River Transport Modeling for Unsteady Flows

Coupled flow-mass transport models are applied to a 17-mile (27.8-km) reach of the Chattahoochee River between Buford and Norcross, Ga. Discharge variations from 550 cfs to 8,000 cfs (15 m³/s to 100 m³/s) in less than 20 min are analyzed. Accompanying stage variations are as much as 7 ft (2 m). Two data sets with 5-min time resolution are used to calibrate and verify the flow model. A longitudinal depth profile at steady low flow is also used in the calibration. Data from a 3-day continuous-injection dye study are used to verify the mass-transport model. The study demonstrates the feasibility of coupled models under highly unsteady flow conditions. The dye study demonstrates the unusual nature of conservative mass transport in unsteady flow. Both the flow and mass-transport models produce answers approaching the accuracy of the data. These are estimated as ±20 min in time, ±15%-20% for discharge, and ±0.5-ft (0.1-m ) for stage.