How much water run off is there per foot of asphalt? (Flooding question).

Calculating how much storm water runoff leaves your parcel is simply a matter of multiplying the percent impervious surface times either the annual precipitation or amount of an individual storm event.

I want to pretend I said that, but I stole it from this PDF

So how much does it rain on average in a year is one thing you calculate. You also calculate how much it rains in rare storm events.

Thanks @Imadethisupwithnoforethought.

I just found an ehow page with a conversion at the moment you posted this. I’m going to double & triple check my calculations now.

It looks like the proposed path would be send an extra 254,053 gallons of water runoff towards my home.

If anyone knows anything to compare 254,000 gallons to, I’d appreciate that, also.

An Olympic style swimming pool holds 660,000 US gal.

And if you are trying to argue against this, I might say something to the effect of “On average it would send 254k gallons to my house every year, but here are the figures for 10, 25, 50, 100 year events in my local.” And do the math on rainfall per foot on those.

Here’s a link to some water tanks, with images, that gives the number of gallons they hold and their dimensions if will help you get a sense of how much 254,000 gallons of water is.

@SpatzieLover – I would use the information gleaned here for sure. But I am curious, aren’t the actual figures dependent on the thickness and density of the asphalt? I know there are certain types of asphalt that is used in horse barns because the liquid waste will soak through instead of pooling or running?

That’s a great point @rooeytoo.

Our Village will do this as cheaply as possibly, so the asphalt will definitely allow for 100% runoff. Also, the road they plan to do this on is a steep hill. I will go out during the next rainfall to photograph the runoff already pouring downhill.

Others from the neighborhood are fighting the pathway due to safety hazards. Besides the steep grade, near the last quarter of the road (coming down hill) there are train tracks. Small children may not be able to stop their bikes in time.

I just found out I have one more week (meeting is re-scheduled) to get my facts together ;)
Anyone looking in on this one, feel free to assist.

You’re undermining the point you’re trying to make, @SpatzieLover. If the area to be covered was, say, a relatively flat field or meadow as it was turned into a parking lot, then your calculation of runoff would be a very high percentage of what could have and probably would have soaked into the ground. (In the exceptional storm events that @Imadethisupwithnoforethought was talking about, even the field would have eventually reached a point of saturation or inundation, at which point all of the added water would turn to runoff, too.)

But if you’re talking about paving a gravel or dirt road on a steep hill, then most of that water is already runoff. (In fact, it includes a lot of eroded material from the hill, too.) So there may, in fact, be a net reduction in total runoff, since there should be less mud and gravel from the roadway washing down.

In addition to the foregoing, a roadway improvement such as paving should definitely include some kind of method to handle runoff, either by periodically shunting it to standing pools at the side of the road where it can eventually seep into the soil, or by installing catch basins and drainage piping, or even drainage ditches and culverts to channel that water to a common point. Otherwise even the asphalt paving will eventually be eroded.

@CWOTUS There is a ravine near the train tracks, mature (80 yr plus) trees and large garden beds that would be removed/altered for the pathway. The off road pathway would go through extremely expensive (well landscaped estate homes) gardens. It is not a flat meadow or a parking lot. I cannot find the grade of the hill online, but I can tell you it’s steep. It’s a difficult walk up, and it’s a good workout for experienced bikers.

The current plans only have new storm sewer piping added to the bill. No catch basins, standing pools, added retention basins or culverts.

I reside in a 100 yr flood plain that currently floods every two years. The Village put in retention ponds (per FEMAs recommendations) near the base of the road that would receive the path, which also fill past capacity during these floods.

Looks like I’ll be on the phone with FEMA sometime today or tomorrow to discuss this matter.