Flow depth,velocity, and sediment motions in a straight widened channel with vegetated floodplains

Environmental Fluid Mechanics - Experiments were conducted in a straight channel that widens from a single channel to a compound channel based on a real navigation channel with a similar shape in a...     

Advent of sheet flow in suction affected alluvial channels

The presence of suction (flow of water from channel to ground water) affects the channel hydrodynamics and increases the bed shear stress. At high bed shear stress in alluvial channels made of the non-cohesive material, sediment transport occurs as sheet flow layer of high sediment concentration. The sediment transport in the form of sheet flow has been observed in the present study when suction was applied to the non-transporting channels designed on incipient motion condition. The erosion of the channel banks contributed to the sheet flow because of the increased channel bed shear stress. An empirical relation for the thickness of sheet flow layer has been developed which includes suction as independent parameter along with others.     

Vegetation cover influence on the morphology and migration patterns of river mouths

The present study focuses on the effects of vegetation cover changes on the dynamic morphology of seven southeastern Mediterranean river mouths. The methodology used comprised monitoring and mapping by GIS techniques, with data derived from historic aerial photographs, which were applied in the investigation of the morphological spatial and temporal migration patterns of the mouths, and subsequent analysis of the vegetation cover changes influencing them. Vegetation cover adjacent to river mouths influences river mouth morphology through five primary mechanisms: a) bank vegetation; b) dune advancement toward the shoreline; c) changes in the beach??s micro-topography; as well as d) long-term continuous channel migration through permanent vegetation patches; and e) channel switching through permanent vegetation patches. The five mechanisms are part of a system of interactions between channel water flow and fluvial processes; coastal sediment transport and coastal processes; and the evolution of plant communities. In the interplay between these factors they all affect and are being affected by one another. In many river mouths artificial channel diversion is often needed due to uncontrolled channel migration. It is demonstrated that vegetation cover can serve as a mean of ??soft?? channel regulation. Therefore, a better understanding of the five influencing mechanisms may aid in controlling and managing river mouth migration patterns. The study contributes to the knowledge about bank vegetation as a tool of ??soft?? channel regulation and thus can contribute to the improvement of coastal zone management.     

菊酯类杀虫剂抗性棉铃虫para型钠离子通道基因的部分序列测定与突变

昆虫神经系统para型钠离子通道是菊酯类杀虫剂的主要靶标，对10多种昆虫的研究表明，钠离子通道基因发生点突变与昆虫对菊酯类杀虫剂的抗性密切相关．通过RT-PCR扩增的方法，我们获得了编码溴氰菊酯抗性和敏感品系棉铃虫钠离子通道域Ⅱ-Ⅳ的cDNA片段．核苷酸序列及推导的氨基酸序列分析结果表明，溴氰菊酯抗性品系棉铃虫钠离子通道基因不存在其它昆虫中报道的kdr和super-kdr抗性突变，但是我们发现了一个甲硫氨酸(M)到亮氨酸(L)的新突变，该突变位于域Ⅱ和Ⅲ之间．由于昆虫钠离子通道高度保守，因此，该突变很可能与棉铃虫对溴氰菊酯的抗性有关．另外，通过PCR扩增的方法，克隆了棉铃虫钠离子通道域Ⅱ-Ⅳ的基因片段，该片段全长7334bp，包含有13个外显子和12个内含子，与果蝇和烟蚜夜蛾相比，内含子在基因中的位置基本一致，进一步表明昆虫钠离子通道基因在进化过程中高度保守．图5表1参22     

Curvature-induced secondary flow in submarine channels

The curvature-driven secondary flow in sinuous submarine channels has been a subject of considerable interest and controversy. Here, results from numerical model studies involving saline flow in laboratory-scale channels are presented. A 3D finite volume model of density and turbidity currents is used and simulations are run with different inflow discharges and channel-axis slopes. The simulation results show strong influence of bend wave length, channel gradient, confinement and cross sectional shape on the structure of secondary flow in submarine channels. Major findings are: (i) reversal of secondary flow in submarine channels is strongly associated with a tight bend characterized by a smaller wave length to width ratio or larger wave number, (ii) for the same inflow condition and planform characteristics, a trapezoidal channel cross section is more favorable to secondary flow reversal than a rectangular cross section, (iii) lateral convection resulting from the interaction between in-channel and overbank flows leads to the reversal of secondary flow in an unconfined channel at a lower channel slope than in a confined channel with the same dimensions, (iv) flow discharge has only nominal effect on the secondary flow in submarine channels.     

Morphology of submarine channel-levee systems in the eastern Bay of Bengal near Andaman region

Multibeam echo-sounder (MBES), sub-bottom profiler, singlebeam tracks and public-domain bathymetry geophysical datasets were analyzed in a Geographic Information System (GIS) for accuracy assessment and further used to update the previous map (Curray et al. Mar Pet Geol 19: 1191–1223, 2003) of submarine channel-levee systems in the eastern Bengal Fan near the Andaman region. Comparative analysis of singlebeam with MBES indicated 2.8 % of root-mean-square difference in bathymetry, and could clearly bring out distinct submarine channel profiles. The channel map revealed that the previously named E6 [updated as E7(1a)] and E7 [updated as E7(1b)] channels emanate from the middle and lower Bengal Fan, respectively and merge together at 91.28°E, 10.64°N where they form a second order channel [E7(2)] that flows southward along the east of Ninety-east Ridge. Similarly, two first order channels [E6(1a) and E6(1b)] emanate from the lower Bengal Fan which merge together at 88.78°E, 06.92°N to form a second order channel [E6(2)] that flows along the west of Ninety-east Ridge towards the equator. This channel was previously thought to be linked to the middle Bengal Fan. High-resolution MBES tracks over the channel shows extensive meandering patterns with the channel gently deepening and widening towards south. Variations in the morphology and stratigraphy indicate that the E7 channel is characterized by a curvilinear course, with higher observed sinuosity wherever gradient decreases.     

Assessment of the Environmental Risk Posed by an Exotic Fish: The Proposed Introduction of Channel Catfish (Ictalurus punctatus) to New Zealand

Abstract: In 1987, a proposal was made to introduce channel catfish (Ictalurus punctatus) into New Zealand for aquaculture. An environmental impact assessment in support of the proposal incorporated details of channel catfish biology, its value to people, diseases, the history of introductions elsewhere, and possible impacts on New Zealand biota should the species become established in the wild. Although information on the environmental impacts of channel catfish introduced to other countries was limited and inadequate for assessing potential effects on freshwater ecosystems in New Zealand, a permit was granted to import fertilized eggs. These were hatched in quarantine, and subject to the favorable outcome of environmental trials were to be released for aquaculture. Subsequently, the requirement for trials was abandoned and instead an independent, two-man review team (the authors) was appointed to advise the Minister of Fisheries on whether the environmental risk posed by channel catfish was acceptable. The team considered the impact assessment and additional submissions from interested parties. The team concluded that there was a high probability that fish would escape and that they would be capable of breeding and growing in a wide range of freshwater environments throughout much of New Zealand. Because channel catfish are omnivorous, they would probably prey on at least some native fish species and introduced salmonids, invertebrates including freshwater crayfish, and aquatic macrophytes. Competition to the detriment of other fishes was also considered likely. Furthermore, the low species diversity and ecological simplicity of fish communities in New Zealand could allow channel catfish opportunities not found within their natural range, as in parts of western North America where they have had significant impacts on freshwater communities. Because the evidence indicated that one or more valued species was likely to suffer a decline in abundance or distribution if channel catfish were introduced, the review team recommended that the environmental risk posed by the fish was unacceptable. The recommendation was accepted by the Minister of Fisheries and all the channel catfish held in quarantine were destroyed. This case clearly demonstrated weaknesses in the importation procedure in New Zealand and subsequently some changes have been made. In general, we recommend a four-step importation procedure comprising (1) environmental impact assessment, (2) publicity, (3) independent review, and (4) implementation. We suggest that the second step is most crucial.     

Laboratory study investigating the regeneration potential of iron particles by and the hydrodynamics of a dam-break generated flow from an infinite reservoir into a channel with an adverse slope

To determine the feasibility of using a dam-break generated flow from the sea into a storm-drain to aid in the regeneration of iron particles that control the production of H₂S in the storm-drain, a laboratory experimental investigation is carried out to measure the regeneration potential and the detailed hydrodynamics of the dam-break generated flow that causes the regeneration. The experiments are carried out using a reservoir of essentially infinite size, the sea, and a channel of limited width and adverse slope 1:20, the storm-drain. The regeneration experiments confirmed the ability of the dam-break generated flow to aid in the regeneration of the iron particles, however the regeneration potential varies from good to poor with distance away from the gate into the channel. The detailed measurements of the hydrodynamics highlighted that the dam-break generated flow from an infinite reservoir diverges little during the first uprush, has much smaller velocities during the first backwash and includes significant free surface waves. An initially wet channel bed reduces the flux into the channel. Close to the gate the flow depth increases more quickly but the velocity, and therefore the regeneration potential, is smaller.     

First signs of changes to a tropical lagoon system in the southeastern Brazilian coastline

Coastal lagoons have long been subjected to anthropogenic disturbances due to the increasingly urbanization pressure along the margins. As a result, the composition, structure and distribution of macrozoobenthic communities are strongly affected by environmental gradients and harsh ecological conditions. This study address the first signs changes in the Piratininga-Itaipu lagoon system, located in the southeastern Brazilian coastline, state of Rio de Janeiro, after the opening of a tidal channel in the most deteriorated area of Piratininga lagoon in April 2008, as part of restoration measures concerning the severe eutrophication of this system. In order to observe changes in environmental conditions and community structure before and after the channel opening, sampling of the benthic macrofauna and abiotic variables were conducted in seven sites, from August 2007 to March 2009. According to statistical tools, no significant differences were shown in both environmental characteristics and community structure before and after the channel at the distinct locations. Nevertheless, local changes in the abundance and composition of species were detected particularly in Piratininga, in the site where the tidal channel was constructed. A remarkable loss in abundance of most opportunistic species and the appearance of six new taxa in Piratininga after the opening of the channel was related to habitat regeneration in virtue of the intrusion of marine waters into the lagoon. However, marine inflow seems to be restricted since no clear evidence of the channel influence was felt in the remaining sites of Piratininga and in Itaipu. Macrobenthic communities were heterogeneous between both lagoons throughout the sampling period, characterized by low diversity and evenness indices. A general increase in salinity and redox potential were observed in Piratininga. Both variables allied to dissolved oxygen appeared to play a major role in structuring the benthic communities. This study is the first contribution to knowledge of taxonomic biodiversity of macrozoobenthic assemblages of Piratininga-Itaipu lagoon system and pioneer in describing apparent ecosystem ameliorations after the channel opening. Therefore, continuous monitoring ought to be carried out in order to evaluate the effectiveness of the tidal channel and follow further recovery of this ecosystem. Macrobenthos were quite heterogeneous between both lagoons throughout the sampling period, with biological attributes showing a remarkable loss in species abundance, mostly opportunistic. The appearance of new taxa, Polydora cornuta, after the opening of the channel was related to improvement of better oxygenation of water and sediments, causing regeneration of habitat. Other resilient taxa with persistant bioturbation activities, such as Allita, Streblospio, Capitella and oligochaetes, dominated the early stages of recovery. This study is the first contribution to knowledge of taxonomic biodiversity of macrozoobenthic assemblages of Piratining-Itaipu lagoon system and pioneer in describing apparent ecosystem ameliorations after the channel opening. Therefore, continuous monitoring ought to be carried out in order to follow further recovery of this ecosystem.     

用大型底栖动物和ODP系统评价珠江的有机污染

采用大型底栖动物需氧有机体百分率ODP(oxygen demander percentage)法对广州珠江前航道、西航道和流溪河的下游段进行河流有机污染评价.结果显示:底栖动物需氧类群密度在三河段间分布确有显著性差异,并根据其ODP可以判断流溪河水质相对较好,水质级别为中国地表水环境质量标准(EQSSW)Ⅳ级,西航道和前航道水质级别都为Ⅴ级.通过测试,这一方法能成功地应用在珠江及流溪河,且该法可以较好地匹配于EQSSW五级评价系统,初步认为ODP系统可以成为一个较好的河流水质生物监测方法.图3表4参13     

Analysis of the velocity field in a large rectangular channel with lateral shockwave

In this work the authors describe the main characteristics of the velocity field of hydraulic jumps in a very large channel where lateral shockwaves occur. Experiments were carried out at the Coastal Engineering Laboratory of the Water Engineering and Chemistry Department of the Technical University of Bari (Italy). Extensive flow velocity measurements were investigated in order to have a clearer understanding of both hydraulic jump development and lateral shockwave formation in a very large channel. Eight experiments were performed in a 4m wide rectangular channel; the experiments differed in the inlet Froude number F ₀ and the jump type. Seven tests were carried out with undular jumps and one with a roller jump. The flow velocity and the flow free surface measurements were taken using a two-dimensional Acoustic Doppler Velocimeter (ADV) and an ultrasonic profiler, respectively. The experimental results can be summarized as follow: (i) the formation of well developed lateral shockwaves similar to those of oblique jumps were observed; (ii) the comparison of the experimental and theoretical data shows that the classic shockwave theory is sufficiently confirmed in the analyzed range of Reynolds number, taking into account the experimental errors and the difference between the theoretical and experimental assumptions; (iii) the transversal flow velocity profiles in the recirculating zone show a good agreement with the numerical simulations presented in literature in the case of a separated turbulent boundary layer over a flat plate. This conclusion enables us to confirm the hypothesis that the lateral shockwaves in the channel are the result of a boundary layer which, as observed, forms on the channel sidewalls.     

五里塘生态农场有机废弃物的资源化生态工艺

上海市松江县五里塘生态农场以沼气工程、氧化沟和鱼塘围网养鸭为主的生态工程,有效地利用了农场产生的大量禽畜粪便并解决了沼液,沼渣的二次污染问题;在沼气池上建玻璃温室保温,使沼气池能正常越冬从而得以周年运转.生态工程的建立,取得了良好的经济、生态和环境效益.     

Determination of the Manning roughness coefficient influenced by vegetation in the river Aa and Biebrza river

The aim of this study was to investigate the variation of channel bed roughness in two rivers, as important parameter in hydraulic modelling especially with regard to flood control. The universities of Ghent (UG) and Antwerp (UA) are conducting scientific research in the river Aa in Belgium and the Biebrza river in Poland in order to better understand the phenomena involved and to come to a more accurate determination of the different parameters influencing flow. In this paper, the determination of the roughness coefficient ‘n’ from the Manning equation is used. This coefficient is not easy to determine and is varying constantly. It is influenced by the meandering character of the river, the bed material and the average grain size, the channel bed forms, the channel obstructions, the geometry changes between sections and the vegetation in the channel. Furthermore, due to these parameters, the roughness of the channel is not equally distributed over the channel, the banks and the floodplains. So, using literature data does not always lead to satisfactory results, due to the different situation in the field (Werner et al. J Hydrol 314:139–157, 2005). Therefore, measurements are necessary to determine the variation of the Manning coefficient. The Manning coefficient is a function of the discharge, but will also vary over the time due to the mentioned influences. In a multidisciplinary research project on the fundamental exchange processes in river ecosystems, hydraulic measurements were performed on a regular base in the river Aa. During these measurement campaigns, velocity and discharge measurements were carried out in multiple cross-sections. Once a month, the discharge and the water levels were measured at the upstream and the downstream end of the test stretch. On the river Biebrza, similar intensive measurement campaigns took place along a 6 km stretch in the upstream part of the river. An accurate determination of the Manning coefficient according a seasonal variation is an important tool in hydraulic modelling.     

Hydrodynamics and bed stability of open channel flows with submerged foliaged plants

In this work we investigate experimentally and numerically the flow structure around foliaged plants deployed in a channel with gravels on the bed under submerged and barely submerged conditions. Velocity and Reynolds stress were measured by using a NORTEK Vectrino profiler. Visual observation shows that the initial motion of gravels is easier to be triggered under the condition of flow with barely submerged vegetation. This is confirmed by the measured velocity, Reynolds stress and total kinetic energy (TKE) profiles. The velocity exhibits a speed up in the near-bed region, and the associated Reynolds stress and TKE increase there. A 3D numerical model is then verified against the experiments and used to investigate systematically the effect of degree of submergence of foliaged plants on the channel bed shear stress. The results show that the maximum bed shear stress occurs when the foliage is situated slightly below the water surface, which can enhance channel bed instability.     

Front velocity and structure of bottom gravity currents with a low volume of release propagating in a porous medium

The paper reports results of large eddy simulations of lock exchange compositional gravity currents with a low volume of release advancing in a horizontal, long channel. The channel contains an array of spanwise-oriented square cylinders. The cylinders are uniformly distributed within the whole channel. The flow past the individual cylinders is resolved by the numerical simulation. The paper discusses how the structure and evolution of the current change with the main geometrical parameters of the flow (e.g., solid volume fraction, ratio between the initial height of the region containing lock fluid and the channel depth, ratio between the initial length and height of the region containing lock fluid) and the Reynolds number. Though in all cases with a sufficiently large solid volume fraction the current transitions to a drag-dominated regime, the value of the power law coefficient, α, describing the front position’s variation with time (x _f  ~ t ^α , where t is the time measured from the removal of the lock gate) is different between full depth cases and partial depth cases. The paper also discusses how large eddy simulation (LES) results compare with findings based on shallow-water equations. In particular, LES results show that the values of α are not always equal to values predicted by shallow water theory for the limiting cases where the current height is comparable, or much smaller, than the channel depth.     

Modeling the longitudinal profiles of streamwise velocity in an open channel with a model patch of vegetation

This paper proposes a model for predicting the longitudinal profiles of streamwise velocities in an open channel with a model patch of vegetation. The governing equation was derived from the momentum equation and flow continuity equation. The model can estimate the longitudinal profiles of velocities both inside and outside a vegetation patch. Laboratory experiments indicate that the longitudinal profiles of velocities inside a patch and in the adjacent bare channel have the same adjustment distance in the longitudinal direction, but the profiles have different trends because the vegetation drag drives the flow from the patch to the adjacent bare channel. The model considers different dimensionless parameters in two flow adjustment regions upstream of and inside the patch. Sixteen sets of experimental data from different sources are used to verify the model. The model is capable of modeling the longitudinal profiles of velocities inside and outside patches of cylinders or cylinder-like plants. Compared to a previous model, the current model improves the modeling accuracy of longitudinal profiles of velocities.

     

Evidence supporting the importance of terrestrial carbon in a large-river food web

Algal carbon has been increasingly recognized as the primary carbon source supporting large-river food webs; however, many of the studies that support this contention have focused on lotic main channels during low-flow periods. The flow variability and habitat-heterogeneity characteristic of these systems has the potential to significantly influence food web structure and must be integrated into models of large-river webs. We used stable-isotope analysis and IsoSource software to model terrestrial and algal sources of organic carbon supporting consumer taxa in the main channel and oxbow lakes of the Brazos River, Texas, USA, during a period of frequent hydrologic connectivity between these habitat types. Standardized sampling was conducted monthly to collect production sources and consumer species used in isotopic analysis. Predictability of hydrologic connections between habitat types was based on the previous 30 years of flow data. IsoSource mixing models identified terrestrial C3 macrophytes (riparian origin) as the primary carbon source supporting virtually all consumers in the main channel and most consumers in oxbow lakes. Small-bodied consumers (<100 mm) in oxbow lakes assimilated large fractions of algal carbon whereas this pattern was not apparent in the main channel. Estimates of detritivore trophic positions based on delta15N values indicated that terrestrial material was likely assimilated via invertebrates rather than directly from detritus. High flows in the river channel influenced algal standing stock, and differences in the importance of terrestrial and algal production sources among consumers in channel vs. oxbow habitats were associated with patterns of flooding. The importance of terrestrial material contradicts the findings of recent studies of large-river food webs that have emphasized the importance of algal carbon and indicates that there can be significant spatial, temporal, and taxonomic variation in carbon sources supporting consumers in large rivers.     

Experimental study of recirculating flows generated by lateral shock waves in very large channels

Due to the lack of data on hydraulic-jump dynamics in very large channels, the present paper describes the main characteristics of the velocity field and turbulence in a large rectangular channel with a width of 4 m. Although a hydraulic jump is always treated as a wave that is transversal to the channel wall, in the case of this study it has a trapezoidal front shape, first starting from a point at the sidewalls and then developing downstream in an oblique manner, finally giving rise to a trapezoidal shape. The oblique wave front may be regarded as a lateral shockwave that arises from a perturbation at a certain point of the lateral wall and travels obliquely toward the centreline of the channel. The experimental work was carried out at the Coastal Engineering Laboratory of the Water Engineering and Chemistry Department of the Technical University of Bari (Italy). In addition to the hydraulic jump formation, a large recirculating flow zone starts to develop from the separating point of the lateral shock wave and a separate boundary layer occurs. Intensive measurements of the streamwise and spanwise flow velocity components along one-half width of the channel were taken using a bidimensional Acoustic Doppler Velocimeter (ADV). The water surface elevation was obtained by means of an ultrasonic profiler. Velocity vectors, transversal velocity profiles, turbulence intensities and Reynolds shear stresses were all investigated. The experimental results of the separated boundary layer were compared with numerical predictions and related work presented in literature and showed good agreement. The transversal velocity profiles indicated the presence of adverse pressure gradient zones and the law of the wall appears to govern the region around the separated boundary layer.     

Steady open channel flows with curved streamlines: the Fawer approach revised

Rapidly varied open channel flows are characterized by curvilinear streamlines, thereby resulting in a pressure field different from the hydrostatic approach proposed in the standard gradually varied flow theory. This problem is related to environmental hydraulic problems such as the undular hydraulic jump and flow over round-crested weirs, for which streamline curvature effects are significant. The inclusion of the curvilinear streamline effect in an extended energy equation was firstly by Fawer. Most of the extended energy equations currently employed are therefore modified forms of the original Fawer approach. The aim of the present study is to highlight and remind engineers of the outstanding theory presented by Fawer. Herein, his approach for steady open channel flow with curved streamlines is revised and compared with experimental observations. Computational methods are presented in detail and based on present results, it can be observed that more recent and complex models for these problems are similar to the original proposal of Fawer, and hardly more accurate in some instances. Based on the proposed study an useful framework for theoretical models for steady open channel flows with curved streamlines is proposed.     

Characteristics of flow structure of free-surface flow in a partly obstructed open channel with vegetation patch

Free-surface flows over patchy vegetation are common in aquatic environments. In this study, the hydrodynamics of free-surface flow in a rectangular channel with a bed of rigid vegetation-like cylinders occupying half of the channel bed was investigated and interpreted by means of laboratory experiments and numerical simulations. The channel configurations have low width-to-depth aspect ratio (1.235 and 2.153). Experimental results show that the adjustment length for the flow to be fully developed through the vegetation patch in the present study is shorter than observed for large-aspect-ratio channels in other studies. Outside the lateral edge of the vegetation patch, negative velocity gradient (\(\partial \overline{u}/\partial z < 0\)) and a local velocity maximum are observed in the vertical profile of the longitudinal velocity in the near-bed region, corresponding to the negative Reynolds stress (\(- \overline{{u^{\prime}w^{\prime}}} < 0\)) at the same location. Assuming coherent vortices to be the dominant factor influencing the mean flow field, an improved Spalart–Allmaras turbulence model is developed. The model improvement is based on an enhanced turbulence length scale accounting for coherent vortices due to the effect of the porous vegetation canopy and channel bed. This particular flow characteristic is more profound in the case of high vegetation density due to the stronger momentum exchange of horizontal coherent vortices. Numerical simulations confirmed the local maximum velocity and negative gradient in the velocity profile due to the presence of vegetation and bed friction. This in turn supports the physical interpretation of the flow processes in the partly obstructed channel with vegetation patch. In addition, the vertical profile of the longitudinal velocity can also be explained by the vertical behavior of the horizontal coherent vortices based on a theoretical argument.