?
"Impervious surfaces have a perviousness of less than 50% and promote runoff of water instead of infiltration into the subsurface. Examples include parking lots, roads, sidewalks, and plazas."
"Perviousness is the percentage of the surface area of a paving system that is open and allows moisture to soak into the ground below."
Perviousness refers only to paving systems, therefore imperviousness refers only to paving systems...rubbish!
I have an existing site. The new building will be built on an underground parking garage which is currently covered by a layer of turf...so what is the pre-construction "perviousness" of this area? What units is it measured in?
I can calculate pre and post developement run-off rates and volumes, but what does that help if I can't determine into which of the 2 cases it falls into.
Does anyone have a quantifiable definition of perviousness?
Amy Rider
Sustainability ManagerKEMA Services
161 thumbs up
May 21, 2010 - 8:09 pm
Keep in mind that this credit looks at the site as a whole not a single paving material. Your pre-development runoff rate should answer the question of whether case 1 or case 2 is applicable by giving you a ratio of runoff to rainfall; thus the perviousness of your site.
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
May 26, 2010 - 5:16 am
What you're saying is that
Run-off Coeffient RC = Imperviousness
right?
This would be a clear quantifiable definition that is not found in the reference guide. I can work with this.
Amy Rider
Sustainability ManagerKEMA Services
161 thumbs up
June 2, 2010 - 4:41 pm
Just to clarify:
Run-off coefficients are for specific materials under specific circumstances (e.g. turf with 1-2% slope) whereas imperviousness is typically applied to an area, or in this case a site, as a whole.
Yes, in this case where your site seems to be of a uniform nature, they are roughly equivalent.
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
June 3, 2010 - 3:10 am
Run-off coefficients are also area weighted when calculating the RC for the whole site. This is standard practice. Thanks for the tips.