Swales

What do they do?

• Slows down the rapid flow of stormwater runoff by ponding water between its slopes, often called berms.
• The ponded water allows pollutants to settle out of the water.
• Once full it will spill over the berm and slowly run into a local water body.
• Remaining water will either evaporate or infiltrate into the soil.

What's the cost?

Grass swales typically cost .50¢ cents per square foot. 
Swale construction costs have not been well studied. Costs will differ from regionally and with site conditions. A best estimate is the one above (based on 2002 dollars). An undated estimate (no older than 2000) suggests a value of $5.50 per cubic foot of storage provided. 

Concerns with Swales

Individual dry swales can only treat a small area.
Limited data exists to gauge their effectiveness.
Aren't applicable to sites with many driveways, cluverts, or extensive sidewalk systems.
Require more maintenance than curb and gutter systems.
They're not the prettiest things to look at.

Benefits of Swales

Traps sediment and other pollutants.
Reduces runoff and promotes infiltration, which in turn, controls peak discharges.
Good option for retrofitting small areas, especially in terms of replacing drainage ditches.
Linear nature makes them effective at treating runoff from highways and residential areas.
May provide groundwater recharge, if design and soils allow for increased infiltration.
One of the cheaper options.