Dike Field Research Articles

Mechanisms of influence of confluence containing spur-dike on microplastic transport and fate

The issue of microplastic pollution is an urgent and undeniable global ecological concern. Due to the intricate three-dimensional (3D) hydrodynamic properties, the confluence is highly susceptible to microplastic contamination, particularly in the presence of spur dike disturbance. The present research goal is to elucidate the underlying mechanism governing the impact of the spur dike on transport and retention fate of microplastic at the confluence. A 3D hydrodynamic-microplastic transport model was developed at the confluence, and the Computational Fluid Dynamics (CFD) coupled with Discrete Element Method (DEM) method was employed to conduct the simulation. The concentration distribution, residence time, and trajectory of microplastics with different sizes and materials, as well as the effect of various angles of the spur dike on them were explored. The results indicate that (i) Large quantities of polyamide and polystyrene microplastics (less than 1 mm in diameter) with a residence time greater than 70 s are present in the flow stagnation zone, flow separation zone, the vicinity of spur dike, and downstream of the flow separation. (ii) The layout of the spur dike will lead to an increase in microplastic concentrations upstream of the spur dike and downstream of the flow separation. (iii) Vorticity and mixing layer thickness play crucial roles in the retention of microplastics within the flow separation and spur dike field. (iv) The dimensionless angles of the spur dike (αN) exhibit a linear and inverse relationship with the dimensionless standard deviations (σN) within the spur dike field. This study will expand our understanding of the transport and retention mechanisms of microplastics, and provide valuable insights for accurately identifying pollution hotspots of microplastic pollution at confluences with spur dikes.

Study of the effect of spur dikes on beach protection based on physical model experiment

The Qiantang River is a typical strong tide estuary, the world-famous tidal bore have huge turbulent energy and causes damage to the sea wall. Spur dike is an important engineering to prevent the seawall foundation from tidal bore. But the tidal bore occurs at the low tide level, strong turbulence, high flow velocity, water level rises sharply. The spur dike height, length, inclination angle need to be determined by physical model according to the specific bore dynamic conditions. In the tests, the fixed-bed model was used to study the tidal current velocity reduction rate and region, the falling current circulation characteristics in dike field. The movable-bed model test is used to study the effect of each test on protecting the beach. This physical model study reveals the rule of spur dike influence on tidal bore dynamics so the effect of spur dike on protecting the beach is clear.

Hydro-morphological behavior around T-shaped spur dikes with downward seepage

The present work experimentally analyses the flow behaviour near the T-shaped spur dike field with no seepage, 5%, and 10% downward seepage. Experiments were aimed at analysing the channel morphology with different discharges. According to the results, downward seepage movement causes significant modification in the channels bed elevation and the development of scour depth. The maximum scour depth is observed at the edge of the first spur dike facing the flow. The rate of scouring also increases with the effect of seepage. Due to downward seepage, the flow distribution is shifted near the channel bed. However, near the channel boundary attained some velocity, significantly enhancing the sediment transport rate. The wake zone between the spur dikes saw very low-velocity magnitudes of positive and negative values. This reveals secondary current generation inside the loop and cross-stream circulation. With an increment of seepage percentage, the velocity, Reynold shear stress, and turbulent kinetic energy magnitude also rise close to the channel’s boundary.

Dike fields as drivers and witnesses of twentieth-century hydrosedimentary changes in a highly engineered river (Rhône River, France)

Hydraulic structures, such as groyne fields, were commonly used to channelize European and North American rivers, thus forming engineered margins on the edges of the active channel. On the Rhône River (France), which was corrected with dike fields (classical groyne fields and specific ones closed with a longitudinal submersible dike; i.e., closed fields) and equipped with numerous dams (mid-twentieth century), the engineered margins have mostly been filled with fine sediments and become terrestrial. On the 11 km bypassed studied reach (middle Rhône), 55.6 ha in 167 dike fields (i.e., 75 % of the cumulated surface) have been subject to terrestrialization (i.e., the transformation of aquatic areas into terrestrial ones). Our study aimed to understand the trajectory of dike fields that serve as both drivers (e.g., inducing in-dike fine sediment trapping) and witnesses (e.g., informing the diversion impact) of hydrosedimentary changes. We combined geohistorical analyses (aerial photographs, riverbed elevations, and water levels) and GIS modeling, as well as topographic and ground-penetrating radar surveys (GPR), to emphasize the terrestrialization patterns and hierarchize the drivers and processes involved. We obtained a classification of diachronic patterns (five types) that highlight local recurrences and specificities (inherited forms) in the terrestrialization trajectory. Topographic and GPR surveys complemented our data, shedding light on the main characteristics of sediment deposits (volumes, thicknesses) and structural units. We also determined that the contribution to terrestrialization of phase 1 (channelized state; 1900s–1970s) is lower than that of phase 2 (channelized and bypassed state; 1970s–2000s), at 42 % (23.4 ha) and 58 % (32.2 ha), respectively. In phase 1, fine sediment deposition leads to deposit construction, triggered by the dike field setting, which shapes spaces with reduced shear stresses. The riverbed incision (induced by channelization) is shallow on this reach, so its contribution (by lowering the water level) is considered negligible. In phase 2, terrestrialization is mainly provoked by the diversion-induced drop in water level that has caused the retraction of the active channel and promoted its abandoned edges as new terrestrial margins. Understanding the evolution of these ecotones made it possible to adjust our recommendations in terms of management and restoration. It also highlights the relevance of strategic dike field reconnections to support the river by recreating gradients of hydrological connectivity (which are favorable to habitat diversity).

Flow Dynamics Around Permeable Spur Dike in a Rectangular Channel

In this study, numerical simulations were performed to investigate the flow and turbulence characteristics of rectangular spur dikes with varying permeability using the Reynolds stress turbulence model developed by three-dimensional (3-D) numerical code FLUENT (ANSYS). In this research, both permeable and impermeable spur dikes were investigated with varying permeability (25%, 37%, 49%, 62%, and 74%) to show its effect on the flow structure. At zh=3.5 cm (where z is the maximum flow depth, and zh is mid of flow depth), 3-D flow velocities and depth-averaged velocities were measured in the horizontal plane. Different velocity profiles and contours, turbulent intensities (T.I), and turbulent kinetic energy (TKE) were analyzed at various selected positions. The findings indicate that an increase in the permeability of the spur dikes up to 74% had a reciprocal effect on the mean stream-wise velocity. Moreover, the recirculation regions created within the impermeable spur dike field decreased with increased permeability of spur dikes. The permeable spur dike head showed a significant decrease in T.I, and TKE compared to the impermeable spur dike. Therefore, to protect the spur dike head from severe turbulent flow during floods and reduce the spur dike field’s recirculation region, a permeable spur dike is preferred.

Long-term river management legacies strongly alter riparian forest attributes and constrain restoration strategies along a large, multi-use river

Many terrestrial ecosystems have undergone profound transformation under the pressure of multiple human stressors. This may have oriented altered ecosystems toward transient or new states. Understanding how these cumulative impacts influence ecosystem functions, services and ecological trajectories is therefore essential to defining effective restoration strategies. This is particularly the case in riverine ecosystems, where the profound alteration of natural disturbance regimes can make the effectiveness of restoration operations questionable. Using the case study of legacy dike fields, i.e., area delimited by longitudinal and lateral dikes, along the regulated Rhône River, we studied the impacts of long-term channelization and flow regulation on environmental conditions and riparian forests attributes along a 200 km climatic gradient. We characterized the imprint of human stressors on these forests by comparing the dike field stands to more natural stands in both young and mature vegetation stages. Across four reaches of the river between Lyon and the Mediterranean Sea, we found that channelization consistently promoted high rate of overbank sedimentation and rapid disconnection of dike field surfaces from the channel. The rapid terrestrialisation of dike field surfaces, i.e., the process by which former aquatic areas transition to a terrestrial ecosystem as a result of dewatering or sedimentation, fostered a pulse of riparian forest regeneration in these resource-rich environments that differs from more natural sites in structure and composition. Within the dike fields, older pre-dam stands are dominated by post-pioneer and exotic species, and post-dam stands support large, aging pioneer trees with a largely exotic understory regeneration layer. These patterns were associated with differences in the relative surface elevation among dike fields, whereas species shifts generally followed the river's longitudinal climate gradient. To enhance the functionality of these human-made ecosystems, restoration strategies should target the reconnection of dike fields to the river by dismantling part of the dikes to promote lateral erosion, forest initiation and community succession, as well as increasing minimum flows in channels to improve connection with groundwater. However, since a river-wide return to a pre-disturbance state is very unlikely, a pragmatic approach should be favoured, focusing on local actions that can improve abiotic and biotic function, and ultimately enhancing ecosystem services such biodiversity, habitat, and recreation opportunities.

Three-Dimensional Flow Characteristics in Slit-Type Permeable Spur Dike Fields: Efficacy in Riverbank Protection

This paper focuses on finding efficient solutions for the design of a highly permeable pile spur (or slit type) dike field used in morphologically dynamic alluvial rivers. To test the suitability of different arrangements of this type of permeable pile spur dike field, laboratory experiments were conducted, and a three-dimensional multiphase numerical model was developed and applied, based on the experimental conditions. Three different angles to the approach flow and two types of individual pile position arrangements were tested. The results show that by using a series of slit-type spurs, the approach velocity of the flow can be considerably reduced within the spur dike zone. Using different sets of angles and installation positions, this type of permeable spur dike can be used more efficiently than traditional dikes. Notably, this type of spur dike can reduce the longitudinal velocity, turbulence intensity, and bed shear stress in the near-bank area. Additionally, the deflection of the permeable spur produces more transverse flow to the opposite bank. Arranging the piles in staggered grid positions among different spurs in a spur dike field improves functionality in terms of creating a quasi-uniform turbulence zone while simultaneously reducing the bed shear stress. Finally, the efficacy of the slit-type permeable spur dike field as a solution to the riverbank erosion problem is numerically tested in a reach of a braided river, the Brahmaputra–Jamuna River, and a comparison is made with a conventional spur dike field. The results indicate that the proposed structure ensures the smooth passing of flow compared with that for the conventional impermeable spur structure by producing a lower level of scouring (low bed shear stress) and flow intensification.

Numerical investigation of flow pattern around a T-shaped spur dike in the vicinity of attractive and repelling protective structures

This piece of research simulated the flow pattern around a T-shaped spur dike in the vicinity of attractive and repelling protective structures in a 90° bend using computational fluid dynamic (CFD) model. This CFD analysis has utilized RNG turbulence model to simulate turbulent flow field, and free surface variations were modeled through finite volume of fluid. A comparison between mean velocities in the main spur dike field was indicative of an acceptable precision of CFD model in simulating flow pattern along river bends in the presence of a spur dike. This numerical model mainly aimed at a scrutinizing study of the process of flow pattern around the T-shaped spur dike, the maximum velocity variations, the secondary flow strength, and the maximum stresses along the bend and the main spur dike field. Results demonstrated an increase in the maximum shear stress in attractive and repelling modes of the protective structure by 23.5 and 17.6%, respectively, in comparison with the vertical mode.

Dynamics of 30 large channel bars in the Lower Mississippi River in response to river engineering from 1985 to 2015

Channel bars are a major depositional feature in alluvial rivers and their morphodynamics has been investigated intensively in the past several decades. However, relatively less is known about how channel bars in alluvial rivers respond to river engineering and regulations. In this study, we assessed 30-yr morphologic changes of 30 large emerged bars located in a 223km reach of the highly regulated Lower Mississippi River from Vicksburg, Mississippi, to the Mississippi-Atchafalaya River diversion. Landsat imagery and river stage data between 1985 and 2015 were utilized to characterize bar morphologic features and quantify decadal changes. Based on bar surface areas estimated with the satellite images at different river stages, a rating curve was developed for each of the 30bars to determine their volumes. Results from this study show that the highly regulated river reach favored the growth of mid-channel and attached bars, while more than half of the point bars showed degradation. Currently, the mid-channel and attached bars accounted for 38% and 34% of the total volume of the 30bars. The average volume of a single mid-channel bar is over two times that of an attached bar and over four times that of a point bar. Overall, in the past three decades, the total volume of the studied 30bars increased by 110,118,000m3 (41%). Total dike length in a dike field was found mostly contributing to the bar volume increase. Currently, the emerged volume of the 30bars was estimated approximately 378,183,000m3. The total bar volume is equivalent to ~530millionmetrictons of coarse sand, based on an average measured bulk density of 1.4t/m3 for the bar sediment. The findings show that these bars are large sediment reservoirs.

Foundering Triggered by the Collision of India and Asia Captured in Xenoliths

AbstractXenoliths that erupted in the SE Pamir of Tajikistan from 1000 to 1050°C and 90 km depth illuminate what happens when crust founders into the mantle. 40Ar/39Ar dating of minerals from the xenoliths and volcanic host rocks of the shoshonitic Dunkeldik pipe and dike field indicates eruption at 11.2 ± 0.2 Ma. U‐Pb and trace element laser‐ablation split stream inductively coupled plasma mass spectrometry of zircon shows that the igneous and metasedimentary xenoliths were likely derived from the crustal section into which they were intruded: the Jurassic‐Cretaceous Andean‐style magmatic arc and its Proterozoic‐Mesozoic host rocks along the southern margin of Asia. Recrystallization of these zircons was extensive, yielding a range of dates down to 11 Ma. The zircons show distinct changes in Eu anomaly, Lu/Gd ratio, and Ti concentrations compatible with garnet growth and minimal heating at 22–20 Ma and then 200‐300°C of heating, ~25 km of burial, and alkali‐carbonate melt injection at 14–11 Ma. These changes are interpreted to coincide with (i) heat input due to Indian slab breakoff at ~22–20 Ma and (ii) rapid thickening and foundering of the Pamir lithosphere at 14‐11 Ma, prior to and synchronous with collision between deep Indian and Asian lithospheres beneath the Pamir.

Early magmatism in the greater Red Sea rift: timing and significance

Throughout the greater Red Sea rift system the initial late Cenozoic syn-rift strata and extensional faulting are closely associated with alkali basaltic volcanism. Older stratigraphic units are either pre-rift or deposited during pre-rupture mechanical weakening of the lithosphere. The East African superplume appeared in northeast Africa ∼46 Ma but was not accompanied by any significant extensional faulting. Continental rifting began in the eastern and central Gulf of Aden at ∼31–30 Ma coeval with the onset of continental flood volcanism in northern Ethiopia, Eritrea, and western Yemen. Volcanism appeared soon after at Derudeb in southern Sudan and at Harrats Hadan and As Sirat in Saudi Arabia. From ∼26.5 to 25 Ma a new phase of volcanism began with the intrusion of a dike field reaching southeast of Afar into the Ogaden. At 24–23 Ma dikes were emplaced nearly simultaneously north of Afar and reached over 2000 km into northern Egypt. The dike event linked Afar to the smaller Cairo mini-plume and corresponds to initiation of lithospheric extension and rupture in the central and northern Red Sea and Gulf of Suez. By ∼21 Ma the dike intrusions along the entire length of the Red Sea were completed. Each episodic enlargement of the greater Red Sea rift system was triggered and facilitated by breakthrough of mantle-derived plumes. However, the absence of any volumetrically significant rift-related volcanism during the main phase of Miocene central and northern Red Sea – Gulf of Suez rifting supports the interpretation that plate–boundary forces likely drove overall separation of Arabia from Africa.

Appropriate CFD Models for Simulating Flow around Spur Dike Group along Urban Riverways

The flow field around spur dike group is complex, noticeable, and widely encountered in the improving progress of urban riverways and coastlines. Detailed investigation on such flow phenomenon is necessary and of applied significance. In contrast to experimental study and field survey, the numerical simulation can provide much more details with relative low cost. Aiming to identify the appropriate Computational Fluid Dynamics (CFD) models for simulating flow around spur dike group, the flow fields around non-submerged and submerged spur dike groups, including eight spur dikes in staggered arrangement, were numerically investigated in this paper with selected CFD models and validated based on corresponding flume test, i.e., two sets of laboratory experiments and observed data collected. The numerical simulations were carried out using Finite Volume Method (FVM) and three turbulence models, i.e., standard k-e model, Reynolds Stress Model (RSM) and Large Eddy Simulation (LES) model. In each model, the free surface boundary was implemented respectively via two approaches, i.e., rigid-lid assumption (RLA) and volume of fluid (VOF) method. The comparisons between the CFD outputs and the observed data from flume experiments show that all three turbulence models are capable to simulate the three-dimensional flow around spur dike group to certain degree. It is noticed that, with comprehensive understanding of simulation accuracy and computational time, aiming to rapidly capture the field characteristics of main flow for non-submerged spur dike group, the standard k-e model under RLA method is recommended, while to achieve the fine simulation of spur dike field especially in backflow zone, LES model under VOF method is appropriate. For submerged spur dike group, comparing to simulation accuracy, simulation cost is not a major factor concerned and LES model based on VOF method is the most suitable one due to the overflow effect of spur dike crest and the backwater effect of spur dike body.

Paleoproterozoic remagnetization in the white sea mobile belt, Karelia: Petro-paleomagnetic evidence and supercomputer modeling

The detailed petro-paleomagnetic study of the Paleoproterozoic eclogite complexes of the White Sea mobile belt in the area of Gridino dike field revealed two stages of rock remagnetization. The Early Paleoproterozoic gabbronorite dikes contain a secondary component of NRM that is pointed NNW with a strong positive slope that is correlated with the post-orogenic collapse of 1.95-1.88 Ga B.P. Formation of the second alternating magnetization phase of rocks is estimated at 1.8-1.75 Ga B.P., which corresponds to the influence of post-orogenic hydrothermal fluids.

Discussion of “Theoretical Analysis of Wing Dike Impact on River Flood Stages” by Fredrik Huthoff, Nicholas Pinter, and Jonathan W. F. Remo

The theoretical analysis proposed in the original paper is an oversimplified method to quantify an extremely complex and dynamic hydraulic problem. The basis of this analysis is the effective n value formula developed by Yossef (2004, 2005). Although these relationships can give insight into the relationship between the effective roughness in the dike zone and submergence, it is only suitable to deduce trends rather than quantify accurate magnitudes of change. Background into the development of Eq. (3) is necessary to understand its limitations. Eq. (3) was developed using a fixed bed physical model scaled to the dimensions and geometry of a dike field in the Dutch River Waal where the wing dike crest elevation, spacing, and length remained fixed throughout the study. There are major differences in geometry and spacing of dikes used by the Dutch and those on the Middle Mississippi River. The dikes used by the Dutch have a much higher crest elevation, closer spacing, and higher contraction ratios. Eq. (3) is dependent on the drag coefficient CD found in Eq. (4). The drag coefficient, CD, was developed using empirical data from the previously mentioned flume study and assumed to be in accordance with a power function. The coefficients a and b in Eq. (4) are only appropriate for the dimensions and channel geometry tested, and are not applicable to the much different geometry and dimensions of the Middle Mississippi River. The limitations of the relationships, which are the foundation of the original paper [Eqs. (3) and (4)], combined with concerns of the applicability of the relationship to the geometrically different Middle Mississippi River, make it difficult for the authors of the original paper to base sweeping conclusions off small changes (decimeter range) in an overly simplistic analysis. Another limitation the conclusions presented by the authors of the original paper is the model used in the original paper is extremely sensitive to the input data; small changes to in bank and/or channel roughness, wing dike crest height, and/or bed level change can have major impacts on the resulting water level difference, including resulting in negative values. An example of the sensitivity of the model to the input values is the resulting water level differences using the input values in the original paper. As detailed in the publisher’s note, the model developed by the authors of the original paper originally had an inadvertent error in the calculation of Manning’s n from Eq. (3). This resulted in effective roughness values in the bank zone with dikes (neff ) of approximately 0.6 s m−1=3, which is an order of magnitude too high. In combination with the correction of the math error, critical input values were changed in the corrected analysis. The bank roughness values (nb) used in the original version were 0.031, 0.032, 0.027, and 0.027 for St. Louis, Chester, Grand Tower, and Thebes, respectively. In the corrected analysis the original n values were decreased by over 40% to physically unrealistic values (Table 1). Had the authors of the original paper only corrected the math error and used the original values of base roughness in the bank zone with resulting calibrated bed changes of−0.80,−0.26,þ0.72, and −0.27 m at St. Louis, Chester, Grand Tower, and Thebes, respectively, the resulting water level difference would have been −0.12 m at St. Louis, þ0.21 m at Chester, þ0.84 m at Grand Tower, and −0.00 m at Thebes for 4Qave. These values of water level change lead to a conclusion on the impact of wing dikes on flood stages that is in conflict with what is presented in the original paper, but shared by many scientists (Watson et al. 2013; USGAO 2011; Huizinga 2009). A major issue with the roughness values used in the corrected paper is the resulting velocity distribution in the channel. Main channel velocities for the no dike, T0, condition at St. Louis were taken from a reconstruction of the model used in the original paper. At Qave, both main channel and bank zone velocities were equal. At 4Qave, the bank zone velocity was 135% of the channel velocity [Fig. 1(a)]. Conversely, had the authors of the original paper used similar roughness values for the channel and bank zone (as done in the

Drag force and associated backwater effect due to an open channel spur dike field

This study focuses on quantifying the flow resistance and the associated backwater effect of a spur dike field in an open channel flow. The work was carried out in a rigid bed flume. The model spur dikes were simulated using two-dimensional (2D), rectangular plates placed in an array along one side of the flume. The results indicate that the arrangement of a spur dike field has a substantial effect on the drag force and hence the backwater effect that is experienced. In general, the total drag force of a spur dike field increases as the number of spur dikes and the relative spacing between them increase. The most upstream spur dike experienced the greatest drag force among the entire spur field, essentially acting as a shield, resulting in a reduction in the drag force of the downstream spur dikes.

Petrology and thermobarometry of eclogite rocks in the Krasnaya Guba dike field, belomorian mobile belt

A sequence of mineral associations was examined in eclogitized basites of the Krasnaya Guba dike field in the Belomorian Mobile Belt. Two morphological types of eclogite and eclogite-like rocks were recognized: (1) eclogite rocks that developed after ferrogabbro dikes and completely replaced these dikes from contact to contact and (2) eclogite-like rocks that developed after gabbronorites in zones of ductile deformations and shearing. According to data mineral geothermobaromety, both rock types were formed within temperature and pressure ranges corresponding to high-pressure and high-temperature amphibolite facies at T = 700 ± 40°C and P = 10.0 ± 0.5 kbar. The peak metamorphic parameters of the host gneisses are analogous. The decompressional stage, which is unambiguously identified by reaction textures, occurred at 630–660°C and 7.9–8.2 kbar. As the temperature and, first of all, pressure decreased, the SiO2 activity in the fluid systematically varied. The eclogitization of the basites took place locally in relation to fluid fluxes, which were restricted to zones of intense deformations, at variable SiO2 activity. The rocks show evidence of two stages of post-eclogite amphibolization. Older amphibolization 1 was coeval with the late prograde metamorphic stage (T = 650°C, P = 10–11 kbar). Younger amphibolization 2 affected eclogitized basite dikes and unaltered gabbronorites (together with their host gneisses) over large areas. This process coincided with decompression (T = 580°C, P = 7–8 kbar) and was likely accompanied by the exhumation of deep zones of BMB to upper-middle crustal levels.

Geological and structural conditions of eclogitization of Paleoproterozoic basic dikes in the eastern Belomorian Mobile Belt

A new, Krasnaya Guba dike field that comprises more than 30 minor gabbronorite intrusions and dikes of “garnet” Fe-gabbro was revealed at the northeastern flank of the Belomorian Mobile Belt. Gabbronorite intrusions were emplaced along the faults that formed as a result of brittle failure under conditions of general extension. As follows from the structural relationships, dikes of Fe-gabbro are younger than gabbronorite. They were formed under compression and shearing. The mobile melt was pumped into gneisses that underwent ductile deformation. Eclogitization in the Krasnaya Guba dike field was selective and affected only dikes of Fe-gabbro and minor gabbronorite intrusions and did not develop in country rocks. The bodies of Fe-gabbro are eclogitized almost completely, whereas gabbronorite is transformed into eclogite only along shear zones. Eclogites were formed after the complete solidification of intrusive bodies, and their localization is controlled by the concentration of deformations owing to the different elastoplastic properties of gabbroic rocks and country gneisses and migmatites. The different degree of fracturing in basic dikes and country gneisses resulted in the concentration of fluid flows in fracture systems within competent rocks. Late Svecofennian pegmatites (1.86 Ga) distinctly crosscut eclogites that experienced retrograde metamorphism. The eclogitic rocks from the Krasnaya Guba are the youngest known in the Belomorye. The age of eclogitization is 2.12–1.86 Ga. A new finding of eclogitic rocks in the Belomorye, as well as previously known similar rocks near Gridino Village, emphasize tectonic activity of the northwestern Umba Collision Zone. The eclogitization of basic rocks in Krasnaya Guba is related to their desilification in shearing zones in the course of interaction of plagioclase with ortho-and clinopyroxene with the formation of garnet-omphacite aggregate and removal of silica. The currently observed texture, structure, and chemical and mineral compositions of eclogitic rocks are products of intense retrograde metamorphism.

Evolution of stress fields in the Por’ya Guba dike field (Kandalaksha Gulf, White Sea)

The tectonic evolution of the Por’ya Guba segment of the White Sea Rift System began in the late Paleoproterozoic, i.e., soon after completion of the Svecofennian collision. The fracture system that controlled localization of the lamproite dike complex was formed under conditions of horizontal compression combined with shear. Subsequently, this system predetermined the location of a rift-graben segment that formed as a result of simple shear. The reactivation of the rift system in the Middle Paleozoic proceeded in two stages. The first stage, when strike-slip movements along previously formed faults predominated, resulted in formation of quartz-carbonate veins bearing base-metal mineralization. The veins that filled the shear fractures opened owing to local reorientation of the stress field. The second stage fitted the transtension conditions, and the Late Devonian alkaline ultramafic dikes of this stage introded into the already existing fracture system, which was oriented at a roughly right angle to the predominant stress orientation.

Chironomid assemblage of a Lower-Rhône dike field: relationships between substratum and biodiversity

The effect of microhabitat on the chironomid (Diptera) community of a large river was tested by trapping floating pupal exuviae after emergence of adults. The spatial distribution of the 33 species or groups of species that were identified was strongly explained by mean particle size, porosity and heterogeneity of substratum. A study of the size, means of respiration, mode of life and feeding habits of the species showed that the substratum-faunal relationships could be explained in part by biological traits of the species present. Two gradients could be defined on the basis of habitat variables: a substratum porosity gradient that was significantly correlated with species richness and diversity, and a second gradient related to particle size that was significantly correlated with biological categories.

Hydrosedimentary classification of natural and engineered backwaters of a large river, the lower Rhône: possible applications for the maintenance of high fish biodiversity

At the end of the last century, the shorelines of the Lower River Rhone were embanked and submersible dikes were built in the main channel, delimiting dike fields. We compared the morphological, hydrological and sedimentary variables of nine dike fields, four sites in dead arms and one in the main channel, in order to describe the fluviosedimentary functioning and to produce a classification of these different natural and engineered backwaters. Within these ‘artificial floodplains’, the different types of backwaters and their long-term changes bear a strong similarity to those observed within a natural floodplain. The long-term frequency of submersion by the water of the main river channel is the first factor that controls the erosion and sedimentation within the different backwaters. Three types of site can be distinguished: the lotic sites are the main channel and one dike field, five dike fields comprise the semi-lotic sites and the cluster lentic sites include the three remaining dike fields and the four dead arms sites. These three groups also differ in their riparian habitats and in their juvenile fish assemblages. As juvenile fish were most abundant in the semi-lotic dike fields, the use of such structures can be envisaged in order to restore high fish diversity in rivers where the former floodplain has disappeared or is no longer accessible to fish because of human modifications. © 1997 John Wiley & Sons, Ltd.