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Chapter 6 - Analysis of the test results 6.4 Analysis of the sediment transport Lets consider the evolution of the sediment transport rates measured at the outlet of the channel during the test (Figure 6.17). The plots for the lowest discharge (on top), show that the sediment transport rate at the outlet decreases approximately with an exponential function 1 . Without ribs, the transport rate stabilizes around an average value after 6 to 8 hours. With ribs, the transport rate stabilizes faster, but on a lower mean value. This indicates, that the transport capacity is significantly reduced by the presence of the macro-roughness (to 35 to 50% for high discharges). Q=180 l/s Q=210 l/s 8000 7000 6000 Q b [g/min] Q=150 l/s 8000 7000 6000 5000 4000 3000 2000 1000 Q b [g/min] 8000 7000 6000 5000 4000 3000 2000 y = 4009e -0.0114x R 2 = 0.8789 y = 2232e -0.0072x R 2 = 0.905 y = 5154.2e -0.0039x R 2 = 0.6137 1000 y = 1641.5e -0.006x R 2 = 0.7306 0 0 60 120 180 240 300 360 420 480 540 Time [min] 0 0 60 120 180 240 300 360 420 480 540 Time [min] B1c B2c B3c B4c B1b B2b B3b B4b Expon. (B1b) Expon. (B2b) Expon. (B4b) Expon. (B3b) Q b [g/min] 5000 4000 3000 B1d B2d B3d B4d 2000 1000 0 0 60 120 180 240 300 360 420 480 Time [min] Figure 6.17: Evolution of the sediment transport rate Q b at S 0 = 0.5%, for 3 measured discharges, without macro-roughness (B1), and with ribs spaced every 4, 2 and 1° (B2 to B4). 1. This was at least the best fit obtained with a commonly used function. page 132 / November 9, 2002 Wall roughness effects on flow and scouring
Analysis of the sediment transport The higher discharges (Figure 6.17 center and bottom) show an almost constant transport rate (a slightly decreasing tendency can be figured out for the case without macro-roughness). This is mainly due to the circumstance that the tests were performed as a series with increasing discharge. The bed was only levelled after the first test (Q = 150 l/s). The next tests continued with the bed topography of the previous test. Since the discharges were high enough to provoke bed changes this was no problem, but as far as the sediment transport is concerned, the bed was already armored; therefore the sediments introduced at the inlet were transported over the armoring layer to the outlet. In the presence of macro-roughness, a natural river, subject to a constant sediment transport rate will compensate the reduced transport capacity in the bend by steepening the slope in the bend: Upstream the bend, the water depth increases locally. Therefore the transport capacity is reduced and sediments accumulate in the inlet reach. The overall channel slope increases, till it is sufficient to pass the bedload across the bend. The increased slope leads to higher velocities in the exit reach, inducing some additional erosion after the curve. This phenomenon was observed during the preliminary tests, for which the sediment transport rate was kept constant (Figure 6.18 and Appendix 3.3.3). For a discharge of 170 l/s respectively 210 l/s the overall bed slope increased by about 40%, respectively 50% from the initial value (0.50%). Difference to initial bed level [mm] 100 0 -100 -200 -300 -400 Comparison of the bed levels with and without macro-roughness - Q = 212.5 l/s Q=212.5 l/s without mr, avg Q=212.5 l/s with mr, avg Q=212.5 l/s, Difference, avg Bend -500 -100 0 5 10 15 20 25 Distance to the inlet [m] Figure 6.18: Changes of the average bed level for the preliminary tests (with and without ribs) at a constant sediment transport rate 500 400 300 200 100 0 Difference between without / with mr [mm] EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 133
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WALL ROUGHNESS EFFECTS ON FLOW AND
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Abstract By applying vertical ribs
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Résumé En disposant des nervures
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Zusammenfassung • Ein markanter S
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Acknowledgments page x / November 9
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Table of contents 4. Smart & Jäggi
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Table of contents 7.6 Summary and c
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Chapter 1 - Introduction 1.1 Contex
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Chapter 1 - Introduction page 4 / N
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Chapter 2 - State of the art 2.1 Fl
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Chapter 2 - State of the art 2.2.2
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Chapter 2 - State of the art Theref
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Chapter 2 - State of the art resolu
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Chapter 2 - State of the art comple
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Chapter 2 - State of the art ORLAND
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Chapter 2 - State of the art 2.6 Me
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Chapter 2 - State of the art page 2
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Chapter 3 - Theoretical considerati
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Chapter 8 - Summary, conclusions an
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Notations K S [m 1/3 /s] roughness
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Notations bed b bf c cr d e g w o o
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References de Vriend H.J. (1981). S
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References Meyer-Peter, E., Favre,
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References Williams, R. (1899). Flu
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References by chapter Keulegan (193
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References by chapter page 214 / No
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Index of Authors Hoffmanns 13 Hoffm
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Index of Authors page 218 / Novembe
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Index of Figures Fig.4.19: Frame po
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Index of Figures 22] 183 Fig.7.15:
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Index of Tables Table 7.3: Table of
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Index of Keywords dimensional analy
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Index of Keywords sediment sampling
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Index of Keywords parameter 77 Pete
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Publications (continued) 1997 Hersb
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