Showing papers on "Silt published in 1980"

A physical model for the transport and sorting of fine‐grained sediment by turbidity currents

Abstract: Turbidite muds in cores from the outer Scotian continental margin, off eastern Canada, contain abundant thin silt laminae. Graded laminated units are recognized in parts of this sequence. These represent single depositional events, and show a regular decrease in modal grain size and thickness of the silt laminae through the unit. A similar fining trend is shown by both silt and mud layers over hundreds of kilometres downslope. Textural analysis of individual laminae allows the construction of a dynamically consistent physical model for transport and sorting in muddy turbidity currents. Hydraulic sorting aggregates finer material to the top and tail regions of a large turbidity flow which then overspills its channel banks. Downslope lateral sorting occurs with preferential deposition of coarser silt grains and larger mud flocs. Depositional sorting by increased shear in the boundary layer separates clay flocs from silt grains and results in a regular mud/silt lamination. Estimates can be made of the physical parameters of the turbidity flows involved. They are a minimum of several hundreds of metres thick, have low concentrations (of the order of 10−3 or 2500 mg 1−1), and move downslope at velocities of 10-20 cm s−1. A 5 mm thick, coarse silt lamina takes about 10 h to deposit, and the subsequent mud layer ‘blankets’ very rapidly over this. A complete unit is deposited in 2-6 days which is the time it takes for the turbidity flow to pass a particular point. These thick, dilute, low-velocity flows are significantly different from the ‘classical’ turbidity current. However, there is mounting evidence in support of the new concept from laboratory observations and direct field measurements.

How rain intensity affects interrill erosion.

Abstract: THE effect of rain intensity on interrill erosion was studied on crop-row sideslopes of farm fields using simulated rainfall. For a wide range of soils and cropping conditions, erosion (E) was related to intensity (I) as the power equation, E = aP. The exponent, b, decreased from slightly above 2.0 for soils of low clay content to about 1.6 for soils with about 50 percent clay. For the low-clay soils (silts, silt loams, loams and sandy loams), the power equation, E = cl2, fit the data well, and the ratios of the coefficients, c, then expressed the relative interrill erodibilities of the different soils. When a certain soil was studied at different crop stages, the c-values in-dicated the relative effect of different cropping condi-tions on interrill erosion.

Effects of Tubificid Oligochaetes on Physical and Chemical Properties of Lake Erie Sediments

Abstract: Laboratory experiments and field observations show that tubificid oligochaetes have important effects on sediment properties. Tubificids pelletize the surface sediment of the western and central basins of Lake Erie and increase the median sediment grain size and settling velocity by two orders of magnitude. The thickness of the pelletized layer in the lake is about 1 – 2 cm. The critical entrainment stress of pelletized sediment in box cores from Lake Erie is about twice that of unpelletized sediment. The difference in entrainment rate of sediment may be even greater. In sandy sediments from the Vermilion River, tubificids have the opposite effect: ultimate grain size in the upper 1 cm of sediment is decreased, water content is increased, and surface sediments are more easily eroded than non-pelletized sediments because the tubificids feed selectively on clay and silt size sediments.

The Northwest Atlantic Mid-Ocean Channel of the Labrador Sea: II. Deposition of parallel laminated levee-muds from the viscous sublayer of low density turbidity currents

Abstract: Varve-like parallel laminated and homogeneous muds on natural levees of submarine channels result from the spill-over of the dilute upper parts of channelized turbidity currents. On the levees of the Northwest Atlantic Mid-Ocean Channel, Labrador Sea, laminated turbidite muds show a systematic upward thickness variation of the granular silt laminae and the clay laminae. In many depositional units the former decrease logarithmically upwards in thickness while the latter increase logarithmically. The origin of parallel lamination in current-deposited, fine-grained sediments is interpreted in terms of the ‘burst-and-sweep model’ for the viscous sublayer of turbulent flows. Individual silt laminae of laminated turbidite muds form due to shear sorting during burst-and-sweep events which have a winnowing effect on the clay fraction. In contrast to sand-transporting flows the viscous sublayer of silt- and clay-carrying dilute turbidity currents cannot be permanently in a turbulent state. The presence of clay laminae suggests that the boundary layer relaminarizes periodically. It is speculated that this is caused by pressure waves with negative pressure gradients favouring the suppression of bursts and sweeps. The observed vertical thickness variations suggest that the periods of laminar boundary-layer flow increase in duration as turbulence of the main current decays. The upward thickness decrease of the silt laminae is explained in terms of decreasing supply rates to the boundary layer from the main flow. A multiple bursting-cycle mechanism is favored over a single bursting cycle mechanism for the formation of individual silt laminae by shear sorting.

Observations on moisture migration in frozen soils

Abstract: Artificially prepared, previously frozen specimens of a clayey silt have been subjected to thermal gradients under both closed and open systems Direct observation of moisture content changes reveals that substantial amounts of water are imbibed by frozen soil under an open system If an ice lens exists within the system, the increase in water content on the cold side of the lens is small compared with that on the warm side Heave rate is governed primarily by the frozen fringe of soil between the warmest ice lens and the frozen–unfrozen interface

Stress strain behaviour of a cohesive soil deposited under water

Abstract: A dilute suspension of soil particles in water will normally flocculate and settle under gravity to form a bed of consolidating soil. The essential difference between the suspension and the soil is that effective stresses exist in the soil. This thesis is concerned with the effective stress behaviour during the sedimentation and consolidation processes. Chapters two and three are a description of the equipment and experimental procedures used to determine void ratio changes, pore pressures and effective stresses during this soil formation process. The density measuring technique developed, using an X-ray source and a scintillation counter, is described in some detail. An experiment consists of monitoring the sedimentation, form an initially uniform suspension, of an inorganic clayey silt in a 102 mm I.D. column. Experimental results are presented and discussed in the next two chapters. Some novel aspects of stress/strain behaviour (in one dimensional consolidation) are revealed. In particular an intermediate zone is identified, which has properties between those of a suspension and a soil in the traditional sense. This zone does have effective stresses, but the effective stress/void ratio relationship is not unique. Theoretical consideration of settling suspensions and soil consolidation is covered in chapter six, forming an introduction to the next chapter, where a large strain soil consolidation theory is modified and applied to the intermediate zone. Differences between the theory and experimental results largely reflect the inaccuracies resulting from the simplifying assumptions necessary to keep the mathematics simple. The expulsion of pore water, rather than soil compressibility, is the major factor in determining consolidation behaviour of the intermediate phase. Finally, some recommendations are made as to how to continue the study and on applications to real problems such as dredging, pollution and land reclamation.

Disintegration of Fan Sediments in Death Valley, California, by Salt Weathering

Abstract: In Death Valley, California, an area of high aridity and great salinity, alluvial fans de scend into the zone of salt accumulation. When the rounded fan gravels come into contact with salt, some of which is sodium sulfate, they are rapidly shattered and reduced in size. Silt and clay are formed by this salt weathering process and this fine material may be subject to deflation processes.

The Effects of Size-Selective Feeding by Oligochaetes on the Physical Properties of River Sediments

Abstract: Tubificid oligochaetes selectively ingest silt- to clay-sized particles at depth within the substratum, transport them vertically upward through their gut, and deposit them as feces at the sediment-water interface. These activities form three distinct sedimentary layers. The sediment-water interface becomes covered with sand-sized fecal pellets. A silt-clay layer forms directly below this. The third layer is a sandy concentrate that represents the zone of tubificid feeding. The upper, pelletized layer is enriched in water content and organic carbon. The high water content of this layer, its irregular surface, and the low density of the constituent pellets destabilize the sediment surface and increase its erodability. In addition, the coarse-to-fine layered structure of the deposits fo ms distinctive biogenic graded bedding that is a potentially useful indicator of low current velocities and low rates of inorganic sediment accumulation in ancient fluvial environments.

Soil Development on Moraines of Taylor Glacier, Lower Taylor Valley, Antarctica

Abstract: Soils were examined on moraines deposited by Taylor Glacier in lower Taylor Valley to determine changes in soil properties with time in an environment of extreme cold and aridity and minimal biologic activity. The soils range in age from 200,000 to 2.7-3.5 million years BP. Soil profiles contain a desert pavement over a weakly expressed B horizon, followed by permafrost, which may be “dry” or ice-cemented. The soils are alkaline (pH 7.5 to 9.0) and are enriched in salts. Based on X-ray diffraction, the salts are mirabilite (Na₂SO₄·10H₂O), tachyhydrite (CaMg₂Cl₆·12H₂0), and halite (NaCl). Electrical conductivity of the salt-enriched zone ranges from 2.8 to 9 mmho cm⁻¹. Sodium and chloride are the dominant ions in 1.5 soil/water extracts. Ion ratios suggest that the bulk of the salts are of marine aerosol origin. The soils are generally gravelly sands, but the amounts of clay (<2 µm) and medium+fine silt (2–20 µm) increase with soil age. Free iron also increases with age. Secondary clay minerals include montmorillonite, vermiculite, and interstratified layer silicates. The predominant clay mineral weathering process is hydration of mica. The age-soil property relationship may have been influenced by microclimatic variations, previous occurrence of lakes dammed by a westward flowing marine ice sheet, mass-wasting processes, and persistent ice-cemented frost table in the solum. The soils are classified as Pergelic Cryorthents, sandy-skeletal, mixed, frigid.

Adsorption‐Desorption of Diuron by Freshwater Sediments

Abstract: Freshwater sediments were collected from eight different locations in California to study the influence of sediment properties on the adsorption and desorption of the model herbicide, diuron (3-(3,4-dichlorophenyl)-l,l-dimethylurea). The sediments were characterized to physical and chemical properties using the same methods used for soils. The sediments had a range of organic carbon of 0.91 to 19% and were high in silt and clay. The adsorption of diuron was described by the Freundlich equation. Adsorption at 25 °C expressed as the Freundlich coefficient, k, was positively correlated to the amount of organic carbon present. There was no correlation with total CEC. The desorption of diuron was evaluated by taking the difference between the slopes of the adsorption and desorption isotherms. As the difference between these slopes increased, less diuron was desorbed. These differences were positively correlated to k and organic carbon. Adsorption at temperatures of 5, 25, and 40°C on three sediments indicated that the adsorption of diuron decreased as the temperature increased. tern of a lake, then it would not only be important to study their chemical and physical properties, but also the potential retention or release of pesticides on these sediments. The capacity of lake sediments to adsorb or desorb pesticides is likely to be one of the most important factors which influence the concentration of a pesticide in the lake water. Since sediments are formed and exposed to conditions different from soils, it is important to know if the adsorption of a pesticide would vary with the same physical-chemical properties as for soils. The study was undertaken to determine the extent to which variation of several sediment characteristics, including organic matter, cation exchange capacity, and pH, affects sorption of the model herbicide, diuron (3(3,4-dichlorophenyl)-l,l-dimethylurea). Diuron is nonpolar herbicide of relatively low water solubility. It is used as both a preand postemergence herbicide. Additional Index Words: herbicides, pesticides, water quality, sediment characteristics, organic matter content, apparent heats of adsorption. Peck, D. E., D. L. Corwin, and W. J. Farmer. 1980. Adsorption-desorption of diuron by freshwater sediments. J. Environ. Qual. 9:101106. Although herbicide-soil interactions have been extensively studied, litte attention has been given to herbicide-sediment interactions. There have been some investigations of insecticide adsorption on lake sediment (Lotse et al., 1968; Vieth and Lee, 1971); however, there appears to be a lack of quantitative studies of the adsorption of herbicides on lake sediments. Poinke and Chesters (1973) have reviewed the literature on pesticide-sediment interactions. With the increasing use of herbicides in modern agriculture, it is likely that some will reach water bodies. Once in the water, it becomes important to understand the physical-chemical dynamics of the herbicide and sediment. It has been shown that sediments can play an important role in affecting the concentration of pesticides in a lake. Earlier investigations have shown (Bridges, 1961) that as the concentration of a given pesticide decreases in the water phase, there is a concomitant increase in the pesticide content of the sediment. As discussed by Chesters and Konrad (1971), these sediments may act as reservoirs, being able to recycle pesticides back into the aquatic environment. If lake sediments are considered as part of the life sys’ Contribution of the Dep. of Soil and Environ. Sci., Univ. of California, Riverside, CA 92521. The research leading to this report was supported by the Univ. of California, Water Resour. Center, as part of Water Resour. Center Project UCAL-WRC-W-485. Received 23 Sept. 1978. 2 Graduate Research Assistant, Graduate Research Assistant, and Associate Professor of Soil Science, Univ. of California, Riverside, respectively. Current address of senior author: Cooperative Extension Service, Univ. of Nevada, P.O. Box 651, Overton, NV 89040. Senior author is presently Soil and Water Scientist, Univ. of Nevada Agricultural Experiment Station, Logandale, NV 89021. MATERIALS AND METHODS The sediments were collected from eight different sites in California. Some of the chemical and physical properties of the sediments are listed in Table 1. The sediments were chosen to represent a number of differing environments and to be high in fine materials and organic matter. The sampling tool used to collect the sediment (Fig. 1) was modification of a tool designed by Barkley (1971). The sediment collection tube consisted of a 60-cm (2A-in) length of PVC pipe, 5 cm in) in diameter, with the circumference of one end beveled to aid penetration into the sediment. As the collection tube was pushed into the sediment, air and water were displaced upward through the tool stem. When the tube was filled with sediment, a rubber stopper was placed in the top of the tool. As the tool was withdrawn from the bottom, a vacuum was created holding the sediment in the tube. The success or failure of collecting a sediment sample depended upon the tool being completely air tight before removing the sediment from the bottom. Sediments were collected in water depths ranging from 0.5 to 4 m. Beyond a 4-m water depth it was difficult to maintain sufficient vacuum to hold the sediment in the tube. A composite sample consisting of 12 to 16 cores, 15 cm in depth, was collected at each site. The 0to 15-cm depth was obtained by sectioning the sediment core after removal from the collection tube. The sediment core slipped easily from the collection tube after releasing the vacuum. Intact 60-cm cores were retained in extra collection tubes for additional studies to be reported another time. Once collected, the sediments were immediately quickfrozen in dry ice. The frozen sediments were freeze-dried and each sampling site cornposited and ground to 60-mesh (0.25 mm). Preliminary investigations showed it necessary to reduce particle size to 60-mesh to provide a reproducible, homogeneous sample for the adsorption measurements. Preliminary screening through a 1-mm mesh removed fresh, undecomposed organic matter. Grinding may be expected to have an effect on the final particle size distribution in the sand fraction but would not be expected to significantly affect adsorption properties of the sediments. Most of the adsorption would be expected to be associated with the smaller size fractions. The chemical and physical properties of the sediments were determined using methods developed for soils. Organic carbon was determined using a programmed microcombustion apparatus in a pure oxygen atmosphere (Allison, 1965). In this method any sediments containing carbonates were treated with sulfurous acid before determining carbon. Total N was determined by the semimicro-Kjeldahl method (Bremner, 1965). The percent LOI (loss on ignition) was determined using a procedure developed for ash content (Sneddon et al., 1971) in which the sample was ignited 375°C for 16 hours. Cation exchange capacity of the sediment was determined using a modified BaCl2 method (Chapman and Pratt, 1961). J. Environ. Qual., Vol. 9, no. 1, 1980 101 Published January, 1980

Sediment transport and a reservoir siltation formula for Zimbabwe-Rhodesia

Abstract: Field measurements of the suspended sediment load of rivers draining five large catchments ranging from the driest to the wettest parts of the country have enabled a reservoir siltation formula to be developed for Zimbabwe-Rhodesia. A few below-average annual runoff events and several above average events were sampled, including a 40-year return period runoff for one catchment. Almost every significant discharge during a three-year period was measured on the rivers concerned. The laboratory analysis was facilitated by use of a turbidity meter for measuring suspended sediment - a method particularly suited to rivers carrying large proportions of clay and silt. Data for bed load transport were obtained by bed material sampling and survey of channel dimensions, carried out during the dry season, followed by the use of Einstein's calculation. The transport of dissolved solids was measured so that total losses from catchments could be compared with soil loss and soil production data. An order of magnitude amount of 40 t / km2 / year is suggested as an upper bracket sediment transport value for design purposes, provided the catchment concerned is not undergoing larger than average soil erosion. This figure is shown to be in agreement with previously published data, based on reservoir siltation rates, for parts of the Limpopo catchment, but several times smaller than data for small catchments under heavy human occupation in Tanzania. A reservoir siltation formula allowing for the low trap efficiencies of small reservoirs is suggested.

The Micromorphology of Young Soils from South-East Iceland

Abstract: The micromorphology of a chronosequence of lowland soils developed on till exposed by the retreating Breidamerkur glacier is compared with that of a palaeosol buried by the advancing glacier about 1700 A. D. and with three soils from the mountainous hinterland. As first exposed the wet highly dispersed till has a dense matrix with a few spherical bubble pores near the surface and an incipient platy structure below the surface layer. In the early years after deglaciation, on freely drained sites, the surface layers develop a porous spongy fabric and silt is gradually washed down into the lower part of the solum to form a dense silty layer. Within thirty years a weakly developed silt droplet fabric is present in the surface layer and reaches a depth of more than 50 cm in the oldest soil of the sequence on the 1894 moraine. A similar weakly developed silt droplet fabric is present in the Fjall palaeosol and in two of the three mountain soils. The weak patchy silt droplet fabric currently developing in these immature Icelandic soils is very similar to the fabric which developed in upland soils at the southern margin of the zone of discontinuous permafrost in the British Isles at the beginning of the present cycle of soil profile development in the Pre-Boreal Period.

The Northwest Atlantic Mid-Ocean Channel of the Labrador Sea: III. Head Spill Vs. Body Spill Deposits from Turbidity Currents on Natural Levees

Abstract: Very poorly sorted, non-laminated thin layers of clayey to silty sand designated as "a"-division of turbidites occur among thinly laminated, fine-grained spill-over deposits on the natural levees of the relatively steep upper channel segment of the Northwest Atlantic Mid-Ocean Channel (NAMOC) of the Labrador Sea. These layers are non-graded and the transition to overlying ripple-cross laminated silt or parallel laminated terrigenous mud is abrupt both in terms of grain-size and sorting. The "a"-division is interpreted as a head spill-over deposit from channelized turbidity currents. Turbulence in the current head causes a wide range of available grain sizes to be thrown up high enough so that lateral overflow over the channel banks can carry them to the levees where they are deposited as massive, ungraded sand layers containing granule to clay-sized material. These head-spill overflows may be supercritical at times. Subcritical body spill from the dilute upper parts of the (same or a later) flow generally deposits thinly laminated (d-division) or homogeneous (e-division) muds which represent the bulk of the spill-over deposits. Locally, body spill deposits include ripple cross-laminated Fine sand and silt (c-division), and rarely a- and b- divisions. The common occur ence of the "a"-division on the upper channel levees (channel slope greater or equal 0.002) and the scarcity of such layers further downchannel (slope gradient less than 0.002) supports Komar's (1972) choice of a critical slope gradient of 0.002 for head spill-over to occur. Although no other modern examples of head spill deposits have been recognized similar, but considerably thicker, deposits are known from ancient levee sequences.

Physical properties of the tundra soil-water system at stordalen, abisko

Abstract: The Stordalen mire at Abisko, North Sweden, is a 25 ha large peatland underlain by silt. Under neath an active layer of seasonal thaw the soils are perennially frozen. In general, the peat shows a low degree of humification. Differences in relative decomposition, as revealed by absorbency deter minations, are related to the micro topography. The organic material shows a very large pore volume, locally more than 96%. The pore shape tends to be vertically oriented in the upper 7-8 cm, whereas a transformation to horizontally oriented pores is evident below 20 cm depth. The peat layer averages 30 cm thickness over the mire and shows a transition layer of 3-20 cm above the mineral soil. Bulk densities of peat are 0.06-0.15 g cm-3 and average 1.3 g cm-3 in mineral soils. Soil water retention curves, showed differences between hummocks and small depressions, though, at the tension of wilting (15 atm), the average moisture content is 15% by volume at both micro sites. Water balance variables and the amount of plant available water were calculated. The soil moisture regime showed a yearly maximum close to saturation in connection with the melting of snow, followed by a decreasing trend throughout the summer. As regards water flow, the physical properties and the microtopographical characters lead to the conclusion that the hummocks act as inflow areas for precipitation. Lateral flow is likely to occur along the frost table whereas small depressions act as "ground water" outflow areas. Accordingly, the physical properties and water flow are strongly related to the microtopography and thus also to the vegetation.

Hydraulic Properties of Coal and Related Materials, Northern Great Plains

Abstract: The hydraulic properties (hydraulic conductivity, specific storage and porosity) of coal and the sediments that surround the coal must be known or estimated to properly evaluate the environmental impact of coal strip mining on ground-water flow systems. Published and unpublished data have been summarized from five study areas in North Dakota, three areas in Wyoming, two areas in Montana and two study sites in Alberta. Coal and sand beds form important aquifers in the region. Both materials have hydraulic conductivities of about 2 × 10−6 m.s−1 and specific storage values on the order of 5 × 10−6 m−1. Aquitards in the region consist of clayey silt to clay bedrock and pebble−loam (in parts of the region). The aquitards are fractured to varying degrees resulting in a wide range of measured hydraulic conductivity values. The aquitards are, in general, 100 to 1,000 times less permeable than the aquifers. Specific storage values of the aquitard material is on the order of 3 × 10−4 m−1. The hydraulic conductivity of strip mine spoils has a six order of magnitude range with a mean of 8 × 10−7 m.s−1. Variability is due to a number of factors including spatial variation of overburden lithology, method of spoil handling and contouring and time of year during which the spoil is handled. The hydraulic conductivity of the coal shows wide spatial variability within a given mine site. Variability of coal hydraulic conductivity between mines within North Dakota is less than within a given mine. Within the Northern Great Plains the permeability of coal may increase slightly from east to west.

Dynamic Properties of Frozen Silt Under Cyclic Loading

Abstract: Cyclic triaxial tests were conducted to evaluate the dynamic properties of laboratory prepared samples of frozen silt under simulated earthquake loading conditions. Dynamic Young's modulus and damping ratio were evaluated for Hanover and Alaska silt at two water contents over axial strain amplitudes from approx 2x10 to the power -3% to 8x10 to the power -2%, temperatures from 30.2 deg F to 14 deg F (-1 deg C to -10 deg C), frequencies from 0.95 cps to 10.0 cps, and confining pressures from 0 psi to 200 psi (0kN/m squared to 1400kN/m squared). Values of dynamic Young's modulus obtained over the range of test parameters were from 1x10 to the 5th power psi to 35x10 to the 5th power psi (7x10 to the 5th power kn/sq m to 245x10 to the 5th power kn/sq m) for Hanover silt and 1x10 to the 5th power psi to 22x10 to the 5th power psi (7x10 to the 5th power kn/sq m to 154x10 to the 5th power kn/sq m) for Alaska silt; values of damping ratio ranges from .02 to .36 for Hanover silt and from .02 to .32 for Alaska silt. The test results indicate that dynamic Young's modulus of frozen silt decreases with increasing axial strain amplitude and temperature, increases with increasing frequency, and is unaffected by confining pressure. The damping ratio increases with increasing axial strain amplitude and temperature and decreases with increasing frequency, water content, and confining pressure. (ASCE)

Organic matter distribution with particle-size in surface horizons of some sombric soils in vancouver island

Abstract: The surface horizons of some Sombric Brunisols and Podzols were fractionated to determine the distribution of the organo-mineral complexes. The soils were dispersed in water using ultrasonics and fractionated by wet sieving and the continuous flow supercentrifuge. The amounts of organic matter and the surface area of each fraction were determined. The weight of organic matter per unit of surface area increased with increasing size of soil separates. A considerable amount of organic matter is associated with the coarser fractions, with 5, 12 and 49% of the total organic matter being associated with the sand, coarse silt and fine silt fractions, respectively.

Benthic macroinvertebrate communities of the athabasca river near Ft. Mackay, Alberta

Abstract: Benthic macroinvertebrates were sampled at six stations in the Athabasca River near Ft. Mackay, Alberta, during the 1977 open water season. Fluctuating discharge of the river effected changes in the nature of the sediments as well as the abundance and composition of invertebrate communities. During most of the summer, coarse sand was dominated by chironomids of the Harnischia complex and an undescribed Orthocladiinae. Stoneflies and mayflies invaded the coarse sand in autumn when decreasing discharge led to greater stability of the substrate. The invertebrate fauna of silt and mud resembled that of similar substrates in lakes. Bedrock supported communities similar to those of stony streams. The average abundances of invertebrates on sand and mud were similar but estimates from mud were somewhat more variable. These results are discussed with respect to substrate stability and compared with published records from other large rivers.

Method and apparatus for removing sediment or other flowable solid material from a bed underlying a body of water

Abstract: This invention relates to a method and apparatus for removing settled sediment from a water-covered bed such as a silt pond. It also relates to a method and apparatus for the transfer of the removed sediment to a disposal site.

Hydrologic and sediment modelling studies in the environmental impact assessment of a major tropical dam project

Abstract: A computer-based study of the impact of the proposed Wabo hydroelectric scheme on the Purari River, Papua New Guinea was carried out. The HEC-6 model, Scour and Deposition in Rivers and Reservoirs developed by the Hydrologic Engineering Centre was used to simulate the effect of the dam on sediment transport and erosion in the lower Purari. Two runs with the model were carried out. The first one was used to establish baseline conditions and the second modelled dam impact. Before the study was carried out, data had to be collected on channel geometry, sediment input, river bed material size composition and hydraulic conditions in the river. Supplementary models also had to be developed to fill in gaps in runoff records and to describe flow in the river during power generation. Results of the investigation indicate that limited erosion will occur because of bed-armouring and the river will adjust towards a new equilibrium condition quite rapidly. The sediment output of the river into the Purari delta will change, load in the clay, silt and sand/gravel fractions decreasing by 22, 53 and 78 per cent respectively.

Clay and silt mineralogy of some soils from qasim, saudi arabia

Abstract: Summary Qasim, some 500 km north-west of Riyadh, is one of the agriculturally important areas of Saudi Arabia. Silt fractions of the soils of the Wadi ar Rimah and side wadis, classified as Torrifluvents, are characterized by high quartz contents and the universal occurrence of kaolinite and felspar, generally with plagioclase predominant; mica, chlorite and hornblende also generally occur. The clay fractions contain mainly palygorskite and a smectite-mica interstratification with smaller amounts of kaolinite, traces of quartz and occasionally chlorite. Palygorskite generally increases in content with depth. Variations in mineralogy probably reflect changes in depositional conditions rather than pedogenic processes; possible reasons for the high palygorskite content are discussed. Sedentary soils (Torriorthents) reflect the mineralogy of the rock from which they were derived. In clay fractions from salt-marsh soils (Salorthids) calcite reacts strongly with smectite-mica during DTA.

Influence of Regolith Properties on Migration of Septic Tank Effluent

Abstract: An investigation of waste-migration patterns from a septic system indicates that complex patterns result from minor variations in regolith adsorptive capacity and texture, local hydrology, and possibly soil microbiology. The existence of multichemical, bifurcating plumes suggest that monitor wells arranged up and downgradient and capable of multilevel sampling are essential to adequately delineate contaminant migration in ground water. The data also indicate that sampling for a single constituent could yield misleading information about the nature and distribution of other ground-water contaminants. The ability for chemical removal by the regolith is in direct response to minor variations in silt- and clay-sized particle content and corresponds to Langmuir adsorption isotherms. Silt- and clay-sized particles are dominantly organic in origin. Minor iron and aluminum hydroxyoxides and clays are present. Substrate samples, when collected at regular intervals and analyzed for adsorbed constituents and textural variability, provide an integrated picture of the distribution of waste chemicals through time. Such samples also provide insight into the mechanics of plume configuration and flow characteristics within the regolith. The study shows that regolith adsorption data are essential to the determination of life expectancy of the regolith as a contaminant treatment system.

Stratigraphy of a Silt-cored Palsa, Atlin Region, British Columbia, Canada

Abstract: Palsas in the Fourth of July Creek valley, B.C., Canada are round- or oval-shaped single hummocks with cores of silty permafrost. Their height ranges from 0.5 to 3 m. The top layer of the studied palsa is only 7 cm peat. In the frozen silt core are found segregated ice lenses up to 15 cm thick. The 0.006-0.02 mm size fraction of the core material of the palsa constitutes 55%. Carbon fourteen datings of the organic material of two different layers in the core of the palsa gave 7470 ± 180 and 7990 ± 180 years B.P. Special attention is given to the unusual sequence of dates obtained and to the pollen stratigraphy. The material is explained by redeposition by flood water into a pond and later uplifting by frost. No evidence of mixing of the original bottom sediments is found.

Size Distribution of Chemically Extracted Quartz Used to Characterize Fine-grained Sediments

Abstract: Silt sized quartz can be chemically separated from deep-sea sediments, by dissolution of other minerals, without altering its particle size. The extracted quartz can then be used for particle size analysis. The resulting polymodal size frequency distributions can be resolved into their individual modal components by means of an analog curve resolver. The method has been applied to three rather different sets of samples to assess its value in interpreting the origin and dispersion of quartz in the deep-sea: samples from Astoria Fan reveal the textural character of fine-grained quartz supplied by the Columbia River; two pairs of samples straddling the Mendocino Escarpment show that this major deep-sea topographic barrier has suprisingly little effect on the textural character of fine-grained detrital quartz; and, the technique reveals systematic temporal variations in the size distribution of quartz in a northeast Pacific core near the southern end of the Gorda Ridge spanning the last 30,000 years.