Processes and rate of retreat of the clay and sandstone sea cliffs of the northern Boulonnais (France)

Retreat of the clay and sandstone cliffs of the northern Boulonnais (France) has been quantified using stereophotogrammetry. The low retreat rate of this coastal strip — 0.08 m/yr between 1939 and 2003 — is far less than that encountered on chalk and clay-chalk cliffs of either side of the Channel, and even less than a previous estimate of 0.17 m/yr regularly quoted in management studies. The retreat rate is closely related to shore platform morphology and dynamics. The shore platform presents 1) a steeply sloping ramp due to the accumulation of flat calcareous megaclasts that reduce marine erosion; 2) upstanding bare platform surfaces, related to tectonic deformation; and 3) thick platform-beaches trapped in troughs. In all three cases, the reflective behaviour of the nearshore protects the cliff foot from the incoming waves. Two critical eroding segments are the result of changes in the platform sedimentary budget. Around Cran Poulet, and between Plage de la Sirène and Pointe de la Courte Dune, the retreat rate is up to 0.25 and 0.15 m/yr, respectively. At Cran Poulet, recession has been facilitated by the extraction of pebble for more than half a century, whereas erosion of the beach at la Sirène is probably linked to severe erosion of the coastline in the adjacent Wissant Bay. Mass movements on the cliff face are essentially shallow-seated translational slides along with small debris falls and mudflows. The instability of the Argiles de Châtillon is greatly diminished by their sandy and silty texture and by the presence of interstratified solid shelly limestone beds that allow steep slopes to develop in rather weak material. The ‘vertical erosion antecedent’ is the erosional mode of the cliff, and its reduced efficiency explains the slow recession of the cliff. This study will help to determine the long term evolution of the Boulonnais coast.     

The development of talus slopes around Lord Howe island and implications for the history of island planation

Lord Howe Island is a small eroded remnant of a Late Miocene shield volcano. A fringing coral reef dissipates wave energy along a portion of the shoreline, but the remainder of the coast is rugged with spectacular high basaltic sea cliffs. This paper investigates the evolution of talus slopes that occur beneath the loftiest cliffs, and places this analysis within the context of a longer history of island planation that has resulted in a wide truncated shelf around the island. During the Last Glacial, when the sea level was lower than at present, talus slopes accumulated around the extent of the island's cliffed coast because material eroded from cliffs by subaerial processes could not be removed by marine processes. The survival of these slopes during the Holocene has depended on a balance achieved between rates of subaerial and marine erosion. This balance is fundamentally influenced by cliff height, as cliffs higher than 200 m are plunging or veneered by talus slopes, whereas lower cliffs have erosional shore platforms. On comparison with published erosion rates from inland basalt scarps it appears that marine processes may account for over 90 per cent of the total cliff retreat that has occurred at Lord Howe Island, yet contemporary coastal morphology attests to the significance of subaerial processes in recent times. It is likely that marine cliffing was very rapid soon after volcanism ceased, but rates of erosion decreased through time as wave energy became increasingly attenuated across a widening planation surface, and as increasing cliff heights yielded greater quantities of talus that provided protection from rapid marine erosion.     

Coastal cliff behaviour: Observations on the relationship between beach levels and recession rates

Beaches dissipate wave energy and regulate the frequency that the cliff foot is subject to wave attack. The relationship between beach levels and cliff recession rates has been established for Pleistocene soft rock cliffs along the North Norfolk and Suffolk coasts, UK. The results suggest that over a decadal timescale, there is a non-linear increase in the average recession rate as the beach profile area above High Water Mark (HWM) decreases. Small changes in beach level can result in significant differences in the recession rates. The impact of a unit change in beach level on the recession rate depends on the initial beach level. On a year-by-year basis, it is possible to divide the beach level and recession relationship into a series of zones with characteristic types of behaviour. At low beach levels there is high to extremely high recession with considerable variability, whereas at high beach levels there is almost zero recession with limited variability. It is concluded that historical recession rates are the product of both the past forcing events and changes in cliff–beach state. Extrapolation of historical rates can be extremely unreliable unless it is supported by an understanding of the dynamic behaviour of the cliff–beach system and the energy inputs over the observation period.     

Beach morphology and change along the mixed grain-size delta of the dammed Elwha River, Washington

Sediment supply provides a fundamental control on the morphology of river deltas, and humans have significantly modified these supplies for centuries. Here we examine the effects of almost a century of sediment supply reduction from the damming of the Elwha River in Washington on shoreline position and beach morphology of its wave-dominated delta. The mean rate of shoreline erosion during 1939–2006 is ~ 0.6 m/yr, which is equivalent to ~ 24,000 m³/yr of sediment divergence in the littoral cell, a rate approximately equal to 25–50% of the littoral-grade sediment trapped by the dams. Semi-annual surveys between 2004 and 2007 show that most erosion occurs during the winter with lower rates of change in the summer. Shoreline change and morphology also differ spatially. Negligible shoreline change has occurred updrift (west) of the river mouth, where the beach is mixed sand to cobble, cuspate, and reflective. The beach downdrift (east) of the river mouth has had significant and persistent erosion, but this beach differs in that it has a reflective foreshore with a dissipative low-tide terrace. Downdrift beach erosion results from foreshore retreat, which broadens the low-tide terrace with time, and the rate of this kind of erosion has increased significantly from ~ 0.8 m/yr during 1939–1990 to ~ 1.4 m/yr during 1990–2006. Erosion rates for the downdrift beach derived from the 2004–2007 topographic surveys vary between 0 and 13 m/yr, with an average of 3.8 m/yr. We note that the low-tide terrace is significantly coarser (mean grain size ~ 100 mm) than the foreshore (mean grain size ~ 30 mm), a pattern contrary to the typical observation of fining low-tide terraces in the region and worldwide. Because this cobble low-tide terrace is created by foreshore erosion, has been steady over intervals of at least years, is predicted to have negligible longshore transport compared to the foreshore portion of the beach, and is inconsistent with oral history of abundant shellfish collections from the low-tide beach, we suggest that it is an armored layer of cobble clasts that are not generally competent in the physical setting of the delta. Thus, the cobble low-tide terrace is very likely a geomorphological feature caused by coastal erosion of a coastal plain and delta, which in turn is related to the impacts of the dams on the Elwha River to sediment fluxes to the coast.     

Comparison of short-term seacliff retreat measurement methods in Del Mar, California

Seacliff retreat has been variously characterized as the recession rate of the cliff top, of the cliff base, and as the bulk recession rate based on volumetric changes of the entire cliff face. Here, these measures of retreat are compared using nine semi-annual airborne LiDAR (Light Detection And Ranging) surveys of southern California seacliffs. Changes in the cliff base location (where the steeply sloping cliff face intersects the beach) include cliff retreat owing to basal erosion, but also reflect changes in beach sand level and basal talus deposits. Averaged over the 2.5 km alongshore study span, the cliff base actually prograded seaward about 12 cm during the 4-year study. Cliff top change was dominated by few, relatively large (several meters) localized retreats. Cliff face changes, that include failures and deposits anywhere on the cliff profile, had a relatively small mean magnitude compared to cliff top changes and were more widely distributed alongshore. However, the similar alongshore averaged, cumulative cliff top and net bulk cliff face end-point retreat (14 and 19 cm, respectively) suggest that mean cumulative cliff top retreat can potentially be a viable surrogate for mean net cumulative cliff-wide erosion (and vice versa) over relatively short time periods. Cliff face erosion occurred repeatedly at some locations, confirming the presence of seacliff erosion hot-spots during the study period.     

Cyclic and Seasonal Beach Response: A Comparison of Oceanside and Bayside Beaches

This study compares the cyclic and seasonal geomorphic responses of oceanside and bayside beaches on Sandy Hook Spit, New Jersey. It was hypothesized that the different nature of onshore wind regimes at bayside and oceanside beaches would cause different types of beach change. On the east-facing ocean shoreline, rates of beach change are related to weather patterns associated with the passage of mid-latitude cyclonic storms. Bayside beach change is related to the influence of the prevailing westerlies. Storm erosion and post-storm deposition is more rapid on oceanside beaches. Swell waves occurring between storms rapidly reinstate pre-storm equilibrium conditions. Lower bay-side wave energies occurring between storms have little effect on profile development, and foreshore slopes inherited from previous storms undergo little change. Bayside beaches therefore appear to be more in equilibrium with storm conditions than oceanside beaches.

Changes in foreshore slope, beach volume, and beach position associated with individual storm events are compared to seasonal trends to test the applicability of the Hayes cyclic model of beach development to both oceanside and bayside beaches. There is evidence of a cyclic trend of develop-ment on the more exposed ocean sites and seasonal development on the more sheltered bayside sites, which suggests that cyclic development may be causally related to the difference among the energy levels of the storm and nonstorm wave regimes.     

An approach to the problem of coastal erosion in quaternary sediments

Approximately 40 per cent of the shorelines of the lower Great Lakes are backed by relatively weak Quaternary sediments, and similar shorelines are found on many middle and high-latitude coasts. The high rates of bluff recession which are characteristic of these areas lead to economic losses through erosion of land and damage to buildings, and may prompt a wide range of measures designed to reduce erosion and protect property. Assessment of the physical problem and of possible solutions to it can best be achieved through a sediment budget approach. This is illustrated through a case study of an area near Grimsby, Ontario. The volume of sediment supply to the beach was calculated from measurements of bluff height and annual recession rates. The potential volume of longshore sediment transport was determined from the wave climate of the area and computer modelling of wave refraction. The relationship between sediment supply and potential transport rate can be used to assess the magnitude of the sediment deficit or surplus at points along the shoreline, and this provides an insight into the controls on alongshore variations in recession rates.     

An empirical–conceptual gully evolution model for channelled sea cliffs

Incised coastal channels are a specific form of incised channel that are found in locations where stream channels flowing to cliffed coasts have the excess energy required to cut down through the cliff to reach the outlet water body. The southern coast of the Isle of Wight, southern England, comprises soft cliffs that vary in height between 15 and 100 m and which are retreating at rates ≤ 1.5 m a^− 1, due to a combination of wave erosion and landslides. In several locations, river channels have cut through the cliffs to create deeply (≤ 45 m) incised gullies, known locally as ‘Chines’. The Chines are unusual in that their formation is associated with dynamic shoreline encroachment during a period of rising sea-level, whereas existing models of incised channel evolution emphasise the significance of base level lowering. This paper develops a conceptual model of Chine evolution by applying space for time substitution methods using empirical data gathered from Chine channel surveys and remotely sensed data. The model identifies a sequence of evolutionary stages, which are classified based on a suite of morphometric indices and associated processes. The extent to which individual Chines are in a state of growth or decay is estimated by determining the relative rates of shoreline retreat and knickpoint recession, the former via analysis of historical aerial images and the latter through the use of a stream power erosion model.     

Erosion and reorientation of the Sapodilla Cays,Mesoamerican Reef Belize from 1960 to 2012

The Sapodilla Cays Marine Reserve in southern Belize includes nine low-relief sand cays that were first surveyed in 1960. The purpose of this study is to reconstruct a 52-year history of the Sapodilla Cays (1960–2012) using a combination of historical topographic surveys, satellite imagery, and additional field data collection. Results suggest that the majority of islands are eroding with some islands having lost over 70% of their area, and many have become swash aligned, which suggests limited sediment availability. The proportion of area lost on each island is related to the width of the reef platform (to the 5?m isobaths) in the direction of the reef edge, while island area is dependent on the width of the reef platform in the direction of the resultant wind. This suggests that the width of the reef platform is a primary determinant of sediment supply between storms that tend to erode the eastern shoreline of the island through refraction along the reef edge. While storm erosion tends to be concentrated along the eastern shoreline through the loss of sediment offshore, alongshore transport to the lagoon shoreline, and the transfer of sediment to the interior of the island, net shoreline retreat is greatest along the lagoon, suggesting that the relatively small winter “northers” and a lack of sediment supply from the reef lagoon are responsible for the observed erosion. Extrapolations based on contemporary loss-rates suggest that the smallest cays will disappear by 2020, while the largest cays will begin to disappear by the end of century.     

Sea cliffs resulting from late Miocene extensional tectonics: the Serra Gelada case study (Betic Cordillera, Spain)

The Serra Gelada sea cliffs are carved in Mesozoic carbonate rocks belonging to the External Zones of the eastern Betic Cordillera (Alicante, SE Spain). Several normal faults with vertical slips of more than a hundred metres have played an important role in the origin of this coastline. Some previous studies propose that the present cliff morphology was mainly originated by Quaternary fault activity. However, the integration of geomorphological features, stratigraphical and sedimentological data, together with the results of the tectonic analysis of fractures occurring in Serra Gelada, and a detailed study of seismic reflection profiles carried out in the adjacent continental shelf, indicate that these normal faults were active mainly during the late Miocene. Therefore, the Serra Gelada sea cliffs represent a tectonically controlled long-term landscape. Thus, normal faults have not significantly modified the Serra Gelada relief since then. Furthermore, the northern part of the Serra Gelada cliff may be considered as an inherited pre-Quaternary relict palaeocliff since it has only undergone very little erosive recession.     

Shore platform development around Lord Howe Island, southwest Pacific

This paper describes the morphology of near-horizontal basalt and calcarenite shore platforms around Lord Howe Island as well as the lithological and process environment in which they occur. The morphology of platforms around the island is highly varied. For instance, they occur at a wide range of elevations between low-tide level and several metres above the highest tides, and their width ranges between just a few metres to over 100 m. However, common to most platforms is that they have near-horizontal surfaces that terminate abruptly in a steep scarp at their seaward edge. Correlations indicate that the variability in platform elevation and width is attributable to variability in key parameters of erosion, such as rock resistance and shoreline water depth. Whereas wider platforms occur in rocks of lesser resistance, platform elevation is shown to increase both as rock resistance and the depth of water at the shoreline increases. Wave exposure has a significant control on platform width in some instances, but an over-arching relationship was not detected. Most platforms around the island appear to have developed over the Holocene and a conceptual model is proposed to account for the varied morphology of platforms that have developed over this period. Resistant plunging cliffs occur along about 18% of the rocky coastline of Lord Howe Island and represent the starting point for the model. Where cliffs have yielded to wave erosion platforms have been initiated at an elevation controlled predominantly by rock resistance and shoreline water depth. At Lord Howe Island narrow platforms up to about 20 m wide in exposed locations generally occur about 4 to 6 m above high tide level, and have apparently been subject to very infrequent erosion. Shore platforms about 20 to 40 m wide have generally formed in less resistant rocks, are somewhat lower at about 2 to 3 m above high tide level, and have been subject to more frequent erosion over the Holocene. Most basalt platforms around the island fall within these first two categories, but there are some instances of platforms about 50 m wide. These platforms have formed in rocks of comparatively low resistance and are relatively low in elevation occurring close to the level of high tide. An interesting feature of some of these wide platforms is that, unlike the morphology of narrower platforms, raised ramparts sometimes occur on their outer edge.     

BEACH RIDGES AND PALEOGEOGRAPHY,CENTRAL BAJA CALIFORNIA,MEXICO

Two spatially separated sets of beach ridges were studied in Baja California. Each set contains several well-preserved beach ridges and each is adjacent to an arroyo mouth. Modern beach ridges mark coastal location whereas abandoned paleobeach ridges record coastal change. Paleobeach ridges were dated by absolute and relative techniques for rate-change, chronology, and longshore correlation purposes. With time, shape becomes more planar, foreslope angle is reduced, and there is a progressive tendency for constituent shingle to disintegrate and develop both rinds and surficial roughness. One paleobeach ridge on the lowest emergent terrace was assigned a 1,150 ± 230 yr. B.P. radiocarbon date whereas another beach ridge 18 km farther north has a partially overlapping date. Older but undated beach ridges also exist on the lowest emergent terrace as well as on the highest persistent terrace. Those beach ridges on the highest persistent terrace are highly weathered and Pleistocene in age. All beach ridges have paleogeographic implications and suggest that shoreline orientation has been generally stable although its position has shifted laterally by up to 720m westward in the last 1,150 ± 230 years.     

Municipal initiatives for managing dunes in coastal residential areas: a case study of Avalon, New Jersey, USA

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).     

RATES OF SLOPE DECLINE,TALUS GROWTH AND CLIFF RETREAT ALONG THE SHOMYO RIVER IN CENTRAL JAPAN: A SPACE‐TIME SUBSTITUTION APPROACH

On the basis of a space–time substitution, rates of cliff retreat and talus development overthe pasttens ofthousands of years were quantified for valley‐side slopes along the Shomyo River with a known rate of waterfall recession. Detailed profiles of the valley‐side slopes were obtained at 19 sites along the river by map reading and field survey, and the morphologic characteristics of the slopes were then measured. By combining the locations of the slopes with the recession rate of Shomyo Falls, it is estimated that the growth rate of the talus slope is 3.9–7.2 mm/yr, the rate of decline of the cliff is 2.0–4.0 × 10^?4 deg/yr, and the retreat rate of the cliff top is 6.2–11.6 mm/yr.     

Tsunami as agents of geomorphic change in mid-ocean reef islands

Low-lying atoll islands appear highly vulnerable to the effects of climate change and extreme natural events. Potentially disastrous effects of future sea-level rise have been inferred in many studies, and the actual impacts of tropical storms on island destruction and formation have been well documented. In contrast, the role of tsunami in the geomorphic development of atoll islands has not been investigated. The Sumatran earthquake of 26 December 2004 generated a tsunami that reached the Maldives 2500 km away, with waves up to 2.5 m high. Observations on the geomorphic changes resulting from the tsunami are detailed here, based on pre-and post-tsunami profile measurements of island, beach and reef topography, and GPS surveys of the planform shape of islands and beaches of 11 uninhabited islands in South Maalhosmadulu atoll, Maldives. Erosional and depositional impacts were observed on all islands and these have been quantified. In general the changes were of a minor nature with a maximum reduction in island area of 9% and average of 3.75%. Rather, the tsunami accentuated predictable seasonal oscillations in shoreline change, including localised erosion reflected in fresh scarps and seepage gullies. Depositional features in the form of sand sheets and sand lobes emplaced on the vegetated island surfaces provide clear evidence that the tsunami waves washed over parts of all the islands. Both erosional scarps and overwash deposits were concentrated at the tsunami-exposed eastern sides of the islands. Impacts on leeward shores were primarily accretionary, in the form of spit and cuspate foreland extension. Whereas the nature and magnitude of intra-and inter-island impacts was variable, an east to west decline in aggregate effects was noted. Detailed consideration of the morphodynamic interaction between the tsunami waves and island morphology, show that this cross-atoll gradient resulted not just from the reduction in tsunami energy as it passed through the atoll, but also from variations in elevation of the encircling island ridge, and the quantity and distribution of sediment in the antecedent beach. A conceptual model identifying the sequence of changes to individual islands supports the observational data and the pattern of geomorphic changes resulting from the tsunami. This model leads to consideration of the longer-term impacts of the tsunami on the future stability of islands. Four scenarios are presented, each of which has a different island-beach sediment budget, and different relaxation time to achieve dynamic equilibrium.     

Bank recession processes,rates, and prediction,Lake Sakakawea,North Dakota,U.S.A.

Bank recession at twenty stations along the eastern end of 286 km long Lake Sakakawea, North Dakota, measured regularly between 1983 and 1986, showed a recession range from essentially zero to almost 9 m yr⁻¹ with an average of 1.6 m yr⁻¹. Between 1969 and 1979, rates were greater, the declining rate a normal consequence of the factors affecting bank recession. Rates of recession vary with seasons, with 78% of the annual recession occurring during the warmer season (May–October) as a result of wave erosion. The remaining 22% occurs during thaw failure in spring.Prediction of ultimate (500 yr) bank recessions attempted using historical recession data, the three years of detailed field measurements, and regression analysis of the erosion factors. An equation, presented here, requires field measurement of bank orientation and height, batch width and composition, and slope of the offshore area. Effective fetch must be measured, as well. The resulting equation for the monthly warm season recession rate is: Rs=154.9 −, (18.8 √ A+ 25.1 √ B +10.1 √ c +6.9 √ 0+5.0 √ E + 1.1 √ F) where: A = sine of angle between bank orientation and dominant wind; B = offshore slope angle; C = beach width; D = bank height; E = effective fetch; and F = percentage of coarse beach clasts. The colder season recession percentage is: Rw = Rs [(2.05D = 0.043G) − 2/100] where D is bank height and G the bank orientation with respect to sun. The total yearly predicted recession (cm yr⁻¹) is: Rt = 6 (Rw) + 6 (Rs) 6 being the number of months of “cold” and “warm” weather at Lake Sakakawea.For this rate to be projected into the future, it is necessary to assume a declining rate of recession, employing a parabolic trend. From this, the ultimate bank recession for Lake Sakakawea is predicted to range up to 542 m.     

Processes reshaping the Nile delta promontories of Egypt: pre- and post-protection

Shoreline positions established from beach profile surveys combined with wave data are jointly analyzed, as a function of their contribution to coastal processes, to investigate the interaction between waves, shoreline orientation and coastal structures along the Nile delta promontories, Rosetta, Burullus and Damietta. Repeated beach profile surveys along the promontory sectors (64 km long in total) have been analyzed to determine rates of shoreline changes prior to construction (1971–1990) and after construction of protective structures (1990–2000). The behavior of coastline pre- and post-construction indicates that coastal erosion fronting protective structures has declined in the case of the seawalls at the tips of the Rosetta and Damietta promontories, or has been partially replaced by sand accumulation in the case of detached breakwaters at Baltim (east of Burullus promontory) and at Ras El Bar (west of the Damietta promontory). As a consequence, downdrift erosion has been initiated in local areas adjacent to these structures in the direction of longshore sediment transport. The 5-km-long seawall protecting the Rosetta promontory has stopped the dramatic erosion of this highly eroded area (formerly shoreward retreated 88 m/year), with adverse local erosion at its west and east ends, being 3 and 13 m/year, respectively. Similarly, the 6-km-long seawall built on the eastern tip of the Damietta Promontory, still under construction, has nearly stopped the severe erosion, which was formerly 10 m/year. The detached breakwaters at both Baltim and at Ras El Bar have accumulated sand at accretion rates of 37 and 14 m/year, respectively. This sand accumulation is associated with downdrift erosion of 25 and 13 m/year at Baltim and Ras El Bar, respectively. Results reaffirm that the original erosion/accretion patterns along the Nile delta promontories have been reshaped due to the massive protective structures built during the last decade. This reshaping along the examined promontories is generally controlled by the temporal variability in the intensity and reversibility of wave directions and associated longshore currents, coastline orientation and by the existing coastal protection structures.     

Holocene sedimentation in Lake Winnipeg, Manitoba, Canada: implications of compositional and textural variations

Two seismic facies were recognized in the sedimentary sequence overlying acoustic basement in Lake Winnipeg. The upper facies, which overlies a regional unconformity, is termed the Lake Winnipeg Sequence. Based on the seismostratigraphy, lithostratigraphy, and radiocarbon dates of approximately 4000 and 7000 yr BP from material collected directly over the unconformity in the southern and northern parts of the lake, respectively, this facies has been interpreted as representing Holocene sedimentation. Results of compositional and textural analyses of the Holocene sediment (Winnipeg sediment) from thirteen long (>2 m) cores indicate a transgressional sequence throughout the basin. In the South Basin, the generally fining upward sequence is characterized at the base by silt-sized detrital carbonate minerals, quartz and feldspar which decrease in concentration upward. In this basin, the high carbonate content and V/Al and Zn/Al ratios are indicative of a Paleozoic and Cretaceous provenance for sediment derived from glacial deposits through shoreline erosion and fluvial transport, via the Red River. Sedimentation in the central part of the lake and the North Basin is attributed to shoreline erosion of sand and gravel beaches. Consequently, the texture of these sediments is generally coarser than in the South Basin, and the composition primarily reflects a Paleozoic and Precambrian provenance. The basin-wide decrease in Ca, total carbonate minerals, dolomite and calcite concentrations upward in the cores is reflected by a decrease in the detrital carbonate component in all but the most northern cores. Other basin-wide trends show an upward increase in organic content in all cores. An increase in grain size near the top of most cores suggests a major, basin-wide change in sedimentation within the last, approximately 900 years in the South Basin.     

Controls on coastal dune morphology, shoreline erosion and barrier island response to extreme storms

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.     

THE MARBLE BEACHES OF TUSCANY*

ABSTRACT. Beach‐nourishment operations designed to replace sediment lost through erosion change the identity and meaning of coastal landscapes. Seven beaches in Tuscany, nourished with marble‐quarry waste, reveal how an industrial byproduct is naturalized by particle rounding and sorting and can become a positive symbol of human‐altered nature. The marble was placed on formerly sandy beaches, resulting in different grain size and color of sediments, beach morphology, and value for human use. The abrasion rate of marble makes the nourished beaches unsatisfactory when viewed solely as protection structures, but the rapid particle rounding and aesthetic appeal of marble increase the acceptability of the beaches for recreation.