Observations of offshore bar decay: Sediment budgets and the role of lower shoreface processes

Long-term, net offshore bar migration is a common occurrence on many multiple-barred beaches. The first stage of the process involves the generation of a longshore bar close to the shoreline that oscillates about a mean position for some time, followed by a stage of net offshore migration across the upper shoreface, and finally a stage of decaying bar form through loss of sediment volume at the outer boundary of the upper shoreface. The phenomenon has been previously documented in the Netherlands, the USA, the Canadian Great Lakes, and in New Zealand, but our present understanding of the morphodynamic processes and sediment transport pathways involved in bar decay is limited. In this paper, long-term, net offshore bar migration is investigated at Vejers Beach, located on the North Sea coast of Denmark where offshore bar migration rates are of the order of 45–55 m a⁻¹. A wave height transformation model confirmed that the decay of the outer bar results in increased wave heights and undertow speeds at the more landward bar potentially causing this bar to speed up its offshore migration. The causes for outer bar decay were investigated through field measurements of sediment transport at the decaying bar and at a position further seaward on the lower shoreface. The measurements showed that a cross-shore transport convergence exists between the bar and the lower shoreface and that the loss of sediment involved in bar decay is associated with a longshore directed transport by non-surf zone processes. At Vejers, and possibly elsewhere, the net offshore migration of bars and the subsequent loss of sand during bar decay is an important part of the beach and shoreface sediment budget.     

Alternate bar development in an alpine river following engineering works

Formation and development of alternate bars in an engineered mountainous reach of the Arc River, France, is studied using photo analysis, 1D modelling and by applying theoretical and empirical models for alternate bar systems. Alternate bars already existed in the 80s in the form of a stable confined wandering system. In 1994, the river bed was flattened after engineering works. However, aerial photographs and cross-sectional profiles show that bars rapidly recovered within a few years. The alternate bar system evolved rapidly with a reduction of the number of bars and so an increasing bar length. The width-to-depth ratio, the slope change, the bend upstream of the reach, and the sediment supplies are the main controls of bar formation and evolution. The system appears to lead to force bars due to the bend but also due to a bridge in the downstream part of the reach. Nevertheless some free mobile bars are still observed in the middle of the reach. A discussion on the alternate bar formation is provided using empirical and analytical models. Finally, impacts of low flows and vegetation seem to be significant in the stabilization of the system toward a confined wandering system as observed before the engineering works.     

Field observations of nearshore bar formation

The formation of an inner nearshore bar was observed during a high‐energy event at the sandy beach of Vejers, Denmark. The bar accreted in situ during surf zone conditions and the growth of the bar was associated with the development of a trough landward of the bar. Measurements of hydrodynamics and sediment fluxes were obtained from electromagnetic current meters and optical backscatter sensors. These process measurements showed that a divergence in sediment transport occurred at the location of the developing trough, and observed gradients in cross‐shore net sediment flux were consistent with the morphological development. The main cause for the flux gradients were cross‐shore gradients in offshore‐directed mean current (undertow) speed which depended upon local relative wave height and local bed slope. Copyright © 2007 John Wiley & Sons, Ltd.     

双向单排配筋剪力墙节点抗震性能试验研究

双向单排配筋剪力墙结构适用于多层住宅结构。为了研究不同构造及不同连梁剪跨比的节点抗震性能,进行了5个连梁与墙肢节点的低周反复荷载试验。较系统地分析了其承载力、刚度及其退化过程、延性、耗能、破坏机制和破坏特征等。试验表明,经过合理设计,这种双向单排配筋剪力墙节点可以满足抗震要求。     

吐哈盆地WM油田辫状河三角洲前缘砂体分析

湖盆可容空间较小时,辫状河三角洲相对发育,水下分支河道横向迁移频繁,易形成席状分布的砂体 (水下分支河道复合体 )。可容空间增大时,辫状河三角洲相对不发育,水下分支河道延伸不远,河道砂体较孤立,河口坝相对比较发育。文中根据水流和波浪强弱提出九种水下分支河道与河口坝的关系图,基本上总结了吐哈盆地WM油田三间房组河口坝的形态;并对水下分支河道复合体的形成机制提出了独特的推理。     

Note on sediment sorting in a sandy bed under standing water waves

We present new quantitative data on the sorting of sediments on a sandy seabed under standing waves. Starting from a flat bed composed of a homogeneous mixture of a coarse and a fine sand with mean diameters 0.11 and 0.21 mm, we observed simultaneous ripple and sand bar formation and sand sorting on the seabed. Over days of wave action, sand bars formed with crests beneath the surface wave nodes and flat plateaus flanked by mounds beneath the antinodes. Bar crests were composed of sand coarser on average than 0.21 mm, while the flat plateaus were covered by sand finer on average than 0.11 mm. Comparison with two experiments involving sand beds of more homogeneous size distributions shows that the mounds are characteristic of the motion of fine suspensions.     

Channel bar dynamics on multi‐decadal timescales in an active meandering river

Occurrence and development of channel bars are major components of the morphodynamics of rivers and their relation to river meandering has been much explored through theory and experimentation. However, field and documentary data of characteristics and evolution over timescales from years to several decades are lacking. Four sets of aerial photographs in the period 1984–2007 were used to map and quantify bar numbers and areas in GIS on an active meandering reach. Bar types were classified. Additional temporal resolution was provided by annual ground photography and mapping for 1981–2010. Analysis was extended backward by use of large scale Ordnance Survey maps from 1873 onwards. As expected, point bars are the most common type but ‘free’ bars of several types are major components of bar deposition. Point bars and attached bars are significantly larger in size than mid‐channel and side bars. Spatial distribution of bars varies down the reach and over time but is related to channel sinuosity, gradient and mobility and to bend evolution. Different types of bar occur in distinctive channel locations, with point and concave‐bend bars in zones of high curvature. Bar activity shows a relation with discharge events and phases and possibly with changing riparian conditions, but superimposed on this is a common sequence of bar evolution from incipient gravel mid‐channel bars to full floodplain integration. This life‐cycle is identified as 7–9 years on average. No evidence for mobility of free bars within the course is found. The results are compared with bar and bend theory; the bars are forced and conform in general to bend theory but detailed variation relates to geomorphic factors and to autogenic sequences of bends and bars. Mid‐channel bars are width induced. Variability of bar occurrence needs to be taken into account in river management and ecological evaluation, including for the EU WFD. Copyright © 2011 John Wiley & Sons, Ltd.     

Sedimentary characteristics of large‐scale lacustrine beach‐bars and their formation in the Eocene Boxing Sag of Bohai Bay Basin,East China

The beach‐bar reservoir play has become an important exploration target within the Bohai Bay Basin, especially in the Boxing Sag within the Dongying Depression, where a large‐scale lacustrine beach‐bar oil pool has been discovered recently. The sedimentary characteristics, distribution patterns and formation mechanisms of beach‐bar sand bodies in the upper fourth member of the Eocene Shahejie Formation (Es4s) in the Boxing Sag were studied in detail based on seismic, well log data and core data. The Es4s in the Boxing Sag is composed of a third‐order sequence consisting of three systems tracts, i.e. a lowstand systems tract, a transgressive systems tract and a highstand systems tract. Beach‐bar sand bodies were deposited widely in the basin during the lowstand systems tract period. The sandy beach‐bars are characterized by siltstones, fine‐grained silty sandstones interbedded with thin mudstone units. The presence of well‐developed sedimentary structures, such as swash bedding, parting lineation, parallel bedding, ellipsoidal mud clasts, ripples, terrestrial plant debris and vertical burrows, suggests that beach‐bars were deposited in a relatively shallow water environment under the influence of strong hydrodynamics. Laterally, the sandy beach facies occurred as a more continuous sheet‐like body around the sandy bar in most parts of the sag. Stratigraphically, beach‐bars were distributed mainly in the lowstand systems tract and they were less well‐developed in the transgressive systems tract and highstand systems tract. Several factors were probably responsible for the occurrence of the large‐scale beach‐bars during the lowstand systems tract period, including: (i) a gentle palaeoslope and relatively weak structural activities; (ii) a shallow‐water condition with a strong hydrodynamic environment; (iii) high‐frequency oscillations of the lake level; and (iv) an abundant terrigenous clastic feeding system with multiple‐point and linear sediment sources.     

Cyclic behavior of precast segmental concrete bridge columns with high performance or conventional steel reinforcing bars as energy dissipation bars

The cyclic behavior of precast segmental concrete bridge columns with high performance (HP) steel reinforcing bars and that with conventional steel reinforcing bars as energy dissipation (ED) bars were investigated. The HP steel reinforcing bars are characterized by higher strength, greater ductility, and superior corrosion resistance compared with the conventional steel reinforcing bars. Three large‐scale columns were tested. One was designed with the HP ED bars and two with the conventional ED bars. The HP ED bars were fully bonded to the concrete. The conventional ED bars were fully bonded to the concrete for one column, whereas unbonded for a length to delay fracture of the bars and to increase energy dissipation for the other column. Test results showed that the column with the HP ED bars had greater drift capacity, higher lateral strength, and larger energy dissipation than that with fully bonded conventional ED bars. The column with unbonded conventional ED bars achieved the same drift capacity and similar energy dissipation capacity as that with the HP ED bars. All the three columns showed good self‐centering capability with residual drifts not greater than 0.4% drift. An analytical model referred to as joint bar‐slip rotation method for pushover analysis of segmental columns with ED bars is proposed. The model calculates joint rotation from the slip of the ED bars from two sides of the joint. Good agreement was found between analytical predictions and the envelope responses of the three columns. Copyright © 2010 John Wiley & Sons, Ltd.     

The bed morphology of upland single‐thread channels in semi‐arid environments: evidence of repeating bedforms and their wider implications for gravel‐bed rivers

Single‐thread, gravel‐bed streams of moderate slope in the northern Negev are characterized by three channel units: bars exhibit steeper than average slopes and poorly sorted mixtures of small–medium cobbles and coarse–very coarse pebbles; flats are associated with more gentle slopes and well‐sorted medium–fine pebbles and granules; and transitional units have intermediate slopes and grain size. In general, all three units are planar, span the full channel width and have well‐defined boundaries. Bars and flats are more common than the transitional units and alternate downstream for distances of several hundred metres, forming sequences that are reminiscent of the riffle–pool structure commonly observed in humid‐temperate gravel‐bed rivers. A notable contrast is the absence of significant bed relief: bars lack crests and flats lack depressions. The relative lack of bed relief in bar–flat sequences is attributed to the high rate of sediment supply from the sparsely vegetated hillslopes which promotes the infilling of depressions and to the erosion of crests under conditions of intense transport. This reduction of bed relief lowers channel roughness, which in turn increases flow velocity and, therefore, the ability of the channel to transmit the large sediment loads it receives. Although our analyses pertain to a semi‐arid river system, the results have wider implications for understanding the adjustment of channel bedform to high sediment loads in other fluvial environments. Copyright © 2012 John Wiley & Sons, Ltd.