The role of bathymetry and directional wave conditions on observed crescentic bar dynamics

Nearshore sandbars are important features in the surf zone of many beaches because they strongly influence the mean circulation and evolving morphology. Due to variations in wave conditions, sandbars can experience cross-shore migration and vary in shape from alongshore uniform (shore-parallel) to alongshore rhythmic (crescentic). Sandbar dynamics have been studied extensively, but existing observational studies usually do not quantify the processes leading to crescentic bar formation and straightening. This study analyses the dynamics of crescentic bar events at the fetch-limited beach of Castelldefels (northwestern Mediterranean Sea, Spain) using 7.5 years of hourly time-exposure video images and detailed wave conditions. The results show that, despite the generally calm wave conditions, the sandbars were very dynamic in the cross-shore and longshore directions. They often migrated rapidly offshore during storms (up to 70 m in one day) and more slowly onshore during post-storm conditions. Crescentic bars were often present at the study site (48% of the time), but only when the sandbar was at least 10 m from the shoreline. They displayed a large variability in wavelengths (100–700 m), alongshore migration speeds (0–50 m/day) and cross-shore amplitudes (5–20 m). Wavelengths increased for larger bar–shoreline distances and the alongshore migration speeds were strongly correlated with the alongshore component of the radiation stresses. Crescentic patterns typically developed during low–medium energetic waves with limited obliquity ( $\theta \lesssim 20$° at 10 m depth), while bar straightening occurred during medium–high energetic waves with strong oblique angles of incidence ( $\theta \gtrsim 15$°). Overall, this study provides further proof for the important role of wave direction in crescentic bar dynamics and highlights the strong dependence of crescentic bar development on the initial bathymetric configuration.     

A laboratory experiment to monitor the contact state of a fault by transmission waves

We performed a series of laboratory experiments in which elastic waves were transmitted across a simulated fault. Two types of experiments were carried out: (1) Normal Stress Holding Test (NSHT): normal stress was kept constant for about 3 h without shear stress and transmission waves were observed. (2) Shear Stress Increasing Test (SSIT): shear stress was gradually increased until a stick-slip event occurred. Transmission waves were continuously observed throughout the process of stress accumulation. We focused on the change in transmission waves during the application of shear stress and especially during precursory slips.It was found in NSHT that the amplitude of transmission waves linearly increased with the logarithm of stationary contact time. The increase amounted to a few percent after about 3 h. Creep at asperity contacts is responsible for this phenomenon. From a theoretical consideration, it was concluded that the real contact area increased with the logarithm of stationary contact time.We observed in SSIT a significant increase in wave amplitude with shear stress application. This phenomenon cannot be attributed to the time effect observed in NSHT. Instead, it can be explained by the mechanism of “junction growth” proposed by Tabor. Junction growth yields an increase in real contact area. It is required for junction growth to occur that the material in contact is already plastic under a purely normal loading condition. A computer simulation confirmed that this requirement was satisfied in our experiments. We also found that the rate at which the amplitude increased was slightly reduced prior to a stick-slip event. The onset time of the reduction well coincides with the onset of precursory slip. The cause of the reduction is attributed to the reset of stationary contact time due to displacement. This interpretation is supported by the result of NSHT. Taking the time of stationary contact in SSIT into account, we may expect the change in wave amplitude to be, at most, only a few percent. The observed slight reduction in increasing rate is, in this sense, reasonable. The static stiffness of the fault also decreases with precursory slip. It was also found that low frequency waves are a better indicator of precursory slip than high frequency waves. This might suggest that low frequency waves with longer wavelength are a better indicator of average behavior of faults. The problem, however, merits a further investigation. The shifts in phase were also found to be a good indicator of the change in contact state of the fault. The changes in both amplitude and phase of transmission waves are unifyingly understood through the theory of transmission coefficient presented by Pyrak-Nolte et al. Rough surfaces have a tendency to give larger stick-slips than smooth surfaces. The amount of precursory slip is larger for rough surfaces than for smooth surfaces. Although it was confirmed by a computer simulation that rough surfaces have larger contact diameters than smooth surfaces, the rigorous relationship between the surface roughness (contact diameter) and the amount of precursory slips was not established.     

大变形有限元法分析边坡稳定性

边坡稳定性分析的重要任务是确定边坡潜在滑面的位置及相应的安全系数,鉴于坡体位移可在现场监测中直接得到,提出边坡潜在滑面确定的位移判据,采用大变形有限元首先计算出边坡的位移场,然后在开挖面的不同高度作塑性区内的水平位移参考线,依次连接参考线上的位移突变点即得到边坡失稳的潜在滑面位置,分析滑面上的应力以及岩体抗剪强度即得到边坡的整体安全系数。     

Slope instability of San Miniato hill (Florence, Italy): possible deformation patterns

The hill of San Miniato (Florence, Italy) has been repeatedly identified in the past as the site of a slow moving, intermittent landslide. Possible hypotheses regarding the mechanisms responsible for past movements and for cracks and fissures present today are formulated based on the comparison of historical information and processed monitoring data. Although it is not possible to completely exclude a large inactive landslide that involves the entire northern slope, it seems more probable that the slide, if it does exist, is no longer active and that present-day damage is due to separate shallow movements concentrated in man-made deposits. This hypothesis can only be verified with a temporal extension of the monitoring activities, necessary in any case for planning possible safeguarding measures for the cultural heritage in the area.     

Real Time Analysis and Forecasting of Strata Caving Behaviour during Longwall Operations

Summary. Discontinuous manual observations and irregular caving characteristics of roof rocks often lead to improper decisions resulting in accidents and production loss. Hence, systematic monitoring of the hanging roof behind the chock shields is necessary for safe and productive mining operations. A real-time application was successfully implemented in an Indian mine for forecasting of hanging roof behaviour to enhance safety and productivity. This paper reports the functioning of real-time TWAP (time weighted average pressure) analysis in the forecasting of hanging roof behaviour in real time.     

Spatial assessment and redesign of a groundwater quality monitoring network using entropy theory, Gaza Strip, Palestine

Using entropy theory, a methodology was developed for the evaluation and redesign of groundwater quality monitoring wells in the Gaza Strip in Palestine. Essential to the methodology is the development of a Transinformation Model (TM) which yields the amount of information transfer and the dependency between the wells as a function of distance. The TM parameters, such as the minimum transinformation and the range, were employed for evaluating the network which revealed that most of the distances between wells were less than the range. It also indicated that a high percentage of redundant information existed in the network. Therefore, the network was reduced by superimposing a square pattern over the monitored area and selecting one well per square block in a stratified pattern. The methodology was tested using the chloride data collected from 1972–2000 from 417 groundwater quality monitoring wells in the Gaza Strip. The number of the groundwater quality monitoring wells in the Gaza Strip was reduced by 53%, while there was 26% redundant information based on the minimum existing distance between wells. This methodology is meant to help monitor the groundwater quality (salinity) in the Gaza Strip.     

Effect of sampling and linkage on fault length and length–displacement relationship

The power-law exponent (n) in the equation: D=cL ⁿ , with D = maximum displacement and L = fault length, would be affected by deviations of fault trace length. (1) Assuming n=1, numerical simulations on the effect of sampling and linkage on fault length and length–displacement relationship are done in this paper. The results show that: (a) uniform relative deviations, which means all faults within a dataset have the same relative deviation, do not affect the value of n; (b) deviations of the fault length due to unresolved fault tip decrease the values of n and the deviations of n increase with the increasing length deviations; (c) fault linkage and observed dimensions either increase or decrease the value of n depending on the distribution of deviations within a dataset; (d) mixed deviations of the fault lengths are either negative or positive and cause the values of n to either decrease or increase; (e) a dataset combined from two or more datasets with different values of c and orders of magnitude also cause the values of n to deviate. (2) Data including 19 datasets and spanning more than eight orders of fault length magnitudes (10⁻²–10⁵ m) collected from the published literature indicate that the values of n range from 0.55 to 1.5, the average value being 1.0813, and the peak value of n _d (double regression) is 1.0–1.1. Based on above results from the simulations and published data, we propose that the relationship between the maximum displacement and fault length in a single tectonic environment with uniform mechanical properties is linear, and the value of n deviated from 1 is mainly caused by the sampling and linkage effects.     

Some Observations on the Use of Data from Historical Documents in Debris-Flow Studies

The knowledge of past events is important for the assessment of debris-flow hazard. Amongst the sources of information, documents from historical archives are particularly important in sites where the debris flows cause damage to urban areas and transportation routes. The paper analyses the availability of historical documents on debris flows in Northeastern Italy and discusses factors that can influence the building of time series from archive data both at regional and single basin scales. An increased number of debris flows was observed in the studied region for the last decades. This could be due both to an increased frequency of the events and to a larger availability of information: the analysis carried out indicates that the latter factor is probably the most influencing. The importance of factors, which affect the collection of data, including the conservation of documents and the presence and fragility of the elements at risk, is stressed in view of a wise use of historical data on debris flows.     

G856磁力仪在地磁台站建设中的应用与实践方法

本文通过泉州地磁台重建工程中的工作实践,阐述了应用G856磁力仪在建筑材料磁性检测、施工过程中磁性跟踪监测的实践方法。     

多时相组合分类法在土地利用动态监测中的应用

介绍了土地利用遥感动态监测的基本概念，简述了遥感土地利用变化信息提取等遥感监测方法．重点探讨了多时相组合分类方法的相关技术。对广西2002年和2003年两个时相的MODIS数据．采用多时相直接分类法对土地利用变化状况进行了遥感动态监测。对不同方式波段组合的试验表明。经过差值、比值处理的波段组合具有较差的试验效果(总体精度只有30％～40％)，而经过PCA变换的波段组合则具有相对较好的试验效果(总体精度超过70％)。