A radiometric density gauge using 662 keV gamma rays has been developed at the GKSS research centre, for the determination of local silt distributions on the beds of waterways. Features of the techniques utilized are transmission geometry, data telemetry via a single conductor cable, a high data-sampling rate, and computer-based on-line data evaluation. The probe was subjected to various in-situ tests and successfully applied for extensive measurements in Hamburg harbour and in the tidal region of the fiver Weser.

INTRODUCTION

The sedimentation of suspended particulate matter and sand in harbours and shipping channels of tidal rivers or estuaries causes a high maintenance dredging cost. Beyond that, removal or deposition of the dredged material raises serious ecological problems because of heavy metal pollution and organic contamination. Sediment in waters has thus attracted considerable research interest. In particular, the density of silt on the beds of waterways has been recognized to be an important parameter describing not only the actual mass distribution, but also indicating the so-called navigational depth. Since the body causing echo-sounding horizons at depth may vary from consolidated mud (or sand) to "dirty water", the latter being no hindrance to navigation, the vertical density profile might be a decision criterion for the necessity of expensive dredging operations.

Therefore, a vessel-borne silt-density gauge has been developed, which may penetrate the ground to depths of up to 1.60 g/cm3, by gravity alone. It differs from other commercially available gauges by the application of transmission geometry which is superior to backscattering arrangements. Also with respect to signal processing, data transfer and on-line evaluation, a state-of-the-art concept has been realized. Details of construction and operation, results of tests and field measurements will be given in the following sections.

IN-SITU SEDIMENT INVESTIGATION METHODS

Since the capabilities of methods other than gamma-ray absorptiometry appear to be widely overestimated with respect to the quantitative analysis of bottom silt, it is necessary to point out a few features of competing methods such as echo-sounding and radiometry. The well known echo-sounding technique utilizes the tune elapsed between emission of an acoustic signal and receipt of the echo to measure the water depth; the strength of the reflected signal is proportional to the gradient of the product of density and the propagation velocity of sound in the reflecting layer. Evidently, for soft transitions between water and the bottom, as is the case with silt, the echo horizon is not well defined. In any case, the reflection is determined by the local change of density with depth, rather than by the absolute value of the density.

This fact was quite well demonstrated by a tank experiment performed in 1967 (Hellema, 1979), the results of which are given m Fig. 1. In that experiment, starting with a well mixed homogenous mixture, the sedimentation was studied for 32 days by dally sampling and echo-sounding using different frequencies. As may be seen in the figure, the acoustic 30 kHz horizon crosses several density levels between 1.20 and 1.27 g/cm3, whereas higher frequency horizons.

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