Definition: long, grassy channels, dry on a daily basis, from which rainwater is filtered into the ground, while at the same time being cleaned by sedimentation of various pollutants. This solution is most commonly used along parking lots, sidewalks or carriageways. The flow of water should be slow, so they are often meandered and various types of dams and weirs are built. The channel can also run under sidewalks or streets, and then culverts are used. They are designed for 10-year rainstorms, lasting up to 24 hours. It is best to use them in areas with a very low soil density, commercial spaces. It is assumed that several such channels will be preferred. They are components that are friendly for reptiles and amphibians living in cities, as they can freely cross them.
Environmental benefits: Water drainage mitigation, rainwater retention and infiltration, reduction of the risk of flooding, rainwater purification, reduction of the urban heat island, air purification, reduction of water runoff to sewage disposal systems. Reptile- and amphibian-friendly solutions.
Social benefits: Enhancing the esthetic appeal of the place, creating diverse urban landscapes.
Economic benefits: Reduced costs associated with rainwater discharge to rain sewage system.
They are part of the creation of green networks in the city: rain gardens, green walls, green roofs, retention tanks.
Construction:
– they should be located at least 3 meters from the building foundations, 15 m from a cesspit and 50 meters from a well. (See Appendix https://www.swbmp.vwrrc.vt.edu/wp-content/uploads/2017/11/BMP-Spec-No-3_GRASS-CHANNELS_v1-9_03012011.pdf for details on storm channels)
– They are built in areas where the slope does not exceed 4%, ideally when the slope is inclined by 2%.
– The channel should have a trapezoidal or parabolic cross-section. The bottom width from 1.2 m to 2.4 m, when the channel used is wider, dams and weirs should be used in addition.
– The inclination of the slopes 4 to 1 makes it easier to cut the grass.
– The minimum height should be 30 cm. A depth of not less than 10 cm above the expected water table.
– The substrate should consist of sandy clay (not more than 20%) with an organic content of 10 to 20%. There should be a drain underneath.
– The soil should be completely covered with grass of not more than 15 cm high. The grass species used must be resistant, first of all, to salinity and drought – but also to high humidity and fast water current. Species such as: Bermuda grass, tall fescue, red fescue can be used. In spaces where leaching of seeds may occur, a biodegradable anti-erosion mat can be used. We sow the grass, we do not use grass from the roll.
– The sump speed should not be more than 30 cm per second. The channel water flow time can be increased by decreasing the channel inclination, increasing the channel width, and more dense sowing of grass. We can also slow down the water by having the channels terraced or building dams.
Examples:
Sources and additional information:
http://dnr.mo.gov/env/wpp/guide/Runoff%20conveyance.pdf
https://megamanual.geosyntec.com/npsmanual/grassedchannel.aspx
https://www.murfreesborotn.gov/DocumentCenter/View/2742/Grass-Channel-223-?bidId=
https://intrans.iastate.edu/app/uploads/sites/15/2020/03/7E-23.pdf