Side Channels to Improve Navigability on the River Waal

Side channels to improve navigability on the river Waal November 2005 Author: Corstiaan van Dam Side channels to improve navigability on the river Waal November 2005 Preface This document is the final report for the MSc. thesis work of Corstiaan van Dam, student at Delft University of Technology, Faculty of Civil Engineering & Geosciences. The work was carried out between February 2005 and October 2005, partly at WL | Delft Hydraulics and partly at RIZA, Arnhem. The work was commissioned by the Ministry of Transport, Public Works and Water Management. It concerns a possible solution to the problem of a fast growing pointbar in the inner bend of the river Waal at Hulhuizen, The Netherlands. I would like to use this opportunity to thank Rijkswaterstaat, Oost-Nederland for the financial support and the supervision by ir. R.H. Smedes, which I also gratefully acknowledge. WL | Delft Hydraulics for the facilities and assistance during my stay there. Especially the advice of dr.ir. H.R.A. Jagers and dr.ir.C.J. Sloff has been indispensable. Dr.ir. E. Mosselman was the supervisor of this study. His encouraging support and guidance are acknowledged with gratitude. Furthermore, I would like to thank prof. dr. ir. H.J. de Vriend (Chairman) and drs. R. Booij for being members of my graduation committee and dr.ir. A. Sieben for being my supervisor at RIZA. Delft, November 2005 Corstiaan van Dam WL | Delft Hydraulics, RIZA, ON, TU Delft i Side channels to improve navigability on the river Waal November 2005 Summary Rivers in the Netherlands are not only meant to safely discharge; water, ice and sediment, they also have an important role in the National economy. Due to the favourable position of the Netherlands on the North Sea, the rivers form important fairways to the hinterland. In the past, this location already gave rise to significant activity in the sea harbours and on the inland waterways. To maintain this favourable position, the economical processes in the harbours and on the rivers should be optimised. With respect to inland navigation, obstacles should be removed as much as possible, to maximise the efficiency of the navigation routes. One of the principle limiting factors for navigation in the Dutch rivers is the limited navigable width in bends. Every year, during flood period, the bed level in the inner bends increases rapidly and limits the width for navigation in the subsequent low-water period. Several studies have been performed on how to reduce this so-called pointbar formation. This report investigates the possibility of a certain type of floodplain intervention to reduce the effect of pointbar formation on the navigable width. Via Multi Criteria Analysis and a rough qualitative assessment, it was decided to study the applicability of side channel for this purpose. Side channels are applied nowadays to increase flood conveyance and biodiversity, but never with the intention to reduce pointbar formation. First, an analytical model (from De Vriend and Stuiksma [1983]) predicting bed deformation in curved alluvial channels was used to get an impression of the morphological activity in the Hulhuizen bend. In order to understand the morphological activity, it is necessary to consider not only the bed topography, but also the water and sediment motion in a channel of transient curvature. The bed level deformation is a combination of the following two phenomena: 1. tilting of the transverse bed slope (axi-symmetrical solution) 2. overshoot phenomenon The analytical model showed that the overshoot phenomenon is likely to occur only at relatively small discharges (sufficiently high width-to-depth ratio). At these low discharges, the damping lengths become negative, which corresponds to exponential growth of the bed level. The corresponding morphological time scale during the actual low-water season (more than nine months), however, is too long to allow this bed evolution to take place. This indicates that the observed navigable width problems directly after flood a period cannot be ascribed to the overshoot phenomenon. At higher discharges, the damping length becomes positive, which implies rapidly decaying amplitude of the overshoot. The model showed that the bed level tends to develop rapidly towards the axi-symmetric solution, without overshooting it. The morphological time scale corresponding to this phenomenon is relatively small compared to the time scale for the overshoot phenomenon. The conclusion from the analytical model is therefore that the tilting of the transverse bed slope, which is the predominant morphological activity during flood, is the most propable cause of the observed navigable width problem. In order to determine the influence of side channel application, the Delft3D SED-online (2D depth-averaged) program was used, in which an existing model of the Waal at Hulhuizen was adjusted to the present conditions. The general morphological phenomenon, caused by withdrawing water from the main channel is a one-dimensional sedimentation wave. This tends to increase the bed level, which is ii WL | Delft Hydraulics, RIZA, ON, TU Delft Side channels to improve navigability on the river Waal November 2005 unfavourable to navigation. On the other hand, the analytical model showed that when the discharge through the main channel is reduced, the pointbar tends to be less pronounced. Furthermore, the analytical model showed that a decrease of discharge coincides with an increase of the morphological time scale. It is the combination of both the one-dimensional sedimentation wave and the tilting of the transverse bed slope, that determines the actual bed level. A number of computations have been made, most of them with constant discharges of 3500 m3/s or 4500 m 3/s. First, a reference computation was made, in which no water is extracted. Subsequently, a computation was made, in which only water was abstracted. Finally, computations were made in which both water and sediment were abstracted Four side channels alignments were taken into account. By a lack of time, however, most computations were executed with the same side channel alignment. The following conclusions are derived from the computations for this side channel: • The offtake location should be located such, that the one-dimensional bed level wave, which starts at the offtake and gradually expands downstream cannot reach the critical cross-section. The results showed a propagation length of 900 m for a simulation period of one year. • The bed level in the inner bend increases less fast if the side channel transports only water. • Lateral withdrawal of water only has not only a positive effect on the rate of pointbar accretion, but it is also able to reduce its maximum height. To reduce the pointbar height at higher discharges, relatively more water needs to be withdrawn than at lower discharge discharges. • Lateral withdrawal of both water and sediment decreases the pointbar height and accretion rate even more than when only water is abstracted. • Yet, the results of this study did not show a significant increase of navigable width within a time span of one year. The computations in this study were mainly executed with a constant discharge and only for a period of one year. It is recommendable to extend the simulation time and execute the computations with a varying discharge, because this corresponds more to reality. It is also recommended and to include the bed level legacy from the previous flood periods. Furthermore, Delft3D-SEDonline should be adjusted to simulate the spilling process over the summer dikes, which makes the modelling of the exchange between floodplain and summer bed possible. Therefore, the computational model should be extended with the floodplain area. These two adjustments will constitute another step forward towards more accurate simulation. WL | Delft Hydraulics, RIZA, ON, TU Delft iii Side channels to improve navigability on the river Waal November 2005 Contents PREFACE ........................................................................................................................... I SUMMARY........................................................................................................................II LIST OF FIGURES ..........................................................................................................VI LIST OF TABLES .........................................................................................................VIII NOTATION ......................................................................................................................IX 1. INTRODUCTION........................................................................................................1 1.1. MOTIVE ......................................................................................................................1 1.2. PROBLEM ANALYSIS ..................................................................................................4 1.3. RESEARCH QUESTION ................................................................................................6 1.4. OBJECTIVE .................................................................................................................6 1.5. APPROACH .................................................................................................................6 2. STATE OF ART ..........................................................................................................7 2.1. RIVER MORPHOLOGY ................................................................................................7

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