Continuous Monitoring of Suspended Sediment Load in Semi-arid Environments

Abstract

Sediment transport in water courses is an indicator of soil eroded from agricultural land, and the intensity of the phenomenon provides a measure of land degradation and the associated reduction in the global soil resource. Suspended sediment load is a useful indicator for assessing the effects of landuse changes and engineering practices in watercourses. The investigation of the trend in the sediment loads has different constraints in terms of available data. Sediment load data are lacking for rivers in many areas of the world, particularly in developing countries where changing sediment yields might be expected (Walling & Fang, 2003). Continuous river monitoring is essential to effectively measure suspended sediment loads during storm events and to accurately describe the sediment transport dynamic. The paper focuses on the application of technologies for continuous monitoring of suspended sediment concentration in rivers in semi-arid environments. In the first part of the paper, methods for measuring suspended sediment concentration are reviewed. Technologies to continuously monitor suspended sediment overcome traditional methods requiring routine collection and analysis of water samples. Among the available instruments, based on optical principles, pressure difference and acoustic backscatter principles, the optical technology largely spread as turbidity is considered a good “surrogate” for suspended sediment (Gippel, 1995; Lewis, 1996; Lenzi & Marchi, 2000; Seeger et al., 2004;) and particularly suitable for high suspended concentrations (LopezTarazon et al., 2009; Gentile et al., 2010). It provides reliable data when the point measurements can be correlated to the river's mean cross section concentration value, the effects of biological fouling can be minimized, and the concentrations remain below the sensor's upper measurement limit (Gray & Gartner, 2009). In the second part of the paper the continuous monitoring of the suspended sediment concentration in a semi-arid watershed is used to analyze the sediment transport dynamics. In semi-arid areas the seasonality of the hydrological processes and the strong interannual variation in precipitation rates enhance the role of infrequent flood events (Soler et al., 2007). As a consequence suspended sediment concentrations in rivers are generally high, as they compensate for the infrequency of runoff in producing high annual unit sediment yields (Walling & Kleo, 1979; Alexandrov at al., 2007; Achite & Ouillon, 2007). In these areas suspended sediment transport provides problems for water-resource management where channels are impounded as high rates of sedimentation occur in reservoirs.