RAINWATER AS THE RESOURCE - Low Impact Development and Sustainable Stormwater Management - page 30

1.7 STORMWATER VOLUME METHODOLOGIES
The land development process results in significantly greater volumes of
runoff and conveys land pollutants to surface waters, but the difficult issue
remains as to how to prevent or reduce this impact. Before we consider
how best to “manage” our runoff, we must decide how much of the net
increase should or can be reduced or prevented. Since our measurements of
the hydrologic cycle are limited to input (rainfall) and output (stream flow),
understanding what happens in between in a watershed or catchment is the initial
issue.
This is certainly not a new problem, and engineers and hydrologists have
developed a number of analytical methods over the past several decades to esti-
mate the amount of runoff produced in a watershed. These various methods
are based on the land that comprises the drainage area, and attempt to repli-
cate the complex process of how surface runoff is produced and the multiple
pathways followed by each raindrop as it follows the energy gradient downhill.
Whatever the algorithm formulated to describe the process, the end result is to
estimate the form of the resulting hydrograph that occurs in the receiving stream
following rainfall. Since this surface flow hydrograph is the end result of the
process, all models “calibrate” or adjust input parameters to replicate this energy
waveform.
Since most of this effort has been undertaken to allow the building of structures
within the stream or river channel, in the form of culverts, bridges, dams, and
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 current stormwater
management strategy of reducing or mitigating runoff volume produces a very
different perspective on these various modeling procedures, and begs the ques-
tion: Which method best estimates the change in runoff volume resulting from
land development?
Many methodologies have been 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 Chapter 6 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 avail-
able for performing stormwater calculations, and these models use one or
more calculation methodologies to estimate runoff characteristics, following
procedures discussed in Chapter 6. To facilitate a consistent and organized
presentation of information throughout the country, to assist design engineers
in meeting the recommended control guidelines, and to 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.