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Sediment traps are crucial elements for flood protection in mountain rivers with high sediment transport capacity. Existing structures often interrupt the channel connectivity. Ideally, a sediment trap should be permeable for bed load during non-hazardous floods and ensure sediment retention during hazardous discharges. A new sediment trap concept, fulfilling these requirements was recently developed and tested in a laboratory flume. A guiding channel trough the deposition area is combined with a slot check dam having an upstream bar screen with bottom clearance. This study aims to validate the proposed concept with a finer sediment mixture on an experimental set-up. Furthermore, we provide improved recommendations for bar screen design regarding minimal bar spacing and the range of applicable clearance heights. Optimal bar spacing and clearance heights of the bar screen are determined through individual tests of the bar screen with steady discharges and varying sediment supply intensity. The best performing bar screen configuration is subsequently tested in combination with a slot check dam using a flood hydrograph to simulate the influence of quasi-unsteady discharge. The proposed concept corresponds to a combined mechanical-hydraulic control and works well for a large range of grain sizes, if the bar screen is correctly adapted.

Sediment detention basins are implemented on mountain rivers to trap solid material that may aggravate the flooding of downstream settlements. However, retention structures built in the past may unnecessarily retain sediment during non-hazardous flood events, resulting in high maintenance costs and sediment deficit downstream. In addition, the so-called spontaneous self-flushing of previously retained sediment during floods has occasionally been observed. Recent research suggests to design sediment detention basins for controlling sediment passage with a guiding channel across the deposition area upstream of a hybrid barrier. Such barriers consist of a check dam with a slot orifice and an upstream bar screen with a bottom clearance in order to benefit from a combined mechanical-hydraulic retention control. The present paper enhances this pioneering research with the help of new experimental data, including a wide range of sediment mixtures and large wood, as well as variable barrier heights. Improved design criteria are provided regarding the bar screen and the basin storage capacity. The functionality of the enhanced concept for sediment detention is illustrated by a case study on a physical model: the protection of the Villard-Bonnot village (France) against torrential hazards. [ABSTRACT FROM AUTHOR]

Recently, Jafarnejad, Franca, Pfister, and Schleiss [2017. Time-based failure analysis of compressed riverbank riprap. Journal of Hydraulic Research, 55(2), 224–235. doi: ] presented an experimental study on the stability of compressed riprap, which means that it is composed of individually placed blocks with a single layer as riverbank protection. The relationship between the time to failure and dimensionless bed shear stress was presented. Here, the authors complement the previous investigation with results regarding the effect of a second riprap layer. In total, 49 tests (28 tests with one and 21 tests with two riprap layers) were performed using a single block size, three longitudinal channel slopes and three different riprap bank inclinations. Under similar conditions, the second layer can significantly delay the time to failure, whereas the block erosion rate was found to increase. The second layer has a more stabilizing role when the bank angle of the riprap is approaching the angle of repose of the blocks. [ABSTRACT FROM AUTHOR]

Methods to design riprap-lined channels usually refer to dumped material. Large blocks placed individually by machinery are used when more stability is required. They offer additional resistance against flow erosion since space between blocks is minimized and interlocking increased. The behaviour of this protection has rarely been studied. An experimental investigation was carried out on the stability of compressed riprap as riverbank protection. Riprap was reproduced by uniform crushed limestones with three block sizes. Tests were conducted for three channel slopes under supercritical flow conditions and for constant bank slope. A time-based analysis allowed establishing relations among time to failure, friction velocity, and dimensionless bed shear stress. The results of 45 tests confirm that the rate of block erosion is significantly reduced with increase in the riprap diameter. The time to failure of the riprap protection depends strongly on the longitudinal slope and on the block sizes. An empirical prediction to estimate the riprap time to failure is shown. [ABSTRACT FROM PUBLISHER]