Difference between revisions of "Rainwater harvesting: Sizing and modeling"
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[[File:Cistern Size.png|thumb|Schematic diagram of the inputs and outputs to a rainwater harvesting cistern]] | |||
===Simple=== | |||
Five percent of the average annual yield can be estimated: | |||
<br><strong>Y<sub>0.05</sub>= A × C<sub>vol, A</sub> × R<sub>a</sub> × e × 0.05</strong> | |||
<br>Y<sub>0.05</sub> = Five percent of the average annual yield (L) | |||
<br>A = The catchment area ( m<sup>2</sup>) | |||
<br>C<sub>vol, A</sub> = The annual runoff coefficient for the catchment | |||
<br>R<sub>a</sub> = The average annual rainfall depth (mm) | |||
<br>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 C<sub>vol, A</sub> of the rooftops to be around 0.8 [http://www.sustainabletechnologies.ca/wp/home/urban-runoff-green-infrastructure/low-impact-development/rainwater-harvesting/performance-evaluation-of-rainwater-harvesting-systems-toronto-ontario/]. This figure includes losses to evaporation, snow being blown off the roof, and number of overflow events. | |||
Five percent of the average annual demand (D<sub>0.05</sub>) can be estimated: | |||
<br><strong>D<sub>0.05</sub> = P<sub>d</sub> × n × 18.25</strong> | |||
<br>D<sub>0.05</sub> = Five percent of the average annual demand (L) | |||
<br>P<sub>d</sub> = The daily demand per person (L) | |||
<br>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<sub>0.05</sub>/ D<sub>0.05</sub> < 0.33, the storage volume required can be estimated: | |||
<br><strong>V<sub>S</sub> = A × C<sub>vol</sub> × R<sub>d</sub> × e</strong> | |||
<br>V<sub>S</sub> = Storage volume required (L) | |||
<br>A = The catchment area (m<sup>2</sup>) | |||
<br>C<sub>vol, E</sub> = The design storm runoff coefficient for the catchment | |||
<br>R<sub>d</sub> = The design storm rainfall depth (mm), and | |||
<br>e = The efficiency of the pre-storage filter. | |||
/*Good catchment selection means that the runoff coefficient, for a rainstorm event (C<sub>vol, E</sub>) should be 0.9 or greater. | |||
When 0.33 < Y<sub>0.05</sub>/ D<sub>0.05</sub>< 0.7, the total storage required can be estimated by adding Y<sub>0.05</sub>: | |||
<br> | |||
<strong>Total storage = V<sub>S</sub> + Y<sub>0.05</sub></strong> | |||
---- | ---- | ||
===STEP Rainwater Harvesting Tool=== | |||
[[File:RWH_tank_capacity_table.jpg| Quick reference table generated using STEP RWH tool, (data for the City of Toronto (median annual rainfall 678 mm). Optimal cistern size is that providing at least a 2.5% improvement in water savings following an increase of 1,000 Litres in storage capacity.]] | |||
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. | |||
<strong>[http://www.sustainabletechnologies.ca/wp/home/urban-runoff-green-infrastructure/low-impact-development/rainwater-harvesting/rainwater-harvesting-design-and-costing-tool/| Rainwater Harvesting Tool]</strong> | <strong>[http://www.sustainabletechnologies.ca/wp/home/urban-runoff-green-infrastructure/low-impact-development/rainwater-harvesting/rainwater-harvesting-design-and-costing-tool/| Rainwater Harvesting Tool]</strong> | ||
---- | ---- | ||
===STEP Treatment Train Tool=== | |||
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. | |||
In a typical configuration: | 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 = <em>doesn't matter, can ignore</em> | |||
**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) | |||
The dimensions of the rainwater cistern can be placed into the fields: | #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 (m<sup>2</sup>) will remain the same throughout the depth. | |||
<strong>[http://www.sustainabletechnologies.ca/wp/low-impact-development-treatment-train-tool/|The Treatment Train Tool]</strong> | <strong>[http://www.sustainabletechnologies.ca/wp/low-impact-development-treatment-train-tool/|The Treatment Train Tool]</strong> | ||
---- | ---- | ||
[[category:modeling]] | [[category:modeling]] |
Revision as of 20:54, 11 August 2017
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Simple[edit]
Five percent of the average annual yield can be estimated:
Y0.05= A × Cvol, A × Ra × e × 0.05
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 number of overflow events.
Five percent of the average annual demand (D0.05) can be estimated:
D0.05 = Pd × n × 18.25
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 Y0.05/ D0.05 < 0.33, the storage volume required can be estimated:
VS = A × Cvol × Rd × e
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.
/*Good catchment selection means that the runoff coefficient, for a rainstorm event (Cvol, E) should be 0.9 or greater.
When 0.33 < Y0.05/ D0.05< 0.7, the total storage required can be estimated by adding Y0.05:
Total storage = VS + 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. 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.