Grain size and topographical differences between static and mobile armour layers

A series of laboratory flume experiments under conditions of sediment starvation (zero sediment feeding) and recirculation were conducted in order to identify the temporal evolution and surface properties of static and mobile armour layers. The experiments were carried out in an 8 m long flume using a bimodal grain‐size mixture (D₅₀ = 6·2 mm) and a range of shear stresses ranging from 4·0 to 8·6 N m^–2. The results confirm that a static armour layer is coarser than a mobile one, and that the grain size of a mobile armour layer is rather insensitive to changes in the imposed flow strength. An analysis of laser scan bed surveys revealed the highly structured and imbricated nature of the static armour layer. Under these conditions the vertical roughness length scale of the bed diminished and it became topographically less complex at higher forming discharges. The topography of mobile armour layers created by rising discharges differed. They exhibited a greater roughness length scale and were less organized, despite the fact that the grain size of the surface material maintained an approximately constant value during recirculation. Also, the mobile armour tended to create larger cluster structures than static armour layers when formed by higher discharges. These differences were mainly due to the transport of the coarser fraction of bed sediments, which diminished to zero over the static armour because of being hidden within the bed, whereas in the mobile armour the coarser particles protruded into the flow and were actively transported, increasing the vertical roughness length scale. Overall, the results show that an examination of the grain size characteristics of armour layers cannot be used to infer sediment mobility and bed roughness. Detailed elevation models of exposed surfaces of gravel‐bed rivers are required to provide critical insight on the sediment availability and sedimentation processes. Copyright © 2011 John Wiley & Sons, Ltd.     

上海陆域古河道分布及对工程建设影响研究

上海地区第四纪时期古河道发育,其中晚更新世以来的古河道对工程建设影响最大.根据大量钻孔及测试资料,研究了晚更新世末期古河道、全新世晚期吴淞江古河道和近代古河道的成因、分布规律及沉积物特征.在此基础上,结合上海市工程建设特点,系统分析了古河道对工程建设,尤其是地下空间开发的影响,成果可为工程勘察、设计、施工提供基础地质依...     

The Spey Bay geomorphological study

The Spey Bay coastline and the estuary of the River Spey exhibit a rapidly evolving planform. They provide an opportunity to study geomorphological changes in a coupled coastal/estuarine system which can be measured over hours and days rather than the years and decades associated with more slowly evolving landscape features. Attempting to understand coastal and estuarine geomorphology provides engineers and earth scientists with an exciting intellectual challenge. This is not, however, simply an academic challenge because the successful management of coastal and estuarine environments depends on improving this understanding. The impetus behind our work in this area is the protection of land-use interests. It is in this context that Babtie Dobbie Limited have been employed in studying the River Spey since 1962. This paper is based on a practical case study performed for coastal and river engineering purposes between 1990 and 1992. The case study was formulated using the results and conclusions of earlier geomorphological studies of the area undertaken over the past 32 years. Knowledge gained from previous studies was crucial in designing the detailed methods and techniques employed in the 1990–1992 geomorphological study. Methods used included historical analyses, photogrammetry, numerical modelling, geographic information systems (GIS) and interpretation of the complex processes which drive landform changes in the estuary and along the adjacent shoreline. The improved understanding of geomorphological processes that has been achieved has enabled engineers to formulate a new management strategy. Central to this strategy are engineering solutions to river and coastal stability and flooding problems which are in sympathy with the environment and which take maximum advantage of our understanding of geomorphological processes.     

Water quality in rivers of western Switzerland: Application of an adaptable index based on benthic invertebrates

Water quality was estimated from 205 samples of benthic invertebrates collected between 1982 and 1986 in 51 rivers of western Switzerland (canton of Vaud). Each sample consisted of the combined list of taxa resulting from one spring sample pooled with one summer sample. Water quality was indicated by total number of taxa and number of taxa intolerant of pollution: i. e. Heptageniidae, Plecoptera, and Trichoptera with a case. Six classes of values were delimited for each of these two variables by cluster analysis. Values from zero to five were attributed to each class. The index of water quality was computed by adding these two values in each sample. According to this index, good water quality was indicated by 42% of samples. This index can be adapted to other rivers because its components are easily modified.     

The influence of microform bed roughness elements on flow and sediment transport in gravel bed rivers: Comment on a paper by marwan A. Hassan and ian reid

Data from Turkey Brook are used to demonstrate that the interaction between gravel bedforms, flow resistance and bedload transport is a dynamic one, both between and within hydrographs. and that creation of a significant component of form drag through construction of microforms (pebble clusters) may precede the eventual break-up of the bed in a transport event. This process of drag augmentation', which can be seen as a feedback mechanism delaying transport and can be likened to dilation of a soil tested in a direct shear apparatus, itself appears to be dependent on the characteristics of turbulence, and therefore involves feedback at a finer resolution than envisaged by Hassan and Reid (1990).     

Major ion chemistry of the Ganga source waters: Weathering in the high altitude Himalaya

A systematic study of the major ion chemistry of the Ganga source waters—the Bhagirathi, Alaknanda and their tributaries—has been carried out to assess the chemical weathering processes in the high altitude Himalaya. Among major ions, Ca, Mg, HCO₃ and SO₄ are the most abundant in these river waters. These results suggest that weathering of carbonate rocks by carbonic and sulphuric acids dominates in these drainage basins. On an average, silicate weathering can contribute up to ∼ 30% of the total cations. The concentration of total dissolved salts in the Bhagirathi and the Alaknanda is 104 and 115mg/l, respectively. The chemical denudation rate in the drainage basins of the Bhagirathi and the Alaknanda is, respectively, 110 and 137 tons/km²/yr, significantly higher than that derived for the entire Ganga basin, indicating intense chemical erosion of the Himalaya.     

Velocity-reversal hypothesis: A model approach

Hydraulic modelling of a riffle-pool-riffle sequence in a straight reach of Dry Creek near Winters, California, indicates that while at low flow the mean (section average) velocity at riffles exceeds that of an adjacent pool, during high flow the mean velocity of the pool exceeds that of adjacent riffles. This supports the velocity-reversal hypothesis which is based on limited field measurements of near-bed flow velocity. A velocity reversal does not occur at the same discharge in both riffle-pool sequences modelled, and a more detailed explanation is necessary to account for complex aspects of channel form and process in riffle-pool sequences. Nevertheless, the velocity-reversal hypothesis helps to explain many of the observations and processes that operate in the riffle-pool environment.     

Variability of bedload transport and channel morphology in a braided river hydraulic model

This paper investigates variability in bedload transport and channel morphology for 11 replicate experimental runs in an approximately 1:50 braided river model. The experiments, each of 90 h duration, were carried out in a 20 × 3m tilting flume. All the experiments started with the same initial conditions. Bedload transport was measured at 5 min intervals in a collection drum at the exit from the flume. The model showed reasonable hydraulic similarity when compared to prototype rivers. Results show that mean bedload transport rates for the 11 runs vary in the range 0·98 to 1·49gs^?1 (mean + 1·21, coefficient of variation 11 per cent). Within-run transport rates commonly vary from close to zero, to two and occasionally three or four times the mean rate. Within the bedload series, several irregular phases of transport intensity can be observed, but time series analysis of the data show little underlying serial structure (an AR(2) autoregressive model is appropriate). Channel patterns are narrow/braided, are established quickly and remain relatively stable throughout the runs, although channel widths increase between 20 and 103 per cent over the 11 runs. Channel behaviour varies from aggradational to transitional between aggradation and degradation. Time-averaged bedload transport rate is weakly correlated with braiding intensity. In general, these results demonstrate that for a given set of controlling variables, bedload transport and channel morphology can be approximately replicated.     

傍河水源地勘探方法的探讨

傍河取水已经成为我国某些城市缓解水资源紧缺状态的一种手段。本文讨论了傍河取水的理论根据和勘探步骤,井提出了相应的评价方法。     

Application of a new river classification scheme to Australia's tropical rivers

A revised typology of Australian tropical rivers was applied to the complete channel network (named and major rivers) shown on 1:250 000 topographic maps for three large drainage basins in northern Australia (Daly River, NT; Fitzroy River, WA; Flinders River, Qld). Reach mapping and classification were conducted using the revised typology. The 12 major river types proposed were: (1) bedrock rivers; (2) bedrock‐confined and ‐constrained rivers; (3) low sinuosity (straight) rivers; (4) meandering rivers; (5) wandering rivers; (6) anabranching rivers; (7) chains of ponds; (8) gullies; (9) floodouts; (10) lakes, swamps, billabongs and wetlands; (11) non‐channelized valley floors; and (12) estuarine rivers. The 12 major river types were developed based on river reach mapping for more than 264 000 km² of tropical Australian catchments. At scales larger than 1:250 000, subdivision of each major river type is recommended. In the Daly and Fitzroy catchments, confined and constrained rivers dominate, whereas in the Flinders and Fitzroy catchments, anabranching rivers dominate. The dominant river types need benchmarking with adequate numbers of control reaches so that channel changes induced by human and natural impacts can be measured by reference to the stability of these controls. Wandering rivers, floodouts and non‐channelized valley floors were rare for the 1:250 000 channel network in northern Australia but need inclusion in national parks.