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H119
Estimating Storm Water Runoff

John Poullain, PE

This two-hour online course provides general guidelines and techniques for estimating storm water runoff from development areas. Storm water management is necessary to control erosion and sediment from construction activities. Estimating stormwater runoff is the first step in designing a stormwater management system. The rational method is primarily discussed in the course. Water quality objectives must be met to avoid discharging pollutants into waterways, creeks and rivers. The necessary data and terms describing design storms such as hydrographs, time of concentration, lag time, duration, manning’s n and runoff coefficient C are discussed. Comparisons between predevelopment and post development conditions on a site hydrograph will show the importance and benefits for stormwater management.

The rational equation was developed from simplified runoff analysis using isochrones, lines of equal travel time. It is the simplest method to determine peak discharges from an area to culvert or other points of interest. It is not as sophisticated as the SCS TR-55 method that can be used for much larger drainage areas (up to 20-sq. mi.) but has commonly been used for sizing sewers. The rational method uses a coefficient (C) based on the soil type, developments and drainage basin slopes. The rainfall intensity can be found from intensity/duration/frequency (IDF) curves for rainfall in the geographical region being analyzed.

The course is based on methodology and data used by the State of Florida but is similar to that used by other states, all of which have variances in formula values and methodology used in applying the formula. Hydrological data can be obtained for different geographical regions from Technical paper No. 40 of the Weather Bureau or from local sources. Local governments usually determine the storm frequency depending on the impact of development. The subject of stormwater drainage is complicated, sometimes controversial and accuracy is dependent on an individual's judgment and experience.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.


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