Scour holes in tidal rivers with heterogeneous subsoil under anthropogenic influence

Scour holes in tidal rivers with heterogeneous subsoil under anthropogenic influence

Koopmans, H.

Uijttewaal, W.S.J. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

2017-04-06

Over one hundred scour holes are located in the Rhine-Meuse delta. These scour holes have relatively large depths and steep slopes and could pose a risk to the stability of river banks and hydraulic structures, especially since some scour holes are still growing. Scour holes develop at locations where either the hydrodynamics or the erodibility of the soil varies locally. In the Rhine Meuse delta, the subsoil stratigraphy is believed to play a large role , as it is composed of alternating layers of poorly erodible clay and peat and highly erodible sand. Local and large scale erosion are therefore not evenly distributed over the river bed and at locations where a top layer of clay or peat becomes too thin, exposure of an underlying layer of sand can result in a scour hole. Anthropogenic activities also have a large impact on the development of scour holes, such as the presence of hydraulic structures (e.g. groynes, bridge piers) and changes in geometry, which cause local changes in hydrodynamics. Moreover, deepening of river beds by dredging may have removed protective clay layers exposing underlying sand bodies. Finally, the closure of the Haringvliet caused higher flow velocities and therefore erosion in the connecting branches in the central part of the delta. In order to judge the potential risk, understanding is needed on the behaviour of scour holes in the Rhine-Meuse delta. A thorough data analysis is therefore performed on a large set of bed topography data. By performing experiments in a scale model more knowledge is obtained on the detailed physical processes influencing scour hole development. In order to predict future scour hole development the applicability of the method of Breusers is tested that predicts the depth and slopes of scour holes behind bed protections. As a first step in this research an attempt is made to develop an objective and generic identification method in order to identify scour holes in the future or in other river branches. Although a method based on the statistical characteristics of the river bed results in a good first approximation to indicate scour zones, the analysis shows that bed irregularities and large variations in scour hole shapes and sizes complicates this method. It is therefore advised to always study scour holes in more detail individually. As the closure of the Haringvliet in 1970 led to erosion in the connecting branches, it is likely that this also influenced the growth of scour holes in that area. After relating the current growth of the scour holes in the delta to the current bed level trends, it is seen that despite these degrading bed levels, the scour holes in the connecting branches do not show the largest growth. This is explained by studying the evolution of scour hole growth over the last five decades in two connecting branches (Oude Maas and Dordtsche Kil). It shows that most scour holes in the area already exist for decades and have reached a stable state. Most of the scour holes show a large growth after 1970 which is likely related to enhanced flow velocities caused by the Haringvliet closure. However, the analysis furthermore shows that only few scour holes developed just after 1970 of which some are expected to have been caused by dredging activities. The influence of the Haringvliet closure on the development of new scour holes seems therefore limited. The analysis of long term evolution of scour holes in the Oude Maas and Dordtsche Kil furthermore shows that the scour hole behaviour varies strongly and does not show similar trends, not even when they are located in the same river reach. This is likely related to the large variations in local conditions such as changes in geometry, the presence of structures, such as bridges and groynes, and the composition of the subsoil.

scour
tidal rivers
heterogeneous subsoil
delta
Rhine-Meuse delta

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Student theses

master thesis

(c) 2017 Koopmans, H,