Hydrologic modeling in Jefferson County has historically been performed using the United States Army Corps of Engineers (USACE) Hydrologic Engineering Center's Flood Hydrograph Package (HEC-1). This model was used to create the hydrologic inputs necessary for the development of the 100-year topographical floodplains during the Flood Insurance Study of 1990 (FIS 90), the result of which was used to create the FEMA Floodplain Maps. The HEC-1 models developed for each watershed during the FIS 90 study became public domain after they were approved by FEMA and they became the foundation for hydrologic modeling in Jefferson County ever since. Watershed response characteristics, in the HEC-1 model, are based on simple mathematical representations of precipitation, abstraction, and routing mechanisms. The FIS 90 HEC-1 models characterized precipitation events as 6-hour duration, NRCS Type II rainfall distributions. Rainfall excess, or runoff, was then calculated by using the NRCS Curve Number method. The hydrograph method typically consists of the SCS Unit Hydrograph that depends on an accurate lag time, or time from start of a storm until a watershed is fully producing runoff.

The FIS 90 models, as mentioned above, became the "official" models in Jefferson County for the development of hydrographs used as inputs into stream models. MSD currently requires the use of these models whenever developers are required to analyze watersheds in excess of 50 acres surface area (smaller watersheds are typically analyzed with the aid of the Rational Method). Over the years modifications have been made to the models to reflect land surface changes and more detailed subbasin delineation; however, the models still closely resemble the ones used to develop the FEMA floodplains. Until further notice, the FIS 90 models (or revised versions of these models) represent the best source of hydrologic information in most watersheds in Jefferson County.

Along with the HEC-1 models mentioned above, stream models were necessary to complete the FIS 90 floodplain-mapping project. In all of the detailed study areas in Jefferson County (stream reaches not analyzed with "approximate" methods), the HEC-2 program (USACE Hydrologic Engineering Center) was used to calculate water surface elevations and floodway encroachments. The HEC-2 program computes one-dimensional, steady, gradually varied flow profiles in streams with any cross sectional geometry. As with the FIS 90 HEC-1 models for hydrologic modeling, the FIS 90 HEC-2 models are the basis of most of the hydraulic modeling currently being performed in Jefferson County.

The Hydrologic Engineering Center is in the process of re-developing its models with a NexGen "next generation" software approach. Current offerings are HEC-HMS (Hydrologic Modeling System) and HEC-RAS (River Analysis System). For hydrologic modeling, HEC-HMS replaces the HEC-1 package. It features a drag-and-drop interface for constructing basin models. The current version is 2.0 and many of the underlying algorithms have been revised. Also important is that much of the input data and all results are stored in a HEC-DSS (Data Storage System) database. While slightly increasing the complexity of data management, it allows better organization of model data and will allow a seamless connection to provide flows to the HEC-RAS (River Analysis System). HEC-RAS is a hydraulic model that fully replaces the HEC-2 model for computing flood profiles and determining floodways. HEC-RAS has a new calculation engine that was built independent of HEC-2, although it can import most HEC-2 model files. The current version is 2.2 and version 3.0, scheduled for release in Fall 2000, will include the UNET engine and support unsteady flow modeling. In the future HEC-RAS will support sediment transport, which is currently available in the HEC-6 package. The HEC-HMS, HEC-RAS, HEC-1 and HEC-2 packages may be downloaded from links to the HEC web site.

Other surface water models used in Jefferson County, to eventually be available through this model database, are SWMM models created to analyze MSD's combined sewer system and the Louisville International Airport drainage system. In addition Hydrologic Simulation Program-Fortran (HSPF) models have been developed in two watersheds. These models are highly specialized and they require a higher degree of modeling effort and expertise than the HEC-1 models. They are unique in that they may also be used to investigate hydraulic and water quality relationships. The HSPF models were developed by Lynn Jarrett (University of Louisville) to study water quality in the South Fork Beargrass and Chenowith Run stream systems.