Runoff Quantity (Non-Technical Summary)

Let’s take a look at how much runoff the 168 Elm Ave. pilot project produced, prior to any green and sustainable landscape improvements and after all improvements were installed. To do so I need to pick a rain event or "design storm" for the calculations. A "design storm" is used in the engineering community for the design and sizing of stormwater systems.

I picked the 2 month* design dtorm, a storm that is expected to reoccur six times throughout a year. This is a relatively common rain event, expected to produce 1.38 inches of precipitation across the property over a period of 24 hours.

Let’s look at the pre-improvement conditions, with the conventional or default landscape treatments, and add up all the runoff from the various surfaces, as depicted to the right.

The output from the engineering model that we used tells me that I should expect a runoff volume of around 980 gallons from the 50 feet by 150 feet lot. Remember, this is the runoff generated from only 1.38 inches of precipitation in a 24 hour period. Those 980 gallons of runoff would fit into 18 rain barrels, as shown in the picture to the right.
Why is this volume (the 980 gallons or 18 rain barrels) significant? It is safe to assume that everybody with the same kind of conventional or default landscape treatments discharges more or less the same amount of runoff. There are 22 properties on my block alone – 22 homes times 980 gallons – so that already would be 21,560 gallons (or 396 rain barrels). Can you see how this starts to add up? (see also picture to the right).

Now, let’s take the same rain storm (1.38 inches across the property over a period of 24 hours) and look at what happens with all the green and sustainable landscape improvements in place: The green roof, the porous pavement, the gravel grass, the rain gardens and the bioswale. The rain barrels and the cistern were excluded from the calculation to go with a worst-case scenario (assuming they would be already full at the time of the rain storm). We again calculated the runoff from all surfaces, as depicted to the right:

We also took the built-in runoff storage into account as well as the soil’s infiltration capacity (see also Soil Permeability).

At this point I should expect a runoff volume of around 97 gallons – or two rain barrels. Compare this to the 980 gallons prior to any improvements! This is a 10-fold reduction in runoff volume.

This kind of runoff reduction and green or sustainable landscape treatment is still the exception. What happens to all the other runoff volume we are still discharging? To find out, go to the next section where we look at the impact on the watershed.

* Rainfall Frequency Atlas of the Midwest, Bulletin 71, Midwetern Climate Center (MCC) Research Report 92-03


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