Erosion rates of alpine bedrock summit surfaces deduced from in situ Be and Al

We have measured the concentration of in situ produced cosmogenic ¹⁰Be and ²⁶Al from bare bedrock surfaces on summit flats in four western U.S. mountain ranges. The maximum mean bare-bedrock erosion rate from these alpine environments is 7.6 ± 3.9 m My⁻¹. Individual measurements vary between 2 and 19 m My⁻¹. These erosion rates are similar to previous cosmogenic radionuclide (CRN) erosion rates measured in other environments, except for those from extremely arid regions. This indicates that bare bedrock is not weathered into transportable material more rapidly in alpine environments than in other environments, even though frost weathering should be intense in these areas. Our CRN-deduced point measurements of bedrock erosion are slower than typical basin-averaged denudation rates ( 50 m My⁻¹). If our measured CRN erosion rates are accurate indicators of the rate at which summit flats are lowered by erosion, then relief in the mountain ranges examined here is probably increasing.

We develop a model of outcrop erosion to investigate the magnitude of errors associated with applying the steady-state erosion model to episodically eroding outcrops. Our simulations show that interpreting measurements with the steady-state erosion model can yield erosion rates which are either greater or less than the actual long-term mean erosion rate. While errors resulting from episodic erosion are potentially greater than both measurement and production rate errors for single samples, the mean value of many steady-state erosion rate measurements provides a much better estimate of the long-term erosion rate.     

坡地系统土壤侵蚀定量评价方法

区域性土壤侵蚀的定量评在涉及到泥沙输移的非连续性难题和众多的非确定性因素，目前应用较普遍的小区定量难以适用。所提出的定量评价方法是利用ＧＩＳ技术、模糊数学、将以分布参数为特征的区域坡地系统划分成若干类具有集中参数特征的基本侵蚀单元，结合ＵＳＬＥ方程来模拟空间上不同侵蚀背景条件下土壤侵蚀的强度，并可以确定影响土壤侵蚀的主要因子及其排序，该方法适用性强，可用于不同空间尺度的土壤侵蚀定量评价。     

Crater formation in soils by raindrop impact

The process of crater formation by the impact of water drops on soil, sand and various other target material was studied. Craters of various shapes and sizes were observed on different target materials or conditions, ranging from circumferential depression to completely hemispherical shape. Crater shape was dependent upon target material, its ?ow stress or shear strength and the presence and thickness of water on the surface. Between 5 and 22 per cent of impact energy was spent on cratering, but the relationship between crater volume and kinetic energy of a raindrop was curvilinear, indicating a lower ef?ciency of impact energy in removing target material as the energy increases. Impact impulse, on the other hand, showed a more linear relationship with crater volume, and the ratio of impulse over crater volume (I/V) remained constant for the entire range of drop sizes, impact velocities, and surface conditions used in this study. Surface shear strength, represented by the penetration depth of fall‐cone penetrometer, appeared to be a key factor involved in this process. An equation was developed which related crater volume to cone penetration depth and impact impulse. Crater volume, which appeared to be a better indicator of the total amount of material dislodged by a raindrop than splash amount, can thus be predicted using this equation. Copyright © 2004 John Wiley & Sons, Ltd.     

Earthquake‐related geomorphic environment and pond sediment information

Shaking during the 1995 Kobe earthquake caused surface material to be more mobile in catchment areas in the Rokko Mountains, Kobe, where there are some active fault lines. As a result, there were many landslides associated with the earthquake. The sedimentation rate in a pond in the mountains increased several fold, then exponentially decreased with seasonality over several years. Six years after the earthquake there were no marked surface movements related to the earthquake, even though the sedimentation rates had increased slightly. A new steady state for the structure of the earthquake‐modi?ed surface had evidently been reached. Copyright © 2004 John Wiley & Sons, Ltd.     

化学腐蚀下砂岩三轴细观损伤机理及损伤变量分析

利用CT识别技术对化学腐蚀下的砂岩进行了三轴加载全过程的即时扫描试验，得到了在不同级荷载作用下砂岩的压密、微裂纹萌生、扩展和破裂的CT图像和CT数，分析了砂岩损伤演化的细观机理。同时，建立了一个基于化学腐蚀影响和CT数的损伤变量模型。     

湖北省土壤侵蚀景观空间异质性及驱动因子分析

地域分异是地球表层大小不等、内部具有一定相似性地段之间的相互分化以及由此产生的差异。为了研究不同区位土壤侵蚀问题，从土壤生态景观及系统论出发，运用地质学、地理学、景观生态学、环境学的理论和研究方法，研究湖北省土壤侵蚀景观空间格局及其驱动因子，使土壤侵蚀问题研究提高到一个新的水平。湖北省土壤侵蚀景观具有南北分带、东西分区，为一不对称的断块一环组合，土壤流呈现向长江、江汉盆地中心轴带辐聚、单流向特点。景观空间异质性形成的首要驱动因子是大地构造背景，以房县一襄樊一广济断裂带为界，南北两侧地壳物质组成和构造发展史存在较明显的差异，现代气候带、降雨量、温热程度及土地利用等差异，造成了湖北省区域土壤地理、土壤生态的分异，形成湖北省土壤生态带、区具有南北分带，东西分区的宏观格局；其次大兴安岭一武陵山深部构造陡变带两侧新构造运动强度差异、大别造山带构造强烈隆升，导致土壤侵蚀强度的西强东弱、南北强中间弱的态势；成土母岩差异性决定了土壤可蚀性的多变；空间上“土壤侵蚀内城区”分布在湖北省的周边地区，经济贫困、管理落后，这一地区的经济水平与水土流失间形成“自反馈作用”，这一现象在我国水土保持、生态建设工作中应该引起重视。     

A Numerical Study on Coastal Defence at Chennai and Related Management Strategies

Chennai coast, right from the inception of Madras harbourin the year 1876, has been experiencinghostile conditions such as (i) coastal erosion, (ii) sandbar formation at the entrance to inlets, (iii) sea water ingression and (iv) change insea bed elevation, etc. In addition, construction of a new satellite harbour, about18 km north of Madras harbour has produced a negative impact on the delicatecoastal features such as (i) Pulicat lake, (ii) Ennore shoals, etc. Construction ofthis satellite harbour has led to the accumulation of sand south of the southbreakwater of the harbour and its accelerated growth is of concern to an inletlocated 2.6~km south of the harbour. `Coastal erosion', a perennial problemassociated with north Chennai sea front for the past 100 years has been addressedin this paper. The paper discusses on a long term solution and details of themethodologies to be adopted for effective management of the coast. Thesolution presented in this paper is based on numerical model study consideringthe nearshore currents and wave induced sediment transports.     

A model of Cs activity profile for soil erosion studies in uncultivated soils of Mediterranean environments

A model to simulate ¹³⁷Cs profiles in soils during the time in which they are being eroded is proposed. The model uses one parameter to characterize the cesium transference in the soil and another to express the erosion rate. To test the model, ¹³⁷Cs profiles of stable and eroded soils were collected at sampling sites located on semi-arid and temperate slopes in the Central Ebro basin, Spain. The ¹³⁷Cs profiles, corresponding to uncultivated soils with natural vegetation cover, were simulated using this model. The ¹³⁷Cs inventories and profiles calculated with the model are very similar to those measured experimentally, and thus it is possible to calculate soil erosion rates in physiographically diverse Mediterranean environments.     

Anabranching in mixed bedrock-alluvial rivers: the example of the Orange River above Augrabies Falls, Northern Cape Province, South Africa

Anabranching is characteristic of a number of rivers in diverse environmental settings worldwide, but has only infrequently been described from bedrock-influenced rivers. A prime example of a mixed bedrock-alluvial anabranching river is provided by a 150-km long reach of the Orange River above Augrabies Falls, Northern Cape Province, South Africa. Here, the perennial Orange flows through arid terrain consisting mainly of Precambrian granites and gneisses, and the river has preferentially eroded bedrock joints, fractures and foliations to form multiple channels which divide around numerous, large (up to 15 km long and 2 km wide), stable islands formed of alluvium and/or bedrock. Significant local variations in channel-bed gradient occur along the river, which strongly control anabranching style through an influence on local sediment budgets. In relatively long (>10 km), lower gradient reaches (<0.0013) within the anabranching reach, sediment supply exceeds local transport capacity, bedrock usually only crops out in channel beds, and channels divide around alluvial islands which are formed by accretion in the lee of bedrock outcrop or at the junction with ephemeral tributaries. Riparian vegetation probably plays a key role in the survival and growth of these islands by increasing flow roughness, inducing deposition, and stabilising the sediments. Less commonly, channels may form by eroding into once-continuous island or floodplain surfaces. In shorter (<10 km), higher gradient reaches (>0.0013) within the anabranching reach, local transport capacity exceeds sediment supply, bedrock crops out extensively, and channels flow over an irregular bedrock pavement or divide around rocky islands. Channel incision into bedrock probably occurs mainly by abrasion, with the general absence of boulder bedforms suggesting that hydraulic plucking is relatively unimportant in this setting. Mixed bedrock-alluvial anabranching also occurs in a number of other rivers worldwide, and appears to be a stable and often long-lived river pattern adjusted to a number of factors commonly acting in combination: (1) jointed/fractured granitoid rock outcrop; (2) erosion-resistant banks and islands; (3) locally variable channel-bed gradients; (4) variable flow regimes.