首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 203 毫秒
1.
Erosion rates are 15–18.5 times higher in places where there is a beach at the cliff foot compared with those where there is no beach. The zone of intense erosion associated with a beach of sand and pebbles was below a height of about 10 cm above the surface, maximum erosion being at 14.5 cm below the surface. Erosion was by corrasion and wedging, a previously unrecognised process which is the quarrying of rock aided by fine-grained particles from the beach. Wedging, perhaps with some corrasion, had a median erosion rate of 11.05 · 10?3 cm tide?1 in Upper Lias shale while the rate for corrasion was 5.79 · 10?3 cm tide?1. A smooth, notched cliff foot may be produced by these processes. Where there was no beach, erosion of the cliff foot was by the sporadic quarrying of blocks of rock and the more continuous micro-quarrying of shale fragments. The cliff foot was dominated by structural planes and had a rough surface. The horizontal erosion rate due to quarrying at one cliff foot was 2.3 cm year?1. Quarrying acts on the cliff foot above the intense zone of erosion at a beach surface. The median erosion rate for this quarrying was 0.91 · 10?3 cm tide?1.  相似文献   

2.
The studies of vertical particle fluxes were performed in 2006–2007 in Dalnezelenetskaya Inlet (the Barents Sea) and Chupa Inlet (the White Sea). The average annual particle flux in Dalnezelenetskaya Inlet was 20–21 cm3/(m2 day), or 1350–1400 mg/(m2 day), while, in Chupa Inlet, it was 17–18 cm3/(m2 day), or 500–550 mg/(m2 day). The sedimentary matter is mostly represented by flocculent detritus less than 100 μm in size. Currently, the estimated sedimentation rate constitutes 0.9–1.1 mm per annum in the DI and 0.3–1.0 mm per annum in the Chupa Inlet.  相似文献   

3.
Volumes of seawater filtered through the intertidal zone were measured on three modally reflective microtidal beaches in Western Australia. The filtered volumes were large, 19 m3 m?1 day?1 and 73 m3 m?1 day?1 on two ‘clean’ beaches but only 0·4 m3 m?1 per tidal cycle on a beach covered in kelp and seagrass wrack. The mean residence times of this water in the interstitial system and its percolation paths were both short, 1–7 h and 2–5 m respectively. Water input was greater across a beach cusp horn than across a cusp embayment. Most input occurred in the upper swash zone where the water table was less than 20 cm deep. Tidal variations in input volumes were evident even with tide ranges of only 20 cm. The inshore zone off these beaches filters on average 0·07 m3 m?2 day?1 at an average depth of 5·5 m under 0·4 m waves of 6·5 s duration. The importance of these procedures in the mineralization of organic materials and the regeneration of nutrients for an inshore ‘lagoon ecosystem’ is estimated and discused.  相似文献   

4.
Altogether three coastal dune fields, one located north and two south of the city of Essaouira, Atlantic Morocco, have been investigated to establish the distribution and overall sand volumes of various dune types. The purpose of the study was to characterize and classify the aeolian landforms of the coastal dune belt, to estimate their sand volumes and to assess the effectiveness of coastal dune stabilization measures. The northern dune field is 9 km long and lined by a wide artificial foredune complex fixed by vegetation, fences and branches forming a rectangular grid. Active and ephemeral aklé dunes border the inner backshore, while some intrusive dunes have crossed the foredune belt and are migrating farther inland. The total sand volume of the northern dune belt amounts 13,910,255 m3. The central coastal sector comprises a much smaller dune field located just south of the city. It is only 1.2 km long and, with the exception of intrusive dunes, shows all other dune types. The overall sand volume of the central dune field amounts to about 172,463 m3. The southern dune field is characterized by a narrower foredune belt and overall lower dunes that, in addition, become progressively smaller towards the south. In this sector, embryonic dunes (coppice, shadow dunes), tongue-like and tabular dunes, and sand sheets intrude from the beach, the profile of which has a stepped appearance controlled by irregular outcrops of old aeolianite and beach rock. The total volume of the southern dune field amounts 1,446,389 m3. For the whole study area, i.e. for all three dune fields combined, a sand volume of about 15,529,389 m3 has been estimated. The sand of the dune fields is derived from coastal erosion and especially the Tensift River, which enters the sea at Souira Qedima some 70 km north of Essaouira. After entering the sea, the sand is transported southwards by littoral drift driven by the mainly north-westerly swell climate and the Trade Winds blowing from the NNE. This sand feeds the beaches along the coast, from where it is blown obliquely onshore to generate the dune fields. The maximum sand input occurs in the north, from where it gradually decreases southwards, this being also reflected in the grain size and dune typologies. The study shows that dune stabilization measures have been reasonably effective along most of the coast, with the exception of a large area immediately north of Essaouira where the almost complete destruction of the plant cover has reactivated sand mobility—this may in the future threaten the city.  相似文献   

5.
The sandy beaches of Anapa Bay Bar are a unique natural resource, but they are gradually being degrade under both natural and anthropogenic factors. The emissions of sand and shelly ground from the adjacent sea bottom partly compensate for this process. The concentration of carbonates may reach up to 50% in the beach sands, and most of these carbonates are of mollusk origin. The major deposit formation role belongs to the key bivalve species: Chamelea gallina (Linnaeus, 1758). The average biomass of this mollusk species reaches up to 450 g/m2 at the depths of 5–10 m. The other two subdominating mollusk species, the bivalve Donax trunculus (Linnaeus, 1758) and the gastropod Rapana venosa (Valenciennes, 1846), may impact as 16 g/m2 and 6 g/m2, respectively. Annually, 350 kg of shelly ground per running meter are newly deposited on Anapa beach.  相似文献   

6.
《Coastal Engineering》2001,42(2):143-153
Beach erosion problems have been solved by adding sand to the beaches along the Gulf of Cadiz. The Gulf is located in SW Spain between the Portuguese border and the Strait of Gibraltar. During the last decade, more than 12×106 m3 of sand have been nourished in 38 restoration operations carried out on 28 beaches. The main characteristics of the nourishment campaigns (year, volume, budget, transport method, sand data, etc.) are presented. Location of sand borrow sites and distance to the beaches are also shown. Monitoring programs have been performed in order to calculate sediment loss rates. These results have been related to the beach length, the berm width and the budget in order to obtain a variety of relationships for maintenance cost as, for example, the total annual cost for each beach. This information is very useful when developing a strategy in coastal zone management. Furthermore, at least in reef-protected beaches, small yearly renourishments similar to the yearly losses, instead of greater nourishments performed with a periodicity of many years, lead to an economical saving, as well as to a better use of the natural resources.  相似文献   

7.
Beach-nearshore profiles combined with beach and surficial sediment samples were analyzed in conjunction with wave, current, littoral drift and sea-level data to determine the effect of bedrock on morphodynamic processes within the littoral zone of Alexandria on the Mediterranean coast of Egypt. This 14.5-km-long littoral cell is bounded by pronounced embayments and pocket beaches separated by headlands which prevent bypassing of beach sands, in effect making this cell a large, semi-closed basin. The compartmented nature of this cell acts together with the rough irregularity of the rocky seafloor to trap a thin veneer of sediment (<3 m thick), showing proportional mixing between two sedimentary provinces. A modern fine-grained sediment facies consisting of mixed carbonate/siliciclastic sand flanks most of the nearshore zone down to a depth of 8–10 m. Beyond this depth, considered to be the depth of closure, a relict late Pleistocene to mid-Holocene coarse-grained facies composed of biogenic carbonate sand is found. Along a short section of the coastline (km 3–6), the coarser sediment also occupies the nearshore zone. Over most of the study area the two sediment types are mixed in various proportions, largest mixing coinciding with poorest sorting. Profile analyses revealed seasonal changes in sediment volume along the coast which closely follow the cyclicity of seasonal changes in wave climate. The present shoreline orientation, headlands and rough, irregular rocky seabed are reflected in the erosion/accretion pattern, sediment characteristics, and the reversibility of longshore currents and littoral drift. Although there is a marked deficiency in the sediment balance, the sand budget for this cell, including artificial material (2.339*106 m3) has increased slightly by 0.041*106 m3 year–1 as a result of engineering works carried out to widen the coastal road (Corniche). In addition to the physical properties of the bedrock (degree of induration), the accelerating sea-level rise during the Holocene and human influences, the modern morphology of the coast, the erosional seabed features in the nearshore zone, and the texture of seabed sediments are all controlled by the original geometry of the coast which consisted of an elevated subaerial ridge.  相似文献   

8.
Different processes erode the plane and ramp, the two morphological elements of the shore platform. On the plane, desiccation of the shale causes contraction of the clay lattices in intertidal periods but wetting by the flood tides produces expansion. These movements crack the shale bedding laminae into polygons about 2 cm in diameter which are removed by waves. Desiccation is enhanced in well-drained areas and there is a positive correlation between altitude and erosion rate producing the smooth-sub-horizontal plane. Erosion is highest during summer. It averages 0.1 cm year?1 but ranges from 0.0 to 0.9 cm year?1.Corrasion is the main erosive process on the ramp. It is most active during winter, the stormy season, but is influenced by beach depth and possibly grain size. A shallow sand and pebble beach less than 5 cm deep allows erosion at all times. Beach depth is the most important influence on corrasion if it brings the ramp within 5 cm of the surface at some times and buries it more than 13.5 cm deep at others. Wave energy is the most important variable if the beach is usually more than 13.5 cm deep. Erosion rates for the zones of 0–5 cm depth, 5–13.5 cm depth, and greater than 13.5 cm depth were respectively 3.94 · 10?3 cm tide?1, 3.26 · 10?3 cm tide?1 and 1.13 · 10?3 cm tide?1.  相似文献   

9.
The benthic faunal spectrum including bacteria, protozoans, meiofauna, wrack epifauna and macrofauna, was quantitatively surveyed on two modally reflective, moderate energy, Western Australian beaches. The more exposed beach had coarser sand, no intertidal macrofauna and a poor interstitial fauna. The less exposed beach had a large deposit of wrack totalling 161 kg m?1 dry mass concentrated on the lower shore. The amphipod Allorchestes compressa was abundant in the fresh wrack comprising most of the macrofauna. There were also fairly abundant small epifauna on the wrack. Dry biomass of macrofauna, epifauna, meiofauna, protozoans and bacteria was 0, 0, 15, 4 and 180 g m?1 on the more exposed beach and 160, 3, 112, 9 and 901 g m?1 on the less exposed beach with wrack. On the latter beach there was an inverse correlation between meiofaunal densities and the densities of protozoans and bacteria, suggesting grazing by the former on the latter. On both beaches meiofauna was concentrated in the mid- to upper beach, protozoans near the surface and bacteria in the mid- to lower beach. It is estimated that bacteria are responsible for most of the secondary production on both beaches.  相似文献   

10.
A diagnostic of coastal erosion and shoreline retreat occurring at a 7-km long sandy beach (Littoral Cell III) located in Todos Santos Bay (Baja California, Mexico) is performed trough the analysis of aerial photographs. Around 82,000 m2 of this sandy beach have been lost in a 20 year period (1985–2005), at a beach loss rate of 2,100 m2 per year. This indicates that coastal erosion is becoming a hazard to human lives and coastal infrastructure. Due to the latter, the implementation of a Shoreline Erosion Management Plan (SEMP) is proposed as the best management approach to deal with the problem. The Littoral Cell III SEMP considers four core policies, eight management strategies and a group of specific measures.  相似文献   

11.
Erosion of sandy beaches is a worldwide problem that elicits innovative geoengineer‐ing techniques to reduce adverse impacts of shoreline retreat. Beach replenishment has emerged as the “soft”; shore‐stabilization technique of choice for mitigating beach erosion. This method of shore protection involves the addition of sand to the littoral sediment budget for sacrificial purposes. Because inland sand sources are often uneconomical or impractical to use, and known nearshore sources are limited, finding adequate quantities of suitable sand on the inner continental shelf is often vital to beach replenishment projects. The technical studies of survey and materials analysis that identify and delineate usable sand sources are sometimes almost as expensive as small‐project dredging, pumping, and placing the sand on the beach as fill. Inadequate quantity or substandard quality of shelf sand, as well as often‐prohibitive overhead expenses, thus compel shoreline managers to seek suitable sand sources offshore.

In the study area off the central‐west coast of Florida, offshore potential borrow areas (PBAs) were identified on the basis of studies conducted in reconnoitory and detailed phases. Sophisticated state‐of‐the‐art equipment used in this investigation provided more detailed subbottom mapping information than is normally obtained with conventional seismic equipment. An example of sand exploration studies was incorporated in a 215‐km2 survey of offshore areas by conducting bathymetric surveys and subbottom seismic profiling, collecting jet probes, grab samples, and vibrocores, and analyzing sediment grading in subsamples from vibrocores. These combined analyses indicated that at least 8.8 ×106 m3 of sand is available in potential borrow areas from 7.0 to 12 km offshore in water depths of 8.0 to 11.5 m. In the PBAs, mean grain size of sand falls into the range 0.13–0.53 mm, sorting averages 0.65–1.31ø, and the overall silt content varies from 3.9–8.5%. High silt contents (13–19%) mapped in some areas make these sedimentary deposits unsuitable as fill for artificial beach renourishment.  相似文献   

12.
张东  潘雪峰  张鹰 《海洋学报》2008,30(2):33-37
以长江口南支河段为研究区,利用LandsatETM+遥感影像反演的1999年水下地形和该河段2002年实测水深数据,结合长江口来水来沙现状和河段特征,分析了河段的冲淤变化和沙体迁移规律,提出了需采取的治理措施。研究结果表明:(1)从1999到2002年南支河段总体发生冲刷,年平均冲刷量为0.1926亿m3,河道内白茆沙、新浏河沙和新浏河沙包的沙头年平均后退467,374和421m;(2)上、下扁担沙之间由于串沟发育,两者有分离的趋势,沙体5m等深线的变动格局显示两沙体沙尾向下移动导致新桥水道上段水深加深、下段南侧水深变浅;(3)固定白茆沙和上、下扁担沙沙体,对维持南支河段航道的航运、稳定南北槽河势具有重要作用。  相似文献   

13.
This paper summarizes the results of over 8 years of data describing the performance of a large beach nourishment project on Perdido Key, immediately adjacent to Pensacola Pass in Escambia County, FL, USA. As a result of a major excavation of the entrance channel to Pensacola Bay, over 7 million m3 of beach-quality sand were placed along the easternmost 7.5 km of Perdido Key, adjoining the entrance channel at Pensacola Pass. The project included the placement of 4.1 million m3 of sand directly upon the shoreline in 1989–1990, followed by the placement of an additional 3 million m3 as an underwater berm just offshore of the beach nourishment project in water depths of roughly 6 m. Monitoring of the performance of the beach nourishment project and the offshore berm has been conducted since 1989, beginning with a pre-construction survey of the project area. Monitoring surveys have been conducted on an annual or biennial basis since that time, with the most recent survey occurring in July/August, 1998. Over 8 years of monitoring data indicate that the beach nourishment project has retained approximately 56% of the original volume placed within the 7.5-km project length. In addition, according to the latest monitoring survey, the dry beach width of the project, initially constructed as 135 m on average, is still 53 m wider than pre-project conditions. Approximately 41% of the originally placed dry planform area remains as of July 1998. The most recent monitoring surveys in 1995, 1997, and 1998 encompass the effects of two major storm systems, Hurricanes Erin (August 1995) and Opal (October 1995). Monitoring of the offshore berm area indicates only a slight landward migration of the berm, accompanied by a minor decrease in volume, over the entire monitoring period. The performance of both the beach nourishment project and the offshore berm appear to be significantly related to the two storm events, particularly Hurricane Opal, and the proximity of the project to the tidal entrance at Pensacola Pass. Comparison of the documented performance of the beach nourishment project to simple existing analytical models of beach-fill evolution have yielded encouraging results in terms of preliminary design aids for future beach nourishment projects in the vicinity of deep tidal entrances.  相似文献   

14.
A simple sand trap is used to measure swash and backwash bedload transport rates on intertidal profiles. Data from sixty-eight beach experiments are used to calculate a mean value of 12.78 kg m?4s?2 for the calibration coefficient in the Bagnold beach equation.  相似文献   

15.
Victoria Beach (Cadiz, Spain) comprises a rocky flat outcrop in its northern zone and a sand-rich southern zone. These natural features allowed for a 5-year monitoring period and subsequent analysis of two different profiles (one in each zone) based on differences in bottom contours. Topo-bathymetric data were analysed using empirical orthogonal functions (EOFs) to determine changes over the short-, medium- and long-term. Several morphologic phenomena were identified (generalised erosion, seasonal or summer–winter tilting of the profile around different hinge points, berm development and its posterior destruction, etc.) in terms of their importance in explaining the variability of the collected data for both profiles. It is worth mentioning that both profiles undergo parallel regression in the medium-term. Thus, the 1st eigenfunction enabled us to identify the true regression of the beach shoreline, independent of seasonal or summer–winter slope changes. Reconstruction of profiles using EOF components demonstrated that though accretion periods in the medium-term were similar for both types of profiles, the accretion speed was much faster in the sand-rich profile than in the reef-protected profile (1.01 m3/day versus 0.33 m3/day). Moreover, the seasonal erosion rate and the subsequent shoreline retreat for the sand-rich profile were much larger than for the reef-protected profile (121 m3/year versus 29 m3/year). Analysis in the short-term (changes induced by a single day's storm) showed an instantaneous tilting of the profile, with the mobilised sand volume being much greater for the sand-rich than for the reef-protected profile (68 m3/m versus 12 m3/m).  相似文献   

16.
《Coastal Engineering》2005,52(2):151-158
A winter storm eroded a small (160,000 m3) beach fill at Torrey Pines State Beach in southern California. The fill, constructed in April 2001, was a 600-m long flat-topped berm, extending from a highway revetment seaward about 80 m, terminating in a 2-m tall, near-vertical scarp. The size distributions of the preexisting and fill beach sand were similar (median ∼0.2 mm). A total of 56 cross-shore transects were surveyed between the revetment and 8 m water depth biweekly along 2.7 km of the beach centered on the fill area. During summer and fall, the incident significant wave heights measured 1 km offshore of the fill usually were below 1 m, the scarp was not overtopped, and the fill did not change greatly. The beach face alongshore of the fill accreted, consistent with the usual seasonal cycle in southern California. During a storm (3 m significant wave height) in late November, erosion began when wave uprushes overtopped the scarp and reached the relatively flat elevated fill, where the overwash flowed alongshore to initially small depressions that channeled the flow seawards. The offshore flow rapidly deepened and widened the channels, which maintained steep vertical faces and eroded by slumping. Thirty hours after the storm began, the shoreward end of the eroded channels had retreated to the highway revetment, leaving uneroded sand peninsulas protruding seawards ∼50 m from the revetment and elevated ∼1.75 m above the surrounding beach. Erosion of the beach adjacent to the fill was much less variable alongshore than within the fill region. During the next few days, the peninsulas eroded almost completely.  相似文献   

17.
Abstract. The secondary production and population dynamics of the mole crab Emerita brasiliensis Schmitt, 1935 (Decapoda: Hippidae) were studied by taking monthly samples from June 1993 to May 1995 at each of three intertidal transects at Prainha beach, Brazil. The lifespan was ca. 8 months for males and females, but females showed higher growth, mortality, secondary production, and turnover rate. The higher production in spring versus autumn and winter was related to intense recruitment during that period. The population production was estimated at between 39.86 and 46.88 g (AFDW) · m?2 · a?1 for the first year (June 93–May 94) and between 150.95 and 156.07 g (AFDW) · m?2 · a?1 for the second year (June 94–May 95); the mean annual biomass was 4.91 and 23.09 g (AFDW) · m?2, respectively. High P/B rates, between ca. 6 and 9 · a?1, reflected the fast growth, high mortality, and low lifespan of the population, characterized by a high percentage of recently recruited individuals.  相似文献   

18.
From July 1981 to July 1982 Umbonium vestiarium (L.) on a north Penang sand shore numbered 573–11 077 m?2 (mean 4126 or 53·1 g dry tissue) near MLWN and 2164–12 414 m?2 (mean 6500 or 59·8 g dry tissue) further downshore. Heavy recruitment of young became evident in June and July 1982 and a closely corresponding cohort of young was present in July 1981. Progression of this cohort indicated that young settling in May–June grew to full size (11–13 mm diameter) by January–March the following year and that virtually all died during their second year, presumably having spawned in March–May. Recruitment of young was chiefly on the lower shore but adults came to be more abundant and predominant on the upper shore. There is some evidence of migration upshore during growth.Production is estimated at 105·4 g dry tissue m?2 y?1 (2118 kj) at the lower shore level and this is almost double the 58·8 g m?2 y?1 (1176·6 kj) at the higher level. These values represent almost the entire secondary production across much of the sand flats.Possible causes of such a marked annual cycle in the very weakly seasonal tropics of the Malacca Strait are considered and it is suggested that monsoonal changes in wind, wave action and salinity might be involved.  相似文献   

19.
Several researchers have investigated morphological changes on the south-eastern Mediterranean coast during the late Holocene. However, very few of these studies include quantitative data covering the last 200 years. In this study, topographical maps, nautical charts and aerial photographs are used to estimate the shoreline migrations and beach–nearshore sand balance over the last 200 years in Haifa Bay, Israel, the northernmost final depositional sink of the Nile littoral cell. The findings reflect two main periods. During the first period, between 1799 and 1928, human intervention along the bay’s coast was negligible, a significant coastal expansion of ∼50 to 150 m (averages of 0.4–1.2 m/year) was measured, and sand accumulation was estimated at ∼70,000 m3 annually in the beach–nearshore area. A dramatic change in the sedimentological pattern was observed during the second period, between 1928 and 2006, following the completion of Haifa Port’s main breakwater (1929–1933). During this period, most of the bay’s coast was in a steady state, with seasonal fluctuations of less than about ±20 m, and slight erosion of ∼7,000 m3 annually. These findings are consistent with previous studies which conclude that from approximately 4,000 years ago until the construction of Haifa Port, sea level remained relatively stable, and a continuous accumulation of Nile-derived sand dried up the Zevulun Plain and shifted the Haifa Bay shoreline westwards to its present location. This long-term trend ceased after completion of the Haifa Port main breakwater.  相似文献   

20.
海滩对风暴的响应及风暴后海滩的恢复过程一直以来都是国内外海滩研究的热点。本文通过对浙江舟山市朱家尖岛东沙海滩地形地貌的现场调查，对比分析了热带风暴"娜基莉"影响下东沙海滩剖面的蚀积变化，探讨了海滩在热带风暴发生后的恢复情况。结果表明，在"娜基莉"影响期间，因风暴浪为向岸浪，东沙海滩几乎遭受全线侵蚀，12个剖面单宽侵蚀总量为73.46 m3/m，其中海滩直线段较两个遮蔽段侵蚀显著。由于海滩在风暴前进方向的左侧，且"娜基莉"距东沙较远，使得东沙海滩普遍侵蚀但强度较小。东沙海滩在热带风暴后的恢复过程中，不同部位的地貌调整和冲淤变化不同，下岬角遮蔽段基本趋于稳定，直线段和上岬角遮蔽段在恢复过程中因受海滩季节性调整的影响呈现持续侵蚀。  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号