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\begin{document}

\title{Dune geometry and the associated hydraulic roughness at the transition
from a fluvial to tidal regime at low river discharge}



\author[1]{Sjoukje Irene de Lange}%
\author[2]{Ryan Bradley}%
\author[3]{Reinier Schrijvershof}%
\author[4]{Daniel Murphy}%
\author[3]{Kryss Waldschlager}%
\author[2]{Ray Kostaschuk}%
\author[2]{Jeremy G. Venditti}%
\author[1]{A.J.F. (Ton) Hoitink}%
\affil[1]{Wageningen University}%
\affil[2]{Simon Fraser University}%
\affil[3]{Wageningen University and Research}%
\affil[4]{School of Environmental Science, Simon Fraser University}%


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  \date{May 21, 2024}


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\begin{abstract}
In deltas and estuaries throughout the world, a fluvial-to-tidal
transition zone (FTTZ) exists where both the river discharge and the
tidal motion drive the flow. It is unclear how bedform characteristics
are impacted by changes in tidal flow strength, and how this is
reflected in the hydraulic roughness. To understand bedform geometry and
variability in the FTTZ and possible impacts on hydraulic roughness, we
assess dune variability from multibeam bathymetric surveys, and we use a
calibrated 2D hydrodynamic model (Delft3D-FM) of a sand-bedded lowland
river (Fraser River, Canada). We focus on a period of low river
discharge during which tidal impact is strong. We find that the
fluvial-tidal to tidal regime change is not directly reflected in dune
height, but local patterns of increasing and decreasing dune height are
present. The calibrated model is able to predict local patterns of dune
heights using tidally-averaged values of bed shear stress. However, the
spatially variable dune morphology hampers local dune height
predictions. The fluvial-to-tidal regime change is reflected in dune
shape, where dunes have lower leeside angles and are more symmetrical in
the tidal regime. Those tidal effects do not significantly impact the
reach-scale roughness, and predicted dune roughness using dune height
and length is similar to the dune roughness inferred from model
calibration. Hydraulic model performance with a calibrated, constant
roughness is not improved by implementing dune-derived bed roughness.
Instead, large-scale river morphology may explain differences in model
roughness and corresponding estimates from dune predictors.%
\end{abstract}%



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