Geoscience Reference
In-Depth Information
25 Stormwater Calculations
25.1 INTRODUCTION *
For the environmental practitioner involved with stormwater compliance programs, March 10,
2003, was a very significant date: It was the deadline for compliance with new National Pollutant
Discharge Elimination System (NPDES) permit applications for municipal separate storm sewer
systems (MS4s) that had previously been exempt, but after this date no longer are. The affected
MS4s include federal and state regulated operators—serving less than 100,000 people—for areas
such as military installations, prisons, hospitals, universities, and other, who are required, after
March 10, 2003, to have complied with the Storm Water Phase II Rule, published December 8, 1999.
Additionally, state regulators may subject certain other sources to regulations, such as municipally
owned industrial sources, construction sites disturbing less than 1 acre, and sources that contribute
to a significant degradation of water quality.
To comply with the new stormwater regulations, environmental engineers are called upon to
design stormwater discharge control systems. In this design phase, several mathematical computa-
tions are made to ensure that the finished stormwater discharge control system meets the regulatory
requirements.
This chapter provides guidelines for performing various engineering calculations associated
with the design of stormwater management facilities such as extended detention and retention basins
and multistage outlet structures. The prerequisite information for using these calculations is the
determination of the hydrologic characteristic of the contributing watershed in the form of the peak
discharge (in cubic feet per second, cfs ), or a runoff hydrograph, depending on the hydrologic and
hydraulic routing methods used. Thus, before discussing the various math computations used in
engineering a stormwater discharge system, we begin by defining general stormwater terms and
acronyms and present a detailed discussion of hydrologic methods.
25.2 STORMWATER TERMS AND ACRONYMS
Anti-seep collar —A device constructed around a pipe or other conduit and placed into a dam,
levee, or dike for the purpose of reducing seepage losses and piping failures along the
conduit it surrounds.
Anti-vortex device —A device placed at the entrance of a pipe conduit structure to help pre-
vent swirling action and cavitation from reducing the flow capacity of the conduit system.
Aquatic bench —A 10- to 15-foot wide bench around the inside perimeter of a permanent pool
that ranges in depth from zero to 12 inches. Vegetated with emergent plants, the bench aug-
ments pollutant removal, provides habitats, protects the shoreline from the effects of water
fluctuations, and enhances safety.
* Much of the information contained in this chapter is adapted from Spellman, F.R. and Drinan, J.E., Stormwater Discharge
Management: A Practical Guide to Compliance , Government Institutes, Lanham, MD, 2003; excerpted from Federal
and State Regulations , Soil Conservation Service (SCS) Technical Release Nos. 20 and 55 (TR-20 and TR-55), and
Virginia Stormwater Management Handbook (1999) and U.S. Corps of Engineers.
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