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the business plan for Bioretention area typical section with stone bed and underdrain. - Taken from the City of Chattanooga Resource Rain
the project. When a low
impact design element is Guide Section 5.3.4 Bioretention.
implemented on a site, it
cannot be looked at as an project type, soil types, and jurisdiction. sideration. A bioretention area functions
“apple to apple” replace- Depending on the low impact design ele- differently than a vegetated swale. De-
ment for stormwater ment being utilized on a project, different pending on the site and conditions some
costs. A detention pond components will be required. Each mu- sites may require treatment trains, which
has different elements nicipality or jurisdiction will have design are multiple practices which work together
and costs than a bioreten- criteria, and sometimes a design manual, to store, treat and infiltrate the stormwater
tion area. Some green for use during the stormwater design of a runoff to achieve improved water quality.
infrastructure can be ex- project. The City of Chattanooga Resource
pensive and labor-inten- Rain Guide or the Metro Nashville Low One important consideration in design-
sive to construct, while Impact Design Manual are two examples ing a stormwater system utilizing green
others are simple and cost of publications that provide information infrastructure is long-term maintenance. If
effective. For instance, on various green infrastructure practices. maintenance is not performed as required,
a subdivision can be Both of these publications include details the system can get clogged or stop func-
designed without curbs and the required design criteria to assist tioning as designed. This can be detrimen-
and gutters, stormwater the engineer in designing the system to tal to the site, or to an adjacent site. Each
inlets, and piping with meet the stormwater ordinances. system will require specific maintenance
vegetated swales being with differing costs associated with that
utilized between the edge Design Considerations maintenance. Bioretention areas have
of pavement and the side- high long-term maintenance costs, while
walk to allow for infiltration and storage. Several things must be considered when vegetated swales have low long-term main-
The swales would direct the stormwater to selecting low impact design practices for a tenance costs. This information should be
a larger facility, such as a naturalized basin project. Most importantly is soils infor- relayed to the developer prior to design
or bioretention area, for further treatment. mation. A Geotechnical Engineer should to ensure he/she adequately plans for the
Another example would be to utilize pervi- provide detailed site information relative required maintenance.
ous pavement for sidewalks or streets to to rock seams or outcroppings, soil types
allow for stormwater storage underground. and infiltration rates. Some soils are not TSS Removal
When comparing these construction costs, suitable for infiltration. Another consider-
all aspects have to be taken into account ation is the size of the drainage area. Some One major benefit that is sometimes
individually before determining if there is a practices are better suited for small drain- overlooked is the total suspended solids
cost increase or savings on the stormwater age areas, in the two- to five-acre range, removal achieved when implementing
component of the project. while others can treat larger areas. Land- green infrastructure. A traditional deten-
scaping plans with water-tolerant plants tion pond is assumed to achieve approxi-
Pre-fabricated stormwater storage units specified are required for some practices mately 60 percent TSS removal. In many
are available from several manufacturers such as bioretention areas, rain gardens, or instances, low impact design practices
for underground detention systems with naturalized basins. are assumed to achieve 80 percent TSS
infiltration, which are allowed in some removal without additional measures.
municipalities to meet low impact design Practice performance is another con-
requirements. These can be a little more Continued on page 11
expensive to purchase; however, they are
typically easier and cheaper to install. If
the project site is land restricted, these
units are great alternatives and space sav-
ers to meet stormwater ordinance require-
ments. Two examples are a stormwater
management chamber system offered by
ADS called “StormTech” or multiple un-
derground storage options from Contech
Engineered Solutions. Another green
infrastructure option is installing a green
roof on a structure, which utilizes space
that is often times ignored and can assist in
meeting LEED specifications and storm-
water requirements.
Low Impact Design Criteria
There are many options for low im-
pact design depending on the location,
View online at tnpublicworks.com 9
