Sediment flux in the gravel-sand transition of the Fraser River

Sediment transport conditions in the gravel-sand transition of the Fraser River, Canada, are examined using field data collected during the falling limb of a spring freshet in June 2007. The gravel-sand transition in the Fraser River is abrupt, defined by a 12 km reach where supply-limited bedforms migrate over a gravelly bed before evolving to channel-spanning dunes that mark the beginning of the sandy reach. Field measurements were obtained at a flow of ~9,000 m3 s1, immediately following a high peak flow of 11,800 m3 s1. Suspended sand concentrations were measured over the supply-limited bedforms, as well as farther downstream in the sand-bedded reach using a point-integrated water sampler. The data indicate that sand concentrations are 2-4X greater in the sand-bed reach, suggesting that local bed composition strongly influences sand concentration of the overlying water column. Suspended sand flux, calculated by coupling flow velocities (acquired using an acoustic Doppler profiler) with measured sediment concentrations, is also spatially variable, increasing by nearly a factor of two from the transitional reach to the sand-bed reach (47,000 T/day to 87,000 T/day). Bedform sediment flux, calculated using multibeam data that defines dune size and translation rate, increases by four-fold from the area of supply-limited bedforms to the downstream sand reach (1500 T/day to 6200 T/day). Field measurements and numerical models are combined to explore the transport conditions associated with this gravel-sand transition. Bedload transport formulae are used to estimate values of skin-friction stress from measured bedform transport rates. The Rouse concentration profile for suspended sediment is fit to river data in order to estimate the transporting shear velocities. We find that the skin-friction stress is relatively uniform over the transitional reach (0.4-1.6 N m-2), indicating that the spatial imbalance in both suspended and bedload sand flux is likely associated with sediment hysteresis, whereby sands are effectively “mined” or removed from the gravel reach during the rising limb and peak of the freshet hydrograph and transported downstream.