"Proceedings, Federal Interagency Workshop,
"Sediment Technology for the 21'st Century,"
St. Petersburg, FL, February 17-19, 1998"

Bed-Image Velocimetry: a Technique for Measuring Bedload Transport in Large Alluvial Rivers

By Marcelo García

Pre-Proposal

Issue/Problem

It is well known that in large alluvial streams, conventional means for measuring bedload transport do not work well. However, the amount of sediment being transported as bedload can be estimated from the mean velocity and mean height of the bedforms moving along the bottom. While the mean height of dunes can be measured with relative ease in both the field and the laboratory, the same cannot be said about the velocity of dunes, particularly in large alluvial streams where three-dimensional effects are important. Swath mapping can provide enough detailed information to assess bedload transport rates over large segments of a river bed. To accomplish this, however, it is necessary to develop an algorithm that can translate bathymetric information into sediment transport rates along the bottom of a given stream. Because of its nature, such algorithm can be named Bed-Image Velocimetry and its development provides the motivation for the research proposed herein.

Beneficiaries

The maintenance of large alluvial rivers, such as the Mississippi and the Atchafalaya, to facilitate navigation, requires substantial amounts of dredging every year. The backfilling of dredged regions takes place mainly by a gravity effect on bedload transport and partly by deposition of suspended load due to decreasing transport capacity. Thus, the possibility of estimating bedload transport rates accurately, would have a direct impact on the our ability to assess backfilling rates as well as the frequency and magnitute of dredging operations.

In addition, deltaic areas in California which are degrading quite rapidly because of lack of sediment input, could benefit greatly if the amount of sediment transported as bedload by rivers that lead to them, could be estimated. Such a source of sediment could be tapped and the material placed in areas where it is needed the most.

Objective

The main goal of this research is to develop a technique, Bed-Image Velocimetry, for predicting the velocity of dunes in large alluvial rivers, and the bedload transport rates associated with them. The final product will be software capable of converting bathymetric information taken at different times into bedload transport rates.

Approach

Bed-image velocimetry requires the use of two acoustic images of the river bottom, taken at different times. Swath maps of a given river bed taken one day apart, will be required. It would be desirable to have maps corresponding to different flow conditions, so that a wide range of bedform characteristics can be observed. Flow discharge, flow depth, sediment size characteristics, and water temperature, corresponding to each swath mapping will also be needed. It would be of interest to consider the possibility of measuring vertical velocity profiles with an ADCP, simultaneously with the bathymetric measurements. The strength of the Bed-Image Velocimetry technique will be tested first with laboratory data.

Funding for Fiscal Year 1999-2000

This project can be completed in 2 years of work. The cost of the research is estimated to be around $50,000 per year.

Personnel

The principal investigator will be Dr. Marcelo Garcia. The research assistant will Mr. Juan Fedele, who has extensive experience with the dynamics of dunes and is currently finishing his Master's research on alluvial roughness, under the supervision of the P.I.

References

Amsler, M.L., and Garcia, M.H. (1997) Discussion on "Sand-Dune Geometry of Large Rivers During Floods," by Julien and Klaasen, Journal of Hydraulic Engineering, ASCE, vol.

Fedele, J.J. (1995). "Dune Velocity in Sand Bed Rivers", IAHR Congress, London.


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