An Overview of Urban Stormwater-Management Practices in Miami-Dade County, Florida

Chin, David

Agencies with jurisdiction over stormwater-management systems in Miami-Dade County, Florida, include the MiamiDade Department of Environmental Resources Management (DERM), South Florida Water Management District (SFWMD), and Florida Department of Transportation (FDOT). These agencies are primarily concerned with minor drainage systems that handle runoff from storms with return periods of 10 years or less (DERM), major drainage systems that handle runoff from storms with return periods of 25 years or more (SFWMD), and runoff from major roadways (FDOT). All drainage regulations require retention of at least a specified water-quality vplume (defined volume of surface nmoft), typically the first inch of runoff. The DERM and FDOT intensity duration frequency (IDF) curves used as a basis for design are si milar but different, with differences particularly apparent for short-duration storms. The SFWMD 25-year 3-day storm incorporates an IDF curve that is substantially different from both the IDF curves of DERM and FDOT. A DERM methodology for designing closed exfiltration systems is applicable to storms of !-hour duration, but is not applicable to all storms with a given T-year return period. A trench design that is applicable to all storms with a given T-year return period is presented as an alternative approach. Stormwater-management systems include minor and major drainage systems. Minor drainage systems typically consist of stormwater inlets to subsurface systems that handle runoff from design storms with return periods of 10 years or less, whereas major drainage systems typically include the above-ground pathways that handle runoff from storms with greater return periods. A clear understandi ng of the role and function of storm water-management systems and the regulatory environment in which these systems are designed provides for realistic expectations of their performance during periods of heavy rainfall. The purposes of this report are to: (1) present an overview of the State and local regulations governing the design of stormwater management systems in Miami-Dade County, (2) describe the types of storm water-management systems that are commonly used, and (3) present an approach for designing closed exfiltration trench systems to accept runoff from all storms with a given T-year returl'!. period. Agency documents should be consulted for detailed design guidance. This report is the result of a cooperative effort between the U.S. Geological Survey (USGS) and SFWMD.