2006年台风“碧利斯”与“格美”的降水差异性比较

对2006年7月登陆,西移影响粤东的路径相似、降水过程强度和范围不同的2次台风过程,从环流形势,台风环流本身、结构以及降水性质等方面进行了对比分析.结果表明,"碧利斯"的降水是一典型的和西南季风相结合的降水,而"格美"的降水仅仅是台风本身环流的降水.同时发现两者在登陆2 d内500 hPa平均高度场、水汽通量垂直剖面图,水汽输送时间序列图上均有显著差异.指出水汽输送的强弱,决定了登陆台风的维持时间和降雨强度.     

Sediment transport patterns at Trafalgar offshore windfarm

A simple analytical model of wave propagation has been developed in order to study the potential sediment transport patterns due to the action of currents and waves in the neighborhood of cylindrical structures as well inside a group of these structures. The attention is focused on the study of Trafalgar offshore windfarm, a case in which it has been necessary to analyze the flow trough porous structures in order to model fish growing-cages planned to be installed at each aerogenerator structure. The results are obtained by averaging over one period of wave in order to evaluate the net potential sediment transport. The analysis of the results reveals how the processes of wave diffraction and reflection give rise to periodic patterns of sediment transport around and between the structures.     

Onshore sandbar migration at Tairua Beach (New Zealand): Numerical simulations and field measurements

We observed the onshore migration (3.5 m/day) of a nearshore sandbar at Tairua Beach, New Zealand during 4 days of low-energy wave conditions. The morphological observations, together with concurrent measurements of waves and suspended sediment concentrations, were used to test a coupled, wave-averaged, cross-shore model. Because of the coarse bed material and the relatively low-energy conditions, the contribution of the suspended transport to the total transport was predicted and observed to be negligible. The model predicted the bar to move onshore because of the feedback between near-bed wave skewness, bedload, and the sandbar under weakly to non-breaking conditions at high tide. The predicted bathymetric evolution contrasts, however, with the observations that the bar migrated onshore predominantly at low tide. Also, the model flattened the bar, while in the observations the sandbar retained its steep landward-facing flank. A comparison between available observations and numerical simulations suggests that onshore propagating surf zone bores in very shallow water (< 0.25 m) may have been responsible for most of the observed bar behaviour. These processes are missing from the applied model and, given that the observed conditions can be considered typical of very shallow sandbars, highlight a priority for further field study and model development. The possibility that the excess water transported by the bores across the bar was channelled alongshore to near-by rip-channels further implies that traditional cross-shore measures to judge the applicability of a cross-shore morphodynamic model may be misleading.     

典型河口浮游动物种类数及生物量变化趋势的研究

依据国家海洋局2004-2006年每年5月和8月的黄河口、长江口和珠江口生态监控区浮游动物的监测结果,对上述河口浮游动物和桡足类种类数及浮游动物生物量分布进行了分析,并将结果与历史资料进行同测区、同季节的对比。结果表明,与全国海洋普查时期相比,2006年黄河口浮游动物种类数下降了50．6％,桡足类种类数下降了53.3％;桡足类在浮游动物中所占比重由1985年的42.4％下降到2006年的32．6％。长江口浮游动物种类数有所上升,但是桡足类种类数由1985年的62种降低到2006年的37种;桡足类在浮游动物中的比重从1958年的54．2％降低到2006年的42．5％。珠江口桡足类在浮游动物中所占的比例由1959年的59．3％下降到54．4％,未发生明显变化。黄河口春季、夏季浮游动物生物量上升趋势不明显,夏季增幅较小;长江口自2000年以后春季浮游动物平均生物量为528．36mg／m^3,是1958年的2．4倍,2005年与2006年夏季浮游动物生物量分别是1958年的2．2和2．7倍,增加趋势十分明显;珠江口春季浮游动物生物量上升趋势最为显著,2004-2006年3年春季浮游动物平均生物量是1959年同期的5．7—8．7倍,1981年、2006年夏季生物量分别比1959年增加了3．1倍和11倍。就这3个河口生物量变化趋势看,珠江口生物量的上升趋势最为明显,长江口次之,黄河口变化幅度较小。     

雷州半岛灯楼角珊瑚礁海区夏季的浮游动物

2004年8月在雷州半岛灯楼角珊瑚礁海区分别使用浅水Ⅰ型和浅水Ⅱ型浮游生物网进行了浮游动物调查,共鉴定浮游动物72种和浮游幼虫19个类群(或类型),其中桡足类的种数最多达45种。结果表明,浮游动物种数分布由近岸往远岸递增,约3/4的种类属于终身浮游生物,1/4属于阶段性浮游生物。中小型的浮游动物种类多,数量大。两种网具采集的浮游动物总种数、总密度、多样性指数、均匀度和中小型优势种的密度差异相当显著,而大型优势种的密度差异不明显。用浅水Ⅱ型网采集的浮游动物总密度平均值为5270ind/m3,是浅水Ⅰ型网的110倍。桡足类占浮游动物总密度的72.45%,其次幼虫占23.41%。浮游动物密度呈斑块状分布,最高达16257ind/m3,底质为珊瑚礁的测站的浮游动物数量一般较多。优势种主要是中小型种类和底栖动物的幼虫,如强额孔雀哲水蚤、蔓足类无节幼虫、细长腹剑水蚤、驼背隆哲水蚤、无节幼虫、双壳类面盘幼虫、桡足幼体、简长腹剑水蚤等。     

The effect of bedform dynamics on computing suspended sediment fluxes using optical backscatter sensors and current meters

The sensitivity of the suspended sediment flux is tested with respect to rapid changes in bed-level across the surf zone of a sandy beach. The suspended flux was computed using a fixed instrument array, but bed-level changes due to ripple migration caused the instrument elevations to be significantly changed during the course of the experiment. The nominal elevations of the instruments were adjusted during data processing (using the MOBS array) to maintain a fixed elevation with respect to bed-level changes. The resultant suspended sediment concentrations and fluxes were significantly different from the unadjusted data, and for the present data set O(35%) less when averaged over the tide. The maximum difference between adjusted and unadjusted fluxes may be O(260%). The results indicate that changes in bed-level, particularly those due to bedform migration, must be accounted for when processing OBS data if reliable estimates of suspended sediment transport are to be obtained in the field.     

Large-scale dune erosion tests to study the influence of wave periods

Large-scale physical model tests were performed to quantify the effects of the wave period on dune erosion. Attention was focussed on 2D cross-shore effects in a situation with sandy dunes and extreme water levels and wave conditions. Besides profile measurements, detailed measurements in time and space of water pressure, flow velocities and sediment concentrations were performed in the near near-shore area. It was concluded that a longer wave period leads to a larger dune erosion volume and to a larger landward retreat of the dune face. Tests with double-peaked wave spectra showed that the influence of the spectral shape on dune erosion was best represented by the T_m − 1,0 spectral mean wave period, better than the peak wave period, T_p. The effect of the wave period on dune erosion was implemented in a dune erosion prediction method that estimates erosion volumes during normative storm conditions for the Dutch coast. More details of the measurements and additional analyses of physical processes are described in an accompanying paper by Van Thiel de Vries et al. [Van Thiel de Vries, J.S.M., van Gent, M.R.A., Reniers, A.J.H.M. and Walstra, D.J.R., submitted for publication. Analysis of dune erosion processes in large scale flume experiments, In this volume of Coastal Engineering.].     

Analysis of threshold and incipient conditions for sediment movement

Prediction of threshold conditions and incipient motion is the essential issue for the study of sediment transport. This work compares existing empirical threshold curves proposed for Shields diagram, a method based on the concept of probability of sediment movement, and an empirical method based on movability number. These methods are used to predict the incipient motion conditions for experimental runs taken from various studies. Most of the experimental data, used in this work, have not been used before in derivation of alternative formulations for Shields diagram and other methods. The empirical threshold curves based on the Shields entrainment function was the least successful at predicting the measured incipient motion conditions, while the use of the movability number gives good predictions of critical shear velocity compared with experimental data.