Which method is recommended for evaluating storm drainage system capacity?

The Rational Method is a widely recognized approach for evaluating the capacity of storm drainage systems. It allows for the estimation of peak runoff from a specific watershed area based on several key factors, including rainfall intensity, the area contributing to the drainage system, and the runoff coefficients for the land use involved. This method is particularly useful for small to medium-sized drainage areas, typically those less than 200 acres, and it provides a straightforward calculation that incorporates local rainfall data.

The Rational Method is favored because it is relatively simple to use and can quickly provide the necessary information to design effective drainage systems. By focusing on peak flow rates, it helps engineers and designers ensure that stormwater management systems can handle expected rainfall events adequately, thus minimizing the risk of flooding and system overload.

Other methods mentioned, such as soil percolation tests, historical flow records, and installing flow meters, provide valuable data but serve different purposes. Soil percolation tests are primarily used to determine the infiltration rates of soils for septic system designs rather than storm drainage capacity. Historical flow records can offer insights into past runoff patterns but may not accurately reflect future storm events. Installing flow meters can measure existing conditions but does not inherently evaluate the system's capacity for new designs or modifications.