From: Erik Mosselman To: ; roger bettess Cc: erik mosselman Subject: Rivers-List: River engineering problems for 21st Century Date: 30 June 2000 10:16 Dear Dr. Bettess, Your took a very nice initiative and I hope that you will receive many contributions to optimize your list. I just would like to suggest to rephrase point 6 "A theoretical justification for Regime theory". I think that the major problem here is to find ways to predict river width adjustment (also implicit in your point 7), and that the most intriguing question regarding regime relations is why the calibration of river width predictors based on a power-law relation between width and some representative discharge almost universally leads to an exponent of 0.5 (although Rhoads (1992) suggests that a bias is introduced because many researchers assume a priori that this exponent should be equal to 0.5). The reason why I propose a rephrasing, is that point 6 has been formulated differently from the other points (a justification of a theory instead of development of theories, predictors, solutions and strategies). Moreover, Regime Theory is controversial as a theory, and I think that its weak points have become sufficiently clear in the 20th Century. So a justification at all cost seems a bit odd. I hope to explain this better below. I think that one should make a clear distinction between Regime Equations and Regime Theory. Regime Equations are good carriers of empirical knowledge and hence indispensable tools for phenomena where the available theoretical knowledge is still insufficient or inadequate. Regime Theory, however, is in its essence founded on two assumptions which are not supported by data or modern theories: (1) "Assumption that equilibrium hydraulic geometry of an alluvial channel is a unique function of water discharge, sediment supply and bed and bank material". This is a misconception because the parameters of the equilibrium hydraulic geometry can take any value from a certain range between an upper and a lower limit (Blench, 1969). The equilibrium hydraulic geometry actually established depends on the initial and boundary conditions which govern the preceding transition towards the equilibrium. (2) "Assumption that the simplicity of regime equations indicates that they represent fundamental laws of nature rather than crude approximations of much more complex interrelationships". Blench (1969) warns specifically against such an assumption, but Inglis (1947) suggests that Lacey, the Father of Regime Theory, did hold this view. These weak points have both been identified already in the sixties by Blench, based on his work in the Punjab, the cradle of Regime Theory. In this respect it is also worth quoting Vanoni (1975), who writes: "One should study Blench's rather thorough exposition of the "regime" philosophy before undertaking use of the regime method". References: Blench, T. (1969), Mobile-bed fluviology. University of Alberta Press, Edmonton. Inglis, C.C. (1947), Meanders and their bearing on river training. Inst. Civil Engrs., Maritime and Waterways Engrg. Div. Rhoads, B.L. (1992), Statistical models of fluvial systems. Geomorphology, Vol.5, Nos.3-5, pp.433-455. Vanoni, V.A. (Ed., 1975), Sedimentation Engineering. ASCE Manuals and Reports on Engrg. Practice, No.54. I hope this provides a positive contribution. Best regards, Erik Mosselman