Civil Engineering Reference
In-Depth Information
a floor drain or the use of a pump and collection tank during the
cleaning process.
•
A rainfall harvesting system consists of the following basic compo-
nents:
— Catchment surface, typically the roof of the building.
— Collection system...the gutters and downspouts that collect and
transfer the rainwater to a storage tank.
— Leaf screens, first-flush diverters, and/or roof washers that re-
move dust and debris from the initial catchment runoff. First-
flush diverters are designed to divert or waste a portion of the
initial rainfall to eliminate contaminants that were on the catch-
ment surface when the rainfall started.
— Storage tanks, called “cisterns.”
— Filtration, to make the water potable, if required by the applica-
tion. Given the local regulatory requirements, either cartridge
filtration or RO may be required. (No filtration is required if the
water is not used for human consumption.)
In theory, about 0.62 gallons/sf of catchment area can be collect-
ed per inch of rainfall. However, considering first flush losses,
evaporation, splash, etc., a value of 0.50 gallons/sf of catchment
area per inch of rainfall is typically used for system sizing. The
catchment area is based on the “horizontal projection” of the
building roof and is independent of roof pitch. To ensure a year-
round water supply, the catchment area and storage capacity
must be sized to meet the water demand through the longest
expected span of continuous dry days.
The capital cost of rainwater harvesting systems is highly
dependent on the type of catchment, conveyance, and storage
tank materials used.
The primary reference for the design of rainwater harvesting
systems is The Texas Manual on Rainwater Harvesting, avail-
able as a free download at http://www.twdb.state.tx.us/publi-
cations/reports/RainwaterHarvestingManual_3rdedition.pdf.
Stormwater Runoff
Surface water runoff occurs when it rains and a percentage of the
rainfall drains into the local watershed streams and rivers. In “green-
fields” or natural, undeveloped land, the surfaces are considered to be
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