Littoral Drift System Research Articles

Changes in the Littoral Drift System of the Uruguayan Coast during the Holocene and its Influence in the Continuing Erosion in Southern Brazil

Dillenburg, S.R.; Barboza, E.G.; Rosa, M.L.C.C.; Caron, F., and Bitencourt, V.B., 2020. Changes in the littoral drift of the Uruguayan coast during the Holocene and its influence in the continuing erosion in southern Brazil. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 453–457. Coconut Creek (Florida), ISSN 0749-0208.In northern Uruguay and southern Brazil, a littoral drift system dominates from southwest to northeast. This littoral drift is a consequence of higher wave-energy, generated in southern latitudes and that has been operating on a long-term scale, playing an important role not only in the evolution of coastal barriers of this region during the Holocene but also in coastal erosion that presently occurs in some parts of this coast. From La Coronilla to Cabo Santa Marta, which is a coastal sector 750 km - along the coast is gently undulating, showing gentle projections located to the south of gentle reentrances, that are dominated by transgressive/stationary and regressive barriers, respectively. In general, the regressive barriers have been developed by sediments brought into the reentrances by long-shore currents. The main source of these sediments had been the transgressive/stationary barriers that occur along the coastal projections. This differentiation of barrier behavior started at around 6-5 ka, at the end of the Postglacial Marine Transgression (PMT), when all rivers were drowned, finishing the delivery of new coarse sediments to the coast. Also, at this time the large rock promontories of the Uruguayan coast were formed and started to retain sediments of the littoral drift system, triggering a long-term coastal erosion in the southernmost coastal projection. Regarding erosion that is occurring in this stretch of coast the important issue to be considered in future coastal planning is that erosion will continue to operate in the long-term but will also be (and certainly has been already) amplified by sea-level rise in the next centuries as projected by the Intergovernmental Panel on Climate Change (IPCC).

Tidal strait to embayment: Seismic stratigraphy and evolution of a rock-bounded embayment in the context of Holocene sea level change

Elongate, rock-framed Santa Catarina Island partly encloses a low-energy marine embayment that communicates with the ocean via narrow, rock-bounded inlets with ebb and flood-tide deltas. The bay shoreline consists of low-energy sandy beaches, strandplains, mangroves and bayhead deltas while the exposed Atlantic shoreline comprises a series of headland-embayment littoral cells in a linked, north-directed longshore drift system.New and previously published seismic stratigraphic investigations enable assessment of the palaeogeographic/geomorphic evolution of the area since the early Holocene. At a sea level of ca. −50 m a sandy mainland-attached shoreline complex (dune and beach) developed on a coastal plain setting some 6–7 km offshore of the present shoreline. North of the island, a barrier island and tombolo were formed in a coastal re-entrant adjacent to the northern entrance to Florianopolis Bay. The open coast beach/dune system was overstepped by rapid sea-level rise during MWP-1B whereas the barrier island and tombolo continued to accrete until MWP-1C when they too were overstepped.The back-island embayment contains up to 40 m of sediment overlying an irregular bedrock surface. It was initially flooded during MWP-1C and the early muddy sedimentation suggests a distal, sheltered marine embayment. The subsequent development of many large bioherms on bedrock and mud suggests a change to more fully marine conditions as sea-level continued to rise. In the North Bay, these were subsequently enveloped by tidal dunes migrating north from the mid-bay. The vertical stacking of 3-D and 2-D sandy bedforms suggests that a tidal strait had developed with strong tidal currents radiating from a constriction in the vicinity of modern Florianopolis City. The strong tidal currents may be linked to the Holocene sea-level high that is well documented for this region.A fall in sea level from this highstand, coupled with accumulation of tidal strait sediments, reduced the tidal prism and strength of tidal currents in the bay. At the same time, regressive beachridge plains developed in open coast and bay environments. This coincided with development of the contemporary tidal inlet complexes at the north and south outlets of the bay. As a consequence, the upper surface of the tidal strait sand deposits experienced tidal ravinement and the bay switched to a sheltered back-barrier environment dominated by deposition of mud and silt which blanket the contemporary bay floor. The tidal inlets sequestered sand from the littoral drift system in ebb and flood-tide deltas, which created breaks in the longshore drift system, causing littoral sediment to accumulate at the north of the island in a series of spits, and promoting strandplain development south of the island.

Distribution of clay minerals in marine sediments off Chennai, Bay of Bengal, India: Indicators of sediment sources and transport processes

Clay mineralogy, texture size and statistical analyses were carried out on surface sediments from the continental shelf of Chennai, Bay of Bengal, India. The purpose of this study is to characterize the clay mineral distribution and its relation to the hydrodynamics off Chennai to identify the sources and transport pathways of the marine sediments. Characterization of clay minerals in coastal sediments by Fourier Transform Infrared (FTIR) spectroscopy has provided the association of quartz, feldspar, kaolinite, chlorite, illite and iron oxides (magnetite and hematite) derived from river catchments and coastal erosion. Kaolinite, chlorite, illite, iron oxides, and organic matter are the dominant minerals in Cooum, and Adayar region. High quartz and feldspar zones were identified in Marina, which are being confined the sand zone and paralleling the coast. The strong relationships among the wave energy density, sand, quartz and carbonate revealed that wave induced littoral drift system play a dominant role in transportation and deposition of sediments in the Chennai coast. The sediment texture and minerals data are in agreement well with the previous results of hydrodynamics and littoral drift models in this region. Multivariate statistical analyses (correlation, cluster and factor analyses) were carried out and obtained results suggested that clay minerals and organic matter are trapped in silt and clay particles, whereas quartz, feldspar and carbonate are associated with sand particles. Results of sediment sources and transport processes from this study will be useful to predict the fate of the pollutants released from land or the potential change in sediment delivery to coastal areas.

Decadal-scale impacts of a segmented, shore-parallel breakwater system

Between 1994 and 1997 nine segmented, shore-parallel, rock-mound breakwaters, were constructed at the meso-tidal beach of Sea Palling, on the North Sea coast of the UK, to provide protection for a low lying hinterland vulnerable to storm surge inundation and a 3.5km section of sea wall that was in danger of collapse due to low beach levels. In this paper we assess the effectiveness of these breakwaters, and of the associated beach recharge events, in stabilising the beaches both within and around the breakwater system, and we examine their impact on the wider littoral drift system. We discuss a ‘trapping’ mechanism whereby sand entering the system as littoral drift is effectively retained at both ends of the system. At the northern (updrift) end, the trapping of littoral drift sediments, aided by a 1.3×106m3 sediment recharge designed to remedy severe gap erosion, lead to the growth of salients into tidal tombolos (effectively changing the X/h ratio (distance offshore/depth) on Pope and Dean's (1986) morphology-existence diagram). The largest tombolo (Tombolo 5) is at the updrift end, is only inundated during large storm surges, and forms a 260m wide barrier to littoral drift and the alongshore supply of sediment to breakwater beaches. In response to a large reduction in littoral drift, shorelines in the centre of the system are steadily retreating, with some embayment shorelines being closer to the seawall in 2005 than they were pre-construction. These shorelines have not reached a steady-state and further recharge will be needed unless the littoral drift supply can be restored. The evidence (nearshore morphology, sediment starved beaches, down-drift recharge, shoreline change patterns and a simple sediment budget) indicates that an estimated 80% of littoral drift sediments are directed offshore at Tombolo 5, bypass the breakwaters and downdrift beaches, but may return to shore ~2.5km downdrift. As a direct result of recharge, Sea Palling makes a less-than-ideal case for assessing the applicability of micro-tidal shoreline response equations to breakwaters in tidal settings. The Sea Palling experience also demonstrates that the use of beach recharge in combination with breakwaters in settings of high littoral drift requires very careful consideration as it has the potential to block littoral drift and starve the local beaches — the opposite effect of that intended by both techniques.

The geomorphology and evolution of a large barrier spit: Farewell Spit, New Zealand

AbstractFarewell Spit is a 25 km long barrier spit that marks the end of a littoral drift system, almost 1000 km in length that runs along South Island, New Zealand. The spit is composed of barchan dunes over 20 m high, sand sheets over 1 km wide and vegetated linear dunes. Analysis of aerial photography indicates a rapid colonization of the spit by vegetation which has expanded in area by 75% since 1950. Vegetation colonization preferentially occurs on the southern side of the spit, with its northern margin characterized by barchan dunes which migrate at rates of up to 64 m/yr. Sand sourced from longshore drift appears to be the primary source of beach sediment, which is then transported into the dune field by the persistent westerly winds of the Roaring 40s. While there has been significant dune roll‐over on the surface of the spit, its overall area has remained much the same for the past 54 years. Occasional cyclone events cause erosion, but this is balanced by aeolian sediment transport. It would appear that extension of the subaerial portion of the spit is related to the development of shells banks at its downdrift end which are periodically welded to the main spit by dune extension. Farewell Spit therefore provides an ideal example of a barrier environment where longshore sediment supply and aeolian transport dominates geomorphic evolution. This differentiates the study site from other barrier environments where overwash or tidal inlet development often characterizes recent landform evolution. Copyright © 2010 John Wiley & Sons, Ltd.

Ebb-Jet Dynamics and Transient Eddy Formation at Tauranga Harbour: Implications for Entrance Channel Shoaling

Abstract In 1992 the entrance channel through the tidal inlet to Tauranga Harbour, which is located along the Bay of Plenty littoral drift system, was deepened from 10 m to 14 m. The deepened channel has become a sediment trap for littoral drift bypassing and tidal current driven sediment transport through the inlet. Since 1992, there has been an increase in maintenance dredging requirements at the inlet, because of sand accumulation along the southeastern border of the entrance channel. Previous studies have identified an ebb tide–induced eddy operating on the eastern side of the ebb-jet as it exits the tidal gorge. In this article, the eddy system has been simulated with a validated two-dimensional hydrodynamic model, detailing time-varying current patterns over the ebb-tidal delta. Particular emphasis is placed on defining the trajectory of the eddy and evaluating its influence on the observed sedimentation patterns. The model results indicate the formation of opposing eddies on either side of the entr...

戻り流れや海浜勾配の影響を取り入れた漂砂量公式の提案

A numerical model for 3-D beach deformation has been developed to evaluate the amount of sediment transport rate in the littoral drift system. The model includes the sediment transport rate formula under the coexistence of waves and currents, considering the effect of bottom slope on the sediment movement. In case of a longshore uniform beach, this model has a solution that total amount of littoral drift is proportional to the longshore component of wave energy flux, similar to existing longshore sediment transport rate formulas. Numerical calculations are conducted to estimate the trapping rates of longshore sediment transport due to a groin or detached breakwater. Furthermore, wave height distributions of irregular wave and bottom friction components in nearshore zone are calculated to evaluate the sediment transport rate vector and total longshore sediment discharge on a bar type beach with a longshore undulation.

Sediment trend analysis of the hylebos waterway: Implications for liability allocations

Sediment trend analysis of the hylebos waterway: Implications for liability allocations

ESTRATIGRAFIA E EVOLUÇÃO DA BARREIRA HOLOCÊNICA DO RIO GRANDE DO SUL NO TRECHO TRAMANDAÍ-CIDREIRA

A barreira costeira de idade holocênica (sistema barreira-laguna IV), no Rio Grande do Sul, não evoluiu de um modo uniforme nos últimos 6 – 5 ka, ao longo dos seus 620km de extensão. O segmento da barreira investigado neste trabalho situa-se entre dois outros segmentos, um ao norte e outro ao sul, onde a barreira holocênica apresenta um caráter regressivo e transgressivo, respectivamente. Com base em dados morfológicos, litológicos de sub-superfície, datações por 14C e simulações de evolução costeira, foi definida uma natureza transgressiva para este segmento da barreira, porém não uniforme ao longo de seus 30 km de extensão, e estabelecida a sua cronologia evolutiva. Em Tramandaí, a barreira permaneceu relativamente estável nos últimos 6 – 5ka, enquanto que em Jardim do Éden e Cidreira a barreira apresentou um comportamento dominantemente transgressivo. Estas diferenças evolutivas são atribuídas a fatores locais que influenciaram no balanço de sedimentos. Como principal fator local é destacada a declividade diferenciada dos substratos da barreira em Tramandaí (0,062°), Jardim do Éden (0,067°) e Cidreira (0,069°), a qual, por sua vez, influenciou nas taxas de translação da barreira durante a elevação do nível do mar, e na energia das ondas incidentes no sistema praial.

Leaky Valves in Littoral Sediment Budgets: Loss of Nearshore Sand to Deep Offshore Zones via Chutes in Barrier Reef Systems, Southeast Coast of Florida, USA

The decrease in longshore sediment volumes from north to south, calculated for littoral cells between inlets, indicates significant loss from the littoral drift system along the Florida east coast or localized deposition. The decrease in littoral drift volume from about 490,000 m3 of sediment over a distance of about 550 km (average diminution rate of about 890 m3 km−1) was traditionally accounted for in terms of alongshore deposition and some unknown degree of speculated offshore transport. With the application of laser airborne depth sounding (LADS) technology to bathymetric survey in southeast Florida (2001–2003), recognition of detailed bottom topography became possible for the first time. Significant in this regard was the bathymetric display of macro- and mesoscale morphological features that showed areal continuity of sandy bottoms and the prominent but disjunctive occurrence of relict barrier coral reef tracts that line the shelf edge. Identification of gaps in the line of barrier reefs showed pathways of cross-shore sediment transport from the inner shelf to offshore beyond the shelf edge. Diabathic sediment transport through these reef gaps robs the littoral sediment package by allowing the chuting of sediments during high energy events or when the backreef flats over fill with sand. The reef gaps are thus leaky valves where littoral sediments are periodically chuted offshore and lost from the littoral drift system.

Sand barrier development in the bight of Benin, West Africa

The morphology, sedimentary facies, stratigraphy and development history of re-entrants and sand barriers in Togo and Benin (West Africa) have been analysed from boreholes and field data, and available radiocarbon dates. Re-entrant deposits range from basal Late Pleistocene fluviatile sands into Holocene estuarine and fluvio-deltaic sands. As sea level stabilized between 6000 and 5500 B.P., the “compartmented” coast associated with these re-entrants favoured a polycellular longshore drift system. As the latter became infilled throughout the bight of Benin between 6000 and 4000 B.P., the littoral drift system evolved into a unicellular system fed by the Volta delta in Ghana, which has been the major sand source for an almost 300 km long outer barrier. In spite of the massive and consistent volume of sand drifting eastward (1.2–1.5×10 6 m 3/yr), progradation slowed down or ceased as equilibrium was achieved between coastal/shoreface geometry and the hydrodynamic regime, resulting in a balance between incoming and outgoing sand, and the downdrift migration of the accumulation sector towards Nigeria. This situation was probably achieved around 1500 to 2500 years ago in Benin, but apparently occurred much earlier in the updrift sector in Togo. This stability has been perturbed by the construction of deepwater ports in 1962 in Cotonou and 1967 in Lomé leading to segmentation of the unidirectional drift and spectacular progradation and erosion, respectively, updrift and downdrift of breakwaters.

Stratigraphy and Holocene evolution of a regressive barrier in south Brazil

The Paranaguá coastal plain is a 600 km2 barrier system located in south Brazil (25° 30′ S) and is largely made up of Pleistocene and Holocene beach ridges. Sea level history in the area is characterized by a gradual, 3.5 m sea level fall in the last 5000 years. Several reverse-circulation drill logs from the Pleistocene and Holocene sectors allowed for the definition of a general morphostratigraphic model of the coastal plain, which proposes the staking of at least two generations of marine deposits upon continental and paleo-estuarine sediments. Analysis of shallow vibro-cores and exposures along back-barrier creeks indicates the existence of four transgressive (estuarine, estuarine channel, overwash and flood-tidal delta) and three regressive (beachface, nearshore and upper shoreface) sedimentary facies in the Holocene section of the coastal plain. The morphostratigraphy suggests that barrier emplacement took place at the last stages of the post-glacial marine transgression, when the back-barrier estuary was squeezed in between the Pleistocene and Holocene barriers. Holocene barrier progradation was significant in both the shore-normal and longshore direction, and was apparently very swift in the last thousand years corresponding to a period when sea level fell by 1.5 m, and when about half of the present barrier volume was accumulated. Higher progradation rates are ascribed to the arrival of sediment yielded by an active littoral drift system, which appears to have eroded large extents of several Holocene barriers along 100 km of coastline.

Les cordons de galets de la Baie des Anges : histoire environnementale et stratigraphique ; évolution morphodynamique récente en réponse à des aménagements/The gravel barriers of the Baie des Anges (French Riviera): environmental and stratigraphie history; recent morphodynamic responses to coastal management

Abstract The gravel barriers of the Baie des Anges (French Riviera) protect the densely developed area of Nice against marine flooding. The general setting of these barriers is that of a steep coastal margin and shelf, and a fetch-limited, low wave-energy, almost tideless environment. In terms of sequence stratigraphy, these gravel barriers, which have yielded radiocarbon ages ranging from 3000 yr B.P. to Present, are part of a highstand systems tract comprising essentially gravelly river-channel and finer-grained floodplain, delta-plain and nearshore deposits. The local sea-level curve shows a strong slow-down in the Holocene transgression at around 4000 yr B.P. This change coincided with active upland erosion by mountain streams exhibiting catastrophic flow regimes that resulted in downstream bedload channel aggradation and seaward flushing of gravel clasts. The present gravel barriers, which may have been formed and dismantled successively during the eustatic transgression, are an outgrowth of these bioclimatic conditions and of the change from transgression to stillstand. Once gravel supply from river mouths to the littoral drift system became efficient, the gravel barriers became consolidated by bi-directional longshore drift with changing drift termini, in response to short, poorly refracted waves coming from a wide zone spanning east to south-west. The barriers seem to have been morphodynamically stable forms as a result of their highly gravelly composition, relative sea-level stability, and the almost tideless, low energy and fetch- limited hydrodynamic setting of the bay. Stability was enhanced by crestal buildup, a drift-aligned plan-form configuration that enabled longshore transfer of fluvial sediment to make good losses to the steep nearshore shelf, and a textural organisation that favoured segregation between coarse and fine clasts, leading, in the long run, to relative enrichment in coarse clasts and resistance to change. The barrier sediment budgets were balanced, with gains from the coastal rivers compensating for the losses to the steep nearshore zone, which also considerably limited progradation. Over the past century, human activities have resulted in the destabilization of the barrier beach sediment budgets, initiating a phase of slow, uneven erosion and textural disorganisation. To counter this erosion, gravel nourishment has been practised on certain portions of beach in the last twenty years. Optimization of these periodic nourishment campaigns however requires not only a fine understanding of the environmental context and mechanisms of morphodynamic development of these gravel barriers but also of the hydrodynamic processes involved in their breakdown.

Holocene evolution of the gravel coastline of East Sussex

The East Sussex coastline has been a sedimentary sink during the Holocene. Therefore, variations in the type and quantity of sediment transported within the coastal system may have been the principal factors determining stratigraphie sequences. The variations in sediment supply have found a morphological expression in periodic prograding and retrograding of coastal barriers. An examination of the origin and development of the present gravel barrier beaches indicates that much of the beach sediment probably has its origin in the Pleistocene, while during the Holocene, variations in the littoral drift system and associated changes in geomorphic processes, especially between reflective and dissipative domains, have exerted a major control upon coastal evolution in East Sussex.

Barrier Island Evolution and Reworking by Inlet Migration along the Mississippi-Alabama Gulf Coast: ABSTRACT

ABSTRACT The five barrier islands along the Mississippi-Alabama coast are located 10 to 14 miles (16 to 23 kilometers) offshore and separate Mississippi Sound from the Gulf of Mexico. The barrier islands in the chain are, from east to west: Dauphin Island, Petit Bois Island, Horn Island, Ship Island, and Cat Island. The islands are low sand bodies situated on a relatively broad Holocene sand platform that extends 70 miles (113 kilometers) from Dauphin Island on the east to Cat Island on the west. The platform varies in thickness from 25 to 75 feet (7.6 to 23 meters) and rests on Holocene marine clays or on Pleistocene sediments. The barrier islands are nourished chiefly by littoral drift from sand sources to the east. The barrier island chain predates the St. Bernard lobe of the Mississippi delta complex, which began to prograde about 3,000 years ago, and continued until it was abandoned approximately 1,500 years ago. In contrast to the other islands, Cat Island at the western down-drift end of the Mississippi-Alabama barrier island chain, is characterized by more than 12 prominent east- west oriented progradational linear ridges. The ridge system of Cat Island is interpreted as a relict of an earlier stage in the life cycle of the barrier platform when there was a more robust littoral drift system and an abundant sediment supply. During the Pre-St. Bernard Delta period of vigorous sedimentation, the islands in the barrier chain probably exhibited progradational ridges similar to those now found only on Cat Island. Presently, only vestigial traces of these progradational features remain on the islands to the east of Cat Island. Unlike Cat Island, which has been protected and preserved by the St. Bernard Delta, the other barrier islands have been modified and reworked during the past 1,500 years by processes of island and tidal inlet migration, accompanied by a general weakening of the littoral drift and a reduction of the available sediment supply. The four tidal passes, or inlets, that separate the barrier islands are broad low areas in the Holocene barrier island sand platform, ranging from 3.5 to 5.8 miles (5.6 to 9.3 kilometers) in width. The water depth in the passes is generally shallow, less than 15 feet (4.6 meters), except in the tidal channels where natural depths of more than 30 feet (9 meters) are often reached. Due to the water depths reached by the tidal channels, the nearly the full thickness of the Holocene island platform east of Cat Island has likely been reworked during the migration of the islands and tidal passes. Additionally, the down-drift islands have become increasingly sediment-starved due to sediment losses in the island passes. Shoals and lobes of sand, commonly found on the north (Mississippi Sound) side of the barrier islands, are interpreted to be the remains of interchannel shoals that mark former tidal inlet positions.

Sedimentary processes operating around the entrance to a river mouth port, Westport, New Zealand

Abstract The paper presents investigations of wave climate, tidal inlet hydraulics, and sand sediment bypassing at the entrance to Westport Harbour, South Island, New Zealand. The results complement and extend those of studies of bar morphologies and sediment characteristics already published. Longshore transport of about 1 × 106 m3/year is directed in a net eastward fashion across the inlet because of an in‐built misalignment of the harbour training walls. Approximately 90% of the drift is bypassed, and has been since 1921, by deflection and splitting of the main sediment streams through the inner and outer bars and a transverse channel aoross the entrance. The outer bar appears to be the submarine, downdrift extension of Carters Breach and river load appears to contribute an order of magnitude less sediment to the complex than annual littoral drift. River sediments and littoral drift are mixed off the harbour and a declining proportion over time is recirculated to cause progradation of North Beach. The tidal compartment contributes little to scour of the entrance because of the predominance of bar bypassing. Contrary to the recommendations of several past studies, it is argued that improvements in navigation depths at West‐port are more likely to be obtained through modification of the littoral drift system than they are from tidal compartment enlargement.

K-type coefficients in the littoral drift system: Discussion

K-type coefficients in the littoral drift system: Discussion

K-type coefficients in the littoral drift system: Variable or constant?

The analysis by Tanner (1979) of the k-coefficient in the littoral drift equation, which led him to believe that it varies between 1.0 and 10 −5, is concluded to be invalid. k appears to be relatively constant for sandy ocean beaches, perhaps decreasing somewhat for gravel beaches or for sandy beaches with much reduced wave energy levels. k should not be made a function of the ratio of the net littoral drift to the gross littoral drift as Tanner has done.

K-type coefficients in the littoral drift system

The quantity q (volume of sand transported past a given point, per unit time; L 3T −1, can be related to P L (the littoral component of wave power, per unit length of coast; MLT −3) by using the expression k γρa (where gamma is the acceleration of gravity, rho is the mass density of sediment, and a is a packing coefficient). In this third expression, three of the values can be taken as constants which provide that the resulting equation will be dimensionally homogeneous: q = kP L/γϱs a The coefficient k is, therefore, dimensionless. Its value, as reported in the literature, ranges from numbers close to (but smaller than) 1.0, down to numbers such as 10 −4 and 10 −5. If one wishes to use P L in order to predict q, and hence dq dx (erosion or deposition, according to the algebraic sign on the derivative), there must be a better assessment of k than a range of four, five, or six orders of magnitude. The coefficient k was originally defined in an instantaneous sense, so it can have values greater than 0.1. For long term calculations, a similar coefficient can be defined as: In K = C t 1P 1 t 2P 2 − 1.0 where C can be taken as −11, and the tP-ratio expresses the extent to which wave power is cancelled in a bidirectional drift system, thus producing values of K in the general range down to 10 −5 or perhaps smaller.

Morphology and dynamics of a barrier breach: a study in stability

A small barrier breach in Kouchibouguac Bay, New Brunswick, is examined with respect to: (a) origin, (b) morphological development, and (c) hydraulic characteristics. The small tidal inlet was formed in 1970 by storm conditions which have a return period of between 5 and 12 years, based upon two different storm intensity indices. Barrier breaching is not a catastrophic event in the temporal sense in the bay and the return period of the initiating storm conditions is close to the average interval between breaching episodes for this barrier system. A threshold barrier morphology is suggested as a more important variable than storm magnitude. Traced from its inception to closure (a period of 6 years), the barrier breach cited revealed: (a) flood domination generating a transfer of large volumes of sedimentary material from the littoral drift system into the lagoon, (b) gradual reduction in tidal prism associated with flood delta growth and infilling of tidal channels, and (c) migration (30 m year−1) and increasing reorientation of the inlet neck under a strong preferred littoral drift direction.