Runoff Quantity (Numeric Summary)

Once all the sustainable landscape treatments described on this Web page were installed, I attempted to quantify the runoff volume reduction for the property. I did so with the help of Tom Price, who is a professional engineer at Conservation Design Forum, Inc. (CDF) I divided the property into different drainage areas (or subbasins), selected four different storm events (or design storms) following Bulletin 71 (MCC Research Report 92-03), and calculated* how much precipitation each drainage area would infiltrate or cause to runoff. The results where very encouraging and are listed below:

24 Hour Design Storm**

Precipitation over 24 hour design storm duration

Modeled surface runoff with sustainable improvements

Total runoff reduction

2 year event

3.04 inches

0.18 inches

94%

10 year event

4.47 inches

0.56 inches

87%

50 year event

6.46 inches

1.59 inches

75%

100 year event

7.58 inches

2.31 inches

70%

Another way to look at the improvements is to compare the runoff volume prior to the sustainable treatments to after the sustainable treatments. In other words, compare the runoff volume for a default landscape treatment against the sustainable landscape treatment:

24 Hour Design Storm**

Modeled surface runoff prior to sustainable improvements

Modeled surface runoff after sustainable improvements

Runoff  reduction

2 year event

1.11 inches

0.18 inches

84%

10 year event

2.16 inches

0.56 inches

74%

50 year event

3.80 inches

1.59 inches

58%

100 year event

4.76 inches

2.31 inches

51%

The results show an encouraging reduction in stormwater runoff. Equally important, the reduced volume of runoff is put back into the natural water cycle: It either infiltrates back into the ground or is evapotranspired back into the atmosphere by the native vegetation.

Most importantly in all of this, think about the potential cumulative effect. What if everybody on the block or in the neighborhood would do something small to treat stormwater responsibly and as a valuable resource? We may hear less frequently about flood warnings, flooding, and flood damage. We may not need to worry as much about that “heavy” rainfall that otherwise would have caused flooding. We would start to adjust our attitude toward rain water, improve the methods with which we treat our land, and begin to work with, rather than against, our natural history. We can have a positive and even restorative impact on the natural water cycle and ultimately improve our quality of life.

* Stormwater model used to calculate runoff: HydroCAD® Stormwater Modeling (http://www.hydrocad.net/)
** Rainfall Frequency Atlas of the Midwest, Bulletin 71, Midwetern Climate Center (MCC) Research Report 92-03

 

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