A Review of “Eden on the Charles,The Making of Boston”     

Arthur Krim 《The Professional geographer》2013,65(3):470-472

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STUDY ON FREQUENCY CODING ANTI—INTERFERENCE SYSTEM FOR DATA TRANSMISSION IN SHALLOW WATER ACOUSTIC CHANNELS     

许肖梅  粘宝卿  刘志鑫  许鹭芬  黄衍镇 《中国海洋湖沼学报》2002,20(4):309-315

Achieving reliable underwater communication in shallow water acoustic channels is a difficult because of the random time-varying nature of multipath propagation,severe amplitude fluctuation,and spatial variability of the channel conditions.This paper describes a new signal processing technique-frequency coding and decoding by means of real-time measurement of signal width,jamming and suppressing multipath interference and using redundant coder.The application of the technique to thd model ZTY-1 status monitor for underwater system of seabed is introduced in this paper.The main principle,the technique specifications and the key techniques of the system are discussed here.Theoretical estimations and experimental results proved that the performance of the system is excellent.The method can be used for some other related low-data-rate data transmission detecting in shallow water acoustic channel.  相似文献   

Abandoned channel fill sequences in the tidal estuary of a small mountainous,dry‐summer river          下载免费PDF全文

Andrew B. Gray  Gregory B. Pasternack  Elizabeth B. Watson  Miguel A. Goñi 《Sedimentology》2016,63(1):176-206

This study proposes a modification of the current model for abandoned channel fill stratigraphy produced in unidirectional flow river reaches to incorporate seasonal tidal deposition. Evidence supporting this concept came from a study of two consecutive channel abandonment sequences in Ropers Slough of the lower Eel River Estuary in northern California. Aerial photographs showed that Ropers Slough was abandoned around 1943, reoccupied after the 1964 flood, and abandoned again in 1974 with fill continuing to the present. Planform geomorphic characteristics derived from these images were used in conjunction with sub‐centimetre resolution stratigraphic analyses to describe depositional processes and their resultant sedimentary deposits. Both abandonment sequences recorded quasi‐annual scale fluvial/tidal deposition couplets. In both cases, tidal deposits contained very little sand, were higher in organic and inorganic carbon content than the sandier, fluvially dominated deposits, and possessed millimetre‐scale horizontal laminations. The two abandonment fills differed significantly in terms of the temporal progression of channel narrowing and fluvial sediment deposition characteristics. Aerial photographic analysis showed that the first abandonment sequence led to a more rapid narrowing of Ropers Slough and produced deposits with a positive relationship between grain size/deposit thickness and discharge. The second abandonment resulted in a much slower narrowing of Ropers Slough and generally thinner fluvial deposits with no clear relationship between grain size/deposit thickness and discharge. The δ¹³C values and organic nitrogen to organic carbon ratios of deposits from the first phase overlapped with Eel River suspended sediment characteristics found for low flows (one to five times mean discharge), while those of the second phase were consistent with suspended sediment from higher flows (seven to ten times mean discharge). When considered together, the results indicate that the early fill sequence recorded a reach experiencing regular fluvial deposition through flow conditions during the wet season, while the latter fill sequence records a reach more disconnected from the main stem in terms of flow and sediment. The major factor affecting the difference in sedimentation between the two fill periods appears to have been the morphology of the upstream river bend in relation to the position of the bifurcation node. During the first fill period, the upstream entrance to Ropers Slough seems to have remained open, in part due to the placement of its entrance on the outside of the mainstem river bend, and despite stronger tidal effects caused by a larger tidal prism and closer proximity to the tidal inlet. By the second fill sequence, the upstream bend morphology had altered, placing the entrance to Ropers Slough on the inner bank of the mainstem bend, which resulted in more rapid plug bar formation. The role of tidal effects in the geomorphic trajectory of the two abandonment sequences is unclear, but appears to have been less important than local bifurcation geometry.  相似文献   

A model of submarine channel-levee evolution based on channel trajectories: Implications for stratigraphic architecture   总被引：3，自引：0，他引：3  

Zoltán SylvesterAlessandro Cantelli 《Marine and Petroleum Geology》2011,28(3):716-727

Channel-levee systems are frequently interpreted as having a long history of cut-and-fill by channel-shaped features of different scales. Results from a simple geometric model based on a centerline migration algorithm combined with a vertical channel trajectory show that an incising-to-aggrading trajectory of a single channel can produce realistic morphologies similar to systems observed on the seafloor and subsurface, including features such as a basal erosional surface, coeval inner and outer levees, internal erosional boundaries, and terraces draped by inner levee deposits. Channel migration results in composite erosional surfaces that are distinct from topographic surfaces, and their formation does not require larger than usual erosional flows. Many submarine channels interpreted as underfit were probably carved by flows similar to the ones that eroded and deposited the entire channel system. We suggest that the features of most submarine channel-levee systems do not require large temporal variations in flow magnitude but can be explained by a simpler model whereby incision, migration and aggradation of a single channel form over time results in an apparently complex system.  相似文献   

Hydraulic modelling of fish habitat in urban rivers during high flows     

D. J. Booker 《水文研究》2003,17(3):577-599

In urban rivers, flow regime and channel morphology are the drivers of physical habitat quality for aquatic species. Peak discharges are increased at high flows as a result of impermeable catchments and channel engineering for flood protection schemes. Hazardous conditions and flashy hydrographs mean that measurement of velocities at high flows is a difficult task. This research uses a three‐dimensional computational fluid dynamics (3D‐CFD) model to simulate hydraulic patterns in two urban river channels. A 3D‐CFD code, called SSIIM, was used to simulate hydraulic conditions in two engineered river reaches of the River Tame, Birmingham, UK. These two sites represent channels with different levels of engineering. Models were calibrated and tested using field measurements. Results show that modelled water surface levels and velocity profiles are well simulated. Calibrated roughness heights are compared with those derived from field measurement of sediment size. Numerical experiments are used to assess the relationship between grid resolution in the vertical dimension and the form of the modelled velocity profiles. Biologists have used laboratory experiments to determine maximum sustainable swimming speeds (MSSS) of fish, often in order to assess what level of a particular pollutant may be tolerable. In this work, simulations of high‐flow hydraulic patterns are used to compare velocity patterns with fish MSSS. Results show that when the water levels rise to fill the first channel of the two‐stage channels at the sites, which occurred 16 times in 2000, MSSS are surpassed in the majority of available habitat, suggesting that excessive velocities at high flows are one factor that limits fish habitat. A comparison between the two reaches shows that there is less available habitat in the more modified reach. Conclusions suggest that an approach that integrates water quality issues and physical channel characteristics must be taken in river rehabilitation schemes, as improvements to water quality alone may not be sufficient to improve habitat quality to the desired level. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

Bed forms and bend-arc spacings of some coarse-bedload channels in Upland Britain     

J. A. Milne 《地球表面变化过程与地形》1982,7(3):227-240

Seventy-eight riffle to riffle and 80 bend spacings along eleven coarse-bedload, low sinuosity stream channels in upland Britain have been surveyed. Frequency distributions of these spacings are notably right-skewed. The most common repeating distances between riffles and bend inflections are between 4 and 6 channel widths although spacings up to 20 widths are also present. Riffle and pool locations around bends at different stages of planform development indicate that change is largely through increased sinuosity between two consecutive riffles of an original straight reach. Observed straight segments exhibit alternating riffles and pools evenly spaced at 4-6 widths, and most bends have similarly spaced riffles at their inflections in plan, with the intermediate pool at their apex. However angular deflections between axial lines joining inflections indicate existing sequences of bends did not develop from a single straight reach. Bends which are significantly longer than 4-6 widths are of low sinuosity and represent variability in naturally irregular planforms rather than arcs in advanced stages of meander growth. Adjusted bed topography around such long bends takes two forms. Either a single riffle-pool cycle is present with one or both bed forms being longer than average, or a 4-6 widths spacing is maintained by more than one riffle-pool sequence. Locally, the cross-section characteristics of riffles and pools are also influenced by planform location.  相似文献   

A model of two-phase bedload transport in an oregon coast range stream     

W. L. Jackson  R. L. Beschta 《地球表面变化过程与地形》1982,7(6):517-527

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SENSITIVITY ANALYSIS OF HYDRAULIC GEOMETRY     

CAO Shuyou  This research was supported financially by the Foundation for Scholars Return from Abroad of State Educational Committee  the National Natural Science Foundation of China 《国际泥沙研究》1997,(3)

I.INTRODUCTIONThehydraulicgeometryofstablenaturalriversisdeterminedbyceftaincontrolfactors.suchassizeandgradationofboundarymaterial-sorting,transportandarmoring.planformofchannel.ballkstability.amongstothers.Ageometricmodelfornonuniformboundarymaterialhasbeenproposed(Cao,1996).Theentropy-basedthresholdbankprofiletheory(CaoandKnight,1996).thenext'solutionofthestablechannelparadox,basedonShionoandKnight's(1991)solutionofthedepthmean-averagedmomentumequationatthejunctionpointsareemployedto…  相似文献   

河口涨潮槽的演变及治理   总被引：16，自引：0，他引：16  

沈焕庭  李九发  金元欢 《海洋与湖沼》1995,26(1):83-89

根据长江口１９５８－１９８７年的地形、水文测量资料和研究成果，对河口涨潮槽的形态特征、水文泥沙特性、形成原因与演变规律作较系统的分析研究。结果表明，涨潮槽呈上口窄下口宽的喇叭形，延伸方向受口外潮波传播方向制约，潮波更多地呈现驻波性质；涨潮流起主导作用，余流方向指向上游，涨潮期含沙量大于落潮期；涨潮槽的水文泥沙特性有明显的大小潮，洪枯季和年际变化，其分布可从口外海滨－直延伸到潮流界；按成因涨潮槽可分  相似文献   

双通道声学水位计     

孙强 《海洋技术学报》2002,21(1):53-56

双通道声学水位计是在 SSA1- 1型声学水位计的基础上 ,通过一台主机带两个探头 ,进行准同步测量而实现的  相似文献