巢湖快速变化风浪场及其环境效应

浅水湖泊风浪过程对于湖泊生态系统具有重要的意义.基于巢湖风场、风浪和水环境参数同步高频观测结果,详细分析了快速变化风场下的风浪快速变化特征及其对湖泊水环境的影响特征.浅水湖泊风浪的有效波高和平均波周期均随风速的快速变化有较好的同步响应规律.在风速快速衰减阶段,相较有效波高,波周期有更好的稳定性.湖泊水体pH、水温、溶解氧会快速响应风浪的变化,随着风浪强度增强,对水体浊度、总磷浓度以及藻密度和生物量的扰动影响逐渐呈现.强烈的风浪扰动引起水体浊度变化的滞后时间可达3 d.快速变化的风浪场下,风浪的强烈扰动会改变水体固有的理化参数分布特征,扰动藻类常规的水体分布规律,风浪强度是造成差异的主要因子.     

太湖不同湖区风浪的季节变化特征

为明晰太湖风浪的空间分布及季节变化,在湖心区设立波浪观测站,利用其记录的波浪数据证明SWAN模型能够较好地模拟太湖风浪.基于所建模型,对2013年自然风场条件下太湖不同湖区风浪季节动态进行模拟分析,结果表明:受岸线、地形和岛屿等地理因素影响,大太湖的风浪总是最强,其有效波高均值为0.523 m;而东太湖风浪最小,有效波高均值为0.305 m.受盛行风场季节变化影响,太湖春、夏季有效波高均值明显大于秋、冬季.太湖波浪的能量主要来源于风场,其有效波高随风速增大而增大,两者呈极显著正相关.而风向则可以通过改变风区长度来影响风浪生消.在偏东风作用下,太湖湖西区的风浪大于东部湖区;而受盛行于冬季的偏北风影响,太湖南部水域风浪要大于北部.同时,太湖风浪的时空分布特征是造成太湖水质参数、沉积物和水生植物空间分布差异的重要原因之一.     

Rapid shoreline flooding enhances water turbidity by sediment resuspension: An example in a large Tibetan lake

Rapid water level rise due to climate change has the potential to remobilize loose sediments along shorelines and increase the turbidity of nearshore waters, thereby impacting water quality and aquatic ecosystem health. Siling Lake is one of the largest and most rapidly expanding lakes on the Tibetan Plateau. Between 2000 and 2017, this lake experienced an increase in water level of about 8 m and a doubling in water turbidity. Here, using this lake as a study site, we used a wave model and high-resolution remote sensing of turbidity (Landsat-8) to assess the potential connection between water-level rise, enhanced wind-driven sediment resuspension and water turbidity. Our analysis revealed that strong bottom shear stresses triggered by wind-generated waves over newly flooded areas were related to an increase in water turbidity. The spatial variability of Siling Lake turbidity showed a strong dependence on local wind characteristics and fetch. Two factors combined to drive the increase in turbidity: (1) high wave energy leading to high bottom shear stresses, and (2) flooding of unvegetated shallow areas. Using a new relationship between wave energy and turbidity developed here, we expect the increase in turbidity of Siling Lake to taper off in the near future due to the steep landscape surrounding the lake that will prevent further flooding. Our results imply that rising water levels along the coast are not only expected to influence terrestrial ecosystems but could also change water quality. The methodology presented herein could be applied to other shorelines affected by a rapid increase in water level. © 2020 John Wiley & Sons, Ltd.     

Hurricane impacts on turbidity and sediment in the Rookery Bay National Estuarine Research Reserve,Florida, USA

The resuspension and transportation of sediments affect the nutrient or contaminants cycles in water column. Therefore, hurricane-induced sediment resuspension in coastal waters may cause environment impacts on coastal ecosystems. In this study, field observed data during a hurricane event from the Rookery Bay National Estuarine Research Reserve (Rookery Bay NERR), Florida, were studied. Based on the relationship between sediment and turbidity, variations of turbidity in the Rookery Bay NERR were discussed in this paper under the impact of the Hurricane Isaac in 2012. Time series analysis indicates that Hurricane Isaac causes the substantial increase of turbidity. Traditional direct regression analysis with raw data was found to result in poor correlation (R2 o 0.41) between wind speed and turbidity due to the impacts of multi-factors in coastal waters of study region during hurricane. Then Hanning Filter was used to remove tidal effect, which shows some improvement of correlations (R2 4 0.6) between wind speed and turbidity. Further improvement was made by introducing a phase factor to account for the lag time between the peak turbidity and peak wind speed, which lead to further improvement of the regression equation between wind speed and turbidity. By using Hanning Filter and phase factor, the regression equation between turbidity and wind speed fits well with observations (R2 4 0.96). Different lag time between wind speed and turbidity at different locations illustrated that wind had different effects on turbidity due to the locations and water depth. Based on the approximate empirical rela-tionship between turbidity and sediment concentration, a regression equation between wind speed and sediment concentration was derived approximately. The equation could be applied to approximately estimate the sediment concentration using wind speed during hurricane, especially with the strong wind speed above 4 m/s. It can be used to provide technical supports for sediment assessment and coastal zone research under extreme clime in the study region.     

Study on the simulation of transparency of Lake Taihu under different hydrodynamic conditions

It was indicated in this study that there were negative relations between the concentrations of suspended solid (SS) and transparency according to the analysis of measured data of Lake Taihu. Their relations in pervious studies were reviewed, which showed that the changes of transparency in Lake Taihu could be reflected by simulating suspended solid concentration (SSC). Measured data showed that the changes of SSC with wind speed were similar at different water depths. SSC increased with the increasing of wind speed. Both wave and lake current of Lake Taihu had positive relations with SSC. However, wave was the main factor affecting sediment suspension, while flow took the second place. In this study, a numerical model coupling lake current, wave and SSC of Lake Taihu was developed. In the SS model, the combined effects of wave and current were included. The amounts of suspended and deposited sediments near the lake bed surface layer were treated separately. The stochastic characteristics of turbulent flow pulsation near lake beds were also considered, and the start-up conditions of sediment suspension were introduced to the model. The model elucidated the mutual exchange processes between sediment particles in SS and active sediments within and on the bed surface layer. Simulated results showed that lake current had relatively significant effects on the SSC at littoral areas of Lake Taihu, while SSC at the central area of the lake was mainly influenced by wave. The changes of transparency with SSC were simulated for Lake Taihu using this model. Calculated results were validated by measured data with good fitness, which indicated that the model is basically suitable for the simulation and prediction of transparency of Lake Taihu.     

Study on the simulation of transparency of Lake Taihu under different hydrodynamic conditions

It was indicated in this study that there were negative relations between the concentrations of suspended solid (SS) and transparency according to the analysis of measured data of Lake Taihu. Their relations in pervious studies were reviewed, which showed that the changes of transparency in Lake Taihu could be reflected by simulating suspended solid concentration (SSC). Measured data showed that the changes of SSC with wind speed were similar at different water depths. SSC increased with the increasing of wind speed. Both wave and lake current of Lake Taihu had positive relations with SSC. However, wave was the main factor affecting sediment suspension, while flow took the second place. In this study, a numerical model coupling lake current, wave and SSC of Lake Taihu was developed. In the SS model, the combined effects of wave and current were included. The amounts of suspended and deposited sediments near the lake bed surface layer were treated separately. The stochastic characteristics of turbulent flow pulsation near lake beds were also considered, and the start-up conditions of sediment suspension were introduced to the model. The model elucidated the mutual exchange processes between sediment particles in SS and active sediments within and on the bed surface layer. Simulated results showed that lake current had relatively significant effects on the SSC at littoral areas of Lake Taihu, while SSC at the central area of the lake was mainly influenced by wave. The changes of transparency with SSC were simulated for Lake Taihu using this model. Calculated results were validated by measured data with good fitness, which indicated that the model is basically suitable for the simulation and prediction of transparency of Lake Taihu.     

Terrigenous transportation to the Okinawa Trough and the influence of typhoons on suspended sediment concentration

The source and transport mechanisms of land-derived Okinawa Trough sediments were studied using the field data of temperature, salinity and turbidity in the East China Seas. The results suggest that there are two primary sediments sources from the Chinese Mainland to the Okinawa Trough: one is the Old Huanghe River submarine delta, and the other is the Changjiang River sediments, which are distributed at the Changjiang River estuary and the off-coast of Zhejiang and Fujian provinces. It is difficult for the Huanghe River suspended sediments to arrive in the Okinawa Trough via the new estuary. Although the Taiwan warm current blocks the seaward terrigenous transportation to a certain extent, part of the coastal suspended sediments are transported to the outer shelf. Suspended particulate matter is unable to get through the barrier of the Kuroshio Current under normal conditions. However, episodic events, such as winter storms, internal-tidal waves and turbidity flows, are capable of transporting suspended particulate matter into the Okinawa Trough. The super typhoon “Ewiniar” induced strong waves and influenced the thermocline depth and suspended sediment concentration of the East China Seas. The typhoon-induced waves pushed the thermocline depth down to around 40 m and caused the resuspension of large volumes of sediments in its path. In the other East China Seas regions, the typhoon-induced swells deepened the thermocline depth by about 5 m and increased suspended sediment concentrations. The typhoon effect on suspended sediment concentration of the East China Seas disappeared within 2 weeks.     

Developing configuration of WRF model for long-term high-resolution wind wave hindcast over the North Atlantic with WAVEWATCH III

The spatial resolution of wind forcing fields is critical for modeling ocean surface waves. We analyze here the performance of the non-hydrostatic numerical weather prediction system WRF-ARW (Weather Research and Forecasting) run with a 14-km resolution for hindcasting wind waves in the North Atlantic. The regional atmospheric model was run in the domain from 20° N to 70° N in the North Atlantic and was forced with ERA-Interim reanalysis as initial and boundary conditions in a spectral nudging mode. Here, we present the analysis of the impact of spectral nudging formulation (cutoff wavelengths and depth through which full weighting from reanalysis data is applied) onto the performance of the modeled 10-m wind speed and wind wave fields for 1 year (2010). For modeling waves, we use the third-generation spectral wave model WAVEWATCH III. The sensitivity of the atmospheric and wave models to the spectral nudging formulation is investigated via the comparison with reanalysis and observational data. The results reveal strong and persistent agreement with reanalysis data during all seasons within the year with well-simulated annual cycle and regional patterns independently of the nudging parameters that were tested. Thus, the proposed formulation of the nudging provides a reliable framework for future long-term experiments aiming at hindcasting climate variability in the North Atlantic wave field. At the same time, dynamical downscaling allows for simulation of higher waves in coastal regions, specifically near the Greenland east coast likely due to a better representation of the mesoscale atmospheric dynamics in this area.     

Modelling hydrodynamics in Yachiyo Lake using a non‐hydrostatic general circulation model with spatially and temporally varying meteorological conditions

In this study, a three‐dimensional (3D) non‐hydrostatic circulation model was applied to study the thermal structure, its evolution and water circulation of Yachiyo Lake in Hiroshima, Japan. The simulations were conducted for 1 month during July 2006. The meteorological forcing variables such as wind stress, surface atmospheric pressure and heat flux transfer through the lake surface were provided by an atmospheric mesoscale model run. The vertical mixing process of the lake was calculated using the Mellor‐Yamada turbulence model. The 1‐month numerical simulation revealed the wind‐induced currents of the lake, two gyres in the mid‐layer, and depth‐averaged monthly mean currents. Further numerical experiments studying the mechanism of the two gyres in the lake showed the important role of topography in gyre formation. The thermal structure of the lake and its evolution both in space and in time as predicted by the model showed very good agreement with the observed values and characteristics of Yachiyo Lake. The internal gravity waves, which are crucial for mixing in the stratified lake, are depicted by the vertical fluctuation of isotherms. Using the non‐dimensional gradient Richardson number, Yachiyo Lake was determined to be stable under strong stratification during the study period, and therefore very sensitive to wind stress. Copyright © 2009 John Wiley & Sons, Ltd.     

考虑采砂影响的鄱阳湖丰水期悬浮泥沙浓度模拟

针对受采砂活动影响显著的鄱阳湖高浑浊水体,结合数值模拟和遥感技术,利用已有的鄱阳湖采砂区遥感监测结果,在构建的鄱阳湖水动力-悬浮泥沙输移模型中添加泥沙点源,对2011年7月1-31日采砂影响下的鄱阳湖丰水期悬浮泥沙浓度进行数值模拟.利用悬浮泥沙浓度实测数据和MODIS影像反演结果对模拟结果的有效验证表明,考虑采砂影响后,悬浮泥沙浓度模拟值与实测值具有强相关关系,确定性系数为0.831,均方根误差为15.5 mg/L,悬浮泥沙浓度空间分布趋势与遥感反演结果基本一致.模拟结果显示,采砂活动对鄱阳湖南部主湖区、河流入湖口影响较小,其主要影响由南向北,经棠荫以西和松门山岛以北航道、入江水道延伸到湖口区域,是鄱阳湖北湖区高浑浊水体形成的重要原因.     

Effect of sediment resuspension on underwater light field in shallow lakes in the middle and lower reaches of the Yangtze River: A case study in Longgan Lake and Taihu Lake

Based on three continuous in situ underwater light field measurement under different wind waves conditions in Longgan Lake, Meiliang Bay of Taihu Lake in July 2003 and littoral zone near TLLER in July 2004, respectively, the effects of sediment resuspension caused by wind waves on PAR diffuse attenuation, absorption coefficients and euphotic depths are analyzed. In Longgan Lake, PAR diffuse attenuation coefficients during small, middle and large wind waves were 1.74, 2.02 and 2.45 m-1, respectively, and the corresponding PAR spectral diffuse attenuations ranged from 0.98 to 2.97, 1.34 to 3.95 and 1.80 to 5.40 m-1, respectively. In Meiliang Bay, PAR diffuse attenuation coefficients were 2.63, 3.72, 4.37 m-1 during small, middle and large wind waves. PAR diffuse attenuation coefficients increased by 41% and 66% from small to middle, large wind waves, respectively. Absorption coefficients integrated over the range of PAR of CDOM, phytoplankton were 0.26, 0.28 m-1; 0.76, 0.49 m-1, respectively during middle and large wind waves. Absorption coefficients integrated over the range of PAR of non-algal particulate matter and total suspended particulate matter increased from 0.94 to 1.73 m-1, and from 1.70 to 2.22 m-1, respectively during middle and large wind waves. Relative contributions of absorption coefficients of non-algal particulate matter to total absorption coefficient integrated over the range of PAR were 44.14%, 65.05%, respectively, during middle and large wind waves. PAR euphotic depths decreased by 0.40, 0.19, 0.20 m from middle to large wind waves in Longganhu Lake, Meliang Bay and littoral zone near TLLER. Significant correlations were found between transparency, PAR diffuse attenuation coefficients, euphotic depths and total suspended paniculate matter, wind velocity, wave height. Most significant correlations were found between transparency, PAR diffuse attenuation coefficients, euphotic depths and inorganic suspended paniculate matter but low correlations for chlorophyll a, dissolved organic carbon. Increase of total suspended paniculate matter, especially inorganic suspended paniculate matter caused by wind waves was the dominant factor affecting underwater light field in shallow lakes in the middle and lower reaches of the Yangtze River based on observations at three stations.     

Monitoring turbidity from above: Deploying small unoccupied aerial vehicles to image in‐stream turbidity

Small unoccupied aerial systems (sUASs) are increasingly applied to study hydrologic processes and water quality. Here, we evaluate a novel application of sUAS to stream turbidity monitoring, with the goal of extending analyses implemented with satellite remote sensing to enable high resolution, rapid collection of turbidity imagery along smaller waterbodies. To accomplish this, we collected multispectral imagery using two sUAS platforms under a range of environmental conditions along a local creek in Syracuse, NY. In addition, we collected in situ turbidity observations immediately after each flight along several transects along the creek, as well as within a clear plume created by a natural spring entering the main channel of the creek. The in situ turbidity values were compared with the mean and standard deviation of several single‐band and multiband indices extracted along similar transects from the sUAS flights. On the basis of data collected across several flights, we found optical metrics obtained from multispectral imagery correlated well with in situ turbidity measurements. Though many optical metrics yielded strong relationships considering only values within the main channel, values associated with the red band were strongly related to turbidity estimates from the main channel as well as lower turbidity values observed in the spring plume. Although there are still limitations of this approach associated with variable field conditions, results from this proof of concept analysis show that sUASs offer a promising avenue for cost‐effective turbidity monitoring.     

An idealized model study of flocculation on sediment trapping in an estuarine turbidity maximum

Flocculation has an important impact on particle trapping in estuarine turbidity maximum (ETM) through associated increases in particle settling velocity. To quantify the importance of the flocculation processes, a size-resolved flocculation model is implemented into an ocean circulation model to simulate fine-grained particle trapping in an ETM. The model resolves the particle size from robust small flocs, about 30 μm, to very large flocs, over 1000 μm. An idealized two-dimensional model study is performed to simulate along-channel variations of suspended sediment concentrations driven by gravitational circulation and tidal currents. The results indicate that the flocculation processes play a key role in generating strong tidal asymmetrical variations in suspended sediment concentration and particle trapping. Comparison with observations suggests that the flocculation model produces realistic characteristics of an ETM.     

A 1D model for tides waves and fine sediment in short tidal basins—Application to the Wadden Sea

In order to simulate the dynamics of fine sediments in short tidal basins, like the Wadden Sea basins, a 1D cross-sectional averaged model is constructed to simulate tidal flow, depth-limited waves, and fine sediment transport. The key for this 1D model lies in the definition of the geometry (width and depth as function of the streamwise coordinate). The geometry is computed by implementing the water level and flow data, from a 2D flow simulation, and the hypsometric curve in the continuity equation. By means of a finite volume method, the shallow-water equations and sediment transport equations are solved. The bed shear stress consists of the sum of shear stresses by waves and flow, in which the waves are computed with a depth-limited growth equation for wave height and wave frequency. A new formulation for erosion of fines from a sandy bed is proposed in the transport equation for fine sediment. It is shown by comparison with 2D simulations and field measurements that a 1D schematization gives a proper representation of the dynamics in short tidal basins.     

Fine sediment dynamics in a shallow lake and implication for design of hydraulic works

Lake Markermeer is a large (680?km²), shallow body of water in the middle of the Netherlands, with a mean water depth of 3.6?m. One of the major problems in the lake is its decreasing ecological value which is, among other reasons, caused by a gradual increase of suspended sediment concentration and associated increase of light attenuation in the water column. A thorough understanding of fine sediment dynamics in the lake is a prerequisite for solving this problem. This paper addresses the 3D nature of near-bed sediment dynamics in Lake Markermeer, based on data sampled from a 1-month field experiment in autumn 2007. The campaign involved the collection of 71 bed samples across the lake. At each location, dual-frequency echo soundings were carried out to assess the thickness of the silt layer, and sediment concentration throughout the water column was measured with an Optical Backscatter Sensor (OBS). Moreover, 2-week time series of wave height, water level, current velocities, and near-bed sediment concentration were collected at a single location. The time series of sediment concentration were measured with a regular OBS and an Argus Surface Meter IV (ASM). During the measurement period, flow velocities ranged between 2 and 15?cm/s, wave heights up to 1.2?m were observed and turbidity levels varied between 40?mg/l to more than 300?mg/l. The ASM data generally showed uniform concentration profiles. However, profiles with steep concentration gradients near the bed were found for wave heights above 0.5?m. The field experiments further revealed pronounced 3D structures near the bed during discrete storms. The results are generalized for a wider range of conditions and across the full water depth through application of a 1DV point model, using a two-fraction representation of the grain size distribution. The fine and coarse fractions are found to resuspend rapidly for wind speeds above 5?m/s and 10??2?m/s, respectively, forming a uniform concentration profile if these wind conditions persists. High-concentration (???g/l) layers near the bed, containing the coarse sediment fraction, only occur at the onset and towards the end of a storm, when wind speed changes rapidly. It is under these conditions that horizontal gradients in layer density or thickness can transport considerable fine sediment. This transport provides an additional mechanism for the infill of, for instance, silt traps and navigation channels.     

Reducing the impacts of flood-induced reservoir turbidity on a regional water supply system

This paper proposed an integrated simulation model to incorporate the impact of flood-induced reservoir turbidity into water supply. The integrated model includes a regional water allocation model and a one-dimensional settling model of cohesive particles based on Kynch’s theory. It simulates the settling of sediment flocculation in a turbid reservoir. The restrictions of water supply during floods is mimicked by simulating turbidity profiles for control points and then quantifying the associated treatment capability of raw water in the regional water allocation model for each time step. This framework can simulate shortages caused by flood-induced high turbidity as well as extended droughts, thus provide a basis for comprehensive evaluations of emergent and regular water supply facilities. A case study of evaluating different measures to mitigate the impact of turbid reservoir on water supply in northern Taiwan is presented to demonstrate the efficacy of the proposed approach.     

Numerical prediction of turbidity currents on unsteady sediment-supply

A three-dimensional(3D) numerical model of unstable turbidity currents is developed based on the mechanism of sediment transport and turbulence theory.In this model,numerical simulation of turbidity currents without subsequent supply of muddy water was conducted using the same parameters as were used in the flume experiments.The evolution process of turbidity currents of completely losing supplies observed in the experiment was simulated by the model;validation of the numerical model and the algorithm was conducted.If momentarily interrupted process is regarded as a special case of the gradually interrupted,based on the preceding numerical simulation validity,it is feasible to simulate the motion law of turbidity currents under losing gradually supplies.By this method in this article,the characteristic of sediment-laden flow of losing gradually supplies was obtained,as well as its relationship between front velocity and sediment concentration.     

A 1D model for tides waves and fine sediment in short tidal basins—Application to the Wadden Sea

In order to simulate the dynamics of fine sediments in short tidal basins, like the Wadden Sea basins, a 1D cross-sectional averaged model is constructed to simulate tidal flow, depth-limited waves, and fine sediment transport. The key for this 1D model lies in the definition of the geometry (width and depth as function of the streamwise coordinate). The geometry is computed by implementing the water level and flow data, from a 2D flow simulation, and the hypsometric curve in the continuity equation. By means of a finite volume method, the shallow-water equations and sediment transport equations are solved. The bed shear stress consists of the sum of shear stresses by waves and flow, in which the waves are computed with a depth-limited growth equation for wave height and wave frequency. A new formulation for erosion of fines from a sandy bed is proposed in the transport equation for fine sediment. It is shown by comparison with 2D simulations and field measurements that a 1D schematization gives a proper representation of the dynamics in short tidal basins.