Existing hydrology

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See the related article on Natural drainage.

Many features in the natural landscape provide the important hydrologic functions of retention, detention, infiltration, and filtering of stormwater. These features include, but are not limited to:

  • highly permeable soils
  • pocket wetlands
  • significant small (headwater) drainage features
  • riparian buffers
  • floodplains
  • undisturbed natural vegetation
  • tree clusters

These features act as sponges and can sometimes be used to buffer the hydrologic impacts created by neighbouring development. They preserve the natural character of the site and in many cases improve the aesthetics and value of the developed property.

All areas of hydrologic importance should be delineated at the earliest stage in the development planning process. Once these areas have been mapped, they can guide the layout of the site.


Preserve stream buffers[edit]

Streams include all sizes of rivers, creeks and ephemeral channels. Landscape buffers provide filtration, infiltration, flood management, and bank stability benefits. Buffers are essentially a no capital cost and low maintenance form of infrastructure, recommended for pollutant removal and to support aquatic and terrestrial riparian habitat [1].

The benefits of buffers diminish when slopes are greater than 25 %, so steep slopes should not be counted as buffer [2]

Preserve areas of undisturbed soil and vegetation cover[edit]

Stockpiling topsoil can damage the structure of the soil. This affects infiltration rate, microbiology and chemistry. The effects can be mitigated by amendment and care in its reuse. Soil compaction by heavy construction machinery is common on many development sites. This significantly reduces the infiltration capacity of the soils and must be minimized.[3] In some instances, the bulk density of construction-compacted soils is similar to values for impermeable surfaces. Native undisturbed soils have a structure that can take hundreds of years to develop. The structure is created by the growth and decay of plant roots and earthworm and insect activity. In addition to destroying the structure during topsoil stripping and stockpiling, biological activity in the soil is greatly diminished. The shallow-rooted turf of lawns and landscaped areas will not provide the same stormwater benefits as the agricultural and native vegetation it replaces.

During construction, natural heritage features and locations where infiltration-based BMPs will be constructed must be delineated. These areas must not be used for stockpiling and must be protected from all vehicular traffic and any sediment accumulation.

Avoid development on permeable soils[edit]

Highly permeable soils (i.e., hydrologic soil groups A and B) function as important groundwater recharge areas. Compacting or paving over these areas will have significant hydrologic impacts. To the greatest extent possible, these areas should be preserved in an undisturbed condition or set aside for stormwater infiltration practices. On sites with a variety of soil types, impervious land cover should be concentrated in areas with the least permeable soils and underlying geology. Where avoiding development on permeable soils is not possible, stormwater management should focus on mitigation of reduced groundwater recharge through application of stormwater infiltration practices.

Preserve existing trees[edit]

Mature stands of deciduous trees can intercept 10 - 20 % of annual precipitation falling on them, and a stand of evergreens can intercept 15 - 40 %.[4] Depending on understory vegetation, soils and topography, tree clusters may only produce surface runoff during major flood event storms. Preserving mature trees will provide immediate benefits in new developments, whereas newly planted trees will take 10 years or more to provide equivalent benefits.Tree clusters can be incorporated into development in many ways, including parking lot interiors or perimeters, private lawns, common open space areas, road buffers, and median strips. Any areas of reforestation or new urban tree plantings need an uncompacted soil volume for allowing the root systems to get air and water. An uncompacted soil volume of 15 to 28 cubic metres is recommended to achieve a healthy mature tree with a long lifespan.[5]

  1. Wenger, S. (1999) A Literature Review on Riparian Buffer Width, Extent, and Vegetation. Institute of Ecology, University of Georgia. Athens, GA
  2. Scheuleer, T (1995) The Architecture of Urban Stream Buffers. Watershed Protection Techniques. 4(1).
  3. http://trieca.com/wp-content/uploads/2016/07/Soil-Mgmt-Guideline-Mar-28-2012.pdf
  4. Cappiella, K., T. Schueler, and Wright, T. 2005. Urban Watershed Forestry Manual. Part 1: Methods for Increasing Forest Cover in a Watershed. Center for Watershed Protection. Ellicott City, MD.
  5. Casey Trees. 2008. Tree Space Design: Growing the Tree Out of the Box. Washington D.C. http://www.caseytrees.org/planning/design-resources/fordesigners/tree-space/index.php