A philosophy of rivers: evolutionary change and survival of the fittest
Prof Gerald C Nanson, School of Earth & Environmental Sciences, University of Wollongong
Prof He-Qing Huang, Institute of Geographical Sciences & Natural Resources Research, Chinese Academy of Sciences, Anwai, Beijing, China
Friday 31 May 2013
3-4 pm Environment 1_N55_Room 1.07
Griffith University, Nathan Campus
Underlying the study of fluvial geomorphology has been the desire to understand the fundamental behaviour of rivers and hence predict their adjustment for purposes of management. Resolving the physical laws governing forces and motions has been prevented by a lack of mathematical closure, hence variational approaches have been adopted whereby assumptions are made regarding possible optimum operating states (extremal hypotheses). However, until recently there has been no logical basis to justify selecting any one of these.
By a mathematical ‘sleight of hand’, the channel form ratio (w/d) has reduced the number of variables and merged Newtonian and variational approaches, showing that rivers are controlled by the least action principle. Alluvial rivers evolve teleomatically and iteratively via various forms of dynamic equilibrium towards progressively more stable states, stationary equilibrium being the most stable and the ultimate attractor. In rivers this is ‘survival of the most stable’, comparable to biological evolution’s ‘survival of the fittest’. In clean water this ultimate state occurs at Froude No 1; in rivers transporting bedload it occurs at H No 0.3.
Correcting earlier versions of the Meyer-Peter Müller bedload equation with the H number, and using field data from the Yangtze River, we show this large river adjusts its channel morphology to maximum flow efficiency (transporting its imposed bedload with the least amount of power). This approach greatly expands an understanding of the dynamics of rivers and shows them to be controlled by least action. This for the first time provides a simple means of quantitatively measuring and defining a river’s equilibrium state. It opens the way for predicting channel adjustment and river management in different environmental settings and shows why rivers are rarely straight but adopt various dynamic forms from steep step-pools to gentle anastomosing systems.
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