Difference between revisions of "Rainwater harvesting: Sizing and modeling"
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*Careful catchment selection means that the runoff coefficient, for a rainstorm event (C<sub>vol, E</sub>) should be 0.9 or greater. | *Careful catchment selection means that the runoff coefficient, for a rainstorm event (C<sub>vol, E</sub>) should be 0.9 or greater. | ||
Finally, when \(0.33<Y_{0.05}/D_{0.05}<0.7\), the total storage required can be estimated by adding Y<sub>0.05</sub>: | |||
<br> | <br> | ||
<math>TotalStorage = V_{S} + Y_{0.05}</math> | <math>TotalStorage = V_{S} + Y_{0.05}</math> |
Revision as of 13:55, 17 August 2017
Simple[edit]
Five percent of the average annual yield can be estimated $$Y_{0.05} = A \times C_{vol,A}\times R_{a} \times e \times 0.05$$ where:
- Y0.05 = Five percent of the average annual yield (L)
- A = The catchment area (m2)
- Cvol, A = The annual runoff coefficient for the catchment
- Ra = The average annual rainfall depth (mm)
- e = The efficiency of the pre-storage filter
- Filter efficiency (e) can be reasonably estimated as 0.9 pending manufacturer’s information.
- In a study of three sites in Ontario, STEP found the annual Cvol, A of the rooftops to be around 0.8 [1]. This figure includes losses to evaporation, snow being blown off the roof, and a number of overflow events.
Five percent of the average annual demand can be estimated:
$$D_{0.05} = P_{d} \times n\times 18.25$$
Where:
- D0.05 = Five percent of the average annual demand (L)
- Pd = The daily demand per person (L)
- n = The number of occupants
Then the following calculations are based upon two criteria:
- A design rainfall depth is to be captured entirely by the RWH system.
- The average annual demand (D) is greater than the average annual yield (Y) from the catchment.
When \(Y_{0.05}/D_{0.05}<0.33\), the storage volume required can be estimated: Where:
- VS = Storage volume required (L)
- A = The catchment area (m2)
- Cvol,E = The design storm runoff coefficient for the catchment
- Rd = The design storm rainfall depth (mm), and
- e = The efficiency of the pre-storage filter.
- Careful catchment selection means that the runoff coefficient, for a rainstorm event (Cvol, E) should be 0.9 or greater.
Finally, when \(0.33<Y_{0.05}/D_{0.05}<0.7\), the total storage required can be estimated by adding Y0.05:
STEP Rainwater Harvesting Tool[edit]
The Sustainable Technologies Evaluation Program have produced a rainwater harvesting design and costing tool specific to Ontario. The tool is in a simple to use Excel format and is free to download.
Rainwater Harvesting Tool
STEP Treatment Train Tool[edit]
Once the size of cistern has been determined, it can easily be modeled in many open source and proprietary applications. For planning purposes, a RWH system could be integrated into a site plan using STEP's Treatment Train Tool. This tool provides a graphical user interface and simplified inputs on the EPA SWMM model. It is free to download. In a typical configuration:
- The catchment (roof) would be 100% impervious
- The rainwater harvesting system would be a 'Storage' Element with the following properties:
- Storage type = No removal
- Lined
- Underlying soil = doesn't matter, can ignore
- Evaporation factor = 0
- Suction head (mm) = 0
- Saturated conductivity (mm/hr) = 0
- Initial soil moisture deficit (fraction) = 0
- The dimensions of the rainwater cistern can be placed into the fields:
- Bottom elevation (m)
- Maximum depth (m)
- Initial water depth (m)
- The Curves table is designed to accommodate ponds of roughly conical dimensions. A rainwater cistern is usually cuboid or cylindrical in shape, so that the area (m2) will remain the same throughout the depth.