Rainwater tanks allow for the simulation of a range of stormwater harvesting and re-use strategies, which may have benefits both for potable water conservation, and also for restoration of flow regimes towards the pre-development level.
Rainwater Tank Properties
The rainwater tank properties dialogue box contains the parameters that describe the basic physical characteristics of the rainwater tank. A description of each of the parameters is given below.
A conceptual diagram of the rainwater tank properties in music is presented below:
Conceptual diagram of rainwater tank properties.
Location
The location name will be displayed under the rainwater tank node icon on the main worksheet.
Inlet Properties
The Inlet Properties define the physical characteristics of the inlet section of the rainwater tank.
Flow is hydrologically routed through the rainwater tank, based on the characteristics defined by the user.
Low Flow Bypass
All of the stormwater that approaches the pond below the user-defined Low Flow Bypass amount (in units of m3/s) will bypass the rainwater tank. Any flow above the Low Flow Bypass (subject to the presence of a High Flow Bypass) will enter and be treated by the rainwater tank.
High Flow Bypass
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 rainwater tank. All of the stormwater flow in excess of the High Flow Bypass amount will bypass the rainwater tank and will not be treated.
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 Properties
The Storage Properties define the physical characteristics of the rainwater tank.
Volume below overflow pipe
Defines the volume in the tank, in kilolitres, below its overflow pipe.
Surface Area
Defines the surface area of the rainwater tank in m2.
Depth above overflow (metres)
Defines the depth of water above the overflow pipe, to the top of the tank (in metres). This allows an ‘extended detention depth’ of the tank to be simulated, so that, for example a ‘leaky tank’ can be simulated, to model the effect on peak flow attenuation. When the tank water level exceeds the maximum depth of the tank, it is assumed to overflow as an unlimited weir.
Outlet Properties
Overflow Pipe Diameter
Defines the diameter of the overflow pipe in millimetres.
Custom Storage-Discharge-Height Relationship
A custom pipe flow, weir flow and storage relationship can be specified to represent custom outlet and storage configurations for rainwater tanks. 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 in the'Custom Storage and Outflow'section.
Re-use properties
There is, of course, an option to re-use water from the rainwater tank, by specifying a demand:
The demand can be distributed using the following options:
To edit the user defined distribution of the annual demand, select the button. The graphical editor shown below allows the user to edit the percentage of the annual demand for each month.
To edit the monthly demand percentage, click and drag the unlocked bar graph for the required month, until the desired value is displayed. To unlock one of the monthly bar graphs, select the image adjacent to the monthly value bar graph. Alternatively to lock the monthly bar graph, select the
image adjacent to the monthly value bar graph. When editing, only the monthly values on which the unlocked
image is displayed will be adjusted.
To enter a user defined timer series, either type in the file path and file name, or click on the Browse button, to locate a previously prepared comma separated value (csv) file which contains the reuse rate (in L per timestep). The re-use file must be a CSV file with a seperate reuse value on each line. The time step for the reuse file must be the same as that used in the meteorology template file. Only a single column of data should be provided in the CSV file. A header row may also be incorporated or may be left blank.
Demand will only be met when there is available storage in the rainwater tank.
You can use the node water balance report to obtain basic information on the overall water balance at the rainwater tank, 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 rainwater tank as shown below.
The button can also be used to save data on the water re-use demand and supply at every timestep should that be required (see below).
It is possible to record flux data for the rainwater tank:
These data can be exported to a text file, for importing and analysis in other software packages:
Note that for small timesteps (e.g. 6 minute), the resulting flux file can be very large.
NOTE: The user also has the option to view a simple summary of the water balance (e.g. evapotranspiration, infiltration, overflow, etc) within a node by using the Node Water Balance item under the Statistics sub-menu; refer to Production and Interpretation of music Output. In many cases this will provide the extra detailed water balance information needed, without the need to handle the large files generated by the flux file option.
The Notes button allows you to record any important details or assumptions for the pond or sedimentation basin (for example, you may provide an explanation of how the volume was calculated, or how the notional detention time was selected). It is good practice to provide notes of any important assumptions, for future reference by others using the model.
Advanced Rainwater Tank Properties
The advanced properties section (opened using the button) of the rainwater tank displays the parameters that describe the hydraulic characteristics for the tank, and the parameters that describe the treatment process in the rainwater tank.
Orifice Discharge Coefficient
The overflow outlet from the tank is modelled as a circular orifice with an invert located at the overflow level. Although a default value of 0.6 is adopted in the model, you can set any appropriate value between 0 and 1 (more information is available from most hydraulics text books).
Number of CSTR Cells
The USTM models the treatment of each of the contaminants using a series of continuously stirred tank reactors (CSTR), each with a first order kinetic model to describe the exponential decay of the contaminant. An infinite number of CSTRs would replicate the effects of plug flow through the rainwater tank. music defaults to 2 CSTR cells for the rainwater tank, 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. To select the most appropriate value, click on the button next to the CSTR Cells entry box. A new dialog box is then opened allowing you to click on the radio button that is next to the shape which most appropriately describes the shape of the rainwater tank as shown below .
k and C* Values
The first order kinetic model adopted in the USTM is described by definition of k, the exponential decay rate constant and C*, the background concentration. The rate at which each contaminant is treated, and the background concentration for each contaminant will be different within a rainwater tank and different values should be adopted for each contaminant. Refer to Universal Stormwater Treatment Model and Selecting Appropriate k and C* Values for the USTM.
C** Values
Does not apply to rainwater tanks.
Threshold Hydraulic Loading for C**
...
The process for undertaking a life cycle costing analysis using music for rainwater tanks is the same as described in Life-Cycle Costing - Constructed Wetlands.
The origin of all of the 'expected' values and algorithms in MUSIC's costing module, as well as the statistical operations used to generate 'upper' and 'lower' estimates for rainwater tanks are explained in Table 1.
Note that the CRC for Catchment Hydrology's survey used to gather real costing information for rainwater tanks from around Australia and to develop the 'size / cost' relationships in Table 1 only involved data from small to medium sized galvanized, colorbond, zincalume and aquaplate aboveground tanks of various shapes. Costing information for polyethylene (plastic) tanks is provided in Taylor (2005b), Costing Information and from several Australian rainwater tank suppliers' web sites.
Table1. Summary of cost-related relationships for rainwater tanks.
Element of Life Cycle Costing Model | Default Option for Estimation in music | Alternative(s) | Notes |
Life cycle | 25 years. | No alternative in music. | Default value from Melbourne Water (2003) and relevant to metal, aboveground tanks. Estimates vary from 10 to 100 years depending on tank materials. It is recommended that users contact their tank suppliers to estimate the life cycle if they have a specific type of tank in mind. Upper and lower estimates have not been provided in music. |
Total acquisition cost (TAC) | TAC/kL ($2004/kL) = 1,354 x e[(-0.1589) x (Vol)] R2 = 0.63; p = <0.01; n = 35. Where: Vol = volume of tank (kL or m3); and e = 2.718. | No alternative size / cost relationships in music. For literature values, see Taylor (2005b) ' Included in Appendix H or contact tank suppliers.* NB: Appendix H includes some approximate acquisition costs for plastic tanks (i.e. 0.5 to 48 kL in size) | Warning: The default relationship should only be applied to galvanized, colorbond, zincalume and aquaplate aboveground tanks, 1 to 10 kL in size. (NB: Underground tanks are generally more expensive to purchase and have longer life cycles.). TAC includes plumbing and installation costs (i.e. to connect tanks to toilets or hot water systems) but not GST. Note that an unexpected finding from the CRCCH dataset is that the predicted TAC costs rise as the tank size moves from 1 to 6 kL in size (as expected), but then slightly falls from 6 to 10 kL (unexpected). Upper and lower estimates derived using a 68% (or 1 standard deviation) prediction interval for the regression. |
Typical annual maintenance (TAM) cost | TAM ($2004) = 90. | No alternative size / cost relationships in music. For literature values, see Taylor (2005b).* | Upper and lower estimates have not been provided in music due to limited data. This TAM estimate is based on: $75 for annual maintenance (from Kuczera and Coombes, 2001), scaled to $2004 using an annual inflation rate of 2%, plus $10 per year for operation and maintenance cost associated with pumps. Estimated using information from: Gardner (2004) and Grant and Hallmann (2003) on electricity use and cost that when combined, indicate pump running costs are typically $0.41 per kL; and Duncan (2004) who suggested a typical residential water usage rate for rainwater tanks would be approximately 20 kL per year. This TAM estimate assumes the tank is plumbed into a house (e.g. for toilet flushing) or the garden and includes a pump. |
Annualised renewal / adaptation cost (RC) | RC ($2004) = $0 | No alternative size / cost relationships in music. | Assumed to be minor and included in TAM. |
Renewal period | N/A (as RC = $0) | ||
Decommissioning cost (DC) | DC ($2004) = $200 | No alternative size / cost relationships in music. | Estimate obtained from a NSW plumber as no data was found in the literature. Given the low cost and effect of discounting in the life cycle analysis, the influence of any error associated with this estimate will be negligible. |
General caveats / notes for this type of device | * Up to date acquisition cost estimates for rainwater tanks are easily obtained from the internet or directly from tank suppliers (for a list of suppliers see www.wsud.org/wsud.htm). This industry is rapidly evolving in Australia, so obtaining current estimates is recommended. | ||