SWALES
You know it when you see one!

 
Swales are simply shallow, low depressions in the ground designed to encourage the accumulation of rain during storms and hold it for a few hours or days to let it infiltrate into the soil. 

 Swales ideally are tree-lined and store wa ter for
the immediate landscape as well as help cleanse
the water as it percolates down. Swales can be installed separately or as part of a larger water rain catchment system with other water conservation measures.

Swales on slight slopes can also be used to direct water away into percolation pits that will charge the groundwater.

Swales dug into even the gentlest of slopes will fill with water after every good rain and allow the moisture to percolate deep into the soil over a day or more. This charges the subsoil over several years allowing the trees planted into the mound to thrive even during the driest of times.

Also known as infiltration swales, biofilters, grassed swales, or in-line biorentention, bioswales are vegetated open channels specifically designed to attenuate and treat stormwater runoff for a defined water volume. Like open ditches, they convey larger stormwater volumes from a source to a discharge point, but unlike ditches, they intentionally promote slowing, cleansing and infiltration along the way.

Grassed Channels

These are similar to a conventional drainage ditch, with the major differences being flatter side slopes and longitudinal slopes, and a slower design velocity for water quality treatment of small storm events. Grass channels are the least expensive option.

Grass channels should be designed to ensure that runoff takes an average of ten minutes to flow from the top to the bottom of the channel.


 

 Wet Swales

These swales intersect the groundwater, and behave almost like a linear wetland cell. The design variation incorporates a shallow  permanent pool and wetland vegetation to provide stormwater treatment. Wet swales are rarely used in residential settings because the shallow standing water is often unpopular with homeowners.

What are the benefits of using swales?

  • Improves water quality
  • Cheaper to construct than piped systems
  • Can be incorporated into the landscape
  • Low maintenance
  • Visible Operation                                       
 Dry Swales

Dry swales incorporate a deep fabricated soil bed into the bottom of the channel. Existing soils are replaced with a sand/soil mix that meets minimum permeability requirements. An underdrain system is also placed under the soil bed. Typically, the underdrain consists of a layer of gravel encasing a perforated pipe. Stormwater treated by the soil bed flows into the underdrain, which conveys treated stormwater back to the storm drain system.

 

 

What makes a good swale?
  • The longer the swale the better. A long and a winding swale allows plenty of time for settlement of solids contained in the runoff. Swales of more than 70m in length give good, consistent performance.
  • No sharp corners. Swales should have gentle curves.
  • Shallow gradients. A swale which curves to and fro with a shallow gradient is better than a swale which is built directly down slope.
  • Shallow swales with no permanent water have better chance of establishing a grass sward.
  • Swales work better when used in conjunction with other filtration techniques like recharge pits, recharge trenches etc.

Design Considerations

Factors to be kept in mind while constructing swales:

Soils

  • Soil infiltration rates between 0.5 and 3.0 inches per hour are preferred.
  • The clay content of the soil should be less than 20 percent, and the silt/clay content should be less than 40 percent.
  • Coarse, highly permeable soils should be avoided because they have shorter infiltration times and are less conducive to supporting growth of vegetation.
  • Impermeable soils facilitate ponding and should be avoided.
  • The bottom of the swale should be at a minimum of two feet above the seasonal water table or bedrock.
  • Less desirable soils can be amended to improve infiltration characteristics.

 

Cross Section Shape

  • Parabolic or trapezoidal cross sections maximize infiltration.
  • Triangular cross sections should be avoided as they concentrate flow and promote channel erosion.
  • Side slopes should be relatively flat (3:1 or flatter).
  • Channel bottom width should be between two feet and eight feet (based on cross-sectional area of flow for the channel).

Slopes and Swale Length

  • Swale gradients (slopes) of one to two percent are recommended. Gradient pictures
  • Swale length should be a minimum of 200 feet to encourage deposition.

Vegetative Cover

  • Vegetation should be limited to perennial grasses and grass-legume mixes.
  • Species of vegetation chosen should have a dense growth habit and be able to tolerate extended periods of flooding.
  • Vegetative species can be selected to target different types of pollutants.
  • Vegetation height should be maintained at a minimum height of three to four inches.

Design Calculations

  • Typical storm intensities should be calculated for each specific site location.
  • Swale design should be based on flow rate, not volume. Runoff should pass from the upstream end to the downstream end of the swale in ten minutes.
  • Swale should be designed to effectively handle runoff from a one-inch, 24-hour storm event and efficiently pass excess runoff from larger storms (e.g., 10-year storm events).

Where should the swale be located?

This will depend on the layout of the site and the amount of space available, but the swale would usually run either parallel with or at 90° to the houses, with the hard-standing draining towards the swale.

Roof water can be directed to the swale via surface drains or via smaller swales.

They can also be constructed to drain areas which flood or hold water for a long time. This water can be directed into a recharge pit that could charge the ground water.

 

Where swales should NOT be located ?

Swales and other stormwater infiltration systems should not be used in

areas where the land around which drains the water into the system is associated with or used for chemical storage, high levels of pesticides, the washing and maintenance of vehicles or equipment, or where wastes are handled.

They also have to carefully used near petrol bunks, garages, mechanic sheds etc. and other areas where oils, grease, and other types of toxic materials can get into the groundwater.

 "I am having a swale of a time ..."