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GENERAL DESCRIPTION Grass channels have long been used for by kbrillhart


                   GENERAL DESCRIPTION
Grass channels have long been used for conveyance, particularly as road-
way drainage. More recently, their benefits as a stormwater best management
practice have been recognized. Grass channels are closer in hydrologic prop-                                                                                                                                                                                                                                         BALANCE
erties to natural zero order headwater streams than drainage systems com-
posed of curb and gutter, inlets, and pipes. Grass channels allow infiltration,                                                                                                                                                                                                                                       WATER
discharge at a lower rate, and reduce pollutant loads. However they are not                                                                                                                                                                                                                                          QUALITY
capable of providing the same level of water balance and water quality benefits
as dry swales, as they lack the engineered soil media and storage volumes of                                                                                                                                                                                                                                         GEOMORPHIC
that best management practice.
                                                                                                                                                                                    STORMWATER TYPICAL PERFORMANCE                                               SITE CONSIDERATIONS
Where development density, topography and soils permit, grass channels are                                                                                                                                                                                                                                             TARGETED
a preferable alternative to both curb and gutter and storm drains as a storm-                                                                                                                    Water Balance Water Quality             Geomorphic                     Available Space
water conveyance system. When properly incorporated into an overall site                                                                                                                                                                                                Grass channels usually consume                POLLUTANTS
                                                                                                                                                                                 Grass Channel   Partially        Yes, if velocity is    No                             about 5 to 15 percent of their contrib-
design, grass channels can reduce impervious cover, accent the natural land-
                                                                                                                                                                                                                  less than 0.3 m/s                                     uting drainage area. A width of at least     BACTERIA
scape, and provide aesthetic benefits.                                                                                                                                                                                                                                   2 metres is needed.
                                                                                                                                                                                                           GENERAL                                                                                                   TSS
                                                                                                                                                                                                        SPECIFICATIONS                                                  Site Topography
                      DESIGN PRINCIPLES                                                                                                                                                                                                                                 Site topography can constrain the
                                                                                                                                                                                 Component               Specification                         Quantity                  application of grass channels. Longi-
                                                                                                                                                                                                                                                                        tudinal slopes between 1 and 4% are
   GEOMETRY AND SITE LAYOUT                                                                                                                                                      Check Dams      Check dams should be con-              Computation of check            recommended. This prevents pond-             METALS
   •    Shape: should be designed with a trapezoidal or parabolic cross                                                                                                                          structed of a non-erosive ma-          dam material needed             ing while providing residence time
        section. Trapezoidal channels will generally evolve into parabolic                                                                                                                       terial such as wood, gabions,          based on surface                and preventing erosion. On steeper
        channels over time initial trapezoidal cross-section design should                                                                                                                                                                                              slopes, check dams can be used.              NUTRIENTS
                                                                                                                                                                                                 riprap, or concrete. All check         area and depth used
        be checked for capacity and conveyance assuming it is a parabolic                                                                                                                        dams should be underlain with          in design computa-
        cross-section.                                                                                                                                                                           filter fabric conforming to local       tions.
                                                                                    PLAN VIEW OF A GRASS CHANNEL                                                                                 design standards.                                                      Drainage Area & Runoff Volume

                                                                                                                                                                                                                                                                                                                                                               GRASS CHANNELS
                                                                                                                                                                                                                                                                        The conveyance capacity should
   •    Bottom Width: between 1 and 3 metres. It should allow for shallow
                                                                                                                                                                                                                                                                        match the drainage area. Sheet flow
        flows and adequate water quality treatment, while preventing flows                                                                                                                         Wood used for check dams                                               to the grass channel is preferable. If
        from concentrating and creating gullies.                                                                                                                                                 should consist of pressure                                             drainage areas are greater than 2
                                                                                                                                                                                                 treated logs or timbers, or                                            hectares, high discharge through the
   •    Longitudinal Slopes: between 1% and 4%. Check dams can be used                                                                                                                           water-resistant tree species                                           channel may not allow for filtering and
        to create the target slope on soils with good infiltration potential and                                                                                                                  such as cedar, hemlock,                                                infiltration, and may create erosive

                                                                                                                                                                                                                                                                                                                        CVC/TRCA SWM DESIGN GUIDELINE MANUAL
        be underdrained in soils with poor infiltration potential.                                                                                                                                                                                                       conditions.
                                                                                                                                                                                                 swamp oak or locust.

   •    Length: parallels the road when used for roadside drainage, and                                                                                                                                                                                                 Soil
                                                                                                                                                                                 Diaphragm       Pea gravel used to construct           Volumetric computa-
        should be equal to or greater than the roadway length. In Hydrologic                                                                                                                                                                                            Grass channels can be applied on
        Soil Group D soils, a minimum residence time of 5 minutes and maxi-
                                                                                                                                                                                                 pre-treatment diaphragms               tion of pea gravel
                                                                                                                                                                                                                                                                        sites with any type of soils. However,
        mum velocity of 0.3 m/s translates to a minimum length of 60 metres.                                                                                                                     should consist of washed,              needed based on                 grass channels situated on Hydro-
                                                                                                                                                                                                 open graded course aggre-              surface area and                logic Soil Group D soils should have
   •    Flow Depth: max. should correspond to two-thirds the height of the                                                                                                                       gate between 3 and 10mm in             depth used in design            a maximum velocity of 0.3 m/s during
                                                                                                                                                                                                 diameter and conforming to             computations.                   the 25 mm storm.
        vegetation. Vegetation in some grass channels may reach heights of
        150 millimeters or more, therefore, a maximum flow depth of 100 mil-                                                                                                                      local design standards.
        limeters is recommended during the water quality storm event.
                                                                                                                                                                                                                                                                        Hotspot Land Uses
   •    Side Slopes: should be as flat as possible to aid in providing pre-                                                                                                            CONSTRUCTION CONSIDERATIONS                                                       Grass channels are not recommended
        treatment for lateral incoming flows and to maximize the channel fil-                                                                                                                                                                                             for hotspots, due to the potential for in-
        tering surface. Steeper side slopes are likely to have erosion gullying                                                                                                                                                                                         filtration of untreated stormwater.
                                                                                                                                                                             Grass channels should be clearly marked before site work begins to avoid
        from incoming lateral flows. A maximum slope of 3:1 (H:V) is recom-
        mended and a 4:1 slope is preferred where space permits.
                                                                                                                                                                             disturbance during construction. No vehicular traffic, except that specifically
                                                                                                                                                                             used to construct the facility, should be allowed within the channel site. Any
                                                                                                                                                                                                                                                                        Underground Utilities
                                                                                                                                                                                                                                                                  U     Utilities running parallel to the grass
   PRE-TREATMENT                                                                                                                                                             accumulation of sediment that does occur within the channel must be re-                    channel should be offset from the cen-
                                                                                                                                                                             moved during the final stages of grading to achieve the design cross-section.               ter line of the channel. Underground
   A pea gravel diaphragm located along the top of each bank can also be                                                                                                     Final grading and planting should not occur until the adjoining areas draining             (below the channel invert) utility cross-
   used to provide pre-treatment of any stormwater runoff that may be enter-                                                                                                 into the channel are stabilized. Flows should not be diverted into the channel             ings are not a problem.
   ing the channel laterally along its length. Gentle side slopes (< 3:1) act as                                                                                             until the banks are stabilized.
   a filter strip to provide pre-treatment for any lateral sheet flow entering the
   channel.                                                                                                                                                                  Preferably, the channel should be installed at a time of year best for estab-              Water table
                                                                                                                                                                             lishment of the vegetation without irrigation. Installation of erosion control             Designers should separate the bottom
   CONVEYANCE AND OVERFLOW                                                                                                                                                                                                                                              of the channel from groundwater by at
                                                                                                                                                                             matting or blanketing to stabilize soil during establishment of vegetation is
   They must be designed for a velocity of 0.3 m/s or less for the water quality                                                                                             highly recommended. If sod is used, it should be placed with staggered                     least 0.6 metres.
   design storm. The channel should also convey the locally required design         PLAN AND PROFILE VIEWS                                                                   ends and secured by rolling the sod. This helps to prevent gullying.
   storm (usually the 10 year storm) at non-erosive velocities.
                                                                                                                                                                             Typical Maintenance Activities for Grass Channels                                          Setback from Buildings
   SOIL MEDIA                                                                                     OPERATIONS & MAINTENANCE                                                   •   Mow grass at least twice yearly (more frequently if desired for aesthetic              Channels should be located a mini-
                                                                                                                                                                                                                                                                        mum of 3 metres from building foun-
   Poor soils, or those with very low organic content, can be amended using                            REQUIREMENTS                                                              reasons) to remove woody material. Maintain minimum grass height of                    dations to prevent water damage.
   compost or other amendments to better support plant growth                                                                                                                    150mm.
                                                                                    Maintenance                                                                              Annually:
   LANDSCAPING                                                                      Preservation of water quality benefits should be assured. Routine roadside ditch
                                                                                                                                                                             •   Inspect pea gravel diaphragms and check dams for clogging
   Grasses that can withstand both wet and dry periods as well as relatively        maintenance practices such as scraping and re-grading should be avoided at chan-
                                                                                                                                                                             •   Inspect grass along side slopes for erosion and formation of rills or glies   ESTIMATED INSTALLATION                                  FACT SHEET
   high velocity flows within the channel should be chosen. For applications         nel locations.
                                                                                                                                                                             •   Remove trash and debris accumulated in the channel
   along roads and parking lots, salt tolerant species should be chosen. Tall-                                                                                               •   Based on inspection, plant an alternative grass species if the original               COST
                                                                                    Owner should be responsible for maintenance of on-site channels. Roadside chan-
   er and denser grasses are preferable, though the species of grass is less
   important than good stabilization. A list of grass species suitable for use in
   grass channels in the CVC/TRCA watersheds is found in the Stormwater
                                                                                    nels in residential areas also generally receive routine maintenance from homeown-
                                                                                    ers. Residents should be educated on correct grass height and fertilizer rates to keep
                                                                                                                                                                                 grass cover has not been successfully established.
                                                                                                                                                                             As needed (Infrequently)
                                                                                                                                                                                                                                                               $32000                                 $105,000
                                                                                                                                                                                                                                                                                                                       LID 11
                                                                                                                                                                             •   Sediment build-up within the bottom of the grass channel should be re-                                                               CROSS REFERENCE: TRCA
   Management Planning and Design Manual.                                           lawns and grass channels from becoming nutrient sources.
                                                                                                                                                                                 moved once it has accumulated to 25% of the original design volume.                                                                   MANUAL, SECTION 3.6.11
                                                                                                                                                                                                                                                               $52000/PER m2 TREATED

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