Hard-core information!!!!

[Jan]

At last some hard-core information on uses and limitations of grassy
swales, both dry and wet!!! Jan

Limitations
Grassed swales are not recommended for large drainage areas.
Grassed swales are a cheap alternative to curbs and gutters;
however their efficacy is dependant on careful
design and construction, a thoughtful selection of plants as
well as provision of dependable maintenance
over time. Badly designed grass swales will not remove
significant quantities of pollutants.
If vegetation is not sufficiently established the swale will not
function.
Given the importance of design and construction, selection of
engineering services and a construction
company should be based on proven, successful past
experience with swales and recommendations from
past clients who have had swales installed.
Wet swales should not be used in high density residential areas
because of potential for mosquito generation
and smells.
Grassed swales are not the best management practice for:
Ultra urban areas because the areas of pervious surfaces
required for swale development are usually
unavailable.
Stormwater hot spots where land use will generate
stormwater with high levels of contaminants. Good
examples of hot spots would be gas stations and
convenience stores.
In cold weather climates, adjustments to design and
maintenance must occur to deal with high snow
loads and significant frost depths.


Applications
Grassed Swales are an appropriate stormwater management practice for
most regions of North America. Swales are a low cost low maintenance
option to remove sediments, nutrients and pollutants. They increase
stormwater infiltration and add a visually aesthetic component to a
site.
Establishment of grassed swales is a potential solution wherever
stormwater needs to be transported from impervious surfaces, slowed
down and allowed to infiltrate into soils.

Typically grassed swales are used as an environmentally preferential
solution or sometimes as an enhancement to the more traditional curb
and gutter based storm sewer system. The linear structure of swales
favors their use in the treatment of runoff from highways, residential
roadways and common areas in residential sub-divisions, along property
boundaries and in and around parking lots.


Generally grassed swales are used to treat relatively small drainage
areas of five acres or less. In highly urbanized areas or other highly
impervious areas grass swales are not recommended unless constructed
in series or function as pretreatment for other stormwater management
practices.
Advantages of correctly designed grassed swales:

Dry Swales
Trap and remove sediments and other pollutants and thus improve
water quality.
Reduce peak runoff velocity and promote infiltration.
Reduce erosion.
Provide for some groundwater recharge if correctly designed.
Are favored for use for treating highway and residential road
runoff because of their linear structure. Good to
use in replacement of existing drainage ditches.
Are best if used in low to moderate density developments.
Are useful as a means of disrupting impervious pavement in
parking areas.
Has ponding of water for only a short time with little increase
in water temperature thus useful in watersheds
containing cold-water trout streams.
Are less expensive to build and maintain (easy to mow) than a
traditional curb and gutter system.

Wet Swales
Function as a linear wetlands
Reduce peak flows and runoff velocity and promote infiltration.
Reduce erosion.
Are easy to design. Can be built in relatively impervious soils
or in seasonally saturated soils or intersecting
water table
Trap and remove sediments and other pollutants with increased
efficiency and thus improve water quality.
Create visually appealing and beneficial habitat between uplands
and surface waters
Are less expensive to build and maintain than a traditional curb
and gutter system
Provide effective pretreatment of stormwater passing through for
further processing by additional stormwater
management practices.