Page 398 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

CHAPTER 10

Low-Impact Development

and Stormwater Issues

tormwater management is an important design and construction practice. Without
proper management, the resulting runoff can cause many problems including
Sflooding and poor water quality. Traditionally, the design of stormwater facilities
is taught in hydrology and watershed management classes. These types of courses are
important to stormwater management, but focus more on the big picture, not individual
construction projects. The purpose of this chapter is to look at stormwater management
from the construction and individual site perspective so that it can be better integrated
into the LEED or green building process. However, the actual design of any stormwater
management system should be done by practitioners experienced in civil engineering
systems, hydrology, and watershed management in addition to these newer concepts.
The concepts that are of most interest for stormwater management are the rate of
stormwater runoff from a site or area, the amount of stormwater runoff from an area,
the pollutant loadings from the runoff, and the impact on erosion and subsequent
pollution that runoff has on the downstream areas. Over the decades many methods have
been developed to estimate runoff. They are used for calculating rates, volumes, and a
concept referred to as the time of concentration. A simplified definition of the time of
concentration is the time it takes for various runoff contributions from a defined site or area
to come together to a point producing a peak rate at that point. Akan and Houghtalen
(2003) list three methods for determining the time of concentration: the SCS (Soil
Conservation Service) time-of-concentration method, kinematic time-of-concentration
formulas, and the Kirpich formula. They also list the following runoff rate calculation
methods: unit hydrograph method, TR-55 graphical peak discharge method and tabular
hydrograph method, the Santa Barbara urban hydrograph method, U.S. Geological Survey
(USGS) regressions equations, the rational method, and the kinematic rational methods.
The concept of percent imperviousness as used by the USGBC, and as discussed in
Chap. 2, is based on the rational method. The rational method is a very old (nearly a
century in use) and fairly simplified method. It is applicable to smaller watersheds,
usually much smaller than 100 acres. Most project areas associated with the LEED
certification process are relatively small (10 acres or less) and therefore fall into this
category. It is mainly a method to estimate peak runoff based on a 2- to 5-year storm.
The calculations for both peak runoff rates and volumes usually require some stormwater
path information, and therefore the other hydrological models are frequently used,
expecially the TR-55 method for larger watersheds. Regardless, the following discussions
concern methods to control runoff from a project, and proper hydrological modeling

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