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Chapter 6 - Analysis of the test results more important than the one without ribs. A possible explanation is that this scour behaves like a combination of the first and second scour. Probably the first scour is essentially due to the flow being reflected on the side wall in the bend (causing the flow to head towards the bottom and increasing the secondary current), and the second one is influenced by the point bar (which is itself influenced by the flow), concentrating the flow at the outer side wall in combination with the main stream. For the highest rib density, the location of the first scour is shifted in the downstream direction due to the roughness of the outer wall, but the point bar forms as usual or is even shifted in the upstream direction (see 6.2.1 c). At the point where the scour now occurs, the two phenomena are superposed and an important amplification of the scour depth. A last observation concerns the erosion after the bend: the presence of macro-roughness causes some additional erosion in the outlet reach. Since the ribs create head losses in the bend, the water is accumulated upstream the bend and accelerated after the bend to pass the same discharge (steepening of the bed slope). Fortunately, this additional erosion is of limited depth and located towards the center of the channel where it does not endanger bank protection structures. If the curve is immediately followed by a bend in the other direction, bank protection measures may be necessary in the following curve. f) Shape of the cross-sections - line bend SCHÖBERL (2002) made an interesting observation in the sinuous channel used by REINDL (1994) (see also § 3.5.2/11). In the scour holes, the cross-section presents a point where the lateral slope abruptly changes. At the inner bank the bed is quite flat and towards the outer bank, an important lateral bed slope can be observed. This abrupt change in the bed slope (see Figure 6.4) will be called in this report as “line bend”. Considering the measured cross-sections (Appendix 7), this line bend can be found, too. But it is mainly observed in the part where the scour develops than at the maximum scour location (Figure 6.9). page 124 / November 9, 2002 Wall roughness effects on flow and scouring
Analysis of the final scour 10° 25° 40° 1300 Water− and bedlevel B01 Q=210 10° 1300 Water− and bedlevel B01 Q=210 25° 1300 Water− and bedlevel B01 Q=210 40° 1200 1200 1200 1100 1100 1100 Level [mm] 1000 900 Level [mm] 1000 900 Level [mm] 1000 900 800 no macro-roughness 800 800 700 Water level Bed level Mean water level Mean bed level Initial mean bed level 700 Water level Bed level Mean water level Mean bed level Initial mean bed level 700 Water level Bed level Mean water level Mean bed level Initial mean bed level 600 6500 1300 6400 6300 6200 6100 6000 5900 5800 5700 Distance to center of bend [mm] Water− and bedlevel B02 Q=210 10° 5600 600 5500 6500 1300 6400 6300 6200 6100 6000 5900 5800 5700 Distance to center of bend [mm] Water− and bedlevel B02 Q=210 25° 5600 600 5500 6500 1300 6400 6300 6200 6100 6000 5900 5800 5700 Distance to center of bend [mm] Water− and bedlevel B02 Q=210 40° 5600 5500 1200 1200 1200 1100 1100 1100 Level [mm] 1000 900 800 700 600 6500 ribs spaced every 4° 6400 6300 6200 6100 6000 5900 5800 Distance to center of bend [mm] Level [mm] 1000 900 800 Water level Bed level 700 Mean water level Mean bed level Initial mean bed level 600 5700 5600 5500 6500 6400 6300 6200 6100 6000 5900 5800 Distance to center of bend [mm] Level [mm] 1000 900 800 Water level Bed level 700 Mean water level Mean bed level Initial mean bed level 600 5700 5600 5500 6500 6400 6300 Water level Bed level Mean water level Mean bed level Initial mean bed level 6200 6100 6000 5900 5800 Distance to center of bend [mm] Water level Bed level Mean water level Mean bed level Initial mean bed level 5700 5600 5500 Figure 6.9: Development of the line bend in the first scour (test B1d on top and B2d at the bottom) Without macro-roughness, the line bend starts at about 10° at a distance around 20% of the channel width from the outer bank to finally disappear between 50% and 80% from the outer bank. In the maximum scour cross-section, the line bend is attenuated and the profile becomes s-shaped. The same process can be observed for the second scour. At 55° the bed has flattened again partially (between the two scour holes). Downstream this point (70°), up to the end of the bend, the development of the line bend can be observed a second time. Figure 6.10: Scheme of the development of the line bend This process can be explained in a schematic way with the development of the secondary current (Figure 6.10). Between 10° and the first maximum scour, the secondary current grows (the inclination of the radial water surface slope also increases slightly which is exaggerated on the schematic view) and leads progressively to a more pronounced scour hole. At the final state, the shape of the scour hole is more s-shaped. This can be explained by the fact that the main secondary current does not “attack” the bed at the outer wall anymore, shifts towards the center, but remaining in the outer half of the channel. EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 125
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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 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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