Proper grading is one the most overlooked issues during design and even at the stage of residential development. Many municipalities do not review the grading plan thoroughly and rely on the Ontario land surveyor or professional engineer’s certificate confirming that the municipalities‘ requirements are met.
While those requirements warrant a proper grading and management of surface runoff, the lack thereof may result in several issues or make the owner, designer, contractor or even the municipality prone to claims. Several issues may happen that the two main issues are encroachments and flooding.
• Encroachment to the right of way of neighbouring property as a result of misalignment during construction
This issue becomes critical when the primary property share a mutual driveway with the neighbouring property or when the developer builds a semi detached or row townhouse.
In this case the structure and/or attachment to a building or structure was inadvertently and partially erected on or over a shared right of way or adjacent neighbouring property.
The builder and surveyor must pay enough attention to avoid claims from the owners later on.
• Flooding of the primary residential dwelling or neighbouring properties.
This issue is mainly the result of improper grading. In recent developments the designers attempt to use the maximum height setback. The finished grading at the garage entrance will be lowered dramatically
and as a result, many properties have the same level grade with the street. Where these houses located at the low point of the street or close to end point of the sub-watershed, they have increased risk of flooding during major storm events. The simple rule of thumb is to keep the surface runoff away from the surrounding buildings. Most municipalities want to see a minimum of 2 percent slope away from the building.
The upset is 5 percent. Ideally we want to sheet drain the water to the municipality’s right of way. In case of split drainage lot where the highest point is somewhere in the middle of the lot, proper measures must be employed to keep the runoff away from the surrounding buildings. Some grading consultants propose French drains in such circumstances. French drains are good contribution, if the native soil has enough infiltration capacity. French drains however are not aesthetically pleasing and Rain Garden is a better alternative for French drain.
Rain gardens are a designed depressed area in the landscape that store runoff from impervious urban areas, like roofs, driveways, walkways and compacted lawn areas. The rain garden should be built below the surrounding elevations. They can be planted with a combination of shrubs, grass and flowers. Rain gardens provide stormwater control at the source, or where the rain falls with absorbent landscape to promote the infiltration.
An“AbsorbentLandscape” is proposed at rain garden to promote infiltration. Minimum depth of absorbent soil for on-site landscaping area should be 300 mm and shall meet the City of Toronto standards for landscaping soil. The range of acceptable soil texture is typically equal to 60-70% sand, 20- 40% silt, 5-10% clay, and 2-5% organic material (PH equal or less than 7.5).
This absorbent landscape allows percolation or infiltration of stormwater through the surface into the soil below where the water is naturally filtered and pollutants are removed. Vegetation in the swale slows the water to allow sedimentation, filtration through the soil matrix and root zone, and infiltration into the underlying native soil, where suitable conditions exist. The rain garden should have soil amendment with mulch, and loose soil as per CVC/TRCA LID fact sheet, to further promote infiltration.
There are other best management practices or stormwater management measures to mitigate the effects of the development.
Their primary contributions are through on site storage by detention or retention of surface runoff. This can be achieved by combination of infiltration or evapotranspiration from the plants. A relatively easy and inexpensive measure is the use of permeable pavers in the driveway and walkways. Permeable pavements are a recognized runoff reducing substitute for normal pavements in development
or redevelopment of parking areas; especially over-flow parking and those associated with office buildings driveways, and low-traffic roads. The permeable pavement area covers the entire driveway. A 50 mm bedding course (No. 2 stone) and 200 mm open graded aggregate (No. 57 stone reservoir) shall be laid under the permeable pavement of to allow some infiltration during minor storm events
