Page 93 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

74 Cha pte r T w o

but also gives the system less time to stabilize with vegetation in between major
storms. Therefore, this credit requires that the controls also restrict the outflow for
smaller one-year storms to the one-year rate. There is still a chance that storms with a
frequency of less than a year will now have the larger impacts of the one-year storms
based on rate, but at least the impacts of potentially increasing the frequency of the
two-year have been drastically reduced.
The two restrictions in Cases 1 and 2 on postdevelopment runoff can be attained with
a combination of many strategies. Surfaces can be chosen with lower runoff coefficients.
Similarly options to detain and infiltrate or use the rainfall such as retention basins,
rainwater harvesting for irrigation or interior uses, or underground infiltration systems
can be used to reduce the postdevelopment runoff. The stormwater control methods
should be chosen based on environmental, societal, and economic criteria, in addition to
local restrictions or preferences on many of the options. These methods and options are
frequently referred to as best management practices (BMPs). There is also a relatively new
and emerging stormwater management concept referred to as low-impact development
(LID) which uses a subset of these BMPs, many for multiple purposes. These are sometimes
also referred to as integrated management practices (IMPs), and the main goal is to mimic the
natural hydrologic cycle as much as possible on the site. Usually, the more the site mimics
the natural hydrological cycle, the fewer the opportunities for anomalies that can disrupt
downstream flows and cause nonpoint source pollution. Chapter 10 deals with other
stormwater management options such as LID that may help eradicate some of the potential
problems of designing to only to a few specific storm rates and volumes.
The stormwater runoff calculations for Case 2 and the first option of Case 1 should be
performed using accepted hydrological models for estimating runoff. The second option of
Case 1 requires that the stream protection measures be described in detail and that similar
accepted runoff calculations be performed for the predevelopment and postdevelopment
runoff rates and quantities. It must be demonstrated that the postdevelopment runoff rates
and quantities will be below critical values for the receiving waterways after the measures
are in place. No specific frequency storm is given for this requirement. It is assumed that at
a minimum, both a one-year and a two-year frequency storm should be used. Again, more
details on methods can be found in Chap. 10.

Stream Channel Protection Many stream protection strategies are now using a form of
control at the headwaters instead of protection within the channel. There are two main
ways to control at the headwaters: either by controlling the hydrology upstream or by
modifying the hydraulics at the stream headwaters.
Controlling the hydrology upstream is similar to other on-site stormwater BMPs.
Usually retention and infiltration is the preferred strategy. An example is the incorporation
of infiltration gardens on projects (bioretention without a subdrain) which may significantly
reduce the cumulative runoff based on typical two-year storm-current regulated detention
requirements. A good reference for this is the North Carolina Department of Environment
and Natural Resources, Division of Water Quality, Stormwater Best Management Practices
Manual, July 2007.
Controlling the hydraulics at the headwaters is usually by some form of energy
dissipation at the culvert outlets. Other than increasing the imperviousness of the
watershed, eroding headwater tributaries caused by culvert scour have typically been
an observed contributor to the initiation of stream departures from stable stream
equilibrium. Energy dissipation techniques are varied and include such items as
channels for level spreaders, plunge pools, and rip rap aprons.