Grading

Expressing slope

Slope gradient

Slope gradients are are common form of expression for steeply sloped areas like the sides of swales. Expressed as Y:X where Y is a single unit of rise and X is the corresponding run. It can be calculated by dividing the rise by the run: e.g.:${\frac {rise}{run}}={\frac {4\ m}{12\ m}}={\frac {1}{3}};we\ say\ the\ slope\ =1:3\ or\ 1\ in\ 3$

Slope percentage

Slope percentage is calculated similarly by dividing rise by run, but then multiplying by 100 to get a percentage: e.g.:$slope=\left({\frac {rise}{run}}\right)\times 100=\left({\frac {4\ m}{12\ m}}\right)\times 100=33\%$ But for many calculations the rise/run isn't multiplied by 100, and instead is left as a decimal. e.g. 0.33 in the example above.

Slope in degrees

Expressing a slope in degrees requires using the tangent trigonometric function. e.g.:$\tan \theta =\left({\frac {4\ m}{12\ m}}\right);\theta =\tan ^{-1}\left(0.33\right)=18.3^{\circ }$

Quick reference table

Normal range of slopes

The Accessibility for Ontarians with Disabilities Act provides a bare minimum to which public spaces must be designed. e.g. cross slope not to exceed 2 %.

Consider designing landscapes to universal design principals instead.

Grading Standards and Critical Gradients

Use	Extreme range (%)	Desirable range (%)
Public Streets	0.5 - 10	1 - 8
Private streets	0.5 - 20	1 - 12
Service lanes	0.5 - 15	1 - 10
Parking areas	0.5 - 8	1 - 5
Parking ramps	up to 20	up to 15
Collector walks	0.5 - 12	1 - 8
Entrance walks	0.5 - 8	1 - 4
Pedestrian ramps	up to 12	up to 8
Stairs	25 - 50	33 - 50
Game courts	0.5 - 2	0.5 - 1.5
Paved gutters	0.25 - 100	1 - 50
Grassed swales	0.5 - 15	2 - 10
Terraces and sitting areas	0.5 - 3	1 - 2
Grassed banks	up to 50	up to 33
Planted banks	up to 100	up to 50

Freeboard

• In swales conveying flowing water a freeboard of 300 mm is generally accepted as a good starting point.
• In bioretention the freeboard is the difference between the invert elevation of the inlet and overflow structure. 150 mm will usually suffice, so long as the inlet will not become inundated during design storm conditions.
• In above grade stormwater planters, the equivalent dimension would be the difference between the invert elevation of the lip of the planter and the overflow structure (150 mm minimum).

Additional Resources

• Planning and Design (Huang, 2012)
• Grading and Earthwork (Matusik and Deibel, 2004)
• Fundamentals of Site Grading Design (Tiner, 2014)
• Slope-Meter, Inc. - external resource, Slope Conversion Table (Slope-Meter Inc. 2009)