Natural drainage

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See the related article on Existing hydrology

The use of natural drainage picks up where stormwater leaves impervious areas. Rather than collect and move stormwater rapidly to a centralized location for detention and treatment, the goal of these strategies is to take advantage of undisturbed vegetated areas and natural drainage patterns (e.g., small headwater drainage features). These strategies will extend runoff flow paths and slow down flow to allow soils and vegetation to treat and retain it. Using natural systems or green infrastructure to provide communities with environmental services is often more cost effective than traditional drainage systems, and they provide more ancillary benefits.


Disconnect impervious areas[edit]

Impervious areas have varying degrees of hydrologic impact depending on their connection to the receiving waterbody. For example, impervious 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 impervious surface runoff to pervious areas are first considered during the site layout stage. Sheet flow should be encouraged from all impervious 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 runoff 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 runoff from impervious 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 runoff 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[edit]

Undisturbed lands have a micro-topography of dips, hummocks and mounds which slow and retain runoff. 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[edit]

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).