Les méthodes de bilan des taux de comptage de traceurs radioactifs appliquées à la mesure des débits massiques de charriage

Abstract

The present scope of quantitative sediment bed load measurement methods is briefly reviewed (semi-empirical formulae and measurements in situ) also the principles of quantitative methods using radioactive elements (time integration methods : total counts and continuous dilution, spatial integration). A new method is presented, in which the integral (N = ρρ nds) of the number of impacts per second n given by the pick-up is calculated for a radioactive cloud representative of the particular form of motion under investigation. The operation is referred to as the "count rate balance" method of measurement. The bed load masse flow is then found by relating N to the depth of the mass in motion E, giving the following relationship : Q = ρ x l x Vm x E where Vm : mean velocity; l: width of moving mass ; ρ : bulk density of the sediment. N is related to E by the pick-up's response function f (z) with depth and the tracer's distribution function Γ (z) with depth. Hence: N = E 0 Γ (z) f (z) dz It is shown that, if ƒ(z) is a linear function, the mean buried depth of the tracer Zm, l'an be found for any value Γ (z). Such detectors have been successfully obtained for 192Ir and l82Ta down to a buried depth of 10 cms. Furthermore, by using two pick-ups with different linear response functions, an exact value can he found for Vm, independently of buried depth effect. The use of more than two pick-ups with different response functions yields additional information, which is discussed. It is found that, for all the assumptions made regarding Γ (z), the rate of flow can be calculated to within 25 %. Where the response function is exponential (which the authors observe experimentally), the rate of flow is known in terms of a factor β varying with Γ (z), ƒ (z) and E. The rate of flow is then calculated for all the assumptions made with an accuracy varying from within a few per cent for moving masses of little depth to within about 30 per cent for mass depths of up to 40 cms. The use of several pick-ups yields additional information which improves accuracy. It is also shown that the use of two pick-ups can be of considerable assistance where time Integration methods are to be applied. Result of two river experiments are described, as follows : 1) On the Stung Sen in 1963. Although the method had not been finalized at the time of obtaining the data, the rate of sediment flow during the river flood was estimated to amount to 800 tons ± 200 tons, which agreed with previous estimates. 2) On the Sienne, a coastal river in the Cotentin peninsula. The rate of flows was estimated to amount to 21 tons ± 5 tons. A systematic possible error study was done for this application, giving special consideration to errors which cannot be dealt with by calculation, for example errors in plotting field measurement data or in estimating mean count rate between two isoactives. These errors were found to be small. As these recommended methods do not require any more special equipment than is used in conventional radioactive tracer work and as they are merely a different way of interpreting measured data, the authors urge anyone using them to systematically work out "count rate balance sheers" from the results obtained, and to try to deduce the depth of moving sediment and possibly the rate of flow from them.