Influence of ship-induced currents on the erosion and permeability of sand-mud mixtures in the Twentekanalen

Nijborg, Niels A. (2016) Influence of ship-induced currents on the erosion and permeability of sand-mud mixtures in the Twentekanalen.

Abstract:Over the last decades there is a trend of ever growing ships worldwide in order to increase their transport efficiency. The draught increases and increasingly larger propellers are built on these modern ships. However, the dimensions of numerous inland navigation channels do not meet the requirements for these larger vessels. A direct consequence is the significant increase in ship-induced flow velocities under and around the ships which could cause severe bottom and bank erosion. Scour induced by the propeller wash has become one of the most important issues for the design and maintenance of navigation channels and harbour structures (Hong, Chiew, & Cheng, 2013). The Twentekanalen in the Netherlands are currently categorized as class IV waterway, however considering the economic development in the hinterland and ongoing deployment of larger vessels Rijkswaterstaat decided to enlarge these waterways to class Va in 2017. On the one hand this research focussed on the increase in ship-induced flow velocities and the consequences regarding bottom erosion in the side channel of the Twentekanalen. On the other hand this research also focussed on seepage which is for a large extent dependent on the resistive bed of the canal. In 2010 (maintenance) dredging activities took place in the side channel of the Twentekanalen where at specific trajectories the resistive sludge layer was removed which resulted in higher groundwater levels in the area behind the dikes causing nuisance for both residents and farmers in terms of flooded basements and reduced crop yield. In 2016 a temporary sludge layer is constructed on the bottom of the side channel to reduce the amount of seepage and consequently lower the groundwater level. Monitoring the groundwater levels revealed that the groundwater levels were lowered, however not sufficient to mitigate problems for farmers and residents. Concluding, Rijkswaterstaat would like to gain more insights in bottom erosion caused by larger ships and the effects of sludge on the bottom of the channel with respect to the hydraulic conductivity. Therefore, the objective of this research was to “to visualize the erosion profile and estimate the equilibrium erosion depth of a sand-clay sediment mixture under the influence of shipping in the axis of the side channel of the Twentekanalen, and to test whether clay particles are infiltrated into the sandy subsoil affecting the hydraulic conductivity”. To achieve this objective two separate laboratory experiments were conducted: (1) a scale model of a ship towed in a flume over a sand-clay mixture in which different parameters are varied and (2) a plastering experiment to determine the effects of clay plastering in a sand filter on the hydraulic conductivity. Findings of the experiments are translated and scaled for the situation of the Twentekanalen. Erosion experiment - The erosion profile in the sand-clay mixture starts to develop directly after a ship has passed, relatively fast in the beginning and reduces with every ship passage eventually resulting in an equilibrium depth. Underneath the ship the bottom erodes and the eroded sediment is transported to the sides of the channel. At small sailing speeds the propeller wash appeared to be dominant and higher efflux velocity of the propeller results in a wider and deeper scour hole. The return current starts to play a role with increasing sailing speeds while simultaneously the impact time of the propeller on the bottom decreases. Moreover, flow velocities at the bottom caused by the propeller wash are decreasing when sailing speed increases assuming a constant efflux velocity. Erosion certainly takes place when large sailing speeds were applied, however a clear scour hole did not develop. Additionally, the erosion depth is dependent on the slope stability of the erosion hole, hence the strength of the sediment and the sand-clay ratio. Comparison with existing erosion formulas for sand seems to give fairly reasonable results, however these are established for continuously rotating propellers located on the same location contradicting to the conducted experiment in this research. This research focussed on erosion by moving ships and might be an explanation for the different empirical coefficients in these formulas. Therefore, more research into moving propellers and subsequent erosion is needed. Furthermore, quantitative research is advised to determine the effects of cohesive properties on the erosion depth under influence of shipping currents and to incorporate specific cohesive properties in an erosion formula. Plastering experiment - Clay plastering appeared of great importance regarding the hydraulic conductivity. Pouring a natural sludge or sand-clay mixture on top of a sandy soil directly influences the hydraulic conductivity. Moreover, the amount of clay particles in the mixture is important, more clay particles clogging more pores and results in even lower hydraulic conductivity. Additionally, it was observed that de hydraulic conductivity decreases when the sludge is stirred with the sandy subsoil. Hence, it is concluded that also the distribution of clay particles within the sandy subsoil is affecting the hydraulic conductivity. The invasion depth of clay particles, affecting the hydraulic conductivity and subsequently the amount of seepage, is therefore an interesting issue to examine in more detail. Translation and recommendation for the Twentekanalen – As a result of larger ships in the future and adjustments to the profile of the channel, the ship-induced flow velocities in the Twentekanalen increase substantially with an increase of 20% regarding the return current and roughly 60% for the propeller wash. Maximum flow velocities on the bottom of 2.9 m/s for moored ships and 1.9 m/s for sailing ships could occur. The occurring ship-induced flow velocities are significantly larger than the critical velocities of natural sludges, thus the application of a stable natural sludge layer on the bottom of the Twentekanalen is very unlikely. Increasing critical erosion velocities e.g. by compacting and or adding more clay are most likely not sufficient. Measures to decrease flow velocities on the bottom by deepening or widening the channel were found not sufficient or expected too costly. Currently, the erosion tracks in the side channel of the Twentekanalen ranges from 0.15 m to 0.5 m found in bathymetry measurements. It is likely that erosion tracks become deeper due to the increasing flow velocities of the larger vessels. The experiments in which reality is best reflected show erosion tracks of 2 - 5 cm corresponding to 0.4 and 1 m in reality (scale 1:20). These results are verified with existing formulas which are validated to similar situations of inland navigation channels in reality. Erosion tracks calculated with these formulas resulted in an erosion range expected in the Twentekanalen of 0.15 - 1 m in accordance with the experiments. Clay plastering is important for the seepage hindrance in the surroundings. Samples of the inserted sludge layer in 2016 revealed that the requirements for physical characteristics were far from met. In particular the clay content was significantly lower than the minimum requirement. It seems that especially the percentage of clay within the sludge is important while the layer thickness is of less importance also taking into consideration the displacement of sludge over the profile. Lastly, it is known that mixing the sand with the sludge consequently causes a better distribution of clay particles resulting in a lower hydraulic conductivity. Therefore, related to the Twentekanalen this would mean that ship-induced currents do not have directly negative consequences for the amount of seepage because it could mix the clay particles with the sandy subsoil.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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