Gravel Bed Research Articles

Modelling restoration of natural flow regimes in dam impaired systems: Biomorphodynamic effects and recovery times

Dams affect the natural flow regime by altering the magnitude, timing and frequency of high and low flows. Many river ecosystems impaired by dams are currently being restored. Restoration success is difficult to quantify and is often assessed by comparing the restored system to an unimpaired static ‘reference’ system. However, restoring a river to past environmental conditions and assessing restoration success by comparing it to a static situation neglects natural system dynamics and non-linear, adaptive system responses. With this modelling study we evaluate long-term changes in river morphology, morphodynamics, riparian vegetation cover and habitat suitability of two fish species and two types of wetland vegetation in a meandering gravel bed river after removal of an upstream dam and complete restoration of the natural flow regime. We assessed the ecological and hydromorphodynamic recovery of systems impaired by two different dam operating regimes and three different time periods the dam was present by comparing these to a dynamic undisturbed situation. Modelling results show that recovery potential depends on how much the system has been changed by the dam and the system state at the start of the restoration, rather than the duration of the pressure. Even if the conditions shortly after restoration are comparable to pre-disturbance conditions, there can still be a time-lag in the system response where the future state of the restored system continues to deviate from the undisturbed situation. When this happens, the system can develop into an alternative dynamic equilibrium where recovery becomes increasingly difficult. These results stress the importance of considering natural variability in restored systems as well as in reference systems, requiring detailed spatio-temporal monitoring to assess restoration effects.

Conservative solute transport processes and associated transient storage mechanisms: Comparing streams with contrasting channel morphologies, land use and land cover

AbstractLand use within a watershed impacts stream channel morphology and hydrology and, therefore, in‐stream solute transport processes and associated transient storage mechanisms. This study evaluated transport processes in two contrasting stream sites where channel morphology was influenced by the surrounding land use, land cover, climate and geologic controls: Como Creek, CO, a relatively undisturbed, high gradient, forested stream with a gravel bed and complex channel morphology, and Clear Creek, IA, an incised, low‐gradient stream with low‐permeability substrate draining an agricultural landscape. We performed conservative stream tracer injections at these sites to address the following questions: (1) How does solute transport vary between streams with differing morphologies? and (2) How does solute transport at each stream site change as a function of discharge? We analysed in‐stream tracer time series data and compared results quantifying solute attenuation in surface and subsurface transient storage zones. Significant trends were observed in these metrics with varying discharge conditions at the forested site but not at the agricultural site. There was a broad range of transport mechanisms and evidence of substantial exchange with both surface and hyporheic transient storage in the relatively undisturbed, forested stream. Changing discharge conditions activated or deactivated different solute transport mechanisms in the forested site and greatly impacted advective travel time. Conversely in the simplified agricultural stream, there was a narrow range of solute transport behaviour across flows and predominantly surface transient storage at all measured discharge conditions. These results demonstrate how channel simplification inhibits available solute transport mechanisms across varying discharge conditions.

Debris‐Flood Hazard Assessments in Steep Streams

AbstractDebris floods most commonly occur in steep mountain channels and on their alluvial fans but can also occur on small gravel bed rivers with watershed areas up to several hundred square kilometers. This became obvious during July 2021 and November 2021 debris floods in northwestern Germany and southwestern British Columbia, Canada. We subdivide debris floods into three categories: those triggered by a supra‐critical bed shear stress ratio, form by dilution from debris flows, and resulting from outbreak floods. This trichotomy challenges traditional hazard assessments; debris floods classify as a fluvial process, yet their destructive mechanics are difficult to characterize. Key hazards interact spatially and temporally in debris floods: inundation, scour, sediment transport and deposition and bank erosion. We describe approaches to quantify hazards by systematically accounting for these processes and introduce a novel approach for hazard quantification and mapping in which flow velocity, depth, and presumed fluid density are combined with the annual event frequency for all event scenarios. The derivative “composite hazard maps” are equally valid for debris floods and debris flows. Isolines of bank erosion based on a probabilistic analysis of a physically based model are added to capture the potential of debris floods to abruptly widen their channels. Substantial challenges remain, specifically in the reliable prediction of sediment transport and progressive bank erosion. Our intention is to homogenize debris‐flood hazard assessments and especially mapping methodologies. This could allow for systematic integration with landuse policies assuring consistency among approaches executed by practitioners acting in this field.

Determination of Skin Friction Factor in Gravel Bed Rivers: Considering the Effect of Large-Scale Topographic Forms in Non-Uniform Flows

Determination of skin friction factor has been a controversial topic, particularly in gravel-bed rivers where total flow resistance is influenced by the existence of small-scale skin roughness and large-scale topographic forms. The accuracy of existing models predicting skin friction factors in conditions where small-scale skin roughness and large-scale topographic forms exist is very low. The objective of this study is to develop a modified model that improves the accuracy of the determination of skin friction factors in gravel-bed rivers. To this end, 100 velocity profile data obtained from eight gravel-bed rivers were utilized to develop an analytical method that considers the momentum thickness of the boundary layer and its deviation in large-scale topographic bedforms in a 1D force-balance model. The results show that the accuracy of the skin friction factors is enhanced when (1) the model is in the form of an exponential function of energy slope, and (2) the deviation of momentum thickness is considered in the model. The proposed model results in high accuracy of the predicted skin friction factors for energy slopes between 0.001 and 0.1, which exist in most gravel-bed rivers with different morphologies. Additionally, this study model was used to modify the classic Einstein–Strickler equation. The modified equation resulted in improved accuracy of the predicted skin friction factors in non-uniform flow conditions even when velocity profiles and energy slope were not available.

Gravity currents with large salinity in a gravel bed valley: an experimental study

Abstract In the current study experiments in a valley (a composite cross-sectional tank consisting of a rectangular upper cross section and a lower trapezoidal) were performed. The purpose of this research is to study the gravel bed in gravity currents with large salinity. The tank is separated in two parts by a vertical gate, which separates the fresh water with the gravel bed from the salt water with the impermeable bed. When the gate opens, a flow is created due to density difference. The salinity densities are large (from 1,050 to 1,090 kg/m3). These specific densities have been chosen to simulate liquid wastes that affect the environment. Compared to experiments in the same tank without a gravel bed, it is observed that the gravel bed reduces the propagation of currents and mainly reduces the current with the smallest density difference at the lowest height. The main conclusion of this study is that the kind of channel bed plays a key role in the propagation of gravity currents. The gravel bed affects significantly the velocity and the distance traveled by the currents in comparison with impermeable bed.

Glaciofluvial sequences recording the Birrfeld Glaciation (MIS 5d\u20132) in the Bern area, Swiss Plateau

In this paper, we document that glaciofluvial gravel sequences and glacial till deposits that are exposed in the Müntschemier and Finsterhennen gravel pits (Swiss Plateau west of Bern) record three glacial advances during the Birrfeld Glaciation, which corresponds to the last glacial cycle. Sedimentological logging shows that both gravel pits expose deposits of glaciofluvial braided river systems. These sediments are overlain by a till that was deposited during the Last Glacial Maximum (LGM). The results of the provenance analysis imply that the sediments were mainly supplied by the Valais Glacier, which originated in the Central Alps. A minor contribution of the material was supplied by the Saane Glacier with sources in the northern parts of the Alps. In addition, the morphometric analysis particularly of quartzite clasts in the till deposits indicate that while some clasts (the angular ones) were eroded and transported by the Valais Glacier from the Central Alps to the depositional site, the majority of the quartzite constituents (the rounded ones) were most likely reworked from the Molasse bedrock or older gravels. This implies that a large fraction of the sediments in the Müntschemier and Finsterhennen gravel pits could represent recycled material from older fluvial gravels and conglomerates that were then reworked by the glaciers as they advanced to the foreland. Based on the sedimentological data and considering published and new optically stimulated luminescence (OSL) chronological data, we propose a landscape evolution scenario where the first glacial advance occurred during Marine Isotope Stage (MIS) 5d. The second glacier advance followed during MIS 4, while the last one during the Last Glacial Maximum (LGM), which corresponds to the MIS 2. The MIS 5d advance is recorded by the lowest unit of the Müntschemier gravel pit and consists of a fining upward sequence made up of an alternation of gravel and sand beds. The MIS 4 advance is recorded by the unit beneath the LGM till at Müntschemier and by the lowermost layer in the Finsterhennen gravel pit. It comprises an alternation of gravel and sand beds, which coarsens and thickens upwards. The LGM advance, finally, resulted in the deposition of amalgamated gravel beds at Finsterhennen, which ended with the construction of a till that is encountered on the top of both gravel pits. Sediments related to the interstadial conditions between MIS 5a and MIS 5b and MIS 3 were not encountered, which suggests that the warmer periods were characterised by non-deposition and/or erosion, which possibly resulted in the observed sedimentary hiatus. Although the chronological results are still preliminary, the available information allows us to suggest that during the Birrfeld Glaciation, the Valais lobe advanced several times to the Swiss Plateau. In addition, the facies associations imply that the eastward expansion of the Valais lobe during the MIS 5d and MIS 4 were most likely shorter than during the LGM.

Effects of substrate types on nitrogen removal efficacy and growth of Canna indica L.

Constructed wetlands (CWs), a cost effective technology for wastewater treatment, consist of substrates and wetland plants, which should be selected carefully to gain highest treatment efficiency. However, studies on plant growth and responses to different types of substrates are very few. This study aims to assess the effects of substrate types on growth and root morphology of Canna indica L. and nitrogen (N) removal. Twenty-four similar sized approximately 1 month old C. indica plants were selected and grown on different substrates (gravel, pumice and biochar). All plants were supplied with a standard growth medium to which 14 mg L-1 NH4+, 14 mg L-1 NO3-, 3 mg L-1 PO43- were added and pH was adjusted to 6.5. The growth solution was renewed every week. The plants were grown under greenhouse conditions for 45 d. Results showed that growth of C. indica was not significantly different among treatments but differences on root morphology were found. Plants grown on pumice had the largest root diameters while plants grown on biochar had the longest roots. In the gravel-filled treatment, C. indica showed the lowest root diameter and root length but formed more internal air space in its roots. It indicates that types of substrate can affect O2 supply and root morphological adaptation. Moreover, the porous substrate bed systems were capable of eliminating more NH4+ than gravel bed systems, with the NH4+ removal rates of 5.6–6.3 mg L-1 d-1 compared to 4.7 mg L-1 d-1, respectively. The results show that porous substrates can act as plant supporting substrates and play important roles in N adsorption. Also, they can improve oxygen supply and stimulate root growth. Thus, application of porous substrates as filter media could help to increase pollutant removal efficacy of CWs.

Asymmetric competition over space use and territory between native brown trout (Salmo trutta) and invasive brook trout (Salvelinus fontinalis).

Interference competition over food and territory can shape population structure and habitat use within and between species. The introduction of invasive species often leads to novel competitive interactions over shared resources and invaders can eventually exclude the native species from preferred habitats. Invasive brook trout (Salvelinus fontinalis) introduced to northern Europe have excluded native brown trout (Salmo trutta) from numerous headwater streams. The fact that invasive brook trout can displace the more aggressive brown trout is puzzling. However, the earlier spawning and hatching of brook trout, compared to brown trout, may lead to unequal competition due to size advantage and prior resident status of brook trout at the fry stage. In this study, we examine the effect of competition between brown trout and brook trout using the natural size distribution of the two species. In two consecutive experiments, we first measured space use and feeding of a fry (age 0+) in the presence of a juvenile (age 1+). In experiment 2, we assessed territorial interactions between the species at the fry stage (age 0+) and if smaller brown trout could compensate the disadvantage by manipulating residence duration. Fry of brook trout feed sooner and spend more time close to the larger individual than brown trout fry. We also found that brook trout fry won most territorial contests against brown trout, and that increased residence duration led to longer and more aggressive interactions. The results suggest that smaller brown trout are displaced to suboptimal habitats in the presence of a larger brook trout. Therefore, the later emergence from gravel beds resulting in the naturally occurring size disadvantage of brown trout at the fry stage may lead to unequal territorial interactions that could explain why brown trout are displaced from preferred habitats in sympatry with brook trout.

From glacial erosion to basin overfill: a 240 m-thick overdeepening–fill sequence in Bern, Switzerland

Abstract. We drilled a 210 m-thick succession of Quaternary sediments and extended it 30 m upsection with information that we collected from an adjacent outcrop. In the 240 m-thick succession we identified 12 different lithofacies, grouped them into five facies assemblages, and distinguished two major sedimentary sequences. A sharp contact at 103 m depth cuts off cross-beds in sequence A and separates them from the overlying horizontal beds in sequence B. Although the lowermost facies assemblage of each sequence includes a till deposited during a period of ice cover, the two tills differ from each other. In particular, the till at the base of sequence A is dominated by large clasts derived from the underlying Molasse bedrock, whereas the till at the base of sequence B has no such Molasse components. Furthermore, the till in sequence A bears evidence of glaciotectonic deformation. Both tills are overlain by thick assemblages of subaqueous, most likely glaciolacustrine and lacustrine facies elements. The cross-bedded and steeply inclined sand, gravel, and diamictic beds of sequence A are interpreted as deposits of density currents in a subaqueous ice-contact fan system within a proglacial lake. In contrast, the lacustrine sediments in sequence B are considered to record a less energetic environment where the material was most likely deposited in a prodelta setting that gradually developed into a delta plain. Towards the top, sequence B evolves into a fluvial system recorded in sequence C, when large sediment fluxes of a possibly advancing glacier resulted in a widespread cover of the region by a thick gravel unit. Feldspar luminescence dating on two samples from a sand layer at the top of sequence B provided uncorrected ages of 250.3 ± 80.2 and 251.3 ± 59.8 ka. The combination of these ages with lithostratigraphic correlations of sedimentary sequences encountered in neighboring scientific drillings suggests that sequence B was deposited between Marine Isotope Stage 8 (MIS 8; 300–243 ka) and MIS 7 (243–191 ka). This depositional age marks the end of one stage of overdeepening–fill in the perialpine Aare Valley near Bern.

Drag Coefficient of Submerged Flexible Vegetation Patches in Gravel Bed Rivers

Vegetation patches and strips either along riverbanks or in channel beds are essential for the protection of erosion and sedimentation processes. In the present study, the drag coefficient Cdv of submerged flexible vegetation patches in gravel bed rivers was investigated. A total of 13 vegetation patches with different densities were studied in disparate reaches of the Padena Marbor and Beheshtabad gravel bed rivers in Iran. Water depths, flow velocities, and particle grain sizes around these vegetation patches were collected. The Saint-Venant equation and various empirical equations for estimating the drag coefficient were applied to study hydrodynamics in the presence of vegetation patches under nonuniform flow conditions. Furthermore, the drag coefficient factor of flexible vegetation was used to represent the flexibility of vegetation patches and drag characteristics, which were explored from the perspective of material mechanics. The results showed that the calculated values of Cdv exhibited nonuniform variations with the increase in the Reynolds number along the streamwise direction due to flow nonuniformity. Two effects caused by flexible vegetation patches were observed, namely, the sheltering effect (for Red>580) and blockage effect (for Red<450). In most of the vegetated patches, the sheltering effect was dominant, which reduced the drag coefficient. Finally, a fitting formula was proposed based on the drag coefficient factor and Cauchy number.

Emulating the natural spawning conditions of Labeobarbus brevicephalus (PISCES: Cyprinidae) from Lake Tana, Ethiopia

Labeobarbus brevicephalus is one of the 17 endemic migratory large African barb species making up a species flock in Lake Tana (Ethiopia). The multiple constructions of dams and irrigation weirs on the tributary rivers of Lake Tana have become detrimental to the upstream spawning migration of these fishes. Considering worst-case scenarios of obstruction of all-natural migratory routes, artificial spawning emulation structures were designed and developed at the Bahir Dar Fisheries and Other Aquatic Life Research Center (BFALRC) to emulate the upstream spawning migration routes and habitats. A pond was constructed to emulate the lake and a canal was connected to the pond emulating tributary rivers. The setups were to enable the fishes to migrate out of the pond and spawn at the upper area (pool) of the canal. At the upstream of the canal, gravel beds were prepared on which the L. brevicephalus could spawn. Running (ready for spawning (stage 5)) male and female L. brevicephalus were placed in the pond. After a day long-acclimatization period lake water was pumped into the canal at a discharge rate of 250–1000 liters/min. Stimulated by the flow, 86 male and 24 female L. brevicephalus moved out of the pond migrated to the upper part of the canal made 16 spawning on the gravel beds, and returned to the pond. The fertilized oocytes (zygotes) were collected and it was possible to hatch them in aquariums in the laboratory.

Analytical model of flow velocity in gravel-bed streams under the effect of gravel array with different densities

The standard exponential and logarithmic laws for flow velocity evaluation cannot be applied for gravel-bed streams due to the effect of large-scale roughness. An analytical model based on a two-layer subdivision approach was developed to predict flow velocity over a gravel array with different densities. The velocity distribution in the gravel layer was derived by solving the Darcy-Forchheimer-Brinkman (DFB) differential equation based on the concept of porous medium flows. The velocity in the surface layer was modeled by the mixing length theory with the length scale modified by taking into account the porous effect of the gravel (ball) arrays. To assess the model, flow velocities over gravel beds simulated by Ping-Pong ball arrays with different densities (or porosities) were measured in a laboratory flume. The results show that the predicted velocities by this subdivision model agree well with experimental data, reflecting that the porosity of the gravel array essentially impacts the flows both in the gravel layer and surface layer. In comparison with the existing models not considering the porous effect of the gravel bed, this model can effectively predict vertical velocity distributions and thereby flow resistances in gravel streams. The permeable interface velocity atop the gravel layer, as the boundary condition for the two-layer model, is sensitive to the bed context effect rather than the hydraulic effect, suggesting that the inflection point of the full velocity profile might be vital in determining the accuracy of the model. Moreover, with other data sources over dense gravel-bed, our model is suitable for data pre-dealt with the spatially averaging, where the spatial non-uniformity effect is great.

Effects of plant density and stem pruning on plant biomass yield and economic benefits in a low-cost gravel bed aquaponic system

ABSTRACT The combined yields of Oreochromis mossambicus and fruit crops – tomato, sweet pepper, and cucumber were investigated in a low-cost designed gravel bed aquaponic system to maximize overall yield. This study was modeled according to bioeconomic principles, which analyzed the experimental setup’s system design and management, biological characteristics, and economic performance. The study was conducted from March 2018 to February 2019. The growth performance of fish was determined using growth rate parameters, length–weight relationship, and condition factor. The effect of plant density and plant stem pruning was used to determine plant yield. Tomato, sweet pepper, and cucumber were subjected to two plant densities (5 and 8 plants.m2). Tomato and cucumber plants were pruned to one and two stems, while sweet pepper plants were pruned into two and three stems. A complete randomized design in triplicate was used. The FCR of 1.25; 97.5% survival rate; specific growth rate of 1.64%, LWR r2 of 0.945, regression coefficient b of 3.1 and condition factor K of 1.93 were recorded for the fish performance. Total yield, marketable yield, unmarketable yield, fruit mass, and number fruits.m2 were recorded. Tomato and cucumber plants pruned into two stems produced significantly (p < .05) higher total and marketable yields than plants pruned to one stem. Sweet pepper plants pruned into three stems produced significantly (p < .05) higher total and marketable yields than those pruned into two stems. A plant density of 8 plants.m2 produced a higher (p < .05) total and marketable yield of tomatoes, sweet peppers, and cucumbers than 5 plants.m2. Biomass yield in aquaponics can be optimized for higher economic viability by manipulating environmental conditions, adopting strategic culture management practices, and targeting higher market prices through scheduled production.

Spatial variability of the erodibility of fine sediments deposited in two alpine gravel-bed rivers: The Isère and Galabre

In mountainous environments, high suspended sediment load during runoff or dam flushing events can lead to important amounts of fine deposits in gravel bed rivers. Fine sediment deposits may contribute to bar elevation, riparian vegetation growth and consequently to bar stabilization. Despite their contribution to the morphodynamic of mountain rivers, the erosion properties of fine sediments in this context is not fully understood.In order to investigate the dynamics of re-suspension of these deposits, field monitoring campaigns were performed to explore both the spatial variability and the controlling factors of the erodibility of fine deposits. A cohesive strength-meter (CSM), along with moisture, grain sizes, geographical position and elevation were used to evaluate both the critical bed shear stress for erosion and erosion rate of fine sediment deposits in two rivers of the French Alps: the Isère and Galabre.The results highlight a large variety of fine sediment deposition areas, which are discontinuous compared to those in estuaries and lowland rivers. A high spatial variability of erodibility was observed on the reach, the bar and the metric scale. While no upstream–downstream trend was observed at the scale of both studied reaches, the locations of the deposits, elevation from the river surface and their moisture were inter-related variables and with the highest correlations to erodibility. Measurements showed that both dry and humid deposits located at the highest and lowest elevation from the river surface respectively, were more easily eroded than intermediate deposits with medium moisture.

Modulating the lateral migration of a gravel bed channel using the coarse tail of the bed material distribution

SummaryMost stream channels around the world have been, to some extent, laterally constrained by non‐erodible banks constructed of concrete or large angular boulders (called riprap). These lateral constraints generally have a negative impact on aquatic ecosystems by reducing stream channel complexity, and limiting the natural disturbance processes that are critical for renewing the physical habitat. In this paper, we test a novel approach to limiting stream channel instability by exploiting the stabilizing effects of the coarsest grains in the bed material distribution, which we call geomorphic river engineering. This approach modulates the rate of bank erosion rather than halting it altogether. It also exploits the geomorphic feedback between bank erosion, channel width, and mean boundary shear stress during a flood event to limit the potential for vertical incision into the stream bed. The basic premise of the geomorphic river engineering approach is to place the coarse sediment on the floodplain adjacent to the channel margins, and rely on bank erosion by the river to recruit the sediment, allowing the river to remain the architect of its own configuration. Our results suggest that the recruitment of a modest volume of coarse sediment to a coarse, gravel bed stream during a flood could potentially dramatically reduce the amount of lateral migration that would otherwise occur, and that the degree of stabilization could be modified by changing the volume of coarse sediment per unit area placed on the floodplain. More generally, our results confirm the fundamental role that the coarse tail of the bed material distribution plays in dictating the channel dynamics of gravel bed streams with cohesionless banks, and indicate the potential for future river stabilization projects to focus on the engineering that can be done by rivers, rather than to them.

Countermeasure against local scouring and tsunami damage by landward forests behind a coastal embankment

In the Great East Japan Tsunami, many coastal embankments collapsed due to excessive local scouring at the embankment toe. Thus, as a countermeasure against local scouring and tsunami damage, the present study performed a series of flume experiments with landward forests models (the combination of a short frontal forest (SF) and a landward tall forest (TF)) behind a coastal embankment model (EM) on a gravel bed. The damming of washed-out trees as debris occurs due to the excessive local scouring around the embankment; if they are properly managed, they could also contribute to reducing the tsunami energy. Thus, the effect of damming by tsunami-borne driftwood on scouring was also investigated. The maximum reductions of the scour depth at the EM toe, maximum scour depth, and scour length were found to be approximately 44.1–65.2%, 30.2–58.6%, and 21.9–41.9% compared with the single embankment system when the SF was placed between the EM and TF (i.e., Case 3). Moreover, damming (i.e., Case 5) also effectively reduced the local scouring around the EM toe. However, at the highest overflow depth, significant scouring was observed beneath the formed driftwood dam, which implies the washout of TF. Both Case 3 and the damming case increased the energy reduction due to the greater shift of the position of a submerged hydraulic jump toe toward the EM crest and an increased backwater rise in front of TF. However, considering the vulnerability of TF, Case 3 is recommended to mitigate a large tsunami.

Wake flow patterns and turbulence around naturally deposited and installed trees in a gravel bed river

AbstractLarge wood structures, such as wood fragments, debris jams, or entire trees, create flow and habitat diversity in rivers. A key flow feature associated with such structures is the wake, characterised by a core zone of reduced velocity and shear layers at its margins. Wakes are largely controlled by geometric and structural properties of the wood. In the present study, the flow patterns and turbulence created by different wood structures were compared at two study sites: naturally eroded and fragmented oaks (Site A) and artificial poplar installations (Site B). Flow and turbulence were quantified using pointwise velocity measurements with acoustic Doppler velocimeters (ADVs) and surface particle tracking velocimetry (SPTV). The measured flow patterns exhibited similarities with shallow porous wakes that feature fluid advection through the structure into the wake core downstream. Two additional features of wood structures were identified in the present study: (i) the growth of the shear layers was hindered by bed friction like for shallow mixing layers and (ii) the presence of a tree stem and sediment deposit in the wake centre delayed or even suppressed the interaction of the shear layers and vortex street formation similar to a wake‐splitter plate. Methodologically, the combined ADV/SPTV measurement approach and the use of analytical models for shallow mixing layers proved to be highly valuable to decipher the complex flow patterns around wood structures in the field.

Busan Geoje Fixed Link

The Busan Geoje Fixed Link project is one of the landmark projects in South Korea, providing a road connection between Gaduk and Geoje Island near to the city of Busan at the Southern tip of the Korean peninsula. The project consists of two Cable Stayed Bridges, an Immersed Tunnel, rock tunnels and it has a total length is over 8 km.This paper will focus on the immersed tunnel part of the link and the particular challenges that the project faced in designing and constructing the tunnel, these challenges required a significant evaluation in immersed tunnel technology. The tunnel consists of 18 tunnel elements and has a total length of 3.6 km. The tunnel location was characterised by the following conditions:•a large depth, maximum depth of the tunnel is 48 m below sea level.•the tunnel is located in a seismic sensitive area.•the tunnel is founded largely on weak marine clay, but parts also on bedrock.•part of the tunnel was constructed on a sub sea embankment which is also founded on a weak marine clay.•the tunnel has been constructed in a location that is facing to the Sea of japan and is exposed to a severe wind and wave environment with large long period swell waves present for part of the year.Because for only a few months a year the wave- and weather circumstances were suitable for immersion, extensive numerical and physical model tests were carried out to predict the behaviour of the immersion system for the expected conditions. These model tests were used to determine the wind and wave climate that the immersion system could cope and a go, no-go range of conditions.Additionally, an accurate, sophisticated short and long term wave and weather forecast system was developed. This system enabled forecasts to be made within a 10 cm wave height accuracy, for periods leading up to the element transport and immersion operations which allowed the immersion operations to be possible within acceptable risks.The tunnel elements (TE) were built in batches in a pre-cast yard (PC). After inundation of the dock the elements were transported to a mooring location close to the PC-yard for fitting out and to wait for favourable immersion weather.The elements were immersed using two catamaran pontoons and placed in a previous dredged trench on the sea bed. A taut mooring configuration was used in order to reduce the wave influenced motions to a minimum. Anchor points were created by pre-installed plate anchors.Due to the total length and installation depth of the tunnel the traditional survey systems using towers and total stations were not suitable. Therefore new survey methods were developed, these included a tautwire system and an Ultra Short Base Line (USBL) acoustic system were used for the positioning of the elements during the immersion operation. Fine positioning of the TE’s under swell wave influence was done using a special designed External Positioning System (EPS) and placed on the pre-laid screeded gravel bed.

Relevancy and comparison of friction factor equations for gravel bed in bedload regime

Relevancy and comparison of friction factor equations for gravel bed in bedload regime

Study on the Water Blocking Capacity and Influencing Factors of Hydrophobic Gravel Beds.

Excess water production is a common and extremely serious problem during the process of reservoir development, especially in the middle and later stages of unconsolidated sandstone reservoirs. High water content ratios will negatively affect the recovery efficiency and equipment safety of oil and gas wells. In this study, a hydrophobic quartz sand proppant was prepared with 1,1,1,3,3, 3-hexamethyldisilazane (HMDS) for gravel packing technology. The capillary resistance of hydrophobic quartz sand beds to water was used to achieve water inhibition and oil enhancement. Then, a theoretical capillary bundle model of a quartz sand bed was developed to predict the capillary resistance and water blocking capacity of the quartz sand bed. An evaluation of the experiment results shows that the activation index is up to 96% and the water contact angle is up to 151° after treatment by 0.6 wt % HMDS, which indicates that the quartz sand after modification has strong hydrophobicity. The calculation results demonstrate that the capillary resistance (CR) and the water blocking capacity (WBC) decrease with increasing quartz sand particle size and porosity of the quartz sand bed. As the water contact angle increases, the CR and WBC are also increased. In addition, to make the WBC of hydrophobic quartz sand bed δ > 20%, the parameters of the quartz sand bed must be satisfied: particle size dg < 50 μm, porosity ϕ < 30%, water contact angle θ > 150°, and production pressure difference Δp < 1 MPa. The results of this study could provide theoretical guidance for the construction parameters of hydrophobic gravel packing in high water-cut oil and gas fields.