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You are here : eLibrary : IAHR World Congress Proceedings : 35th IAHR Congress - Chengdu (2013) : THEME 2 - JOHN F. KENNEDY STUDENT PAPER COMPETITION : Experimental Study on Bedload Pulses in a Steep Flume with Boulders
Experimental Study on Bedload Pulses in a Steep Flume with Boulders
Author : Tamara Ghilardi
In recent years, it has been observed that bedload transport fluctuates over time in steep rivers, with wide grain size distributions. This phenomenon is perceived even under constant sediment feed and water discharge. Bedload pulses are periodical and are a consequence of grain sorting. Furthermore, along with bedload transport, other parameters fluctuate in time as flow velocity and bed morphology. The influence of macro-roughness elements on these fluctuations has not been studied yet, even though it is widely accepted that the presence of large relatively immobile boulders, such as erratic blocks often present in mountain streams, has a strong impact on flow conditions and sediment transport. In the present work, the influence of the presence of these boulders on sediment transport and flow conditions is investigated with a set of laboratory experiments carried out in a 8 m long, 0.25 m wide tilting flume (analyzed slopes S range between 6.7% and 13.1%). Constant sediment and water discharge were supplied into the flume at the upstream section, for tests duration reaching 250 minutes. Experiments included the use of various boulder diameters (D=0.075-0.125 m) and dimensionless distances between them (?\D=2-5, where ? is the average distance between boulders). Sediment transport, protrusion of several and bulk mean flow velocities are measured regularly during experiments. Bedload pulses or fluctuations, well-behaved periodically, are clearly visible in the measurement of sediment transport at the downstream section. Fluctuations in boulder protrusion and average velocity are also detected. Auto-correlation analysis shows that the duration of the cycles is similar for all the mentioned variables. A detailed analysis of period and amplitude of bedload transport is carried for 16 experiments with similar sediment supply. For high flow conditions, the amplitude of the bedload pulses decreases. The period and amplitude of these fluctuations is clearly related to the stream power and to the presence of boulders, considering the number of boulders and their protrusion. This article shows that for higher stream power 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 fluctuations.
File Size : 258,877 bytes
File Type : Adobe Acrobat Document
Chapter : IAHR World Congress Proceedings
Category : 35th IAHR Congress - Chengdu (2013)
Date Published : 18/07/2016
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