Alligator Creek Watershed Management Plan

Status: Complete

Digital topographic information (DTI) is the fundamental starting point for the watershed model. The main source of digital data for this project is airborne Light Detection and Ranging (LiDAR) data. LiDAR data is collected from airplane-mounted equipment which sends laser pulses to the ground to collect high definition topographic information. The DTI is used to generate a digital elevation model (DEM). After reviewing the DEM, areas of concern are documented. These areas can be addressed later in the project, during model development or floodplain mapping, or these areas can be reprocessed to enhance the quality of the data. The DEM for this project was reprocessed following SWFWMD’s Hydro Enhancement Specification to provide the best possibility that all meaningful hydrologic features are discovered and used accurately for the watershed evaluation and subsequent modeling.

Status: Complete

The next step in developing a watershed model consisted of assembling and developing the GIS data needed to evaluate the watershed. An efficient watershed evaluation process required using GIS for storing the Digital Terrain Model that was used to characterize the drainage within the watershed; storing spatial background data (land use, soils, hydraulic structure location, etc.) needed for engineers to develop a good understanding of the watershed; storing, documenting, and archiving the data collected during the watershed evaluation; and establishing relationships between data sets that allows for the efficient processing, reviewing, and quality control of data.

GIS processing of the DEM was performed to develop an initial characterization of drainage within the watershed. GIS tools were used to develop initial catchments and the watershed connectivity. Urban watersheds like Alligator Creek need extra attention to detail during this process. Portions of the watershed do not drain based on topography but instead based on subsurface storm water networks. Considering both the surface drainage and subsurface pipe networks was essential to correctly developing catchments for this watershed.

Significant drainage features often require site visits to confirm model input data. These features were identified by reviewing the catchments, and inspecting recent aerial photography, topography, construction as-built drawings, and storm water asset inventories. With approximately 1000 catchments at an average catchment size of approximately 5.5 acres, the watershed contains over 1000 significant features. Once visiting all required structures, catchment delineations were verified against observations in the field. The final step during watershed evaluation was creating the schematic representing the connections between catchments. Both the surface and subsurface connections are represented in the schematic. The model will use these connections to determine how water flows between catchments.

Status: Complete

With the model schematic completed, the first step in the Watershed Management Plan phase is to parameterize the model. Parameterization is divided into a number of sub-tasks, some of which include parameters determined from the DEM, through other GIS data sources, through conventional survey, or from site visits performed in the previous phase. Some examples of parameters are: catchment runoff, catchment storage volume, pipe inverts, and channel cross-sections.

Once the model was fully parameterized in GIS, it was loaded into an ICPR model. The ICPR model is calibrated using a major storm event of recent memory. The Alligator Creek model was calibrated to the July 7, 2011 event which generated 6.4 inches of rainfall on average watershed wide. The model was then verified to the March 31, 2011 event which generated 3.9 inched of rainfall on average watershed wide. Model results can be compared to measured peak stage values or observed flooding. Any observed flooding accounts from past flooding events can be used to help calibrate the model. The calibrated model is used to run design storm events. The design storms represent a given rainfall amount distributed over time based on a rainfall distribution that has been specially developed for Florida’s weather patterns. The results from these model runs are used to assess the flood risk of the parcels in the watershed. This design storm model was peer reviewed by RS&H to make sure that sound modeling techniques were used and that the model results were reasonable.

The design storm model results are also used to provide the City with an assessment of the flooding level of service in the watershed. The City will also be provided with the results of a pollutant loading model for the watershed. Once the results of the water quantity studies (flood risk and level of service) and water quality study (pollutant loading) are complete, best management practice (BMP) alternatives analysis can begin. The BMP alternatives analysis is one of the important results of the watershed management plan as it identifies opportunities for the City and SWFWMD to address the flooding, water quality, and other problems identified during the project. By merging the spatial design storm results and spatial pollutant loading estimates, potential BMP sites are quickly identified and prioritized.