25 October 2004
much more efficient and cost effective alternative to discharging directly into the stream
AIR FORCE: Air Force Instruction (AFI) 32-7080 lays the framework for P2
implementation. Compliance by all Air Force installations is required. Air and water
pollutant reduction is one of the six P2 program elements. P2 is mandated at the Major
Command (MAJCOM) level, and the Air Force Center for Environmental Excellence is
the primary provider of P2 technical support services. Installations must implement P2
management plans and conduct regular P2 opportunity assessments, which should be
based on existing waste stream management plans when they exist.10
ARMY: P2 is a required element in the Army's Sustainable Project Rating
Tool (SPiRiT); compliance with SPiRiT is now mandatory for MILCON construction
projects. P2 plans for Army installations are developed from opportunity assessments
of existing waste stream data and are designed to maximize environmental compliance.
The U.S. Army Environmental Center provides P2-related technical and policy
DESIGN GUIDANCE AND STANDARDS
Methods to Determine Effectiveness. Stormwater projects are typically
designed with a particular objective in mind, such as flood control or water quality
improvement. Such projects typically require that the designer evaluate the
effectiveness of the proposed treatments at meeting the stated objectives.
A number of hydrologic models have been developed to model surface runoff
from a given drainage area. Because conventional models are primarily concerned with
computing flow rates or flood hydrographs at a point of interest, this approach to
hydrologic analysis must be modified in cases where not all of the runoff from a given
site converges to a single point. Typical watershed models take into account general
land cover and stream channel characteristics. To account for LID features and runoff
management devices, refinement of the analysis may be desirable. A variety of tools
are freely available from public agencies:
5-5.1.1 Natural Resources Conservation Service (NRCS). The NRCS, formerly
called the Soil Conservation Service, has been developing runoff models for decades.
The NRCS models TR-20 and TR-55 account for variations in land cover and the
velocity of water movement across a watershed. Of particular interest are the
determination of a drainage area's curve number (CN) and time of concentration (Tc).
The value of CN reflects the degree to which land surface conditions will generate
runoff, while the value of Tc indicates how quickly the runoff will converge at a particular
point downstream. TR-20 and TR-55 are popular for watershed modeling but are
generally not recommended for predicting runoff from small storms.
5-5.1.2 Federal Highway Administration (FHWA). The FHWA has developed a
variety of software packages, primarily concerned with channel and pipe hydraulics.
Air Force, 1994.