Coupled modelling of storm surge,circulation and surface waves in a large stratified lake

The wind-driven nature of large lakes suggests that the accuracy of meteorological inputs is essential for hydrodynamic modelling. Moreover, coupling between the meteorological inputs and density stratification may also influence the simulated lake behavior. To investigate wind-driven large lake processes, a high-resolution coupled Delft3D-SWAN model was applied to Lake Ontario to simulate storm surges, surface waves, and circulation during two recent storm events. In both events, the sustained wind speeds approached 20 m s^?1; however, variations in wind direction and duration altered the lake's surface wave and storm surge responses. The influence of different atmospheric inputs was investigated by comparing results from two spatially varied atmospheric models: the Rapid Refresh (RAP) and the High-Resolution Deterministic Prediction System (HRDPS). Hydrodynamic simulations using HRDPS were marginally better, with maximum root mean squared errors (RMSE) between modelled and observed water levels of 0.07 m, compared to 0.08 m with RAP. Predictions of the magnitude and timing of the maximum wave heights varied based on wind fields, with differences between predicted peak wave heights of up to 0.4 m. Both events occurred during a stratified period, allowing for a comparative evaluation of the influence of baroclinic and barotropic processes on the simulated surface wave and storm surge results. Simulations including the vertical density gradient gave a better representation of current velocities with depth and resulted in an improved prediction of peak storm surge magnitudes and surface water level behavior following the storms, reducing the RMSE by up to 12%.     

Hydrostatic versus nonhydrostatic hydrodynamic modelling of secondary flow in a tortuously meandering river: Application of Delft3D

Given the importance of pressure gradients in driving secondary flow, it is worth studying how the modelled flow structures in a natural river bend can be impacted by the assumption of hydrodynamic pressure. In this paper, the performance of hydrostatic versus nonhydrostatic pressure assumption in the three‐dimensional (3D) hydrodynamic modelling of a tortuously meandering river is studied. Both hydrostatic and nonhydrostatic numerical models were developed using Delft3D‐Flow to predict the 3D flow field in a reach of Stillwater Creek in Ottawa, Canada. An acoustic Doppler velocimeter was employed to measure the 3D flow field at a section in a sharp bend of the simulated river at two flow stages. The results of the Delft3D hydrostatic model agreed well with the acoustic Doppler velocimeter measurements: The hydrostatic model predicted reasonably accurately both the streamwise velocity distribution across the section and the magnitude and location of the primary secondary flow cell. The results of the Delft3D nonhydrostatic approximation showed that the model was not conservative and could not accurately generate either the secondary flow or the streamwise velocity distribution. This study illustrated the superior performance of the hydrostatic over nonhydrostatic 3D modelling of the secondary flow using Delft3D. Several possible reasons for unfavourable performance of the nonhydrostatic version of Delft3D are discussed, including the pressure correction technique employed in Delft3D. Considering the uncertainties that may arise in both modelling and field measurements, the 3D hydrostatic Delft3D model was capable of reasonably predicting the river bend flow structures in the studied meandering creek.     

The last snapshot of natural pelagic fish assemblage in Lake Turkana,Kenya: A hydroacoustic study

A hydroacoustic survey and supplementary gillnet investigation were carried out in the open water of the central part of Lake Turkana in September 2009. Overall acoustic fish density and biomass were assessed as 1381 ind./ha and 30 kg/ha, respectively. The fish density estimate was lower than the results from two previous investigations in the 1970s and 1980s (long-term average 3739 ind./ha), but the biomass remained relatively unchanged (long-term average 25.4 kg/ha). A decreasing gradient in pelagic fish density from the western to eastern shore of the lake was observed. Fish were distributed unevenly within the water column. During the day, a majority of fish aggregated in the mid-water layers (10–12 m below the water surface), creating on echograms the so-called Midwater Scattering Layer. This feature dissipated completely during dusk and the majority of fish occurred in the surface layers at night. These diel vertical fish migrations influenced day and night hydroacoustic estimates of the total fish abundance. Synodontis spp., Lates spp. and Schilbe uranoscopus dominated the catch of both mid-water and bottom gillnets installed in open water areas. Hydrocynus forskalii and Brycinus spp. contributed significantly to the catch of mid-water gillnets while Bagrus bayad and the endemic Haplochromis macconneli occurred only in the catch of the bottom gillnets.     

Nutrient concentrations in the surface sediments of Lake Chivero,Zimbabwe: A shallow,hypereutrophic, subtropical artificial lake

A lake‐wide survey in Lake Chivero, Zimbabwe, was performed to (i) examine the spatial distribution of three sediment grain sizes; (ii) determine their nutrient concentrations; and (iii) analyse the relationships between the measured surface sediment variables in the three sediment grain sizes. The results of this study indicated sand, silt and mud sediments occupied about 24, 22 and 54%, respectively, of the lake area. All the measured physicochemical variables varied significantly among the three sediment grain sizes (P < 0.05, anova ). High values for pH, dissolved oxygen and NO₃ were observed in the sand sediments, while water depth, electrical conductivity, soluble reactive phosphorus, NH₄, total phosphorus, total nitrogen and total organic carbon were observed in the mud sediments. In the silt sediments, most measured variables showed intermediate values. Intervariable correlation indicated varied correlation patterns among the three sediment grain sizes. Information generated in this study is essential for an accurate delineation of the spatial extent of nutrient and contaminant distribution in the sediment of Lake Chivero and will facilitate the ability of lake managers to derive a comprehensive nutrient budget of the lake as a means of establishing realistic target conditions for lake restoration.     

太湖涉水建设项目防洪影响控制性技术指标初探

为规范太湖流域涉水项目的建设,保障流域防洪安全,在调查分析太湖流域主要涉水项目建设状况的基础上,提出了太湖流域涉水建设项目防洪影响控制性技术指标体系,初步构建了以"防洪安全"为目标层,以"建筑物形式、行洪畅通、河势稳定、水利工程安全、防洪标准、规划相适性、其他影响"等7个方面为准则层,以23项具体指标为指标层的防洪影响控制性技术指标体系。指标体系的构建为太湖流域涉水建设项目的设计、审批与管理提供了技术支撑。     

A high resolution study of spatial and temporal variability of natural and anthropogenic compounds in offshore Lake Superior sediments

An 8 km² area representative of deep offshore basins in Lake Superior was surveyed with multi-beam sonar and a high-frequency seismic-reflection system to create a high-resolution bathymetric map of the lake floor morphology, which is dominated by ring-shaped depressions attributed to the dewatering of glacial-lacustrine clays. Ten multi-cores were recovered from the survey area. The cores were scanned for magnetic susceptibility (MS), dated by ²¹⁰Pb and analyzed for water content, total organic carbon (TOC) and nitrogen (TON), biogenic silica (BSi), and total (THg) and methyl (MeHg) mercury. MS profiles varied considerably, inferring substantial centennial-scale differences in sedimentation history among the core sites. Concentration profiles of the analyzed constituents displayed differences of about ± 15% TOC, ± 40% BSi, ± 50% THg and ± 50% MeHg. Total mercury and methylmercury concentrations were typical of past measurements, and the mean THg accumulation rate (12 μg/m² year) was similar in magnitude to that of atmospheric Hg deposition. Sediment mass accumulation rates (MAR) ranged among the cores between average values of about 50 g/m² year in the ring centers to as high as 180 g/m² year between rings. Temporal variation in MAR within cores varied considerably on a decadal scale as well. Sediment redistribution by bottom currents over the complex morphology of the Lake Superior basin is not uniform in space and time, and indicates that a single core from any given area in the lake may not reflect the true history of environmental conditions that extend even a few hundred meters beyond the core site.     

典型风况下兴凯湖波流耦合数值模拟研究

兴凯湖是中俄界湖,湖区流场和波浪场的观测数据及相关研究工作十分匮乏。针对兴凯湖典型风况下波流耦合数值模拟问题,在兴凯湖风场及湖水位特征分析的基础上,首次采用Delft-3D模型对兴凯湖流场和波浪场进行耦合数值模拟,分析了流场和波浪场的结构,并与太湖实测流场进行了对比分析。另外,利用相关规范中的公式计算了流场和波浪场的相关水力要素,验证了数值计算结果。经分析认为,波流耦合作用是兴凯湖北岸泥沙淤积、湖岸冲蚀破坏的主要原因。研究成果对兴凯湖北岸的防护工程建设及水资源保护具有重要意义。     

历史视角下的湿地演变与恢复保护——以永年洼为例

湿地的恢复与保护是近年来环保领域的热点。用历史视角研究湿地及其周边的演化历程,能极大地丰富对湿地的认识,使湿地的恢复和保护更有针对性。本文以华北地区典型的受高强度人类活动长期影响的自然湿地永年洼为例,尝试从历史视角审视其演变,证明这一方法的有效性,并指出:(1)广府古城的历史长于永年洼的历史,永年洼始终受高强度人类活动影响;(2)永年洼在1949年以后的剧烈萎缩主要是因为水利工程控制来水量和围垦造田,永年洼仍具备成为湿地的条件;(3)永年洼以防洪功能为主、生态功能为次,永年洼的恢复保护应借建设湿地公园的契机,保障湿地来水的水量和水质。     

Causal mechanisms of the deep chlorophyll maximum in Lake Superior: A numerical modeling investigation

The deep chlorophyll maximum (DCM) is a near ubiquitous feature in Lake Superior during the summer stratified season. Previous studies have elucidated observable characteristics of the DCM in Lake Superior but the physical and biological mechanisms controlling the creation and maintenance of the DCM remained unclear. We use a three-dimensional hydrodynamic model of Lake Superior coupled to an ecological model to perform sensitivity runs exploring the influence of photoadaptation, photoinhibition, zooplankton grazing, and phytoplankton sinking on the vertical distribution of chlorophyll in the water column. The role of a nutricline in determining the presence and nature of the DCM is also explored. The presence of the DCM is dependent upon the presence of thermal stratification in the model. The sensitivity runs reveal that photoadaptation plays a primary role in determining the depth of the DCM in the model while zooplankton grazing and phytoplankton sinking affected the magnitude but not the presence or depth of the DCM. Photoinhibition showed negligible effects on chlorophyll concentration distribution. The presence of a nutricline in the model is also a necessary condition for the formation of the DCM and it influences both the depth and magnitude of the DCM.     

The effectiveness of small-to-medium river basin organizations in China: a case study of the Erhai Lake Basin Management Commission

Drawing on empirical evidence from a case study conducted in the upper Mekong catchment, this article assesses the performance of the Erhai Lake Basin Management Commission, an organization mandated to control pollution and integrate the diverse interests of those individuals and communities living and working near Erhai Lake. Clear local rules, strong enforcement, practical protocols, clear responsibilities and extensive mobilization have been crucial to the success of this small river basin organization. However, the insufficient involvement of numerous important stakeholders in decision-making processes has decreased the commission’s capacity to address water issues.     

The role of the Yala swamp lakes in the conservation of Lake Victoria region haplochromine cichlids: Evidence from genetic and trophic ecology studies

Lake Kanyaboli, an isolated satellite lake of Lake Victoria, has been suggested as a potential refugium for haplochromine cichlids that have gone extinct in the main basin of Lake Victoria. Mitochondrial DNA (mtDNA) molecular markers, as well as feeding ecology studies, were employed in this study to re‐evaluate the evolutionary and ecological significance of six common Lake Kanyaboli haplochromines. The mtDNA marker revealed high genetic variability within four of the six haplochromine cichlids. Five haplotypes were discerned in Astatoreochromis alluaudi (n = 27), seven in Lipochromis maxillaris (n = 29), five in Astatotilapia nubila (n = 12) and 11 in the endangered Xystichromis phytophagus (n = 205). A haplotype genealogy suggests that Lake Kanyaboli harbours mtDNA haplotypes that could have been lost or not sampled in Lake Victoria, or could have arisen in situ. Lipochromis maxillaris appears to have undergone a recent demographic expansion. The pairwise F_STs indicated that only the comparison between X. phytophagus and A. nubila led to a non‐significant F_ST value. All other comparisons were significant at the 0.01 level, indicating the genetic distinctiveness of the haplochromines in the satellite lake. This could suggest that the lake harbours ‘pure’ relict populations of the haplochromines and therefore that Lake Kanyaboli can be considered a ‘genetic reservoir’. Gut content analysis of the six haplochromine species revealed that eight different food items were consumed. No single species fed exclusively on a single food item, but certain food items contributed higher proportions of the fish diet for each fish species. Resource partitioning therefore could be discerned within this haplochromine community. Thus, Lake Kanyaboli and similar satellite lakes provide an opportunity for conservation of both genetic and trophic diversity threatened by introduction of exotics in the Lake Victoria basin. Lake Kanyaboli should be recognized and conserved as important evolutionary significant units for Lake Victoria region haplochromine species.     

基于情景分析技术的太湖流域洪水风险动因与响应分析研究初探

利用情景分析技术对未来可能影响太湖流域洪水灾害的洪水风险动因和响应进行分析研究,初步确定了影响太湖流域未来洪水风险的动因与响应,在动因与响应的识别和描述的基础上,对动因、响应与洪水风险之间的关系进行了分析,并对动因的相对重要性进行大致排序,分析了太湖流域为应对未来洪水风险所采取的响应的影响及其可持续性,总结归纳了太湖流域未来可能发生的极端异常洪水灾害事件。结果表明,降雨等自然气候和城市化等经济社会因素是影响太湖流域未来洪水风险的主要动因;环湖大堤加固工程、城市地下蓄排系统等是可持续性评价指标较好的响应措施。针对太湖流域可能发生的极端异常洪水灾害事件较多,并且影响重大,提出对这些极端异常洪水灾害事件需要更加深入研究,以便得到更进一步的结论。     

Temporal and spatial variability of phytoplankton in Lake Poyang: The largest freshwater lake in China

The composition and both the temporal and spatial distribution of phytoplankton were studied in Lake Poyang; samples were collected every 3 months from January 2009 to October 2011 at 15 sites. The phytoplankton community was found to belong to seven groups, with Bacillariophyta dominating. No significant difference was observed in the phytoplankton community structure at any of the sites (p = 0.2371), except one site; however, the structure was significantly different with regard to annual and seasonal trends (p = 0.0001 and p < 0.0001, respectively). Aulacoseira granulata, Synedra acus, Fragilaria virescens, and Cryptomonas erosa were the main contributors to the dissimilarity in temporal distribution. Although the nutrient concentrations for 3 years combined were relatively high (mean total nitrogen was 1.719 mg L^− 1 and mean total phosphorus was 0.090 mg L^− 1), phytoplankton biomass was low (mean total biomass of 0.203 mg L^− 1). The underwater light condition, as indicated by the Secchi depth, was shown to be the principal limiting factor in regulating the growth of phytoplankton, and the transparency coincided with biomass variation on a seasonal level. The effect of nutrients on phytoplankton may be concealed by the water level, which varied over a wide range among different seasons. However, the annual trend for the biomass was associated with the nutrient concentration, which increased yearly and initiated the development of phytoplankton. The biomass is high in the south and low in the north, which may be the result of greater underwater light climate and high nutrient concentrations in the southern area.     

Burrowing mayfly (Ephemeroptera: Ephemeridae: Hexagenia spp.) bioturbation and bioirrigation: A source of internal phosphorus loading in Lake Erie

Traditional lake eutrophication models predict lower phosphorus concentrations with decreased external loads. However, in lakes where decreased external phosphorus loads are accompanied by increasing phosphorus concentrations, a seeming “trophic paradox” exists. Western Lake Erie is an example of such a paradox. Internal phosphorus loads may help explain this paradox. We examined bioturbation and bioirrigation created from burrowing mayfly, Hexagenia spp., as a possible source of internal phosphorus loading. Phosphorus concentrations of experimental microcosms containing lake sediments, filtered lake water, and nymphs (417/m²) collected from western Lake Erie were compared to control microcosms containing sediments and lake water over a 7-day period. Phosphorus concentrations in microcosms containing Hexagenia were significantly greater than microcosms without nymphs. Further, we estimate the soluble reactive phosphorus flux from the sediments due to Hexagenia is 1.03 mg/m²/day. Thus, Hexagenia are a source of internal phosphorus loading. High densities of Hexagenia nymphs in western Lake Erie may help explain the “trophic paradox.” Furthermore, Hexagenia may be a neglected source of internal phosphorus loading in any lake in which they are abundant. Future studies of phosphorus dynamics in lakes with Hexagenia must account for the ability of these organisms to increase lake internal phosphorus loading.     

Declining commercial fish catches in Lake Victoria's Winam Gulf: The importance of restructuring Kenya's aquaculture programme

The total weight of commercial fish (metric tons) caught in the Winam Gulf area of Lake Victoria in Kenya declined from 3300 mt to 760 mt between 1995 and 2009. Associated with this decline, and ranked in descending order, the species richness of the commercial fish catch was limited to seven major groups (Rastrineobola argentea; Lates niloticus; Protopterus aethiopicus; Labeo victorianus; Haplochromis species; Clarias gariaepinus; Bagrus domac). Numerous hypotheses ranging from eutrophication, inadequate control of the exotic water hyacinth and overfishing were advanced to explain this decline. The Kenyan government, working in partnership with international organizations, implemented multiple restoration and management programmes in response, including aquaculture. Unfortunately, success for this implemented programme has been very limited because important socio‐economic driving factors were overlooked. To provide a better understanding of the issues, 150 members of the Winam Gulf fishing community, ranging in age from 20 to 40 years, were surveyed. The results of this survey identified poverty and inadequate education as two major issues requiring substantial attention. The survey also suggested that if societal changes are to occur, it will rest upon changing the beliefs of the youth in the Winam Gulf area. Restructuring Kenya's aquaculture programme, with a focus on the youth, provides a pathway for increasing the literacy level of the community, as well as for reducing poverty.     

Trophic evolution of Lake Lugano related to external load reduction: Changes in phosphorus and nitrogen as well as oxygen balance and biological parameters

Lake Lugano is located at the border between Italy and Switzerland and is divided into three basins by two narrowings. The geomorphologic characteristics of these basins are very different. The catchment area is characterized by calcareous rock, gneiss and porphyry; the population amounts to approximately 290 000 equivalent inhabitants. The external nutrient load derives from anthropogenic (85%), industrial (10%) and agricultural (5%) sources. The limnological studies carried out by Baldi et al. (1949) and EAWAG (1964) revealed early signs of eutrophication, with a phosphorous concentration of about 30–40 mg m^–3 and an oxygen concentration of less than 4 g m^–3 in the deepest hypolimnion. Subsequently Vollenweider et al. (1964) confirmed these data and was the first to point out the presence of a meromictic layer in the hypolimnion of the northern basin. From the 1960s, as a result of an increase in the population and internal migration, the lake became strongly eutrophic with the P concentration reaching 140 mg m^–3 and the oxygen in the hypolimnion reduced to zero. Fifty‐five per cent of the P was from metabolic sources and 45% from detergents and cleaning products. In 1976, a partial diversion of waste water from the northern to the southern basin was begun, and gradually eight waste water treatment plants came into operation using mechanical, chemical and biological treatments. In 1986, Italy and Switzerland began to eliminate the P in detergents and cleaning products. Since 1995, the main sewage treatment plants have improved their efficiency by introducing P post‐precipitation, denitrification and filtration treatments. The recovery of the lake is due to be completed by the year 2005. Altogether, during the last 20 years recovery measures have reduced the external P load from about 250 to 70–80 tonnes year^–1; the goal to be reached is 40 tonnes year^–1. In‐lake phosphorous concentrations have decreased from 140 to 50–60 mg m^–3, with the target at 30 mg m^–3. Dissolved oxygen concentration is satisfactory only between the depths of 0 and 50 m, falling rapidly to zero in the deepest layers. Below a depth of 90 m, high CH₄, HS^–, NH₄⁺, Fe²⁺ and Mn²⁺ concentrations exist. Primary production has decreased from 420 to 310 g C_ass m^–2 year^–1, notwithstanding an increase in the thickness of the trophogenic layer. Structure and dynamic biomass show marked changes: phytoplankton dry weight has decreased from 16 to 7 g m^–2, while zooplankton dry weight has increased from 3 to 4.5 g m^–2. Chlorophyll concentration has fallen from 14 to 9 mg m^–3 and Secchi disk transparency has increased from 3.5 to 5.5 m. The current sources of the external load are uncollected small urban conglomerations, storm‐water overflows from outfall sewers, and the residual load from sewage treatment plants, particularly those without P post‐precipitation.     

基于最大日负荷量的流域污染控制措施——以太湖新孟河流域为例

以最大日负荷量（TMDL）计划理念为基础,利用太湖新孟河流域水文水质、污染源资料,对照水功能区的水质要求,对基于月水平的流域最大日负荷量进行了计算分析,提出了基于TMDL值的污染控制措施。结果表明：该流域全年TMDL值变化幅度较大,面源为该区域水质的主要污染源;面源污染负荷量在不同月份差异明显;根据污染物削减量的计算结果提示,并非每个月都要按照统一的削减标准进行排污控制,可根据TMDL计划科学利用和调控水环境容量,既促进环境保护又推动社会经济的和谐发展。     

Examining 20th century water quality and ecological changes in the Lake of the Woods, Ontario, Canada: A paleolimnological investigation

Comparisons between diatom assemblages preserved in modern and pre-industrial sediments were made to provide a lake-wide paleolimnological assessment of environmental change in 17 sites located throughout the Ontario portion of the Lake of the Woods (LoW). Diatom changes were consistent across all sites, although the magnitude of these changes varied along a trophic gradient (i.e., main channel sites versus isolated bays). The most notable taxonomic shift was towards a higher relative abundance of small, centric Cyclotella taxa and other planktonic, pennate diatoms (e.g., Asterionella formosa, Fragilaria crotonensis) in the modern sediments, with corresponding lower relative abundances of heavily silicified Aulacoseira taxa and small benthic Fragilaria taxa. To aid in determining whether changes in nutrients can explain the diatom trends, weighted-averaging partial-least-squares (WA-PLS) techniques were used to develop a diatom-based inference model to reconstruct changes in total phosphorus (TP) concentrations. Diatom-inferrred TP (DI-TP) reconstructions revealed that 88% of the sites showed either no change or a slight, but not significant decline in DI-TP since pre-industrial times. Diatom-based inferences suggest that TP concentrations at many sites in the Ontario portion of the LoW were moderately elevated in nutrients prior to any major human disturbances (i.e., pre-1850). Results suggest that substantial increases in temperature over the last few decades, and the associated changes to ice cover and water column properties, have likely exacerbated the effects of existing stressors on the system and were key factors influencing a lake-wide restructuring of the diatom communities over the past ca. 150 years.     

The role of coastal processes in the management of the mouth of the River Murray,Australia: Present and future challenges

The Murray–Darling Basin is the largest river system in Australia to enter the sea. Prior to regulation of water flows, the Murray Mouth remained open to the sea even during droughts. An open mouth assists in sustaining the ecology of a Ramsar listed wetland and enables the flushing of salt, nutrients, and suspended sediments to the sea. Construction of barrages designed to prevent saltwater ingress has separated the estuary into two sections, a set of freshwater lakes and a saline tidal lagoon area, creating a unique set of management challenges. Under current overextraction of water resources upstream, river flows have been largely insufficient to counter wave and tide processes, which propel marine sands thereby constricting the Murray Mouth. Dredging has been required to maintain an open entrance. Managing this system is part of a 2012 agreement between state and federal governments, through the Murray–Darling Basin Plan. This plan recognizes a healthy end of system; however, the hydrological models failed to factor in the power of the sea in blocking the Murray Mouth. The plan requires that there will be sufficient river flows for the mouth to be open 95% of the time without dredging. Currently, sand ingress from the sea requires dredging most of the time. It is unlikely that there will be sufficient river flows to counteract continued sequestration of marine sand into the mouth. Sea level rise and decreased rainfall in the southern half of the Basin under climate change conditions will require a review of management options to prevent the long‐term degradation of the end of system.     

SPARROW模型的传输过程研究——以新安江流域总氮为例

建立新安江流域总氮的SPARROW模型,以土-水传输因子(LDF)表示降雨、坡度、气温3个土-水传输变量的影响,结合农业源、林草地源、生活源3个污染源排放系数,分析总氮经过土-水传输之后到达河道的比例(LDR),由此揭示总氮的非点源污染特征。结果显示坡度的影响在整个流域范围内差异相对较大,LDF为0.86~1.06,因而对3类污染源进入河道的传输比差异亦较大。综合考虑3个土-水传输变量作用下,子流域60土-水传输因子最大,而子流域225最小,因此若制定减排措施要求入河减少量相同,管理上会优先考虑子流域60所在的地区。研究采用改进的河流衰减方程同时描述水文和非水文因素的影响,代替河流分级衰减系数,引进传质系数作为模型模拟参数,削减速率与流量呈负相关关系,且大部分河段削减速率均在以往文献研究范围之内,表明改进的传质速率用于新安江流域总氮模型具有可行性。