infiltration system.
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Defines the surface area of the infiltration system in m2. The hydrologic routing analysis calculates the volume of water in storage during a storm event by multiplying the depth of water below the overflow weir by this surface area.
Extended Detention Depth
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Defines the maximum depth of water ponding above the infiltration media before flow starts to discharge over the outflow weir. The Extended Detention Depth is defined as a depth in metres.
Filter Area
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Evaporative Loss
Evaporation from the infiltration system basin can be modelled by defining the evaporative loss rate, defined as a percentage of the daily Potential Evapotranspiration data contained in the Meteorological Template used to create the model. The water that evaporates is lost from the catchment. Contaminants in the water that is lost to evaporation remain within the infiltration system, until they are infiltrated, or overflow, whichever occurs first.
Infiltration Properties
The Infiltration Properties define the filtration characteristics of the infiltration system.
Unlined Filter Media Perimeter
Defines the perimeter of the infiltration media, in metres. For example, for a rectangular system, the perimeter would be the 2 • length + width. The perimeter is necessary, because MUSIC takes into account the infiltration that will occur through the sides of the system.
NOTE: MUSIC does not explicitly allow lining to the base or walls of an infiltration system. If you wish to include lining, then you can either (a) take the lined part into account when calculating the Perimeter of the Infiltration Media (ie. subtract the lined component) or (ii) use the Bioretention node, as it allows a complex system with part of the infiltration area lined.
Filter Area
Defines the surface area of the infiltration media. Where the infiltration system’s extended detention zone has non-vertical sides, the infiltration area (filter area) will normally be smaller than the pond area above it.
Depth of Infiltration Media
Defines the depth of the infiltration media in metres. The depth is from the base of the infiltration system to the top surface of infiltration media.
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Infiltration into the underlying soil is given by the exfiltration rate (mm/hr). Representative exfiltration rates for different soil types are provided in the table below. The water that exfiltrates from the infiltration system is lost to the treatment train, and does not re-enter the system downstream. Contaminants in the water that is lost to infiltration are also removed, along with the infiltrated water and are also lost from the catchment. Representative infiltration rates for different soil types are shown in the following table. NOTE that in MUSIC 4 and above, exfiltration from the ponding zone of the infiltration system is also taken into account.
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Soil Type | Median Particle Size (mm) | Saturated Hydraulic Conductivity | |
(mm/hr) | (m/s) | ||
Gravel | 2 | 36000 | 1 x 10-2 |
Coarse sand | 1 | 3600 | 1 x 10-3 |
Sand | 0.7 | 360 | 1 x 10-4 |
Sandy loam | 0.45 | 180 | 5 x 10-5 |
Sandy clay | 0.01 | 36 | 1 x 10-5 |
In theory, the infiltration rate specified in MUSIC should be the rate which limits its performance. In most cases, this will be the infiltration rate of the surrounding soils, although in some cases (e.g. in areas with very sandy soils) the limiting rate could be that of the filter media itself.
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Evaporation from the infiltration system basin can be modelled by defining the evaporative loss rate, defined as a percentage of the daily Potential Evapotranspiration data contained in the Meteorological Template used to create the model. The water that evaporates is lost from the catchment. Contaminants in the water that is lost to evaporation remain within the infiltration system, until they are infiltrated, or overflow, whichever occurs first.
Outlet Properties
Overflow Weir Width
Defines the width of the overflow weir for the infiltration system. The Overflow Weir Width is defined as a length in metres.
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You can use the node water balance report to obtain basic information on the overall water balance at the infiltration system, including basic information on reuse at the node. To do this, select node water balance from the list of available reporting boxes by right clicking on the node.
A reporting window will then be presented showing the water balances for various inflows and outflows at the infiltration system as shown below.
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Defines the porosity (voids ratio) of the infiltration media. The following general values are recommended:
Media Type | Typical Porosity |
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Loamy sand | 0.35-0.4 |
Sandy loam | 0.35-0.4 |
Sand | 0.3-0.4 |
Gravel | 0.3-0.4 |
Scoria | 0.5-0.6 |
The porosity affects the performance of the infiltration system by determining the volume of water which is necessary to “fill” the filter media, thus potentially creating an overflow event.
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