Search

The transport of coarse sediment during floods often exhibits hysteresis patterns from changes in flow fields, channel geometry, bedforms, or sediment supply conditions. Flume experiments that simulate hydrographs tend to confirm that hysteresis is a consequence of the progressive organization of surface sediments in terms of grain protrusion, imbrication, orientation, and roughness. Hysteretic patterns are also highly dependent on the kind of sediment supply conditions, and the type of simulated hydrograph. A factor that has not been investigated extensively is the effect of the timing and sequencing of floods on bedload transport. Depending on its magnitude and duration, each flood leaves the channel bed in a different condition, which influences the bedload transport of the next event, representing the river bed's memory of past floods, which can determine future responses to natural disturbances. In this study, I investigated the effects of different sequences of events, i.e., the flood history, on sediment transport through a series of flume experiments that simulated three types of stepped and symmetrical hydrographs (ranging from short-duration/high-magnitude to long-duration/low-magnitude events) under sediment recirculation conditions. Hydrographs were simulated as a sequence of the same event, and with events in different sequences, in order to explore the effects of different antecedent conditions on sediment transport. The results show that a previous event decreases the rates of sediment transported by a certain hydrograph by around 40% if a high-magnitude event precedes another one and around 70% if a low-magnitude event precedes another event of similar magnitude. A low-magnitude event does not affect the rate of sediment transported by a subsequent high-magnitude flood, but a high-magnitude event reduces the sediment transported by a subsequent long-duration/low-magnitude event. [ABSTRACT FROM AUTHOR]

Tropical cyclones can generate huge precipitation leading to flash floods in coastal and island areas. The consequences for infrastructures and populations can be dramatic as large amounts of sediments are transported downstream. The ReNovRisk project aims studying the erosion and sediment transport in flooding rivers of La Réunion Island in the Indian Ocean during tropical cyclones. In this frame, we analysed recordings made at three seismic stations installed along a river in the Salazie Basin on the northern side of La Réunion Island, together with hydrological and meteorological data to characterize microseismic noise during the tropical cyclone Dumazile (March, 2018). We evidenced a very good correlation (r2 ~ 0.9) between the seismic noise amplitude and the water level, particularly during the rising limb of the river flood at high frequencies (>1 Hz). We also observe a clear influence of the station distance to the river on the noise amplitude, with strong noise attenuation with distance, particularly at high frequency, suggesting that the sediment transport monitoring has to be performed by stations located very close to the riverbed. We evidence a clockwise hysteresis between the power spectral density and the water level in both low and high frequency ranges dominated by water turbulence and sediment transport, respectively. Polarization measurements performed in multiple frequency ranges show an overall weak polarization, suggesting that the noise sources generated by the flooding river are not focused in the single point but instead, that the whole riverbed is emitting noise that reach the station from numerous backazimuths. [ABSTRACT FROM AUTHOR]

The landscape development in the Kosi River basin in the central Himalaya is strongly influenced by large tectonic and climatic gradients. Dynamic behaviour of the Kosi River in the alluvial part has been well-documented and is strongly linked to high sediment supply from the upstream hinterland area. Hence, reach scale erosion mapping in the hinterland area is a major issue, which can be tackled by highlighting sediment generating areas within the Kosi River basin. Sediments generated through hinterland erosion are routed downstream and spatially distributed in lower reaches of the Kosi River. The spatial distribution of energy available for sediment transport is governed by stream power (function of discharge and slope). This sediment transport competency approach has been applied in this paper to unravel erosion at basin/reach scale using spatial distribution pattern of specific stream power (SSP). A physically-based semi-distributed hydrological model, Soil and Water Assessment Tool (SWAT) has been used to simulate the reach scale stream power variability through estimation of channel slope and discharge. Stream power distribution pattern for different months was extracted at reach scale, through SWAT model by utilizing basic inputs such as precipitation, temperature, elevation, land use land cover and soil data. We observed that stream power is higher in channel reaches of the Sun Kosi and Arun. Downstream variation in SSP plot shows that mid-channel reaches in the Higher Himalaya have higher stream power in the Kosi River basin. The results also highlight a significant role of channel slope on stream power distribution pattern. Spatial distribution of slope data shows higher channel slope in western and main channel of the Kosi River that also results in higher stream power. This increased fluvial incision event is manifested in the spatial distribution of sediment yield. Further, since the estimation of SSP incorporates spatial variation in channel width, the peaks in specific stream power shift downstream in comparison to stream power distribution due to spatial variability in channel width. The reaches of high specific stream power and stream power therefore represent the major erosional zones, and therefore this approach can be used as a proxy for sediment erosion to understand sediment dynamics in the Kosi River basin. [ABSTRACT FROM AUTHOR]