地震海啸的激发与传播

本文围绕海啸的激发和传播两个方面简述了有限矩形源产生的地表形变场、浅水波浪理论和Boussinesq方程等基本理论,讨论了震源参数对海啸传播的影响,比较分析了海啸的数值模拟方法,介绍了我国地震海啸的研究进展等。     

地震海啸传播过程数值模拟研究

运用数值模拟的方法,对常水深4 000 m,8.0级纯倾滑直立断层触发的地震海啸,在大洋中传播的前2 000 s各时刻的形态以及峰值进行研究.结果表明,地震海啸产生后的前1 000 s左右的时间内,存在数值震荡,幅值先快速衰减,再逐渐增大,此后,幅值随着时间的推移缓慢衰减.     

Tsunami propagation in rivers of the Japanese Islands

On 26 May 1983 the rivers of the Japanese Islands were invaded by a tsunami from the Sea of Japan. Water levels in five large rivers were measured and were highest midway towards the uppermost point of inundation. The level at the uppermost point was approximately the same as that at the river mouths. As a result of resonance, a standing wave develops in the rivers. The period of the invading tsunami was found to be 80 min. This continental shelf oscillation resulted in a long-wave propagation of the tsunami.The water levels showed another peak in the neighbourhood of estuarine inflows. The period of this wave was estimated to be 20 min; its formation was attributed to generation near the tsunami source.The cross-section and longitudinal profiles of the rivers, together with water flow, complicated the observed water level profiles.     

南海潜在海啸传播过程的数值模拟：初始海啸衰减的影响

准确预估南海海啸风险是有效防灾减灾的前提.前人一般把弹性半无限空间背景下解算出来的海底位错直接等同于初始海啸分布,继而开展海啸传播过程研究.由于断层破裂并非瞬时完成,破裂过程会导致初始海啸波高小于海底位错量,即初始海啸衰减.本文基于高精度地形和高密度网格,求解非线性浅水方程,分别针对马尼拉断裂带的南段、中段和北段,构建南海海啸传播数值模型,试图定量考察初始海啸衰减作用对南海海啸的影响.模拟结果表明一定幅度的初始波高衰减将导致几乎相同幅度的海啸波高衰减,相应的偏差可以忽略.在保守的初始海啸衰减幅度（10%）下,模拟结果显示我国东南沿海、越南东部沿海和巴拉望岛为海啸危险区.另外,模型显示科里奥利力导致的波高变化幅度小于5 cm且其分布样式符合预期,这进一步佐证了数值模型的可靠性,也表明在实际南海海啸模拟中可以忽略科里奥利力进而提高计算效率.结合前人的沉积学认识和本文的数值模拟结果,本文认为南澳岛、西沙东岛和越南绥和周边曾同时遭受海啸侵袭,产生海啸的断裂带最有可能是马尼拉断裂带南段.后续有必要加强南澳岛、西沙东岛和越南东部的沉积学研究,识别更早的海啸事件,以期有力约束南海下次海啸事件的发生年份.

     

Gender of large river deltas and parasitizing rivers

Deltas are the most dynamic part of large rivers and the characteristics of deltas reflect the basic nature of morphodynamics,ecology and anthropogenic influence.The authors investigated many deltas of...     

海啸传播模型与数值模拟研究进展

海啸在浅水大陆架的传播问题由于其非线性作用和浅水效应而变得十分复杂,然而目前成熟的海啸传播理论及数值模拟结果在这方面与实际并不一致.本文比较分析了可用来模拟大陆架海啸传播的浅水波模型和数值方法,并提出对我国东海陆架边缘可能发生的近海海啸需要开展数值试验研究.     

Simulated groundwater interaction with rivers and springs in the Heihe river basin

Surface water and groundwater in the Heihe river basin of China are interconnected and the pattern of water resources exploitation has a direct effect on the interaction of groundwater and surface water, especially on a downstream oasis. A three‐dimensional groundwater flow simulation model with eight model layers was established to simulate the regional groundwater flow in the multilayered aquifer system and the interaction among the rivers, springs, and groundwater. The model was calibrated not only with historical water levels but also with the investigated baseflow and spring flux. The simulation results of the numerical model match reasonably well with the observed groundwater levels, baseflow to rivers, and spring flux. The numerical simulation also demonstrates that the hydraulic connection between the river and the aquifers has transferred from the coupling to decoupling at some reaches. It is suggested that there is a vital need to reduce groundwater withdrawal and to rationalize the use of both groundwater and surface water in order to maintain sustainable development in the study area. Copyright © 2007 John Wiley & Sons, Ltd.     

Effects of bathymetry on tsunami propagation: Application of ray tracing to tsunamis

Ray tracing of seismic surface waves is applied to tsunami propagation to examine bathymetric effect along its propagation path. Computations are made for trans-Pacific tsunamis and for near-field tsunamis in the Japan Sea. For tsunamis across the Pacific Ocean, the comparison to a uniform ocean shows that focusing and defocusing, due to bathymetry, are significant for some combinations of source and receiver. For example, the refraction of rays is predominant at the East Pacific Rise for the tsunami from Chile. The tsunamis in the Japan Sea are strongly affected by the shallow Yamato Rise. The predicted arrival time and amplitude distribution generally agree with the observations from an actual tsunami. Since the computation can be made very quickly, the method is useful for preliminary analysis of tsunami propagation, such as in an operational warning system or in the determination of computational area for finite-difference computation.     

The legacy of river channel modification in wadeable,lowland rivers: Exploring overdeep rivers in England

Many lowland fluvial systems are suspected to possess a morphological legacy from a long history of channel modifications as a consequence of limited energy and sediment supply to facilitate recovery. We explore the extent of such modifications using a regionally extensive dataset of physical habitat surveys compiled by non-specialist surveyors. Representative photographs for each surveyed site were used to quality check channel width, depth and bed grain size information derived from Modular River Physical (MoRPh) surveys. Following checking, 1659 surveys were retained for analysis from alluvial sites, almost entirely in England. The photographs were also inspected for evidence of clear ‘overdeepening’ that would preclude frequent overtopping of the lower bank top. Results indicated that almost one-third of the sites were overdeepened, that width-to-depth ratios defined using the active bed width showed stronger discrimination of overdeepening than bankfull width, that highly statistically significant identification of overdeepened channels was possible in channels up to 10 m wide and with only minimal differences attributable to channel bed materials. Stepwise regression analysis estimated relationships between channel width-to-depth ratios and channel size for overdeepened and non-overdeepened channels. We demonstrate that large data sets collected by numerous non-specialist surveyors can, with careful filtering, generate statistically robust results of geomorphological value over areas larger than is otherwise practicable. Furthermore, we reveal a notable legacy of overdeepening in the analysed lowland rivers, which presents a significant ‘hydromorphological’ management challenge.     

Remote sensing of rivers: the emergence of a subdiscipline in the river sciences

This article reports on the special issue of Earth Surface Processes and Landforms dedicated to remote sensing of rivers. This emerging subdiscipline of river science has grown at a rapid rate in recent years because of: (a) the growing desire and need for data to document and explore the full range of spatial and temporal variations in river systems; (b) evolving technologies that enable lower cost data acquisition, processing and analysis at reach to catchment to continental scales; and (c) the increasing engagement of river scientists with GIScience. The convergence of these factors and the ever growing number of practitioners speaks to the need for more communication among researchers, a major reason for creating this special issue. The 12 articles in the volume cover a broad spectrum of applications that use a variety of platforms and sensors, ranging from photogrammetric mapping of riffle‐pool morphology beneath forest canopy using a camera mounted on a hand held pole to satellite‐based synthetic radar mapping of subcontinental scale hydrology of large rivers. In this overview each of the 12 articles is briefly summarized. Based on these works and other research, it is concluded that the time for more widespread application of river remote sensing techniques is now. To promote more widespread use of remote sensing techniques for river science and management, the following are advocated: (a) developing stand alone or plug‐in software products that enable non‐expert users to implement these new methods, (b) incorporating remote sensing of rivers training into classes, workshops, and on‐line tutorials; and (c) promoting more intentional and formal collaboration among members of the river remote sensing community. Copyright © 2010 John Wiley & Sons, Ltd.     

The effect of tide on the hydrology and morphology of a freshwater river

How does river hydrology and morphology change due to tidal influence? We contend that this is a question of particular consequence to many earth surface disciplines, but one that has not been adequately addressed. Previous studies have relied on gradients in channel morphology and stratigraphy to infer energy regime of channels. However, in tidal rivers geomorphology influences the energy regime while the energy regime influences morphology; thus, geomorphic and stratigraphic patterns do not fully resolve the mechanisms which lead to change. We addressed this problem by comparing measurements of hydraulic energy and channel morphology along a tidal gradient to predictions of these characteristics in the absence of tides, and attributed the differences to tidal processes. Measurements of discharge, channel area, and energy dissipation (in kJ day^–1) were made over a 24·8 hour period at four sites spanning the non‐tidal to tidal freshwater Newport River, NC. We then predicted those characteristics under non‐tidal conditions using hydraulic geometry relationships and literature values from coastal plain rivers. Discharge was enhanced more than 10‐fold by tide, and this tidal effect increased from upstream to downstream along the tidal gradient. Cross‐sectional area increased three‐fold due to tide. Energy dissipation measured in the upper tidal river was four‐fold lower than predicted to occur in the absence of tide because tides decreased average velocity and discharge. Energy dissipation measured downstream was similar to that predicted to occur without tides, although there was large uncertainty in predicted values downstream. While this limited dataset does not permit us to make broad generalizations for definitive models, it does provide a proof‐of‐concept for a new approach to addressing a critical problem at the interface of fluvial and coastal morphology. Copyright © 2013 John Wiley & Sons, Ltd.     

The nation-state and the river: Spaces and times on Dutch rivers, 1795-1814

Nothing indeed demonstrates more forcefully the extent to which Dutch water management corresponds to the nature and the needs of the Dutch people and their land, and how it has emerged there from in a natural fashion than the fact that the revolutionaries of 1795, despite being so intoxicated by their unexpected victory and sudden power that they overturned everything … nonetheless refrained from laying hands on institutions whose extreme antiquity would in those days have provided more of an excuse to abolish than to preserve them (J.W. Welcker, De Noorder-Lekdijk Bovendams en de doorsteking van den Zuider-Lekdijk bij Culemborg 1803-1813. Een bijdrage tot de geschiedenis van den Nederlandschen Waterstaat geschetst en met onuitgegeven stukken toegelicht (‘s-Gravenhage: 1880), p. 2).     

Particle size distribution of bed materials in the sandy river bed of alluvial rivers

The particle size distribution of bed materials in the sandy river bed of alluvial rivers is important in the study of topics such as friction, river bed evolution, erosion, and siltation. It also can reflect the dependency relation between river bed sediment and flow intensity. In this paper, the critical pattern of sediment movement in the near-wall region of a sandy river bed was analyzed. According to the principle of momentum balance, the critical settling-rising condition of bed material in a sandy river bed was found to be instantaneous turbulent velocity equal to 2.7 times the sediment settling velocity in quiescent water. Based on a vertical instantaneous turbulent velocity with a Gaussian distribution, a theoretical relation for calculating the particle size distribution of bed materials in a sandy river bed without pre-known characteristic grain sizes was developed by solving a stochastic equation. The for-mula is verified using measured data, and the results show that the proposed formula was in accordance with the measured data. This study has theoretical significance and practical value for determining the bed material particle size distribution of the sandy bed of alluvial rivers.     

Impact of flow variability on the morphology of a low-energy meandering river

Few studies have precisely documented the response of stream channels to short-term flow variability. This paper examines the impact of sequential flows of various magnitudes on the morphology of a low-energy river in northeastern Illinois, U.S.A. Between June 1986 and November 1988 channel cross-sections were surveyed on a semiannual basis at 26 locations along a 7.2 km stretch of the Des Plaines River. During this period an estimated 100-year flood, several bankfull flows, and an extreme low flow associated with a severe drought occurred. The response of the river channel to each of these events was relatively minor. Mean changes for the reach were generally less than 3 per cent for mean depth and less than 1 per cent for width. Statistical analysis indicates that net changes in width and depth over the entire period were not significantly different from zero. This lack of geomorphic response is attributable to low stream power, low hydrologic variability, fine bed materials, and cohesive banks along this stretch of river. Although dramatic changes in channel morphology did not occur, subtleties in geomorphic response were observed that reflect the temporal ordering of hydrologic events.     

The impact of beaver dams on the morphology of a river in the eastern United States with implications for river restoration

Restoration projects in the United States typically have among the stated goals those of increasing channel stability and sediment storage within the reach. Increased interest in ecologically based restoration techniques has led to the consideration of introducing beavers to degraded channels with the hope that the construction of beaver dams will aggrade the channel. Most research on beaver dam modification to channels has focused on the long‐term effects of beavers on the landscape with data primarily from rivers in the western United States. This study illustrated that a role exists for beavers in the restoration of fine‐grained, low gradient channels. A channel on the Atlantic Coastal Plain was analyzed before, during, and after beaver dams were constructed to evaluate the lasting impact of the beaver on channel morphology. The channel was actively evolving in a former reservoir area upstream of a dam break. Colonization by the beaver focused the flow into the channel, allowed for deposition along the channel banks, and reduced the channel width such that when the beaver dams were destroyed in a flood, there was no channel migration and net sediment storage in the reach had increased. However, the majority of the deposition occurred at the channel banks, narrowing the channel width, while the channel incised between sequential beaver dams. The study indicated that where channels are unstable laterally and bank erosion is a concern, the introduction of beavers can be a useful restoration tool. However, because of the likelihood of increased channel bed erosion in a reach with multiple beaver dams, they may not be the best solution where aggradation of an incised channel bed is the desired result. Copyright © 2014 John Wiley & Sons, Ltd.     

Channel morphology and bedload pulses in braided rivers: a laboratory study

Bedload pulses in gravel-bed rivers have been widely reported in recent years and attempts have been made to relate them to channel morphology. Bedload transport and channel morphology were measured in a small-scale generic model of braided gravel-bed streams. Two experiments are described in which braided channels developed in a 14 m × 3 m sand tray. Total bedload output from the tray was weighed every 15 minutes. Stream bed geometry was surveyed every four hours. Pulses were observed in the bedload output time series, and were qualitatively related to the channel morphology immediately upstream of the measuring section. The Bagnold (1980) bedload equation generally overpredicts measured bedload transport rates when applied to channels that were in equilibrium or aggrading. Underprediction occurred when applied to degrading channels. Aggradation was associated with channel multiplication and bar deposition. Channel pattern simplification occurred when degradation took place, and bars emerged from the water flow. Development of phases of aggradation and degradation is dependent upon the three-dimensional geometry of the stream beds. Spatial and temporal feedback loops can be identified, enabling links between channel morphology and bedload transport rate to be directly identified.     

Channel morphology and sediment movement in a tidal river,pitt river,British Columbia

The Pitt River is a meandering river channel linking the Fraser River estuary and Pitt Lake. The lake acts as a temporary reservoir for tidally diverted Fraser River flow. Stage level can fluctuate 2 m in Pitt River and as much as 1.2 m in Pitt Lake on a tidal cycle. Stage data from three locations in the system, used in conjunction with velocity measurements (profiles and tethered meter), revealed large tidal and seasonal variations in discharge. Calculations indicate that during the flood, basal shear stress peaks earlier in the cycle and reaches higher values than during the ebb. Thus, sediment moves farther forward on a flood flow than it moves back on the succeeding ebb. An upstream movement of sediment in Pitt River from the Fraser River is indicated by: (1) the identical mineralogy of the two rivers, (2) a decrease in median grain size from the Fraser to Pitt Lake, and (3) a predominance of flood-oriented bedforms in the river channel. A delta, 12 km² area, has accumulated at the lower (draining) end of the lake. Studies of the river channel using hydrographic charts revealed regular meanders (λM = 6100 m) and evenly spaced riffles and pools which appear to be scaled to the strongest flow, winter flood current (2400 m³/s). The winter flood is thus considered to be the effective discharge. Meander point bars are accreting on the ‘upstream’ side indicating deposition by the flood-oriented flow. The three dimensional geometry of the large-scale bedforms which cover the sandy thalweg of both river and delta channel was determined by echo sounding and side-scan sonar. Three distinct sizes (height/spacing = 0.8 m/10–15 m; 1.5 m/25–30m; 3m/50–60 m) of large-scale bedforms (sand waves) were found; their linear relationship of height vs. spacing on a log-log plot suggests a common genesis. Their occurrence by size does not appear to be related to depth of flow but rather to their position in the channel with respect to large scale features which alter flow.     

The response of river mouths to large-scale variations in sea level and river runoff: Case study of rivers flowing into the Caspian Sea

Regularities in the response of the mouths of major rivers, flowing into the Caspian Sea, to large-scale variations in its level and river water runoff and sediment yield are considered. Changes in the morphological structure and hydrological regime of the Volga, Terek, Sulak, Ural, and Kura mouths have been analyzed in both geological past and separately for three modern periods: a considerable drop in Caspian Sea level before 1978, its abrupt rise in 1978–1995, and a relative stabilization in the subsequent years. Specific features were identified in the hydrological-morphological processes in different mouths, caused by the differences in river sediment yields, and the slopes of delta surface and mouth nearshore beds. Some theoretical and methodological approaches were verified in the analysis and evaluation of the processes under consideration. The obtained results of studies of the mouths of rivers flowing into the Caspian Sea can be regarded as examples and analogues in the assessment of processes, which take place at the mouths of other Russian and world rivers at present and can take place in the future under anticipated natural and anthropogenic variations in sea level and river runoff.     

Impact of a CO2-induced climatic change on river flow variability in three rivers in Belgium

A hydrological conceptual model developed by the Royal Meteorological Institute of Belgium has been run to assess the potential hydrological impacts of an hypothetical doubling of the atmospheric CO₂ concentration. The simulated scenarios were derived from the predictions of climatic change currently provided by General Circulation Models (GCMs). Three typical drainage basins in Belgium have been selected for assessing their common responses and bringing out possible specific behaviours attributable to catchment characteristics. The study dealt essentially with modifications of the streamflow and with alterations of the flood and low-flow regimes. In catchments with prevailing surface flow the considered change in climate could induce:

• An increase in flood frequencies during the winter season together with a strengthening of the extreme river stages leading to greater flooding risks;
• A decrease in streamflow during the summer season and, as a corollary, increased risks of water pollution;
• Possible restraints, in summer and autumn, on water availability from local groundwater storages.

In catchments with high infiltration rate and with strong aquifer the impact could be:

• An increase in groundwater storage, bringing about an increase in the base flow throughout the year, which in turn involves increased flood risks;
• A reduction of the number of low-stage occurrences in summer, resulting in reduced river pollution;
• A possible increase in water availability from the aquifers.