Natural drainage

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See the related article on Existing hydrology

The use of natural drainage picks up where stormwater leaves imperviousA hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil. areas. Rather than collect and move stormwater rapidly to a centralized location for detentionThe temporary storage of stormwater to control discharge rates, and allow for sedimentation. and treatment, the goal of these strategies is to take advantage of undisturbed vegetated areas and natural drainage patterns (e.g., small headwaterReferring to the source of a stream or river. drainage features). These strategies will extend runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. flow paths and slow down flow to allow soils and vegetation to treat and retain it. Using natural systems or green infrastructureNatural vegetation and vegetative technologies in urban settings such as: urban forests; green roofs; green walls; green spaces; rain gardens; bioswales; community gardens; natural and engineered wetlands and stormwater management ponds; and porous pavement systems. These systems are designed to provide multiple benefits, such as moderate temperatures, clean air and water, and improve aesthetics. to provide communities with environmental services is often more cost effective than traditional drainage systems, and they provide more ancillary benefits.

Strategies

Disconnect imperviousA hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil. areas

ImperviousA hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil. areas have varying degrees of hydrologic impact depending on their connection to the receiving waterbody. For example, imperviousA hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil. areas such as parking lots that drain directly to a concrete gutter and storm sewer will have a much greater impact than parking lots graded to drain to densely vegetated pervious areas. Roof leaders or downspouts, parking lots, driveways, sidewalks, and patios should be disconnected from the storm sewer and directed towards stabilized pervious areas where possible (see downspout disconnection).

Opportunities for directing imperviousA hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil. surface runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. to pervious areas are first considered during the site layout stage. Sheet flow should be encouraged from all imperviousA hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil. surfaces draining to pervious areas. In cases of concentrated flow, the flow can be broken up with level spreaders or flow dissipating riprap. Use the following guidance for the pervious runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. receiving areas:

  • Undisturbed densely vegetated areas and buffers – A hydrologist and/or ecologist should be consulted before designing a site to drain to sensitive natural heritage features like pocket wetlands.
  • Landscaped and disturbed areas – With the proper treatment, the landscaped areas of the site can accept runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. from imperviousA hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil. areas. Deep tilling or soil aeration is recommended for topsoil that has been replaced or compacted by construction equipment. Former agricultural lands tend to develop a “hardpan” or compacted layer 0.5 - 1 m below the soil surface from repeated plowings and farm equipment. Breaking up the hardpan may improve infiltration rates. Soil amendments can be applied to hydrologic soil group (HSG) C and D soils to encourage runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. absorption. Use deep rooting vegetation in landscaped areas when possible which will maintain and possibly improve the infiltration rates over time.

Preserve or create micro-topography

Undisturbed lands have a micro-topography of dips, hummocks and mounds which slow and retain runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface.. Site grading smooths out these topographic features. Micro-topography can be restored in areas of ornamental landscaping or naturalization. Any depressed areas should drain within 48 hours, or they may provide breeding habitat for mosquitoes. See dry ponds, enhanced swales and grading].

Extend drainage flow paths

Slowing down flows and lengthening flow paths allow more opportunities for stormwater to be filtered and infiltrated. Extending the travel time can also delay and lower peak flows. Where suitable, flows should be conveyed using vegetated open channels such as enhanced grass swales or bioswales).