NPL recently led a project to measure the typical underwater noise levels radiated by dredgers during marine aggregate extraction. The project was funded by the Marine Aggregate Levy Sustainability Fund (MALSF) as Project 09/P108 under the Marine Environmental Protection Fund (MEPF).

The project was delivered by a consortium consisting of the National Physical Laboratory, Loughborough University, Gardline Environmental Ltd, and the Institute of Sound and Vibration Research (ISVR) of the University of Southampton.

This project has provided an extensive study of the noise generated by the UK's fleet of trailing suction hopper dredgers during marine aggregate extraction. The objectives of the work were (i) to develop a suitable methodology for measuring underwater noise radiated by dredgers, (ii) undertake measurements on UK dredgers at up to four sites and report, whilst disseminating the results to the wider stakeholder community. In the report, data is presented for 6 vessels, measured across 3 different areas around the UK's coast, with one vessel being measured in two different areas.

The Source Levels (a measure of the acoustic noise output) of six dredging vessels have been estimated, and an investigation undertaken into the origin of the radiated noise. To achieve this, a methodology was established which employed hydrophone measurements made as a function of range from the source coupled with Propagation Loss modelling to establish the one-third-octave band Source Levels for each vessel. Measurements were made at frequencies up to at least 48 kHz, with data taken up to 100 kHz for four of the dredgers, and with some data obtained up to 200 kHz.

Considering the results in context, the noise radiated at frequencies less than 500 Hz is similar to that of a merchant vessel travelling at modest speed. An interesting feature of the results is that, while extracting aggregate, the vessels generate higher levels of noise at frequencies above 1 kHz than a typical merchant vessel. Analysis of the measured data for differing operation modes leads to the conclusion that the major source of this higher frequency noise is the impact/abrasion of the aggregate material passing through the draghead, suction pipe and pump (possibly with some additional contribution due to cavitation noise).

This is clear from figure below which shows Received Levels measured for the same dredger under different operational modes (full dredging, draghead lifted with pump on, draghead lifted with pump off). This means that the overall noise output level is partially dependent upon the aggregate being extracted, and results indicate that extracting gravel is noisier than extracting sand.

View the final report and find out more about the Marine Aggregate Levy Sustainability Fund

For more information please contact Stephen Robinson