Determination of river-bed transport with Iridium-192 in the upper reaches of the River Lerma

The upper reaches of the river Lerma discharge at an average rate of 29 m3/s into the Solis barrage, which is one of the largest in the Mexican high plateau. This barrage covers an area of 105 ha and is subject to problems of sedimentation, which result in an estimated reduction in useful volume of about 4.8x106 m3 per year. Its hydraulic characteristics are those of a torrential river; during the rainy season, almost instantaneous variations in its flow rate of 15 m3/s to 70 m3/s are recorded, and during some hours the flow rate even reaches 100 m3/s to 300 m3/s. Since radioactive tracers have proved very useful in studying river-bed transport in this type of river, one kilogramme of glasses labelled with 0.685 Ci of 192Ir was used to determine the quantity of solid material carried by the river to the Solis barrage. The grain size of the tracer injected (0.84-1.10 mm) was determined from a mathematical model for size distribution of the river-bed sediment. The experiment lasted 232 days and covered the whole floodwater period (June to October). A unit solid flow rate of 2.25+-0.66t/m.d was estimated on the basis of the spatial integration of the tracer. For the purpose of comparison, various semi-empirical formulae (Einstein, Meyer Peter-Mueller, Schoklitsch, Kalinske, and others) were used to calculate river-bed transport. Agreement was found between some of these results and those obtained using the radiotracer. (author)