Sediment transport in high‐speed flows over a fixed bed: 2. Particle impacts and abrasion prediction

Single bed load particle impacts were experimentally investigated in supercritical open channel flow over a fixed planar bed of low relative roughness height simulating high‐gradient non‐alluvial mountain streams as well as hydraulic structures. Particle impact characteristics (impact velocity, impact angle, Stokes number, restitution and dynamic friction coefficients) were determined for a wide range of hydraulic parameters and particle properties. Particle impact velocity scaled with the particle velocity, and the vertical particle impact velocity increased with excess transport stage. Particle impact and rebound angles were low and decreased with transport stage. Analysis of the particle impacts with the bed revealed almost no viscous damping effects with high normal restitution coefficients exceeding unity. The normal and resultant Stokes numbers were high and above critical thresholds for viscous damping. These results are attributed to the coherent turbulent structures near the wall region, i.e. bursting motion with ejection and sweep events responsible for turbulence generation and particle transport. The tangential restitution coefficients were slightly below unity and the dynamic friction coefficients were lower than for alluvial bed data, revealing that only a small amount of horizontal energy was transferred to the bed. The abrasion prediction model formed by Sklar and Dietrich in 2004 was revised based on the new equations on vertical impact velocity and hop length covering various bed configurations. The abrasion coefficient k_v was found to be vary around k_v ~ 10⁵ for hard materials (tensile strength f_t > 1 MPa), one order of magnitude lower than the value assumed so far for Sklar and Dietrich's model. Copyright © 2017 John Wiley & Sons, Ltd.     

Colluvial and alluvial sediment archives temporally resolved by OSL dating: Implications for reconstructing soil erosion

Colluvial and alluvial sediments represent important geoarchives to reconstruct long-term soil erosion and to gain insight into the complex system of sediment cascades and sediment fluxes within a catchment. In this respect, the temporal information of sediment archive formation is essential and achievable through optical stimulated luminescence (OSL) dating. In this study, colluvial and alluvial sediments from a mesoscale catchment in northern Bavaria were investigated and dated by OSL. Insufficient sediment bleaching was detected for some of the samples. In these cases, the method proposed by Fuchs and Lang [Fuchs, M., Lang, A., 2001. OSL dating of coarse-grain fluvial quartz using single-aliquot protocols on sediments from NE Peloponnese, Greece. Quaternary Science Reviews 20, 783–787.] was applied for equivalent dose (D_e) determination. The calculated OSL ages are in stratigraphic order and their accuracy is confirmed by ¹⁴C age control. Based on the chronostratigraphies for alluvial and colluvial archives, there is a dominant synchronous sedimentation history for the Medieval and Modern period, but initiation of Holocene sedimentation occurred at different times in the upper and lower parts of the catchment. The latter phenomenon might be explained by the decoupled sediment fluxes between both the slope–channel system and the upper and lower catchment.     

Regional Considerations of Coastline Change,Tsunami Damage and Recovery along the Southern Coast of the Bay of Izmit (The Kocaeli (Turkey) Earthquake of 17 August 1999)

Co-seismic phenomena along the south coastline included liquefaction, subsidenceand tsunami. Construction on areas composed of fluvial and alluvial sediments aswell as unconsolidated fill increased the risk by creating potential for amplificationof seismic waves. Cyclic mobility liquefaction was common along the coastline, andlevel-ground liquefaction was observed. Flow liquefaction is held forth as a possibilityin the Deirmendere submarine landslide. Damage to structures was markedly more in areas of unconsolidated sediments. One or more tsunami struck immediately after the event; the uniformity of tsunami impact indicating a wave coming from 310° suggests that submarine faulting was the major source of tsunami. Over 800,000 m² of subsidence resulted from sediment slumping, fault controlled subsidence, and possibly post-liquefaction sediment compaction. After a brief period of post-event abandonment, reclamation and use of coastal areas is well underway. This creates a tension between human desires pushing for quick and inexpensive re-inhabitation of the coastal areas, and the needs for zoning and building codes for risk reduction. In this high-risk area suchcontrary cultural mandates cannot yield ideal results. It is suggested that an alternativemodel of immediate post-event creation of parks and natural areas that would yield benefit is preferable in coastal areas rather than the enforcement approach currently favored.     

A simple analytical method for calculating temperature perturbations in a basin caused by the flow of water through thin, shallow-dipping aquifers

Fluid flow in sedimentary basins can be driven by a variety of mechanisms. Commonly, flow is localized in relatively thin aquifers or structures and often these permeable units may be approximated by a series of straight planar segments with shallow dip. A simple formula is derived that describes temperature perturbations in a straight aquifer segment with shallow dip. The temperature is determined everywhere along the segment if the flow rate and temperature at one point is known. “Daisy chaining” the solutions allows easy calculation of the temperature perturbation in a basin caused by fluid flow through a shallow-dipping network of linear fault and aquifer segments. The approach may find many applications but is here illustrated the a number of examples related to the origin of Mississippi Valley-type PbZn deposits. The cases considered emphasize the “salinity problem” encountered by models that hypothesize these deposits formed as the result of cross-basin hydrological flow.     

Phytoplankton of the Barents Sea at the Polar Front in Spring

Oceanology - This paper describes the comprehensive studies of phytoplankton carried out in April 2018 in the northwestern Barents Sea, in the Polar Front region (74.8°–76.2° N), at...     

Fractional Crystallization and Liquid Immiscibility Processes in the Alkaline-Carbonatite Complex of Juqui (So Paulo, Brazil)

The Juqui circular intrusion, which is Cretaceous in age (130–135Ma), crops out in the Precambrian gneissic basement in Brazilover an area of 14 km2. It consists of olivine clinopyroxen-itecumulates (with minor olivine gabbros) in the northeastern sector(74 vol.%), whereas ijolites-melteigites-urtites (4%) and nephelinesyenites with minor essexites and syenodiorites (21%) outlinesubannular concentric patterns with an Mg-carbonatite core (1%), in the southwestern part of the complex. Petrographical, bulk rock, and mineral compositional trendsindicate that the origin of the complex can be largely accountedfor by shallow-level fractional crystallization of a carbonatedbasanitic parental magma. Such a magma was generated deep inthe subcontinental lithosphere by low-degree partial meltingof a garnet-phlogopite peridotite source. Mass-balance calculations in agreement with field volume estimatespermit definition of several fractionation stages of the magmaticevolution under nearly closed-system conditions, with inwarddevelopment of zonally arranged side-wall cumulates. These stagesinvolved: (1) fractionation from basanite to essexite magma(liquid fraction F = 33–5%) by crystallization of olivineclinopyroxenite plus minor olivine alkali gabbro cumulates;(2) derivation of the least differentiated mafic nepheline syenite(F = 5–5 %) from essexitic magma by subtraction of a syenodioriteassemblage; (3) exsolution of a carbonatite liquid (5%) froma CO₂-enriched mafic nepheline syenite magma, which also underwentcontinuous fractionation giving rise to ijolite-melteigite-urtitecumulates. The proportion of cumulus clinopyroxene and biotiteand intercumulus nepheline and alkali feldspar in these lastrocks, as well as the absence of alkalis in carbonatite, maybe attributed, at least in part, to loss of alkali-rich hydrousfluids released during and after the unmixing formation of thetwo conjugate liquids. The KD values determined for Mg-carbonatite/nepheline syeniteare lower (1–4–2–9) for light rare earth elements(LREE) than for REE from Eu to Yb (4–6–7–8),in contrast to recent experimental results (Hamilton et al.,1989). A possible explanation is that Juquia Mg-carbonatiterepresents an alreadydifferentiated magma, which underwent extensivefractionation of LREE-enriched calcite. In this way, the highvariability of K₀ REE patterns observed in several alkaline-carbonatitecomplexes can also be accounted for. The remarkably constant initial ⁸⁷Sr/⁸⁶Sr ratios (mostly between0–7052 and 0–7057) support the interpretation ofthe intrusion as having been generated by fractrional crystallizationand liquid immiscibility from a common parental magma. Iligherisotopic ratios (0–7060–0–7078), found mainlyin dykes and in the border facies of the intrusion, may be dueto contamination by the gecissic basement.     

Seasonal soil water storage changes beneath central Amazonian rainforest and pasture

Evaporation and infiltration were compared for tropical rainforest and pasture, near to Manaus, Brazil from October 1990 to February 1992 using measurements of soil water storage over a depth of 2 m. The soil is a clayey oxisol of low water available capacity. In both of the dry seasons studied, the maximum change in soil water storage in the forest was 154 mm and in the pasture it was 131 and 112 mm. Similar behaviour of the soil water reservoir below forest and pasture in the wet season implied that differences in evaporation and drainage were small. In the dry season, soil water storage behaviour in the upper metre of the soil was similar but there were marked differences in the second metre. The pasture took up little water from below 1.5 m but the forest appeared to utilise all of the available water in the 2 m profile in both seasons.

The water balance of the 2 m profile showed that the pasture evaporation rate was equal to that of the forest until storage had decreased 80 mm from the maximum. There was then a decline in pasture evaporation rate to 1.2 mm day⁻¹ as the storage decreased by a further 50 mm. In contrast, the forest uptake rate remained above 3.5 mm day⁻¹ until storage had decreased 140 mm from the maximum (within 15 mm of the extraction limit), before declining abruptly to less than 1.5 mm day⁻¹. There was strong evidence that the forest was able to abstract water from depths greater than 3.6 m.

Spatial variability of soil water storage was significantly greater beneath the pasture than beneath the forest, particularly following rainfall events in the dry season. This was largely the result of redistribution of rainfall as local surface runoff. There was no evidence of redistribution or runoff in the forest.     

Two‐dimensional physically based finite element runoff model for small agricultural watersheds: I. Model development

Soil erosion by water is the root cause of ecological degradation in the Shiwalik foothills of Northern India. Simulation of runoff and its component processes is a pre‐requisite to develop the management strategies to tackle the problem, successfully. A two‐dimensional physically based distributed numerical model, ROMO2D has been developed to simulate runoff from small agricultural watersheds on an event basis. The model employs the 2‐D Richards equation with sink term to simulate infiltration and soil moisture dynamics in the vadoze zone under variable rainfall conditions, and 2‐D Saint‐Venant equations under the kinematic wave approximation along with Manning's equation as the stage‐discharge equation for runoff routing. The various flow‐governing equations have been solved numerically by employing a Galerkin finite element method for spatial discretization using quadrilateral elements and finite difference techniques for temporal solutions. The ROMO2D computer program has been developed as a class‐based program, coded in C + + in such a way that with minor modifications, the model can be used to simulate runoff on a continuous basis. The model writes output for a runoff hydrograph of each storm. Model development is described in this paper and the results of model testing and field application are to be presented in a subsequent paper. Copyright © 2008 John Wiley & Sons, Ltd.