Downdrift Coast Research Articles

Impact of ebb-delta dynamics on shoreline evolution along inlet-interrupted coasts

Shorelines adjacent to tidal inlets are highly dynamic landforms affected by oceanic (e.g., sea-level rise) and terrestrial (e.g., fluvial sediment supply) processes. Climate change is thus expected to have substantial physical impacts on these inlet-interrupted coasts. Numerical simulation of such impacts requires a holistic approach where at least the major governing processes that affect the local sediment budget are considered. The Generalized-Scale-aggregated Model for Inlet-interrupted Coasts (i.e., G-SMIC) is such a model that is capable of holistically simulating the evolution of inlet-interrupted coasts over multi-decadal to century time periods. However, in its present form, G-SMIC does not consider the effects of ebb-delta dynamics in its computations. Here, we improve the model to include ebb-delta dynamics and pilot the improved model (G-SMIC+) at two selected case study sites in Vietnam (Thu Bon estuary) and Wales, United Kingdom (Mawddach estuary). Model hindcasts of G-SMIC+ at both case study locations show reasonable agreement with available records of shoreline variations. The evolution of the two inlet-estuary systems was assessed over the 21st century under four of the IPCC’s sixth assessment report climate scenarios (viz., SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5). Results show that both systems switch between sediment exporting and sediment importing systems over the study period (2031 - 2100). Moreover, while the inclusion of ebb-delta dynamics may decrease the erosion volumes of the up-drift shoreline by up to 37% and 46% at Thu Bon and Mawddach estuaries, respectively (by 2100, relative to 2030), the down-drift coast is only affected in a noticeable way at the Mawddach estuary, where the accretion volume is projected to reduce by ~50%. As a result, the ebb-delta effect decreases the up-drift shoreline retreat by up to 37% and 48% at Thu Bon and Mawddach estuaries, respectively, while it reduces shoreline progradation of the down-drift coast of Mawddach estuary by up to ~50%. These results highlight the importance of including ebb-delta dynamics in modelling efforts to assess the climate change responses of inlet-interrupted coasts worldwide.

Geomorphological control of sandy beaches by a mixed-energy tidal inlet

Coastal areas adjacent to tidal inlets are highly valuable areas for marine ecosystems where a wide range of social and economic activities can be found. These areas can be differentiated from open coast due to their specific morphological behaviour caused by a complex nearshore bathymetry and interactions between wave and tidal forcing. The understanding of these interactions is usually limited by the lack of a systemic approach and the scarce availability of comprehensive datasets covering subtidal, intertidal and supratidal areas at large spatial and temporal scales. Here, the complementary use of different bathymetric and topographic datasets and the development of innovative methods using satellite imagery, offers the opportunity to develop a comprehensive understanding of the timescales and the key processes involved in the dynamics of tidal inlets and its control on the downdrift coast. The use of satellite-derived bathymetric maps, computed over the last two decades, showed two sequences of 8 and 9 years where large sandy shoals migrated along the tidal inlet and welded onto the downdrift coast. Although the study period covers one of the most energetic winter over the last decades, the most significant sediment volume changes (±300 m3/m) observed along the three kilometres of beach located south to the inlet were mostly attributed to the migration and welding of these sandy shoals, whereas the offshore wave forcing was of secondary importance. It was also demonstrated that these migrating and welding events had an impact on the pattern of the nearshore bathymetry and sandbars down to 10 km south to the inlet. Primary welding events near to the inlet are associated to the formation of kilometre-long and alongshore uniform nearshore sandbars that subsequently migrate further down the coast causing secondary welding events. The ability to understand and define the spatial and temporal boundaries at which beach behaviour is controlled by a local tidal inlet gives the opportunity to develop sediment compartment approach in order to make accurate predictions of future beach behaviour.

Primary drivers of multidecadal spatial and temporal patterns of shoreline change derived from optical satellite imagery

Understanding and predicting shoreline change along sandy coasts requires continuous (in both time and space) long-term (decades) shoreline data at good spatial (e.g. 100 s of metres) and temporal (e.g. months) resolution. Publicly available satellite imagery can now provide such time series. However, satellite-derived shorelines (SDS) are associated with uncertainties, particularly at high-energy meso-macrotidal coasts, which challenge the assessment of long-term trends and interannual variability. In this paper we address the 1984–2020 time- and space-evolution of 269 km of high-energy meso-macrotidal sandy coast in southwest France using uncertain (no tide and runup correction) SDS data. The shoreline trends are validated with field data collected over the period 2008–2019. Over 1984–2020, the shoreline eroded by 0.55 m/yr with maximum erosion (accretion) reaching 15.61 m/yr (6.94 m/yr), with the largest changes observed along coasts adjacent to the inlet and estuary mouths. We show that, away from the presence of ebb-tide deltas and swash bars affecting offshore wave transformation and nearshore circulation, the long-term shoreline trend is well explained by the gradients in longshore drift computed from a regional wave hindcast and an empirical longshore transport formula. By averaging the yearly SDS along the entire coastline, we find that interannual shoreline variability is well correlated with the winter West Europe Pressure Anomaly (WEPA), which outscores the other conventional teleconnection pattern indices. WEPA even explains >80 % of the space-averaged shoreline variability over the recent period 2014–2020 when more and higher quality satellite images are available. A more local assessment of the links between climate indices and shoreline response shows that correlation with all climate indices dramatically drops downdrift of the large-scale estuary mouths and inlets. This suggests that along this 10–20 km stretch of downdrift coast, shoreline response is controlled factors internal to the estuary mouth/inlet system. The rest of the coast is mostly controlled by factors external to the system, which are primarily the variability in winter-mean wave height correlated to winter WEPA index. Overall, we demonstrate that an adapted space-averaging of uncorrected (noisy) SDS dataset can allow addressing the time- and space variability of shoreline change and their primary drivers including large-scale climate patterns of atmospheric variability. We also advocate that such SDS analysis can be performed along any coastline in the world in order to guide future model development and application.

Emerging Downdrift Erosion by Twin Long-Range Jetties on an Open Mesotidal Muddy Coast, China

Downdrift shoreline recession associated with the construction of a shore-crossing hard structure represents one of coastal erosional hotspots that must be addressed for an integrated, sustainable coastal zone management. To prevent siltation within the navigation channel, two rubber-mounted jetties were installed at the Sheyang River mouth on the open mesotidal muddy coast in Jiangsu province, China, in October 2013. The north jetty is 7.9 km long, while the south jetty is 7.8 km long. The net longshore sediment transport is from the north to the south due to flood-tide dominance. As disclosed by high-resolution satellite images, a 36-km-long downdrift shoreline stretch had experienced remarkable retreats at alongshore varying rates by March 2019. The eroding shoreline planform does not resemble a classic “S” shape, a crescentic shape, or a parabolic shape but an irregularly indented curved shape. Transect topographic survey also reveals an almost immediate response of the downdrift coast from the original accretionary scenery to an erosional regime, with the erosion front translocating downcoast at a much faster speed than a normal speed of 1–1.5 km/yr. Using FVCOM and SWAN, 2DH process-based numerical simulations are performed to simulate the flow, the sediment transport, and the yearly-magnitude accretion/erosion distribution in the jetty-affected area by a representative tidal force and an annual-magnitude wave force. The results demonstrate that the reciprocal tidal flow is predominantly responsible for the muddy sediment accretions at downdrift intertidal and surf zones shallower than a 4.0-m isobath, whereas big wind waves play a decisive role in triggering and developing the downdrift erosional process. The predicted spatial extent of the downdrift erosional segment matches closely the actual eroding front. The loss of the net annual longshore sediment transport volume, i.e., 3.08 million m3 due to the blockage by the twin jetties is recovered from a much larger spatial extent than the 36-km-long retreating shoreline stretch. With regard to the Bruun model, the one-line model, the headland-bay model, and the 2DH numerical model, the potential maximum recession length and the planform shape of the downdrift erosional shoreline arc are further elaborated to gain new insights into the spatial and temporal impact of a hard structure on the adjacent shoreline and flat (beach).

Multi-decadal dynamics of the Saloum River delta mouth in climate change context

The coast of Senegal is subject to climate variability that affects coastal dynamics. The Sangomar spit bounding the Saloum River delta provides a striking example. The spit dynamics has been characterized by complex changes involving both erosion and accumulation. Using a methodological approach combining remote sensing and data analysis in a Geographic Information System (GIS) we calculated a mean annual erosion rate of −3.55 m of the spit shoreline from 1954 to 1987. In 1987 the spit was breached by storm in its most eroded sector (- 4.59 m/year). This breach widened significantly to attain 5.25 km in 2018 and has now become the Saloum River mouth. The breach has resulted in exacerbated erosion of adjacent shorelines, especially at the down-drift coast, where the retreat attains a peak of −20.16 m/year, the average erosion rate remaining at −3.56 m/year for the entire coast. Extension of the spit tip has favoured progradation of the Sangomar Pointe which has lengthened by 5.74 km between 1954 and 2018, diverting gradually the Saloum River former mouth towards the South, and corresponding to a 5,185,009 m3 sedimentation. Extrapolation of shoreline mobility reveals high erosion rates due to sea-level rise by 2050 and 2100.

Unraveling the Mechanisms That Cause Cyclic Channel‐Shoal Dynamics of Ebb‐Tidal Deltas: A Numerical Modeling Study

The morphodynamics of many ebb‐tidal deltas include cyclic patterns of channel deflection and breaching associated with sandy shoals migrating and attaching to the downdrift coast. These channel‐shoal dynamics result from the complex combination of tides and waves; however, their precise role and the physical mechanisms forcing the cyclic patterns are unknown. Here, we use Delft3D/SWAN to simulate their periodic behavior and to study the underlying mechanisms during different phases of the cyclic behavior. The model forcing was such that different ratios between tidal prism and littoral drift were obtained. Two different types of cyclic behavior were modeled, namely, ebb tidal delta breaching and outer delta breaching. The typical time scale of modeled cyclic behavior generally increases with increasing tidal prism and decreasing littoral drift. No cyclic channel‐shoal dynamics were found for high‐ratio tidal prims/littoral drift. Classically, channel deflection and shoal growth have been attributed to waves and the breaching solely to tidal currents. We confirm that shoal formation and channel deflection are forced by wave‐induced currents and sediment concentrations. However, the subsequent shoal growth can only be explained by tides and waves operating simultaneously. Moreover, the breaching of ebb tidal deltas is not a tide‐only effect because waves entrain the sediment that is transported by the ebb flow. In fact, there exists an optimum wave height; that is, larger waves cause further channel rotation rather than channel breaching. Furthermore, temporal variations in the sediment exchange between the back‐barrier basin and the sea are found to be an inherent feature of the modeled ebb tidal delta breaching.

Multi-Decadal to Short-Term Beach and Shoreline Mobility in a Complex River-Mouth Environment Affected by Mud From the Amazon

On the 1500 km-long mud-dominated Guianas coast of South America, between the mouths of the two mega-rivers, the Amazon and the Orinoco, debouch numerous small rivers draining the humid tropical/equatorial Guiana Shield. The geomorphic development of the mouths of these rivers reflects interactions among water discharge, fluvial sediment load, and the alongshore migration of Amazon-derived mud banks alternating with inter-bank areas. The mouth of the Maroni River, astride the French Guiana-Suriname border, shows advanced estuarine infill and geomorphic development characterized by a western (downdrift) side comprising numerous recent cheniers and an eastern (updrift) side bound by an old (> 2000 years B.P.) chenier. A multi-decadal analysis of the beach bounding this chenier shows little net overall mobility notwithstanding significant decadal to sub-decadal variation. The overall stability reflects the diversion of sand supply from the Maroni River towards the downdrift coast and limited sand supply by the smaller Mana River further east, and the south bank of which was contiguous with this beach. The variability in beach multi-decadal mobility reflects the influence, on waves, of alongshore-migrating banks (strong wave dissipation, limited beach mobility) and inter-bank areas (limited wave dissipation, larger beach mobility), highlighted by a comparison, in the current bank phase, of offshore and inshore waves. Erosion of the beach between 2011 and 2017 coincides with the sealing of the mouth of the Mana by muddy progradation in 2011 and mouth relocation several kilometers eastward. The morphodynamics of the beach and shorter-term fluctuations in budget are related to: (1) interaction with estuarine sand dunes mobilized by strong tidal currents on the adjacent shallow shoreface, (2) the influence of the Maroni channel, and (3) rapid encroachment of the leading edge of the shore-attached mud bank on the eastern part of the beach. The beach morphodynamics and evolution highlight, thus, embedded levels of influence: the Maroni at the local scale, and the net westward sediment-transport system and bank and inter-bank alternations that affect the Guianas coast at a regional scale. The recent erosion poses a threat to the local communities by reducing beach space available for recreation and turtle-nesting.

Long-Term Evolution of Morphology at Loc An Estuary, Vung Tau, Vietnam

In the last decade, the downdrift coast of the Loc An river mouth, Vung Tau province, Vietnam was subject to severe erosion. This was mainly due to the downdrift migration of the river mouth which had reached in total several kilometers over the past 30 years. This study presents the migration of the Loc An river mouth and the related consequences using long-term satellite images and field data. According to wave data analysis, the waves were separated in two different monsoon seasons, northeast and southwest and the characteristics of these waves in the two seasons are noticeably different. This led to contrary dominant directions of the longshore sediment transport between the two monsoon seasons. The river mouth migrated in the southwest direction around 3000 m in the period from 1979 to 2017 (80 m/year). This migration caused the erosion and the collapsing of the geotextile jetties on the right side of the river mouth. The change rate of the surface area of the updrift sand spit is utilized to estimate the longshore sediment rate which equals the order of 2.0 × 105 m3/year. It was noted that a re-occurrence of the breaching would be very difficult in the future due to the strengthening of the back side of updrift sand spit and the operation of the Song Ray reservoir. The migration caused the two jetties to become obsolete, which were constructed to stabilize the river mouth and ensure a smooth passage for the fishing boats. The primary action would be to remove the jetties, however, if this action proves too difficult to be actualized, immediate action like dredging deposited sediment and closing the gap would need to be taken in order for the jetties to become functional again. Additionally, this study illustrates the usefulness of using satellite images to investigate the long-term evolution of the morphology which is still active and where there is still a large amount of change such as in the river mouth area. This is very useful for the research carried out at this specific place or in the country which has limitations in the field and observed data.

Modeling the growth and migration of sandy shoals on ebb‐tidal deltas

AbstractCoherent sandy shoals that migrate toward the downdrift coast are observed on many ebb‐tidal deltas. In this study, processes that cause the growth and migration of shoals on ebb‐tidal deltas are identified. Moreover, the effect of the incident wave energy and the tidal prism of an inlet on the migration speed of these shoals is investigated. For this, a numerical morphodynamic model with an idealized geometric setup is employed. The model computes the bed level evolution due to local erosion and deposition of sand driven by tides and waves. Analysis of model results shows that shoals grow when there is a local imbalance between the bathymetry and the wave conditions, which in this study was imposed by manually breaching the ebb‐tidal delta or by adding storms to the wave forcing. There are thresholds for shoal formation that depend on the distribution of the sand and the incident wave energy. Wave refraction over the shoals leads to focusing of wave energy and increased wave energy dissipation around the location of the local minimum water depth. This generates residual currents over the shoal and increased skin friction toward the local minimum water depth, which together create a sand transport pattern that induces the growth and migration of the shoal. Sand transport due to asymmetric waves contributes to keeping the shoal a coherent structure. The shoal migration speed increases with increasing incident wave energy and decreasing tidal prism; this is because tidal residual currents oppose the wave‐driven residual currents that cause shoal migration.

Impact of detached breakwaters on shoreline evolution: a case study on the Portuguese West Coast

Araújo, M.A.V.C., Di Bona, S., Trigo-Teixeira, A., 2014. Impact of detached breakwaters on shoreline evolution: a case study on the Portuguese west coast. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 041–046, ISSN 0749-0208.The Portuguese West Coast is facing severe erosion problems that threaten both the population andthe immediate seaside properties. Some locations, such as the Vagueira region, are especially vulnerable. In an attempt to reverse the present erosional trend, a coastal defense scheme involving two detached breakwaters was introduced in a phased manner. In this work, the impact of those two detached breakwaters on the down-drift coast is investigated. The study area extends from south of the Costa Nova village southwards to Mira. The shoreline evolution is predicted, for various decades, using the GENESIS model. The present time shoreline position, from which future situations are predicted, was obtained using LIDAR data acquired in 2011. Two different future scenarios are investigated: the ‘do-nothing’ scenario, which assumes that no further engineering interventions will be performed on the coast; and the ‘detached breakwaters protected’ scenario, with the first breakwater introduced in 2011 and the second in 2026. Depending on sediment supply, two types of simulations are carried out: calibrated simulations, which assume that the amount of sediments in the future will be equal to those in the present situation; and extreme simulations, which assume an acute shortage in sediment supply. Results show that if no further interventions are executed at the Vagueira region, the entire coastal stretch will continue to erode, which could lead to extreme consequences in some locations, depending on sediment supply. If two detached breakwaters are introduced at different intervals, a new accreting trend is predicted at the Vagueira region, reversing the present erosional phase. However, the erosional trend will continue southwards, unless a 3rd detached breakwater is introduced in Mira.

Decadal morphological response of an ebb-tidal delta and down-drift beach to artificial breaching and inlet stabilisation

The morphodynamic response of a mixed-energy ebb-tidal delta (Guadiana estuary, southern Portugal) and its down-drift barrier island (Canela Island) to channel relocation and stabilisation by jetties is examined using a series of sequential bathymetric maps and vertical aerial photographs spanning five decades. Morphological analysis indicates that the ebb delta is in an immature state, characterised by weak sediment bypassing. Landward-migrating shoals on the swash platform have been produced by the jetty-induced artificial bank-breaching and by the collapse of the eastern portion of the delta. The welding of these shoals has largely controlled the evolution of the coast, with local accretion and erosion lasting for years, and large amounts of regional accretion occurring over decades due to sand accumulation against jetties located further down-drift. These observations provide insights into the potential response of a coast to very large, locally concentrated sand nourishment in the form of shoals. The main effects of the jetties on the coast are observed at the centre of Canela Island, with the production of an erosion hot-spot associated with a temporally persistent and divergent longshore transport providing sand to the adjacent areas. Significant accretion is anticipated for the next decade along the entire island due to the ongoing attachment of the presently observed shoals. After the depletion of this sediment source, and in the context of weak sediment bypassing, the most severe down-drift erosion induced by the jetties is predicted to occur some decades after their construction. This study demonstrates that the geomorphic response of an ebb-tidal delta to jetty construction must be considered at multiple temporal and spatial scales when assessing the impacts of jetties on the down-drift coast.

APPLYING AN ANALOGUE FOR A CONCEPTUAL MODEL FOR THE DEVELOPMENT OF A MEGA NOURISHMENT

This paper discusses the effectiveness of a mega-nourishment project. Mega-nourishment is a new technology for beach nourishment that has recently been developed. It arises as an alternative to beaches where a structural erosion problem is observed and there is the need for continuously nourishments works. A pilot project was implemented near The Hague (The Netherlands) in October 2011, called the Sand Engine [Mulder 2000], however the impacts of such project are widely unknown. From this perspective the study of a natural coastal area that was subjected to similar conditions can generate know how about the impacts of such a project. One of these natural examples is Ameland: one of the Frisian Islands in the Wadden Sea. Ameland experienced large natural nourishment coming from the ebb tidal delta. The nourishment shape and magnitude were in the same order of magnitude of to the Sand Engine project, leading to a comparison point. In the Ameland case this nourishment occurred on 1990`s and on 1993 achieved a similar shape to the Sand Engine pilot project and currently almost all the sediment is already spread. So from the natural case we can assume that the Sand Engine is going to take around 2 decades to spread all the nourished sediment, a special care should be taken in the down-drift coast to avoid early erosion.

Morphodynamics and Sand Bypassing at Ameland Inlet, The Netherlands

Abstract In this paper, we review and analyze the cyclic morphodynamics and sand-bypassing processes at the Ameland Inlet, The Netherlands. The inlet is located between the islands of Terschelling and Ameland in the Dutch Wadden Sea. The bypassing rate and sand storage have a periodicity of 50 to 60 years in connection with the migration of the channels in the inlet and the transformation of the gorge between a one-channel and a two-channel system. These developments modulate the volumes of the shoal and bypassing bars and produce an attenuating sand wave toward the downdrift coast. An aggregate model is developed to describe the inlet morphology at this intermediate timescale. The model is based on a modified equilibrium-volume approach, which allows erosion and accretion of the shoal and bars to occur about their long-term equilibrium. The model results depict observed morphologic behaviors of the inlet and corroborate the measured modulation of the downdrift coastline. The simulation of the effect of t...

An aggregate model for the adaptation of the morphology and sand bypassing after basin reduction of the Frisian Inlet

To study the adaptation of the morphology of the Frisian Inlet after basin reduction an aggregate model is developed. In the model, especial attention is given to the sand transport to the down-drift coast. In developing the model the inlet system is divided into three elements, the ebb tidal delta, the Zoutkamperlaag and the tidal flats. Based on observations during the first 18 years after basin reduction the adaptation time scale of the tidal flats is expected to be much larger than that of the ebb tidal delta and the Zoutkamperlaag, essentially reducing the inlet schematization to a two-element system. The dependent variables in the model are the sand volume of the ebb tidal delta and the water volume of the channel. The governing equations are non-linear and for quantitative accurate results are solved numerically. To demonstrate the nature of the solution the equations are linearized assuming the morphological state is close to equilibrium. The linearized equations are solved analytically and the solution is applied to a hypothetical case where the tidal prism of the Frisian Inlet is reduced by 10%. From the analytical solution it follows that the adaptation of the volumes of the two elements, delta and channel, is governed by two system time scales. These system time scales are functions of two local time scales. The local time scales pertain to the adaptation of one element assuming the other element has reached equilibrium. Because there are two system scales the adaptation of the volumes of the delta and channel is not exponential and is not necessarily monotonic. For example, initially the transport of sand to the down-drift coast is larger than the long-shore sand transport entering the inlet system from the up-drift coast, then becomes smaller and after some time increases again to reach the value of the up-drift long-shore sand transport. Comparison of the numerical solution for the actual reduction in tidal prism of 30% with the analytical solution for the 10% reduction in tidal prism shows qualitatively the same results.

Sediment transport pattern at the Barra Nova inlet, south Portugal: a conceptual model

The Barra Nova inlet, in south Portugal, is known to migrate progressively southeastwards under wave action. The morphodynamics of this system during a representative year suggests that this long-term evolution is dependent on a seasonal behavior of the tidal inlet which can be described through a three-stage model of post-storm, transition and extended fair-weather conditions. Processes involved in this evolution indicate that the historical migration of the Barra Nova is not dependent on the longshore drift forcing constricting the channel on the updrift coast, but rather on the adjustment of the system to a major erosion of the downdrift coast during short storm events.

River-beach interaction on mixed sand and gravel coasts: a geomorphic model for water resource planning

A distinctive sequence of river mouth offsetting and freshwater lagoon development is described from mixed sand and gravel coasts on the east coast of the South Island, New Zealand, particularly the Rakaia river. Though total annual sediment load exceeds 4 × 10 6t, the Rakaia is a ‘small’ river in that the coarse fraction nourishing beaches and the regional longshore drift is insufficient to maintain either the river mouth or the adjacent coasts against long-term erosion by the sea. Dominance of marine over fluvial processes is an important overall control of the lagoon and spit development sequence. Long, narrow spits grow generally, but not exclusively northward for up to 3 km at the Rakaia under the influence of longshore drift in low to moderate stages of river flow. Under higher river flows the spits are breached, the lagoon circulation alters and a ‘slug’ of the regional drift sediments temporarily stored in the spits is bypassed to the downdrift coast. River-injected materials contained in flood formed subtidal deltas also contribute to the drift and to marked modulation of the regional net longshore sediment transport which displays a ‘pulsational’ character at downdrift sites. The river mouth lagoons are non-tidal and non-saline owing to generally steep bed gradients in the braided rivers and to low tidal range, though backwater effects are observable and some salt enters by storm overwash and spray. Permeability variations within enclosing spits are important in both the process of mouth offsetting and in flood breaching. A range of potential and actual environmental impacts on such river mouths relating to use of freshwater resources for activities such as hydroelectric power generation and irrigation is discussed in the light of the fact that the coastal lagoons present important concentrations of recreational, wildlife and fisheries values. Irrigation drawoff and hydroelectric power generation can affect the magnitudes of river flow and its temporal pattern at the coast, as well as delivery of coarse sediments to the spits enclosing the mouth, thus markedly altering the interaction with marine processes in the river mouth region. Water resource development on the river has the potential to increase freshwater flooding adjacent to the mouth, to cause protracted closure of the mouths and to accentuate coastal erosion.

PREDICTION OF 3-D BEACH CHANGES ON THE FUJI COAST

The temporal and spatial beach changes on the Fuji Coast are elucidated through this study, and the 1i t toral transport rate on the downdrift coast of Tagonoura Port is evaluated. In particular, it is concluded that a large amount of sediment eroded from the downdrift beach of Tagonoura Port is carried away toward a zone deeper than 20m. The second aim of this study is to devel op a numerical model whi ch can predict three-dimensional beach changes. For this purpose, a model in which the vertical distribution of littoral transport is taken in to account and the effect of the wave-dissipating breakwaters on the beach changes can be evaluated is developed.

A DETACHED BREAKWATER SYSTEM FOR BEACH PROTECTION

Three segmented, detached breakwaters were constructed In the fall of 1977 at Lakeview Park, Ohio, on Lake Erie, to protect a beachfill to be used for recreation and shore protection. This paper documents the design procedures which established the configuration of the breakwaters and the beachfill, and determined the need for a terminal groin. The beachfill has been monitored by aerial photography and bathymetric profiling. During the second year, a storm of near design intensity generated severe waves concurrently with high Lake levels and eroded the updrift beach; however, the initial beach configuration was partially restored by natural processes during the following summer season. The project has functioned well, with very little loss of sand from the system and without adverse impacts on the downdrift coast.

The Mississippi river improvements

The morphodynamic response of a mixed-energy ebb-tidal delta (Guadiana estuary, southern Portugal) and its down-drift barrier island (Canela Island) to channel relocation and stabilisation by jetties is examined using a series of sequential bathymetric maps and vertical aerial photographs spanning five decades. Morphological analysis indicates that the ebb delta is in an immature state, characterised by weak sediment bypassing. Landward-migrating shoals on the swash platform have been produced by the jetty-induced artificial bank-breaching and by the collapse of the eastern portion of the delta. The welding of these shoals has largely controlled the evolution of the coast, with local accretion and erosion lasting for years, and large amounts of regional accretion occurring over decades due to sand accumulation against jetties located further down-drift. These observations provide insights into the potential response of a coast to very large, locally concentrated sand nourishment in the form of shoals. The main effects of the jetties on the coast are observed at the centre of Canela Island, with the production of an erosion hot-spot associated with a temporally persistent and divergent longshore transport providing sand to the adjacent areas. Significant accretion is anticipated for the next decade along the entire island due to the ongoing attachment of the presently observed shoals. After the depletion of this sediment source, and in the context of weak sediment bypassing, the most severe down-drift erosion induced by the jetties is predicted to occur some decades after their construction. This study demonstrates that the geomorphic response of an ebb-tidal delta to jetty construction must be considered at multiple temporal and spatial scales when assessing the impacts of jetties on the down-drift coast.