From: Erik Mosselman To: Subject: Fw: Rivers-List: Regime Theory and Problems of the 21st Century Date: 07 July 2000 14:17 Dear fellow researchers, Despite the mono-disciplinary zeal that I may have exhibited in giving my opinion on Regime Theory, I would like to give first of all my whole-hearted support to the discussers who advocated more integrated approaches which include ecology in particular. I think that these approaches provide the real challenges for the 21st century, not only in terms of good river management but also in terms of exciting science. The interactions between hydromorphology and biology yield new monodisciplinary research questions to the disciplines involved, but they give also rise to phenomena which cannot be understood in the context of a single discipline alone. For those phenomena, field measurements, laboratory experiments, modelling and theoretical analyses require an interdisciplinary approach. Then back to the discussion on Regime Theory. I would like to thank Dr Robert Millar for engaging in the discussion and for his attempt to conclude somewhere in the middle between opposite views. I fear that we will not reach a complete agreement, but at least we have given sufficient arguments for anybody interested to build his or her own opinion. Below follows a reaction to Dr Millars last e-mail: (1) "Channels with the same Q and Qs, but different bank stability will each have different (but still single-valued) equilibrium geometry". The magnitude and the duration of an extreme flood (with a recurrence interval much larger than the time to adapt to an equilibrium) may determine the amount of bank erosion during that extreme flood, and hence the resulting river width. Different extreme floods produce different overwide channels. The narrowing through bench formation or bank advance under ordinary conditions is in my view not a matter of bank stability, and will not always result in the same equilibrium state. (2) "I would interpret the differences between the growing and dying channels as reflecting the differences in erodibilty of the sediments making up the banks, that is consolidated floodplain sediment versus unconsolidated silts". In the case of the Brahmaputra-Jamuna, the channels include low-water channels within a wider bed of unconsolidated sediments. For the behaviour of those channels, consolidated floodplain sediments do not play a role (channels may migrate laterally hundreds of meters per year, thus reworking the wider river bed). (3) "Dr Mosselman's Discussion published in J. Hydr. Eng. (2000) critical of extremal hypotheses cites the "Illusion of Progress" paper by G. Griffiths (1984, Water Res Res., 20 (1), 113-118). This study is often used by critics of the approach. However, what is usually overlooked is that Griffiths' analysis is fundamentally flawed because the "wide channel approximation" that was used did not account for the effect of river bank resistance on transporting capacity (see discussion by H. Chang, 1984, WRR, 20 (11), 1767-1768). Thus, general dismissal of "extremal based" approaches based on Griffiths' analysis not justified". Of course it is attractive for people like me to cite the paper by Griffiths, because it has such a provocative title. I will not react to the remark on the "flaw", but just point out that my dismissal is not only based on that paper. Personnally I find also the theoretical considerations described by Nagel (1961) very important (Nagel, E., 1961, The structure of science. Second Ed. 1979, Hackett, Indianapolis). Best regards, Erik Mosselman