Wall roughness effects on flow and scouring in curved channels with gravel bed

Wall roughness effects on flow and scouring in curved channels with gravel bed In the narrow valleys in Alpine regions, rivers frequently flow across constructed zones, passing through villages and cities. Due to limited space, the protection from high floods often needs to be ensured by protection walls. During floods, these protection walls may be endangered by scour phenomena, especially if they are located in bends. In the past, the potential danger of underscoured structures was reduced by sufficient foundation depth. By providing roughness elements such as vertical ribs at the surface of walls located at the outer side of river bends, the local erosion along the foot of the wall can be considerably decreased, reducing the cost of wall foundations. Such observations were made during the review and optimization of several flood protection projects with hydraulic model tests in Switzerland. A literature review showed, that no systematic study of the influence of these ribs on scour and flow in bends was performed up to this day (§ 2). This research project covers this gap by investigating the development of the scour as a function of main parameters. The study is based on an experimental investigation in a 90° bend with a radius of 6 m including measurements of the velocities, the water level, the bed topography, the sediment characteristics, the grain size distribution of the armoring layer, the discharge and the bed load (§ 4 and 5). The large set of tests covers a wide range of discharges, bed slopes, rather high Froude numbers, but in subcritical regimes, many rib spacings and depths. The formation of two scour holes was observed. Without macro roughness, a first scour hole occurs in the prolongation of the inner sidewall of the entry reach at the outer wall and a second scour hole appears at the end of the 90° bend. The analysis of the performed tests results in the following conclusions: Most existing scour formulae considerably underestimate the scour depth in mountain rivers with coarse gravel bed (§ 3). Significant oscillations, both of the free water surface (stationary waves) and of the scour depth were observed, especially for the second scour hole. The two scour holes have different reasons. The first one is essentially due to the change of the main flow direction (impact on the wall) and the induced secondary current, whereas the second one is mainly due to increased velocity fluctuations after the point bar formed at the inner side of the channel (§ 6). In the average flow field in a bend, the highest main velocities are shifted from the centerline close to the surface toward the outer wall and then towards the bottom (§ 6). At the first scour, the highest main velocities are found next to the ground. A significant grain sorting process is observed over the cross-section due to the use of coarse gravel mixture, resulting in the accumulation of coarse sediments at the outer wall and depositions of fine material on the inner bank. By applying vertical ribs on the outer sidewall, an important impact on the scour process and on the flow field can be observed (§ 6). The macro-roughness has the following effects: The scour depth along the outer sidewall is significantly reduced and the prominent scours almost disappear. With increasing wall roughness, the first scour hole shifts in the downstream direction whereas the second one remains at about the same position. Significant oscillations of the water surface and the scour depth, observed without macroroughness, are reduced by about 50% and the scour develops in a "smoother" way. The flow field undergoes a pronounced modification: the highest velocities are kept away from the outer sidewall, reducing the scour at the bottom of the outer wall foundations. Along the outer wall at the free surface, an secondary flow cell at the outer bank can be observed. The importance of this cell shows an important correlation with the bank protection effect. Optimum rib spacing is essential since an inappropriate spacing may lead to an important increase in scour depth. The transport capacity in the bend is reduced. In natural rivers, this phenomenon is compensated by a steepening of the bed slope. The upstream and downstream extremities of the bend are influenced by the ribs: upstream of the bend, the water depth increases due to the head losses in the bend, and downstream, some additional erosion is found in the center of the channel. The grain sorting process is not significantly influenced by the presence of the ribs beside an increase of the area of the coarse sediment zone. This report furthermore presents a new empirical scour formula for the estimation of the maximum scour depth for mountain rivers with wide grain size distribution (§ 7), established with physically based parameters, which are the ratio mean water depth to channel width, a dimensionless ratio combining the mean velocity with the hydraulic radius and finally the friction angle of the bed material. The lateral bed profile in the maximum scour cross section can also be computed with this equation. An estimation of the maximum scour depth in the presence of macro-roughness on the outer wall can be obtained with a formula depending on the rib spacing, the hydraulic radius, the Froude number and the difference between the dimensionless shear stress and the critical Shields parameter (§ 7). Finally recommendations for hydraulic engineers (§ 8.3) are given to facilitate the application of vertical ribs on outer banks serving as macro-roughness.

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