Barrier Islands

Algarve barrier islands: A noncoastal-plain system in Portugal. Journal of Coastal Research, 5(2): 239-261 (1989).
The small barrier island system of the Algarve (South Portugal) exhibits several unique characteristics that differ from more widely studied systems. The islands exist on a noncoastal-plain coast as a result of a platform on the inner shelf which acted as a cape during the sea level rise. Initial spits became islands as the platform was transgressed, and the islands are transgressive in appearance, although west-to-east lateral processes (inlet migration, spit growth) dominate. The islands exist under moderate to high wave energy and mesotidal conditions. Flood-tidal deltas are dominant and inlet migration is rapid. Aeolian and overwash processes are important to vertical growth of the islands. The largest islands have widened as a result of flood-tidal delta Incorporation. After incorporation, aeolian processes modify the delta surface into a dune field before stabilization by vegetation.
Backbarrier dissection by spring tide (4 m.) flooding works in opposition to the constructional effects of delta Incorporation, over-wash and dune formation. An extensive network of erosional channels has developed on backbarriers as a result of erosion by the ebbing flood. Channel location is controlled by antecedent morphology such as incorporated delta channels, interdune areas, and troughs between incorporated recurved spits. Once established, the channels incise, erode laterally and extend into the island, sometimes joining with overwash passes, and forming important conduits of sediment transport to the lagoon/marsh. The lagoonward sediment transport is partially offset by floating sand and shells carried toward the island interior on the incoming flood tide. Spring tide flooding also results in vegetation kills and groundwater contamination.

This study systematically classifies washover dynamics with reference to coastal changes along the Ria Formosa barrier islands (Southern Portugal). Identification of washovers using a sequence of 11 sets of aerial photographs dated between 1947 and 2001 allowed a classification to be developed based on: (1) overwash evolution (increasing, decreasing, or constant overwash processes); (2) the mechanisms promoting washover formation (exceptional to infrequent oceanographic conditions, washout processes, structural erosion, inlet dynamics, and human interventions); and (3) the mechanisms promoting washover cessation (berm development, structural erosion, dune development, inlet dynamics, and human interventions). A total of 369 different washovers were observed along the Ria Formosa barriers during the study period, with 209 washovers being formed in various types of dune morphology and 303 being obliterated. The number of washovers was relatively stable from 1947 to 1972, and increased dramatically between 1972 and 1976 probably as a result of the development of immature inlet margins and downdrift starvation. From 1976 to 2001, washover occurrences declined and their spatial dimensions decreased, leading to a decrease in overwash activity over this time. Overall, the dominant formation mechanisms of washovers in the Ria Formosa were inlet dynamics (accounting for 57% of washovers formed) and structural erosion (20%), with human intervention mechanisms accounting for 12%. The cessation of washovers was dominated by dune development (33% of the washovers obliterated) followed by inlet dynamics (24%) and structural erosion (19%), while human intervention mechanisms accounted for 13%. The classification should be of use for the coastal management of barrier systems including the definition of overwash-prone areas and the determination of the relative importance of the mechanisms contributing to washover formation and cessation.

This study systematically classifies washover dynamics with reference to coastal changes along the Ria Formosa barrier islands (Southern Portugal). Identification of washovers using a sequence of 11 sets of aerial photographs dated between 1947 and 2001 allowed a classification to be developed based on: (1) overwash evolution (increasing, decreasing, or constant overwash processes); (2) the mechanisms promoting washover formation (exceptional to infrequent oceanographic conditions, washout processes, structural erosion, inlet dynamics, and human interventions); and (3) the mechanisms promoting washover cessation (berm development, structural erosion, dune development, inlet dynamics, and human interventions). A total of 369 different washovers were observed along the Ria Formosa barriers during the study period, with 209 washovers being formed in various types of dune morphology and 303 being obliterated. The number of washovers was relatively stable from 1947 to 1972, and increased dramatically between 1972 and 1976 probably as a result of the development of immature inlet margins and downdrift starvation. From 1976 to 2001, washover occurrences declined and their spatial dimensions decreased, leading to a decrease in overwash activity over this time. Overall, the dominant formation mechanisms of washovers in the Ria Formosa were inlet dynamics (accounting for 57% of washovers formed) and structural erosion (20%), with human intervention mechanisms accounting for 12%. The cessation of washovers was dominated by dune development (33% of the washovers obliterated) followed by inlet dynamics (24%) and structural erosion (19%), while human intervention mechanisms accounted for 13%. The classification should be of use for the coastal management of barrier systems including the definition of overwash-prone areas and the determination of the relative importance of the mechanisms contributing to washover formation and cessation.

The asymmetric Sf. Gheorghe lobe is the only active lobe in the Danube delta associated with a river-mouth bar (and related barrier islands and spits) that has continuously displayed cyclic development during the last 1500 years. During the early cycles, the Sf. Gheorghe distributary experienced a significant increase in sediment load (by an order of magnitude) as a result of the successive avulsions occurred 1.5-1.2 ka, which redistributed more water and sediment captured from the neighboring branches of Împuţita (southern distributary of the Sulina arm) and Dunavăţ. Morphological and sedimentological analyses together with a newly obtained chronology throw light on the multiple ridgeset (10) structure of Sf. Gheorghe lobe, each of them (excepting the first one) following a common evolutionary pattern reflected by the cyclic succession of the recurring stages: i) subaqueous mouth bar building, ii) barrier island emergence, and iii) transformation into a barrier spit with several secondary spits. The spits become encased into the muddy deltaic plain as narrow sandy ridges building out on the downdrift side of the lobe as a barrier-marsh plain, whereas the updrift side constantly accreted fed by longshore currents, forming a classic beach ridge plain. The size of each ridgeset increased exponentially with every new cycle due to the constant lengthening of the coastline as the downdrift side of the lobe advanced seaward through a series of progressively larger similar quadrilaterals, yielding a corresponding geometric progression of the delta front size. Even though each newly formed ridgeset (cycle) had a longer lifespan (the latest cycles lasting 4-5 times longer than the first ones: 200-440 years versus 50-80 years), the evolutionary model remained unchanged as long as the balance between wave-and river-borne sediments, defined by the sedimentary index (Si), remained constantly low (Si ≤ 0.1), whereas the mean advance rates of the river mouth remained constant at c.10 m/year. Abrupt changes occurred within the last cycle (since the beginning of the 20th century) as a consequence of human-induced depletion of sediment supplied by the Danube flow, and is mainly expressed by the complete cessation of the updrift coastal progradation and the prevalence of erosion in front of the river mouth. These changes, which engendered a threefold increase of the Si (0.37 at present), are reflected by the recent (1930s-present) river mouth dynamics, characterized by cessation of its long-term seaward expansion in favor of downdrift migration. This is indicative of the transition of the Sf. Gheorghe mouth from an asymmetric to a deflected wave-influenced delta morphology whose current developments mark a significant change in the multicentennial cyclic evolutionary pattern. The asymmetric to deflected transition reflects the net increase in the influence of wave-driven sediment circulation on river mouth morphology, corresponding to a critical Si threshold of ca. 0.2 (≤0.2: asymmetric; N0.2: deflected).

The Ria Formosa barrier island system is the most remarkable physiographic unit of the Algarve, south Portugal. This system composed of sand spits, beaches and dunes constitutes a Natural Park that has undergone artificial nourishment under the pro- ject “Rehabilitation of the lagoon system of Ria Formosa”. The main objective of this study is to evaluate the effects of a beach and dune rehabilitation program in particle size and morphology in two contrasting sites of the system (Ancão Peninsula and Tavira Island). A monitoring program was performed from June 1999 until September 2000, in order to determine the mor- phological and grain size changes along these beaches, before and after nourishment. The results of particle size analysis show the predominance of coarse sand at Ancão Peninsula and medium sands in Tavira Island. Nourished beaches presented coarser sands and bimodality, due to the heterogeneity of the dredged sediment used for nourishment. The nourished profiles, with increased sand stock, quickly developed natural profile morphology and proved to be able to decrease the effects of acting storms, namely avoiding dune erosion and overwashes. The soft interventions conducted at Ria Formosa proved to be useful on protecting the natural system, being a positive indication for future interventions at sensitive areas prone to marine erosion.

Overwash-Prozesse und washover-Ablagerungen sind weltweit an vielen Küsten verbreitet und gehören zum geomorphologischen Formenschatz vieler Dünen- und Barriereinseln. Auf der Nordseeinsel Sylt sind rezente washover-Areale besonders im Bereich der südlichen
Inselspitze (Hörnum Odde) zu finden. Zwei während der Sturmflutsaison 2006/2007 aktive washover channel an der Westküste der Hörnum Odde wurden im Vorfeld deskriptiv
geomorphologisch analysiert und für Georadarmessungen (GPR) ausgewählt. Um an diesen Stellen eine Abtrennung der südlichen Oddespitze zu verhindern, wurden Sandersatzmaßnahmen
in Form von zwei die Eingänge der Dünentäler schließenden Dünenverwallungen durchgeführt. Auf diese Weise wurden die washoverbedingten Sedimentstrukturen vor anschließender bioturbater, äolischer und sturmflutbedingter Aufbereitung bewahrt. Auf der Grundlage der ursprünglichen, durch den overwash-Prozess geschaffenen Sedimentstrukturen, wurden typische Reflexionsmuster und Reflexionsgeometrien analysiert, die eine Identifizierung von washover-Sedimenten und -Strukturen im Radargramm erlauben. Zu
den typischen Reflexionen zählen konkave, rinnen- bis wannenförmige Reflexionen die eine Geländedepression abbilden und morphologisch einem washover channel entsprechen. Der Talverlauf wird durch einen Reflektor mit starker Amplitude markiert. Dem Querschnitt des
Geländeverlaufs folgend werden annähernd horizontale, subparallele Sedimente abgelagert. An der Basis sowie an den Rändern des washover channel sind im Radargramm vermehrt Diffaktionshyperbeln nachzuweisen, die das dort abgelagerte Treibgut abbilden. Die Daten
zeigen, dass die Georadarmethode bei der Untersuchung von washover-Strukturen und Sedimenten bedeutende Vorteile bietet. Die Ergebnisse lassen sich zudem zur Identifizierung und Abgrenzung unbekannter washover-Sedimente und Strukturen im sandigen Sedimentationsraum einer Dünen- und Barriereinsel, die aus dem gleichen geomorphologischen Prozesssystem hervorgehen, heranziehen.

Tillmann T. & Wunderlich, J. (2013): Washover-Events im Süden der Insel Sylt. Geo-Öko, 34: 177-215.

Weblink:
https://www.uni-goettingen.de/de/2013/479170.html

The aim of the present paper is to analyse the recent morphological evolution of the sandy barriers of Ria Formosa, a multi-inlet system located in South Portugal, to assess evolution regimes and related controlling factors and to identify resilience mechanisms in response to natural and artificial drivers of change. The data collected comprise aerial photographs and wave buoy and hindcast time-series, covering the period from the 1950s to 2014. The results show that the barriers have either been growing, or remaining stable. The growth patterns were either promoted by natural mechanisms, or triggered by stabilization works and supported by natural factors (e.g. longshore transport, shoal attachment). The presence of a broad marsh platform in the backbarrier was found to promote barrier stability, while the sustainance of transgressive barriers is advocated by frequent overwash, combined with low depths in the backbarrier lagoon and localised replenishment of sand. These long-term evolution regimes and their relation to artificial and natural factors show that the barriers of Ria Formosa have been resilient during the time-frame of the study, either absorbing disturbances (Armona and Tavira), or adapting to change while maintaining their functions (rest of the barriers).

Hurricane flooding is a leading natural threat to coastal communities. Recent evidence of sea level rise coupled with potential future global warming indicate that sea level rise will accelerate and hurricanes may intensify over the coming decades. In regions fronted by barrier islands, the protective capacity of these islands may diminish as they are degraded by rising sea level. Here we present a hydrodynamic and geospatial analysis of the relative role of barrier island degradation on potential future hurricane flooding. For the City of Corpus Christi, Texas, USA, hurricane flooding is projected to rise between 20% and 70% by the 2030s, resulting in an increase in property damages and impacted population. These findings indicate that adaptive management strategies should be developed and adopted for mitigating loss of natural barrier islands when these islands act as protective features for populated bayside communities. Finally, this study illustrates a method for applying models to forecast future storm protection benefits of barrier island restoration projects.