From: Robert Millar To: ; roger bettess Subject: Rivers-List: Regime Theory Date: 30 June 2000 18:56 Roger: I agree with your view that a set of specific independent variables (discharge, sediment load, bank stability parameters and valley slope) should in concept yield a single-value equilibrium or regime geometry. In practice, the actual river geometry will fluctuate somewhat about this supposed single value, due in part to adjustment to past events (eg large floods), and also because in rivers, discharge and sediment supply are quite unsteady, and are probably better thought of as ranges, rather than as a single value (eg bankfull discharge). At any rate, application or equilibrium or regime concepts generally requires averaging over suitably long periods, say 10-20 years or more, and therefore fluctuations tend to average out. I disagree with Dr Mosselman's statement that equilibrium geometry is dependent upon initial conditions. I consider that the equilibrium or regime condition is in fact independent of initial conditions. I also consider that the equilibrium is equivalent to an optimum state, which has been characterized previously as the minimum slope, stream power or maximum sediment transporting capacity. Ideas that you are of course very familiar with Roger. Bank stability is an essential constraint on development of the optimum geometry. Finally, with regard to the original 12 problems, I wonder whether (2) Adequate representation of turbulence, and (4) An accurate sediment transport theory might be better placed on the list of 12 problems to be solved by the end of the new millenium! Best Regards Rob Millar P.S. If I can be so bold as to dabble in some self-promotion, those interested in regime theory and "extremal hypotheses" might care to look over a paper that was published recently in Water Resources Research (Millar, 2000: Influence of bank vegetation on alluvial channel patterns", WRR 36 (4), 1109-1118). An approach is developed using Parkers meandering-braiding theory coupled with extremal-based hydraulic geometry model to derive a new theoretical meandering-braiding transition that includes bank vegetation. *************************************************************************** Dr. Robert Millar Assistant Professor University of British Columbia Department of Civil Engineering 2324 Main Mall, Vancouver, BC, Canada, V6T 1Z4. Phone + (604) 822 2775 Fax + (604) 822 6901 email: millar@civil.ubc.ca Web Page: http://www.civil.ubc.ca ***************************************************************************