Coastal Accretion Research Articles

Natural and Human Impacts on Coastal Areas

Coasts are the most densely populated regions in the world and are vulnerable to different natural and human factors, e.g., sea-level rise, coastal accretion and erosion processes, the intensification of sea storms and hurricanes, the presence of marine litter, chronic pollution and beach oil spill accidents, etc. Although coastal zones have been affected by local anthropic activities for decades, their impacts on coastal ecosystems is often unclear. Several papers are presented in this Special Issue detailing the interactions between natural processes and human impacts in coastal ecosystems all around the world. A better understanding of such natural and human impacts is therefore of great relevance to confidently predict their negative effects on coastal areas and thus promote different conservation strategies. The implementation of adequate management measures will help coastal communities adapt to future scenarios in the short and long term and prevent damage due to different pollution types, e.g., beach oil spill accidents, through the establishment of Environmental Sensitivity Maps.

Evaluating and predicting coastal shoreline changes in the Doi Duong - Phan Thiet beach area by remote sensing and GIS technology

Assessing the shoreline variability is paramount in understanding the erosion and coastal accretion processes. The potential impact of erosion on coastal communities, particularly in coastal regions, amplifies the vulnerability to coastal disasters. In this study, Landsat satellite imagery captured from 1990 to 2022 was utilized, in conjunction with Geographic Information System (GIS), to scrutinize alterations in the shoreline of Doi Duong Beach - Phan Thiet. The satellite images were independently processed using aspect-ratio methods and thresholding techniques to determine the prevailing condition of the coastline. The rate of shoreline change was quantified using the DSAS tool. The investigation’s findings reveal a dynamic interplay between erosion and accretion, with accretion dominance explicitly evident within the areas surrounding the Ca Ty - Phu Hai river mouth. The average annual shoreline change rate is 27.1 m/yr for the Ca Ty River mouth and 16.5 m/yr for the Phu Hai River mouth. The most significant disparity between the recorded coastline in 1990 and 2022 measures 868 m. Additionally, the projections for the coastline until 2032 indicate that accretion remains the dominant process. This research shows that combining geospatial techniques and numerical models is a reliable way to study shoreline change trends.

Coastal erosion and accretion on the Caribbean coastline of Costa Rica long-term observations

Several studies show erosive processes on the Caribbean coastline of Costa Rica. These processes may be accelerated as a result of an increase in the sea level. This study covered the entire Caribbean coastline of Costa Rica between 1986 and 2019. This allowed reporting new sites with accelerated coastal accretion and erosion processes, as well as confirming other processes that have already been identified. The methodology was based on shoreline rising images obtained from Landsat and on their processing in a Geographical Information System supported with the “Digital Shoreline Analysis System” (DSAS) tool. The presence of important accretion areas related to the mouth of significant fluvial systems, such as Colorado, Parismina, Matina, Estrella, and Sixaola rivers is highlighted. Moreover, eight beach sectors showing erosion problems have been analyzed. The beach located to the south of Isla Portillos and beaches located on Cahuita Point stand out due to their intensity. Additionally, an intense beach erosion process has been detected at Soropta in Panamá. It should be highlighted that six of the eight sites showing erosion are located within protected zones. This confirms the little anthropic interference on these processes. It was not possible to find a difference between the north and the south of the Caribbean Coast in terms of the magnitude of coastal erosion. The contribution of the rivers is a key factor in the North Caribbean while the responses to the 1991 earthquake conditioned the behavior on the South coast. The results may serve as a basis for management plans or adaptation to climate change proposals.

Modeliranje i analiza promjene obale u Sidi Abdel Rahman obalnom pojasu, Egipat

Estimating the coastline or shoreline is one of the crucial components in determining coastal accretion and erosion and in analysing coastal morph dynamics. In recent years, the socioeconomic pressure has increased on the Sidi Abdel Rahman coast area with the impact of climate change, which has led to the destabilisation of the coast. Therefore, this study aims to investigate the shoreline changes along the coast in the study area using remote sensing data for the period 2007 to 2021, modeling the shoreline change through the study area, and predicting shoreline change in the period from 2021 to 2024. It also suggests the type of protection required for the area from waves and sea currents.

DIFFERENCES OF COASTALLINE CHANGES IN THE AREA AFFECTED BY LAND COVER CHANGES AND COASTAL GEOMORPHOLOGICAL SOUTH BALI 1995 - 2021

The South Bali coast is prone to abrasion due to its geographical position facing the Indian Ocean. High sea waves and currents in the south of Bali will erode beaches whose lithology and morphology are prone to abrasion. Land cover conditions that do not support coastal protection will also affect the high abrasion of the southern coast of Bali. This study aims to analyze the shoreline changes in South Bali from 1995-2021. The analytical method used is the Digital shoreline analysis system (DSAS), with data from Landsat 5 TM, Landsat 7 ETM+, Landsat 8 OLI/TIRS, and Sentinel 2A. The analysis results show that the area directly facing the waves is relatively high, with volcanic rock formations, and there is no mangrove as coastal protection. The lack of good coastal management shows the area with the highest abrasion. It was found in the western part of Tabanan Regency, eastern Gianyar, and southern Badung. Meanwhile, the average coastal accretion was relatively high in the neck of South Bali, in areas where the land cover was mangrove and adjacent to river mouths, which experienced much sedimentation.

Shoreline Change Assessment in the Coastal Region of Bangladesh Delta Using Tasseled Cap Transformation from Satellite Remote Sensing Dataset

Bangladesh is a global south hotspot due to climate change and sea level rise concerns. It is a highly disaster-prone country in the world with active deltaic shorelines. The shorelines are quickly changing to coastal accretion and erosion. Erosion is one of the water hazards to landmass sinking, and accretion relates to land level rises due to sediment load deposition on the Bay of Bengal continental shelf. Therefore, this study aimed to explore shoreline status with change assessment for the three study years 1991, 2006, and 2021 using satellite remote sensing and geographical information system (GIS) approaches. Landsat 5, 7 ETM+, and 8 OLI satellite imageries were employed for onshore tasseled cap transformation (TCT) and land and sea classification calculations to create shore boundaries, baseline assessment, land accretion, erosion, point distance, and near feature analysis. We converted 16,550 baseline vertices to points as the study ground reference points (GRPs) and validated those points using the country datasheet collected from the Survey of Bangladesh (SoB). We observed that the delta’s shorelines were changed, and the overall lands were accredited for the land-increasing characteristics analysis. The total accredited lands in the coastal areas observed during the time periods from 1991 to 2006 were 825.15 km2, from 2006 to 2021 was 756.69 km2, and from 1991 to 2021 was 1223.94 km2 for the 30-year period. Similarly, coastal erosion assessment analysis indicated that the results gained for the period 1991 to 2006 and 2006 to 2021 were 475.87 km2 and 682.75 km2, respectively. Therefore, the total coastal erosion was 800.72 km2 from 1991 to 2021. Neat accretion was 73.94 km2 for the 30-year period from 1991 to 2021. This research indicates the changes in shorelines, referring to the evidence for the delta’s active formation through accretion and erosion processes of ‘climate change’ and ‘sea level rise’. This research projects the erosion process and threatens land use changes toward agriculture and settlements in the coastal regions of Bangladesh.

Assessment of coastal variations due to climate change using remote sensing and machine learning techniques: A case study from west coast of India

Climate change and other environmental disturbances are causing sea level rise all over the world. Due to sea-level rise and other unprecedented atmospheric phenomena caused by climate change, Indian coasts are vulnerable to coastal erosion. The Kerala coast, at the southern tip of India's west coast, has experienced a sea change in the last decade. To address coastal variations, we investigate coastal erosion, coastal accretion, and shoreline changes (from 2006 to 2020) along this coast between Pozhiyoor and Anchuthengu (58 km). To monitor the status and predict changes along the coast, remote sensing, GIS, field checks, and machine learning tools were used. The data analysis reveals that the shoreline configuration, rate of beach accretion, and erosion have all changed significantly. When the rate of erosion in the 58-km-long coastal stretch was examined, it was discovered that approximately 42 km face acute erosion, 13 km face accretion, and approximately 3 km face neither accretion nor erosion and remain in equilibrium. According to the estimates, approximately 2.62 km2 of land has been eroded away from the shore over a 14-year period, while 0.7 km2 of land has been accreted. At locations where the influence of river discharge or groynes is minimal, the normal rate of accretion in the stretch is around 1–2 m/y. Nonetheless, the rate of accretion increased to 5–8 m/y where groynes or river mouths, or both, have a significant influence on shoreline stability. The normal rate of erosion in the stretch under consideration, however, is around 5 m/y. With a rate of 10.59 m/y, Pozhiyoor had the highest erosion rate. According to the data, the rate of erosion is faster between Pozhikkara and Veli. The increase in shoreline changes is primarily may be due to increased cyclone occurrences in the Arabian Sea, the formation of swell waves, changes in wave energy, and sea level rise due to climate change. The slow rate of sediment discharge by rivers, groyne construction, groyne spacing, groyne length, long shore currents, and preferential northward sediment transport all play a role in the formation and destruction of beaches along India's west coast. Using machine learning techniques, a prediction model for the year 2027 was created using data from the previous 14 years (2006–2020). According to the model, almost the entire stretch will experience severe erosion, with the rate remaining consistently high between Shanghumugham and Anchuthengu. As a result, soft and appropriate engineering solutions are required for coastal stability all along the beach, particularly at these two locations after a feasibilty study.

Assessment of Coastal Landscape Along Kelantan Coast Using Geospatial Techniques

Abstract The social and economic development from the coastal area benefits human life. However, growing human and environmental pressures at coastal areas also bring significant impacts on coastal erosion and coastline changes. Nowadays, geospatial technology which utilized remote sensing and GIS techniques has been widely used to detect coastal erosion for controlling the development and coastal region sustainability. Thus, this study was conducted to assess the coastal erosion and accretion for the potential risk zone based on the characteristics of the landscape and land use land cover (LULC) at the Kelantan Coast using land use classification and GIS spatial interpolation techniques. In this study, the vulnerability level of the coastal profile at the Kelantan coast was determined from the beach profile survey and the Inverse Distance Weighting (IDW) interpolation method. Then, the Land Use Land Cover (LULC) along a 1 km buffer zone were classified by performing a supervised classification method on Sentinel 2 satellite images year 2020. The effect of these parameters on coastal erosion was determined from separated five management units (MU 1, MU 2, MU 3, MU 4, MU 5) along the Kelantan Coast based on the Malaysian Department of Irrigation and Drainage (DID). It is found that MU 3 (0.8%) and MU 5 (0.6%) were the area with the lowest average slope profile percentages which is located at Pantai Sabak and Pantai Kemayang respectively. Thus, the infrastructure, LULC and coastal communities in Pantai Sabak and Pantai Kemayang were potentially vulnerable to erosion. This finding supports the significant use of geospatial techniques of important decision-making to protect and mitigate steps toward sustainable coastal management along the coastline.

Prograding Shoreline, Emerging Gravel Bars, and Mangrove Recolonization in 40 Years on a Mixed Siliciclastic-Carbonate Coast in Southwestern Luzon, Philippines

Changes in shoreline positions provide a historical record for identifying causal mechanisms and adaptive responses of coastlines to a wide spectrum of sea-level change that arises either from natural or anthropogenic disturbances. This paper presents a case study of coastal change during a period of the steady rise in sea level that is coupled with frequent storm events. Observations from multiple sources including field survey, topographic map, and series of satellite images were combined and analyzed to determine shoreline changes along the western coast of Calatagan, Batangas from 1979–2018. The Digital Shoreline Analysis System (DSAS) extension tool of ArcGIS was used to estimate magnitudes of shoreline changes, either erosion or accretion. Surface sediment samples were acquired in March and April 2010 to assess the predominant grain size and composition. Historical shoreline positions show that the western coast of Calatagan exhibits discrete erosion, but predominantly coastal accretion trend over a period of nearly 40 years. Both land progradation and the emergence of sand-gravel bars after 1992 occurred in a period of rising sea level across the South China Sea basin, including the West Philippine Sea. Land progradation is attributed to an increase in sediment input to the coast from agricultural activities in unconsolidated volcanic terrain. Notably, overlapping events of storms between 1993–1995 and the 1994 tsunami in the Verde Island Passage (VIP) may have emplaced and reworked a large amount of reef front material onto the reef flat forming emergent bars. Newly accreted lands and emergent bars were subsequently colonized by Avicennia and Sonneratia mangroves. This study highlights that in certain areas, local factors such as increased sediment supply from agricultural activities, storms, and possibly tsunami can modulate shoreline changes in response to modern-day sea-level rise.

Monitoring shoreline shift under rapid sea-level change on the Caspian Sea observed over 60 years of satellite and aerial photo records

The Caspian Sea is the world’s largest inland water body and endures rapid fluctuations in sea level and shoreline changes. The present study investigates and analyzes shoreline changes in the two areas of Tajan and Sefidrud on the southern coasts of the Caspian Sea using aerial photos, affixed with TM, ETM, and OLI images from Landsat satellites, as well as images rendered by Google Earth, all within the time period from 1955 to 2018. Shoreline changes were analyzed using the waterline algorithm on high resolution aerial photos and images from the Google Earth engine. Shoreline extraction on Landsat images was conducted after comparing six different indices, of which the wetness index exhibited the least mean error (1.3 m). Calculations on shoreline changes were undertaken in 4 time periods using the DSAS extension in Arc GIS 10.3 including 1955–1977, 1977–1995, 1995–2002 and 2002–2018. Coastal accretion on the Tajan shoreline after decreases in sea level were evident in the first, third, and fourth time periods, while coastal erosion was discernible in the second time period during significant increases in sea level. The situation, however, was different for the coasts of Sefidrud. The shoreline was divided into 14 distinct parts where the shoreline change rate was inconsistent with the actual changes in sea level. The dismatch between sea level and shoreline changes in the Sefidrud region points to the existence of a different set of factors, in addition to the sea level, which significantly affect the status of the coastlines.

Evaluation of the Effects of Cement and Lime with Rice Husk Ash as an Additive on Strength Behavior of Coastal Soil.

Coastal accretion and erosion are unavoidable processes as some coastal sediments undergo modification and stabilization. This study was conducted to investigate the geotechnical behavior of soil collected from Bagan Lalang coast and treated with lime, cement, and rice husk ash (RHA) to design a low-cost alternative mixture with environmentally friendly characteristics. Laboratory tests were carried out to analyze the physical properties of the soil (Atterberg limits and compaction properties), together with mechanical characteristics (direct shear and unconfined compressive strength (UCS) tests) to determine the effect of different ratios of stabilizer/pozzolan on the coastal soil and the optimum conditions for each mixture. Part of the purpose of this study was also to analyze the shear behavior of the coastal soil and monitor the maximum axial compressive stress that the treated specimens can bear under zero confining pressure. Compared to the natural soil, the soil treated with lime and rice husk ash (LRHA) in the ratio of 1:2 (8% lime content) showed a tremendous increase in shear stress under the normal stress of 200 kPa. The strength parameters such as the cohesion (c) and internal friction angle (ϕ) values showed a significant increase. Cohesion values increased considerably in samples cured for 90 days compared to specimens cured for 7 days with additional LRHA in the ratio of 1:2 (28%).

Shoreline Dynamics in East Java Province, Indonesia, from 2000 to 2019 Using Multi-Sensor Remote Sensing Data

Coastal regions are one of the most vulnerable areas to the effects of global warming, which is accompanied by an increase in mean sea level and changing shoreline configurations. In Indonesia, the socioeconomic importance of coastal regions where the most populated cities are located is high. However, shoreline changes in Indonesia are relatively understudied. In particular, detailed monitoring with remote sensing data is lacking despite the abundance of datasets and the availability of easily accessible cloud computing platforms such as the Google Earth Engine that are able to perform multi-temporal and multi-sensor mapping. Our study aimed to assess shoreline changes in East Java Province Indonesia from 2000 to 2019 using variables derived from a multi-sensor combination of optical remote sensing data (Landsat-7 ETM and Landsat-8 OLI) and radar data (ALOS Palsar and Sentinel-1 data). Random forest and GMO maximum entropy (GMO-Maxent) accuracy was assessed for the classification of land and water, and the land polygons from the best algorithm were used for deriving shorelines. In addition, shoreline changes were quantified using Digital Shoreline Analysis System (DSAS). Our results showed that coastal accretion is more profound than coastal erosion in East Java Province with average rates of change of +4.12 (end point rate, EPR) and +4.26 m/year (weighted linear rate, WLR) from 2000 to 2019. In addition, some parts of the shorelines in the study area experienced massive changes, especially in the deltas of the Bengawan Solo and Brantas/Porong river with rates of change (EPR) between −87.44 to +89.65 and −18.98 to +111.75 m/year, respectively. In the study areas, coastal erosion happened mostly in the mangrove and aquaculture areas, while the accreted areas were used mostly as aquaculture and mangrove areas. The massive shoreline changes in this area require better monitoring to mitigate the potential risks of coastal erosion and to better manage coastal sedimentation.

Accretion–Erosion Dynamics of the Yellow River Delta and the Relationships with Runoff and Sediment from 1976 to 2018

Timely understanding of the coastal accretion–erosion dynamics of the Yellow River Delta (YRD) can not only deepen the understanding of the evolution of the delta but also provide scientific support for water-sediment regulation (WSR) in the lower reaches of Yellow River and the implementation of a protection strategy for the Yellow River Estuary. In this long-term study, Landsat images from 1976 to 2018 were acquired, and the cloud processing platform of the Google Earth Engine was used for extraction of coastlines. On the basis of these coastlines, the area and accretion–erosion dynamics were analyzed. Then, after statistical analysis of the interannual and intra-annual variations in runoff and sediment, we discuss the relationship between the accretion–erosion dynamics and the annual runoff and sediment. The results show that (1) the coastline of the YRD lengthened first and then shortened, and the average annual growth rate was 1.48 km/a. (2) The land area of the YRD showed a significant accretionary trend before 1996, with an average annual growth rate of 28.60 km2/a. Then, the area gradually decreased from 1997 to 2001. After WSR was implemented in 2002, the accretion–erosion dynamics gradually became smooth, with an annual growth rate of 0.31 km2/a. (3) After WSR, the maximum annual sedimentation decreased by 79.70%. The average annual sediment discharge accounted for only 6.69% from November to March of the following year during the non-flood season. (4) With the continuous decrease in sediment discharge, the determination coefficient (R2) between the cumulative accretion–erosion area of the estuary and the annual sedimentation decreased from 0.98 in 1976–1996 to 0.77 after 2002. Overall, although WSR has maintained a steady increase in delta land area, it cannot change the long-term decrease in the land area of the delta. The insights gained from our study can provide some references for related coastline research, and will be useful to science community and decision makers for coastal environmental monitoring, management, protection, and sustainable development of the YRD.

Analysis of shoreline changes in Vishakhapatnam coastal tract of Andhra Pradesh, India: an application of digital shoreline analysis system (DSAS)

ABSTRACT Coastline or Shoreline calculation is one of the important factors in the finding of coastal accretion and erosion and the study of coastal morphodynamic. Coastal erosion is a tentative hazard for communities especially in coastal areas as it is extremely susceptible to increasing coastal disasters. The study has been conducted along the coast of Vishakhapatnam district, Andhra Pradesh, India with the help of multi-temporal satellite images of 1991 2001, 2011 and 2018. The continuing coastal erosion and accretion rates have been calculated using the Digital Shoreline Analysis System (DSAS). Linear regression rate (LRR), End Point Rate (EPR) and Weighted Linear Regression (WLR) are used for calculating shoreline change rate. Based on calculations the district shoreline has been classified into five categories as high and low erosion, no change and high and low accretion. Out of 135 km, high erosion occupied 5.8 km of coast followed by moderate or low erosion 46.2 km. Almost 34.7 km coastal length showed little or no change. Moderate accretion is found along 30.5 km whereas high accretion trend found around 17.8 km. The outcome of shows that erosion is prevailing in Vishakhapatnam taluk, Ankapalli taluk, Yellamanchili taluk whereas most of the Bhemunipatnam coast is accreting. Natural and manmade activities and phenomena influence the coastal areas in terms of erosion and accretion. The study could be used for further planning and development and also for disaster management authority in the decision-making process in the study area.

Signatures of late Quaternary land-sea interactions and landform dynamics along southern Kerala coast, SW India

The Kerala coast displays a spectrum of varied geomorphic features that are evolved as a function of Late Quaternary land-sea interaction. The southern Kerala coast, south of Achankovil Shear Zone (ASZ) has been characterised by cliffed coast with entrenched estuarine basins, pocket beaches, promontories, older and younger strand plains, etc., that are evolved as a result of the transgressive-regressive phases to which the coast has been subjected during the late Quaternary Period. In spite of having varied geomorphic features, lacuna in comprehending the evolutionary phases of the coastal landform features is the primary impediment that bolstered us to unravel the Late Quaternary land-sea interactions and landform changes of the coast between Thottapally and Vizhinjam. Moreover, the last two centuries are known to have been witnessing many natural as well as human induced environmental perturbations in the area.The present study attempts to reconstruct the late Quaternary landform dynamics based on the satellite data while the last century changes were extracted from the Digital Shoreline Analysis System (DSAS). The evidences based on geomorphic variations suggest that the present study area hosts two distinct paleo-coastlines – 1) coincides with the eastern boundary of the older strand plains (accreted during Late Pleistocene) and, 2) marks the boundary between older and younger strand plains (accreted during Early–Middle Holocene). A sector-wise analysis reveals that the northern half of the study area is characterised by many coast perpendicular estuarine basins with well-developed Bay Head Deltas in its fluvial end, and Flood Tide Islands near the estuarine mouth. On the contrary, the southern half is characterised by pocket beaches and coast parallel backwater bodies developed during the regressive phases of the sea. A high-resolution study of the shoreline changes during the period, 1920–2018 reveals that the younger strandlines are vulnerable to severe coastal erosion and shoreline retreat at many places compared to coastal accretion. The study stresses the imminent need for continuous monitoring and implementation of site-specific mitigation measures for the conservation and management of this coast known for its outstanding natural beauty and strategic beach placer deposits.

Coastal accretion and sea-level rise in the Cuban Archipelago obtained from sedimentary records

Sea-level rise (SLR) is one of the most pervasive consequences of global warming, and the Cuban Archipelago is threatened by current and future SLR. In order to support adaptation plans, it is essential to have reliable information about sea-level change during the last decades at the local scale, particularly in the most vulnerable regions. Here, we use sedimentary records to evaluate coastal accretion and to estimate the relative sea-level rise (RSLR) in two vulnerable coastal sites in central Cuba: Cayo Santa María (CSM) and Península de Ancón (PA). Both sites showed sediment sections with a continuous record of sediment accretion as a result of relative SLR and tropical storms. The sedimentary process was different between CSM and PA owing to differences in geomorphology and primary mineral composition. Sedimentary records also showed recent impacts of anthropogenic activities, likely increasing the vulnerability of the shoreline to SLR. The estimated RSLR values agreed with tidal gauge records, although they spanned a much longer time period (CSM: 0.5 ± 0.1 mm a−1, span of 38 years; PA: 1.5 ± 0.3 mm a−1, span of 92 years). Our results confirm that this methodology may be used to estimate the RSLR in places where data by instrumental records do not exist.

Coastline changes and their effects on land use and cover in Subang, Indonesia

Indonesia is at high risk of coastal erosion. Therefore, is important to understand the current status over a large area in order to devise protection strategies. This study measured the coastline changes and examined the land use and cover affected by coastal changes in Subang, Indonesia, using Landsat images from 1990 and 2018. The modified normalized difference water index (MNDWI) was used to separate the water and non-water features of the images, and land use and cover were classified using object-based image analysis based on the red, green, and blue bands and normalized difference vegetation index (NDVI). The results revealed considerable coastal erosion in Legonkulon. In the most extreme case, the coastline had moved 2361.46 m inland, and the total area lost to erosion was 1012.25 ha. This area was mostly covered by fishponds (983.34 ha) in 1990. Given that the fishery sector is the main livelihood of communities in the study area, the disappearance of fishponds might have affected their income and worsened their poverty. We found marked coastal accretion in Blanakan. In the most extreme case, the coastline had moved 1695.61 m seaward from 1990 to 2018, adding a total area of 1856.62 ha. The new areas were used as fishponds (1738.95 ha) in 2018. Although the accretion exceeded the erosion, the distributions differed regionally. Therefore, a regional protection strategy is necessary.

Non-native mangroves support carbon storage, sediment carbon burial, and accretion of coastal ecosystems.

Mangrove forests play an important role in climate change adaptation and mitigation by maintaining coastline elevations relative to sea level rise, protecting coastal infrastructure from storm damage, and storing substantial quantities of carbon (C) in live and detrital pools. Determining the efficacy of mangroves in achieving climate goals can be complicated by difficulty in quantifying C inputs (i.e., differentiating newer inputs from younger trees from older residual C pools), and mitigation assessments rarely consider potential offsets to CO2 storage by methane (CH4 ) production in mangrove sediments. The establishment of non-native Rhizophora mangle along Hawaiian coastlines over the last century offers an opportunity to examine the role mangroves play in climate mitigation and adaptation both globally and locally as novel ecosystems. We quantified total ecosystem C storage, sedimentation, accretion, sediment organic C burial and CH4 emissions from ~70year old R. mangle stands and adjacent uninvaded mudflats. Ecosystem C stocks of mangrove stands exceeded mudflats by 434±33MgC/ha, and mangrove establishment increased average coastal accretion by 460%. Sediment organic C burial increased 10-fold (to 4.5MgCha-1 year-1 ), double the global mean for old growth mangrove forests, suggesting that C accumulation from younger trees may occur faster than previously thought, with implications for mangrove restoration. Simulations indicate that increased CH4 emissions from sediments offset ecosystem CO2 storage by only 2%-4%, equivalent to 30-60MgCO2 -eq/ha over mangrove lifetime (100year sustained global warming potential). Results highlight the importance of mangroves as novel systems that can rapidly accumulate C, have a net positive atmospheric greenhouse gas removal effect, and support shoreline accretion rates that outpace current sea level rise. Sequestration potential of novel mangrove forests should be taken into account when considering their removal or management, especially in the context of climate mitigation goals.

Mangroves and shoreline erosion in the Mekong River delta, Viet Nam

The question of the rampant erosion of the shorelines rimming the Mekong River delta has assumed increasing importance over the last few years. Among issues pertinent to this question is how it is related to mangroves. Using high-resolution satellite images, we compared the width of the mangrove belt fringing the shoreline in 2012 to shoreline change (advance, retreat) between 2003 and 2012 for 3687 cross-shore transects, spaced 100 m apart, and thus covering nearly 370 km of delta shoreline bearing mangroves. The results show no significant relationships. We infer from this that, once erosion sets in following sustained deficient mud supply to the coast, the rate of shoreline change is independent of the width of the mangrove belt. Numerous studies have shown that: (1) mangroves promote coastal accretion where fine-grained sediment supply is adequate, (2) a large and healthy belt of fringing mangroves can efficiently protect a shoreline by inducing more efficient dissipation of wave energy than a narrower fringe, and (3) mangrove removal contributes to the aggravation of ongoing shoreline erosion. We fully concur, but draw attention to the fact that mangroves cannot accomplish their land-building and coastal protection roles under conditions of a failing sediment supply and prevailing erosion. Ignoring these overarching conditions implies that high expectations from mangroves in protecting and/or stabilizing the Mekong delta shoreline, and eroding shorelines elsewhere, will meet with disappointment. Among these false expectations are: (1) a large and healthy mangrove fringe is sufficient to stabilize the (eroding) shoreline, (2) a reduction in the width of a large mangrove fringe to the benefit of other activities, such as shrimp-farming, is not deleterious to the shoreline position, and (3) the effects of human-induced reductions in sediment supply to the coast can be offset by a large belt of fringing mangroves.

Coastline change and offshore suspended sediment dynamics in a naturally developing delta (Parnaíba Delta, NE Brazil)

Continuous coastal erosion has been observed on the shoreline of major river deltas worldwide as a result of relative sea level rise and anthropogenic impacts. However, the relative contribution of sea-level rise on deltaic coastline retreat (i.e. separately from anthropogenic interference) has not been clearly defined, due to a lack of research on naturally developing deltas.The Parnaíba River Delta, located on the NE Brazilian coast is one of the few examples of a river delta that is still developing in almost natural conditions in contrast to many heavily impacted deltas worldwide. The wave- and tide-dominated delta is subjected to delta switching processes with two active distributaries in the east and remnant tidal channels in the west. To address the decadal evolution of the delta in terms of suspended sediment transport and shoreline changes remote sensing techniques applied on orbital sensors were used. Satellite images from LANDSAT missions 2 to 8 were used to assess shoreline change between 1972 and 2016. Moreover, maps of suspended sediment concentration calculated from water leaving reflectance revealed the importance of Parnaíba River as major sediment supplier for the adjacent coast.Sufficient sediment supply from the Parnaíba River (PR) maintains shoreline stability and progradation of the delta in the region of the distributaries despite of a reservoir located 700 km upstream, which is supplied by only 25% of the drainage basin. Westward longshore drift of the sediment yielded by the Parnaíba River is an important transporting agent distributing river sediment along the coast. During ebb-phase, the PR discharge pushes the suspended sediment plume as far as 10 km offshore and the runoff forms a hydrodynamic barrier preventing alongshore transport of sediment from the east to the west part of the delta shoreline. During flood-phase and high tide, the river sediment plume is pushed shoreward by tidal forces and westward by longshore currents. A high turbidity zone extends from the river-mouth plume of the main distributary to the western tidal channels. Turbid water enters the tidal channels during flood-phase and leaves during ebb-phase. Coastal accretion or stability characterizes the eastern active part of the delta while moderate erosion prevails in the western abandoned part damped by the westward directed longshore sediment transport from the PR mouth.