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Presenter: Richard David Williams, Aberystwyth University, United Kingdom, email@example.com
Date: Wednesday April 24th, 2013
Venue: Geography Building Planning Studio (GLB2.01)
Numerical morphological modelling of braided, gravel-bed rivers is gaining momentum as a technique to inform environmental management. In particular, “full process” models, such as Delft3d, are being used to simulate the morphodynamics of real world braided rivers using high resolution topography to provide initial boundary conditions. Whilst considerable attention has been paid to developing the hydraulic and sediment transport algorithms associated with such models, comparatively less attention has been paid to validating model predictions using field observations at the reach scale.
This seminar will first provide an overview of a unique dataset that records the morphological evolution of a 2.5 km long reach of the braided, gravel-bed Rees River, New Zealand, during a sequence of high-flow events during an eight-month period. Digital Elevation Models (DEMs) were constructed using a fusion of mobile Terrestrial Laser Scanning and optical-empirical bathymetric mapping. The resulting topographic models are characterised by mean vertical errors of 0.03-0.12 m in exposed and inundated areas of the model respectively.
The second section of the seminar discusses the performance of a two-dimensional model (Delft3d) to predict braided river flow, using the high-resolution Rees River DEMs as boundary conditions. Hydraulic predictions were validated using a dense set of depth and velocity measurements acquired using an acoustic Doppler current profiler (aDcp) for a 300 m braid bar unit, and reach-scale aerial photos of low and high flows. Model performance was good for a range of flows, with minimal bias and relatively tight error distributions. The results demonstrate the accuracy of two-dimensional modelling for the prediction of braided river flow, and show accurate results can be achieved with spatially uniform calibration of roughness and eddy viscosity.
The final section of the seminar considers whether a suitably parameterised morphodynamic model (Delft3d) can predict patterns of erosion and deposition that were observed during a c.250 cumec high-flow event. Maps of observed channel change from the Rees River dataset are used to validate model performance. A wide-ranging sensitivity analysis also provides insight into model behaviour. Results show that total predicted volumes of erosion and deposition are within 10% of observed volumes. Moreover, histograms show similar distributions for simulated and observed morphological change. Patterns of erosion and deposition shown on maps of morphological change also show a striking similarity between predictions and observations. These results indicate that numerical model results are acceptable for simulations of single high-flow events, at the reach scale.
Tuesday 25th February, 5.15 pm, AH.1, AgHort Lecture Block, Riddet Road, Turitea Campus
Mark G Macklin, PhD – Professor of Fluvial Geomorphology
Institute of Agriculture and Environment
“Floods in the Anthropocene: myths, mud, and metals”
Floods annually account for more deaths, economic loss, and damage to ecosystem and human health than any other environmental hazard. It is widely believed that they are becoming more frequent and that this is linked to human-induced climate change. Drawing on nearly 35 years of research undertaken in Africa, the Americas, Asia, Australasia and Europe, Professor Mark Macklin will explore the relationship between extreme hydrological events (floods and droughts) and changing environmental conditions beginning with the first river civilizations, more than 5,000 years ago, up to the early 21st century including major floods that have affected many parts of the world in the last decade or so. In terms of size and frequency recent flood events are shown not to be unprecedented but because of floodplain encroachment by urban and infrastructure development, and recycling of historical and modern pollutants, their societal impacts are increasing. Recommendations for reducing flood risk using river sedimentary archives, new community-based and citizen-science approaches are outlined. These may offer the only viable options we have for adapting to future climate change impacts.
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Last updated on Tuesday 16 August 2016