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Chapter 6 - Analysis of the test results Flow direction Figure 6.13: Flow separation behind the point bars (LEFT: at 45°, test C4d, RIGHT: at 90°, test C1d) Figure 6.14: Vertical view on the water surface waves; ribs-spacing 4°, test C2c page 128 / November 9, 2002 Wall roughness effects on flow and scouring
6.3 Analysis of the evolution of the scour Analysis of the evolution of the scour As mentioned in the discussion of the preliminary tests (§ 5.2.1 2) a), the erosion first starts almost at the end of the bend at a discharge of about 120 l/s (for a bed slope of 0.5%); then it moves quickly in the upstream direction with increasing discharge to stabilize or rather oscillate around its final position at about 30° to 35°. Considering the evolution of the scour on the outer side wall without ribs (Figure 6.15), it can be seen that the first scour hole armors quickly and gets stable. But the second one needs much more time. The scour deepens and is filled again, until the scour finally stabilizes at the end of the test. This is probably the moment when the armoring layer is well formed in the second scour, too. The phenomenon is the same, independent from the discharge. Analyzing the same cases but with macro-roughness (Figure 6.16) reveals that the first scour takes some more time to be formed, but without oscillations. The second scour shows some oscillations, but with a significantly reduced amplitude (reduction of about 50%). The study of these plots (Figures 6.15 and 6.16) rises a question: Is the final scour the maximum scour ? In order to answer this question, we have to remind ourselves that the maximum scour is in general not located directly at the outer side wall but at about 10 to 20 cm from the wall. Some sediments remain just next to the outer bank. The height of these sediments was frequently twice the maximum grain size diameter d max . During the development of the scour, this depositions became sometimes like a steep outer bank that collapsed from time to time. This can in part explain the oscillations along the outer side wall. The amplitude of the oscillations in the second scour is up to 3 times d max (or 50% of h m ) for the tests without macro-roughness and up to 1 times d max (or 15% of h m ) with ribs. With vertical ribs the oscillations can only be observed for the lowest discharge (Q = 150 l/s). At higher discharges, the oscillations seem to disappear. Turning back to our question, we see that the given final scour depth may be about one maximum grain size diameters too small for the measurements without macro-roughness, but this is a part of the uncertainty: if only one big sediment grain moves, the scour will locally increase by this order of magnitude. For the tests with macro-roughness, it can be assumed that the final scour depth corresponds to the maximum scour, since the deepest scour is not found at the outer bank, and since the oscillations (for the smallest discharge only!) are less that the maximum grain size diameter; for higher discharges they were too small to be measured (if there were any). The other rib-spacings (4° and 1°) that are not given on Figures 6.15 and 6.16 behave like the mentioned case (2°) with macro-roughness. In conclusion, it is evident that the macro-roughness contributes to a smoother development of the scour holes, reducing considerably (for small discharges) and even suppressing (for higher discharges) the oscillations of the scour depth, especially in the second scour hole, where the phenomenon is more pronounced. EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 129
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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 4 - Experimental setup and
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Chapter 7 - Establishing an empiric
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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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