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1.
The response of a barrier island to an extreme storm depends in part on the surge elevation relative to the height and extent of the foredunes which can exhibit considerable variability alongshore. While it is recognized that alongshore variations in dune height and width direct barrier island response to storm surge, the underlying causes of the alongshore variation remain poorly understood. This study examines the alongshore variation in dune morphology along a 11 km stretch of Santa Rosa Island in northwest Florida and relates the variation in morphology to the response of the island during Hurricane Ivan and historic and storm-related rates of shoreline erosion. The morphology of the foredune and backbarrier dunes was characterized before and after Hurricane Ivan using Empirical Orthogonal Function (EOF) analysis and related through Canonical Correlation Analysis (CCA). The height and extent of the foredune, and the presence and relative location of the backbarrier dunes, varied alongshore at discrete length scales (of ~ 750, 1450 and 4550 m) that are statistically significant at the 95% confidence level. Cospectral analysis suggests that the variation in dune morphology is correlated with transverse ridges on the inner-shelf, the backbarrier cuspate headlands, and the historical and storm-related trends in shoreline change. Sections of the coast with little to no dune development before Hurricane Ivan were observed in the narrowest portions of the island (between headlands), west of the transverse ridges. Overwash penetration tended to be larger in these areas and island breaching was common, leaving the surface close to the watertable and covered by a lag of shell and gravel. In contrast, large foredunes and the backbarrier dunes were observed at the widest sections of the island (the cuspate headlands) and at crest of the transverse ridges. Due to the large dunes and the presence of the backbarrier dunes, these areas experienced less overwash penetration and most of the sediment from the beachface and dunes was deposited within the upper-shoreface. It is argued that this sediment is returned to the beachface through nearshore bar migration following the storm and that the areas with larger foredunes and backbarrier dunes have smaller rates of historical shoreline erosion compared to areas with smaller dunes and greater transfer of sediment to the washover terrace. Since the recovery of the dunes will vary depending on the availability of sediment from the washover and beachface, it is further argued that the alongshore pattern of dune morphology and the response of the island to the next extreme storm is forced by the transverse ridges and island width through alongshore variations in storm surge and overwash gradients respectively. These findings may be particularly important for coastal managers involved in the repair and rebuilding of coastal infrastructure that was damaged or destroyed during Hurricane Ivan.  相似文献   

2.
The Ria Formosa lies in southern Portugal, extending for about 55 km. It is a true barrier island system, comprising mainland, backbarrier lagoons, inlet deltas, barrier islands, barrier platforms and shoreface. Of the Ria Formosa system, which covers a total area of 163 km2, 20 km2 are occupied by salinas and aquaculture ponds. The main water reservoir of thesalinas and the extensive aquaculture ponds behave like small lagoons where there are one or more openings to a tidal channel. These small lagoons have the advantage of being easy to study and to model. An ecological model was developed to estimate the potential production of the gilthead seabream in the Ria Formosa, in an extensive aquaculture regime. This model was based on information concerning chemical and physical factors (forcing functions) and secondary production estimates from four sites in the Ria Formosa with different environmental conditions, where extensive aquaculture is practised. Published information on optimal growth parameters of the gilthead was used to build the model, which was based on some assumptions concerning the detrimental effect of an excessive increase of salinity and other environmental factors both on gilthead growth (one state variable) and on prey production (divided into two state variables), which is essentially benthic macrofauna. Reservoirs with low water renewal undergo large environmental fluctuations. The growth of gilthead or other fish can be difficult, not only because of the adverse environmental conditions but also because of the low secondary production. The maximum yield of fish is predicted to be less than 6 g m−2. In reservoirs where the water exchanges on almost every tide, the maximum yield of gilthead can be in excess of 22–25 g m−2. Nevertheless, the high productivity of these sites in terms of secondary production of benthos and fish, as well as primary production, can lead to oxygen depletion during the night or at day-break. An attempt to further improve fish production by fertilisation or by adding food could provoke a rapid deterioration of the water quality and endanger all production.  相似文献   

3.
Plant species distributions often have been attributed to landform characteristics or their associated geomorphic processes. This complicates interpretation of vegetation patterns in that geomorphic processes shape, and are shaped by, landforms. To characterize the biogeographic impacts of this interaction, I used principal components analysis (PCA) to examine hypotheses regarding the structure of variation among soil properties in active barrier-island dune systems. Dune soils and vegetation were sampled on two well-recognized barrier-island morphologies. On low-profile, wave-dominated microtidal barrier islands (South Core Banks, North Carolina) frequent overwash exerts a greater control on the distribution of soil properties. On mixed-energy mesotidal barrier islands (Sapelo Island, Georgia), overwash is less frequent, and the distribution of soil properties is shaped by a complex dune topography. Nontrivial principal components on both islands captured an equivalent amount of variance in the soil data. However, there were inter-island differences in the dimensionality of these nontrivial principal components, and differences in the distribution of variance and factor loadings. Suites of topography-modifying species, unique to each island, were uniform in the strength of their individual correlation with local edaphic variability. I posit that soil variance structure is a useful criterion to distinguish the relative influence on vegetation patterns of soil properties expressed through landforms (Sapelo Island) versus sediment transport processes (South Core Banks). [Key words: dune vegetation, barrier islands, principal components analysis (PCA), overwash.]  相似文献   

4.
We report here on a bare barchan dune in Israel that converted over the last 60 years to a shrub-covered parabolic dune due to changes in land use. Thirty nebkhas (sand mounds) that were formed by sand trapped around shrubs growing on the dune were monitored during winter of 2004–2005. The rates of erosion or deposition were measured at five points in the nebkhas by erosion pins. All nebkhas were shown to undergo erosion or deposition activity. Nebkhas on the windward slope of the dune experienced primarily erosion. Those on the lee slope grew slowly by the light accumulation of sand. The largest nebkhas were found on the dune crest; they built up through the accumulation of sand that was eroded from the windward slope. There were no significant differences between the rates of erosion/deposition of the five study points placed in each nebkha. However, there were significant differences between the rates of erosion/deposition of the nebkhas on the three dynamic segments of the dunes (windward, crest and lee). A change in dune dynamics was observed by the emergence of shrubs on the crest. These shrubs trapped sand and increased the crest height. The sand trapped on the crest was not deposited on the lee side. In that case, the dune becomes narrower, higher, with a concave shape (of the windward slope), during the transformation from a barchan to a parabolic dune.  相似文献   

5.
Morphological analysis of the Fortore River coastal plain and the Lesina Lake coastal barrier integrated with radiocarbon age data indicates that the evolution of the coastal landscape has been strongly affected by a number of strong earthquakes and related tsunamis which occurred during the last 3000 years. The first seismic event struck this coastal area in the V century BC. It produced strong erosion of the Fortore River coastal plain and significant emersion of Punta delle Pietre Nere, as well as the large tsunami responsible for the development of the Sant'Andrea washover fan. The second event occurred in 493 AD; it induced severe erosion of the Fortore River coastal plain and triggered the large tsunami that hit the Lesina Lake coastal barrier, producing the Foce Cauto washover fan. Then later in 1627, an earthquake was responsible for the further coseismic uplift of Punta delle Pietre Nere, the subsidence of Lesina village area and the development of a tsunami which produced two washover fans.Morphological analysis points out that seismic events strong enough to control the morphological evolution of local coastal landscapes show a statistical return period of about 1000 years. These major events produced important coseismic vertical movements and large tsunamis. However, the correct identification of the tectonic structure responsible for the generation of these strong earthquakes is still an unsolved problem.  相似文献   

6.
The Boao coastal system along the eastern coast of Hainan Island is a dynamic delta-tidal inlet-barrier formed during the late Holocene. The delta developed inside a shallow lagoon barred by a sandy barrier with a narrow, shallow tidal inlet opening. Two major distributary channels separated by small islands characterize the delta. The lagoon is silting up receiving and trapping sediments from both the river and, in minor measure during storms, through the tidal inlet opening and barrier washovers. The barrier at the tidal inlet is highly dynamic and changes its form, accreting (migrating spit) against the inlet during fair-weather conditions and being eroded during storms and river floods.The delta has almost completely filled the lagoon and major concerns exist on the effect that ongoing large development plans may have on the environment. These concerns include the effect on floods and rate of siltation once banks of the islands have been stabilized and floodwater and sediment load are impeded from spreading over the lowlands, and the effect of increasing pollutant loads from the new facilities on the ecosystems of the increasingly restricting lagoon water and on the seashores.  相似文献   

7.
1 IntroductionThe objectives of this paper are to analyze the principal characteristics of the W anquan Riverdelta that has developed in a narrow , shallow lagoon, and to determ ine the effect that theever-increasing hum an activities of the area m ay hav…  相似文献   

8.
The Boao coastal system along the eastern coast of Hainan Island is a dynamic delta-tidal inlet-barrier formed during the late Holocene. The delta developed inside a shallow lagoon barred by a sandy barrier with a narrow, shallow tidal inlet opening. Two major distributary channels separated by small islands characterize the delta. The lagoon is silting up receiving and trapping sediments from both the river and, in minor measure during storms, through the tidal inlet opening and barrier washovers. The barrier at the tidal inlet is highly dynamic and changes its form, accreting (migrating spit) against the inlet during fair-weather conditions and being eroded during storms and river floods. The delta has almost completely filled the lagoon and major concerns exist on the effect that ongoing large development plans may have on the environment. These concerns include the effect on floods and rate of siltation once banks of the islands have been stabilized and floodwater and sediment load are impeded from spreading over the lowlands, and the effect of increasing pollutant loads from the new facilities on the ecosystems of the increasingly restricting lagoon water and on the seashores.  相似文献   

9.
The western coast of Hainan Island exhibits a savanna landscape. Many types of sand dunes, including transverse dune ridges, longitudinal dune ridges, elliptical dunes, coppice dunes, and climbing dunes, are widely distributed in the coastal zone. In winter, high-frequency and high-energy NE winds (dominant winds) are prevalent, with a resultant drift direction (RDD) of S35.6°W. In spring, low-frequency and low-energy SW secondary winds prevail, with a RDD of N25.1°E. Wind tunnel simulations revealed that the airflow over the dune surface is the main factor controlling the erosion and deposition patterns of dune surfaces and the morphological development of dunes. In the region's bidirectional wind en-vironment, with two seasonally distinct energy levels, the airflow over the surface of elliptical dunes, barchan dunes, and transverse dune ridges will exhibit a transverse pattern, whereas the airflow over longitudinal dunes ridges exhibits a lateral pattern and that over climbing dunes exhibits a climbing-circumfluent pattern. These patterns represent different dynamic processes. The coastal dunes on the western coast of Hainan Island are influenced by factors such as onshore winds, sand sources, coastal slopes, rivers, and forest shelter belts. The source of the sand that supplements these dunes particularly influences the development pattern: when there is more sand, the pattern shows positive equilibrium deposition between dune ridges and dunes; otherwise, it shows negative equilibrium deposition. The presence or absence of forest shelter belts also influences deposition and dune development patterns and transformation of dune forms. Coastal dunes and inland desert dunes experience similar dynamic processes, but the former have more diversified shapes and more complex forma-tion mechanisms.  相似文献   

10.
新月形沙丘顶部稳定性是风沙地貌学尚未解决的科学问题。研究新月形沙丘的顶部稳定性,对于绿洲边缘风沙运动规律揭示、防沙工程建设和沙区生态环境保护等具有重要的现实意义。选择民勤沙区新月形沙丘,通过测定沙丘各部位风速、风蚀风积和粒度等,分析了新月形沙丘顶部稳定机理。主风向（NW）作用是新月形沙丘最高点与沙脊线重合、沙丘前移和高度降低的过程;反向风（SE）作用是沙丘最高点与沙脊线分离、沙丘背风坡风蚀与沙丘增高的过程。由于研究区以NW风为主,新月形沙丘沿NW-SE方向前移,SE风只能风蚀减缓沙丘背风坡的坡度。人为干预将会阻止或减少从迎风坡向沙丘顶部输送沙量,使得新月形沙丘背风坡尤其是背风坡上部风蚀过程增强,新月形沙丘逐渐过渡为抛物线形沙丘。  相似文献   

11.
乌兰布和沙漠东北缘地表风沙流结构特征   总被引：13，自引：4，他引：9  
在国家林业局磴口荒漠生态站长期监测的基础上,利用多种积沙仪,对乌兰布和沙漠东北缘流动沙丘、油蒿半固定沙丘、白刺半固定沙丘、油蒿固定沙丘、白刺固定沙丘5种典型下垫面近地面（0～100 cm）的风沙流输沙量进行了实地观测和对比分析。结果表明：（1） 输沙率（q）随高度（h）增加呈幂函数（q=ah-b,R2≥0.8409）规律衰减,随风速（v）增大呈幂函数（q=avb,R2≥0.9256）规律增加,42.8%～70.7%的输沙量分布在10 cm高度内,67.6%～90.0%的输沙量分布于30 cm高度内。当地表植被盖度达到40%以上时,输沙率下降至无植被覆盖地表输沙率的6.6%以下,可有效阻止地表风蚀。（2） 沙物质主要由粒径为50～250 μm的细沙和极细沙构成,各高度层风蚀物粒度组成服从单峰态分布,峰值在100～250 μm。随高度增加,风蚀物粒径范围趋于变窄,粒径趋于更细。（3） 起沙风多出现在WSW和NW方向,占全年起沙风的53.19%。风沙流中跃移输沙、蠕移输沙的空间分布在理论上应与风向频率分布基本一致,差异性主要由各方位风的强度和持续时间等因素导致。研究结果可为该区域防沙工程设计提供理论参考。  相似文献   

12.
海南岛西海岸沙丘形成动力过程的风洞模拟试验   总被引：3，自引：0，他引：3  
The western coast of Hainan Island exhibits a savanna landscape. Many types of sand dunes, including transverse dune ridges, longitudinal dune ridges, elliptical dunes, coppice dunes, and climbing dunes, are widely distributed in the coastal zone. In winter, high-frequency and high-energy NE winds (dominant winds) are prevalent, with a resultant drift direction (RDD) of S35.6°W. In spring, low-frequency and low-energy SW secondary winds prevail, with a RDD of N25.1°E. Wind tunnel simulations revealed that the airflow over the dune surface is the main factor controlling the erosion and deposition patterns of dune surfaces and the morphological development of dunes. In the region’s bidirectional wind en- vironment, with two seasonally distinct energy levels, the airflow over the surface of elliptical dunes, barchan dunes, and transverse dune ridges will exhibit a transverse pattern, whereas the airflow over longitudinal dunes ridges exhibits a lateral pattern and that over climbing dunes exhibits a climbing-circumfluent pattern. These patterns represent different dynamic processes. The coastal dunes on the western coast of Hainan Island are influenced by factors such as onshore winds, sand sources, coastal slopes, rivers, and forest shelter belts. The source of the sand that supplements these dunes particularly influences the development pattern: when there is more sand, the pattern shows positive equilibrium deposition between dune ridges and dunes; otherwise, it shows negative equilibrium deposition. The presence or absence of forest shelter belts also influences deposition and dune development patterns and transformation of dune forms. Coastal dunes and inland desert dunes experience similar dynamic processes, but the former have more diversified shapes and more complex formation mechanisms.  相似文献   

13.
通过连续动态观测,获取毛乌素沙地南缘典型半固定沙丘表面风沙输移物,测量了风沙活动中不同沙丘部位的风蚀深度,并对风沙输移物及其养分进行了初步分析。结果表明:风沙活动期间沙丘顶部的风蚀深度最大,沙丘迎风坡次之,而丘间地的风蚀深度最小;沙丘顶部和迎风坡的输移物以中沙和细沙为主,养分含量较低,丘间地颗粒较细且养分含量较高;土壤养分含量与颗粒大小存在显著的负相关关系。迎风坡和丘顶处在风蚀亚环境而难以被植被固定,进而加速了风蚀过程;丘间地风沙活动较弱,风积物中养分含量较高的细颗粒物质促进了植被的发育,因而减弱了局地风速。  相似文献   

14.
半干旱区不同类型沙丘风沙流结构特征   总被引：4，自引：1，他引：3  
采用两种阶梯式集沙仪和小型气象站于2017年4—5月对科尔沁沙地流动沙丘、半固定沙丘和固定沙丘0~75 cm气流层风沙流的总输沙量、输沙率、粒径组成分布和风蚀特征值进行了观测。结果表明：（1）随着高度增加,总输沙量下降,随着风速增加,总输沙量上升;92.20%~95.60%的输沙量发生在0~21 cm高度。（2）总输沙率（Q）流动沙丘>半固定沙丘>固定沙丘,将Q与地上200 cm风速进行函数拟合,流动沙丘幂函数最佳（R2=0.986）,半固定（R2=0.990）和固定沙丘（R2=0.956）指数函数最佳。（3）将各高度的输沙率与地上200 cm风速进行函数拟合,流动沙丘（R2≥0.905）和半固定沙丘（R2≥0.968）拟合度幂函数好于指数函数,固定沙丘（R2≥0.923）指数函数优于幂函数。（4）在一定高度下,3类沙丘输沙率均随着风速的增加而增加;在一定风速下,输沙率随着高度的增加而逐渐递减。（5）3类沙丘的特征值随着风速的增加呈现出逐渐递增的趋势;流动沙丘以λ>1为主,表现出持续侵蚀输送沙粒的能力;半固定沙丘当风速>9.0 m·s-1时逐渐出现侵蚀状态;固定沙丘以λ<1为主,近地表风沙以堆积状态为主。（6）3类沙丘主要由粒径为0.1~0.25 mm的细沙构成,在0~30 cm高度,细沙占输沙量的50.09%~85.11%,在30~75 cm高度,细沙占输沙量的43.53%~75.53%。  相似文献   

15.
A field study was conducted to ascertain the amount of protection that mesquite-dominated communities provide to the surface from wind erosion. The dynamics of the locally accelerated evolution of a mesquite/coppice dune landscape and the undetermined spatial dependence of potential erosion by wind from a shear stress partition model were investigated. Sediment transport and dust emission processes are governed by the amount of protection that can be provided by roughness elements. Although shear stress partition models exist that can describe this, their accuracy has only been tested against a limited dataset because instrumentation has previously been unable to provide the necessary measurements. This study combines the use of meteorological towers and surface shear stress measurements with Irwin sensors to measure the partition of shear stress in situ. The surface shear stress within preferentially aligned vegetation (within coppice dune development) exhibited highly skewed distributions, while a more homogenous surface stress was recorded at a site with less developed coppice dunes. Above the vegetation, the logarithmic velocity profile deduced roughness length (based on 10-min averages) exhibited a distinct correlation with compass direction for the site with vegetation preferentially aligned, while the site with more homogenously distributed vegetation showed very little variation in the roughness length. This distribution in roughness length within an area, defines a distribution of a resolved shear stress partitioning model based on these measurements, ultimately providing potential closure to a previously uncorrelated model parameter.  相似文献   

16.
The characteristics of foredunes created in a municipal management program on a developed barrier island are evaluated to identify how landforms used as protection structures can be natural in appearance and function yet compatible with human values. Shoreline management zones include a naturally evolving, undeveloped segment; a noneroding, developed segment; eroding and noneroding segments of an “improved beach” where dunes have been built by artificial nourishment; and a privately built, artificially nourished dune on the shoreline of an inlet.A disastrous storm in 1962 resulted in an aggressive program for building dunes using sand fences, vegetation plantings, purchase of undeveloped lots, and sediment backpassing to maintain beach widths and dune elevations. The present nourished and shaped foredune in the improved beach is higher, wider, and closer to the berm crest than the natural dune. Restricted inputs of aeolian sand keep the surface flat and poorly vegetated. A stable section of this engineered shore has a wider beach, and sand fences have created a higher foredune with greater topographic diversity. The cross shore zonation of vegetation here is more typical of natural dunes, but the environmental gradient is much narrower. The privately built dune is low, narrow, and located where it could not be created naturally. Foreshore and aeolian sediments in the undeveloped segment and the improved beach are similar in mean grain size (0.16–0.21 mm) and sorting (0.31–0.39φ), but sediment on the surface of the nourished dune is coarser (28.1% gravel) with a more poorly sorted sand fraction (1.30φ) representing lag elements on the deflation surface.Willingness to enhance beaches and dunes for protection has reduced insurance premiums and allowed the municipality to qualify for funds from the Federal Emergency Management Agency (FEMA) to replace lost sediment, thus placing an economic value on dunes. Success of the management program is attributed to: (i) timing property-purchase and dune-building programs to periods immediately after storms (causing residents to accept high dunes that restrict access or views); (ii) instituting a vigorous education program (reminding residents of hazards during nonstorm periods); (iii) maintaining control over local sediment supplies (to keep pace with erosion and create new shoreline environments); (iv) investing private and municipal economic resources in landforms (qualifying them for external funds for replacement); and (v) maintaining, augmenting, or simply tolerating biodiversity and natural processes (retaining a natural heritage).  相似文献   

17.
Holocene coastal evolution in New South Wales has been interpreted essentially as the unfolding of the impact of marine transgression. Sea level on this coast supposedly reached its present height at 6–6.5 ka, and varied < 1 m since then. The early Holocene rise of the sea has been considered the key factor (“forcing function”) in dune migration, coastal sand barrier development, and the evolution of estuaries. Episodic storminess during the late Holocene has been seen as an important, though secondary, factor in beach erosion and dune mobilisation. An alternate interpretation presented here challenges the concept of the marine transgression as the primary “forcing function”. It (a) attributes early Holocene dune mobilisation to climate rather than the rising sea; (b) shows that the sea reached its present level by 7 ka and rose to at least + 2 m until 1.5 ka; (c) links late Holocene dune activity to local disruption of vegetation rather than to regional episodic storminess; (d) demonstrates a fall of 2°C in sea surface temperature after 3 ka that coincides with the onset of barrier erosion; (e) recognises the imprint of at least three tsunamis in the coastal sedimentary record.  相似文献   

18.
This study presents a model of cyclic spit accretion for the southern shoreline of Plum Island, Massachusetts, since 2,500 years B.P. A three-dimensional analysis of 325 cores taken from the sand unit underlying Stage Island Pool on Plum Island and bivariate plots of the graphic statistics derived from 40 sand samples permitted identification of three subsurface environments: beach/dune ridge, interdune, and washover/ tidal channel. The cyclic recurrence of these environments is proposed as the mechanism leading to the formation and growth of the southern end of the island since at least 2,500 years B.P.  相似文献   

19.
基于CASS和ArcGIS的新月形沙丘属性参数提取   总被引：2，自引：1，他引：1  
在塔克拉玛干沙漠腹地选取新月形沙丘演变监测区,运用华测X90 GNSS接收机RTK工作模式,获取精度达毫米级的沙丘表面高时空分辨率三维坐标,应用地藉成图软件CASS 9.0和地理信息数据分析软件ArcGIS 10.0对三维坐标数据进行处理,提取新月形沙丘的坡长、坡角、高度、宽度、脊线长度、两翼距离和夹角、体积、表面积和投影面积、坡度坡向等属性信息数据,多期监测数据叠加分析,求算沙丘侵蚀堆积量、移动速度和移动方向等沙丘演变信息。结果表明：在第1次和第2次监测期间,代表性新月形沙丘处于堆积增大状态,堆积量51.4 m3,移动速度16.0 m·a-1左右,移动方向226°45',沙丘移动方向与主风向NE一致。基于CASS和ArcGIS的新月形沙丘属性参数提取,是量化研究沙丘形态演变过程和位置迁移动态、明确沙丘演变中各属性参数间数值关系的有效途径。  相似文献   

20.
Jespersen, Margot und Rasmussen, Erik: Jordsand—Erosion und Akkumulation einer Hallig. Geografisk Tidsskrift 75: 13–23. København, juni 1, 1976.

Since 1972 the authors have carried out investigations of erosion and accumulation on the island of Jordsand in the Danish Wadden Sea. The investigations supported by the Danish State Research Council for Natural Sciences are not yet finished, and this paper is a midway report including preliminary results and working hypotheses.  相似文献   

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