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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 get traped and filtered out

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 with parallel swales used if greater widths are necessary

External links

• Save the Swales - WHY MANAGE RUNOFF? (the site of a local government in Florida).
• http://www.wikihow.com/Dig-Swales
• Alameda County Stormwater Technical Guidance has more details for the design of Bioswales and other best management practices.

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