Strength and erosion mechanism of soft bedrock under high shear stresses

Bedrock erosion plays a critical role in mountain river channel evolution, especially, where there are hydro-projects with high velocity flow conditions. In this study, laboratory experiments are carried out to investigate strength and erosion mechanism of soft bedrock under high shear stresses by high velocity open channel flows. Prototype bedrock from the upper reaches of Yangtze River is employed as testing samples. New observed dataset is generated concerning the flow, bedrock strength and evolution of bedrock erosion, which can be exploited to evaluate bedrock erosion rate and test mathematical bedrock erosion models.


Introduction
Bedrock erosion processes are believed to play important roles in the evolution of mountain river channels and their associated hydro-projects. During the last decade, there has been a rapid advancement in the development of models of mountain landscape evolution. These models have contributed to the understanding of the fundamental mechanisms underlying landscape evolution [1][2][3][4][5][6].
Generally, the main mechanisms responsible for bedrock erosion including hydraulic scour (plucking of rock fragments by fluid shear stress or differential fluid pressure) and abrasive scour (interaction between rock and moving sediment particles) [7]. It is possible that hydraulic scour may dominate on one reach, whereas abrasive scour may dominate on another. Alternatively, both mechanisms may operate concurrently to erode the bedrock on some rivers. A discussion of how to qualitatively identify the erosion mechanisms operating on a reach was presented by Whipple et al. (2000) [8]. A review was also provided by Lai and Greimann (2009) [9]. However, it is cautioned that the mechanisms are interrelated. Hydraulic scour can occur without the assistance of sediment particles but is influenced by sediments through particle-rock collision and sediment deposition. Similarly, abrasive scour is also strongly linked to hydraulic flow characteristics such as shear stress and turbulence.
In this study, only the hydraulic processes are considered, and particular attention is paid to the bedrock erosion in the upper reaches of the Yangtze River. Clear water alone can cause bedrock scour by instantaneous or progressive failure of closed-end rock joints or by dynamic ejection of single rock blocks [10]. Hydraulic scour may be an important mechanism for weak or highly fractured rock, as reviewed by Sklar and Dietrich (2004) [4]. Additional review was also provided by Tomkin et al. (2003) and Sklar and Dietrich (2006) [11,12]. In the upper reaches of Yangtze River, many alluvial rivers are characterized by the presence and exposure of soft bedrock that consists of mainly mudstone and sandstone with relatively lower compressive strengths. During flood season, the floods have become more and more frequent. These floods significantly erode the exposed soft bedrock and lead to damage to bridge pier foundations, diversion structures, and other infrastructure. Evaluation of bedrock erosion rate requires quantification of bedrock characteristics, together with hydraulic processes. Key geomorphic controls, such as rates of river incision or hillslope erosion, are often unknown, as valid data describing the relationships between rates of bedrock erosion and hydraulic processes are not sufficient.
Here we present 18 runs of experiments on soft bedrock erosion. A flume is purposely designed to generate high velocity open channel flow, and prototype bedrock including mud-rock and sand-rock from the upper reaches of Yangtze River are employed as testing samples. For each run, detailed measurements of the flow and bedrock erosion processes are conducted.

Experimental Set-Up
The experiments were conducted in a tilting bed flume at Changjiang River Scientific Research Institute ( Figure 1). The tilting flume is made by Plexiglas, has a width of 0.4 m, a length of 20 m long and a bed slope fixed at 0.035. In order to relate experimental studies to field conditions, as we propose to do, the upstream of this flume is equipped with a high head water tank, by which high velocity open channel flows with a maximum value of 6.0 m/s can be generated. In the test section of the flume, there is a notch designed to fix bedrock samples. Before each run, a bedrock sample is inserted into the notch, flush with the surface. The flat sample area exposed to the flow is round with a diameter of 7.0 cm. The magnitude of erosion was determined by weighing the erodible sample block before and after the experiment and obtaining the mass loss.

Field Survey
The upper reaches of the Yangtze River cover a region from the Three Gorges Dam to the River's origin. The total area is about 1005 thousand km 2 with a total population of 160 million, which accounts for 58.9% of the area and 35% of the population in the valley, respectively [13,14]. Within this area, 90% or more of the land is mountains and plateaus, in which the ecosystem is very fragile and subject to soil and bedrock erosion due to inappropriate use of land. On the other hand, the Yangtze River Basin is of great economic significance, with its economic productivity amounted to almost half of China's Gross National Product (GNP) during the 1990s. In addition, within the Yangtze River Basin, bedrock erosion problems associated with energy projects such as the Three Gorges Dam (TGD) and the South-to-North Water Diversion Project [15,16].
Soft bedrock samples including mud-rock and sandrock were collected from the main river channel within the area of Chongqing Province .These relatively weaker sample bedrocks eroded more readily than other hard rocks, and also more susceptible to hydraulic erosion, thus shortening the experimental duration needed to produce measurable wear. Samples of bedrock are taken at possible foundation locations, boring method was used, and the erodible samples were cast into cylinder blocks with diameter of 7.0 cm and later cut into flume-ready samples. All samples were kept in a sealed box after sampling until laboratory analysis (Table 1).

Results and Discussion
We conducted 18 runs of erosion experiments in which we varied hydraulic condition and bedrock strength ( Table 2). The procedure for each experiment was as follows: An erodible sample was weighed and placed in the flume, its top surface level with the flume bed, and the flume was filled with still water until sample was completely water-saturated (after 48 hour this work).
During the run, we documented the depth, velocity and slope of the flow. The duration of the experiment was chosen to be long enough to obtain significant wear, but not so long that the erodible sample degraded significantly lower than the flume bed. At the end of each run, the sample was removed, examined for wear marks, and weighed on subsequent days until they equilibrated to their dry weight. Typically only one run was obtained from the same sample before it was replaced due to extensive wear, but when possible, we obtained multiple runs from the same sample to test repeatability. To analyze the erosion process with different conditions, we define the erosion rate as the ratio of the eroded mass and the erosion duration, and calculate the bed shear stress by ghJ U W , where U is the density of water, g is the gravity acceleration, h is the flow depth, and J is the water slope. Figure 2 shows the plot of erosion rate versus bed shear stress for different bedrock samples. It can be seen that erosion rate generally increases with the increase of shear stress for all samples, and the increasing trend decreases with the increase of tensile strength.   2008) conducted experimental study of bedrock erosion by granular flows, and explored the relationship between erosion of a synthetic bedrock sample and variables such as grain size, shear rate, water content, and bed strength [17]. It is interesting to see that their work also suggested that the bedrock erosion rate increases with the increase of shear rate and decreases with the increase of tensile strength. Particularly, our experiments employed prototype bedrock samples and high velocity open channel flow condition, which is closer to natural scale. It is acknowledged that to make the analysis of bedrock erosion more mechanistic, more field measurements are needed.

Summary
Experiments on soft bedrock erosion have been conducted. A flume is purposely designed to generate high velocity open channel flow, and prototype bedrock including mud-rock and sand-rock from the upper reaches of Yangtze River are employed as testing samples. New observed dataset is generated concerning the flow, bedrock strength and evolution of bedrock erosion, which can be exploited to evaluate bedrock erosion rate and test mathematical bedrock erosion models.