Environmental Engineering Reference
In-Depth Information
8.0
7.0
Runoff values for the 1
″
and 1.5
″
storms generated
using the small storm hydrology methodology (Pitt,
1994) and runoff values for the storm generated
using the SCS runoff curve number method (CN = 98
for impervious and CN = 73 for woods, C soils, fair
condition).
6.0
5.0
4.0
2.77
2.89
3.0
2.43
2.26
2.01
2.0
1.13
0.95
1.0
0.0
1 inch
rainfall
1.5 inch
rainfall
2-yr storm
(3.27
5-yr storm
(4.09
10-yr storm
(4.78
50-yr storm
(5.51
100-yr storm
(7.5
″
)
″
)
″
)
″
)
″
)
Figure 1-19
Runoff volume increase from impervious cover on HSG C soil.
soil, while the better-drained (B) soil produces a 2.60-in. runoff volume increase.
Thus, a volume control guideline must be based on the net change in runoff
volume for a given frequency rainfall, in order to be equitable throughout the
country on any given development site.
When the balance of a developed site is cleared of existing vegetation, graded,
and recompacted, it also produces an increase in runoff volume. Traditionally, if
the original vegetation were replaced with “natural vegetation” such as a lawn,
the runoff characteristics would be considered to be equivalent to the original
natural vegetation, but the fact is that the disturbance and compaction destroy
the permeability of the natural soil and increase soil density, which has a direct
impact on permeability.
Lawnscapes
actually produce significant runoff during
rainfall, laden with nutrients, pesticides, and herbicides.
Consideration of runoff volume control has focused on the frequent rainfalls
that comprise a major portion of events in most parts of the country. Since the
2-year rainfall comprises some 95% of the total rainfall in most regions, control
of the 2-year event becomes the recommended basis for stormwater management
as a
control guideline
. It is considered unreasonable to design any stormwater
volume LID measure for greater than a 2-year event. The increase in runoff
volume from the 100-year rainfall after site development is so large that it is
impractical to require management of this total increase in volume. During such
extreme events, the cumulative impact of this volume from largely impervious
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