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Field Investigation on Hydroabrasion in High-Speed Sediment-Laden Flows at Sediment Bypass Tunnels
(2020)
Wear due to sediment particles in fluid flows, also termed hydroabrasion’ or simply ‘abrasion’, is an omnipresent issue at hydraulic structures as well as in bedrock rivers. However, interactions between flow field, particle motion, channel topography, material properties and abrasion have rarely been investigated on a prototype scale, leaving many open questions as to their quantitative interrelations. Therefore, we investigated hydroabrasion in a multi‐year field study at two Swiss Sediment Bypass Tunnels (SBTs). Abrasion depths of various invert materials, hydraulics and sediment transport conditions were determined and used to compute the abrasion coefficients kv of different abrasion models for high‐strength concrete and granite. The results reveal that these models are useful to estimate spatially averaged abrasion rates. The kv‐value is about one order of magnitude higher for granite than for high‐strength concrete, hence, using material‐specific abrasion coefficients enhances the prediction accuracy. Three‐dimensional flow structures, i.e., secondary currents occurring both, in the straight and curved sections of the tunnels cause incision channels, while also longitudinally undulating abrasion patterns were observed. Furthermore, hydroabrasion concentrated along joints and protruding edges. The maximum abrasion depths were roughly twice the mean abrasion depths, irrespective of hydraulics, sediment transport
conditions and invert material.
In order to prevent reservoir sedimentation, sediment bypass tunnels can be an efficient countermeasure restoring sediment continuity of impounded rivers. Although supercritical open channel flow conditions in these tunnelsprevent tun-nel blockage, in combination with the high bypassed sediment volume it may lead tosevere abrasion damages on inverts. Consequently,wear termed hydroabra-sionoccurs. Based on laboratoryexperiments, a theoretical model was devel-oped to predict abrasion rates and service life timeof sediment bypass tunnels. Insituexperiments are further conducted for model calibration to provide an abrasion prediction approach for field applications.Finally,recommendations concerning the hydraulic design of the tunnel as well as the structural design ofthe invertareprovided.
The derivation of an abrasion prediction model for concrete hydraulic structures valid in supercritical flows is presented herein. The state of the art saltation-abrasion model from Sklar and Dietrich (2004) is modified using the findings of a recent research pro-ject on the design and layout of sediment bypass tunnels. The model correlates the im-pacting parameters with the invert material properties by an abrasion coefficient kv. The value of this coefficient is verified by a similarity analysis to bedrock abrasion in river systems applying a correlation between the abrasion rate and the bed material strength. A sensitivity analysis reveals that the saltation-abrasion model is highly dependent on an adequate estimation of kv. However, as a first order estimate the proposed model en-ables the practical engineer to estimate abrasion at hydraulic structures prone to super-critical flows.
Supercritical sediment-laden open channel flows occur in many hydraulic structures including dam outlets, weirs, and bypass tunnels. Due to high flow velocities and sediment flux severe problems such as erosion and abrasion damages are expected in these structures (Jacobs et al., 2001). Sediment bypass tunnels (SBT), as an effective measure to decrease reservoir sedimentation by bypassing sediments during floods, are exceptionally prone to high abrasion causing significant annual maintenance cost (Sumi et al., 2004; Auel and Boes, 2011). The Laboratory of Hydraulics, Hydrology and Glaciology (VAW) of ETH Zurich conducted a laboratory study to counteract these negative effects (Auel, 2014). The main goals of the project were to analyze the fundamental physical processes in supercritical flows as present in SBTs by investigating the mean and turbulence flow characteristics (Auel et al., 2014a), particle motion (Auel et al., 2014b; 2015b), and abrasion development caused by transported sediment. Besides new insights into the three listed topics, paramount interest is given to their inter-relations and the development of an easily applicable abrasion prediction model (Auel et al., 2015a). This paper presents selected results on the second topic, i.e. the analysis of saltation trajectories of single coarse particles in supercritical flow.
Worldwide, a large number of reservoirs impounded by dams are rapidly filling up with sediments. As on a global level the loss of reservoir volume due to sedimentation increases faster than the creation of new storage volume, the sustainability of reservoirs may be questioned if no countermeasures are taken. This paper gives an overview of the amount and the processes of reservoir sedimentation and its impact on dams and reservoirs. Furthermore, sediment bypass tunnels as a countermeasure for small to medium sized reservoirs are discussed with their pros and cons. The issue of hydroabrasion is highlighted, and the main design features to be applied for sediment bypass tunnels are given.
Bedload transport and hydro-abrasive erosion at steep bedrock rivers and hydraulic structures
(2018)
In this paper typical bypass efficiencies of sediment bypass tunnels (SBTs) used to counter reservoir sedimentation are described, distinguishing between two layouts of the tunnel intake. It results that SBTs are an effective measure to reduce the sedimentation of dam reservoirs, particularly of type (A) with intake at the reservoir head. The hydroabrasive wear of tunnel inverts is significant and
has to be mitigated by using adequate invert liners. The invert abrasion can be estimated based on an abrasion model where a correct input value of the bed material resistance coefficient is paramount to limit model uncertainties. Based on abrasion measurements at prototype SBTs typical values of the material resistance coefficient are recommended for high-strength concrete, natural stones and steel liners. The field experiences gathered so far and the comparison of various invert materials suggest granite pavers as a promising lining material for severe abrasion conditions.
This paper describes the design of the new tunnel invert lining of the 9-foot tunnel at Mud Mountain Dam, Washington, USA. The tunnel diverts all bed load sediments into the tailwater. Major invert abrasion has been observed in the existing steel lining. The new invert design consists of 0.59 m2 and 0.79 m2 granite blocks that are 0.25 m thick and placed tightly together along the tunnel. Stability analysis showed factors of safety ranging from 1.2 to 2.6 against uplift. This will be achieved with strip drains placed in the bedding material along the tunnel. A service-design-life analysis was performed using abrasion prediction modelling.
This model was based on abrasion measurement data acquired from granite field tests at Pfaffensprung sediment bypass tunnel, Switzerland. The estimated annual abrasion depths for the granite were approximately 0.50 mm/year for average sediment transport conditions.
A major drawback of sediment bypass tunnels is the potential for severe invert abrasion due to intense bedload sediment transport. This paper briefly describes the abrasion phenomena as well as the available models used to predict invert material loss. The application and calibration is demonstrated on the basis of the Mud Mountain sediment bypass tunnel, Washington, USA.
