velocity was computed to be as large as 2.5 m/s at an approximate water depth of 1 m in the final ..... 2.5 m/s; h = water depth = 1.0-1.5 m; kâ = bed roughness = 0.001 m; ..... 0.55 m for tidal peak velocities increasing from 1.3 to 1.8 m/s, and de-.
SEDIMENT TRANSPORT OF F I N E SANDS AT H I G H VELOCITIES By Leo Voogt, 1 Leo C . van Rijn, 2 a n d J a n H . van den Berg 3 ABSTRACT: The predictive capability of the sediment-transport formulas of Ackers-White, Engelund-Hansen, and van Rijn at high flow velocities is investigated. Total-load transport at high velocities over a fine-sand bed (100-400 (xm) is studied by analyzing data from the literature and data from new flume and field measurements. Flume experiments were performed to measure flow-velocity and sandconcentration profiles in a self-eroding flow initially free of sediment. The flow velocities were up to 2.7 m/s. A two-dimensional vertical mathematical model for suspended sediment was applied to compute the equilibrium transport rates. Finally, predicted transport rates are compared with measured transport rates under flume and field conditions. The formulas of Engelund-Hansen and van Rijn predict transport rates that are in reasonable agreement with the measurements. INTRODUCTION
Two large tidal channels in the Eastern Scheldt estuary (in the southwestern part of The Netherlands) were closed in the autumn of 1986 and in the spring of 1987. The closure method consisted of pumping fine sand (100400 (j,m), which was dredged elsewhere in the estuary, through pipelines in the closure gap of the channel. A successful closure requires that the sanddelivery capacity be larger than the eroding capacity of the (tidal) flow in the channel, especially in the final closure stage. Mathematical and physical flow models were used to predict flow velocities and water levels in the two closure gaps, after which the eroding capacity of the flow was predicted with various sediment-transport formulas. Because the maximum depth-averaged velocity was computed to be as large as 2.5 m / s at an approximate water depth of 1 m in the final closure stage, the question was raised whether the applied sand-transport predictors could be used in such extreme circumstances. Therefore, a literature study was carried out to find field data of sedimenttransport rates at high velocities for verification of the applied formulas. The outcome of this study was that very few data were available with velocities higher than 2 m / s . It was therefore decided to perform supplementary flume experiments in a large flume at Delft Hydraulics, Emmeloord, The Netherlands. A two-dimensional vertical mathematical model for suspended-sediment transport was applied to analyze the results of the flume experiments and to compute the equilibrium transport rates. Two extensive field surveys were carried out one year later during the construction of the sand-closure dams to further verify the applied sedimenttransport formulas. 'Proj. Engr., Ministry of Transp. and Public Works, Van Leeuwenhoekweg 20, 3316 AV Dordrecht, The Netherlands. 2 Sr. Engr., Delft Hydr., P.O. Box 152, Emmeloord, The Netherlands. 3 Researcher, Univ. of Utrecht, P.O. Box 80.115, Utrecht, The Netherlands. Note. Discussion open until December 1, 1991. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on June 27, 1988. This paper is part of the Journal of Hydraulic Engineering, Vol. 117, No. 7, July, 1991. ©ASCE, ISSN 0733-9429/91/0007-O869/$l.O0 + $.15 per page. Paper No. 25945. 869
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This paper presents the analysis of data from literature on sediment transport at nigh velocities over a fine-sand bed (100-400 (xm) and of data from new flume experiments and two new field surveys. A detailed description of the flume experiments is given. Finally, a comparison between measured and predicted transport rates for these four data sets is presented. AVAILABLE DATA
First, field data available in literature (Peterson and Howells 1973) were analyzed. They are from large North American rivers with water depths between 1 m and 25 m, grain diameter of bed material between 100 and 300 u,m, and water temperature between 5° C and 25° C. The total data set consists of 119 measurements, of which 38 are in the velocity range between 1.5 and 2.5 m/s. This latter subset (hereafter called data set 1) was used to compute the sand-transport rate with the formulas of Engelund and Hansen (1967), Ackers and White (1973), and van Rijn (1984b) and then compared with the measured transport rates. These formulas were chosen because they yielded good results in a study by van Rijn (1984b). Fig. 1 shows the ratio of predicted and measured transport rates as a function of depth-averaged velocity for data set 1. The Ackers-White formula predicts values that are significantly too large. About 50% of the predicted transport rates are within a factor of two of the measured transport rates. The Engelund-Hansen and the van Rijn methods have scores of 80% and 90% within a factor of two of the measured transport rates. FLUME EXPERIMENTS
nspor"t rate
Approach Since only relatively few data were available in the literature (with only 6 data in the velocity range of 2.0-2.5 m/s; see Fig. 1), a flume study was
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To minimize the generation of a large scour hole downstream of the fixed concrete bottom, the sand bed was given an overheight (gradually decreasing in longitudinal direction), see Fig. 4. The sand bed surface in each test was laid under an initial slope equal to the expected water surface slope of that test. After the preparation of the sand bed, the flume was filled with water and the bed-level surface was recorded using an echo sounder. Each test consisted of an adjustment period during which equilibrium flow conditions were established in the inflow section, and a measuring period during which the flow velocity and sand concentration profiles were determined. The test period was kept as short as possible to prevent the generation of a large scour hole downstream of the fixed bed, which would result in varying boundary conditions. After stopping the flow, bed-material samples were taken along the flume (grab sampler) and bed levels were recorded (echo soundings). In all, nine tests were executed. The flume data are given in Table 1. Measuring Equipment The water-surface level in the inflow section was measured with a staff gage (reading accuracy = 0.02 m). The water depths in the test section were determined by sounding the bed surface and the water surface with a vertically adjustable staff gage attached to the measuring carriage during the test. The accuracy of this latter method is about 0.02 m. Therefore, accurate values of the water surface slope could not be obtained. Flow velocities were measured in sections 32, 45, 60, 75, and 90 (see Fig. 3). In section 32, an Ott-type propeller meter was used in the centerline of the flume at a height of 0.4 m above the fixed bed. The measuring period was 60 s. In the sections 45 and 60, an acoustic Doppler instrument attached to a measuring carriage wa§ used. In the vertical direction, seven or eight measurements were carried out successively, starting near the bed. The vertical positioning of the measuring instrument relative to the movable bed surface was controlled by a mechanical sensor in contact with the bed. The measuring period varied from 60 s (test Tl) to 16 s (test T7). In sections 75 and 90, an acoustic Doppler instrument was used to measure the velocities in the near-bed region (3 points), while a set of four Ott-type propeller meters was used to simultaneously measure the velocities in the upper region. The vertical positioning was controlled by a mechanical sensor similar to that used in the sections 45 and 60. The measuring period varied from 873
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Downloaded 08 Jul 2009 to 131.211.65.150. Redistribution subject to ASCE license or copyright; see http://pubs.asce.org/copyrig
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