Swales, also know as bioswales are a shallow troughlike depression that's created to carry water during rainstorms or snow melts. -- Wiktionary. They are rapidly becoming a staple in the design of sustainable urban landscapes due to their number of advantages. Many governments are begining to require bioswales along with other "best management practices"
Advantages
- A means of groundwater recharge, a favorite of Bill Mollison permaculture pioneer.
- Asthetically pleasing
- They treat water flowing through them
- oils in the water stick to the foliage where they break down over time.
- sediments are trapped and filtered out
Uses
Swales covered or partially covered with grass can be used as walking paths - however they will be unsuitable during wet periods as they will be underwater, or soft and muddy.
Types of swale
The swale acts as a filter with soil as the medium. It stores rainwater/stormwater until it is filtered through the soil.
Dry swales are above the groundwater, such that they only hold water above the soil surface temporarily.
Wet swales intersect the groundwater, and behave almost like linear constructed wetlands. Plant choice differs accordingly, compared to dry swales.
Related techniques
A "swale" can be considered a kind of bioretention cell or rain garden, but shallower, in a channel shape, and covered with grass or other plants.
Design recommendations
- Single swales can not treat areas greater than 10 acres.
- Impractical on steep slopes.
- Requires thick vegitation.
- The swale should have a treatment area larger than 4% of the impervious surface it treats.
- The treatment depth should not exceed 2/3 the depth of the grass in the swale
- The dimensions of the swale should be checked with Manning's equation and a value of .25 for "Manning's n"
- Care should be taken in the design of the inlet and outlet of the swale
- A 6% down slope grade is ideal to insure that the velocity of the water in the swale does not become excessive.
- the side slopes of the swale should not exceed 3 to 1 (three feet over for every one foot down)
- The swale should have a maximum treatment width of 10ft, parallel swales should be used if greater widths are necessary.
Techniques for increasing capacity
If the soil is very permeable, the runoff infiltrates easily. If not, either an underdrain may be used, or holding capacity increased, or it is accepted that above a certain level of rainfall runoff, overflow will occur.
If an underdrain is used, the emphasis is then on stormwater management and it should be expected that the potential for groundwater recharge is greatly reduced. The underdrain would be a perforated pipe in a gravel layer below the bottom of the swale.
Another way to reduce the likelihood of this level of runoff is to increase the absorption capacity of upstream parts of the catchment, through improvements in soil, and/or adding more retention devices such as additional swales (perhaps small ones) and rain gardens.
Interwiki links
External links
- [ Virginia DCR Stormwater Design Specification No. 10 Dry Swales v1.5 – DRAFT, July 1, 2009] Very detailed, describing dry swales using underdrains. (Last accessed 4 November 2009. Please replace link with later/final version when it becomes available.)
- Save the Swales - WHY MANAGE RUNOFF? (the site of a local government in Florida).
- Alameda County Stormwater Technical Guidance has more details for the design of Bioswales and other best management practices.