Low Impact Development Manual for Michigan - OSEH - University ...

12. Release stored water between storm events for the
necessary storage volume to be available.
13. Positive outlet for overflow should be provided a
few inches from the top of the cistern and sized
to safely discharge the appropriate design storms
when the cistern is full.
14. Rain barrels require a release mechanism in order
to drain empty between storm events. Connect
a soaker hose to slowly release stored water to a
landscaped area.
15. Observation risers should be at least six inches
above grade for buried cisterns.
16. Reuse may require pressurization. Water stored has
a pressure of 0.43 psi per foot of water elevation.
A 10-foot tank when full would have a pressure of
4.3 psi (0.43*10). Most irrigation systems require
at least 15 psi. To add pressure, a pump, pressure
tank, and fine mesh filter can be used, while this
adds to the cost of the system, it makes the system
more versatile and therefore practical.
17. Capture/reuse can also be achieved using a
subsurface storage reservoir which provides
temporary storage of stormwater runoff for reuse.
The stormwater storage reservoir may consist of
clean uniformly graded aggregate and a waterproof
liner or pre-manufactured structural stormwater
storage units.
Stormwater Functions and
Calculations
Volume reduction
In order to keep storage costs to a minimum, it makes
sense to size the storage tank so that it does not greatly
exceed the water need. Where this is done, especially
where a high-volume demand greatly exceeds runoff
(e.g., irrigation or industrial makeup water), then runoff
volume reduction for a particular storm can be assumed
to equal the total volume of storage.
Where the captured water is the sole source for a
particular operation (e.g., flushing toilets) the user does
not want the stored water to be depleted before the
next runoff event that replenishes it. In that case, the
appropriate volume to store will be relatively easy to
calculate based on the daily water need. After water
need is determined, use the table below to choose which
structure will be large enough to contain the amount of
water needed. The amount replenished by a particular
storm is equal to the volume reduction.
Additional Volume Reduction
Considerations
For storage vessels that are not drained down completely
before the next runoff event, the volume available
to be filled by a particular storm may be difficult
to calculate. Typical LID sizing criteria is based on
the volume that goes to storage during a particular
storm. That volume can be subtracted from the runoff
volume, and the designer/developer can size the storage
unit to achieve the targeted volume reduction. But
sizing criteria under these capture and reuse circumstances
may become need based. The designer/builder
may estimate the volume removal for a particular
storm, but estimates should be realistic given the use
rate and storm runoff frequency. The estimate can
be based on an average available storage capacity or
preferably on a water balance analysis based on actual
rainfall statistics.
Available Volume for Capture (gallons) = Runoff Coefficient
(unitless) x Precip (inches) x Area (SF) x 1
foot/12 inches x 7.4805 gallons/1 cubic foot
OR
V = 0.62 x C x P x A
Where
V = available volume for capture (gallons)
0.62 = unit conversion (gal/in./square foot)
C = volumetric runoff coefficient (unitless), typically
0.9 to 0.95 for impervious areas
P = precipitation amount (inches)
A = drainage area to cistern (square feet)
Sizing the tank is a mathematical exercise that balances
the available collection (roof) area, annual rainfall,
intended use of rainwater and cost. In other words,
balance what can be collected against how the rainwater
will be used and the financial and spatial costs of
storing it. In most areas of the country, it’s possible to
collect 80 percent of the rain that falls on the available
roof area. (The 20 percent reduction accounts for loss
due to mist and heavy storms that release more rain than
LID Manual for Michigan – Chapter 7 Page 153