Chapter 10: Storm Drains

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Section 1: Introduction

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Overview of Urban Drainage Design

Proper drainage of a roadway in an urban region can be more difficult than draining roadways in sparsely settled rural areas for the following reasons:

  • heavy traffic and subsequent higher risks
  • wide roadway sections
  • relatively flat grades, both in longitudinal and transverse directions
  • shallow water courses
  • absence of side ditches and a presence of concentrated flow
  • the potential for costly property damages that may occur from ponding of water or from flow of water through built-up areas
  • a roadway section that must carry traffic and act as a channel to carry the water to some disposal point.

The flow of water along a roadway can interfere with or halt highway traffic. These conditions require sound and consistent engineering principles and the use of all available data to achieve an acceptable drainage design. The primary aim of urban drainage design is to limit the amount of water flowing along the gutters or ponding at the low areas to rates and quantities that will not interfere with traffic. You can accomplish this goal by placing inlets at appropriate locations to prevent large concentrations of runoff. The most destructive effects of an inadequate drainage system are damage to surrounding or adjacent properties, deterioration of the roadway components, and hazard and delay to traffic caused by excessive ponding in sags or excessive flow along roadway grades.

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Overview of Storm Drain Design

Although the design of a storm drain system entails many conventional procedures, certain aspects of a storm drain system design require judgment. You must establish design parameters and criteria, decide layout and component location and orientation, take responsibility for using appropriate design tools, and ensure comprehensive documentation.

The development of a storm drain design requires a trial and error approach:

  1. Analyze a tentative storm drain system.
  2. Compare the system to design criteria.
  3. Evaluate the system economically and physically.
  4. Revise the system if necessary.
  5. Analyze the revised system.
  6. Make the design comparisons again.
  7. Repeat this process until you develop a storm drain system that satisfies the technical function of collecting and disposing of the runoff and costs the least amount of money.

The proper design of any storm drainage system requires accumulation of certain basic data, familiarity with the project site, and basic understanding of the hydrologic and hydraulic principles and drainage policy associated with that design.

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