Feature

The Sounds of Gravel Rock

We can learn a lot by listening to the sound of moving rock.

Helen L. Rouse

Looking at how coarse sediments, such as gravel, move along a riverbed or a beach is useful for gaining an understanding of the physical processes that form these environments, as well as the management implications of what can be done within those environments.

In the past, methods of measuring sediment transport -- such as morphological surveying, trapping and water sampling -- have been less sophisticated than those developed for measuring the nature and rate of water flows. Optical and acoustic techniques have more recently been developed to measure sediment transport at sufficiently high frequencies to enable direct comparison of flows and sediment movement.

One such acoustic technique involves the principle of self-generated noise (SGN), or listening to the noise made as sediment moves. SGN is the result of collisions between grains during movement of gravel-sized sediments. Hydrophones (underwater microphones) are used to take non-intrusive sound samples of this bedload transport, typically at a high frequency of more than one sample per second.

Past work by a variety of authors has led to two major conclusions. First, the intensity of the SGN acoustic signal increases with the transport rate -- things get louder as more gravel moves. Second, the frequency of the SGN acoustic signal is inversely proportional to the diameter of the moving particles; that is, the bigger the individual gravel, the lower the sound they produce as they move.

This work showed that it is important to calibrate a hydrophone system before using it in the field to ensure that sediment movement is estimated as accurately as possible. Both the audio frequency and the intensity of the noise will vary in a natural environment, depending on impact duration and velocity, grain size, shape and the type of material involved. Calibration of any acoustic system needs to be done with sediment typical of the field site. It is also important to be confident that only the SGN part of the acoustic signal is calibrated. For this reason it can be helpful to use a filter in the hydrophone system which narrows the band of frequencies measured to those in which SGN collisions are occurring.

In addition, background noise should also be removed from the hydrophone signal. These can come from a variety of sources including hydrodynamic (waves and turbulence), other physical sources (such as ship noise) or be biological in origin (such as the sounds made by sea birds or dolphins).

The relationship between acoustic intensity and bedload transport rate was central to some of my research in the UK, where underwater hydrophones were used to monitor the acoustic intensity of SGN collisions on the seabed.

A filtered hydrophone system was used to collect SGN data, which was then calibrated to produce a bedload transport rate time series showing how the sediment travelled along the seabed over a period of time. Simultaneous measurements of nearbed pressure fluctuations were measured using pressure transducers and then calibrated to produce a surface wave time-series which showed how the wave action changed over time. An average background noise level was calculated from the individual time series, and the background noise removed from the acoustic signal.

This approach enabled the first direct comparison of surface waves and bedload transport. Examining a chart of the two time series provided new information regarding sediment movement. Parts of the wave normally associated with low velocities and shear stresses coincide with significant peaks in sediment transport. This suggests that another factor is involved in the offshore sediment movement, possibly changes in slope resulting in more movement in some areas than others.

Provided that the acoustic signal is properly calibrated and filtered, the SGN technique offers vast opportunities for either high-frequency comparisons of flow and bedload, or long-term automatic monitoring of bedload transport rates.

Funding has been received from the Lotteries Science Fund, Hawkes Bay Regional Council, University of Canterbury's School of Engineering and ECNZ to enable the development and testing of a new hydrophone and filter system. This work will enable improved methods for calibration of the signal and for differentiating between the SGN signal and background noise.

New Zealand Applications

In the New Zealand environment, the SGN technique could be useful in improving understanding of gravel movement in braided rivers or on the gravel beaches common to many areas.

This approach would let regional councils quantify gravel movement rates and be more informed about the consent levels they set for gravel extraction from coasts and rivers. They will also be able to put a figure to the elusive "river input" value which is now estimated in the calculation of coastal sediment budgets that allow regional councils to judge whether a coastal area is likely to experience erosion.

Users such as ECNZ could improve estimates of bedload movement rates at certain flows which may allow the corporation to keep dam intake gates open at greater levels without the fear of gravel accumulation.