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Stormwater Drainage Issues

Published October 11, 2017 by Sean M. Riley, John A. Bologna, David J. Michniewicz,

We have been asked by several clients what can be done to mitigate stromwater drainage issues in the light of the August 19, 2017 storm.

First, consider the following facts:

  • Parts of the Outer Cape received 8" or more of rain over a four to six hour period, making it a "freak" event;
  • The storm magnitude was on the order of a 500-year to 1000-year storm event;
  • Stormwater drainage systems are normally designed for a 25-year storm event - 5.75" of rain in a 24-hour period (or 6.88" for 100-year event) in most Cape communities.

For comparison, for arid areas in the State of Arizona, the range is approximately 2.5" in a 24-hour period, while in the rainy parts of Washington State and Northern California, the design storm is on the order of 10" in a 24-hour period. Civil engineers are tasked to design storm drainage systems to collect and convey storm runoff to an suitable on site area during a designated storm event. Although the design of a storm drain system entails many conventional procedures, the design also requires engineering judgment. The proper design of any storm drainage system requires accumulation of basic data, familiarity with the project site, and understanding of the hydrologic and hydraulic principles and drainage policy associated with that design. The drainage system needs to handle runoff from a designated storm event that defines the rainfall intensity and duration. A common standard used in many communities is the 25 year storm event, which in lay terms, defines the maximum 24-hour precipitation event with a probable recurrence interval of once in 25 years, as defined by the National Weather Service.

Therefore, mitigation for future events might involve a combination of waterproofing the "basement" areas, modifying site grading, and installing new or enlarged stormwater drainage systems. Property owners will need to weigh the cost of mitigation measures against the risk of another storm event of the same or similar magnitude.