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===
===Rapid===
The following approximations 
<p>Total cistern volume can be estimated by multiplying the depth of design storm the catchment area. <br>One millimeter of rain landing on 1 m<sup>2</sup> results in 1 L of runoff. <br>
Five percent of the average annual yield can be estimated:
For example, the 90th percentile event in Barrie is 26 mm, so every 1 m<sup>2</sup> of rooftop will generate 26 L during this storm event and a 2000 m<sup>2</sup> building generates 54,000 L of runoff. <br>
<math>Y_{0.05} = A_{c} \times C_{vol,A}\times R_{a} \times e \times 0.05</math>
The designers have three choices:-
{{plainlist|Where:
<ol>
*''Y<sub>0.05</sub>'' is five percent of the average annual yield (L)
<li>Construct a suitably sized concrete vault underground to capture all of the water</li>
*''A<sub>c</sub>'' is the catchment area (m<sup>2</sup>)
<li>Alter the slope of the roof to create two or more catchments, the smaller catchments may be diverted to plastic or fiberglass cisterns</li>
*''C<sub>vol, A</sub>'' is the annual runoff coefficient for the catchment
<li>Design 1. or 2. Slightly undersized for this storm, but with additional capacity in an infiltration system to capture overflow. Examples include [[Bioretention Cells|bioretention cells]] or [[Infiltration Chambers|infiltration chambers]]. </li> 
*''R<sub>a</sub>'' is the average annual rainfall depth (mm)
</ol>
*''e'' is the efficiency of the pre-storage filter}}
</p>
*Filter efficiency (''e'') can be reasonably estimated as 0.9 pending manufacturer’s information.<br>
 
*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 a number of overflow events.
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<gallery mode="packed" widths=300px heights=300px>
Cistern Size.png| Schematic diagram of the inputs and outputs to a rainwater harvesting cistern
</gallery>
</panelInfo>
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----
----
 
Five percent of the average annual demand can be estimated:
===STEP Rainwater Harvesting Tool===
<math>D_{0.05} = P_{d} \times n\times 18.25</math>
</div>
{{plainlist|Where:
<div class="col-md-8">
*''D<sub>0.05</sub>'' is five percent of the average annual demand (L)
<p>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.</p>
*''P<sub>d</sub>'' is the daily demand per person (L)
</div>
*''n'' is the number of occupants}}
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<panelWarning>
[[File:Connect the Drops.PNG|200 px|STEP Rainwater Harvesting Tool|link=http://www.sustainabletechnologies.ca/wp/home/urban-runoff-green-infrastructure/low-impact-development/rainwater-harvesting/rainwater-harvesting-design-and-costing-tool/]]
</panelWarning>
</div>
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----
----
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:
<math>V_{S} = A_{c} \times C_{vol,E}\times R_{d} \times e</math>
{{plainlist|Where:
*''V<sub>S</sub>'' is the volume of storage required (L)
*''A<sub>c</sub>'' is the catchment area (m<sup>2</sup>)
*''C<sub>vol,E</sub>'' is the design storm runoff coefficient for the catchment
*''R<sub>d</sub>'' is the design storm rainfall depth (mm), and
*''e'' is the efficiency of the pre-storage filter.}}


===The Treatment Train Tool===
*Careful catchment selection means that the runoff coefficient, for an individual rainstorm event (''C<sub>vol, E</sub>'') should be 0.9 or greater.
</div>
----       
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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>'':
<p> Once the size of cistern has been determined, it can easily be modelled in many open source and proprietary applications. </p>
<math>TotalStorage = V_{S} + Y_{0.05}</math>
<p> In addition to the cistern size, this watershed scale modelling requires input information about draw down. time i.e. the rate of use. </p>
</div>
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<btnPrimary>The Treatment Train Tool</btnPrimary>
</div>
<div class="col-md-12">
----
----
==STEP Rainwater Harvesting Tool==
[[File:RWH_tank_capacity_table.jpg|thumb|500 px|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.<br>
{{clickable button|[[File:Connect the Drops.PNG|200 px|link=http://www.sustainabletechnologies.ca/wp/home/urban-runoff-green-infrastructure/low-impact-development/rainwater-harvesting/rainwater-harvesting-design-and-costing-tool/]]}}


===See Also===
==STEP Treatment Train Tool==
This list will be other 'Sizing and Modelling' pages
{{Clickable button|[[File:TTT.png|300 px|link=http://www.sustainabletechnologies.ca/wp/low-impact-development-treatment-train-tool/]]}}<br>
 
'''[[Rainwater harvesting: TTT]]'''
----


===External Links===
==Cistern dimensions==
<ul>
[[File:Cistern dimensions.jpg|thumb|The blue area indicates the only usable volume in this rainwater harvesting cistern, the depth between the pump inlet and the overflow.]]
<li>[https://www.epa.gov/water-research/storm-water-management-model-swmm SWMM5.1] </li>
The connections into and out of a rainwater cistern can have a dramatic effect on the actual usable volume. The only usable water within the cistern is that above the height of the pump intake, and below the invert of the overflow outlet. This dimension can easily become constrained where the outlet must lie beneath the frost line and where a high powered pump is required to elevate the water many storeys.  
</ul>
* The depth of storage between the elevation of the inlet and overflow is unusable space, so the overflow should be located towards the top of the cistern.
----
* The depth of storage beneath the pump inlet is unusable, but may also be a useful zone for sediment to settle. A custom or cast-in-place vault could minimise this unused volume by tapering towards the the base. 
<table class="table table-hover table-condensed table-bordered">
[[category:modeling]]
<td class="text-center"><i class="fa fa-envelope-open-o"></i>&nbsp;&nbsp;[[Special:SpecialContact|<strong>SEND US YOUR QUESTIONS & FEEDBACK ABOUT THIS PAGE</strong>]]</td>
</table>

Latest revision as of 20:52, 20 February 2018

Schematic diagram of the inputs and outputs to a rainwater harvesting cistern

Simple[edit]

The following approximations Five percent of the average annual yield can be estimated:

Where:

  • Y0.05 is five percent of the average annual yield (L)
  • Ac is the catchment area (m2)
  • Cvol, A is the annual runoff coefficient for the catchment
  • Ra is the average annual rainfall depth (mm)
  • e is 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:

Where:

  • D0.05 is five percent of the average annual demand (L)
  • Pd is the daily demand per person (L)
  • n is the number of occupants

Then the following calculations are based upon two criteria:

  1. A design rainfall depth is to be captured entirely by the RWH system.
  2. 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 is the volume of storage required (L)
  • Ac is the catchment area (m2)
  • Cvol,E is the design storm runoff coefficient for the catchment
  • Rd is the design storm rainfall depth (mm), and
  • e is the efficiency of the pre-storage filter.
  • Careful catchment selection means that the runoff coefficient, for an individual 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]

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.
Connect the Drops.PNG

STEP Treatment Train Tool[edit]

TTT.png
Rainwater harvesting: TTT

Cistern dimensions[edit]

The blue area indicates the only usable volume in this rainwater harvesting cistern, the depth between the pump inlet and the overflow.

The connections into and out of a rainwater cistern can have a dramatic effect on the actual usable volume. The only usable water within the cistern is that above the height of the pump intake, and below the invert of the overflow outlet. This dimension can easily become constrained where the outlet must lie beneath the frost line and where a high powered pump is required to elevate the water many storeys.

  • The depth of storage between the elevation of the inlet and overflow is unusable space, so the overflow should be located towards the top of the cistern.
  • The depth of storage beneath the pump inlet is unusable, but may also be a useful zone for sediment to settle. A custom or cast-in-place vault could minimise this unused volume by tapering towards the the base.