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Sediment transport in the Koiliaris River

Nerantzaki Sofia

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Summary

This work presents a study of sediment transport in a complex Mediterranean watershed (i.e. the Koiliaris River Basin of Crete) consisting of temporary flow tributaries and karstic springs. Monitoring of suspended sediment concentration in such watersheds is of utmost importance due to first flash events, when large quantities of sediments and pollutants are carried downstream. Up to now, the estimation of river suspended sediment was carried out by laboratory filtration of grab water samples. This technique provides only a rough estimation of the sediment transport. To overcome this, as well as the lack of representative sediment transport data during extreme flow events (e.g. flash floods), an automated sediment sampling device (Sediment Trap), which allows for flow weighted sampling, using turbidity and water level as a trigger for the initiation, has been developed and is presented in this study. The device is undergoing testing to ensure that it can provide accurate estimates of sediment yield, especially during a flash flood event. Field measurements of turbidity were correlated with suspended solids concentrations derived from grab sampling, and an empirical curve between turbidity and suspended sediment concentration was developed. In addition, X-Ray Fluorescence (XRF) analysis was carried out to determine the chemical characterization of the samples, for a comprehensive understanding of the sediments, and the grain size distribution was determined by a laser diffraction particle size analyzer. The automated system will be used to collect data for the calibration of model simulations of the hydrology and sediment transport of the Koiliaris River watershed. Specifically, both daily flow data (2005-2014) and monthly sediment concentration data (2011-2014) were used to calibrate the Soil and Water Assessment Tool (SWAT) model, designed to simulate the hydrology, sediment yield and water quality of ungauged watersheds, augmented with a karst flow model in order to simulate the contribution of the extended karst to the spring discharge in the basin. The results showed good agreement between observed and model values for both flow and sediment concentration.

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