Modelling the Morphodynamic Response To Flow Regulation In An Artificial Reservoir

Reservoir sedimentation is the gradual deposition of sediment transported by inflowing rivers in artificial lakes. Monitoring this accumulation is essential to preserve storage volume and protect infrastructure, such as turbine components. Numerical models are powerful tools to address this goal, as they can simulate morphodynamic processes across both temporal and spatial scales. In this study, we developed a Delft3D-FM model of Lake Seminole (USA) to answer two questions: (1) how does deposition change between regular flow and flood conditions? and (2) what is the impact of upstream flow regulation on sediment deposition?

Created in 1954 with the construction of the Jim Woodruff Dam, Lake Seminole sits at the confluence of the Chattahoochee and Flint rivers. This location is an ideal site because the reservoir is divided into two distinct arms:

• The Chattahoochee arm, which is fed by the heavily regulated Chattahoochee River and experiences strong daily discharge fluctuations due to hydropeaking.
• The Flint arm, which is fed by the minimally regulated Flint River and exhibits a natural flow regime.

Here we develop a morphodynamic Delft3D-FM model coupled with the Real-Time Control (RTC) module. Time-variable discharge inputs from USGS gauges define the upstream boundary conditions, while water levels at the dam define the downstream boundary. The hydrodynamic model is calibrated using OpenDA by adjusting Manning’s roughness coefficients based on Signature 1000 ADCP velocity measurements. The sediment transport model is calibrated using suspended sediment concentrations collected via Teledyne ISCO water samplers.

Model results show that regular flow conditions lead to net deposition, while flood conditions generate temporary and deep scouring, resulting in net erosion. Additionally, in the Chattahoochee arm, daily discharge fluctuations driven by hydropower redistribute fine sediments throughout the reservoir during peak flows. As discharge drops, sediments settle but are quickly remobilized by the subsequent peak flows. Overall, this study illustrates how hydraulic conditions drive morphological change in Lake Seminole and underscores the significant impact of river regulation on reservoir sedimentation.