Olav van Duin

Om beter tegemoet te kunnen komen aan de telkens dynamischer en complexer wordende processen rond waterbeheer moeten de gereedschappen die bij het oplossen van waterhuishoudkundige problemen gebruikt worden meegroeien met die veranderende realiteit. Dit betekent vooral dat de waterbeheerder meer en meer de mogelijkheid moet hebben om effectief, efficient, flexibel en dynamisch gebruik te maken van de aanwezige gereedschappen. Een belangrijk vooruitzicht voor het gebruik van modellen in waterbeheer is het zogeheten nieuwe modelleren. Deze visie beoogt een reproduceerbaar, flexibel en transparant modelleerproces te volgen om hiermee zowel betere resultaten te krijgen als beter communiceerbare resultaten. De concepten van het nieuwe modelleren en andere algemene vereisten voor waterbeheer worden toegepast op het idee van een beslissingsondersteunend systeem (BOS). Hiermee wordt een leidraad voor de ontwikkeling van dergelijke systemen gegeven die aan de geformuleerde idealen moet voldo...

In the present study, two bed-load transport models are introduced in an existing idealized dune model. These allow for the modeling of the spatial lag between the sediment transport rate and bed shear stress along dune surfaces. This lag is an important factor in determining transitions between bedform regimes. Results of the original dune model (using an equilibrium transport formula) are compared with (1) a new model version that directly models spatial lag with a relaxation equation and (2) a new model version including pick-up and deposition processes. Both bed-load models use mean particle step length as an important parameter, which is varied to assess which value is appropriate for the dune regime. Laboratory experiments are simulated with the model. This shows that the results are best with the pick-up and deposition model version, combined with a step length of 25 times the particle diameter. It is furthermore shown that in principle the model is also able to wash out fully grown dunes, by increasing the step length parameter - See more at: http://ascelibrary.org/doi/10.1061/%28ASCE%29HY.1943-7900.0001254#sthash.to4wr5CU.dpuf

In this paper we derive a new morphological model, with an extended version of the sediment transport model for the mean step length (the average distance travelled by sediment particles), in which this mean step length depends on the mean bed shear stress. This model makes the step length increase with increasing flow, in line with previous experimental results. To account for suspension and the large-scale turbulent structures in rivers, the step length also depends explicitly on water depth. This approach enabled modelling of the transition from dunes to the upper-stage plane bed. It was shown that by increasing the step length, the lag between shear stress and bed load transport rate increases, and the dunes eventually become smoother and lower, until finally the dunes wash out. The newly adopted model approach is tested successfully with a synthetic data set from the literature, where plane bed conditions are indeed reached in the model, similar to the results of a more advance...

The river bed of lowland rivers typically consists of river dunes. These dunes are the major source of roughness for the river flow and so, changes in their dimensions affects the roughness significantly. During floods these dunes grow in length and height. Numerical model studies and laboratory experiments have shown that for extreme discharges these dunes will wash out, i.e upper stage plain bed (USPB), decreasing the water level. In this study we investigate at which locations along the river Rhine, flattening of the dunes or USPB are most likely to occur, using the data-based relationship found by Naqshband et al. (2014a). The flood hydrodynamics is generated by WAQUA. Next, we will apply the steplength approach of van Duin et al (2017) to model this transitions on river scale and assess the water level reduction. Potentially, this USPB may lead to a self-regulation process in rivers decreasing water levels at the extreme discharges which may occur due to climate changes.

Often river bed form modelling is done with an equilibrium bed load transport formula like that of Meyer-Peter & Müller (1948). However, a physically more correct way would be to model it with separate models for the sediment pickup and deposition processes as described by Nakagawa & Tsujimoto (1980). Besides the physics of the sediment transport itself, using such a method allows for the modelling of higher-order processes as well like spatial lag in bed load transport. As shown by Shimizu et al. (2009) applying the aforementioned pickup and deposition model in a dune evolution model, makes is possible to model dunes well. Specifically it made it possible to determine a transition to upper stage plane beds, as well as capturing hysteresis well. In this paper we will explore the effect of using different kinds of bed load models in a relatively simple dune evolution model. The Nakagawa & Tsujimoto (1980) bed load model, will be implemented in the dune evolution model of Paarlberg et...

The bed of a river goes through various stages of development as the flow strength increases. Starting from a plane bed, first ripples will appear, followed by dunes, a transition stage and then an upper-stage plane bed (Richards, 1982). After these regimes the bed can evolve further, but for lowland rivers only the mentioned regimes are relevant. Dunes are the most common bedforms in lowland river channels consisting of sand and gravel. Dunes influence water levels significantly, because they impose roughness on the flow. In general, increasing bedform dimensions lead to increasing water levels. While there are various models that describe the interaction between flow, sediment and bed form properties well, there are few that are able to describe all the transitions from a lower-stage plane bed through an upper-stage plane bed. Also, there is a need for fast calculation of bedform evolution and the effects that they have on hydraulic roughness, in order to be useful for operational...

Water level forecasts with a high river water discharge depend on accurate predictions of the evolution of river dunes. For flood earlywarning systems complexity in sub models is a drawback as it leads to (too) large computation times. Therefore the aim of this study is to develop a relatively simple dune evolution model that works well in the dune regime, and which has the potential to predict the upperstage plane bed as well. To reach this goal we will study the influence of using certain bed load transport formulations on the resulting dune morphologies. For this we will investigate whether alternative bed load transport models within the computationally cheap model of Paarlberg et al. (2009) lead to good results.

ABSTRACT For flood management modelling of lowland rivers it is important to understand the interaction between river flow and bed forms, specifically dunes. In dune evolution models commonly equilibrium transport formulae like that of Meyer-Peter and Müller (1948) are applied. However, the lag between flow properties and sediment transport is considered a principal cause of bed instability (Nakagawa & Tsujimoto, 1980). Their pickup and deposition model is used by Shimizu et al. (2009) to model bed load transport in a dune model. Because the properties of step length under dune conditions is highly uncertain they derived a conceptual model for this important parameter. Sekine & Kikkawa (1992) have made a numerical model of saltation of particles for flat bed conditions and compared it to experimental data. They show that step length strongly correlates with the ratio of friction velocity to settling velocity. Now, we have undertaken experiments to determine step lengths under dune conditions. For a series of dunes the motion of particles has been captured with a high-speed camera. This has been done along the length of the dune to get an idea of the spatial variation. In this paper the first results of the data analysis are presented. The variation of step length distribution along a dune is small, which is against expectation. We hypothesize that this is due to the fact that the relation Sekine & Kikkawa (1992) have found disregards turbulent fluctuation, which is not relevant for a flat bed but is relevant for a dune.

The hydraulic roughness of the main channel of most lowland rivers is dominated by bed forms. River bed forms act as roughness to the flow, thereby significantly influencing the water levels, which are essential for flood forecasting. We compared a time-lag model and a physically based pickup and deposition model to predict dynamic bed form evolution during a flood wave in the flume and the field. The results showed that the explicit computation of bed form and associated roughness predictions perform equally well as a calibrated model for the flume case, but slightly less for the field case. We were able to explain a large part of the roughness of the main channel that is normally calibrated. Using a physically-based roughness prediction improves the accuracy of the modelled water levels for operational flood forecasting.

ABSTRACT Introduction During floods, bedforms develop on the river bed. Dunes are the most common bedforms in lowland river channels consisting of sand and gravel. They have heights of 10-30% of the water depth and lengths around 10 times their height. River bedforms influence water levels significantly, because they impose roughness on the flow. In general, increasing bedform dimensions lead to increasing water levels. Knowledge about bedform evolution and associated roughness is still limited. Therefore this new PhD project aims at gaining that knowledge. Bedform evolution The bed of a river goes through various stages of development (regimes) as the flow strength increases. Starting from a plane bed, first ripples will appear, followed by dunes, a transition stage and then an upper-stage plane bed (Richards, 1982). After these regimes, the bed can evolve further, starting with antidunes. However, for lowland rivers, only the above mentioned regimes are relevant. In Figure 1 a schematic representation of the occurrence of these bed forms for various combinations of sediment size and flow velocity is shown. Figure 1: schematic representation of bed-phase stability plot, corrected for temperature(source: Southard, 1991, original Southard & Boguchwal). On the horizontal axis D 10 in mm is plotted, on the vertical axis flow velocity in m/s is plotted.

Coastal areas are generally intensely used areas with high population density and economic activity. On a basin scale the tide directly determines water levels and currents in a basin. These flow characteristics furthermore determine the shape of the basin itself, for example the forming and evolution of tidal sandbanks, which in turn influences the flow pattern. Because of its importance for various human and natural activities the modelling of tidal flow has been studied by many authors in the past. This has lead to depth-averaged (2DH) and 3D models amongst others The first analytical 3D-model that describes tidal flow in a semi-enclosed basin using Kelvin and Poincare modes with partial slip was created for this research. For this the method devised by Mofjeld (1980) for 3D tidal flow along a single coast with viscosity and no-slip was extended, thereby following Taylor’s approach (1921). As a reference situation the Northern Part of the North Sea was modeled and the properties ...