The measurement of sand transport by means of radioactive tracers

Abstract

Experiments are described which have for their object the development of a method of measuring the sand transport in rivers by means of radioactive tracers. The method used is analogous to that which has been used for the measurement of water discharge in which the mean velocity is determined from the rate of displacement of the centroid of a marked cloud of fluid. In these experiments the spread of radioactive sand has been measured from a transverse strip of the bed of a flume in which a steady flow of water and sand is maintained. The distribution of active grains differs from that of a cloud of fluid, however, in that the time scale of the motion is longer due to the slower drift velocity of the particles. Also the particles at low levels in the ripples are at rest for a very much longer time than those in the upper levels so that the spread of the sand particles is much larger than is the case for fluid flow of the same mean velocity and depth. If the motion were allowed to continue for a sufficiently long time it is probable that the distribution would become normal and centred about a point moving with the centroid, but the time for such development would be impracticably long and the transport rate must be deduced from the distributions in the early stages of the motion when the dilution is not very great. In this stage an attempt has been made to explain the shape of the distribution curves in terms of a simple model of the mechanism of transport in which it has been assumed that every particle has an equal chance of moving a certain hop length. Whilst the distributions given by such a model have a general resemblance to those obtained, the similarity is not quantitatively affirmed. A more complicated model which takes into account both the statistical variation of the hop length of the particles and the times that particles spend at different levels in the ripples appears to be necessary. The experiments have shown that the transport rate can be deduced from the mass distribution of tracer particles initially laid to a depth sufficient to cover all levels of movement of the bed ripples, or by measuring the velocity of the centroid of the activity distribution together with an independent measurement of the ripple movements at a point.