"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.
Workshop Contributions
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