Existing hydrology

There are many features in the natural landscape that 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, and
• 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.

Strategies

Preserve stream buffers

These include intermittent and ephemeral channels. Buffers provide filtration, infiltration, flood management, and bank stability benefits. Unlike stormwater ponds and other structural infrastructure, buffers are essentially a no capital cost and low maintenance form of infrastructure. In general, the literature recommends stream buffers for pollutant removal and support of aquatic and terrestrial riparian habitat (Wenger, 1999). The benefits of buffers diminish when slopes are greater than 25%; therefore steep slopes should not be counted as buffer (Schueler, 1995).

Preserve areas of undisturbed soil and vegetation cover

Typical construction practices, such as topsoil stripping and stockpiling, and site grading and compaction by construction equipment, can considerably reduce the infiltration capacity (and treatment capacity) of soils. In some instances, the bulk density of construction compacted soils is similar to values for impermeable surfaces. Native undisturbed soils have a structure that takes many, if not hundreds of years, to develop. The structure is created by the growth and decay of plant roots, 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 that it replaces. During construction, natural heritage features and locations where infiltration-based SWMPs will be constructed should be delineated and not subject to construction equipment or other vehicular traffic, nor stockpiling of topsoil.

Avoid development on permeable soils

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

Mature stands of deciduous trees will intercept 10 to 20% of annual precipitation falling on them, and a stand of evergreens will intercept 15 to 40% (Cappiella, 2005). Depending on understory vegetation, soils and topography, tree clusters may only produce surface runoff for 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 (Figure 3.2.1). Any areas of reforestation or new urban tree plantings need an uncompacted soil volume that allows 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 (Casey Trees, 2008).

The temporary storage of stormwater to control discharge rates, and allow for sedimentation.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.

Referring to the source of a stream or river.

The technique of removing pollutants from runoff as it infiltrates through the soil.

Defined as those plants (including grasses, herbaceous species, shrubs, vines and trees) that have historically existed within a particular area. Native plants have co-evolved with the local ecosystems and natural processes.

A soil classification system based on the ability to convey and store water; divided into four groups: A,B,C,D

Increases in groundwater storage by natural conditions or by human activity. See also artificial recharge

The inflow of surface water to a groundwater reservoir or aquifer.

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

Any form of rain or snow.

The portion of water precipitated onto a catchment area, which then 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.