Task 11 – Basin Storage/Discharge Analysis

Land use changes in Lee County, as a result of development, may impact the hydrology of the region by decreasing the available ground surface over which water can infiltrate and by decreasing the time it takes for water to reach the discharge point in a watershed. Therefore, it is important to establish maximum discharge coming from each watershed during extreme rainfall events, such that standards for future development account for discharge beyond this rate.

The MIKE SHE/11 model that was used to develop the SLCFMP uses a physics-based approach for calculating channel flow, overland flow, and flow through the groundwater. Water movement over the land surface, as well as subsurface flows, is calculated at each cell for each timestep. Therefore, water is free to cross over watershed divides, as long as the topography and water surface elevation supports that movement. In this regard, the traditional definition of basins was not followed in the model, which instead allowed topography to largely control runoff, with the exception of user-defined boundaries such as major roads, berms, or levees that are known impediments to sheet flow.

The SFWMD sub-basin map, the Lee County basin map, and the Lehigh Acres basin map were combined and modified to improve the representation of the regional basins (for the purposes of this study, the delineated basins will be herein referred to as sub-watersheds to avoid confusion with the basin modeling approach). The sub-watersheds were then fine-tuned in specific areas to 1) split the sub-watershed to include major boundaries such as roads, 2) refine the boundary to match existing roads and watershed divides that may not have existed when the original maps were developed, and 3) to develop a naming system useful for this study. In total, 98 sub-watersheds were identified.