Installing Grass-Lined Ditches
A grassed ditch is a structure designed to collect runoff water and direct it towards buffer zones[1]. It involves many issues, notably soil loss, flooding, and infrastructure degradation. The main objectives of the grassed ditch are to :
- Reduce runoff and diffuse erosion.
- Increase water infiltration.
- Reduce pollutant transfer.
There are different types of grassed ditches :
- Transfer ditches : they facilitate water passage.
- Notched ditches : they delay water arrival downstream of the watershed and promote infiltration on highly permeable soil.
- Ditches associated with a bank : these are retention/diversion zones for runoff upstream of the watershed.
Sizing a grassed ditch
The size of the ditch must be adapted for an annual flood.
- The longitudinal slope must be less than 2%.
- The lateral slope must be 30% on the runoff side and 50% on the other side.
- The width of the strip must be more than 3m before the ditch.
- The bottom of the talweg is a flared V with a width between 4 and 6m and a depth of about 1m.
Ditch maintenance
- Maintenance should be avoided between April and July.
- One mowing per year is sufficient, and phytosanitary products must not be used to avoid their circulation.
- An annual cleaning may be necessary depending on deposits observed in ditch sections.
Cost
- Simple : €6 to €11 per linear meter excluding tax.
- Notched : €15 to €82 per linear meter excluding tax.
Subsidies may be provided by departments.
Maintenance : €0.76 per linear meter (2016).
Regulations
The grassed ditch is classified as a Surface of Ecological Interest (SIE) : 1 linear meter of ditch = 6m² SIE with width <6m.
The distance from dwellings must be greater than 5 to 10m and the ditch/property boundary distance must be greater than 50cm.
The construction of a grassed ditch is subject to declaration or authorization under the Water Law.
Appendices
Sources
- ↑ Institution Adour, 2016 : Approach to combating soil erosion : From preparation to implementation https://www.gesteau.fr/sites/default/files/gesteau/content_files/document/guideerosion_sagemidouze_2016.pdf
