Modification of Canal Flow due to Stream-Aquifer Interaction

Unsteady canal flow in an integrated canal-flow-groundwater-flow system is analyzed by solving the coupled equations governing canal flow, groundwater flow and the seepage between them. Analytical solu- tions are obtained for the coupled system for small water-level disturbances using Fourier analysis methods and complex variables. Dimensionless parameter groups characterizing the aquifer, the canal, and the sediment layer are identified using the governing equations and the solution. The influence in the aquifer and the semipermeable bottom sediment layer due to disturbances in canal flow is studied. The analytical solutions are compared to numerical solutions obtained using the MODFLOW model and the Hydrologic Simulation Engine of the South Florida Regional Simulation Model. Results of the analysis are useful in determining the range of aquifer, sediment, and canal characteristics for which stream-aquifer interaction is important. The results can be used to determine the conditions for which the canal is hydraulically disconnected from the aquifer because of the sediment layer. The analytical solution is useful to understand the propagation characteristics of small-amplitude water-level disturbances in the canal and the aquifer. The characteristics studied include the amplitude decay constant and the speed. The solution can be used to design benchmark problems that can be used to evaluate integrated canal-flow-groundwater-flow models. The results of the study can be used to estimate the space and time steps needed in the canal and the aquifer when simulating stream-aquifer interaction. As part of the overall effort to restore the Everglades eco- system in South Florida, and to meet its regional water man- agement responsibilities, the South Florida Water Management District (SFWMD) (SFWMD 1999) and other organizations have developed a number of mathematical models to simulate the water management system. One important requirement of these models is the capability to simulate stream-aquifer in- teraction or stream-wetland interaction. A significant part of the South Florida landscape is covered with a network of ca- nals that extends for thousands of miles through wetlands, ag- ricultural areas, and urban areas. The behavior of water levels in the canals, when areas adjacent to canals are subjected to water-level changes, is not completely understood. The influ- ences of the highly conductive surface aquifer and the less conductive bottom sediment layer of the canal on the overall hydrology are also not completely known. These behaviors influence a significant part of South Florida hydrology. The current study provides a method to understand the basic pa- rameters that govern the problem and to obtain an analytical solution. Stream-aquifer and stream-wetland interactions have previ- ously been studied by a number of researchers. The study by Pinder and Sauer (1971) was conducted by using a coupled numerical model for canal flow and 2D groundwater flow. The objective was to study the influence of bank storage on the modification of flood waves. The example used by Pinder and Sauer (1971) served as a benchmark test for integrated mod- els such as MODBRANCH (Swain and Wexler 1996) and MODNET (Walton et al. 1999). These models consist of a MODFLOW model (McDonald and Harbaugh 1988) coupled with either the BRANCH model (Schaffranek 1987) or the UNET model (HEC 1996). The comparison between the so- lution obtained by Pinder and Sauer (1971) and the solutions obtained using MODBRANCH or MODNET was not perfect 1