Connecting a Community Through Innovative Modeling of the Withlacoochee River

The Southwest Florida Water Management District (SWFWMD) began the watershed initiative program to provide comprehensive understanding of watersheds. The Withlacoochee Watershed Initiative delved into providing an understanding of the complexities of the Withlacoochee River and utilizing a modeling approach which uncovered many of the longstanding uncertainties surrounding the river. The Withlacoochee is a unique riverine system, winding 160 miles northward through eight Central Florida counties before eventually discharging into the Gulf of Mexico. The Withlacoochee River was called by its earliest inhabitants 'We-Thalko-Chee' or 'Little Big Water', characterizing the great fluctuations between drought and flood that are commonly exhibited by the river due to naturally changing hydrologic conditions. This complex system covering over 2000 square miles was one of the least understood in the region and yet because of its beauty, drew homeowners flocking to its banks. While alterations over the past 130 years have attempted to control this natural wonder to suit commercial navigation, industry, and private needs, the river continues to plot its own course. True to its name, homeowners on the Withlacoochee have experienced extreme drought followed by lasting floods, exemplified in Figure 1. In recent decades, public meetings and workshops involving stakeholders, special interest groups and residents identified several critical issues and ideas to either restore or further alter portions of the system. To comprehensively address those concerns, a first of its kind computer model of the entire Withlacoochee River and its surrounding watershed was developed. This included an unprecedented effort to map the entire bottom of the Withlacoochee River from the Green Swamp to the Gulf of Mexico and calibrating the model to the devastating 2004 Hurricane Season.

METHODOLOGY
To address the unique characteristics of the river and watershed, a comprehensive model was developed using the Interconnected Channel and Pond Routing software version 4 (ICPRv4) created by streamline technologies in Orlando, Florida. This computer model can simulate one-dimensional flow in channels as well as two-dimensional flow in wetlands and marsh areas, both of which are prevalent throughout the Withlacoochee River watershed. This flexibility was necessary to account for the river conditions which were alternatively a wide expansive marsh transitioning to a deeply channelized river section. High resolution LiDAR data supplemented with field work was used to characterize the hydraulic parameters in the watershed, exemplified in Figure 2. This detailed data combine with automated techniques for understanding and delineating the watershed boundaries served to guide the model development process. Additionally, the model was able to account for the volume and timing of contributing inflows from the surrounding watershed using dynamic rainfall inputs and varying initial water levels. This was a critical element since water levels and flow along the river are a direct result of spatially varied rainfall and the watershed's antecedent conditions. In the low condition, water is contained within the main channel and rainfall on the surrounding land will infiltrate prior to contributing flows to the river. In the high condition, water levels are higher and the surrounding land is inundated. Subsequently, when rainfall occurs, runoff immediately causes river levels to rise and flows to increase, since the watershed's soils are already saturated. Because the ICPRv4 model simulates hydrology and hydraulic conditions in parallel, these dynamic changes in soil moisture can be accurately simulated.

RESULTS
Calibration The river model was calibrated to over 20 USGS gauging stations over a three month period in 2004, which started in drought conditions until three hurricanes crisscrossed the watershed, sustaining flooding conditions for months. Despite the numerous calibration parameters available in ICPRv4, initial model results would not converge to the measured data. This prompted additional field work and ground truthing of model assumptions. During this process, it was learned that significant portions of the terrain, particularly in marsh area incorrectly depicted ground conditions. It was learned that the model was highly sensitive to storage and overland flow roughness within 2D zones. Correcting the terrain lead to a more accurate flow regime and ultimately a calibrated model. Resulting hydrograph depicting the rise in water level between modeled and measured data is seen in Figure 3. The model accurately depicted the more than two week lag time between headwater and discharge into the Gulf of Mexico. As seen in the graph, following each hurricane, water conditions sharply rise, yet do not fall quickly, rather remain high, setting an inundated condition subsequent storm events. In this watershed, the reality of highly variable initial conditions became a key feature in understanding flood levels throughout the watershed and inflow tributaries. Scenarios Working with SWFWMD staff and stakeholders throughout the watershed, both past and present concerns were documented, ultimately resulting in 19 scenarios as shown in Figure 4. These scenarios are designed to determine what effect historical or future changes might have on water levels and flows throughout the entire river and watershed. Results indicated how management strategies can optimize the balance between retaining water high for recreation, while being able to evacuate it prior to catastrophic events. Results indicated which river alterations would have had a positive impact on flooding conditions during the hurricane events, and which would not.

CONCLUSION AND DISCUSSION
In the initial phases of the project, the public was skeptical of the overall process, accusing the water management district of mismanagement of the resource and being the cause of both extended drought conditions, as well as, the prolonged flooding conditions. Through a series of public meetings, transparency of the modeling approach, and ultimately demonstrating that the calibrated model would accurately predict conditions in the watershed, a trust was built with these concerned citizens. This trust extended to the results of the 19 model scenarios including those demonstrating existing water management procedures and simulations demonstrating proposed alterations to those procedures. The modeling effort and the project itself are a first of its kind floodplain mapping using a 2D model and providing an understanding of a complex river system. But what makes this project truly unique is its ability to transcend the algorithms and modeling complexities and connect with the public. This calibrated model and this project was able to heal the wound of an irate public skeptical of the water management district practices. These results now serve as the best available data in the region and the model approach for large watershed studies in the Water Management District.