Interpreting event-based suspended sediment concentration and flow hysteresis patterns

Abstract

This study analyses the hysteresis relationship between suspended sediment concentration (SSC) and flow (Q) during runoff events to investigate the effect of hydrological factors and catchment characteristics on suspended sediment dynamics. Continuous records of flow and suspended sediment concentration, proxied by turbidity, collected from 17 catchments across New Zealand with different size, land cover and erosion terrain characteristics were analysed for this purpose. We first classified the hysteresis patterns in terms of their shape and direction during events, then analysed hysteresis indices and related them to event hydrology and catchment characteristics. In total, 1553 events were analysed. The results indicate some clear differences between pasture- and forest-dominated catchments, with pasture-dominated catchments having event flow peaks generally lagging the peak of sediment concentration whereas in forest-dominated catchments the opposite occurred. Moreover, pasture catchments typically showed higher ratios of flow-weighted concentration on rising to falling stages of hydrographs compared with largely forested catchments. This shows that the sources of sediment respond faster during flood events in pasture-dominated catchments. A principal component and classification analysis of hydrological and sediment-related variables showed that the main variables controlling the hysteresis patterns within each catchment were flood total runoff and flood duration. These results are of immediate value to river management programmes and policies concerned with mitigating suspended sediment delivery to impacted waterways and to developing and testing event-scale catchment-based suspended sediment routing models. Future research could aim to reduce ambiguity in interpreting Q-SSC hysteresis driving factors by statistical analysis of larger datasets spanning more catchments and by using other information streams such as high spatio-temporal resolution sediment tracing.