Environmental Engineering Reference
In-Depth Information
multiple benefits. A designer must determine how each site element can serve multiple
functions (e.g., aboveground water tanks could provide stored water, shade, privacy
screening, and noise abatement).
A design of a water saving, integrated site should focus on three general components:
(1) site grading, and opportunities for structures, materials, and strategies for (2) passive
systems, and (3) active systems. These are briefly discussed in the following sections.
21.5.1 Site Grading
There are two types of rainwater harvesting techniques, passive and active. Where active
systems deal with capturing rainwater and storing it in cisterns or tanks, passive systems
work with land contours and gravity to collect, detain, and slow down stormwater while
routing it through a site. Rainwater is low in salt and contains some nitrogen, both benefi-
cial to plant growth. Implementation of passive rainwater harvesting techniques typically
does add costs to a traditional site design. Although, incorporating passive concepts into a
site design early in the process aids in reducing costs and maximizing long-term benefits.
The first step in incorporating harvesting structures and techniques into a design is to
analyze the site's watershed, which includes off-site land that contributes stormwater runoff
to or receives stormwater runoff from the site. The overall goal is to manage small amounts
of water at higher elevations and as it passes through a site to reduce stormwater volume
at lower elevations of a site. Organizing the site into multiple small watersheds where the
stormwater is slowed and allowed to disperse is the method most used to attain this goal.
This reduces the stormwater's erosive nature and sediments transport off-site. The storm-
water should be exposed to as much soil surface as possible, using passive rainwater har-
vesting structures to increase infiltration and natural soil storage of the site's stormwater.
The following table identifies several low-impact development objectives and their
corresponding development technique, which support rainwater harvesting and storm-
water reuse concepts (Table 21.2).
21.5.2 Structures, Materials, and Strategies for Passive Systems
Relatively simple structures can be incorporated on a site to enhance its harvesting
potential, including microbasins, swales, French drains, and rain gardens. These are
discussed below, followed by descriptions of some of the common materials and strategies
used in passive systems: porous pavement, mulches, and use of Xeriscape guidelines.
21.5.2.1 Microbasins
Microbasins are small catchment areas that are best for low low stormwater volumes,
slowing stormwater and allowing infiltration rates to increase. These basins can be located
parallel to one another or in an alternating pattern allowing overflows to be slowed and
allowing additional infiltration. The microbasins can be tree wells, planter islands—with
curb cuts allowing stormwater to enter—or just small depressions next to a path or drive.
Microbasins can be designed with several variations including multiple sizes and shapes
for supporting planting groups with similar water needs.
21.5.2.2 Swales
Swales are best for stormwater occurring in low to medium low volumes. These are not
large channels for moving water, but are small depressions meant to slow sheet flow and
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