Sediment transport and the freshwater modification of tidal hydraulics approaching a fjordic sill: the Falls of Lora, Loch Etive, western Scotland, UK

The Falls of Lora (FoL), Loch Etive, illustrates a highly energetic tidal site across a shallow (mean depth = 14m) fjordic sill. The FoL hydrodynamics have been investigated using an Acoustic Doppler Current Profiler (ADCP), a numerical flow model, water level data, and meteorological data. Additionally, four bathymetric timeseries datasets (from February 2014, September 2020, November 2021 and June 2022) provided alternative indication of flow variability by monitoring the geometry of submarine sediment bedforms. A correlation of 0.22 (-0.2 < 97.5% confidence interval < 0.2) was found between catchment-integrated precipitation and residual water height (with effects of tide and atmospheric pressure removed). Residual water height varied by 0.77m. Artificially increasing the water height by up to 0.6m at the model’s up-fjord open boundary to emulate the effects of precipitation increased (decreased) peak depth-averaged flow speed of the ebb (flood) jet by 0.44m/s (0.33m/s) whilst modifying residual flow fields around the FoL. The freshwater-modified residual flow correlated with the westward displacement of a flow transverse sediment ridge observed in two bathymetric surveys conducted after periods of relatively high precipitation. Additionally, in one of the surveys associated with increased precipitation, wavelength stretching in the sediment dunes was observed uniquely at the western approach to the FoL, dominated by the freshwater-enhanced ebb flow. Increasing the water levels of the up-fjord boundary from 0.1 to 0.6m resulted in an increased mean bedload transport of 2.28x10-5 m3/s (370% increase) for these dunes dominated by the ebb current. Whilst, Loch Etive has an anomalously high freshwater influence on sill flow compared to other Scottish fjords, it is predicted that freshwater inputs are a significant influence on sill hydrodynamics and nearby sediment transport pathways in other fjordic systems.