Gradients In Alongshore Sediment Transport Research Articles

Seasonal changes in beach resilience along an urbanized barrier island

Beach width, dune height, and vegetation coverage are key parameters to assess beach resistance and resilience to storms. However, coastal development often causes beach ecosystem degradation due to poor coastal management. We propose a Coastal Resilience Index from Remote Sensors (CRIfRS) for urbanized coasts based on aerial photogrammetry. The study area, located along a 7.8 km stretch of coast on a barrier island, is characterized by persistent alongshore sediment transport and the presence of coastal structures and beach-front houses. Contrary to previous studies, we focus on anthropogenic perturbations (coastal urbanization and coastal structures), instead of hydrodynamic conditions (storms), since erosion in this region is mainly associated with alongshore sediment transport gradients induced by coastal structures. Thus, the CRIfRS is based on the relation of three indicators that affect the beach functionality for coastal protection: beach width, coastal structure influence area, and vegetation coverage. The CRIfRS was divided into five categories: Very Low resilience (VL), Low resilience (L), Medium resilience (M), High resilience (H), and Very High resilience (VH). The CRIfRS presented an important spatial and temporal variability due to changing environmental conditions and the deployment of new coastal structures. For the study period, the percentage of the coast within the VL and L resilience classification increased, whereas the percentage of the coast classified as M, H, and VH resilience decreased. During the winter storm season, the resilience increased mainly due to the cross-shore transport whilst during mean wave conditions (i.e., sea-breeze conditions) the long-shore transport becomes more persistent and thus the coastal structures play an important role interrupting the sediment flux. Additionally, the CRIfRS trajectory shows an overall increase of the L resilience and an overall decrease of the H resilience values. This study highlights the important role of anthropogenic perturbations on the assessment of coastal resilience for highly urbanized coasts. The CRIfRS can help to improve the coastal management by assessing the coastal protection capability of beaches considering both natural and anthropogenic factors.

Assessing Morphologic Controls on Atoll Island Alongshore Sediment Transport Gradients Due to Future Sea-Level Rise

Atoll islands’ alongshore sediment transport gradients depend on how island and reef morphology affect incident wave energy. It is unclear, though, how potential atoll morphologic configurations influence shoreline erosion and/or accretion patterns, and how these relationships will respond to future sea-level rise (SLR). Schematic atoll models with varying morphologies were used to evaluate the relative control of individual morphological parameters on alongshore transport gradients. Incident wave transformations were simulated using a physics-based numerical model and alongshore erosion and accretion was calculated using empirical formulae. The magnitude of the transport gradients increased with SLR: initial erosion or accretion patterns intensified. Modeled morphologic parameters that significantly influenced alongshore transport were the atoll diameter, reef flat width, reef flat depth, and island width. Modeled atolls with comparably small diameters, narrow and deep reef flats with narrow islands displayed greater magnitudes of erosion and/or accretion, especially with SLR. Windward island shorelines are projected to accrete toward the island’s longitudinal ends and lagoon due to SLR, whereas leeward islands erode along lagoon shorelines and extend toward the island ends. Oblique island, oriented parallel to the incident deepwater wave direction, shorelines are forecast to build out leeward along the reef rim and toward the lagoon while eroding along regions exposed to direct wave attack. These findings make it possible to evaluate the relative risk of alongshore erosion/accretion on atolls due to SLR in a rapid, first-order analysis.

Modeling Shoreline Response to a Submerged Breakwater at Anmok Beach in Korea with Single Line Theory

Since a submerged breakwater (SBW) was built and the beach was nourished at Anmok in the east coast of Republic of Korea at October, 2014, the shoreline in the lee of the SBW has accreted about 25 m during the first seven months after construction. The shoreline evolution showed two distinct patterns which were studied in this paper. A strong local accretion behind the submerged breakwater was observed in March 2015 and a smoother shoreline with accretion that extended up to Gangneung Harbor breakwater in May 2015. The UNIBEST coastline model (developed at Delft Hydraulics) was applied for the investigation of the observed shoreline undulation patterns which were generated by alongshore sediment transport gradients that were induced by the SBW and nourishment. Nearshore wave conditions were computed for this purpose at nine locations in the nearshore with the Delft3D-wave model. Two detailed wave scenarios with different crest height of the SBW were taken into account to represent the transmission of waves at the SBW which were validated with field measurements. The coastline model is able to reproduce the observed shoreline evolution patterns when wave transmission at the SBW is represented well, which is the case for a wave scenario with a lowered effective crest level of the SBW. Initially, accretion takes place predominantly at the northern side of the scheme, which was similar to the observed shoreline shape of the March 2015 situation. This local accretion is a result of the low sediment transport capacity behind the SBW due to sheltering of the wave energy, which initially hinders the redistribution of sediment to the South (i.e. area in-between Gangneung Harbor and SBW). After some months, a redistribution of sediment will take place behind the SBW which results in a smoother shoreline pattern which is similar to the May 2015 situation. The rate of change of the shoreline accretion is controlled by the absolute transport rates at the coast (i.e. wave energy), but is often of lesser importance since an adjustment towards a new shoreline equilibrium may take place within relatively short time scales (i.e. months to a few years). In addition to the wave energy, it was found that the relative angle of the incoming waves (α) is most relevant for the final shoreline shape. The shoreline evolution at future SWB structures may therefore be predicted by precisely estimating the α directly after construction of the SBW.

The Influence of a Seawall on Postnourishment Evolution in a Sea-Breeze-Dominated Microtidal Beach

ABSTRACT Medellin, G.; Marino-Tapia, I., and Euan-Avila, J., 2015. The influence of a seawall on postnourishment evolution in a sea-breeze-dominated microtidal beach. The postnourishment beach evolution is evaluated on a sea-breeze-dominated microtidal beach in Yucatan (Mexico). The study area has experienced severe beach erosion due to both alongshore sediment-transport gradients during calm conditions and offshore sediment transport during extreme wave events. Therefore, a shoreline restoration program, including beach nourishment and the removal of groynes, was carried out in 2002. Bimonthly subaerial/intertidal beach profiles measured during 7 years (2002–09), in front of a seawall and at an adjacent beach (nonwalled), are employed to analyze the beach nourishment evolution. Empirical orthogonal function (EOF) analysis of the shoreline time series is performed using the data extracted from the 50 field surveys. The temporal evolution of the EOF analysis shows the following: (1) an abrupt increase in b...

The influence of sea level rise and changes in fringing reef morphology on gradients in alongshore sediment transport

AbstractClimate‐change‐induced alterations to coral reef ecosystems, in combination with sea level rise, have the potential to significantly alter wave dissipation across reefs, leading to shifts in alongshore sediment transport gradients and alterations to tropical coastlines. We used Delft3D to model schematized profiles of two reef flat widths based on the south Molokai, Hawaii coast. Simulated anthropogenic modifications include incremental degradation of the reef structure as well as sea level rise. Our findings indicate that sea level rise has a greater relative effect on wave energy flux and alongshore sediment transport over a wide flat, whereas both reef degradation and sea level rise exert similar influence over a narrow flat. These results suggest reefs that vary in width alongshore are more likely to experience changes in alongshore sediment transport gradients, and therefore shifts in shoreline erosion and accretion patterns, than more uniform reef systems.

OPTIMAL RESPONSES TO SHORELINE CHANGES: AN INTEGRATED ECONOMIC AND GEOLOGICAL MODEL WITH APPLICATION TO CURVED COASTS

AbstractThe placement of sandy fill on an eroding beach, also known as “beach nourishment,” is a soft‐structure response to property loss often undertaken by coastal property owners. Here, we examine optimal beach nourishment along a curved shoreline with a model that couples geomorphic shoreline evolution with human responses to that evolution. The model includes the exogenous effects of shoreline recession due to sea‐level rise and the endogenous shoreline changes due to gradients in alongshore sediment transport. Human responses (nourishments) to shoreline changes that are not constant along the coast will change the shoreline curvature, thereby influencing local rates of shoreline change. We examine the net benefits of two alternative strategies to undertake soft structural responses to erosion: (i) the decisions of property owners to respond independently and (ii) the decisions of property owners to coordinate their individual responses. Under plausible parameterization, we find that a strategy to coordinate can dominate a strategy to act independently, and the benefits of coordination increase with greater shoreline curvature. This integration of economic and geological models demonstrates how feedbacks between human activities and natural processes can affect the evolution of the shoreline.

The influence of sea level rise and changes in fringing reef morphology on gradients in alongshore sediment transport

The influence of sea level rise and changes in fringing reef morphology on gradients in alongshore sediment transport

CHRONIC BEACH EROSION INDUCED BY COASTAL STRUCTURES IN CHELEM, YUCATÁN

The Yucatan coastline has been experiencing beach erosion during the past few decades; the erosion has reached critical points at some locations such as the Chelem beach, located near the Progreso Pier. Despite this problem, only few studies have been devoted in order to investigate the role of coastal structures on explaining the high erosion rates reported at this location. Therefore, the aim of this work is to evaluate the effects of the Progreso Pier on the wave transformation and alongshore sediment transport in the study area. Field surveys were conducted in a monthly basis in order to estimate the erosion rates and wave conditions with an ADCP installed at 8 m water depth. The field information confirmed the high erosional trends (O(1)m/year) that explain coastal infrastructure damage and property losses. The wave measurements were employed as the forcing of a wind-wave numerical model (MIKE 21 SW). The numerical model is implemented in the study area for two different scenarios, with and without the pier; this was to estimate nearshore wave conditions. Subsequently, the nearshore wave climate at 5 m water depth is employed for the calculation of alongshore sediment transport rates in 5 points that are representative of the littoral drift along the study area. The modeling results show that the pier acts as a large scale wave-sheltering structure that induces important alongshore sediment transport gradients under mean wave conditions, decreasing the capacity of the waves to recover the beach. On the other hand, during winter storms, when the direction of the waves is from the NNW, the structure does not seem to play an important role on wave transformation into the study. As a result, the Progreso Pier enhances beach erosion in the Chelem area by inducing algonshore gradients in sediment transport and decreasing the beach recovery capability

Wave climate, coastal sediment budget and shoreline changes for the Danube Delta

Since its formation, approximately 12 000 years ago, the coastal zone of the Danube Delta experienced alternating periods of accretion and erosion related to the natural process of lobe abandonment caused by river avulsion. Over the last 150 years, the effects of human interventions modified these natural trends. Regulation of the Danube River (e.g. the Iron Gates barrages facilitating navigation and controlling river discharge), construction and extension of the Sulina entrance jetties and dredging of the Sulina mouth bar for navigational purposes are examples of human interventions that influenced and still influence the actual state of the Danube Delta coastal zone. Most importantly, these interventions diminished the quantity of sediments reaching the coast and, consequently, led to a severe retreat of the shoreline, especially in the Sulina–Sf. Gheorghe coastal sector. To investigate and quantify the recent sediment budget and associated shoreline changes along the Danube Delta coastal zone we constructed a hindcast of the wave climate, the alongshore transport capacity and the resulting sediment budget using a numerical coastline model. On a local scale, it was found that overwash plays an important role; i.e. the observed evolution of Sahalin spit island shows a cross-shore landward migration that is much larger than could be explained by the calculated alongshore sediment transport gradients. On a delta-scale however, the resulting alongshore sediment transport gradients and associated sediment budgets comply very well with the observations, confirming the dominant role of wave-induced alongshore transport and giving confidence in our predictions regarding the shoreline position for the next 25 and 50 years. The results of this study indicate the potential for broader applicability of the model to other deltas, especially those in wave-dominated settings.

High‐angle wave instability and emergent shoreline shapes: 2. Wave climate analysis and comparisons to nature

Recent research has revealed that the plan view evolution of a coast due to gradients in alongshore sediment transport is highly dependant upon the angles at which waves approach the shore, giving rise to an instability in shoreline shape that can generate different types of naturally occurring coastal landforms, including capes, flying spits, and alongshore sand waves. This instability merely requires that alongshore sediment flux is maximized for a given deepwater wave angle, a maximum that occurs between 35° and 50° for several common alongshore sediment transport formulae. Here we introduce metrics that sum over records of wave data to quantify the long‐term stability of wave climates and to investigate how wave climates change along a coast. For Long Point, a flying spit on the north shore of Lake Erie, Canada, wave climate metrics suggest that unstable waves have shaped the spit and, furthermore, that smaller‐scale alongshore sand waves occur along the spit at the same locations where the wave climate becomes unstable. A shoreline aligned along the trend of the Carolina Capes, United States, would be dominated by high‐angle waves; numerical simulations driven by a comparable wave climate develop a similarly shaped cuspate coast. Local wave climates along these simulated capes and the Carolina Capes show similar trends: Shoreline reorientation and shadowing from neighboring capes causes most of the coast to experience locally stable wave climates despite regional instability.