Experimental Study on Landslides of Loose Sediment Slope Induced by Stream Bed Incision

Uplift of the Qinghai-Tibetan Plateau has resulted in rapid incision of rivers along the margin of the plateau. Landslides occur frequently as a consequence of increasing bank slope and potential landslide energy due to stream bed incision or lateral bank erosion on the concave banks at bends. The Fencha Gully is on the eastern margin of the Qinghai-Tibetan Plateau and is developing on a huge landslide body. Flume experiments were conducted on the base of the field investigation to study the mechanism of landslides induced by stream bed incision. The experiments were designed with a length scale ratio of 1:20. Landslides and stream bed incision with loose sediment were observed and analyzed. The results show that landslides are induced as a result of stream bed incision. The potential landslide energy is defined, which increases quickly with an effective incision depth coupling vertical incision and lateral bank erosion. The occurrence of landslides can be attributed to increasing incision depth and potential landslide energy. Results indicate that the critical effective incision depth is 4.0–6.0 m. A critical value of the potential landslide energy is found from the experiments. Landslides occur if the potential energy exceeds the critical energy, which is 2.24×104 t·m/s2 for the Fencha Gully. The incision depth and potential energy of landslides from the Fencha Gully agree well with the results.

From steady streaming to oscillations: the role of subglacial drainage and temperate ice in ice-stream dynamics

<p>The majority of Antarctic ice is discharged through fast-flowing ice streams. Some of these ice streams exhibit variations in velocities and ice stream discharge on decadal to centennial time scales, but the factors controlling these variations are still insufficiently understood.  Using computational models of ice flow and hydrology, we predict the existence of two dynamical regimes: stable ice streaming associated with high hydraulic permeability of the bed, and ‘binge-purge’ oscillations associated with low permeability.</p><p>Observations indicate that the fast-flow of ice streams is enabled by meltwater lubricating the ice stream bed, and models suggest that this lubrication is the result of a positive feedback between fast flow, heat dissipation at the ice stream bed and meltwater production within the ice. In particular, recent studies have highlighted that heat dissipation in temperate ice stream margins, which are regions of high lateral strain, can contribute significantly to the subglacial water balance. However, the role of this meltwater flux in ice stream dynamics remains unclear. Here, we investigate the roles of subglacial drainage and feedbacks between fast flow and heat dissipation in ice-stream evolution. </p><p>The ice is modelled as a vertically uniform plug flow. Water flow at the bed is modelled as a Darcian system whose hydraulic transmissivity increases with decreasing effective pressure. Dynamical feedbacks in the energy balance include both frictional heating along the bed and lateral shear heating. Within our model, two distinct dynamic regimes can be identified: if the hydraulic permeability of the bed is sufficiently high to evacuate all meltwater produced at the ice stream bed and in its margins, a moderately-fast steady ice stream forms. Conversely,`binge-purge’ oscillations between fast and stagnant flow emerge when the hydraulic permeability of the bed is too low to evacuate the meltwater produced within the ice stream. Topographic controls can suppress this oscillatory behaviour, while the formation of temperate ice in ice stream margins amplifies it.</p>

A Two-Dimensional Depth-Averaged Sediment Transport Mobile-Bed Model with Polygonal Meshes

A polygonal-mesh based numerical method is developed to simulate sediment transport in mobile-bed streams with free surfaces. The flow and sediment transport governing equations are depth-averaged and solved in the two-dimensional (2D) horizontal space. The flow and sediment transport are further coupled to the stream bed changes so that erosion and deposition processes are simulated together with the mobile bed changes. Multiple subsurface bed layers are allowed so that bed stratigraphy may be taken into consideration. The proposed numerical discretization is valid for the most flexible polygonal mesh type which includes all existing meshes in use such as the quadrilateral-triangle hybrid mesh. The finite-volume method is adopted such that the mass conservations of both water and sediment are satisfied locally and globally. The sediment transport and stream bed processes are formulated in a general way so that the proposed numerical method may be applied to a wide range of streams and suitable for practical stream applications. The technical details of the numerical method are presented; model verification and validation studies are reported using selected cases having physical model or field measured data. The developed model is intended for general-purpose use available to the public.

Characterising the bed of Rutford Ice Stream, West Antarctica, using reflection seismic profiles

<p>Ice stream flow is predominantly controlled by sliding over the bed, deformation within the bed and deformation within the ice column. The significance of processes at the bed, now and in the future, remains uncertain due to a lack of knowledge of conditions at the ice stream bed. In the Austral summer of 2018/19, as part of the BEAMISH Project, three holes were drilled to the bed of Rutford Ice Stream to install instruments in the ice column and at the bed, and also sample the bed. Prior to drilling, three seismic profiles were acquired across the bed access sites. These data therefore provide a rare opportunity to compare in situ measurements of ice stream bed conditions with seismic reflection data. The seismic line acquisition was also repeated one year later to investigate any changes at the bed following the drilling and connection to the bed. We will use a combination of imaging, acoustic impedance calculation and wide-angle reflection amplitude variation to characterise the bed conditions using the seismic data.   </p>

Evaluating of the effect of thinning on suspended sediment runoff in a cypress and cedar plantation forest using Fukushima-derived Cs-137, Cs-134 and Pb-210ex

<p>Skid trail and heavy machinery for forest practice becoming more common for effective forest practices, but these causes soil disturbance in the forest, leading to a sudden increase in the amount of suspended sediment during and post thinning. The discharged sediment can flow into the river and may cause downstream water pollution. To evaluate the effect of thinning on sediment production, sediment fingerprinting techniques can be an effective tool for proper forest practices. In Tochigi prefecture in Japan, in addition to the Cs-137 by global fallout and Pb-210ex, additional FRN, the Fukushima-derived Cs-137 and Cs-134 on March 2011 are available, but few studies are available for combining use of Fukushima-derived radiocesium and fallout Pb-210ex. Therefore, the objective of this study is to determine the transport of the fine sediment in the forest pre- and post- thinning with using fingerprinting techniques.</p><p>The study area is Mt. Karasawa, located 180 km southwest of the Fukushima nuclear power plant in Tochigi prefecture. The fallout inventory of Cs-137 and Cs-134 is 8 kBq/m<sup>2</sup>(Kato et al, 2012). The study site has two catchments which are called K2(17ha) and K3(9ha) respectively and the observation period was from August 2010 to August 2019. In K2, strip thinning was performed with heavy machines from June to October 2011 while randomly thinning without heavy machines was applied for K3 from January to March 2013.Soil samples were collected from the slope surface, skid trail and stream bed, which are the possible sources of suspended sediment. The suspended sediment concentration was measured based on the data of the turbidity censor installed in the stream. The particle size distribution and radionuclide concentration of sediment collected from SS sampler and soil samples are also measured. Hysteresis analysis based on suspended sediment concentration and flow rate and fingerprinting using Fukushima-derived Cs-137, Cs-134, and Pb-210ex was applied to determine the contribution of the slope surface layer and streambed to suspended sediment. By using the difference in the depth distribution of Cs-134 and Cs-137, the production source depth of suspended sediment was estimated.</p><p>In the K2 catchment where strip thinning was performed with heavy machinery, suspended sediment concentration during high flow period was rapidly increased up to 2833 mg/L during thinning period and then decreased down to 503 mg/L. On the other hand, in the controlled catchment(K3), no increase in suspended sediment concentration was observed during the same period. By using End- Members Mixing analysis, we found that the contribution of suspended sediment from hillslope increased (from 22% to 50%) more than the stream bed (30%) in the thinning period. Since 2014, the trend has reversed and in 2019, the contribution from the streambed is dominant (50%), and the contribution from slope is decreasing (28%).</p>

The influence of topography of stream bed with the clogging layer on the stream-groundwater interaction and the groundwater flow patterns beneath stream bed

<p>Studies have indicated that the streambed with the clogging layer affects the interaction between surface water and groundwater. When the streambed covered by a clogging layer, the decrease of the groundwater table can transform the state of stream-groundwater from the connection into disconnection. When the stream-groundwater interaction reaches the state of disconnection, the infiltration rate is independent with the groundwater level beneath the streambed. In this study, we show the effects of the topography of the streambed and clogging layer on the infiltration and groundwater flow patterns beneath the streambed by numerical simulations. The results show that the clogging layer and the change of topography of streambed affect the development of the unsaturated zone, flow path, and residence time beneath the streambed.</p>