Quantification and modelling of bedform dynamics in unsteady flows

It has long been highlighted that important feedbacks exist between river bed morphology, sediment transport, and the turbulent flow field, and that these feedbacks change in response to forcing mechanisms. However, almost all of these previous models assume that steady uniform flow conditions determine bed equilibrium.

The incorporation of these dynamically evolving processes is key to quantifying better the evolving roughness, in time and/or space, in response to changes in flow and will provide knowledge that is of significance for improved morphodynamic modelling over large spatial and temporal scales, more reliable flood predictions, enhanced and informed engineering design and management, international shipping operations, and dredging and environmental management.

Funded by the Natural Environment Research Council (NERC), this project generated a new quantitative process-based understanding of bedform adjustment to unsteady flows using both laboratory and field measurements, and linked these new data to the development and application of a new numerical model of flow-bedform-roughness response. Its aims were to:

quantify the morphology, flow fields, concurrent sediment transport and topographic change over mobile beds in a laboratory flume under a range of unsteady flow conditions (pre-determined hydrographs of different temporal scales) and identify the dynamic interactions between flow, sediment transport and the bed morphology in response to change.

> quantify, in the laboratory, turbulent flow fields over fixed bedforms (that are informed by scaling from 1) under a range of disequilibrium conditions to generate a high-resolution, quantitative hydraulic context from which to explain flow-form-flow interactions during unsteady flow.

> derive and quantify, from the outputs of 1 and 2, the processes and implications of bed response to unsteady flow in terms of both form drag and bed roughness.
validate the experimental relationships determined from (1-3) at the field scale through repeat detailed field surveys of bathymetry and flow during the annual floods (April/May) and base-flow conditions (September/October) on the Mississippi River (north of St Louis).

> develop a fully three-dimensional numerical model that predicts the forcing mechanisms to capture the dynamics of dune migration including the feedbacks between sediment transport, dune morphology and flow structure.

> use the results derived from 5 to determine process-coupling parameterisation for inclusion of physically-based dynamic forcing mechanisms within generic morphodynamic models applied over large areas (>10 km2) and longer time periods (>full flood hydrographs).

Field site

Fieldwork was performed on a 10 km reach of the Mississippi River near St. Louis (MO, USA) that had a relatively uniform flow. The identified reach has widths of ~800 to 1200 m and mean depths of ~12 m at mean annual flow. The river has a meandering planform pattern and a sandy bed with ubiquitous dune bedforms. The site is easily accessible and has a flood hydrograph that provides periods of relatively well-sustained high and low flows. Fieldwork was based at the Jerry F. Costello Field Station which is part of the National Great Rivers Research and Education Center. Three fieldwork periods were undertaken that captured:

> base-flow conditions during one season, and
> a rising limb, a floodwave, and falling limb during two field seasons.

Flood waves are predicted and forecast on the Mississippi, typically occurring due to spring snowmelt in the upper basin. The necessary field logistical support (survey boats, multibeam sonar) at the site is guaranteed through collaborative involvement of the USGS Illinois Water Science Center and University of Illinois at Urbana-Champaign

The project findings resulted in the following publications.

Hardy, R.J., Marjoribanks, T.I., Parsons, D.R., Reesink, A.J., Murphy, B., Ashworth, P.J. and Best, J.L. (2014) Modelling time dependent flow fields over three dimensional dunes. In A.J. Schleiss, G. de Cesare, M.J. Franca and M. Pfister (Eds.), River Flow 2014 (pp. 1045-1052). London, UK: Taylor & Francis Group.

Reesink, A.J.H., Parsons, D.R. and Thomas, R.E. (2014) Sediment transport and bedform development in the lee of bars: Evidence from fixed- and partially-fixed bed experiments. In A.J. Schleiss, G. de Cesare, M.J. Franca and M. Pfister (Eds.), River Flow 2014 (pp. 1179-1186). London, UK: Taylor & Francis Group.

Reesink, A., Parsons, D., Ashworth, P., Hardy, R., Best, J., Unsworth, C., McLelland, S. and Murphy, B. (2013) The response and hysteresis of alluvial dunes under transient flow conditions. In. V. Van Lancker and T. Garlan (Eds.) MARID IV Abstract Volume (215-220). Bruges, Belgium

Unsworth, C.A., Parsons, D.R., Reesink, A.J.H., Best, J.L., Ashworth, P.J. and Hardy, R.H. (2013) Flow structures over fixed 2D bedforms in transient states. In. V. Van Lancker and T. Garlan (Eds.) MARID IV Abstract Volume (265-270). Bruges, Belgium.

Conference abstracts

Invited talk - Hardy, R.J., Parsons, D.R., Ockelford, A.M., Ashworth, P.J., Reesink, A.J. and Best, J.L. (2015) Numerical modeling and applications of river fluxes in large basins under changing environmental conditions. Presented at: AGU Fall Meeting, American Geophysical Union, 14-18 December 2015, San Francisco, USA

Ockelford, A., Parsons, D.R., Hardy, R.J., Ashworth, P.J. and Best, J.L. (2015) Exploring the relationship between hydrograph characteristics and the time evolution of sand bed morphology. Presented at: AGU Fall Meeting, American Geophysical Union, 14-18 December 2015, San Francisco, USA

Hardy, R.J., Marjoribanks, T.I., Parsons, D.R., Reesink, A.J., Murphy, B., Ashworth, P.J. and Best, J.L. (2014) Modelling time dependent flow fields over three dimensional dunes. Presented at: River Flow 2014, International Conference on River Hydraulics, 3-5 September 2014 Lausanne, Switzerland

Hardy R., Parsons D., Ashworth P., Reesink A. and Best J. (2014) Modelling flow and shear stress fields over unsteady three dimensional dunes. Presented at: EGU European Geosciences Union General Assembly, 27 April – 02 May 2014, Vienna, Austria

Reesink A.J., Parsons D.R. and Thomas R.E. (2014) Sediment transport and bedform development in the lee of bars: Evidence from fixed- and partially-fixed bed experiments. Presented at: River Flow 2014, International Conference on River Hydraulics, 3-5 September 2014 Lausanne, Switzerland

Hardy R.J., Reesink A.J.H., Parsons D.R., Ashworth P.J. and Best J. (2013) Flow fields over unsteady three dimensional dunes. Presented at: AGU Fall Meeting, American Geophysical Union, 9-13 December 2013, San Francisco, USA

Reesink, A., Parsons, D., Ashworth, P., Hardy, R., Best, J., Unsworth, C., McLelland, S. and Murphy, B. (2013) The response and hysteresis of alluvial dunes under transient flow conditions. Presented at: Marine and River Dune Dynamics - MARIDIV, 15 April - 17 April 2013, Bruges, Belgium.

Reesink, A., Parsons, D., Ashworth, P., Hardy, R., Best, J., Unsworth, C., Simmons, S., McLelland, S. and Murphy, B. (2013) Effects of depth and water surface slope on dune development. To be presented at: International Conference on Fluvial Sedimentology 10, 14 – 19 July 2013, Leeds, United Kingdom.

Unsworth, C.A., Parsons, D.R. and McLelland, S.J. (2013) On the scaling of dune bedforms: flow depth and shear stress. To be presented at: International Conference on Fluvial Sedimentology 10, 14 – 19 July 2013, Leeds, United Kingdom.

Unsworth, C.A., Parsons, D.R., Reesink, A.J.H., Best, J.L., Ashworth, P.J. and Hardy, R.H. (2013) Flow structures over fixed 2D bedforms in transient states. Presented at: Marine and River Dune Dynamics - MARIDIV, 15 April - 17 April 2013, Bruges, Belgium.