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

Image-Based Instrumentation for Monitoring Processes in Flows with Alluvial Sand

By M. Muste and A. Kruger


Incomplete theoretical knowledge of the coupling between liquid and particle phases in sediment-laden flows precludes a reliable prediction and analysis of processes in flows with alluvial sand. Nearly all existing knowledge of these transport processes, both at fundamental level as well as with respect to global flow characteristics (e.g., velocity and sediment concentration distributions, turbulence characteristics), has been provided by measurements. Modeling attempts and analytical approaches also rely heavily on empirical correlations. Despite the importance of measurements, their effectiveness was limited by the ability to obtain reliable data.

The main limitation of the available measurement instruments for sediment-laden flows is their intrusive nature. In addition, the existing instrumentation does not differentiate between liquid and sediment; practically, it is not known what flow fraction the measurements are made on, i.e., water or sediment. The recent implementation of the Acoustic Doppler Velocimetry (ADV) for point velocity measurements in laboratory and field conditions represented a visible progress in the area of sediment-related technologies. Despite their improved accuracy and nonintrusive nature, ADV measurements do not differentiate the flow fractions in the mixture of water with sediment.

The advent of image-analysis techniques showed great promise in overcoming most of these deficiencies. They have emerged as the best flow diagnostics tools, providing whole-field (multipoint) quantitative information in clear-water and sediment-laden flows alike. Extended research efforts are conducted at Iowa Institute of Hydraulic Research (IIHR) to develop such instrumentation. The following presentation is aimed to familiarize the potential user with two techniques from this category, namely, Particle Tracking Velocimetry and Large-Scale Particle Image Velocimetry.

The Particle Tracking Velocimetry (PTV) system presented herein is aimed at providing full diagnostic on the mean and turbulence characteristics in flows with natural suspended sediment. The PTV system allows measurement of three important properties of sediment-laden flows, namely, liquid and sediment velocities, sediment size and concentrations. Velocity information is measured simultaneously but separately for the two flow fractions. The PTV system comprises a laser light sheet generator, video-based recording equipment, and specialized software. The instrument has been successfully used at IIHR for velocity measurements in laboratory conditions. Figure 1 shows an instantaneous vector field obtained using the PTV system in a submerged water jet loaded with alluvial sand. Currently, a new submerged design for the illumination and recording units of the PTV system is under consideration to further improve the capabilities of the technique for measurements in flows with high concentrations of suspended sediment. Its submerged feature also facilitates measurements near the free surface and near the bed. New software will be developed to include sediment sizing and concentration measurements. The added features have already been tested at IIHR in separate equipment settings and were found to work successfully. The new configuration for the PTV system will combine these features in one setting, creating a powerful nonintrusive tool for field and laboratory measurements in complex flow environment where there are no alternative measurement techniques.

Large-Scale Particle Image Velocimetry (LSPIV) is an extension of Particle Image Velocimetry (PIV) that aims at providing velocity fields spanning large flow areas in laboratory or field conditions. Additional data, such as mappings of large-scale flow structures and discharges are readily obtainable using LSPIV. The configuration of the LSPIV system presented herein included a video camera and a package of specialized software. While the image- and data-processing algorithms are similar to conventional PIV, adjustments are required for illumination, seeding procedures, and pre-processing of the recorded images. Implementation of video-based LSPIV in a practical situation, namely free-surface velocity measurements in a natural stream, is presented. Figure 2 shows the free-surface vector field obtained upstream of USGS Gage 05454220 on Clear Creek stream, near Oxford, IA. Special problems encountered, as well as the selection and adjustments of the parameters to properly solve them, are briefly examined. LSPIV has proven to be a reliable, flexible, and economically efficient flow diagnostic tool that can be employed successfully in surveillance planning, design, hazard warning, operation, and management of water-related activities. The new technique may shed some light on critically important sediment-transport related processes, such as streambank erosion, interaction between the main channel and overbank flows during floods, the impact of floodplain flows on riparian vegetation and habitat, and the role of river structures in stream restoration and preservation of the river ecosystem.

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