Shoal Research Articles

The Role of Beach Bars in Wrocław’s Local Music Scene

Abstract The paper discusses Wrocław’s beach bars, i.e. open-air riverside facilities which (as the name suggests) mostly possess artificial sandy beaches. This importance of this type of venue, relatively new in Wrocław, for the local music life is constantly growing. The text characterises these urban spaces (which have been of only marginal interest to Polish scholars to date) and defines their place in the city’s popular music scene in relation to traditional forms of that repertoire’s presence. In his survey of local musical practice, the author refers to the city’s sonic identity (soundscape), which is co-created by urban artists. The paper covers the period of the author’s field research in 2019–2021 as well as his later observations which indicate potential directions of beach bars’ development and confirm their growing role in shaping Wrocław’s local music scene.

Impact of structural geometry of tilted fault blocks on hydrocarbon entrapment and deposition of syn-rift clastic reservoirs: Belayim Marine field (Gulf of Suez rift)

Belayim Marine oilfield is representative of the Gulf of Suez fields with hydrocarbon production from pre-rift and syn-rift reservoirs. Detailed subsurface mapping using a complete 3D seismic and borehole dataset throws light on the trapping mechanisms of tilted fault blocks in rift basins where the strike of the tilted block is parallel to the bounding fault. For such one-way tilted fault traps, linkage of rift-parallel and rift-oblique faults at the updip sides of the tilted blocks is necessary for trap integrity of laterally continuous reservoirs like the Nubia Sandstone. For laterally discontinuous reservoirs like tidal sand bars aligned normal or oblique to the block-bounding faults, a combination trap can be laterally closed by lateral change in facies of the reservoir unit along fault strike. Fault sealing of the proved hydrocarbon traps is mainly through juxtaposition. Shale smearing is valid for thick shale units with reasonable fault displacement and shale gouge is a proven sealing mechanism especially in clay-rich units. The ductile nature of syn-rift sediments leads to fault-propagation folding above the main-bounding faults of tilted fault blocks where folding is the trapping mechanism of these syn-rift reservoirs.The structural geometry of tilted fault blocks also controls the distribution of syn-rift reservoirs as the linkage of rift-parallel and rift-oblique faults controls the entry of main valleys transporting syn-rift clastics into the basin. Longitudinal depressions sculptured at the crests of tilted fault blocks represent another location for deposition of syn-rift reservoirs.

Sedimentation Characteristics of the Fluctuating Backwater Area at the Tail of Cascade Reservoirs: A Case Study of the Three Gorges Reservoir

The construction of cascade reservoirs is associated with considerable uncertainty in sedimentation in the fluctuating backwater area of the terminal reservoir and poses challenges to water safety. The sedimentation characteristics under the influence of multiple factors in the main urban area of the Chongqing river section were analyzed as a case study for the operation of cascade reservoirs in the Jinsha River via the utilization of a large dataset spanning back to the normal storage of the Three Gorges Reservoir. The results of this study indicate that, owing to factors such as upstream water, sediment inflow, reservoir operation, and river sand mining, this river section experienced deposition on the sand bars and erosion in the main channel. The rate of sedimentation increased with sediment inflow, peak flow rate, and duration, while the location of sedimentation shifted as the concentration ratio changed. These results may provide technical support not only for the operation of the Three Gorges Reservoir, but also for the governance of the fluctuating backwater areas of other cascade reservoirs.

Investigating the mineralogical and geochemical composition of fluvial sediments of Watari river system, Kano, Nigeria

A key objective of this research is to conduct a mineralogical and geochemical screening of the fluvial sediments in Watari river. Data collected from this study can be used to further enhance scientific understanding of the fluvial processes in the Watari basin as well as the larger Hadejia-Jama' are basin. The geological data can also be used as input information for the prediction of sedimentation rates in the Watari dam, Kano. An intensive field was conducted to explore thebedforms (sand bars, ripples, and sand filled channel or dunes) of the proximal channel environments, platform geometry and sinuosity.X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses were used to study the mineralogy and chemical composition of the collected samples, respectively. The Watari river can be considered as braided due to the presence of smaller multi-threaded channels, braid or channel bars. The lithology of exposures observed at the river bank show a series of depositional cycles that comprise mudrocks (light and dark), sandstone, conglomerate and a basal quartzitic gravel deposit. The wacke and sub-litheranite rocks were sourced from mafic and intermediate igneous rocks. The relatively high La and Ce in the mudrocks can be attributed mainly to the adsorption of Ce onto Fe-oxides which are abundant in the fluvial sediments eroded from lateritic soils. From the bivariate plot of Al2O3/SiO2 against Fe2O3+MgO, the sandstone facies fall under the passive continental margin, while the quartzite and conglomerate facies are grouped under the foreland basin, and the light mudrock facies is classified under the continental island arc. In addition, the plot of Al2O3/(CaO + Na2O) against Fe2O3+MgO shows the sandstone and quartzite facies fall under the foreland basin. The relatively high concentration of feldspar and muscovite mica in the siliciclastic facies is indicative of close proximity to parent material, and low degree of weathering and diagenesis. The variability in the mineral composition points toward different cycles of fluvial sedimentation, diverse provenance, and wide range of source materials.The geochemical data further buttresses the variability in provenance, wider geographic area of the sediments or re-deposition and reworking by fluvial actions.

Lithological and diagenetic variation of mixed depositional units in the middle permian saline lacustrine deposition, Junggar Basin, NW China

Mixed depositional reservoirs are widely distributed in Junggar Basin, NW China. These reservoirs are featured by complex lithology, ultra-low permeability and extremely heterogeneous pore structures that markedly impact field development efficiency. This study illustrated the distribution of lithological combinations within a single sand bar in the Middle Permian Lucaogou Formation, Jimsar Sag, Junggar Basin and revealed the diagenetic control on the reservoir quality of a single sand bar based on thin sections, scanning electron microscopy (SEM), helium porosity and air permeability measurement, pressure-controlled mercury injection (PMI), rate-controlled mercury injection (RMI), X-ray computed tomography (CT), and collected and published data. The results show that the heterogeneity of microscopic pore structures within a single sandbar is controlled by the distribution of various lithological combinations and different levels of diagenetic alteration. The results show that there are mainly three types of lithology developed in a single sand bar. The reservoir quality of dolomitic siltstone reservoirs is the best, followed by silty dolomite reservoirs. The reservoirs quality of dolomicrite reservoirs is the worst. Three main lithological combinations can be identified within a single sand bar. Controlled by climate, lake level fluctuation and provenance, lithological combination A, characterized by blended mixing of dolomitic siltstone, silty dolomite and dolomicrite, is mainly developed in the middle of single sand bar. Lithological combination B, characterized by saltatory mixing of interbedded dolomitic siltstone and dolomicrite, and lithological combination A developed on the side of the sand bar near the shallow lake, while lithological combination C with blended mixing of interbedded dolomitic siltstone and silty dolomite developed on the side near provenance. Strong compaction is the main factor of the decrease of reservoir quality of sand bar reservoirs. Carbonate cementation promotes the densification of reservoirs. The irregular flaky clay minerals lead to the exponential decline of permeability, and dissolution is the main kind of diagenesis to improve reservoir quality. The dolomitic siltstone reservoirs in the middle of a single sand bar have the best reservoir quality because the overlying dolomicrite layers resist compaction, resulted in a certain amount of primary pores remained. Besides, the dolomitic siltstone reservoirs are far from the sand-mudstone interfaces, which leads to the low carbonate cement content. Furthermore, abundant dissolution pores in dolomitic siltstone reservoirs improve the reservoir quality. These research results are crucial to reservoir evaluation and development in similar mixed depositional tight reservoirs.

Global mapping of river sediment bars

AbstractRecently, deep learning has been increasingly applied to global mapping of land‐use and land‐cover classes. However, very few studies have addressed the problem of separating lakes from rivers, and to our knowledge, none have addressed the issue of mapping fluvial sediment bars. We present the first global scale inventory of fluvial gravel bars. Our workflow is based on a state‐of‐the‐art fully convolutional neural network which is applied to Sentinel‐2 imagery at a resolution of 10 m. We use Google Earth Engine to access these data for a study site that covers 89% of the Earth's surface. We count 8.9 million gravel bars with an estimated area of 41 000 km2. Crucially, the workflow we present can be executed within a month of highly automated processing and thus allows for global scale, monthly, monitoring of gravel bars and associated rivers.

Background Topography Affects the Degree of Three‐Dimensionality of Tidal Sand Waves

AbstractOffshore tidal sand waves on the sandy bed of shallow continental shelf seas are more three‐dimensional (3D) in some places than others, where 3D refers to a pattern that shows variations in three spatial directions. Such sand waves have crestlines that meander, split or merge. The degree of three‐dimensionality seems to vary especially when large‐scale bedforms, such as tidal sand banks, are present underneath the sand waves. Understanding this behavior is important for offshore activities, such as offshore windfarm construction or the maintenance of navigation channels. In this study, the degree of three‐dimensionality of sand waves at five sites in the North Sea is quantified with a new measure. Results show that tidal sand waves on top of tidal sand banks are more two‐dimensional (2D) than those on bank slopes or in open areas. These differences in sand wave pattern are supported by numerical simulations performed with a new long‐term sand wave model. The primary cause of these differences is attributed to the deflection of tidal flow over a sand bank, which causes sand wave crests to be more aligned with the bank at its top than at its slopes. It is subsequently made plausible that the different patterns result from the competition between two known mechanisms. These mechanisms are nonlinear interactions between sand waves themselves (SW‐SW interactions) and nonlinear interactions between sand banks and sand waves (SB‐SW interactions). On bank tops, SB‐SW interactions favor a 2D pattern, while SW‐SW interactions, which elsewhere produce a 3D pattern, are less effective.

Experimental Study on the Backflow Mechanism of Proppants in Induced Fractures and Fiber Sand Control Under the Condition of Large-Scale and Fully Measurable Flow Field.

The proppant backflow in the process of flowback has a great significant effect on gas field development. Therefore, the study of proppant backflow is of great significance for the development and production of gas wells. At present, the physical simulation methods for proppant backflow mainly include the tube perforation model, the slot model, an API standard flow tester, and a large-scale flowback apparatus. The current experimental methods are unable to observe the backflow of proppants during the process of the flowback test. In addition, the only characterization parameter for proppant backflow is the liquid flow rate corresponding to the sand discharge in the diversion chamber called critical velocity, which is too simple and single to accurately characterize the movement state of proppants during the flowback process. In this paper, a physical simulation method of proppant backflow in fractures based on the measurement of flow field was proposed. It can realize the observation and fine description of the proppant backflow state and movement rule. In addition, the process of proppant backflow can be quantitatively described by a multidimensional characterization parameter. The research shows that (1) the proppant backflow is closely related to the shape of the sand bank formed during the proppant placement and the irregular voids formed; (2) the fiber increases the strength of the proppant pack significantly; (3) the critical velocity with fiber increased by 2.25 times compared with the critical velocity without fiber, the optimum fiber concentration was 0.8%, and the fiber length was 12 mm; (4) the full fiber injection was selected as the best injection mode by the experiment; and (5) the whole process of flowback can be divided into two stages. In the strong fluid shear stage, the effect of fiber sand control is more significant. However, when the flowback enters the stage of slow erosion, the difference in the sand control effect under different parameters is no longer significant.

A Beach Profile Evolution Model Driven by the Hybrid Shock-Capturing Boussinesq Wave Solver

An existing Boussinesq wave model, solved in a hybrid format of the finite-difference method (FDM) and finite-volume method (FVM), with good merits of stability and shock-capturing, was used as the wave driver to simulate the beach evolution under nearshore wave action. By coupling the boundary layer model, the sand transport model, and the terrain updating model, the beach evolution model is established. Based on the coupled model, the interaction process between sandbars and waves was simulated, reproducing the process of the original sand bars diminishing, new sandbars creating, and finally disappearing. At the same time, the formation and movement process of sand bars under solitary and regular waves are numerically simulated, in the breaking zone, the water bottom has a larger shear stress, which promotes the sediment activation, transport and erosion formation, and near the breaking point, the decrease of sand-carrying capacity is the main reason for the formation of sandbars, the numerical model can accurately describe the changes in the shoreline profile under wave action.

Morphological Modelling to Investigate the Role of External Sediment Sources and Wind and Wave-Induced Flow on Sand Bank Sustainability: An Arklow Bank Case Study

Offshore anthropogenic activities such as the installation of Offshore Renewable Energy (ORE) developments and sediment extraction for marine aggregates have been shown to disrupt current flow, wave propagation, and sediment transport pathways, leading to potential environmental instability. Due to the complexity of the interconnected sediment transport pathways in the south-western Irish Sea combined with an increase in planned anthropogenic activities, the assessment of this risk is imperative for the development of a robust marine spatial plan. Subsequently, this study uses two-dimensional morphological modelling to build upon previous studies to assess the dependency of Arklow Bank’s local sediment transport regime on external sediment sources. Additionally, scenario modelling is used to identify vulnerable areas of this offshore linear sand bank to wind and wave-forcing and to examine the nature of this impact. A sediment budget is estimated for Arklow Bank, whereby seven source and nine sink pathways are identified. New evidence to support the exchange of sediment between offshore sand banks and offshore independent sand wave fields is also provided. The areas of the bank most vulnerable to changes in external sediment sources and the addition of wind- and wave-induced flow are analogous. These high vulnerability zones (HVZs) align with regions of residual cross-flow under pure current conditions. The restriction of sediment sources off the southern extent of Arklow Bank impacts erosion and accretion patterns in the mid- and northern sections of the bank after just one lunar month of simulation. Where tidal current is the primary driver of sand bank morphodynamics, wind- and wave-induced flow is shown to temporarily alter sediment distribution patterns. Wind and wave-induced flow can both accelerate and decelerate the east-west fluctuation of the upper slopes of the bank, yet the nature of this impact is inconsistent due to the misalignment of the directionality of these two forces. The methods and new knowledge derived from this study are directly applicable to tidally-dominated environments outside the Irish Sea.

Sequential riparian vegetation alteration in Japanese river landscapes

Bare gravel bed channels have been a common landscape feature of the middle reaches of rivers throughout Japan. They are disappearing these days because of vegetation colonization. Vegetation coverage of the riparian zones in the midstream reaches of rivers distributed throughout Japan was investigated using aerial photo images taken in 1946, 1975, and 2010. It was then compared with hydrological and water quality records, locations of dams and weirs, residential conditions of the surrounding areas, sediment yields, and forest coverage of the upstream mountainous areas. Riparian vegetation coverage has substantially increased in the last 70 years, from nearly 0 to 30-40%. Herbaceous vegetation coverage is higher than tree coverage. The total vegetation coverage did not correlate with water quality parameters. However, vegetation coverage had a significant negative correlation with the upstream basin's sediment yield and a slight negative correlation with the maximum flood peak flow rate. It was positively correlated with the terrestrial area fraction between levees. Riparian vegetation coverage was high within the 2 km downstream of dams or weirs, intercepting sediment flows. These results indicated that the gravelly sediment supplied from the upstream and deposited in the midstream at flood time has been the major contributor to keeping the vegetation coverage low rather than flushing by flood water. Reforestation since World War II has reduced the coarse sediment supply into the midstream river channels. This reforestation, together with gravel mining and intercepting sediment flow by dams and weirs, has caused the recent increase in vegetation coverage of the riparian zones. Human intervention can affect the vegetation conditions in riparian zones by decreasing sediment supply to river channels.

Anthropogenic Geomorphology of the Magoye Riverine Landscape, Zambia

Aims: This study sought to investigate the role of humans in modification and creation of landforms in river channels with specific focus on the Magoye River. The objectives of this study were to: document geomorphic characteristics of Magoye River, assess anthropogenic activities and landuse/cover change in the buffer zone and, examine key anthropogenic river landforms. Study Design: This study was inspired by analytic eclecticism research philosophy and adopted mixed methods, particularly concurrent research design. Methodology: The landcover images were analysed using image processing tools in ArcGIS 10.4 for the periods 1990, 2005 and 2020. Descriptive statistics were used to quantitatively visualize the changes in land cover/use. The data was collected using field observation, photography, GPS and a Likert scale tool and, analysed using descriptive statistics, specifically frequency graphs showing mean and standard deviation. Results: The results showed that sand mining and brick moulding accounted for almost 68% of human activities in the 11.48 km2 delineated buffer zone by 2020, compared to 35% in 1990. These punctuated creation of sand conical heaps, stone bunds, pot holes and pools, shallow wells on the river bed, gullies induced by water accessed points, which weakened river banks. Generally, sand mining and brick moulding were the most severe in the buffer zone and they created wide range of deformations riverbanks and beds. The findings further revealed that Magoye River had geomorphologically evolved into Reservoir River covering 80% on the upstream (139.4km) and Sand Bank River accounting 20% on downstream (27.6 km). Conclusion: The study concludes that, the catchment and buffer zone have undergone degradation propelled by anthropogenic activities, which have punctuated channel morphological degradation. Although the Magoye River channel was highly damaged, it was not beyond regeneration if restoration measures, were collaboratively identified and implemented with the local communities.

Geomorphic Response of a Coastal Berm to Storm Surge and the Importance of Sheet Flow Dynamics

AbstractDuring a storm, as the beach profile is impacted by increased wave forcing and rapidly changing water levels, sand berms may help mitigate erosion of the backshore. However, the mechanics of berm morphodynamics have not been fully described. In this study, 26 trials were conducted in a large wave flume to explore the response of a near‐prototype berm to scaled storm conditions. Sensors were used to quantify hydrodynamics, sheet flow dynamics, and berm evolution. Results indicate that berm overtopping and offshore sediment transport were key processes causing berm erosion. During the morphological evolution of the beach profile, two sand bars were formed offshore that attenuated subsequent wave energy. The landward extent of that energy was confined to the seaward foreshore, inhibiting inundation of the backshore. Net offshore‐directed transport was dominant when infragravity motions increased in the swash zone. Conversely, the influence of incident‐band motions on sediment transport was relatively greater in the inner‐surf zone. Near‐bed flow velocities and sheet flow layer thicknesses were larger in the swash zone than in the inner‐surf zone. This paper also provides a valuable analysis between morphology‐estimated total sediment transport rates and rates derived from in situ measurements. Sheet flow dynamics dominated foreshore cross‐shore sediment processes, constituting the largest portion of the total sediment transport load throughout the berm erosion.

Development and characterization of expressed sequence tag-simple sequence repeat markers for Anaphalis margaritacea var. yedoensis (Asteraceae).

RNA-sequencing was used to develop 16 microsatellite markers for the pearly everlasting, Anaphalis margaritacea var. yedoensis (Franch. et Sav.) Ohwi (Asteraceae), which inhabits gravel bars throughout the Japanese archipelago. The mean number of alleles for these 16 markers in two populations in the Hokkaido and Shizuoka Prefectures, was 3.5 and 4.0, respectively, while the mean expected heterozygosity was 0.525 and 0.560, respectively, with a significant genetic differentiation between the two populations. All markers could also be amplified in two conspecific taxa, A. margaritacea var. margaritacea and var. angustifolia, whereas 11 of the 16 markers were amplifiable in two congeneric species, A. sinica and A. alpicola. These newly developed microsatellite markers will support understanding of population genetics and mating systems in A. margaritacea var. yedoensis, and several will potentially be of use in similar studies in other Anaphalis species.

Interactions between diurnal winds and floodplain mosaics control the insect boundary layer in a river corridor

Insect flight along river corridors is a fundamental process that facilitates sustainable succession and diversity of aquatic and terrestrial insect communities in highly dynamic fluvial environments. This study examines variations in the thickness of the insect boundary layer (i.e., the pre-surface atmosphere layer in which air velocity does not exceed the sustained speed of flying insects) caused by interactions between diurnal winds and the heterogenous habitat mosaics in the floodplain of a braided river. Based on advective–diffusive theory, we develop and test a semi-empirical model that relates vertical flux of flying insects to vertical profiles of diurnal winds. Our model suggests that, in the logarithmic layer of wind, the density of insect fluxes decreases exponentially with the altitude due to the strong physical forcing. Inside the insect boundary layer, the insect fluxes can increase with the altitude while the winds speed remains nearly constant. We suggest a hypothesis that there is a close correspondence between the height of discontinuity points in the insect profiles (e.g. points with abrupt changes of the insect flux) and the displacement heights of the wind profiles (e.g. points above which the wind profile is logarithmic). Vertical profiles were sampled during three time-intervals at three different habitat locations in the river corridor: a bare gravel bar, a gravel bar with shrubs, and an island with trees and shrubs. Insects and wind speed were sampled and measured simultaneously over each location at 1.5-m intervals up to approximately 17 m elevation. The results support our working hypothesis on close correspondence between discontinuity and displacement points. The thickness of the insect boundary layer matches the height of the discontinuity points and was about 5 m above the bare gravel bar and the gravel bar with shrubs. Above the island, the structure of the insect boundary layer was more complex and consisted of two discontinuity points, one at the mean height of the trees’ crowns (ca. 15 m), and a second, internal boundary layer at the top of the shrubs (ca. 5 m). Our findings improve the understanding of how vegetation can influence longitudinal and lateral dispersal patterns of flying insects in river corridors and floodplain systems. It also highlights the importance of preserving terrestrial habitat diversity in river floodplains as an important driver of both biotic and abiotic (i.e., morphology and airscape) heterogeneity.

Grain size estimation in fluvial gravel bars using uncrewed aerial vehicles: A comparison between methods based on imagery and topography

AbstractGrain size assessments are necessary for understanding the various geomorphological, hydrological and ecological processes that occur within rivers. Recent research has shown that the application of Structure‐from‐Motion (SfM) photogrammetry to imagery from uncrewed aerial vehicles (UAVs) shows promise for rapidly characterising grain sizes along rivers in comparison to traditional field‐based methods. Here, we evaluated the applicability of different methods for estimating grain sizes in gravel bars along a study reach in the Olentangy River in Columbus, Ohio. We collected imagery of these gravel bars with a UAV and processed those images with SfM photogrammetry software to produce three‐dimensional point clouds and orthomosaics. Our evaluation compared statistical models calibrated on topographic roughness, which was computed from the point clouds, and to those based on image texture, which was computed from the orthomosaics. Our results showed that statistical models calibrated on image texture were more accurate than those based on topographic roughness. This might be because of site‐specific patterns of grain size, shape and imbrication. Such patterns would have complicated the detection of topographic signatures associated with individual grains. Our work illustrates that UAV‐SfM approaches show potential to be used as an accessible method for characterising surface grain sizes along rivers at higher spatial and temporal resolutions than those provided by traditional methods.

Hyporheic exchange and nitrogen cycle processes under the dual effects of flooding and heterogeneous streambed

In this study, we investigated the effects of media heterogeneity and flood processes on hyporheic exchange and nitrogen cycle processes in hyporheic zone adjacent to a natural in-stream gravel bar. We conducted field experiments and set a reactive transport model of surface-groundwater exchange based on the monitoring results. The flood process is obtained based on field monitoring, and the heterogeneous medium field is constructed using the SAR3DC method based on in-site experiment results. The results showed that both the heterogeneity of the medium and the flood process can enhance the spatial variability of velocity distribution, resulting in an increase of exchange flux, a decrease in lag time, and an extension of the migration path. For heterogeneous media, it is easier to form specific channels for high velocity flow, and the flow velocity within them is more sensitive to changes in media and hydrodynamic conditions, the direction of hyporheic exchange can be reversed in some specific channels. For the nitrogen cycle processes, the heterogeneous medium condition determines the source-sink functions and the direction of nitrate nitrogen reduction in hyporheic zone by providing a wider reaction space for reactants, reducing the lag time to 1/40 and increasing the migration path by 4 times. The flood process can increase the infiltration amounts of reactants by 5 times and reduce the lag time (time of particle entry into the hyporheic zone to migrate out) to 1/6. In general, under the effects of media heterogeneity and flood processes, the reaction rates and nitrate nitrogen reduction rates during the nitrogen cycle processes are decreasing and the hyporheic zone will face greater pollution risks.

Geomorphologic evidences and hydrologic reconstruction of Holocene catastrophic flood events in the Yarlung Tsangpo Grand Canyon, Eastern Himalaya

Catastrophic floods from breaching of natural dams on the Namche Barwa and Giala Peri play an important role in shaping high-mountain landforms in the eastern Himalayan syntaxis. However, there is limited knowledge of geomorphologic evidences and magnitude of outburst flooding in the Yarlung Tsangpo Grand Canyon. In this paper, geomorphic impacts of high-magnitude outburst floods (104–106 m3/s) were analyzed using palaeoflood hydrology technology. Well-preserved 2018 Sedongpu (SDP) flood and 2000 Yigong (YG) flood sand bars are charactered by grey sands with parallel laminations, whereas megaflood geomorphic evidences in the Grand Canyon are dominated by massive slackwater deposits and lateral gravel bars. The palaeoflood events were dated to 4.3–2.4 ka, corresponding to the known Holocene megafloods in the lower Yarlung Tsangpo River. These flood ages are obviously younger than most of remnant lacustrine deposits during the late Pleistocene, but consistent with the ages in the top part of palaeolake and glacial moraines of the Namche Barwa. The chronological data help to confirm the highest probability phases of outburst floods from moraine-dammed lakes. The repeated megaflood peak discharges range from 0.5 to 1.1 × 106 m3/s, which are approximately 5–10 times greater than the maximum peak discharge of 2000 YG flood (9.5 × 104 m3/s) and 20–40 times than that of 2018 SDP flood (2.5 × 104 m3/s) in the Medog reach. Megaflood waters are always deepest (∼200 m) and fastest (∼50 m/s), which are at least twice times larger than 2000 YG flood. Higher shear stress (>3.3 kPa) and stream power per unit area (>168 kw/m2) occurred in narrower channel and sharp bend with higher velocity. Our model also predicts some potential erosion and deposition landscapes resulted from floodwaters routing through the Grand Canyon. These results address the geomorphic impacts of catastrophic flood events with magnitudes (104–106 m3/s) in the Yarlung Tsangpo Grand Canyon. Our new palaeoflood geomorphologic evidences provide a key constraint on the age and magnitude of Holocene megafloods in the largest Himalayan River.

The Blackwater is not a Back Water: Locating the Mesolithic and its Environment at Eversley Quarry, Fleet Hill Farm, Finchampstead, Berkshire

Archaeological fieldwork at Eversley Quarry, Fleet Hill Farm, Finchampstead, Berkshire documented evidence of Mesolithic activity, associated with paleoenvironmental deposits, on the Blackwater River floodplain, a river for which activity of this period was previously unknown. The discovery evolved from initial recognition of worked flint artefacts across a well weathered, stripped subsoil surface in part of the site. Additional material was collected subsequently from the summit of an adjacent low knoll. The findings were of sufficient extent and importance to warrant supplementary archaeological fieldwork using a gridded test pit strategy to evaluate the Mesolithic potential in remaining parts of the site. This resulted in the identification of additional clusters of worked flints, which were preserved in situ.The clusters were predominantly of Mesolithic date but also included Neolithic and Bronze Age artefacts, indicating prolonged use of the landscape. Concentrations were consistently located on slightly elevated sand bars flanking palaeochannels of a formerly braided river system. The contemporaneity of the palaeodrainage and Mesolithic activity has been confirmed by radiocarbon dates from peat that formed during the Holocene. The collective results mark a significant contribution to knowledge of the Blackwater River valley, a major communications artery in the Mesolithic period linking the west end of the Wealden Greensand to the Rivers Thames and Kennet. These findings also highlight the importance that river valleys can make to locations that have been less well studied but nevertheless enjoyed prolonged use.

Effects of the spacing between two hydrokinetic turbines on the bedforms by numerical simulations

Accelerating hydrokinetic renewable energy development towards endurance requires investigating interactions between the hydrokinetic turbine and its surrounding physical environment. Interactions between hydrokinetic turbines (HT) and mobile sediment bed are considered as a critical area of assessment, however limited research studies have been published to address this issue. Hill et al. (2015) have shown experimentally that the presence of either single or multiple turbines and the rotation of the blades affect the bed morphology. Musa et al. (2019) have investigated experimentally the local effect of streamwise aligned turbines on the bedload, they found as a result that the geomorphic effects are stronger with increasing shear stress due to the presence of the rotors, inducing an alternating scour-deposition phenomenon. Chen et al. (2017) have investigated the influence of rotor blade tip clearance (distance between blades and seabed) on the scour rate of pile-supported horizontal axis current turbine. The results suggest that the decrease in tip clearance increases the scour depth, hence more sediment transport. Recently, Khaled et al. (2021) have studied the impact of hydrokinetic turbine on erodible sand banks, they showed numerically a significant interaction between the confinement of the turbine and its impact on the near bottom.
 
 In the present issue, we study the impact of two interacted turbines on the near bedform morphology. A modelling framework is derived to predict the significant transport induced by the turbines, such as the Euler-Euler (EE) multiphase model for sediment transport and the Blade Element Method (BEM) to model the forces generated by the turbines, using the open source platform OpenFOAM and the library SedFoam (Chauchat et al. 2017). A phase of validation is presented for the combined model (EE and BEM) using experimental results of Hill et al. (2015). The present study consists in considering one sediment class, sand of diameter of 0.25 mm, and two horizontal axis turbines with an axial flow direction corresponding to the riverine case. The approach is configured with four different axial inter-turbines distances. The wake distribution behind the second turbine is altered by the wake of the upstream turbine in all configurations (fig. 2). This interaction promotes the erosion under the turbines and the deposition along the axis of the turbines in the wake due to relation between the dynamics of ripples generation and the wake effects of turbines (fig. 3).