Impervious Precipitation
What are the dynamics that influence watershed behavior?
Through an investigation of historic precipitation patterns, soil and geological composition, and indigenous plant species, we can better predict watershed behavior with regard to stormwater runoff. Further, the amount of impervious surface in a watershed will greatly affect stormwater generation and impact. For a general comparison of impervious surface to runoff volume, see the illustration below.
As the amounts of impervious surface escalate to 90 % for a fully urbanized area, so does the relative amount of runoff and associated pollutants.
(illustration credit: Lower Columbia River Field Guide to Water Quality Friendly Development)
Rainfall:
Precipitation patterns affect the behavior and characteristics of a watershed. As you can see from the precipitation prediction graph below, variations of rainfall can be predicted for specific areas.
While this difference may seem minor, the (comparative 'extra') half inch of rain which falls, for example, on a five acre parking lot could translates to an additional 68,000 gallons, (or approximately 9,000 cubic feet) of water in this storm event.
Thus, there are no "cookie cutter" approaches when it comes to stormwater management. The precipitation duration and amounts, ratio of impervious surface to natural land, and other issues such as soil type and vegetation can all inform watershed behavior.
In this prediction graph, a 25 year 2 hour storm*, will deposit between 2.0 and 2.5 inches of rain in the northern Vermont region. This information offers a basis for stormwater planning and watershed analysis, as we can begin to gauge relative volumes and rates of precipitation in extreme events. (For historical precipitation data, visit NOAA)
*( A ’25-year storm’ does not return every 25-years. It is a storm that – on average, over the long-term – has a return frequency of 1/25 = 0.04 or 4% of returning in any given year.)
For more information on impervious surface development in Burlington, please visit the City of South Burlington website, or follow this link: A complete illustration of the impervious surface ratios for South Burlington, VT