Influence of reversing currents on the erosion stability and bed degradation of widely graded grain material

Physical model tests were done in a recirculating flume to investigate the overall erosion stability of widely graded bed material in estuarine and coastal conditions by means of simulating tidal flow conditions with reversing currents. As a result of the reversing flow conditions, previously protected sediment eventually became exposed again, leading to bidirectional displacement processes depending on the flow direction. Furthermore, eroded sediment fractions were slightly finer due to flow in the initially applied direction rather than under the subsequently applied flow in the reverse direction. This indicates higher critical shear stresses, and, thus, erosion stability for the initial flow direction. In comparison to the erosional pattern found when subjecting the material to unidirectional currents, this study finds an even higher erosional stability for sediment fractions smaller than the median (d₅₀) diameter of the parent bed material under reversing current conditions. Overall, no significant damage or failure of the bed was observed after subjecting the material to reversing currents, indicating only a small amount of bed degradation, and, thus, high potential for scour and bed protection under the tested flow conditions.     

Insights into bedload transport processes of a large regulated gravel‐bed river

A comprehensive monitoring programme focusing on bedload transport behaviour was conducted at a large gravel‐bed river. Innovative monitoring strategies were developed during five years of preconstruction observations accompanying a restoration project. A bedload basket sampler was used to perform 55 cross‐sectional measurements, which cover the entire water discharge spectrum from a 200‐year flood event in 2013 to a rare low flow event. The monitoring activities provide essential knowledge regarding bedload transport processes in large rivers. We have identified the initiation of motion under low flow conditions and a decrease in the rate of bedload discharge with increasing water discharge around bankfull conditions. Bedload flux strongly increases again during high flood events when the entire inundation area is flooded. No bedload hysteresis was observed. The effective discharge for bedload transport was determined to be near mean flow conditions, which is therefore at a lower flow discharge than expected. A numerical sediment transport model was able to reproduce the measured sediment transport patterns. The unique dataset enables the characterisation of bedload transport patterns in a large and regulated gravel‐bed river, evaluation of modern river engineering measures on the Danube, and, as a pilot project has recently been under construction, is able to address ongoing river bed incision, unsatisfactory ecological conditions for the adjacent national park and insufficient water depths for inland navigation. Copyright © 2017 John Wiley & Sons, Ltd.     

Does the permeability of gravel river beds affect near‐bed hydrodynamics?

The permeability of river beds is an important control on hyporheic flow and the movement of fine sediment and solutes into and out of the bed. However, relatively little is known about the effect of bed permeability on overlying near‐bed flow dynamics, and thus on fluid advection at the sediment–water interface. This study provides the first quantification of this effect for water‐worked gravel beds. Laboratory experiments in a recirculating flume revealed that flows over permeable beds exhibit fundamental differences compared with flows over impermeable beds of the same topography. The turbulence over permeable beds is less intense, more organised and more efficient at momentum transfer because eddies are more coherent. Furthermore, turbulent kinetic energy is lower, meaning that less energy is extracted from the mean flow by this turbulence. Consequently, the double‐averaged velocity is higher and the bulk flow resistance is lower over permeable beds, and there is a difference in how momentum is conveyed from the overlying flow to the bed surface. The main implications of these results are three‐fold. First, local pressure gradients, and therefore rates of material transport, across the sediment–water interface are likely to differ between impermeable and permeable beds. Second, near‐bed and hyporheic flows are unlikely to be adequately predicted by numerical models that represent the bed as an impermeable boundary. Third, more sophisticated flow resistance models are required for coarse‐grained rivers that consider not only the bed surface but also the underlying permeable structure. Overall, our results suggest that the effects of bed permeability have critical implications for hyporheic exchange, fluvial sediment dynamics and benthic habitat availability. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Large grains matter: contrasting bed stability and morphodynamics during two nearly identical experiments

While the stabilizing function of large grains in step‐pool streams has long been recognized, the role they play in gravel‐bed streams is less clear. Most researchers have ignored the role of large grains in gravel‐bed streams, and have assumed that the median bed surface size controls the erodibility of alluvial boundaries. The experiments presented herein challenge this convention. Two experiments were conducted that demonstrate the significant morphodynamic implications of a slight change to the coarse tail of the bed material. The two distributions had the same range of particle sizes, and nearly identical bulk d₅₀ values (1.6 mm); however the d₉₀ of experiment GSD1 was slightly finer (3.7 mm) than that for experiment GSD2 (3.9 mm). Transport rates during GSD1 were nearly four times greater than during GSD2 (even though the dimensionless shear stress was slightly lower), and the channel developed a sinuous pattern with well‐developed riffles, pools and bars. During GSD2 the initial rectangular channel remained virtually unchanged for the duration of the experiment. The relative stability of GSD2 seems to be associated with a slightly larger proportion of stable (large) grains on the bed surface: at the beginning of GSD1, 3.5% of the bed was immobile, while almost twice as much of it (6.1%) was immobile at the beginning of GSD2. The results demonstrate that the largest grains (not the median size) exert first‐order control on channel stability. Copyright © 2017 John Wiley & Sons, Ltd.     

Gravel‐bed river evolution in earthquake‐prone regions subject to cycled hydrographs and repeated sediment pulses

Sediment often enters rivers in the form of sediment pulses associated with landslides and debris flows. This is particularly so in gravel‐bed rivers in earthquake‐prone mountain regions, such as Southwest China. Under such circumstances, sediment pulses can rapidly change river topography and leave the river in repeated states of gradual recovery. In this paper, we implement a one‐dimensional morphodynamic model of river response to pulsed sediment supply. The model is validated using data from flume experiments, so demonstrating that it can successfully reproduce the overall morphodynamics of experimental pulses. The model is then used to explore the evolution of a gravel‐bed river subject to cycled hydrographs and repeated sediment pulses. These pulses are fed into the channel in a fixed region centered at a point halfway down the calculational domain. The pulsed sediment supply is in addition to a constant sediment supply at the upstream end. Results indicate that the river can reach a mobile‐bed equilibrium in which two regions exist within which bed elevation and surface grain size distribution vary periodically in time. One of these is at the upstream end, where a periodic discharge hydrograph and constant sediment supply are imposed, and the other is in a region about halfway down the channel where periodic sediment pulses are introduced. Outside these two regions, bed elevation and surface grain size distribution reach a mobile‐bed equilibrium that is invariant in time. The zone of fluctuation‐free mobile‐bed equilibrium upstream of the pulse region is not affected by repeated sediment pulses under the scenarios tested, but downstream of the pulse region, the channel reaches different fluctuation‐free mobile‐bed equilibriums under different sediment pulse scenarios. The vertical bed structure predicted by the simulations indicates that the cyclic variation associated with the hydrograph and sediment pulses can affect the substrate stratigraphy to some depth. Copyright © 2017 John Wiley & Sons, Ltd.     

Monitoring gravel augmentation in a large regulated river and implications for process‐based restoration

The artificial gravel augmentation of river channels is increasingly being used to mitigate the adverse effects of river regulation and sediment starvation. A systematic framework for designing and assessing such gravel augmentations is still lacking, notably on large rivers. Monitoring is required to quantify the movement of augmented gravel, measure bedform changes, assess potential habitat enhancement, and reduce the uncertainty in sediment management. Here we present the results of an experiment conducted in the Rhine River (French and German border). In 2010, 23 000 m³ of sediments (approximately the mean annual bedload transport capacity) were supplied in a by‐passed reach downstream of the Kembs dam to test the feasibility of enhancing sediment transport and bedform changes. A 620‐m‐long and 12‐m‐wide gravel deposit was created 8 km downstream from the dam. Monitoring included topo‐bathymetric surveys, radio‐frequency particle tracking using passive integrated transponder (PIT) tags, bed grain size measurement, and airborne imagery. Six surveys performed since 2009 have been described (before and after gravel augmentation, and after Q₂ and Q₁₅ floods). The key findings are that (i) the augmented gravel was partially dispersed by the first flood event of December 2010 (Q₁); (ii) PIT tags were found up to 3200 m downstream of the gravel augmentation site after four years, but the effects of gravel augmentation could not be clearly distinguished from the effects of floods and internal remobilization on more than 3500 m downstream; (iii) linear and log‐linear relationships linking bedload transport, particle mobility, and grain size were established; and (iv) combined bathymetry and PIT tag surveys were useful for evaluating potential environmental risks and the first morpho‐ecological responses. This confirmed the complementary nature of such techniques in the monitoring of gravel augmentation in large rivers. Copyright © 2017 John Wiley & Sons, Ltd.     

Adjustment processes of mountainous rivers affected by tilting uplift: Laboratory experiments and a case study of Yakushima Island,Japan

Mountainous river basins are one of the main sources of sediment. Over long time scales, sediment production is sustained by the persistent dissection of river basins, which is promoted by tectonic activity. The response or adjustment of rivers to forcing factors such as uplift is based on the concept of the graded river and a feedback mechanism between the incision and uplift. Although the development of graded rivers under natural circumstances has been discussed for a long time, knowledge about the transition of river basins under heterogeneous uplift is not enough. To understand the development of a river basin with a non‐uniform uplift rate, two simple cases are examined: landward and seaward tilting uplift, where the uplift rate varies linearly in space. For our study, laboratory experiments were conducted and the results were compared with those of natural river basins; two river basins in Yakushima Island were selected for this purpose. In both the laboratory and Yakushima, the longitudinal profile of the river basin under landward‐tilting uplift has a convex‐up zone and a specific knickpoint is formed at the upstream end of this zone. This knickpoint is inactive with respect to migration and incision owing to the insufficient cumulative uplift to the equilibrium state. It was also observed in both the experimental and natural cases that the profile of the river basin under seaward tilting is unlikely to have such a convex‐up zone in the long term, and will instead have a smooth concave profile. Therefore, the spatiotemporal pattern of dissection differs depending on the type of tilting uplift, which suggests that sediment production also varies in time and space according to the type of uplift.     

沉管隧道卵石与碎石垫层力学变形特性对比研究

国内外沉管隧道先铺基础多采用碎石作为垫层材料,目前尚无采用卵石的先例。卵石和碎石在表面光滑度、排列接触方式、颗粒间天然空隙率等物理特性的差异性将影响其力学性能表现。通过物理模型试验和数值模拟计算,对卵石和碎石垫层的力学变形特性进行对比分析。研究表明：（1）两种材料垫层压缩曲线均呈两阶段反弯曲线变化趋势,相同荷载条件下卵石垫层压缩量较碎石高,总体割线模量较碎石低约30%。（2）垫层厚度由0.8 m变为1 m时,卵石垫层割线模量增加了13.0%,碎石垫层割线模量增加了2.2%;卵石垫层力学变形性能对垫层厚度的变化较碎石垫层更敏感。（3）预压荷载由52.5 kPa增加到84 kPa时,卵石垫层割线模量增加了23.5%,碎石垫层割线模量增加了7.6%;预压荷载越大,卵石垫层能更早达到拐点从而表现出更稳定的力学性能;增加预压荷载对卵石垫层整体力学变形性能的改善较碎石垫层更明显。（4）随沟宽增大,垫层模量在前期再压缩阶段出现模量提高,而在全加载期内总体表现为模量降低;在全荷载范围内卵石垫层对垄沟尺寸变化的敏感度低于碎石垫层。（5）碎石垫层的整体力学性能优于卵石垫层,但两种垫层材料对结构沉降和受力状态的影响相对有限;在对施工偏差敏感度充分分析的基础上,卵石可替代碎石作为沉管隧道垫层材料。（6）开展卵石级配试验研究,获取最佳卵石级配,是未来研究方向。     

桂林岩溶盆地第四纪粘土砾石层沉积与漓江袭夺

通过对桂林岩溶盆地内第四纪粘土砾石层调查研究,认为:①粘土砾石层成因不属冰川沉积或湖相沉积,而是山地河流相沉积和再沉积产物。前者时化属中更新世,后者成因复杂,已发现坡积相、洪积相,时代属晚更新世;②漓江溯源侵蚀过程中发生多次袭夺;③岩溶盆地的发育与漓江袭夺控制和影响粘土砾石层的沉积和再沉积。     

Additives for Slurry Shields in Highly Permeable Ground

Summary. For tunneling projects in saturated soils tunnel boring machines (TBMs) with slurry shields are widely used. However, in coarse, highly permeable soils the suspension penetrates the ground and the required support pressure cannot be built up. For the Zimmerberg Base Tunnel near Zurich permeabilities much greater than 10⁻³ m/s were expected. This value is usually considered as the critical limit for the applicability of slurry shields. Therefore it was aimed to find additives for the bentonite suspension which would allow it to attain a higher suspension pressure. For these investigations an apparatus has been developed. It allows the maximum attainable support pressure for a given suspension and a ground to be determined. It reflects the real situation, produces reproducible results and is insensitive to the inevitable variation of individual parameters. In the tests the additives polymer, sand and vermiculite were studied and their effects on the attainable maximum suspension pressure investigated. For the best combination, i.e. with a well defined proportion of the individual components, suspension pressures could be attained which were about 10 to 20 times higher than those with an ordinary bentonite suspension. Successful excavation of the Zimmerberg Base Tunnel proved the validity of the laboratory tests containing 200 kg bentonite per m³ water.