Changes

[[File:Cistern Size.png|thumb|Schematic diagram of the inputs and outputs to a rainwater harvesting cistern]]

[[File:Cistern Size.png|thumb|Schematic diagram of the inputs and outputs to a rainwater harvesting cistern]]

===Simple===

===Simple===

Five percent of the average annual yield can be estimated:

Five percent of the average annual yield can be estimated:

<math>Y_{0.05} = A_{c} \times C_{vol,A}\times R_{a} \times e \times 0.05</math>

<math>Y_{0.05} = A_{c} \times C_{vol,A}\times R_{a} \times e \times 0.05</math>

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Then the following calculations are based upon two criteria:

Then the following calculations are based upon two criteria:

#A design rainfall depth is to be captured entirely by the RWH system.

# 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.  

# 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:

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>

<math>V_{S} = A_{c} \times C_{vol,E}\times R_{d} \times e</math>

==Cistern dimensions==

==Cistern dimensions==

[[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.]]  

[[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.]]  

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. Communication and understanding between design engineers is paramount in large installations. Quick notes:

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 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 may minimise this unused volume by tapering the base.   

* 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.   

[[category:modeling]]

[[category:modeling]]