Waves and tides responsible for the intermittent closure of the entrance of a small, sheltered tidal wetland at San Francisco, CA     

Daniel M. Hanes  Kristen WardLi H. Erikson 《Continental Shelf Research》2011,31(16):1682-1687

Crissy Field Marsh (CFM; http://www.nps.gov/prsf/planyourvisit/crissy-field-marsh-and-beach.htm) is a small, restored tidal wetland located in the entrance to San Francisco Bay just east of the Golden Gate. The marsh is small but otherwise fairly typical of many such restored wetlands worldwide. The marsh is hydraulically connected to the bay and the adjacent Pacific Ocean by a narrow sandy channel. The channel often migrates and sometimes closes completely, which effectively blocks the tidal connection to the ocean and disrupts the hydraulics and ecology of the marsh. Field measurements of waves and tides have been examined in order to evaluate the conditions responsible for the intermittent closure of the marsh entrance. The most important factor found to bring about the entrance channel closure is the occurrence of large ocean waves. However, there were also a few closure events during times with relatively small offshore waves. Examination of the deep-water directional wave spectra during these times indicates the presence of a small secondary peak corresponding to long period swell from the southern hemisphere, indicating that CFM and San Francisco Bay in general may be more susceptible to long period ocean swell emanating from the south or southwest than the more common ocean waves coming from the northwest. The tidal records during closure events show no strong relationship between closures and tides, other than that closures tend to occur during multi-day periods with successively increasing high tides. It can be inferred from these findings that the most important process to the intermittent closure of the entrance to CFM is littoral sediment transport driven by the influence of ocean swell waves breaking along the CFM shoreline at oblique angles. During periods of large, oblique waves the littoral transport of sand likely overwhelms the scour potential of the tidal flow in the entrance channel.  相似文献   

Transport of mecoprop from agricultural soils to an adjacent salt marsh     

Fletcher CA  Scrimshaw MD  Lester JN 《Marine pollution bulletin》2004,48(3-4):313-320

Salt marshes are important ecological areas and play a significant role in coastal flood defence schemes. In many areas of the UK they are adjacent to agricultural areas utilised for the growth of cereal crops, for which mecoprop is used as a selective herbicide in the control of broad-leaf weeds. This study measured concentrations of mecoprop in soils, drainage ditch waters and sediments and salt marsh sediments over a period of 138 days following spring application. Soil concentrations of up to 1827 microg/g were recorded after application, which demonstrated a half life for mecoprop of from 9 to 12 days, with first order kinetics. However, a major rainfall event 9 days after application resulted in significant transport of herbicide to the salt marsh via subsurface field drains, drainage ditches and discharge sluice. Mecoprop concentrations of up to 386 microg/l observed in water samples were above UK guidelines.  相似文献   

Disequilibrium Between Short Term and Long Term Exhumation Rates in the Gongga Granite, Western Sichuan     

Kristen L. COOK  Yuan-Hsi Lee  Arjun Heimsath 《《地质学报》英文版》2013,87(Z1):38-38

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Geochemical Changes in Metal and Nutrient Loading at Orplands Farm Managed Retreat Site, Essex, UK (April 1995–1997)     

C.L Macleod  M.D Scrimshaw  R.H.C Emmerson  Y.-H Chang  J.N Lester   《Marine pollution bulletin》1999,38(12):1870-1125

Salt marshes have recently been considered to be a major part of the coastal system and have played a key role in the development of the UK coastal management strategy. Managed Retreat (MR) is a process aimed to restore salt marshes by realignment of the seawalls allowing tidal inundation of low value agricultural land. The resultant marshes are expected to function both as an integral part of the flood defence system and as an ecological conservation area. We report on the effects of salt marsh restoration on metal and nutrient loading of the sediment at the Orplands Farm MR site, Essex, UK. Surficial grab and sediment cores were collected from the two fields that comprise the site. The heavy metals, Cd, Cr, Cu, Hg, Pb, Ni and Zn were analysed to determine changes in anthropogenic inputs to sediments. The major ions, Al, Ca, Fe, K, Mg, Mn and Na were also monitored to identify changes in sediment geochemistry. Analysis of the cored sediments after inundation for Na and Sr demonstrated that penetration of estuarine water had, within 2 yr of exposure, reached an average depth of 20 cm. The study observed that input of heavy metals had occurred to the sediments with the most significant being that of Pb, however increases were also observed for Cr and Cu. However, concentrations of Cd in the MR sediments decreased from 1995 to 1997. For the major metals within both fields it was found that the dominant changes were those of enrichment of marine associated metals, Ca, K, Mg and Na via inputs from tidal inundation. The concentration of Ca in the sediments was further enriched by the deposition of carbonates to the sediments. One field demonstrated a significant loss of Fe from sediments which corresponded to changes in redox potential of the sediments. Differences observed in geochemical profiles between the two fields of the site were attributed to differences in land use prior to flooding.  相似文献   

Post-noon two-minute period pulsating aurora and their relationship to the dayside convection pattern     

S. E. Milan  T. K. Yeoman  M. Lester  J. Moen  P. E. Sandholt 《Annales Geophysicae》1999,17(7):877-891

Poleward-moving auroral forms, as observed by meridian-scanning photometers, in the vicinity of the cusp region are generally assumed to be the optical signature of flux transfer events. Another class of quasi-continuous, short period (1–2 min) wave-like auroral emission has been identified, closely co-located with the convection reversal boundary in the post-noon sector, which is similar in appearance to such cusp aurora. It is suggested that these short period wave-like auroral emissions, the optical signature of boundary plasma sheet precipitation in the region 1 field-aligned current system, are associated with ULF magnetohydrodynamic wave activity, which is observed simultaneously by ground magnetometer stations. This association with ULF wave activity is strengthened by the observation of several harmonic frequencies in the pulsation spectrum, each an overtone of the fundamental standing wave resonance frequency.  相似文献   

Rapid adjustment of shoreline behavior to changing seasonality of storms: observations and modelling at an open‐coast beach          下载免费PDF全文

Kristen D. Splinter  Ian L. Turner  Mika Reinhardt  Gerben Ruessink 《地球表面变化过程与地形》2017,42(8):1186-1194

An 8‐year time series of weekly shoreline data collected at the Gold Coast, Australia, is used to examine the temporal evolution of a beach, focusing on the frequency response of the shoreline to time‐varying wave height and period. Intriguingly, during 2005 the movement of the shoreline at this site changed from a seasonally‐dominated mode (annual cycle) to a storm‐dominated (~monthly) mode. This unexpected observation provides the opportunity to explore the drivers of the observed shoreline response. Utilizing the calibration of an equilibrium shoreline model to explore the time‐scales of underlying beach behavior, the best‐fit frequency response (days^?1) is shown to be an order of magnitude higher post‐2004, suggesting that a relatively subtle change in wave forcing can drive a significant change in shoreline response. Analysis of available wave data reveals a statistically significant change in the seasonality of storms, from predominantly occurring at the start of the year pre‐2005 to being relatively consistent throughout the year after this time. The observed change from one mode of shoreline variability to another suggests that beaches can adapt relatively quickly to subtle changes in the intra‐annual distribution of wave energy. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

2004年2月11日Cluster卫星和CUTLASS雷达同时观测的磁通量传输事件   总被引：1，自引：0，他引：1       下载免费PDF全文

张清和  刘瑞源  黄际英  M. W. Dunlop  胡红桥  胡泽骏  Y. V. Bogdanova  M. Lester 《地球物理学报》2008,51(1):1-9

本文分析了2004年2月11日11:00～11:40 UT期间Cluster卫星簇的磁通门磁力计FGM）、等离子体电子及电流试验仪（PEACE）和CUTLASS 芬兰雷达对多个磁通量传输事件(FTEs)的同时观测. 在此期间,Cluster卫星簇位于北半球外极隙区附近,并于11:18 UT左右穿出磁层顶进入磁鞘,四颗卫星同时观测到了多个FTEs, 其出现具有准周期性,周期约为130 s. 利用Cluster四颗卫星的多点同时观测数据,采用最小方向微分法和时空微分方法,我们推断这些FTEs是尺度大小约为(0.87～1.81)R_E的准二维结构,其运动方向为东北方向,与Cooling模型预测方向基本一致. CUTLASS芬兰雷达在相应的电离层区域观测到了明显的“极向运动雷达极光”结构,这些结构与Cluster卫星簇观测的FTEs有着很好的对应关系,它们是FTEs的雷达观测特征.  相似文献   

Geomasking sensitive health data and privacy protection: an evaluation using an E911 database   总被引：2，自引：0，他引：2  

Allshouse WB  Fitch MK  Hampton KH  Gesink DC  Doherty IA  Leone PA  Serre ML  Miller WC 《国际地球制图》2010,25(6):443-452

Geomasking is used to provide privacy protection for individual address information while maintaining spatial resolution for mapping purposes. Donut geomasking and other random perturbation geomasking algorithms rely on the assumption of a homogeneously distributed population to calculate displacement distances, leading to possible under-protection of individuals when this condition is not met. Using household data from 2007, we evaluated the performance of donut geomasking in Orange County, North Carolina. We calculated the estimated k-anonymity for every household based on the assumption of uniform household distribution. We then determined the actual k-anonymity by revealing household locations contained in the county E911 database. Census block groups in mixed-use areas with high population distribution heterogeneity were the most likely to have privacy protection below selected criteria. For heterogeneous populations, we suggest tripling the minimum displacement area in the donut to protect privacy with a less than 1% error rate.  相似文献   

Isotope hydrology of a tropical coffee agroforestry watershed: Seasonal and event‐based analyses          下载免费PDF全文

Kristen Welsh  Olivier Roupsard 《水文研究》2018,32(13):1965-1977

Stable isotope variations are extremely useful for flow partitioning within the hydrologic cycle but remain poorly understood throughout the tropics, particularly in watersheds with rapidly infiltrating soils, such as Andisols in Central America. This study examines the fluctuations of stable isotope ratios (δ¹⁸O and δ²H) in the hydrologic components of a tropical coffee agroforestry watershed (~1 km²) with Andisol soils in Costa Rica. Samples were collected in precipitation, groundwater, springs, and stream water over 2 years. The local meteoric water line for the study site was δ²H = 8.5 δ¹⁸O + 18.02 (r² = 0.97, n = 198). The isotope ratios in precipitation exhibited an enriched trend during the dry season and a notable depletion at the beginning of the wet season. The δ¹⁸O compositions in groundwater (average = ?6.4‰, σ = 0.7) and stream water (average = ?6.7‰, σ = 0.6) were relatively stable over time, and both components exhibited more enriched values in 2013, which was the drier year. No strong correlation was observed between the isotope ratios and the precipitation amount at the event or daily time‐step, but a correlation was observed on a monthly scale. Stream water and base flow hydrograph separations based on isotope end‐member estimations showed that pre‐event water originating from base flow was prevalent. However, isotope data indicate that event water originating from springs appears to have been the primary driver of initial rises in stream flow and peak flows. These results indicate that isotope sampling improves the understanding of water balance components, even in a tropical humid location, where significant variations in rainfall challenge current modelling efforts. Further research using fine‐scale hydrometric and isotopic data would enhance understanding the processes driving spring flow generation in watersheds.  相似文献   

Pi 2 pulsations and the eastward electrojet: A case study     

Mark Lester  Karl-Heinz Glaßmeier  Jörg Behrens 《Planetary and Space Science》1985,33(3):351-364

We report the results of a case study of two Pi 2 pulsations observed near the eastward electrojet by the Scandinavian Magnetometer Array. The power of the two Pi 2 pulsations, calculated using a standard Fast Fourier Transform method, peaks near the centre of the eastward electrojet. For both events there is a strong latitudinal gradient in the power poleward of the equatorward border of the electrojet. The sense of polarisation is predominantly clockwise at the northern stations and anticlockwise at the southern stations although the reversal from clockwise to anticlockwise does not occur at a constant latitude. For the first event the polarisation reversal occurs at higher latitudes in the western half of the array; for the second the polarisation reversal occurs at higher latitudes at the edges of the array. The polarisation reversal does not appear to be related to the location of the eastward electrojet. Equivalent current vectors of the Pi 2 pulsations, obtained by rotating the band pass filtered data through 90°, exhibit clear vortex structures in both events. The vortices change sense of direction at half the period of the Pi 2 pulsation. A simple model for the ionospheric electric field in accord with the field line resonance theory reconstructs the basic features of the observed Pi 2 equivalent current system. We thus conclude that Pi 2 signatures in the region of the eastward electrojet and far away from the auroral break-up region are governed by the field line resonance mechanism.  相似文献