A full scale propeller wash erosion test on heterogeneous cohesive material

Karelle, Y. and Sharpe, P. and Pizzo, G. and Haerens, P. and Albanese, V. (2016) A full scale propeller wash erosion test on heterogeneous cohesive material. In: ICSE 2016 (8th International Conference on Scour and Erosion), 12-15 September 2016, Oxford, UK.

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Abstract

As part of a life-cycle cost analysis for the construction of a harbour basin, a full scale erosion test on a dredged slope was undertaken using a test vessel. The objective of the test was to estimate the erosion rate of the in-situ material in order to predict the final profile of the dredged slope at the end of the design life following exposure to propeller wash generated by various manoeuvring vessels. Based on theoretical formulae, a test protocol of percentages of the installed engine power to be applied was defined, aiming to generate a steady propeller flow on the slope as a function of the tide. A pre-installed anchoring point ensured the test vessel maintained a steady position during the test and the pre-defined percentages of the installed engine power were applied as a function of the tidal levels measured during the test. Several interim bathymetric surveys were undertaken in order to monitor and record the progress of the erosion over the tested slope. Orientation and location of the vessel were monitored by satellite positioning, tides were recorded by installed gauges and actual flow velocities logged by a current meter. A time stamped geo-database was developed to model the theoretical distribution of the velocities at any moment of the test. Through the analysis of the tests results, a curve defining the erosion rate as a function of the velocity at the slope was derived from the model and was verified by comparing generated cross-sections against actual surveys. Results were then extrapolated to represent the situation at the end of the design life of the harbour in order to assess whether protection of the slopes was required to preserve the integrity of the marine structures adjacent to the harbour basin. Results were also used to define an appropriate monitoring program for the dredged slopes which had potential for scour generated by propeller wash.

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