Print Email Facebook Twitter Bed level changes in the Waal during floods Title Bed level changes in the Waal during floods Author Van Denderen, R.P. Contributor Uijttewaal, W.S.J. (mentor) Blom, A. (mentor) Giri, S. (mentor) Sieben, A. (mentor) Stecca, S. (mentor) Faculty Civil Engineering and Geosciences Department Hydraulic Engineering Date 2014-08-20 Abstract Bed level changes during a flood can have large influence on the safety against flooding and the navigational function of a river. In 1997 measurements were carried out in the Waal during a flood. These measurements present an average decrease of the bed level in the order of 10 cm over a 10 km reach which is much larger than expected from the current morphological models. Two reasons for this bed level decrease are suggested: the unsteady sediment load and the influence of a measuring error. The goal of this thesis is to find the main processes which affect the average bed level during floods in rivers like the Waal. The maximum influence of the unsteady sediment load is estimated using multiple empirical relations for the bed load transport, the bed particle velocity and the suspended load transport. The analysis shows that the maximum change of the sediment load can cause a bed level change in the order of 0.7 mm during the 1997 flood in the Waal. Therefore it is concluded that the influence of unsteady sediment load is too small to significantly affect the bed level and to explain a 10 cm decrease of the bed level. The bed level measurements in the Waal during the 1997 flood were carried out with a single-beam echosounder. This means that the water depth is measured and that the water level is used to estimate the bed level. During the 1997 flood the water level was based on a linear interpolation between station Dodewaard, which is in the upstream part of the measuring reach, and measuring station TielWaal, which is downstream of the reach. In addition, the water level at station Dodewaard was not measured but estimated based on a regression relation. Both the linear interpolation and the regression relation are a source of inaccuracies. Therefore these water levels are compared to WAQUA-results for the 1997 flood and water level measurements during the 2011 flood. From this comparison is concluded that a combination of both sources of inaccuracies can be responsible for an average bed level decrease of 10 cm. The incorrect estimation of the water level is therefore assumed to be the main contributor to the large-scale bed level decrease during the 1997 flood in the Waal. The WAQUA-result of the 1997 flood is used to correct the bed level. The corrected measurements are compared with results from a Delft3D model. The model shows similar trends to the measured bed level but the quantitative differences are large. These large differences could be caused by the inaccuracies in the measurements or by the assumptions made in the Delft3D model. The model and the measurements show that the spatial variation of the river geometry has the largest influence on the bed level changes during floods. As a reference case the bed level changes at the Pannerdensche Kop during the 1997 and the 1998 flood are studied. During neither of the floods the measured bed level shows a large-scale bed level change. However, local bed level variations are large and are caused by spatial variations of the river width. Therefore the bed level measurements at the Pannerdensche Kop confirm that the spatial variation of the river geometry has a large influence on the bed level changes during a flood. The main process which affects the average bed level in the Waal is the river geometry. The bed level measurements and the model result show that a spatial variation of the river width has a large effect on the bed level during a flood. A new bed level measuring campaign has to be carried out to be able to validate the morphological models during floods. The current measuring techniques and measuring guidelines make it possible to capture the bed level changes during a flood in the Waal more accurately. Subject Waalfloodbed level changessingle-beam echosounder1997 flood1998 flood To reference this document use: http://resolver.tudelft.nl/uuid:06c25338-de69-4dc2-b8e4-c9e32423361a Part of collection Student theses Document type master thesis Rights (c) 2014 Van Denderen, R.P. Files PDF RPvanDenderen_bed-level-c ... floods.pdf 6.97 MB Close viewer /islandora/object/uuid:06c25338-de69-4dc2-b8e4-c9e32423361a/datastream/OBJ/view