An Analysis of Rainfall Distributions in Orange County, Florida

The Orange County Subdivision Regulations require that the peak flow resulting from a storm shall be calculated using Orange County rainfall distribution, which was derived using three severe storms in 1956 and 1960. The Federal Emergency Management Agency (FEMA) previously accepted the use of Orange County rainfall distribution in establishing the 100-year flood elevations in Orange County. Recently, FEMA requested Orange County to provide justifications for the use of the Orange County rainfall distribution. Since 1986, Orange County has been collecting rainfall data in 15-minute increments. Fourteen storms were extracted from these rainfall data with depths ranging from 4.6 to 7.6 inches and with durations ranging from 14 to 25 hours. The peak flows from these storms were calculated using the SCS method for various times of concentration and runoff curve numbers. They were compared with the peak flows calculated for the same depths and duration but using different storm distributions such as Soil Conservation Service (SCS) Type II, SCS Type II Florida Modified, Florida Department of Transportation (FDOT), and South Florida Water Management District distributions. The analysis showed that for practical values of times of concentration and curve numbers, Orange County rainfall distribution and SCS Type II Florida Modified, distribution predicted peak flows closer to the peak flows generated by the actual storms. The 100-year elevations of Shingle Creek located in an 82 square mile watershed in Orange County were calculated using these distributions. The differences in these elevations are within the computational accuracy proving that Orange County rainfall distribution can be used in determining the 100-year elevations.