Bioturbation and erosion rates along the soil-hillslope conveyor belt,part 1: Insights from single-grain feldspar luminescence

The interplay of bioturbation, soil production and long-term erosion–deposition in soil and landscape co-evolution is poorly understood. Single-grain post-infrared infrared stimulated luminescence (post-IR IRSL) measurements on sand-sized grains of feldspar from the soil matrix can provide direct information on all three processes. To explore the potential of this novel method, we propose a conceptual model of how post-IR IRSL-derived burial age and fraction of surface-visiting grains change with soil depth and along a hillslope catena. We then tested this conceptual model by comparison with post-IR IRSL results for 15 samples taken at different depths within four soil profiles along a hillslope catena in the Santa Clotilde Critical Zone Observatory (southern Spain). In our work, we observed clear differences in apparent post-IR IRSL burial age distributions with depth along the catena, with younger ages and more linear age–depth structure for the hill-base profile, indicating the influence of lateral deposition processes. We noted shallower soils and truncated burial age–depth functions for the two erosional mid-slope profiles, and an exponential decline of burial age with depth for the hill-top profile. We suggest that the downslope increase in the fraction of surface-visiting grains at intermediate depths (20 cm) indicates creep to be the dominant erosion process. Our study demonstrates that single-grain feldspar luminescence signature-depth profiles provide a new way of tracing vertical and lateral soil mixing and transport processes. In addition, we propose a new objective luminescence-based criterion for mapping the soil-bedrock boundary, thus producing soil depths in better agreement with geomorphological process considerations. Our work highlights the possibilities of feldspar single grain techniques to provide quantitative insights into soil production, bioturbation and erosion–deposition. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.     

Erodibility study of sediment in a fast-flowing river

Determination of sediment stability in the field is challenging because bed shear stress (BSS), a determining factor of sediment erosion, can’t easily be directly measured. To tackle this challenge and reliably assess sediment erodibility in a fast flowing river, a standalone underwater camera system and a new insitu flume (ISF) were developed and applied in this study. The camera system was used to record sediment movement and the new ISF was used for measuring critical bottom shear stress (CBSS). The camera can be deployed alone in water to record videos or take pictures with light emitting diode (LED) lighting and flexible schedule settings. The ISF is based on the concept that the amount of force needed to erode the same particle under different flow conditions should be similar. Two high resolution Acoustic Doppler Current Profilers (ADCP) also were deployed in the field to collect velocity-depth profiles which are used by conventional methods to calculate BSS with the law of the wall. The sediment erodibility was then assessed based on the comparison between the obtained CBSS and BSS and then further verified with the recorded observations from the deployed camera. The results reveal that the widely used conventional method can produce large uncertainties and is not adequate to provide meaningful conclusion under these conditions.     

Critical movement of large rocks in currents and waves

Cobbles, boulders, and rocks often are used in a bed protection layer near a structure to protect the underlying sand bed against erosion by combined current and waves. The design of a bed protection layer consisting of loose rocks (rubble mound) requires knowledge of the stability and movement (as bed load) of very coarse materials. If some movement (or damage) is acceptable, the rock diameter can be designed to be smaller. This paper addresses the stability and movement of very coarse materials (cobbles, boulders, and rocks) based on the concept of the critical Shields mobility number. It is shown that the bed load transport of large cobbles, boulders, and rocks can be described by the equations of Meyer-Peter and Mueller (MPM) and Cheng. Both are valid for relatively small Shields mobility numbers. New and general equations for the design of a bed protection layer (including some permissible damage) in conditions with a current with or without waves are proposed based on the Shields mobility parameter and the bed load transport equation of Cheng. Laboratory and field data of critical velocities for pebbles, cobbles, boulders, and rocks have been analyzed and compared to the computed results of the proposed equations. Practical applications are given to demonstrate the general applicability of the proposed equations.     

Cyclic liquefaction screening of sand with non-plastic fines:Critical state approach

There have been significant advances in the application of critical state,CS,in liquefaction potential assessment.This was done by comparing state parameter,j with estimated characteristic cyclic stress ratio,CSR due to an earthquake.A cyclic resistance ratio,CRR curve,which can be determined from cyclic liquefaction tests,separates historical liquefied and non-liquefied data points(j,CSR).On the other hand,the concepts of equivalent granular state parameter,j*,which was developed for sands with fines,can be used in lieu j to provide a unifying framework for characterizing the undrained response of sands with non/low plasticity fines,irrespective of fines content(fc).The present work combines these two propositions,and by merely substituting j*for j into the aforementioned CS approach to capture the influence of fc.A series of static and cyclic triaxial tests were conducted,separately and independently of the concept of j*,for sand with up to fc of 30%.The clean sand was collected from Sabarmati river belt at Ahmedabad city in India which was severely affected during the Bhuj earthquake,2001.The experimental data gave a single relation for CRR and j*which was then used to assess liquefaction potential for a SPT based case study,where fc varies along the depth.The prediction matched with the field observation.     

The critical rainfall characteristics for torrents and debris flows in the Wenchuan earthquake stricken area

Critical rainfall assessment is a very important tool for hazard management of torrents and debris flows in mountainous areas. The Wenchuan Earthquake 2008 caused huge casualties and property damages in the earthquake-stricken area, which also generated large quantities of loose solid materials and increased occurrence probabilities of debris flows. There is an urgent need to quantify the critical rainfall distribution in the area so that better hazard management could be planned and if real time rainfall forecast is available, torrent and debris flow early-warning could be issued in advance. This study is based on 49-year observations （1954-2003） of up to 678 torrent and debris flow events. Detailed contour maps of 1 hour and 24 hour critical rainfalls have been generated （Due to the data limitation, there was insufficient 10 minute critical rainfall to make its contour map）. Generally, the contour maps from 1 hour and 24 hours have similar patterns. Three zones with low, medium and high critical rainfalls have been identified. The characteristics of the critical rainfall zones are linked with the local vegetation cover and land forms. Further studies and observations are needed to validate the finding and improve the contour maps.     

Detection of the Kuroshio frontal instable processes (KFIP) in the East China Sea using the MODIS images

The Kuroshio frontal instable processes (KFIP) in the East China Sea (ECS) not only have a great impact on the hydrologic characteristics,the pollutants drift,the distribution of seafloor sediment and the ships navigation of the ECS,but also are closely related to the climate changes of the coastal areas of the ECS.However the frequency and area of occurrence of the KFIP have not been studied fully and detailedly.Because of its high spatial and temporal resolution,MODIS data is a kind of very good data source for surveying and researching the KFIP in the ECS.The aim of this study is to detect the KFIP in the ECS by using MODIS data,and to study the frequency and region of occurrence of the KFIP in the ECS.The selection has coverage of level 2 data of MODIS SST and Kd490 ranging from July 1,2002 to June 30,2009 of the ECS when there was no cloud impact or little.By using of the data,the minimum standard of the Kuroshio temperature fronts and the diffuse attenuation coefficient (Kd490) fronts of the ECS are given.Based on these standards and the curvature distinguish methods,the standard of curvature distinguish for the KFIP in the ECS are put forward.By making use of this standard,we study a total of 2073 satellite-derived images,and discover that as long as there is no cloud impact from January to May and October to December,the KFIP in the ECS are surely found in MODIS satellite images.From June to September,the frequency of occurrence can also reach to 82.9% at least.Moreover,it is obtained that there are three source regions of these instability processes,namely,(26°N,121.5°E) nearby,(27°N,125°E) nearby and (30°N,128°E) nearby.The differences of the characteristics of these instability processes which are generated in different regions are analyzed in the present study.     

现代黄河水下三角洲粉土的临界水力坡降探讨

通过室内试验，测定不同密实度粉土的临界水力坡降。由试验结果可以看出，粉土的临界水力坡降与一般砂土有着较大差异，一般耗生土临界水力坡降icr＝（ρ-ρw）/ρw，变化范围在0.8-1.5之产，而现代黄河水下三角洲粉土的临界水力坡降为9.5-16.5。同时对土体进行受力分析，考虑颗粒间的凝聚力，对以上结果产生的机理,工找出临界水力坡降与其物理性质指标之间的关系。     

SHEAR STRESS FOR INITIATION OF MOTION OF NON-UNIFORM SEDIMENT MIXTURES

Experiments on initiation of motion of different fractions of non-uniform sediment mixtures are reported. Four sediment mixes and reference transport method to define the threshold of bed particle movement are used in this analysis. Factors controlling the initiation of motion are analyzed. It is found that relative size of different grains with respect to a median size greatly affects the initiation of movement. The incipient motion of non-uniform sediment is very much dependent on the absolute size for coarser fractions. Considering the factors controlling the initiation of motion the experimental data sets along with the other available data yield a new relationship for the calculation of critical shear stress of non-uniform sediments. A set of equations have found that all the data points fall into a single line for the finer and for coarser fractions.     

Seismic stability of retaining walls with surcharge

The use of pseudo-static methods for the computation of soil thrust acting on retaining walls under seismic condition is well established in the design of such structures. Although different methods, based on the limited displacement concept, have been developed in the last 20 years, the most common design method is still the method derived from the theory developed by Mononobe and Okabe. However, the Mononobe–Okabe method presents a basic shortcoming: the solution is based on the limit equilibrium of the soil wedge without taking into account the presence of the wall. In the paper a new solution based on the pseudo-static equilibrium of the soil–wall system is presented. The developed solution takes into account the effect of the presence of the wall and it is applied to soil–wall systems with surcharged backfills. Formulas are provided to calculate directly the yield acceleration and the inclination of the failure surface. The effect of the intensity of the surcharge and of its distance from the wall is investigated and the results are compared to those obtained in the case of soil–wall systems without surcharge.     

破裂临界状态下大理岩的剪切波分裂特征

以山东莱州大理岩为样品对岩样在破裂临界状态下的剪切波分裂特征进行了观测和分析,在两个样品上,得到破裂临界状态下的系列记录,结果表明,在岩石崩溃之前的临界状态下,即使载荷不变,剪切波分裂时间延迟也可能会有涨落现象,即剪切波分裂时间延迟可能会有一个下降的过程。