Temporal evolution of bedload in a steep channel over a long duration experiment

Bedload fluctuations over time in steep rivers with wide grain size distributions have been observed, even under constant sediment feeding and water discharge. Observed bedload pulses are periodical and a consequence of grain sorting. Along with bedload, flow velocity and bed morphology fluctuate in time as well. The presence of large relatively immobile boulders, such as erratic stones often present in mountain streams, have an impact on flow conditions. Their influence on these bedload fluctuations has not been studied yet, namely in what concerns the frequency and amplitude of bedload fluctuations. The influence of boulders on sediment transport and flow conditions is herein investigated by means of laboratory experiments carried out in a 8 m long and 0.25 m wide tilting flume. The detailed analysis of one set of measurements obtained during a 12.8 hours (774 minutes) test is presented in this paper. For this experiment 12 boulders of diameter D=0.075 m were randomly placed in the flume, corresponding to 3% of the flume surface initially occupied by boulders. The flume slope was set to 6.7%. Input sediment discharge was constant and equal to qs,in=0.0563 x10-3 m2/s and the liquid discharge was fixed at q=1.68 x10-2 m2/s. Sediment transport and boulder protrusion are measured regularly during the experiment. Furthermore, bulk mean flow velocities are measured every 15 minutes. Periodical bedload pulses are clearly visible on this long duration experiment. Bulk velocity and boulder protrusion are linked with the fluctuations of the bedload transport as demonstrated by correlational analysis. The period and the phase of the fluctuations are similar for all the measured variables. Observations indicate that the detected periodical fluctuations correspond to different bed states. Grain size distribution through the channel, varying in time and space, is clearly influencing these bedload pulses. During low sediment discharges, coarse stable riffles are formed. On the other hand, in high sediment fluxes the destruction of riffles followed by bed fining is observed. A detailed description of the experimental observations and an analysis of amplitude and frequency of fluctuations for this long duration test are presented in this article.

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

Sediment transport and flow conditions in steep rivers with large immobile boulders

Tamara Ghilardi

In steep mountain rivers, morphological features such as large mostly immobile boulders, strongly influence flow behavior and bedload transport. Without taking into account these large immobile boulders, which are acting as macro-roughness elements, classical bedload transport formula largely overestimate the sediment volume transported. The effect of randomly distributed boulders creating a cascade morphology on the sediment transport capacity and bedload fluctuations in steep mountain channels was studied for the first time by Dr. Tamara Ghilardi with systematic laboratory experiments. A wide grain size distribution was used in the experiments for the movable bedload material present between the large boulders, as it can be found in typical alpine torrents. Boulder size and density was varied in the experiments using shapes of boulders like the ones found in nature. Special attention was also given for the first time on the evolution of the sediment transport characterized by bedload pulses. This allows a new insight on the sediment transport processes in steep channels having large immobile boulders. Finally a new sediment transport formula is suggested which takes into account the presence of such large boulders.

EPFL - LCH2014

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

Tamara Ghilardi

EPFL2013