The optimal meander planform shape from minimization of entropy production

AU over the world natui'al lowland rivers and streams are winding tlii'ough the sediments deposited in earlier times. These meandering rivers and streams show remarkable geometric similaiity: the ratio of the meander length to the width of the channel is constant. The process of meandering is thus independent of scale. Many theories and models ai'e proposed to explain the process of meandering, but still meandering is not completely understood, due to the complexity of the processes involved. The valley slope, banldull discharge, sediment transported, vegetation growth, and bed material all influence the processes of meandering. Moreover they are rarely uniform and the conditions usually change with time, because of changing geology, climate or activities by man. Consequently, the sinuosity of a meandering channel is difficult to predict. To explain the behavior of meandering channels, St0lum [1996, 1998] proposed that meandering can be described with the self-organizing process: clusters of cut-offs of the meander planform tend to cause a transition from active meandering with many cut-off events into a stable, more ordered state without cut-offs. After some time, the channel may change into the active state, if a cluster of cut-offs happens at for example high dischai'ges. In this way stabihzed meandering channels with constant planform shape can be found in most natm-al river reaches, i f sufficient time is present to adapt to the conditions. For a meandering river the timescale of the chamiel in stable state is likely to be hundreds of years. In nature, these stabihzed meandering channels show two asymmetries in their planform resulting from the interaction of flow and form: an upvalley skew of the meandering channel's minima and maxima and a delayed inflection point. Another planform characteristic is fattening with respect to a first order sine-generated cm-ve. In this research attempts were made to derive this optimal planform shape given the sinuosity from minimization of entropy production. Entropy is constantly produced in an open channel by friction. Prigogine [1945] stated that linear thermodynamic systems close to equilibrium evolve toward a stationary state characterized by the minimum entropy production compatible with the constraints imposed on the system. This statement is extended by Reiser [1996, 1998] to be valid for open, non-Unear systems like meandering channels as well

Dispersion and dynamically one-dimensional modelling of salt transport in estuaries

An estuary forms the transition between the  ocean/sea and a river and within its boundaries  fresh and salt water mix. Fresh water intake  points may be located within the reach of salt  intrusion. In order to justify political and  managerial decisions it is thus necessary to  understand and be able to predict the process  of salt intrusion in estuaries.  For one-dimensional dynamic simulation of  the hydrodynamics and salinity intrusion the  modelling suite SOBEK is available. In the  Netherlands this software is used to evaluate  the impact of for example measures and  climate change on salinity intrusion in the  Dutch Rhine Meuse Delta (RMD). Recent  validations of SOBEK have mainly focused on  water levels and discharges, while less  attention was payed to its capability to describe  salt transport. Therefore the objective of this  research is to obtain a better understanding of  dynamic one-dimensional modelling of salt  transport and improve the governing  formulations with the newest scientific insights  A.J.F. Hoitink, T.V. de Ruijsscher, T.J. Geertsema, B. Makaske, J. Wallinga, J.H.J. Candel, J. Poelman (Eds.) NCR days 2017, Febr. 1-3, 2017. Book of abstracts, NCR publication 41-2017.Water Resource