Characterising the fine sediment budget of a reach of the River Swale,Yorkshire, U.K. during the 1994 to 1995 winter season

Suspended sediment budget dynamics for a 55 km reach of the lowland River Swale, Yorkshire, U.K. are investigated for the period October 1994 to June 1995. Particular attention is paid to 11 storm events occurring between October 1994 and April 1995. Each of these storms produced significant suspended sediment transport. Variations in sediment dynamics, for example suspended sediment concentrations, hysteresis patterns and storm peak lag times through events and between the upstream and downstream ends of the reach are examined. Net sediment loss from the reach occurred during the extremely wet four month winter period from December 1994 to April 1995. Patterns of reach sediment storage are concluded to represent a combination of channel bed erosion and/or deposition, bank erosion and floodplain deposition. The implications of these patterns for sediment modelling are discussed.     

Entrainment,deposition, and transport of fine-grained sediments in lakes

Recent work on the settling, diffusion, entrainment, and deposition of fine-grained sediments in fresh water is reviewed and synthesized. Particular attention is given to the dependence of these processes on sediment properties such as particle size. The application of this knowledge to the analysis and numerical modeling of sediment transport is also discussed. Much of the work is concerned with the Great Lakes and, more specifically, with the Western Basin of Lake Erie.     

土壤保持服务:概念、评估与展望

土壤保持服务作为一项重要的生态系统调节服务,是防止区域土地退化、降低洪涝灾害风险的重要保障。针对在全球范围内影响最大的土壤水蚀,基于土壤侵蚀、运移和输出过程,对土壤保持服务的概念、评估方法进行了梳理总结。土壤保持服务是指生态系统防止土壤流失的侵蚀调控能力及对泥沙的储积保持能力。土壤保持服务的评估往往是基于通用土壤流失方程RUSLE(Revised Universal Soil Loss Equation),以潜在土壤侵蚀量(裸地时土壤侵蚀量)与实际土壤侵蚀量之差,作为指标衡量。由于生态过程具有尺度依赖性,对土壤保持服务的有效评估,需要采用多尺度方法。土壤保持服务与人类需求紧密相关,在未来土壤保持服务研究中应强调连接土壤保持服务与人类福祉,对土壤保持服务产生、流动、使用的全过程及土壤保持服务时空动态与人类福祉变化的关系进行探究。     

3D-numerical modelling of cohesive suspended sediment in the Western Scheldt estuary (The Netherlands)

A cohesive sediment transport model considering the effects of flocculation, deposition and erosion is used in an attempt to simulate the suspended sediment distribution in a mesotidal estuary. The numerical model solves the three-dimensional (3D) advection-diffusion equation using a two-time level scheme, and a semi-implicit finite difference approach. The transport model is coupled to a 3D-barotropic hydrodynamic model for the simulation of the major tidal components reproducing the non-linear effects. An application of these models in the Western Scheldt estuary is described. The results of the different tests show that the adopted approach provides a useful basis for a good understanding of the physical processes involved in sediment transport and for the study of practical problems. The sensitivity of the model to key parameters controlling the simulation of bed sediment/water exchanges, shows the importance of a good definition of bottom sediment characteristics and the importance of further development of a consolidation algorithm.     

Hydrology,hydraulics, and geomorphology of the Upper Mississippi River system

The Upper Mississippi River system has been modified with locks, dams, dikes, bank revetments, channel modifications, and dredging to provide a nine-foot navigation channel. These activities have changed the river's characteristics. The historical changes in the hydrologic, hydraulic, and geomorphic characteristics were assessed and related to navigational development and maintenance activities in the Upper Mississippi River system. The hydrologic, hydraulic, and geomorphic features studied include river discharges, stages, sediment transport, river position, river surface area, island surface area, and river bed elevation. Water and sediment transport effects on dredging were also estimated. It was found that the general position of the Upper Mississippi River system has remained essentially unchanged in the last 150 years except for specific man-made developments in the river basin. The stage, velocity, sediment transport, and river and island areas were altered by development of the 2.75-m navigation system. Dredging requirements are strongly related to mean annual water discharge. Years in which water discharges were great were generally the years during which large volumes of sediment needed to be dredged from the channel. The backwater areas are experiencing some deposition. With implementation of erosion-control measures in major tributaries and upland areas, better confinement of disposed dredged materials, and better maintenance practices, the sedimentation and pertinent problems in the main channel, as well as in the backwater areas, may be reduced with time.     

Morphological controls on medium-term sedimentation rates on British lowland river floodplains

This paper outlines a novel two-stage procedure for estimating medium-term (ca. 40 years) rates of overbank sedimentation on British lowland floodplains, and exploring the relationship between floodplain morphology, floodwater hydraulics and fine sediment storage. The first stage utilises a two-dimensional hydraulic model that solves the depth-averaged shallow water equations over a high resolution topographic grid. This model is used to predict distributed patterns of flow depth and velocity within floodplain reaches approximately 0.4–0.6 km in length for floods of varying magnitude. These hydraulic data are then used, in conjunction with a simple sediment transport and deposition model, to estimate rates and patterns of floodplain sedimentation. This procedure was applied to a series of eight sites on the floodplain of the River Culm, Devon, UK, with contrasting morphological characteristics (e.g., channel sinuosity, channel numbers and dimensions, bankfull discharge, floodplain width, etc). The hydraulic modelling procedure was validated using ground and oblique aerial photography of floodwater inundation patterns. Estimates of medium-term sedimentation derived from ¹³⁷Cs analysis of floodplain sediment cores were used to calibrate the sediment deposition model. Results indicate that within-reach variability in sedimentation rates reflects small-scale topographic controls on local flow characteristics and sediment transport and deposition processes. In contrast, between-reach variations in total sediment storage are largely a product of downstream changes in gross valley floor morphology and flood frequency. Preliminary estimates of the total sediment flux to the floodplain of the River Culm for the basin as a whole are consistent with previous estimates and highlight the importance of floodplain sedimentation as a component of the overall catchment sediment budget.     

Flow and sediment transport in vegetated waterways: a review

One of the most important waterway components is vegetation, which play a pivotal role in the flow and sediment transport. Vegetation environment and characteristics, including vegetation porosity or density, shape, flexibility and vegetation height, are significantly affected in vegetated channels. Various vegetation positions and arrangements such as patches significantly affect the sediment deposition rate and flow turbulence. This paper reviews recent works conducted on vegetated open channels, which include the effect of different vegetation arrangements and vegetation characteristics on mass transport and turbulence structure. Studies based on laboratory, field works, and modeling, have been reviewed based on previous methods used by different researchers. Methods used in vegetation porosity evaluation, rate of flow and sediment transport properties are presented. In short, flow and transport depend on the vegetation properties and flow structure conditions.     

Autochthonous outer ramp sedimentation: The Alamogordo Member of the Lake Valley Formation,New Mexico

Summary Paleozoic carbonate ramp sedimentation has generally been described in terms of downlapping clinoforms composed of allochthonous sediment derived from shallower environments. However, during transgressive episodes when carbonate sediment production is low and down slope sediment transport by gravity becomes inactive, autochthonous carbonate sediment accumulates in vertical stacks of essentially in situ sediment. Autochthonous outer ramp deposition is probably a part of many Paleozoic ramp strata, but has heretofore not been recognized because of the general absence of adequate exposures. Evidence of autochthonous, in situ deposition and preservation of sediments in a starved setting is well displayed in the Alamogordo Member of the Lake Valley Formation in south central New Mexico. This evidence includes: 1) beds and bed sets that are individually continuous and traceable along ramp slope for 32 km, 2) down-ramp sequential distributions of depth-sensitive organisms and assemblages but patchy distribution of rock types, 3) lack of sedimentary structures indicative of transport, 4) well preserved, unabraded fossils, 5) the common occurrence of fossils in life position, 6) beds traceable into and through mounds 7) bed thickness trends ascribed to biotic productivity, and 8) geopetal structures in original position. Integrated paleontologic, sedimentologic, and stratigraphic data provide information about depositional processes and setting. The depositional slope was approximately 0.5^o based on the distribution of fossil algae; this is comparable to dips reported for other Mississippian homoclinal ramps. An oxygen minimum zone may have impinged on the ramp during a major flooding event. Shifts in biotic gradients from bed to bed reveal transgressive-regressive patterns that would not be resolvable without detailed paleontological evidence. The Alamogordo Member formed as a result of transgressive and early highstand starved carbonate sedimentation along a narrow, homoclinal outer ramp. The surface of maximum flooding and the boundary between the TST and HST are within the Alamogordo Member.     

Quantifying Burial,the Transport of Matter from the Lake Biosphere to the Geosphere

Burial is one of the most fundamental processes in contexts of massbalance calculations for substances (such as nutrients, organics, metals and radionuclides) in lakes. Substances can leave a lake by two processes, outflow, i.e., the transport to a downstream system, and burial, i.e., the transport by sedimentation from the lake biosphere to the geosphere. This work gives for the first time, to there best of the author's knowledge, a review on the factors and processes regulating burial and presents a general model for burial. This approach accounts for bottom dynamic conditions (i.e., where areas of fine sediment erosion, transport and accumulation prevail), sedimentation, bioturbation, mineralisation, and the depth and age of the bioactive sediment layer. This approach has been critically tested with very good results for radiocesium, radiostrontium, many metals, calcium from liming and phosphorus, but it has not been presented before in a comprehensive way. This model for burial is meant to be used in massbalance models based on ordinary differential equations (i.e., box models) in contexts where burial is not a target y‐variable but a necessary model variable (an x‐variable). This means that there are also specific demands on this approach, e.g., it must be based on readily accessible driving variables so that it is not too difficult to use the model in practice within the context of an overall lake model. The factors influencing burial, e.g., the deposition of materials and the depth of the bioactive sediment layer, are also needed in calculations of sediment concentrations and to determine amounts of substances or pollutants in sediments. To carry out such calculations, one also needs information on sediment bulk density, water content and organic content. This paper also presents new empirical models for such calculations to be used in the new model for burial.     

Sediment resuspension and transport patterns on a fringing reef flat,Molokai, Hawaii

Corals are known to flourish in various turbid environments around the world. The quantitative distinction between clear and turbid water in coral habitats is not well defined nor are the amount of sediment in suspension and rates of sedimentation used to evaluate the condition of reef environments well established. This study of sediment resuspension, transport, and resulting deposition on a fringing reef flat off Molokai, Hawaii, uses a year of time-series data from a small, instrumented tripod. It shows the importance of trade winds and ocean wave heights in controlling the movement of sediment. Sediment is typically resuspended daily and the dominant controls on the magnitude of events (10–25 mg/l) are the trade-wind-generated waves and currents and tidal elevation on the reef flat. The net flux of sediment on this reef is primarily along the reef flat in the direction of the prevailing trade winds (to the west), with a secondary direction of slightly offshore, towards a zone of low coral abundance.These results have application to reef studies and reef management in other areas in several ways. First, the observed resuspension and turbidity results from fine-grained terrigenous sediment that appears to be trapped and recycled on the reef flat. Thus corals are subjected to light attenuation by the same particles repeatedly, however small the amount. Secondly, the measurements show high temporal variability (from daily to seasonal scales) of sediment resuspension, indicating that single measurements are inadequate to accurately describe conditions on a reef flat.Communicated by: P.K. Swart     

A numerical model for fine suspended sediment transport in the southwest lagoon of New Caledonia

A hydrodynamic-transport coupled model is used to understand the transport of fine suspended sediments in the southwest lagoon of New Caledonia. The hydrodynamic model is briefly presented and the circulation due to the tide and to averaged trade wind forcings is analyzed. The transport model for fine suspended sediment is described. Parameters involved in this model (settling velocity, critical shear stresses, erosion rate coefficient, Schmidt number) are discussed and a calibration procedure is proposed. Using the resultant parameters, the erosion and deposition areas predicted by the sediment-transport model are in very good agreement with the distribution of the percentage of mud at the seabed. The sensitivity of the model to the different sedimentary parameters is studied, and the influences of the tide and wind on deposition and erosion are discussed. The influence of the wind is dominant in seabed exchange processes in shallow areas and produces large erosion rates where the water depth is less than 20 m. The tide controls the particulate transport, vertical mixing, and deposition rates in the areas where the influence of the wind is weak.     

Deposition of Cryptosporidium oocysts in streambeds

The transfer of Cryptosporidium oocysts from the surface water to the sediment beds of streams and rivers influences their migration in surface waters. We used controlled laboratory flume experiments to investigate the deposition of suspended Cryptosporidium parvum oocysts in streambeds. The experimental results demonstrate that hydrodynamic interactions between an overlying flow and a sediment bed cause oocysts to accumulate in the sediments and reduce their concentrations in the surface water. The association of C. parvum with other suspended sediments increased both the oocysts' effective settling velocity and the rate at which oocysts were transferred to the sediment bed. A model for the stream-subsurface exchange of colloidal particles, including physical transport and physicochemical interactions with sediment grains, accurately represented the deposition of both free C. parvum oocysts and oocysts that were attached to suspended sediments. We believe that these pathogen-sediment interactions play an important role in regulating the concentrations of Cryptosporidium in streams and rivers and should be taken into consideration when predicting the fate of pathogens in the environment.     

Biostabilization and Transport of Cohesive Sediment Deposits in the Three Gorges Reservoir

Cohesive sediment deposits in the Three Gorges Reservoir, China, were used to investigate physical and geochemical properties, biofilm mass, and erosion and deposition characteristics. Biofilm cultivation was performed in a recirculating flume for three different periods (5, 10 and 15 days) under ambient temperature and with sufficient nutrients supply. Three groups of size-fractionated sediment were sequentially used, including 0–0.02 mm, 0.02–0.05 mm and 0.05–0.10 mm. Desired conditions for erosion and deposition were designed by managing high bed shear stress at the narrow part of upstream flume and low shear stress at the wide part of downstream flume. Biostabilization and transport characteristics of the biofilm coated sediment (bio-sediment) were strongly influenced by the cultivation period, and the results were compared with clean sediment. The bio-sediment was more resistant to erosion, and the mean shear stress was increased by factors of 2.65, 2.73 and 5.01 for sediment with 5, 10 and 15 days of biofilm growth compared with clean sediment, resulting in less sediment being eroded from the bed. Simultaneously, the settling velocity was smaller for bio-sediment due to higher organic content and porosity (i.e., lower density). Additionally, there was a smaller probability of deposition for sediment with a longer cultivation period after erosion, resulting in more retention time in aquatic systems. These results will benefit water management in natural rivers.     

Simulating the recovery of suspended sediment transport and river-bed stability in response to dam removal on the Elwha River,Washington

U.S. Department of the Interior is planning to remove two high dams (30 and 60 m) from the Elwha River, which will allow the river to erode sediment deposits in the reservoirs, and ultimately restore the river ecosystem. Fluvial sediment transport and deposition paradoxically represent ecological disturbance and restoration. A one-dimensional, movable boundary sediment-transport model was applied at a daily time step to simulate changes in river-bed elevations and particle-size distributions and concentrations of suspended sediment. The simulations included a three-year dam removal period and a four-year recovery period. Simulated concentrations of suspended sediment recover rapidly during the recovery period. Simulated bed elevation and particle-size distributions are stable for much of the river during the recovery period, but high flows periodically disturb the river bed, causing changes in river-bed elevation and particle-size distribution, especially during autumn, when summer/autumn chinook salmon are incubating in redds. Although the river bed will become increasingly stable after dam removal, episodic high flows will interrupt recovery trends. Productivity and diversity of the ecosystem may be lower because of excess sediment immediately after dam removal but should increase during recovery above current levels as the river. Monitoring of the recovery of the Elwha River ecosystem can target ecologically significant physical parameters indicating the transition from a sediment transport-limited state to a supply-limited state.     

全氟辛酸在沉积物-水界面污染及吸附迁移行为研究进展

全氟辛酸(PFOA)是一种重要的全氟化表面活性剂,具有环境持久性、高毒性和生物累积性等特征,成为当前备受关注的新污染物。沉积物是PFOA的重要环境储蓄库,被污染后的沉积物可作为长期污染源造成上覆地表水及周边地下水污染。本文结合近年来国内外研究,对地表水和沉积物中PFOA的来源、污染现状和界面吸附迁移行为进行了综述。结果表明: 地表水体和沉积物中PFOA最主要的污染来源为工业废水排放。全球范围地表水体和沉积物中PFOA污染水平普遍为ng·g^-1和ng·L^-1数量级,且国内部分地区污染形势更为严峻。沉积物组分、水化学条件、有机质及表面活性剂等因素均影响沉积物中PFOA的吸附行为,但目前在吸附主控机制方面仍然存在争议。沉积物中PFOA迁移行为的研究正处于起步阶段,相关报导还较为缺乏,对于迁移机理的认识还很不足,今后需在该方面加大研究力度。     

A study of seed dispersal by flood flow in an artificially restored floodplain

Riverine floodplains play many important roles in river ecosystems. However, many floodplains have suffered degradation or loss of ecological function due to excessive river improvements or through changes in agricultural systems. As a result, many floodplain restoration projects are being conducted worldwide. One of the many methods being implemented to restore floodplain vegetation is flood water seed dispersal. In this technique, precisely estimating the effect of seed dispersal by flood water is important in order to achieve successful floodplain revegetation. Here, we focus our attention on sediment transport by flood water into the Azamenose Swamp, a restored floodplain. We attempt to estimate the function of seed deposition in the restored floodplain and explain how the seeds are deposited in the floodplain by flood water. The result suggests that the restored floodplain functions as a more appropriate deposition site for seeds than the riverbanks of the main river. It was also found that the distance from the inflow site and the weight of the sediment were related to seed deposition.     

Historical changes in anthropogenic lead fallout in southern Ontario,Canada

A quantitative historical record of anthropogenic lead deposition has been determined for the Haliburton region of southern Ontario from a) the relative change in lead concentration in undisturbed sediment cores, and b) the total anthropogenic lead deposition measured in the region. The first parameter was measured on two sediment cores from a meromictic lake. Total lead deposition was assumed to be the average of that measured for eight other lakes in the study area. The validity of this historical record was tested on sediment cores from nearby dimictic lakes. Predicted lead concentration profiles closely resembled observed profiles.     

Wave and sediment dynamics along a shallow subtidal sandy beach inhabited by modern stromatolites

To help define the habitat of modern marine stromatolites, wave-dominated flow and sediment transport were studied in the shallow subtidal region (1-2 m depth) along the slightly concave, windward face of Highborne Cay, Exuma, Bahamas - the only face of the cay that includes a population of stromatolites concentrated near the region of highest curvature of the beach. Wave energy impacting this island's most exposed beach was driven by local wind forcing which increases largely in response to the passage of atmospheric disturbances that typically affect the region for periods of a few days. Although some wave energy is almost always noted (maximum horizontal orbital speeds at the bottom are rarely <10 cm s(-1)), wave conditions remain comparatively calm until local winds increase above speeds of approximately 3-4 m s(-1) at which point maximum wave speeds rapidly increase to 50-80 cm s(-1). Stromatolites, which are largely restricted to the shoreward side of a shallow platform reef, are sheltered by the reef beyond which wave speeds are one to four times higher (depending on tidal stage). Moreover, stromatolite populations are predominantly found along a region of this wave-exposed beach that experiences comparatively reduced wave energy because of the curved morphology of the island's face. Maximum wave speeds are 1.4 to 2 times higher along more northern sections of the beach just beyond the locus of stromatolite populations. A quantitative model of sediment transport was developed that accurately predicted accumulation of suspended sediment in sediment traps deployed in the shallow subtidal zone along this beach. This model, coupled with in situ wave records, indicates that gross rates of suspended sediment deposition should be two to three times higher northward of the main stromatolite populations. Regions of the beach containing stromatolites nevertheless should experience significant rates of gross suspended sediment deposition averaging 7-10 g cm(-2) day(-1) ( approximately 4-6 cm day(-1)). Results suggest that one axis of the habitat of modern marine stromatolites may be defined by a comparatively narrow range of flow energy and sediment transport conditions.     

Deposition of Cryptosporidium Oocysts in Streambeds

The transfer of Cryptosporidium oocysts from the surface water to the sediment beds of streams and rivers influences their migration in surface waters. We used controlled laboratory flume experiments to investigate the deposition of suspended Cryptosporidium parvum oocysts in streambeds. The experimental results demonstrate that hydrodynamic interactions between an overlying flow and a sediment bed cause oocysts to accumulate in the sediments and reduce their concentrations in the surface water. The association of C. parvum with other suspended sediments increased both the oocysts' effective settling velocity and the rate at which oocysts were transferred to the sediment bed. A model for the stream-subsurface exchange of colloidal particles, including physical transport and physicochemical interactions with sediment grains, accurately represented the deposition of both free C. parvum oocysts and oocysts that were attached to suspended sediments. We believe that these pathogen-sediment interactions play an important role in regulating the concentrations of Cryptosporidium in streams and rivers and should be taken into consideration when predicting the fate of pathogens in the environment.