From: bram van prooijen To: Subject: Re: Rivers-List: Problems for 21st Century Date: 03 July 2000 13:16 Dear river researchers, Although I am not a specialist in morphology, I would like to mingle with the discussion about the regime theory. I read the book "It's better to live in uncertainty", a Dutch book of Henk Tennekes (a turbulence specialist). This book deals (in a popular way) with the uncertainties in life and with examples from weather forecasting. The problem in weather forecasting is the growth of instabilities. A small difference in initial condition can cause a "totally different" final condition. This means that, due to a uncertain initial condition, even with a "perfect model", the forecasting will be poor for over 10-15 days. The important issue is the stability of the system. If the system is very stable, differences in the initial conditions are not important, but if the system is (strongly) unstable (like the weather system) it is very sensible for small differences in initial conditions. Therefore I think Mosselman is principally right and Millar and Bettess are just right if the system is stable. As I mentioned earlier I am not specialized in morphology, so I cannot give criteria to determine the stability of the system. I think it will be a great challenge for the next century not just to deal with averaged quantities, but also with fluctuations. This is already implicitly expressed in the items 2,3,4, 5,7 and 9 . Item 8 is a good one for the next millenium. best regards, Bram van Prooijen 2 Representation of turbulence > An adequate description of turbulence is required > > 3 Determination of the discharge coefficient for a structure > of arbitrary > shape > > 4 An accurate sediment transport theory > In the 20th century it seemed that no theory could reliably break the > barrier of providing predictions within a factor of 2 about > 70% of the time. > We need more accurate theories or an explanation of why they > cannot be > achieved. > > 5 Prediction of local and general scour > > 7 Determination of the dominant physical processes > responsible for the plan > form of rivers > > 9 Effect of cross-sectional shape on resistance coefficients in > open channel flow > ========= ADDRESS: ir. Bram van Prooijen PhD-student Delft University of Technology Faculty of Civil Engineering and Geosciences Subfaculty of Civil Engineering Laboratory of Fluid Mechanics PO BOX 5048 2600 GA DELFT The Netherlands TELEPHONE (+31)15 278 33 65 TELEFAX (+31)15 278 59 75 E-MAIL B.C.vanProoijen@CT.TUDelft.NL ====