Sediment traps with guiding channel and hybrid check dams improve controlled sediment retention

Sediment traps with partially open check dams are crucial elements for flood protection in alpine regions. The trapping of sediment is necessary when intense sediment transport occurs during floods that may endanger urban areas at downstream river reaches. In turn, the unwanted permanent trapping of sediment during small, non-hazardous floods can result in the ecological and morphological degradation of downstream reaches. This study experimentally analyses a novel concept for permeable sediment traps. For ensuring the sediment transfer up to small floods, a guiding channel implemented in the deposition area of a sediment trap was systematically studied. The bankfull discharge of the guiding channel corresponds to a dominant morphological discharge. At the downstream end of the guiding channel, a permeable barrier (check dam) triggers sediment retention and deposition. The permeable barrier consists of a bar screen for mechanical deposition control, superposed to a flow constriction for the hydraulic control. The barrier obstructs hazardous sediment transport for discharges that are higher than the bankfull discharge of the guiding channel without the risk of unwanted sediment flushing (massive selfcleaning).

Hydro-morphological processes through permeable sediment traps at mountain rivers

Sebastian Schwindt

Sediment traps are used for the protection of urban settlements at rivers in mountainous regions. These structures aim at the retention of sediment in the case of hazardous floods, but existing sediment traps tend to retain sediment also when the discharge is not hazardous to the downstream urban regions. This excessive retention of sediment causes an interruption of the river continuum that may lead to channel incision and the morphological depletion of downstream reaches. Another problem is the remobilization of formerly deposited sediment during a flood, which is addressed here in terms of the unwanted flushing of sediment traps. This research project aims at the development of sediment traps which are permeable up to a certain flood, but not susceptible to unwanted sediment flushing. Typical sediment traps consist of a retention area upstream of a barrier or check dam equipped with openings. The barrier can trigger the retention of sediment in the deposition area either by hydraulic control or by mechanical control. The hydraulic control leading to deposition is achieved by check dams with one or more openings constricting the flow vertically and/or laterally. Improved formulae for the estimation of the discharge capacity of such constrictions have been experimentally obtained for rough, turbulent upstream flow conditions with bed load under varying channel slopes. The constriction-induced head loss and reduction in the bed load transport capacity based on the bed shear stress are analyzed as a function of the upstream flow depth and discharge. The experiments show that the flushing of upstream sediment deposits may occur at open-crested slit check dams or close-crested slot check dams, but only when the latter are overtopped. The mechanical control leading to sediment retention is achieved by screens with vertical bars. The horizontal space between the bars corresponds to the characteristic grain size of traveling bed load. The required bottom clearance under such screens was optimized here in view of the possibility of bed load transfer for small (flood) discharges on the one hand, and the ensured clogging of the screen for high (flood) discharges on the other hand. This optimum bottom clearance height was found to be 1.75 times the characteristic grain size that is transported during floods. Once the bar screen was clogged, the unwanted sediment flushing could not occur anymore. However, the clogging depends on the estimation of the characteristic grain size. The experimental study shows that the combination of mechanical and hydraulic control structures provides a reliably working solution for permeable sediment traps. Smaller bed load-laden discharges can pass unhindered through such combined barriers. For higher discharges, the hydraulic control causes backwater which reduces the influence of the characteristic grain size on the clogging of the bar screen. Moreover, the bar screen prevents unwanted sediment flushing through the hydraulic control. The implementation of a guiding channel across the retention area is introduced and was experimentally verified as being a pertinent structural tool for improving the eco-morphological flow continuum. Finally, the design of a permeable sediment trap is described based on an optimal interaction between a guiding channel and a barrier combining the mechanical control by a bar screen and hydraulic control by a slot check dam.

EPFL2017

Design optimization of permeable sediment traps for fluvial bed load transport

Mona Jafarnejad Chaghooshi, Anita Hélène Roth, Anton Schleiss, Sebastian Schwindt

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.

2018

Bottom slope influence on flow and bedload transfer through contractions

Mário Jorge Rodrigues Pereira Da Franca, Anton Schleiss, Sebastian Schwindt

In-stream open check dams are essential for flood protection at mountain rivers that are characterized by steep slopes (>1%) and sediment-laden flood discharges. These structures comprise an opening acting as a lateral or vertical flow contraction to force sediment retention when floods occur. Otherwise, the contraction should not affect the run-off. Design criteria for the discharge capacity referring to the size and geometry of the opening were previously established. This experimental study reviews the existing formulae for the discharge capacity and analyses the beginning of sediment deposition, with varying channel slopes. The effects of the channel slope on backwater, sediment deposition and local head losses are relevant when free surface flow conditions persist in the opening. A channel slope-sensitive correction factor is introduced for calculating the discharge capacity. The sediment transfer rate through the contraction decreases with increasing backwater and is most sensitive for trans-critical flow conditions. The findings are validated against a case study in the Swiss Alps.

2019

Hydro-morphological processes through permeable sediment traps at mountain rivers

Sebastian Schwindt

EPFL2017