LID DESIGN CALCULATIONS AND METHODOLOGY - Low Impact Development and Sustainable Stormwater Management - page 111

6
LID DESIGN
CALCULATIONS
AND METHODOLOGY
6.1 INTRODUCTION TO STORMWATER 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 a critical 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 attempts to replicate 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 resultant hydrograph that occurs in the receiving stream follow-
ing rainfall. Since this estimated surface flow hydrograph is expected to mimic
the hydrograph observed, all models “calibrate” or adjust input parameters to
replicate this energy waveform.
Initially, most of this effort was undertaken to allow the building of structures
within the stream or river channel, in the form of culverts, bridges, dams, and