Period and amplitude of bedload pulses in a macro-rough channel

It is known that bedload fluctuates over time in steep rivers with wide grain size distributions, even under conditions of constant sediment feed and water discharge. Bedload fluctuations, which are a consequence of grain sorting, are periodic and are related to fluctuations in the flow velocity and channel-bed morphology. The presence of large relatively immobile boulders, such as erratic blocks that are often present inmountain streams, has a strong impact on flow conditions and sediment transport. However, their influence on bedload fluctuations has not been studied. Sediment transport fluctuations were investigated in this study in a set of 38 laboratory experiments carried out on a steep tilting flume under several conditions of constant sediment andwater discharge for three different slopes (S= 6.7%, 9.9%, and 13%). Sediment transport, bulkmean flowvelocities, and variables describing the channel-bed morphologywere measured regularly during the experiments. Periodic bedload pulses were clearly visible in all of the experiments, alongwith flowvelocity and channel-bed morphology fluctuations. Correlation analysis showed that the durations of these cycleswere similar, although theywere not necessarily in phase. The pulses were characterized by their amplitude and period as a function of various boulder spatial densities and diameters.We could showthat for higher streampower the fluctuations decrease in cycle duration and in amplitude. The presence of boulders increases the stream power needed to transport a given amount of sediment, thus decreasing the bedload fluctuations.

Bed load fluctuations in a steep channel

Tamara Ghilardi, Mário Jorge Rodrigues Pereira Da Franca, Anton Schleiss

Abstract: Bedload transport rate fluctuations have been observed over time in steep rivers and flumes with wide grain size distributions even under constant sediment feeding and water discharge. The observed bedload transport rate pulses are periodic and a consequence of grain sorting. Moreover, the presence of large, relatively immobile boulders, such as erratic stones, which are often present in mountain streams, has an impact on flow conditions. The detailed analysis of a 13 hours laboratory experiment is presented in this paper. Boulders were randomly placed in a flume with a steep slope (6.7%), and water and sediment were constantly supplied to the flume. Along with the sediment transport and bulk mean flow velocity, the boulder protrusion, boulder surface, and number of hydraulic jumps, which are indicators of the channel morphology, were measured regularly during the experiment. Periodic bedload transport rate pulses are clearly visible in the data collected during this longduration experiment, along with correlated fluctuations in the flow velocity and bed morphology. The links among the bulk velocity, the time evolution of the morphology variables, and the bedload transport rate are analyzed via correlational analysis, showing that the fluctuations are strongly related. A phase analysis of all observed variables is performed, and the average shapes of the time cycles of the fluctuations are shown. Observations indicate that the detected periodic fluctuations correspond to different bed states. Furthermore, the grain size distribution through the channel, which varies in time and space, clearly influences these bedload transport rate pulses. Finally, known bedload transport rate formulae are tested, showing that only the application of a drag shear stress allows a correct estimation of the time fluctuations.

American Geophysical Union2014

Sediment Transport and Flow Conditions in Steep Rivers with Large Immobile Boulders

Tamara Ghilardi

Recent flood events in Switzerland and across Europe pointed out several deficiencies in hazard assessment, planning, and prediction methods for flood risk mitigation. A good understanding of the dynamics of mountain rivers, grounded on a sound physical-based theoretical framework, is primordial both from an environmental and a safety point of view. Although mountain rivers control sediment supply to lowland rivers, relatively few studies have been carried out on steep mountain channels, mainly during the last two decades. While these studies provide a multitude of sediment transport equations, generally with similar forms, most of them does not take into account the extreme conditions characterizing alpine torrents. For these latter, the presence of macro-roughness elements, such as large relatively immobile boulders, disrupts the flow and alters channel roughness. Moreover, bedload fluctuations have been observed over time in steep rivers and flumes with wide grain size distributions, even under constant sediment feeding and water discharge. This research project investigates the impact of randomly distributed boulders (cascade morphology) on the sediment transport capacity and bedload fluctuations in steep channels. This is done by means of 41 laboratory experiments, carried out on a tilting flume at the Laboratory of Hydraulic Constructions (LCH) at Ecole Polytechnique Fédérale de Lausanne (EPFL). The influence of several boulder sizes and distance between roughness elements is investigated for three flume slopes (S=6.7%, 9.9% and 13%). Sediment transport, bulk mean flow velocities and variables describing the morphology were assessed regularly during the experiments. Firstly, the detailed analysis of a 13 hours laboratory experiment is presented. Periodical bedload pulses are clearly visible on this long duration experiment, along with correlated flow velocity and bed morphology fluctuations. The relation among bulk velocity, morphology variables time evolution and bedload transport is investigated by correlation analysis, showing that fluctuations are strongly linked. Visual observations indicate that the detected periodical fluctuations correspond to different bed states. Furthermore, the grain size distribution through the channel, varying in time and space, clearly influences these bedload pulses. For all the experiments, bedload pulses were then characterized by their amplitude and period. It is shown that for higher stream power the fluctuations decrease, both in duration of a cycle and in amplitude. The presence of boulders increases the stream power needed to transport a given amount of sediments, thus decreasing the fluctuations. The impact of increasing channel slopes on sediment transport is well known. The present research shows that it is also indispensable to take into account the presence of boulders in the estimation of the sediment transport capacity, since it is strongly decreasing with dimensionless boulder distance. Sediment transport capacity is better estimated when taking the liquid discharge as basis parameter instead of bed shear stress. The critical discharge for incipient motion is shown to be dependent not only on the channel slope but also on the dimensionless distance between boulders. A sediment transport formula based on excess discharge and taking into the presence of boulders is herein developed.

EPFL2013

Replenishment of sediment downstream of dams

Elena Battisacco

Dams on rivers alter the sediment continuum, trapping water and sediment in the upstream reservoirs. River reaches downstream are affected by several negative effects, such as bed incision, reduction of the river morphological variability, development of an armoured layer and depletion of ecological habitat for fish. The replenishment of sediment technique has been used since the 1970s to mitigate the lack of sediment transport in the downstream reaches of dams. The method is mainly used to re-establish a sediment continuum, restore a natural bed morphology and to recover spawning grounds for fish. Even if both experimental and field tests have been performed in the past, there is still a lack of knowledge regarding the necessary flow released from the bottom outlet of the dam, the amount of sediment and the configuration of the replenished deposits. This research aims at filling these gaps by means of experimental tests at the Laboratory of Hydraulic Constructions at Ecole Polytechnique Fédérale de Lausanne (EPFL). The main characteristics of an alpine stream were reproduced in a channel facility in terms of grain size distribution, slope and hydraulic conditions. The sediment replenishment technique was investigated as an influence to the geometrical configurations and volume of deposits, as well as applied discharge. The experiments showed that the bed morphological pattern created by the eroded replenishment material was linked to the initial geometrical arrangement of the replenishment volumes. When applying parallel configurations of replenishment volumes, a general bed fining was obtained with material spread over the entire channel width. Alternating replenishment positioning lead, in turn, to the creation of bed morphological patterns. The wavelength of these bed forms was seen to be related to the replenishment length. Both constant discharge and transient flow were investigated. Three submergence conditions of the replenishment were tested: unsubmerged, completely submerged and over-submerged. A complete submergence was optimal to obtain a complete erosion of the volumes with transport of the material along the channel and persistence of the material on the channel bed. Four slopes for the increasing and decreasing limb of triangular-shaped hydrographs, having the same maximum discharge, were tested and the results compared with the constant discharge cases. A discharge having a triangular distribution in time is similar to the operational condition of dams when releasing an artificial flood. Transient flows with steep rising limbs lead to a reduction of 70% of water consumption compared to a constant flow, although achieving the erosion of the replenished material. The application of the complete hydrographs showed to be counter-productive in terms of local effect of the sediment replenishment. These hydrographs may be useful for reaching longer distance impacts in sediment replenishment. Lastly, the effect of consecutive replenishment of sediments was investigated. A second replenishment can increase the deposition heights and volumes downstream. As expected, a second replenishment affects a longer downstream channel reach. Thus, consecutive replenishments are useful in field applications to have effects at longer downstream distances.