Long term bed degradation in rivers: causes and mitigation

Intro

Studying the causes of autonomous bed degradation and prediction of short- and long-term effects of mitigating measures.

Overview

The upstream Dutch Rhine is experiencing bed degradation at a rate of 2 cm per year. This erosion causes severe problems to navigation and river management: non-erodible reaches hamper navigation and in-channel structures are destabilized. Such ongoing bed degradation is also observed in the German Rhine, the Elbe River, and the Danube River.

Natural and constructed non-erodible river reaches are responsible for spatially varying degradation rates, which reduce vessel draft and freight volume with severe economic consequences given the annual volume of goods transported by ship on the Rhine: 200 million tons at the German-Dutch border and 310 million tons on the Dutch Rhine. Long-term degradation poses threats to flood control, because bed degradation leads to an increase in both the migration speed of flood waves and the flood water level, and may also affect the distribution of water and sediment over the downstream branches of bifurcation points.

Understanding the causes of bed degradation is required to properly identify effective sediment management projects. Furthermore, a reliable prediction of the short-term and long-term effects of different mitigation measures is necessary to appropriately design and maintain sediment management projects. To that end, the first step toward understand the response of a river system to changes in flow and sediment transport regime and/or sea level is to identify the reference equilibrium state. In this way, we set up schematic mathematical models to understand how influential typical aspects of river engineering, e.g. channel narrowing, changes to hydrology, changes to upstream sediment flux, are on the ensuing bed degradation rate.

The next phase of the project involves research and evaluation of mitigation strategies to alleviate the causes and/or the symptoms of bed degradation. In the past few years, Rijkswaterstaat has commissioned two pilot studies on longitudinal dams (in the Waal River) and a sediment nourishment (in the Bovenrijn). These field scale experiments are evaluated and paired with analysis from mathematical models to evaluate the effectiveness of different mitigation strategies.

Special attention is given to mitigate impassable conditions at non-erodible reaches in the Waal River. Future studies will include a schematic investigation of mitigation measures specifically designed to alleviate these navigation impediments.

Progress

A Phd-Candidate and Postdoctoral Researcher, both at Delft University of Technology, are working on the project. Meles Siele is researching the relative importance of causes on river degradation, while Matt Czapiga is working on mitigation strategies to alleviate problems of bed degradation.

Products

Webpages

Blom, A. (2016). Bodemerosie in de Rijn. Online magazine Flows, Delft University of Technology, http://flowsplatform.nl/#/bodemerosie-in-de-rijn-1476873029138____151,152,155,184,163____

Blom, A. (2016). Bed degradation in the Rhine River. Online magazine Flows, Delft University of Technology, http://waterviewer.tudelft.nl/#/bed-degradation-in-the-rhine-river-1479821439344____47    

MSc Theses

1. Berkhout – Waves arising from Hirano’s conservation model for mixed-size sediment morphodynamics: An Experimental Study
2. Emmanouil – Analysis of Measured Data of Sediment Nourishments in the Rhine River
3. Emmanouil – Morphodynamic Trends in the Freely Flowing Rhine
4. S. Hiemstra – Development of a Methodology to Assess Functional Performance of the Dutch Rhine
5. W. van Linge – Hydraulic Evaluation of Longitudinal Training Dams
6. Ravenstijn – Behaviour of nourishments in quasi 3-dimensional graded sediment models
7. Rudolph – Measures for Mitigating the Ongoing Bed Degradation in the Rhine River – A First Assessment Using Idealized Models of the Waal Branch
8. van Weerdenburg – Measured change in bed elevation and surface texture near longitudinal training dams in the Waal River

BSc Theses

9. Lokin – Coarse and Fine Sediment Augmentations in Rivers
10. van Weerdenburg – The feasibility of reduced flow equations for long-term morphodynamic modeling of sediment augmentation in lowland rivers

Related NCR Partners

Delft University of Technology

Rijkswaterstaat

Deltares