> Water > Constructed wetlands and built structures for water management > SWALE

I/ General description and characterization of the NBS entity I.1 Definition and different variants existing Definition Swales are broad, shallow, earthen channels designed to slow runoff, promote infiltration, and filter pollutants and sediments in the process of conveying runoff. Swales are often densely planted with a variety of trees, shrubs, and grasses along the bottom and sides of the channel. Two primary vegetated swale design variations exist: dry swales and wet swales.

Dry swales: Wet swales: Dry swales are designed with highly Wet swales are essentially linear wetland cells. Their permeable soils and an underdrain to allow design often incorporates shallow, permanent pools the entire stormwater volume to convey or or marshy conditions that can sustain wetland infiltrate away from the surface of the swale vegetation, which in turn provides potentially high shortly after storm events. Dry swales may pollutant removal. A high water table or poorly be designed with check dams that act as flow drained soils are a prerequisite for wet swales. The spreaders and encourage sheet flow along drawback with wet swales, at least in residential or the swale. Because of their highly permeable commercial settings, is that they may promote soil and conveyance capability, dry swales mosquito breeding in the shallow standing water. are more applicable for urban environments. Infiltration is minimal.

Dry swale Wet swale (Source: Sustainable storm water management) (Source: Sustainable storm water management)

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Grass swales: Enhanced Vegetated Swales: Grass swales are essentially conventional In addition to the required elements of a Vegetated drainage ditches. They typically have milder Swale, the Enhanced Vegetated Swale includes an side and longitudinal slopes than their aggregate bed or trench, wrapped in a non-woven vegetated counterparts. Grass swales are geotextile, which substantially increases volume usually less expensive than vegetated control and water quality performance, although swales. However, they provide far less costs also are increased. infiltration and pollutant removal opportunities. Design of grass swales is often rate-based, as opposed to volume-based.

Vegetated swales along residential area (left) and along road (right) (Source: Pennsylvania Stormwater Management Manual)

I.2 Urban challenges and sub-challenges related + impacts Main challenges 02| Water management and quality - Removal of urban pollutants through and sub-challenges > 02-1 Urban water management infiltration and vegetative filtering targeted by the > 02-2 Flood management - Reduction of runoff rates and volumes NBS (by increasing flow path lengths and channel roughness) -Decrease of stormwater volume through infiltration Co-benefits and 04| Biodiversity and urban space - Local wild grass and flower species can challenges > 04-1 Biodiversity be introduced for visual interest and to foreseen > 04-2 Urban space development and provide a wildlife habitat regeneration - Swales catch pollutants, which are > 04-3 Urban space management concentrated into a limited and dedicated 05| Soil management zones > 05-1 Soil management and quality Possible negative - - effects

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II/ More detailed information on the NBS entity II.1 Description and implication at different spatial scales Scale at which the The NBS is implemented at the neighbourhood scale. Published standards suggest NBS is implemented that the optimal length of vegetated swales is between 30m and 60m (local scale). Impacted scales Neighbourhood scale. II.2 Temporal perspective (including management issues) Expected time for The NBS is directly effective right after its implementation. the NBS to become fully effective after its implementation Life time The lifetime is generally large, around 50 years (The Bay Area Stormwater Management Agencies Association, 1997). Sustainability and A priori, no major impact associated with the life cycle of the NBS. The use phase is life cycle the phase most likely to generate impacts (for example, positive impacts regarding the services provided by the NBS). Management Compared to other stormwater management measures, the required upkeep of aspects (kind of vegetated swales is relatively low. In general, maintenance strategies for swales interventions + focus on sustaining both the hydraulic and pollutant removal efficiency of the intensity) channel, as well as maintaining a dense vegetative cover. Interventions occurring annually (semi-annually the first year) or 48 hours after every major storm event include: - Inspecting and correcting erosion problems, damage to vegetation, and sediment and debris accumulation - Inspecting vegetation on side slopes for erosion and formation of rills or gullies - Inspecting for pools of standing water; dewater and discharge to a sanitary sewer - Mowing and trimming vegetation to ensure safety, aesthetics, proper swale operation, or to suppress weeds and invasive vegetation - Soil clogging by sediments and possible scraping - Inspecting for litter; to be removed prior to mowing - Inspecting for uniformity in cross-section and longitudinal slope - Inspecting swale inlet (curb cuts, pipes, etc.) and outlet for signs of erosion or blockage II.3 Stakeholders involved / social aspects Stakeholders Landowner (private or public) involved in the decision process Technical - Specialized green spaces management firms and gardeners: easy NBS to stakeholders & implement (mechanical digger) networks - The technical stakeholders network for this kind of NBS is well identified Social aspects No particular social bottleneck II.4 Design / techniques/ strategy Knowledge and know- A major concern when designing vegetated swales is to make sure that how involved excessive stormwater flows, slope, and other factors do not combine to produce Or key points for erosive flows, which exceed vegetated swale capabilities. Use of check dams success can enhance swale performance in such situations. Materials involved - If the infiltration capacity is compromised during construction, the first several feet shall be removed and replaced with a blend of topsoil and sand to promote infiltration and biological growth. - Natural wood OR sand, gravel, and sandy loam or stones for check dams, gravel and pipes for underdrain system, if required - Seed and vegetate: dense and diverse selection of native, close-growing, water-resistant plants with high pollutant removal potential.

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II.5 Legal aspects related In France, rainwater management is regulated by several articles of the civil code. In particular, some areas have to delimited: • areas for which soil sealing must be limited, • areas for which collection, storage or even treatment are necessary, • flood risk areas (in order to implement flood prevention practices). II.6 Funding Economical aspects Range of cost The cost of installing and maintaining swales varies widely with design variability, local labour / material rates, real estate value, and contingencies. In general, swales are considered relatively low cost control measures. The Bay Area Stormwater Management Agencies Association (1997) gives construction costs (per linear foot) from $4.50 to $8.50 (from seed) or from $15 to $20 (from sod), annual operations and maintenance costs (per linear foot) of $0.75, and a total annual cost (per linear foot) from $1 (from seed) to $2 (from sod). Costs, which include activities such as clearing, grubbing, levelling, filling, and sodding (if required), may range from $8.50 to $50.00 per linear foot depending on swale depth and bottom width (South-eastern Wisconsin Regional Planning Commission, 1991). Origin of the funds Depending of the owner of the land (can be public or private). (public, private, public- private, other)

II.7 Possible combinations with other kinds of solutions (other environmental friendly solutions or conventional ones) Check dams and berms can be installed across the flow path of a swale in order to promote settling and infiltration. Check dams are recommended for vegetated swales with longitudinal slopes greater than 3%. Check-dams create a series of small, temporary pools along the length of the swale, which make it much more effective at mitigating runoff. The frequency and design of check- dams in a swale will depend on the swale length and slope, as well as the desired amount of storage/treatment volume.

Check dams along a vegetated swale (Source: Pennsylvania Stormwater Management Manual)

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III/ Key elements and comparison with alternative solutions III.1 Success and limiting factors

Success factors - The effectiveness of a vegetated swale is directly related to the contributing land use, the size of the drainage area, the soil type, slope, drainage area imperviousness, proposed vegetation, and the swale dimensions. - Swales are most efficient when their cross-sections are parabolic or trapezoidal in nature. Swale side slopes are best within a range of 3:1 to 4:1 and shall never be greater than 2:1 for ease of maintenance and side inflow from sheet flow. - Swales are easy to incorporate into landscaping - Low capital cost - Maintenance can be incorporated into general landscape management - Pollution and blockages are visible and easily dealt with. Limiting factors - The soil base for a vegetated swale must provide stability and adequate support for proposed vegetation. When the existing site soil is deemed unsuitable (clayey, rocky, coarse sands, etc.) to support dense vegetation, replacing with approximately 30 cm of loamy or sandy soils is recommended. Swale soils shall also be well drained. - Swales are not suitable for steep areas or areas with roadside parking - Limited opportunities to use trees for landscaping - Risks of blockages in connecting pipe work III.2 Comparison with alternative solutions Grey or Vegetated swales provide a cost-effective and an environmentally superior conventional alternative to conventional curb and gutter conveyance systems, including solutions associated underground storm sewers. counterpart

Close NBS • Rain/infiltration gardens • De-sealed areas and associated systems • Constructed wetland for phytoremediation • Constructed wetland for wastewater treatment • Use of terraces

IV/ References IV.1 Scientific and more operational references (presented jointly) Coulon, A., El-Mufleh, A., Cannavo, P., Vidal-Beaudet, L., Béchet, B., Charpentier, S. (2013) Specific stability of organic matter in a stormwater infiltration basin. Journal of Soil and Sediments 13, 508-518 Coulon, A., Cannavo, P., Charpentier, S., Vidal-Beaudet, L. (2014) Clogging process of stormwater infiltration basins quantified by image analysis. Journal of Soil & Sediments, DOI 10.1007/s11368-014- 0951-z (IF 2013 : 2.107, Quartile Q2 en sciences du sol) . (IF: 2.107, Quartile Q2 en sciences du sol) IV.2 Sources used in this factsheet

Pennsylvania Stormwater Management Manual, Section 5 - Structural BMPs

V/ Author(s) Name Institution / company Writer/ reviewer Pyrène Larrey-Lassalle Nobatek/INEF4 Writer Patrice Cannavo Agrocampus Ouest Reviewer Ryad Bouzouidja Agrocampus Ouest Reviewer Marjorie Musy Cerema Reviewer

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