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991.
992.
Environmental impacts of dredging on seagrasses: a review 总被引:2,自引:0,他引:2
Main potential impacts on seagrasses from dredging and sand mining include physical removal and/or burial of vegetation and effects of increased turbidity and sedimentation. For seagrasses, the critical threshold for turbidity and sedimentation, as well as the duration that seagrasses can survive periods of high turbidity or excessive sedimentation vary greatly among species. Larger, slow-growing climax species with substantial carbohydrate reserves show greater resilience to such events than smaller opportunistic species, but the latter display much faster post-dredging recovery when water quality conditions return to their original state. A review of 45 case studies worldwide, accounting for a total loss of 21,023 ha of seagrass vegetation due to dredging, is indicative of the scale of the impact of dredging on seagrasses. In recent years, tighter control in the form of strict regulations, proper enforcement and monitoring, and mitigating measures together with proper impact assessment and development of new environmental dredging techniques help to prevent or minimize adverse impacts on seagrasses. Costs of such measures are difficult to estimate, but seem negligible in comparison with costs of seagrass restoration programmes, which are typically small-scale in approach and often have limited success. Copying of dredging criteria used in one geographic area to a dredging operation in another may in some cases lead to exaggerated limitations resulting in unnecessary costs and delays in dredging operations, or in other cases could prove damaging to seagrass ecosystems. Meaningful criteria to limit the extent and turbidity of dredging plumes and their effects will always require site-specific evaluations and should take into account the natural variability of local background turbidity. 相似文献
993.
储层非均质性研究--以葡萄花油层组PI1~PI4小层为例 总被引:2,自引:0,他引:2
储层非均质性对油气田的勘探和开发效果影响很大。结合测井资料二次解释的孔隙度(φ)、泥质含量(Vsh)、粒度中值(Md)、束缚水饱和度(Swi)等资料,借助BP网络对渗透率进行预测。对萨尔图油田中部葡萄花油层组的非均质性进行了定量半定量的研究,认为萨尔图油田中部葡萄花油层组PI2小层层内非均质性最严重,与沉积微相密切相关。单砂层垂向上渗透率的变化以正韵律和复合式韵律为主,局部发育反韵律模式。经对葡萄花油层组PI1—PI4系统研究,层内非均质性强弱依次为PI2a、PI3、PI2b、PI1、PI4;层间非均质性在PI3和PI4间表现得最强,其它相对要弱;各小层平面非均质性相差无几,整体表现为较强的非均质性。 相似文献
994.
995.
996.
根据泰尔系数和基尼系数可按地域和收入来源进行分解的特性,对我国1997—2009年间各省份农民收入时空差异进行地域和因子结构双重解析。结果表明,省际间农民收入呈小幅波动增大态势;三大地带内差异是构成农民收入差异的主导部分,尤其是中部地带内省际间差异和贡献居首要地位;因子结构分解则显示工资性收入对总体非均衡程度的贡献最高。着力加快中、西部地区的工业化、城镇化进程,推进农业剩余劳动力向城镇转移或向非农产业转移,提升其工资性收入水平,并从政策、财政和科技等方面支持落后地区加快发展家庭经营是缓解农村地区省际收入差距并促进农村经济区域协调发展的有效途径。 相似文献
997.
基于流速衰减特征的泥石流冲出距离预测方法 总被引:2,自引:0,他引:2
泥石流冲出距离的确定是泥石流灾害评价及防治中的关键环节。基于泥石流在堆积区流速的变化特征,提出了泥石流流速"衰减系数"的概念。通过对泥石流流速衰减系数的定义和计算公式的推导,提出了一种基于泥石流流速衰减系数来确定泥石流冲出距离的新方法,并以四川省泸定县干沟泥石流为实例进行计算和验证。研究结果表明:当干沟泥石流在堆积区的流速大于初始(沟口)流速的15%时,流速的衰减程度极为显著;而当堆积区流速小于初始速度的15%时,流速的衰减极为缓慢。研究结果对泥石流灾害的危险性评价及防灾减灾具有一定的理论和实践意义。 相似文献
998.
Variation of the Sectional Drag Coefficient of a Group of Buildings with Packing Density 总被引:2,自引:2,他引:0
Reynolds-averaged Navier–Stokes (RANS) simulations of turbulent flow over groups of buildings with different packing densities
are reported. The results for a selected packing density are compared with direct numerical simulations (DNS) previously validated
against wind-tunnel data. The present study is focused on average properties of the flow, especially on the drag coefficients,
and is a first attempt to provide information on these parameters (their values are not generally known) for a range of packing
densities, for a given staggered arrangement of cubes using RANS methods. However, some of the limitations of RANS have come
to light. Hence, it is recommended that such simulations are ‘calibrated’ against experimental or DNS data, as is done here. 相似文献
999.
Aerodynamic Parameters of Regular Arrays of Rectangular Blocks with Various Geometries 总被引:8,自引:8,他引:0
The aerodynamic effects of various configurations of an urban array were investigated in a wind-tunnel experiment. Three aerodynamic
parameters characterising arrays—the drag coefficient (C
d
), roughness length (z
o) and displacement height (d)—are used for analysis. C
d
is based on the direct measurement of the total surface shear using a floating element, and the other two parameters are
estimated by logarithmic fitting of the measured wind profile and predetermined total drag force. The configurations of 63
arrays used for measurement were designed to estimate the effects of layout, wind direction and the height variability of
the blocks on these parameters for various roughness packing densities. The results are summarised as follows: (1) The estimated
C
d
and z
o of the staggered arrays peak against the plan area index (λ
p
) and frontal area index (λ
f
), in contrast with values for the square arrays, which are less sensitive to λ
p
and λ
f
. In addition, the square arrays with a wind direction of 45° have a considerably larger C
d
, and the wind direction increases z
o/H by up to a factor of 2. (2) The effect of the non-uniformity of roughness height on z
o is more remarkable when λ
f
exceeds 20%, and the discrepancy in z
o is particularly remarkable and exceeds 200%. (3) The effect of the layout of tall blocks on C
d
is stronger than that of short blocks. These results indicate that the effects of both wind direction and the non-uniformity
of the heights of buildings on urban aerodynamic parameters vary greatly with λ
p
and λ
f
; hence, these effects should be taken into account by considering the roughness packing density. 相似文献
1000.
以中国西南重要的国际河流澜沧江为研究对象,借鉴"基尼系数"的构建思路,建立了水沙年内分配均匀度计算模型,并通过下游水沙与上游天然水沙和区域降水的关系,建立了河流水沙年内分配均匀度回归还原模型。还原结果与实测对比表明:漫湾、大朝山电站建设以来,澜沧江径流年内分配均匀度年均升高1.14%,变幅在-7.32%~10.81%之间,输沙年内分配均匀度年均降低0.23%,变幅在-18.98%~26.76%之间;漫湾、大朝山电站同时运行期间,澜沧江径流年内分配均匀度年均升高3.90%,输沙年内分配均匀度年均降低0.42%;大朝山水电站建设后,澜沧江径流年内分配均匀度升高而输沙年内分配变化不明显,说明大朝山水电建设后河流水沙关系发生了变化,但变化较小。该成果不仅能为相关研究提供方法借鉴,也为水电开发对澜沧江水沙年内分配影响程度的科学评估提供了新的依据。 相似文献