Abstract

C-100 Basin is a 40.6 square mile (105 km2) area in southern Miami-Dade County (Florida) with an approximate elevation range of 5 to 15 ft (1.5 to 4.6 m) NGVD. The basin is an urbanized area of the county, with a western boundary only a few miles from the Florida Everglades, and it drains eastward to Biscayne Bay through the C-100 canal and its tributary canals. There are three control structures in the basin which maintain groundwater levels, provide flood relief, and prevent saltwater intrusion from the bay.

Development in South Miami-Dade County (including the C-100 Basin) has heightened concerns about the impact of stormwater runoff on the quality and quantity of water discharged into Biscayne National Park via the canals in these areas. The primary objective of this planning effort is to improve the quality, quantity and periodicity of freshwater discharges to, and prevent degradation of, Biscayne National Park. Parts II and III of the National Pollutant Discharge Elimination System (NPDES) permit requires implementation of a comprehensive Stormwater Management Master Plan for Miami-Dade County to provide pollutant load reduction and water quantity control by the construction/retrofitting of stormwater management systems. Unique modeling techniques were used in order to accurately calibrate the model of the C-100 Basin for both event and continuous simulations. The basin challenges and model solutions are presented in this chapter.

The basin contains extensive areas of French drains that have been designed for the 5 y storm. For other Miami-Dade County basin models, the DCIA had been calculated for each sub-basin and then adjusted to a value based upon the percent of the area covered by French drains. The area containing French drains were assigned 0% imperviousness. This provided excess infiltration for design storms of higher volumes than the 5 y storm, resulting in the forced calibration of some parameters beyond reasonable values. The solution used for the C-100 Basin was to separate each sub-basin into sub-catchments with and without French drains. The areas with French drains are given 0% imperviousness, using Horton parameters with a maximum infiltration volume equal to the 5 y storm. The other sub-catchment is given Green-Ampt infiltration parameters and the calculated DCIA.

The basin has substantial lateral groundwater flow through an extremely transmissive aquifer providing baseflow during extreme events that far exceeds the volumes that may be provided using the groundwater routines in SWMM. This baseflow was added to the model using nodes with runoff not linked to the rest of the model, but with groundwater linked to nodes dispersed along the C-100 canal and its tributaries.

The control structures in the basin have operating procedures that are generally determined by headwater stage; however, the rules change from wet season to dry season. The operations of these structures were modeled using combinations of bendable weirs and time-varying orifices.

The basin was simulated for the wet, average and dry year for both the present and future basin conditions for surface runoff, groundwater interflow contribution, canal hydraulics and surface runoff water quality generation and BMP removal. The water quality model of basin C-100 consisted of 14 water quality constituents along with 125 BMPs. The BMPs were simulated using the Transport layer of SWMM. The three existing year models were compared to monitored data using the Nash/Sutcliffe statistic for evaluating model “robustness”.

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