The impact of submersed aquatic vegetation on the development of river mouth bars     

Sara Lera  William Nardin  Lawrence Sanford  Cindy Palinkas  Roberto Guercio 《地球表面变化过程与地形》2019,44(7):1494-1506

This work is inspired by the sudden resurgence of the submersed aquatic vegetation (SAV) bed in the Chesapeake Bay (USA). Because the SAV bed occurs at the mouth of the Bay's main tributary (Susquehanna River), it plays a significant role in modulating sediment and nutrient inputs from the Susquehanna to the Bay. Previous model studies on the impact of submersed aquatic vegetation on the development of river mouth bars lacked a complete mechanistic understanding. This study takes advantage of new advances in 3D computational models that include explicit physical-sedimentological feedbacks to obtain this understanding. Specifically, we used Delft3D, a state-of-the-art hydrodynamic model that provides fine-scale computations of three-dimensional flow velocity and bed shear stress, which can be linked to sediment deposition and erosion. Vegetation is modeled using a parameterization of hydraulic roughness that depends on vegetation height, stem density, diameter, and drag coefficient. We evaluate the hydrodynamics, bed shear stresses, and sediment dynamics for different vegetation scenarios under conditions of low and high river discharge. Model runs vary the vegetation height, density, river discharge, and suspended-sediment concentration. Numerical results from the idealized model show that dense SAV on river mouth bars substantially diverts river discharge into adjacent channels and promotes sediment deposition at ridge margins, as well as upstream bar migration. Increasing vegetation height and density forms sandier bars closer to the river mouth and alteration of the bar shape. Thus, this study highlights the important role of SAV in shaping estuarine geomorphology, which is especially relevant for coastal management. © 2019 John Wiley & Sons, Ltd.  相似文献   

粉煤灰——漂珠复合低密度水泥浆固井试验与应用     

贾成前  冯波  李毅  白雪峰 《世界地质》2011,30(1):138-144

针对吴起地区固井中一些低压地层由于破裂压力低,不能承受常规水泥浆液柱压力,出现固井水泥浆漏失、水泥返高达不到设计要求的问题,开展了低密度水泥浆体系研究。通过室内试验,对减轻材料粉煤灰、漂珠及二者复合体系进行了较系统的研究,确定了适合的配方。研究选用增稳剂ZW 加入低密度体系,提高了低密度水泥的强度。实验结果及现场实践表明: 用减轻材料配制低密度水泥浆及降低水泥浆液柱压力,是确保低压易漏地层固井水泥浆返高的有效途径,对提高固井质量和延长油井寿命都将起到积极作用。  相似文献   

Stable and radio-isotope analysis to determine recharge timing and paleoclimate of sandstone aquifers in central and southeast Libya     

Mohamed Al Faitouri  William E. Sanford 《Hydrogeology Journal》2015,23(4):707-717

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A review of the assessments of the major fisheries resources in South Africa     

CL de Moor  SJ Johnston  A Brandão  RA Rademeyer  JP Glazer  LB Furman 《African Journal of Marine Science》2015,37(3):285-311

The waters off South Africa's coastline boast a rich mix of commercially fished species. Quantitative assessments of these marine resources have developed from simple methods first applied in the 1970s, to models that encompass a wide range of methodologies. The more valuable resources have undergone regular assessments in recent decades, with frequencies closely related to the management approach employed for each fishery. Many of these assessments form the operating models used to simulation-test candidate management procedures. This paper provides a comprehensive review of the assessments of 11 of the most important fisheries resources in South Africa. Some assessments use simple biomass dynamics models, whereas others are a hybrid of age- and length-based models, each designed to model the specific characteristics of the resource and fishery concerned. Many of the assessments have been disaggregated by species/stock and/or area as related multispecies/stock/ distribution hypotheses have arisen. This paper explores the similarities and differences in the data available and the methods applied. The review indicates that, whereas the status of three of these resources cannot be estimated reliably at present, the status of six resources is considered to be reasonable to good, whereas that of abalone Haliotis midae and West Coast rock lobster Jasus lalandii remains poor.  相似文献   

Blowing in the wind. II. Creation and redistribution of refractory inclusions in a turbulent protoplanetary nebula     

Jeffrey N. Cuzzi  Sanford S. Davis  Anthony R. Dobrovolskis 《Icarus》2003,166(2):385-402

Ca-Al rich refractory mineral inclusions (CAIs) found at 1-6% mass fraction in primitive chondrites appear to be 1-3 million years older than the dominant (chondrule) components which were accreted into the same parent bodies. A prevalent concern is that it is difficult to retain CAIs for this long against gas-drag-induced radial drift into the Sun. We reassess the situation in terms of a hot inner (turbulent) nebula context for CAI formation, using analytical models of nebula evolution and particle diffusion. We show that outward radial diffusion in a weakly turbulent nebula can overcome inward drift, and prevent significant numbers of CAI-size particles from being lost into the Sun for times on the order of 10⁶ years. CAIs can form early, when the inner nebula was hot, and persist in sufficient abundance to be incorporated into primitive planetesimals at a much later time. Small (?0.1 mm diameter) CAIs persist for longer times than large (?5 mm diameter) ones. To obtain a quantitative match to the observed volume fractions of CAIs in chondrites, another process must be allowed for: a substantial enhancement of the inner hot nebula in silicate-forming material, which we suggest was caused by rapid inward drift of meter-sized objects. This early in nebula history, the drifting rubble would have a carbon content probably an order of magnitude larger than even the most primitive (CI) carbonaceous chondrites. Abundant carbon in the evaporating material would help keep the nebula oxygen fugacity low, plausibly solar, as inferred for the formation environment of CAIs. The associated production of a larger than canonical amount of CO₂ might also play a role in mass-independent fractionation of oxygen isotopes, leaving the gas rich in ¹⁶O as inferred from CAIs and other high temperature condensates.  相似文献   

Palaeoenvironmental significance of Late Permian palaeosols in the South‐Eastern Iberian Ranges,Spain     

RAÚL DE LA HORRA  Ma ISABEL BENITO  JOSE LÓPEZ‐GÓMEZ  ALFREDO ARCHE  JOSÉ F. BARRENECHEA  JAVIER LUQUE 《Sedimentology》2008,55(6):1849-1873

The Late Permian (Wuchiapingian) Alcotas Formation in the SE Iberian Ranges consists of one red alluvial succession where abundant soil profiles developed. Detailed petrographical and sedimentological studies in seven sections of the Alcotas Formation allow six different types of palaeosols, with distinctive characteristics and different palaeogeographical distribution, to be distinguished throughout the South‐eastern Iberian Basin. These characteristics are, in turn, related to topographic, climatic and tectonic controls. The vertical distribution of the palaeosols is used to differentiate the formation in three parts from bottom to top showing both drastic and gradual vertical upwards palaeoenvironmental changes in the sections. Reconstruction of palaeoenvironmental conditions based on palaeosols provides evidence for understanding the events that occurred during the Late Permian, some few millions of years before the well‐known Permian‐Triassic global crisis.  相似文献   

Some Characteristics of a Propagating Brittle Tensile Crack     

Merle E. Hanson  Allan R. Sanford 《Geophysical Journal International》1971,24(3):231-244

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Soft X-ray diagnostics of the kinematics of high-velocity clouds     

D. A. Bone  T. W. Hartquist  P. Sanford 《Astrophysics and Space Science》1983,92(1):173-176

We have made near-infrared photometric observations of nine β-Cephei and eight δ-Scuti stars inJ, H, andK bands. The observed fluxes are in good agreement with those expected according to their spectral types. We conclude that these stars do not have any anomalous emission in these near-infrared bands.  相似文献   

A Test of the In Situ Permeable Flow Sensor at Savannah River, SC     

Sanford Ballard  Glenn T. Barker  Ralph L. Nichols 《Ground water》1996,34(3):389-396

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Microbial activity and chemical weathering in the Middendorf aquifer,South Carolina     

Jungho Park  Robert A. Sanford  Craig M. Bethke 《Chemical Geology》2009,258(3-4):232-241

We use reactive transport modeling to better understand the kinetics of chemical weathering in the Cretaceous Middendorf aquifer of South Carolina, USA, and the relationship of this process to subsurface microbial activity. We constructed a model accounting for the kinetics of mineral dissolution and precipitation, ion exchange, and the CO₂ and bicarbonate produced by iron reducing and sulfate reducing bacteria in the aquifer. We then fit the model to observed trends in the chemical composition of groundwater along the aquifer by adjusting the rate constants for the kinetic reactions considered. The modeling portrays weathering in the Middendorf as a slow process by which groundwater gradually reacts toward equilibrium with minerals in the aquifer. The rate constants predicted are 6 to 7 orders of magnitude smaller than measured in laboratory experiments and 3 to 4 orders of magnitude less than those inferred from weathering rates in soils. The rate constants are smaller even than expected by projecting observed trends with the duration of weathering to the geologic age of the Middendorf. Weathering is driven largely by biological activity: about half the acid consumed is CO₂ derived from the recharge area, and about half is supplied by iron reducing bacteria in the aquifer; only about 1% of the acid is of atmospheric origin, from CO₂ dissolved in rainwater.  相似文献