The morphology and evolution of tidal sand bodies in the macrotidal Gulf of Khambhat,western India

Tidal sand bars and tidal sand ridges are extensively developed in the macrotidal Gulf of Khambhat, offshore western India. The inner and outer regions of the gulf are characterised by the development distinct tidal sand bodies with discrete geometries and dimensions. The outer gulf ridges are long, narrow, curvilinear and several metres high (∼20 m). They are asymmetric in cross-section and migratory in nature, forming ‘ribbon’ like sand bodies separated by tidal channels. Active dunes on these ridges indicate the presence of sand and their orientation parallel to palaeo-shorelines supports a tidal origin. In contrast to the outer gulf tidal sand ridges, sand bars associated with macrotidal estuaries flanking the Gulf of Khambhat typically have an elongate to diamond shape and are only hundreds of metres in width and a few kilometres length. These tidal sand bars occur in the estuary mouths and within the tidally influenced fluvial reaches of the rivers flowing into the gulf. The height of these sand bars is in the range ∼1–3 m. Due to high tidal ranges and bi-directional flow the sand bars do not develop significant height and are formed between the mutually evasive ebb and flow channels. Their bi-directional foresets and the presence of abundant mud drapes associated with the dunes within in-channel sand bars indicate a tidal origin.The Gulf of Khambhat acquired the present configuration in the last few thousand years since the Pleistocene sea-level lowstand (last glacial maximum, ∼18 ka) when the entire continental shelf was subaerially exposed and rivers down-cut into the coastal plain. With increasing sea-level rise, the exposed shelf was drowned, flooding parts of the Modern western Indian peninsula, and large tidal sand ridges formed in the outer gulf. After the fall of sea-level at 2 ka the gulf acquired the Modern configuration with multiple estuaries on both coastlines, rivers supplied the embayment with sandy sediment, and tidal sand bars formed in the Modern estuaries.Quantitative data gathered from the Modern Gulf of Khambhat indicates that for the P50 case, a vertical drill hole will encounter tidal sand bodies (ridges and bars combined) of approximate dimensions 1700 m long, 470 m wide and 1.5 m high, with a spacing of 400 m. In subsurface hydrocarbon reservoirs, where data is sparse and only limited amount of core is available, this quantitative dataset can be useful to constrain subsurface geocellular models. Also, the overall geometry, distribution and aspect ratio of the tidal sand ridges and tidal sand bars can be used to identify ancient counterparts through seismic geomorphology or in core.     

工程锚杆注浆质量无损检测技术研究与应用

采用应力波一维波动理论,对清江水布垭工地21根模型锚杆和74根工程锚杆进行了测试与研究,初步形成了工程锚杆无损检测方法,建立了波形特征与锚杆注浆密实度之间的关系,并从锚杆的有效锚固长度、注浆密实度及缺陷大小和位置三个方面评价锚杆的施工质量,确定了该工程锚杆无损检测验收标准。     

Effect of advective and diffusive sediment transport on the formation of local and global bottom patterns in tidal embayments

Bathymetric field data of tidal basins reveal two main classes of bottom patterns: (1) tidal bars, located near the entrance of the basin (length scale determined by the embayment width) and (2) global channel-shoal patterns which scale with the basin length. Previous models were able to describe only either one of these patterns. In this paper it is shown that both of them can be investigated within the framework of an idealised model of a rectangular tidal embayment, with fixed side walls and an erodible bed. The water motion is described by the depth-averaged shallow-water equations and is forced by a prescribed vertical tide at the seaward entrance. Sediment is transported as suspended load and only realistic values of the bottom friction parameter are considered. By assuming the ratio of embayment length over tidal wave length to be small, the model allows for a morphodynamic equilibrium, characterised by a spatially uniform tide moving over a bottom which slopes upwards toward the landward boundary. This equilibrium is unstable for a range of values of the model parameters, such that growth of bedforms occurs. Both global and local bottom patterns are found. In this study particular emphasis is laid on the mechanism governing the growth of a new type of localised bottom pattern. These patterns consist of small bars located near the entrance of the basin, resembling multiple row bars, and are found when advective sediment fluxes prevail over diffusive sediment fluxes. The formation process of these new bedforms is discussed in detail. The results agree well with field data. Comparison of the results with those obtained with a process-based, numerical model shows that, although the idealised model is strongly simplified, it is capable of producing the essential morphodynamics. Therefore, the idealised model is a useful tool to investigate mechanisms of bottom pattern growth.Responsible Editor: Iris Grabemann     

Morphodynamik am Mehrerauer Seeufer (Bodensee)

The shore west of the town Bregenz situated at Lake Constance (Mehrerauer Seeufer) offers habitats for relictic floristic elements, such as the Lake Constance forget-me-not (Myosotis rehsteineri, Wartm.). During the last decades, several-meter-high gravely bars developed at the beach, and moved into landward direction. Sedimentological investigations were carried out in the littoral region, at the beach, and in the periodically flooded lagoons, in order to display the reasons for the sediments movements. Regular surveys of the bars exhibited them to move episodically. The translocation probably occurred at high water stages, especially during storm events. Extended well sorted, gravely beds were classified as tempestites, whereas poorly sorted, coarse gravely plasters seaward of the bars were characteristic of long-term erosion of fines. One reason for the observed morphodynamical processes probably were deep hollows in the littoral region, which had been the results of sand and gravel mining: Storm waves were moving towards the beach with low friction, and therefore were barely decelerated. In addition, the artificial progradation of the mouth of the River Bregenzer Aach, and the deepening of its riverbed evoked a reduction in inputs of fine particulate matter from the River Bregenzer Aach, which in turn caused a decrease in substitution of losses of the fine-grained binding agent due to erosion. The morphodynamics at the shore of Mehrerau are based on an interaction between anthropogenic reasons and natural storm events.     

Quantification of nearshore morphology based on video imaging

The Argus network is a series of video cameras with aerial views of beaches around the world. Intensity contrasts in time exposure images reveal areas of preferential breaking, which are closely tied to underlying bed morphology. This relationship was further investigated, including the effect of tidal elevation and wave height on the presence of wave breaking and its cross-shore position over sand bars. Computerized methods of objectively extracting shoreline and sand bar locations were developed, allowing the vast quantity of data generated by Argus to be more effectively examined. Once features were identified in the images, daily alongshore mean values were taken to create time series of shoreline and sand bar location, which were analyzed for annual cycles and cross-correlated with wave data to investigate environmental forcing and response.These data extraction techniques were applied to images from four of the Argus camera sites. A relationship between wave height and shoreline location was found in which increased wave heights resulted in more landward shoreline positions; given the short lag times over which this correlation was significant, and that the strong annual signal in wave height was not replicated in the shoreline time series, it is likely that this relationship is a result of set-up during periods of large waves. Wave height was also found to have an effect on sand bar location, whereby an increase in wave height resulted in offshore bar migration. This correlation was significant over much longer time lags than the relationship between wave height and shoreline location, and a strong annual signal was found in the location of almost all observed bars, indicating that the sand bars are migrating with changes in wave height. In the case of the site with multiple sand bars, the offshore bars responded more significantly to changes in wave height, whereas the innermost bar seemed to be shielded from incident wave energy by breaking over the other bars. A relationship was also found between a site's mean wave height and inner sand bar location; sites with the highest wave heights tended to have sand bars farther from shore than those with relatively low wave heights.     

Quasi-three-dimensional modelling of the morphology of longshore bars

A morphological quasi-three-dimensional (Q3D) area model for barred coasts has been developed. The model combines a two-dimensional depth integrated model for wave-driven currents with a model for undertow circulation currents. The combined model makes a simultaneous simulation of the bar-forming processes associated with the undertow and the horizontal wave-driven circulation currents, which may cause instabilities of the bar and the formation of rip channels. Situations with normal and oblique wave incidence are considered. Compared to the depth integrated approach the Q3D model produces less pronounced alongshore irregularities for obliquely incident waves. For normal incident waves the Q3D model produces a crescentic bar while the depth integrated model predicts almost straight sections of the bar interrupted by rip channels. The sensitivity to variation of wave angle and beach slope is further investigated.     

An application of Miles' theory to Bragg scattering of water waves by doubly composite artificial bars

In the present paper, Miles' (1981) theory is implemented to derive formulae for describing the Bragg scattering of water waves for doubly composite artificial bars with different shapes, spacings, relative bar heights, relative bar footprint and the number of bars. The theory has clear advantage in estimating Bragg reflection coefficient for practical applications concerning coastal problems. Experiments of Bragg reflections over doubly composite rectangular artificial bars have also been performed in a wave flume. Key parameters that may lead to the optimal selection of a doubly composite artificial bar are studied. Theoretical solutions are seen to compare fairly well with the numerical computations and the laboratory experiments. Our simulated results reveal that the Bragg resonance for doubly composite artificial bars effectively increases the bandwidth of the reflection coefficient.     

Morphodynamics of intertidal bar morphology on a macrotidal beach under low-energy wave conditions, North Lincolnshire, England

The morphological changes of multiple intertidal bars (ridges) on a macrotidal beach were examined under low-energy wave conditions during a spring-to-spring tidal cycle. The morphological response was coupled to the tidal water level variations and related residence times for swash processes and surf (breaking waves and bores) over the cross-shore profile. Spring tides induced a large spatial variation in water lines and small residence times for distinct processes. Neap tides narrowed the intertidal area and increased the time for certain processes to work on the sediment at one location. The observed morphological changes could be coupled to the stagnation of processes at a certain bar crest position. The action of surf (breaking waves and bores) played the major role in the onshore migration of the intertidal bars and the simultaneous erosion of the seaward flank. Swash action, responsible for the generation and migration of intertidal bars in microtidal settings, was not the dominant process in causing the observed morphological changes. Intertidal ridges on macrotidal beaches cannot be considered swash bars as suggested by most previous investigations into these morphological features.     

双向单排配筋混凝土低矮剪力墙抗震性能试验研究

双向单排配筋混凝土低矮剪力墙适用于多层住宅结构。对4个原型的剪跨比为1.0配筋混凝土低矮剪力墙进行了低周反复荷载试验研究,包括1个双向双排配筋混凝土低矮剪力墙和3个双向单排配筋混凝土低矮剪力墙。其中1个双向单排配筋混凝土低矮剪力墙加设暗支撑,用以研究暗支撑对这种新型墙体的作用。在试验研究的基础上,对比分析了各剪力墙的刚度及其衰减过程、承载力、延性、滞回特征、耗能能力及破坏特征。试验表明,经过合理设计,这种双向单排配筋混凝土低矮剪力墙可以满足多层住宅结构抗震要求。     

Nearshore intertidal topography and topographic-forcing mechanisms of an Amazon-derived mud bank in French Guiana

The intertidal topography in the vicinity of the contact zone between a longshore-migrating Amazon-derived mud bank and the muddy terrestrial shoreline in French Guiana was defined from a combination of satellite-based SPOT images, airborne lidar data and high-resolution total station ground surveying of a 75,000 m² plot. The three approaches, at different scales, were carried out at different periods. Digital elevation models generated from these three techniques, however, converge in highlighting the topographic micro-scale (centimetre-scale) variability of the mud bank surface while showing meso- to macro-scale features that reflect the dominance of wave activity in mud bank mobilization and attachment to the terrestrial shoreline. These features are bar-like longshore forms that develop in the intertidal zone from the shoreward drift of gel-like mud that accompanies wave damping. The features progressively become consolidated through mud drying out associated with the formation of cracks that are important in mangrove colonization and ecological changes. Fluid-mud accumulations formed from high concentrations of mud trapped in the troughs behind these linear bar forms generate flat featureless surfaces that tend to mask topographic heterogeneity of the mud bank surface. Dewatering of these lower zones by progressive mud consolidation complements tidal water discharge in providing a mechanism for the formation of the numerous channels that dissect the linear bar features, especially in the upper intertidal contact zone with the terrestrial shoreline. This dissection in the upper intertidal zone generates an intricate topography that replaces the original linear bar forms. The innermost bar forms a ‘suture’ zone with the terrestrial shoreline. Reworking of this bar by high-energy waves may lead to mud dispersal over old terrestrial mangrove substrates, resulting in stifling of mangrove pneumatophores. Mud reworking at the narrow trailing edge of the mud bank in the subtidal and lower intertidal zones leaves behind a flat bed that will eventually be completely eroded by waves in the course of mud bank migration.