Technical Results
Lag Time
In addition to increasing the volume and rate of runoff, the
addition of impervious surfaces to a watershed decreases the amount
of time that it takes to see a response to a precipitation event
at the watershed outlet. With LID, the goal is to preserve the predevelopment
hydrologic function of a watershed, which includes the lag time.
There are several different methods to quantify lag time.
Low
impact development (LID)
had a significantly greater centroid lag-to-peak, centroid lag, lag-to-peak
and peak lag-to-peak times than traditional development. An example
of one storm can be seen in Figure 1. Runoff lag times
for the LID watershed were significantly greater than for the traditional
watershed for small (< 25.4 mm) but not large (> 25.4
mm) storms; short duration (< 4hr) but not long duration (> 4
hr) storms; and low antecedent moisture condition (AMC) (< 25.4mm)
storms but not high AMC (> 25.4 mm) storms.
More detail
on lag times and runoff thresholds can be found in the following
reference:
Hood, M., Clausen, J.C., and Warner, G. 2007. Comparison
of stormwater lag times for low impact and traditional residential
development. Journal of the American Water Resources Association, volume 43(4).
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| Figure 1. Stormwater response for a
September 15, 2003, 11.2 mm rainfall.
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