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Chapter 8 - Summary, conclusions and recommendations 8.2 Outlook and further research One of the main problems with which researchers are confronted when developing numerical models, is their validation. Only a small number of physical tests are available for that kind of work, especially for 3D problems like flow and sediment transport in river bends. The present study furnishes a very precious data set including the free water surface, the bed topography, the average 3D velocity field as well as grain size distributions of the armoring layer and sediment transport rates. About 1500 datapoints were measured for each test. Such detailed data is needed for the verification and testing of numerical 3D-models. Further treatment of the data, especially concerning the measured velocities, could be done. Due to the large amount of records, the velocities were only analyzed in eight selected cross-sections over the bend for about 10% of the available test configurations. But the velocities were recorded in 3 directions on a 9 x 9 cm grid over the whole channel with <strong>12</strong>8 data points over the measurement depth. By treating and analyzing this available raw data, additional insight into the scour process and the effect of macro-roughness on erosion in bends can be obtained. All the tests with macro-roughness were performed with ribs applied to vertical outer walls. The influence of the shape, as well as the type of macro-roughness were not examined in this research study. Therefore, other rib shapes and types of roughness e.g. rock-dumps, need further testing. It would be interesting to study the influence of bank protections realized by a rip-rap, designed for natural rivers. At this stage, the influence of inclined outer walls is not well known. The action of macro-roughness on the scour process, for example applied in trapezoidal cross sections could be further investigated. With additional research, these gaps can be closed in order to provide a better understanding of the flow in bends in general and especially of the scour process in natural rivers with banks equipped with macro-roughness. page 198 / November 9, 2002 Wall roughness effects on flow and scouring
Recommendations for practical engineers 8.3 Recommendations for practical engineers a) Advantages and inconveniences of macro-roughness on outer walls Vertical ribs placed on the outer side wall with an optimum spacing have the following advantages: • The maximum scour depth is significantly reduced (20 to 40% for an optimum rib-spacing). • Prominent scour holes almost disappear. • Important oscillations of the bed level during development of the scour are considerably reduced or even suppressed for high discharges. • The amplitude of the stationary surface waves is diminished by half. But at the same time, the mean water level increases, resulting in about the same maximum water levels in the curve as without ribs. • High main velocities are shifted from the outer wall to a distance of about an average water depth from the outer bank. • The armoring layer zone next to the outer wall due to grain sorting is doubled, now covering about half of the channel. But the macro-roughness on the outer wall has also some negative effects, such as: • A significantly reduced transport capacity in the bend. This reduction is compensated in natural rivers by a steepening of the bed slope obtained by depositions upstream of the curve (see also preliminary tests). • Some additional erosion can be found after the bend in the center of the channel. But with the possible exception of bridge piers, no civil engineering structures are endangered by this limited local erosion. b) Estimation of the maximum scour depth, transversal bed slope and scour locations If existing scour formulae are applied to mountain rivers, attention needs to be paid to the fact that most of them underestimate the maximum scour depth by a factor of up to 2.4. In order to adjust the maximum scour obtained using existing equations, the result can be multiplied with a correction factor given in Table 3.7 on page 64 (in general R 2 < 0.8 , except for Peter’s formula with a higher correlation). Without macro-roughness, equation 7.7 can be used to compute the maximum scour depth and the transversal bed slope. This formula combines both, a good prediction of the maximum scour depth and a good fit with the cross-section at the maximum scour locations. It is difficult to compute the reduction of the scour depth directly. Therefore formula 7.63 is proposed to compute the scour depth in the presence of vertical ribs. The location of the two scour holes can also be predicted by two relations (equations 7.54 and 7.57). The prediction of the first scour gives quite good results ( R 2 = 0.83 ), but the forecast of the second one is more difficult ( R 2 = 0.60 ). During the development of the scour holes, important bed fluctuations were observed for the tests without macro-roughness. They can reach amplitudes of up to 50% of the mean water depth <strong>EPFL</strong> Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 199
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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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Page 216 and 217: Notations K S [m 1/3 /s] roughness
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Page 220 and 221: References de Vriend H.J. (1981). S
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Page 224 and 225: References Williams, R. (1899). Flu
Page 226 and 227: References by chapter Keulegan (193
Page 228 and 229: References by chapter page 214 / No
Page 230 and 231: Index of Authors Hoffmanns 13 Hoffm
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Page 240 and 241: Index of Keywords dimensional analy
Page 242 and 243: Index of Keywords sediment sampling
Page 244 and 245: Index of Keywords parameter 77 Pete
Page 246: Publications (continued) 1997 Hersb
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