Definition:
These are grass channels of trapezoidal, triangular or parabolic cross-section, built mainly on residential settlements or on roads. They are designed to slow down the surface rain water drain. They should account for between 3 and 10 % of the drainage volume, depending on the surface area of impermeable areas. They allow the water to be taken from the five-year rainstorms. Historically, the structures were located along roads and walkways.
– They are involved in sedimentation and infiltration of rainwater into the ground and its purification (water purification is mainly achieved by sedimentation of sludge from rainwater, and micro-organisms in the roots of the grass growing on the channel are also important here). The water in the channel should not last longer than 72 hours.
– They also reduce erosion.
The establishment of dry retention channels in urban areas has many advantages:
Environmental benefits: Rainwater retention, rainwater purification, reduction of water flow to drains, reduction of the urban heat island (reduction of air temperature), air purification.
Social benefits: Improvement of the esthetics of the place.
Economic benefits: Reduction of costs associated with rainwater discharge to rain sewage system.
Building of the connections: Artificial swamps, retention ponds, bioswales.
The design should be tailored to the conditions of the site. They are located on small slopes. They should be as long as possible – about 30 m. It is also important to have a suitable slope of the structure, which should be between 1.5% and 2.5% – such a slope has a good effect on the future development of vegetation. The water flow rate in the channel should be between approximately 0.8 m/s and 0.9 m/s. The channel slopes must be slightly shaped and the channel itself should be sufficiently wide (to store water). The designed depth of the channel should be such that the water level does not exceed 2/3 of this value. In addition, these structures are enriched with various types of dams to slow down surface runoff (design details on the website: https://stormwater.pca.state.mn.us/index.php?title=Requirements,_recommendations_and_information_for_using_swale_side_slope_as_a_BMP_in_the_MIDS_calculator)
Drainage will be an important element of channel construction. We lay the drainage on the aggregate. The drainage must be covered with 10 cm sandy clay to increase the infiltration of the water. On permeable soils there is no need to build drainage.
Turf grass, resistant to drought and densely growing, will serve a good purpose. Appropriate agro-technical procedures are required before the grass is planted. After the channel has been made, we apply grass to the structure, which is appropriate for development in this area. The area should be turf on at least 90%. Sometimes we install irrigation systems to keep the turf in good condition.
The designed channel must also be adequately insulated, so it should not be designed where we have a shaded space. It should stay dry most of the time.
At times, you may need to use the appropriate irrigation system. They should also have many entry points – side tributaries.
Sources and additional information:
https://www.waternz.org.nz/Attachment?Action=Download&Attachment_id=844
https://www.ncagr.gov/SWC/costshareprograms/CCAP/documents/Chapter8-VegetatedSwales.pdf
https://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=17766.wba
https://doee.dc.gov/sites/default/files/dc/sites/ddoe/publication/attachments/Section%203.9%20%20Open%20Channel%20Systems.pdf
http://chesapeakestormwater.net/wp-content/uploads/downloads/2012/02/DCR-BMP-Spec-No-10_DRY-SWALE_Final-Draft_v1-9_03012011.pdf
https://cvc.ca/wp-content/uploads/2012/02/lid-swm-guide-chapter4-4.9-dry-swales.pdf
https://www.environment.fhwa.dot.gov/Env_topics/water/ultraurban_bmp_rpt/3fs10.aspx
https://www.newpaltz.edu/sustainability/bioretention-areas-and-dry-swale/
https://suffolkcountyny.gov/portals/12/Construction%20BMPs/PostConstruction%20BMPs/Open%20Channel%20Practices/Dry%20Swale.pdf
http://scdhec.gov/sites/default/files/docs/Environment/docs/wqc-drySwales.pdf
Examples:
