Difference between revisions of "Rainwater harvesting: Sizing and modeling"

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<li>The design storm rainfall depth (R<sub>d</sub>, in mm)</li>
<li>The design storm rainfall depth (R<sub>d</sub>, in mm)</li>
<li>The design storm runoff coefficient for the catchment (C<sub>vol</sub>), and </li>
<li>The design storm runoff coefficient for the catchment (C<sub>vol</sub>), and </li>
<li> the efficiency of the pre-storage filter (η). </li><ul>
<li> the efficiency of the pre-storage filter (η). </li></ul>
Good catchment selection means that the runoff coefficient (β) should work out to be 0.9 or greater.  
Good catchment selection means that the runoff coefficient (β) should work out to be 0.9 or greater.  
Filter efficiency can be reasonably estimated as 0.9 pending manufacturer’s information.  
Filter efficiency can be reasonably estimated as 0.9 pending manufacturer’s information.  

Revision as of 20:33, 26 June 2017

Rapid[edit]

Five percent of the average annual yield (Y0.05, in L) is calculated as the product of:

  • The catchment area (A, in m2)
  • The runoff coefficient for the catchment (Cvol)
  • The average annual rainfall depth (R, in mm)
  • the efficiency of the pre-storage filter (η), and
  • 0.05

Five percent of the average annual demand (D0.05) is calculated as the product of:

  • The daily demand per person (Pd, in L)
  • The number of occupants (n), and
  • 0.05

The following calculation is based upon two criteria:

  1. A design rainfall depth is to be captured entirely by the RWH system.
  2. The average annual demand (D0.05) is greater than the average annual yield (Y0.05) coming from the catchment.

When Y0.05/ D0.05 <0.33
The total storage volume required (VS, in L) is calculated as the product of:

  • The catchment area (A, in m2)
  • The design storm rainfall depth (Rd, in mm)
  • The design storm runoff coefficient for the catchment (Cvol), and
  • the efficiency of the pre-storage filter (η).

Good catchment selection means that the runoff coefficient (β) should work out to be 0.9 or greater. Filter efficiency can be reasonably estimated as 0.9 pending manufacturer’s information.

When 0.33 < Y0.05/ D0.05 <0.7
The total storage required is the sum of VS and Y0.05.


Total cistern volume can be estimated by multiplying the depth of design storm the catchment area.
One millimeter of rain landing on 1 m2 results in 1 L of runoff.
For example, the 90th percentile event in Barrie is 26 mm, so every 1 m2 of rooftop will generate 26 L during this storm event.
A 2000 m2 building would generate approximately 54,000 L.
The designers have three choices:-

  1. Construct a suitably sized concrete vault underground to capture all 54,000 L of the water
  2. Alter the slope of the roof to create two or more catchments, the smaller catchments may be diverted to plastic or fiberglass cisterns
  3. Design 1. or 2. slightly undersized for this storm, with additional capacity in an infiltration system to capture overflow. Examples include bioretention cells or infiltration chambers.

<panelInfo>

</panelInfo>


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.

In a study of three sites in Ontario, STEP found the annual Cvol 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.

<panelWarning> STEP Rainwater Harvesting Tool </panelWarning>


The Treatment Train Tool[edit]

Once the size of cistern has been determined, it can easily be modelled in many open source and proprietary applications.

In addition to the cistern size, this watershed scale modelling requires input information about draw down. time i.e. the rate of use.

<btnPrimary>The Treatment Train Tool</btnPrimary>


See Also[edit]

This list will be other 'Sizing and Modelling' pages


External Links[edit]


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