Back unsteady-bedforms-hero
Completed projects

Bedforms in unsteady flows

The
Challenge

All rivers and estuaries exhibit temporal variations of discharge over time scales varying from seconds to years. The beds of sandy rivers and estuaries respond to these changes and organise over a wide range of spatial and temporal scales, primarily through adjustment of a variety of bedforms such as ripples, dunes and bars.

These bedforms, and the drag they produce, are the key component of overall bed roughness and they thus have a significant influence on the water depth for a given discharge. For example, research on the Rhine has shown that water depth varies by up to 2 m depending upon the presence of bedforms and their evolution during flooding events. However, almost all numerical flood models ignore the effects of discharge variations on bed morphology and roughness and thus can make grossly erroneous water depth predictions.

The
Approach

Lead researchers

dan-parsons steve-simmons

Project funded by

nerc-logo

Project partners

Durham University

Fulcrum Graphic Communications Inc.

University of Brighton

University of Illinois

University of Southampton

This project combined laboratory experiments at the Total Environment Simulator with repeat detailed field surveys of bathymetry and flow on the Mississippi River North of St Louis, MO, USA and numerical modelling.

The laboratory experiments quantified flow fields, sediment transport rates, morphology and topographic change over mobile sandy beds under a range of different hydrographs. These initial experiments were used to build models of bedforms, over which turbulent flow fields were then measured. The repeat field surveys of bathymetry and flow on the Mississippi were conducted during annual floods and base-flow conditions and were used to validate the experimental results.

A fully three-dimensional numerical model was then developed to capture the dynamics of dune migration including the feedbacks between sediment transport, dune morphology and flow structure. The results of this model were then used to parameterise generic morphodynamic models applied over large areas (>10 km2) and longer time periods (>full flood hydrographs).

The Impact

The incorporation of dynamically evolving processes is key to quantifying the evolving roughness better, in time and/or space, in response to changes in flow and has provided 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.

Other completed research projects