Environmental Engineering Reference
In-Depth Information
other hydraulic structures. The key value measured (or estimated) by the hydro-
logic modeling analysis has been the
peak rate
of flow that will result during a
given rainfall at a specific point in the drainage system. The stormwater manage-
ment strategy of reducing or mitigating runoff volume recommended in this topic
produces a very different perspective on these various modeling procedures, and
begs the question of which method best estimates the change in runoff
volume
resulting from land development.
There have been many methodologies developed to estimate the total runoff
volume, the peak rate of runoff, and the stream hydrograph produced from land
surfaces under a variety of conditions. In this chapter we describe some of the
methods that are most widely used throughout the country. It is certainly not
a complete list of procedures, nor is it intended to discourage the use of new
and better methods as they become available. There are a wide variety of both
public- and private-domain computer models available for performing stormwater
calculations, and these models use one or more calculation methodologies to
estimate runoff characteristics, following procedures discussed here.
To facilitate a consistent and organized presentation of information throughout
the region, assist design engineers in meeting the site design criteria recom-
mended, and help reviewers analyze project data, a series of worksheets are
provided in Appendix A for design professionals to complete and submit with
their development applications. It should be noted that since the traditional focus
of stormwater management in most cities has been related to hydromodification
and peak rate, the methods in this chapter for addressing volume, recharge, and
quality may be relatively new in some areas of the region.
6.2 EXISTING METHODOLOGIES FOR RUNOFF
VOLUME CALCULATIONS
Runoff Curve Number Method
The runoff curve number method, developed by the U.S. Soil Conservation Ser-
vice [SCS; now the Natural Resources Conservation Service (NRCS)], is perhaps
the most commonly used tool in the country for estimating runoff volumes. In
this method, runoff is calculated based on precipitation, curve number, watershed
storage, and initial abstraction. When rainfall is greater than the initial abstraction
(i.e.,
P>I
a
), runoff is given by [1]
I
a
)
2
(P
−
Q
v
=
(P
−
I
a
)
+
S
where
Q
v
=
runoff (in.)
P
=
rainfall (in.)
I
a
=
initial abstraction (in.)
S
=
potential maximum retention after runoff begins (in.)
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