Alluvial Gravel Research Articles

Woodchip denitrification wall technology trialled in a shallow alluvial gravel aquifer

Woodchip denitrification walls are a tried and tested groundwater nitrate remediation concept in shallow sandy aquifer conditions. There are however no published case studies of them having been applied in heterogeneous, fast-flowing gravel aquifers. Such a pilot study is being made in a shallow alluvial gravel aquifer on the Canterbury Plains, New Zealand, as part of an assessment of whether denitrification walls represent a viable edge-of-field nitrate mitigation option for the New Zealand hydrological landscape. Hydrogeological conditions at the field study site were characterised using a suite of investigative methods, the results from which informed design and placement of an experimental woodchip denitrification wall that was installed in November 2018. The average specific flux in the target gravel aquifer is estimated at 2.7 m/d, and 3.1 m/d through the woodchip wall itself, owing to its hydraulic efficiency. These groundwater fluxes are significantly higher than conditions reported for pre-existing denitrification wall case-studies. Monitoring of the groundwater chemistry over the first year of the denitrification woodchip wall's operational life has shown how the woodchip initially leached labile dissolved organic carbon and created a redox plume in which methanogenic conditions existed. Even though dissolved organic carbon concentrations have restored to background levels, the woodchip wall remains effective at nitrate reduction. The measured nitrate removal rate of between 4.2 and 5.4 g N removed/m3 wall/d is higher than what had previously been predicted from controlled lab-scale studies of the wall media and ranks towards the higher end of published removal rates for denitrification walls. Whilst there is direct evidence that heterotrophic denitrification is contributing to the observed nitrate removal, on the basis of chemical indicators, it is assumed other reactive process, such as dissimilatory reduction to ammonia, anammox, and possibly nitrate-reducing Fe(II)-oxidising reduction reactions may also be contributing to the overall removal of nitrogen in the system. Indications are the woodchip wall is enhancing emission of methane gas, albeit at rates less than what is typically reported for constructed wetlands that are an alternative nitrate-remediation option. Emission of the more potent greenhouse gas nitrous oxide from the woodchip denitrification wall has so far been immeasurably low. Longer-term study of the woodchip denitrification wall is continuing.

Eigenmodels to forecast groundwater levels in unconfined river-fed aquifers during flow recession

Low-land alluvial gravel aquifers are formed from, and tend to be recharged, by rivers. These interconnected river - groundwater systems can be highly dynamic with groundwater levels following the seasonality of the hydrological regime of the river. The associated groundwater resources are regularly under stress during summer periods when abstractive demand is high and recharge is low. Predicting lead-times for critical groundwater levels allows for a more flexible and adaptive groundwater management. An eigenmodel approach is proposed here as a way of making such predictions, fast and efficiently. The eigenmodel is a mathematical concept that represents the hydraulic function of a groundwater aquifer as a set of conceptual linear reservoirs, arranged in-series. River recharge, land surface recharge, and groundwater abstraction for irrigation are considered as model forcings. The eigenmodel approach is demonstrated on three wells of the unconfined Wairau Aquifer in the Marlborough District of New Zealand, which are used for water resources management. Individual eigenmodels were calibrated to historic data and predictive uncertainty bounds were determined by Markov chain Monte Carlo sampling. Hindcasting of past recession periods showed a low predictive error of the models and a good coverage of the predictive uncertainty bounds. The main advantage of the approach is a 4-orders of magnitude higher computational efficiency compared to a numerical benchmark model. This allows for probabilistic simulation in operational forecasting of groundwater levels. The framework is implemented as a web application for 30-day operational forecasts that comprises automatic data downloads and model input generation, stochastic simulation, uncertainty estimation, visualization, and daily updates on a website.

Water tracking in surface water, groundwater and soils using free and alginate-chitosan encapsulated synthetic DNA tracers

Investigating contamination pathways and hydraulic connections in complex hydrological systems will benefit greatly from multi-tracer approaches. The use of non-toxic synthetic DNA tracers is promising, because unlimited numbers of tracers, each with a unique DNA identifier, could be used concurrently and detected at extremely low concentrations. This study aimed to develop multiple synthetic DNA tracers as free molecules and encapsulated within microparticles of biocompatible and biodegradable alginate and chitosan, and to validate their field utility in different systems. Experiments encompassing a wide range of conditions and flow rates (19cm/day-39km/day) were conducted in a stream, an alluvial gravel aquifer, a fine coastal sand aquifer, and in lysimeters containing undisturbed silt loam over gravels. The DNA tracers were identifiable in all field conditions investigated, and they were directly detectable in the stream at a distance of at least 1km. The DNA tracers showed promise at tracking fast-flowing water in the stream, gravel aquifer and permeable soils, but were unsatisfactory at tracking slow-moving groundwater in the fine sand aquifer. In the surface water experiments, the microencapsulated DNA tracers' concentrations and mass recoveries were 1-3 orders of magnitude greater than those of the free DNA tracers, because encapsulation protected them from environmental stressors and they were more negatively charged. The opposite was observed in the gravel aquifer, probably due to microparticle filtration by the aquifer media. Although these new DNA tracers showed promise in proof-of-concept field validations, further work is needed before they can be used for large-scale investigations.

Surface traces and related deformation structures of the southern Sanyi Fault, Taiwan, as deduced from field mapping, electrical-resistivity tomography, and shallow drilling

The southern Sanyi Fault extends across a highly urbanized area in central-west Taiwan. It is within 1–2 km from and generally parallels the northern Chelungpu Fault, which last ruptured in 1999 (Mw = 7.6 Chi-Chi Earthquake) and caused about 2400 fatalities. Several questions thus arise: is the Sanyi Fault likewise active? what is its geometry and extent? and should it be zoned or otherwise designated to mitigate risk through restrictions on development or engineering design? We address these questions by mapping river-exposed outcrops, by interpreting nine, near-surface geophysical surveys (electrical resistivity tomography [ERT]), and by compiling and assessing existing shallow borehole data. Outcrops in the Dajia River show that the Sanyi Fault is mainly a low-angle thrust that juxtaposes poorly lithified sandstone over alluvial gravels. Fault branches are common both in the hanging wall and footwall. To the south, the fault is not marked by a break in topography and its trace has not been clearly mapped within the alluvial plain, in part motivating the current effort. Here the ERT surveys identify the hanging-wall sandstone by a relatively low resistivity of <100 Ωm, whereas the footwall conglomerates and alluvial gravels generally exceed ~300 Ωm and ~ 100 Ωm, respectively. Borehole data likewise constrain the extent and character of the Sanyi Fault, which we identify as extending across the eastern part of Fengyuan, a city with a population now exceeding 160,000. The fault system is clearly imaged in two of five surveyed sites, and we integrate these data with drilling data to map the Sanyi fault surface trace as a ~ 400 m wide zone in the city of Fengyuan. Compiling previously documented paleoseismic data at one site, the Sanyi Fault system likely slipped in the thousand-year range throughout the Holocene. The Sanyi Fault is therefore ‘active’ and warrants zoning or other forms of mitigation to minimize potential, seismically induced loss of life and property.

Effect of open-framework gravel on suffusion in sandy gravel alluvium

Some accidents of dams are attributed to suffusion around open-framework gravel (OFG) in sandy gravel alluvium. However, whether OFG can be really an internal seepage exit of fine particle migration in the field or not? How much the effect of OFG on suffusion is? These problems have not been investigated yet. This paper presented a list of flume-scale suffusion tests to investigate suffusion at the tip of a cutoff wall in sandy gravel alluvium with OFG. The results indicate that suffusion first initiates at the downstream side of the tip of the cutoff wall in the test without OFG, while in the tests with OFG, it initiates at the upstream side of OFG, and then both mainly progress backward to the upstream side. OFG significantly decreases the hydraulic gradients at the initiation of suffusion and at blowout. In addition, the experimental results also confirm that OFG is likely to be an internal seepage exit of fine particle migration in the field. A large number of fine particles and some coarse particles in the sandy gravel are eroded into OFGs in the tests, and OFG significantly increases the settlement induced by blowout; meanwhile, it makes the settlement unstable after blowout.

Soil contamination by tar in the alluvial sediments: case study of the brownfield remediation project in the Czech Republic

The paper aims to analyse the remediation of soils contaminated by black coal tar and other contaminants in relation to risk limits in the different geological environments. The research was implemented as a case study in one of the most expensive remediation projects of contaminated soils in alluvial sediments, in the locality of a former coking plant Karolina in an industrial city of Ostrava in north-east of the Czech Republic. Based on the level of risk, three contaminated geological environments (layers) were selected, which were examined for chemical limits that must be complied with after remediation using ex situ thermal desorption. Around 1.5 million tons (10,720 wagons of 72 m3 and 14 m) of contaminated soils were dug out, which were subsequently treated ex situ by means of thermal desorption. In total, 12,200 t of contaminants were removed, among which non-polar extractable substances (NPES) amounted to 67.94% (8289 t), benzene to 12.25% (1494 t), naphthalene to 11.27% (1375 t), polycyclic aromatic hydrocarbons (PAH) to 6.43% (784 t), benzo(a)pyrene to 1.11% (135 t), phenol to 0.82% (99 t), arsenic to 0.16% (20 t) and mercury to 0.02% (3 t). The most hazardous in terms of contact with humans was the top layer of quaternary alluvial fine-grained soils (3664 t of removed contaminants, 30% of all contamination—layer I). However, the most contaminated layer was the lowest, permeable layer of quaternary alluvial gravel sediments (7479 t of removed contaminants, 61% of all contamination—layer III). The contamination penetrated into the highly permeable layer due to gravity and could not migrate further due to the abundance of impermeable Miocene grey–blue clay.

Stuck in a loop: investigating fabric patterns in the Stone Age gravel sequence at Canteen Kopje, Northern Cape Province, South Africa

Canteen Kopje has long been known for its alluvial diamond-bearing gravel deposits, and historically these have featured frequently in archaeological literature over the last century. Studies have shown the site preserves a stratified gravel sequence with assemblages of Fauresmith mixed into the upper gravels, Victoria West Acheulean (1.2 ± 0.07 Ma) and two Early Acheulean assemblages (1.51 ± 0.8 Ma and > 1.51 Ma). Although previous geomorphological studies provide an understanding of site formation, most recently describing the gravels as a splay deposit at the terminal point of a palaeo-channel, or loop, there have been debates regarding their origin and whether they represent colluvial and/or alluvial deposition. This palaeo-channel facilitated the interaction between two nearby koppies and the palaeo-Vaal River, which subsequently influenced the nature, composition and character of the gravel deposits found across the site. Yet no study has investigated the visibly dipping surface of the gravels, nor sought to explain gravel fabric patterns that are preserved in exposed sequences. Through recording fabric data, this study confirms that the gravels retain an easterly dip direction, which provides strong evidence for the east-to-west flow of water from the upstream direction of the palaeo-channel. These findings challenge earlier models suggesting the upper gravels at Canteen Kopje are colluvial, and have implications for understanding the preserved Earlier Stone Age assemblages. This research also provides a better understanding of geomorphological processes involved in site formation and investigates the overall role that colluvial processes have played at Canteen Kopje.

Estimation of fast groundwater flow velocity from thermal response test results

This study proposes a practical approach for the simultaneous determination of both the groundwater velocity and the effective thermal conductivity of the soil by using the moving line source theory. It is especially the process for verifying that the theory can be applied to the area where the groundwater velocity exceeds 100 m/y. Thermal response test (TRT) is carried out in Kazuno City, Japan having alluvial gravel deposits of 100 m in its depth. A borehole thermal resistance is calculated by a numerical simulation considering the effect of the groundwater velocity under the condition in which an inside borehole was filled with the porous material having a high permeability. A radius of an equivalent single pipe instead of the borehole radius is applied to the calculation model to calculate the circulating fluid temperature. The method can calculate the temperature variations of the equivalent pipe wall, reflecting a grout thermal resistance affected by the groundwater velocity. The groundwater velocity and the effective thermal conductivity of the soil are determined by using root mean square error between the calculated results and the TRT data. As a result, the groundwater velocity and the effective thermal conductivity of soil calculated by the moving line source theory were 120 m/y and 4.7 W/(m K), respectively.

Umbrella Arching and Compensation Grouting in Order to Protect Settlement-Sensitive Buildings over Large Shotcrete Excavations in Gravel

The Tunnel Oberau, with a length of 2.9 km, forms the core of the local bypass of Oberau. Between the two massifs Kirchbichl and Muhlberg, the tunnel is situated in the valley of the Giesenbach in alluvial gravel with only a small overlap underneath settlement-sensitive buildings. In this paper, the measures in the tunnel to minimize the settlements of the buildings during the shotcrete excavations are described. Basis for this planning were extensive 3D-FE-calculations. Furthermore, under two buildings a compensation grouting measure was carried out, in order to compensate the occurring settlements. This paper describes how the required time and the amount of injection material (grout) could be reduced during the compensation grouting in highly permeable gravel.

A case study for identification of organic–silt bottom sediments in an artificial lake formed in gravel alluvium in the geotourism locality of Slnečné Jazerá in Senec (Bratislava, Slovakia)

This case study aims to identify the cubic capacity and geometry of the geological body of silt–organic sediments in the environment of a former gravel pit situated in a drainless depression of the alluvium of the Čierna voda River. It is located in the well-known geotourism locality of Slnečné Jazera in Senec, in the SW of Slovakia. To identify the body, electrical resistivity tomography was combined with the use of sonar. The research shows that this approach is appropriate for a number of activities that are subjects of engineering-geological investigations. The cubic capacity and geometry of specific aqueous engineering-geological environments must be determined in connection with the need for the management, control, quantification and extraction of selected sedimentary bodies. In addition, the management of sustainable development of reservoirs, sedimentation basins, industrial ponds, settling pits and natural pools for recreation (the subject of the case study) is important to control the limit amounts of sediments in such environments. The results of this study may be applied in analogous engineering-geological conditions. The drainless depression Slnečné Jazera contained a body of silt–organic sediments amounting to 23 000 m 3 (41 Olympic-size pools of 25 m × 12.5 m × 1.8 m). The maximum thickness of the bottom sediments was about 6.3 m on the alluvium with an articulated morphology owing to the submerged digging of gravel. The proposed approach improved the control of extraction of the sedimentary body and optimized the engineering-geological conditions in this geotourism locality.

Improvement of an Alluvial Gravel Geotechnical Properties with a Clayey Soil for the Road Construction

The aim of this study is to characterize the alluvial gravel of the Niari river in Congo, to improve its geotechnical properties by stabilizing it with a clayey soil and to find out its utilization in the road construction according to the approved standards in most African countries. The results show that the grain size distribution of the alluvial gravel fits in the standard envelope of the base layer and its resistance to friction of 23% is within the allowed limits. However, its fines content and plasticity index are respectively 4 and 0%, out of the allowed range. Adding the clayey soil increases the CBR index from 0 for the alluvial gravel to 142% for a mixture of 15% of the clayey soil, while the sand equivalent decreases from 80 to 42%. The mixtures of 5, 10 and 30%, respectively of the clayey soil content could be used as the subbase layer in the road construction; that of 15% as the base layer for T3 and T4 traffics and those of 20 and 25% as the base layer for T1 and T2 traffics. Besides, it is found that the CBR index of the mixture of the CBR index is strongly correlated to its density and the relationship between them is parabolic.

Quantifying the restoration success of wood introductions to increase coho salmon winter habitat

Abstract. Large wood (LW) addition is often part of fish habitat restoration projects. However, there is limited information about the spatial–temporal variability in hydraulic changes after LW additions. We investigated reach-scale hydraulic changes triggered after the addition of LW that are relevant to juvenile coho salmon survival. We used Nays2DH, an unsteady two-dimensional flow model, to quantify the patterns and magnitudes of changes of stream velocity and shear stress in three alluvial gravel reaches. The study sites are located in low-gradient reaches draining 5 to 16 km2 in the Oregon Coast Range. Survivable habitat was characterized in terms of critical swim speed for juvenile coho and bed stability considering the critical shear stress required to mobilize the median bed particle size. Model predictions indicated that survivable habitat during bankfull conditions, measured as the area with velocity below the critical swim speed for juvenile coho, increased by 95 %–113 % after the LW restoration. Bed stability also increased between 86 % and 128 % considering the shear stress required to mobilize the median bed particle size. Model predictions indicated more habitat created in the larger site; however, considering that wood would move more frequently in this site there appears to be a trade-off between the timing and the resilience of restoration benefits. Overall, this study quantifies how the addition of LW potentially changes stream hydraulics to provide a net benefit to juvenile salmonid habitat. Our findings are applicable to stream restoration efforts throughout the Pacific Northwest.

On palaeomagnetic dating of fluvial deposits in the section of Neporotove gravel quarry on the Middle Dniester

The paper presents the results of palaeomagnetic studies of Quaternary noncemented deposits from the section of Neporotove on the VIIIth terrace of the Dniester River valley with abnormal thickness (more than 20 m) of the channel alluvium near the village of Neporotove, located on the right bank of the Dniester River. Alluvial facies of river terraces’ deposits are valuable palaeogeographic archives but not quite complete. They are affected by denudation and often do not contain fossils. The possible way for their dating is provided by a palaeomagnetic method performed on suitable for sampling underlying and overlying beds. The alluvial sequence consists of four units. Unit I is composed of inclined gravel-pebble layers with the sand filler with a visible thickness of about 8 m. Unit II has bedded over the denudated surface of Unit I; it consists of light-yellow laminated aleurit loam, 5-30 cm thick underlaid by 0.5-1.5 bed of fine sand. We consider Unit II to be lacustrine deposits accumulated in quiet water. Unit III is represented with inclined or sub-horizontally layered gravel-pebble-boulder deposits with up to 10 m of visible thickness. It includes boulders and blocks of sedimentary rocks up to 1.0 in diameter, considered as drop-stones. In the roof of the gravel-pebble Unit III, there is the white carbonate layer, probably, the illuvial horizon of the palaeosol (mr1), which transited up into the brownish-red horizon A (Unit IV). Unit IV – dark-red sandy-gravel horizon, pedosediment, probably partly the reworked material washed into ice wages in the roof of Unit III. As a result of alternating field stepwise demagnetisation of natural remanence of sediments, we determined that loamy Unit II, which separates members of gravel stratum, and sandy Unit IV, which overlays packs of gravel alluvium, bear characteristic remanent magnetisation (RM) with normal geomagnetic polarity. Taking into consideration lithology and two-fold structure of terrace gravel alluvium as well as palaeomagnetic results, we assume the lacustrine deposits of Unit II was formed during Jaramillo palaeomagnetic subchron corresponding to the end of Shyrokyne Stage, and a pedosediment of Unit IV was formed during Martonosha Stage. The upper alluvial suite of a terrace (Unit III) was generally deposited during a cold stage,directly prior to the onset of lacustrine loam sedimentation, while the lower alluvial member (Unit I) dates from the preceding warm stage. Thus, we defined the geological age of the whole channel alluvium strata of the VIIIth Dniester terrace as Shyrokyne-Pryazovya Stage.

A geochemical and micro-textural comparison of basalt-hosted chalcedony from the Jurassic Drakensberg and Neoarchean Ventersdorp Supergroup (Vaal River alluvial gravels), South Africa

South African basalt-hosted chalcedonies provided an ideal case study, to (a) achieve information about the origin of mineral-forming fluids, the process of chalcedony formation and weathering alteration and (b) to verify if the geochemical and micro-textural characterization of chalcedony allowed multiple provenances to be distinguished. For the first time, Drakensberg chalcedonies from the Karoo Jurassic basalts and Windsorton chalcedonies from the alluvial environment of the Vaal River (traversing the Allanridge basaltic–andesitic lavas) were investigated by optical microscopy, laser ablation inductively coupled plasma mass spectroscopy and isotope ratio mass spectrometry (16O, 18O). The results showed that the compositional differences observed in Drakensberg and Windsorton chalcedonies could not be explained by changes in host rocks composition. The tholeiitic basaltic–andesitic lavas of the Allanridge Formation (Pniel Group, comparison term for Windsorton samples) and the tholeiitic basaltic–andesitic lavas of the Golden Gate Unit (Drakensberg Group, comparison term for Drakensberg specimens) proved to be very similar, also in terms of alteration degree. Conversely, both geochemical and oxygen isotope compositions clearly supported a relatively low temperature, hydrothermal origin for all the investigated chalcedonies, further indicating that differences mostly occurred during their formation (fluid circulation). Moreover, several characteristics suggested deposition in non-equilibrium conditions, although oxidizing conditions were most likely. Weathering processes were effective in the alluvial environment only, leading to a wider compositional heterogeneity of Windsorton chalcedonies with respect to Drakensberg samples. Lastly, a correlation between color and chromophores (Fe and Mn) amounts was lacking while a correlation between the geochemical composition and the texture was clearly observed in several specimens.

Hidrogeološke osobenosti Babušničke (Lužničke) kotline u istočnoj Srbiji

The Babušnica basin near the city of Pirot is a mountain depression composed of Late Oligocene (Ol3) and Pliocene (Rl) sediments. There are alluvial (al) formations and terraces (t) along the Lužnica River, which flows through the depression. From a hydrogeological perspective, the Late Oligocene and Pliocene sediments have not been explored in detail. What is known is that the shallow ("first") aquifer is formed in the upper Pliocene strata (sediments up to a depth of 20 m). The hydraulic conductivity of the water-bearing interbeds and lenses of fine-grained sands and gravels is in the range from K=6,5h10-6 to 1,1h10-5 sm/s. The water-bearing capacity of the alluvial gravels and sands (al) of the Lužnica River is somewhat higher than that of the Pliocene sediments (K=1h10-5 - 1h10-3 cm/s). Hydrochemical analyses of groundwater from the Babušnica basin, conducted from 2013 to 2017, frequently revealed non-compliance with drinking water standards. In most cases, the samples exceeded threshold values for nitrites (NO2 ), the ammonium ion (NH4), oxygen demand (KMnO4 ), iron (Fe), and manganese (Mn). In addition, groundwater from the Pliocene sediments exceeded permissible boron (V) concentrations. As a result, public water supply in the town of Babušnica and 24 villages in Babušnica Municipality relies on groundwater extracted along the southwestern edge of the Babušnica basin and the Lužnica springs in Ljuberađa.

An investigation into structural discretisation as a first-order and pilot framework to understand groundwater–stream water connectivity at a reach scale

This study undertook a structural discretisation as a first-order and pilot framework to understand groundwater–stream connectivity at a local to reach scale. Structural discretisation was used as the categorisation of hydrogeological landscapes, where areas having relatively uniform geology and hydrogeological characteristics were conceived of as a single-hydrologic landscape. The results of the study led to the development of reach-scale groundwater–stream typologies, a new paradigm recommended as a first-order and pilot tool to establish a groundwater–surface water interaction framework in similar settings. The typology on the southeastern side reach of the study area is the lateral contact type where groundwater enters the riparian area aquifer from the terrestrial area aquifer through the laterally continuous alluvial gravel sequence. Therefore, over-abstraction from the terrestrial and/or riparian area aquifer may result in the loss of baseflow into the river. The confined contact-type typology operates on the northern reach where the groundwater–surface exchange primarily takes place between the stream and the riparian area aquifer because, due to low-hydraulic conductivities, the terrestrial area aquifer only permits little groundwater to enter or leave the terrestrial area aquifer. In this case, progressive over-abstractions from the riparian area will primarily draw water originating from the stream through transmission losses, resulting in progressively induced stream infiltration. Consequently, groundwater–surface water interaction typologies can also play a key role in the formulation of conjunctive water resource management for greater water supply security and stability.

Heat contamination in groundwater sourced from heat pump for heating in Bratislava (Slovakia)’s historic centre

The aim of the study is to evaluate the warming effect (thermal contamination) of city centres with high building density. This phenomenon has a number of scientific and practical implications. The case study was conducted in Bratislava (Slovakia), in the geological environment of alluvial gravels of the Danube River, where a permanent warming of the subsurface in the city centre is experienced, which is mainly affected by high building density and underground utilities. Thanks to this thermal contamination, the heat pumps efficiency is greater in winter, while in summer, their effectiveness is lower. Economically, this negative phenomenon in the case of heat pumps has a positive effect, since it increases energy efficiency during the heating period. This phenomenon is not universal and depends on the specific boundary conditions of the geological settings and hydrogeological conditions.

Flume-scale experiments on suffusion at bottom of cutoff wall in sandy gravel alluvium

This paper presents a series of flume-scale experiments to investigate suffusion at the bottom of a cutoff wall in an internally unstable sandy gravel alluvium. The initiation, progression, and potential failure of suffusion and the interactive effects of geomechanical and hydraulic conditions with the evolution of suffusion were investigated in this particular application. Temporal and spatial development of pore pressure, earth pressure, and settlement demonstrated suffusion was a multi-phase (involving pore water, fine and coarse fractions) and multi-field (involving seepage, seepage-induced fine-fraction variation, and stress–deformation) coupling phenomenon. Suffusion initiated at the downstream side of the tip of the cutoff wall and then generally progressed backward to the upstream side. The monitored earth pressure provided an evidence of the heterogeneous stress distribution in internally unstable soil. Two linear empirical formulas for average hydraulic gradients at the initiation of suffusion and at blowout were derived based on the flume-scale model experiments.

Tectonic, eustatic and climatic controls on marginal‐marine sedimentation across a flexural depocentre: Paddy Member of Peace River Formation (Late Albian), Western Canada Foreland Basin

AbstractIn north‐central Alberta and adjacent British Columbia, clastic strata of the middle to late Albian Peace River and Shaftesbury formations were deposited in alluvial to shallow‐marine environments across the foredeep of the Western Canada Foreland Basin. A high‐resolution, log and core‐based allostratigraphic framework for the Paddy Member of the Peace River Formation established nine allomembers, PA to PI, bounded by flooding surfaces and apparently equivalent non‐marine surfaces. Within the estimated 2 Myr. duration of the Paddy, allomembers allow the evolving palaeogeography and changing relationship between accommodation and sedimentation rates to be analysed on time‐steps on the order of 105 years. Paddy strata fill an arcuate depocentre ca 300 km wide, across which the rocks thin eastward from 125 m to ca 5 to 10 m. The northern part of the basin is occupied by muddy, offshore marine deposits that pass abruptly southward into a linear, WSW‐ENE‐trending body of sandstone deposited in a wave‐dominated barrier‐strandplain, at least 350 km long. Extending &gt;200 km to the south of the strandplain was a region of shallow brackish to freshwater lagoons and lakes that graded to the SW into alluvial facies. Within the lagoon region, few‐m thick, elongate and patchy sandstones represent river‐dominated deltas. In allomembers PA to PG, these sandstones are concentrated in the west and south, implying supply from the western Cordillera. In allomembers PH and PI, sandstones are mainly in the east and have a distinctive, quartz‐rich composition. They can be correlated eastward into the coeval Pelican Formation, and were sourced probably from the Canadian Shield on the opposite side of the basin. In the western foredeep, alluvial rocks comprise aggradational, unconfined floodplain deposits with ribbon sandstones, dissected, on at least nine separate levels, by palaeovalleys that are confined to the proximal foredeep. Valleys are 10 to 30 m deep, few km wide, and filled with multi‐storey channel‐bars of pebbly coarse sandstone or conglomerate. Valleys cut down from well‐developed interfluve palaeosols that record a falling and then rising water table. Alternating aggradation and degradation, and advance and retreat of the alluvial gravel front is attributed to cycles of varying rainfall intensity, rather than tectonism or eustasy. Apparently, coeval transgressive‐regressive successions in the lagoon and marine regions are attributed to few‐m scale eustatic changes. On the NE margin of the basin, tidal sandstone fills a northward‐opening estuary cut on the basal Paddy unconformity. This sandstone contains the first well‐documented specimens of Gnesioceramus comancheanus (Cragin), proving contemporaneity with at least part of the marine Joli Fou Formation to the east. Paddy allomembers change shape upward from short blunt wedges, through more acutely tapered wedges, to sheets. This change reflects initially rapid flexural subsidence, attributed to active thickening of the adjacent orogenic wedge. A waning rate of deformation permitted wider dispersal of sediment across the basin, driving broad isostatic subsidence beneath increasingly sheet‐like rock bodies. A major hiatal surface, VE3, records non‐deposition or subtle erosion attributed to erosional unloading and uplift of the adjacent orogen. A subsequent marine transgression is attributed to renewed thickening of the tectonic wedge that triggered deposition of marine mudstone that thickens westward from 0 to &gt;110 m over 300 km. A postulated Milankovitch‐band climatic control on both local gravel supply (via fluctuating rainfall), and shoreline movement (via ?Antarctic glacio‐eustasy or groundwater storage), might account for cycles of alternating incision and aggradation in the alluvial realm. The same mechanism may also explain why shallow‐marine units such as the Cretaceous Viking and Cardium formations contain abundant conglomerate in lowstand shoreface deposits (higher river discharge), yet have highstand shorelines dominated by sandstone (lower river discharge).

Latest Quaternary rapid river incision across an inactive fold in the northern Chinese Tian Shan foreland

This work focuses on the incision process over the Tuostai anticline, a fold of the proximal structure Belt I in the northern Chinese Tian Shan foreland, where the Sikeshu River has incised deeply into the alluvial gravels and the fold's underlying bedrock strata. Field investigation and geomorphic mapping define five terraces of the Sikeshu River (designated as T1 to T5 from oldest to youngest) preserved within the Tuostai anticline. 10Be surface exposure dating and optically stimulated luminescence dating constrain stabilization of the highest three terrace surfaces at about 80 ka (T1), 16 ka (T2), and 15 ka (T3), respectively. Around 16 ka, the calculated river incision rates significantly increase from <2 mm/yr to >6 mm/yr. Undeformed longitudinal profiles of terraces T2, T3 and T4 over the Tuostai anticline suggest that this structure may have been tectonically inactive since stabilization of these three terraces. We thus think that the observed rapid river incision over the Tuostai anticline has not been largely forced by tectonic uplift. Instead, the progressively warmer and wetter palaeoclimatic condition within the Tian Shan range and its surrounding area during the period of ∼20–10 ka may have enhanced river incision across the Tuostai anticline. A reduced sediment/water ratio might have lowered the gradient of the Sikeshu River.