Laura del Río

Error determination in the photogrammetric assessment of shoreline changes

The evaluation of error or uncertainty in shoreline change studies is an issue of prime importance for providing an adequate framework for calculated rates of change and to allow the establishment of threshold values above which the rates would be significant. In this note, a practical, easy-to-use method is presented to estimate error involved in the calculation of shoreline changes on aerial photographs, including the three most used types of shoreline indicators: high water line, dune/cliff toe and cliff top. This approach takes into account the specific characteristics of each shoreline proxy, such as relief in the case of the cliff top or tidal oscillations in the case of the high water line. At the same time it includes the error components that are independent from the proxy, basically related to the technical aspects of the process such as photo scanning and georeferencing. A practical example of application of the method is provided for several types of data inputs, based on shoreline changes around the Bay of Cádiz (SW Spain).

Image classification methods applied to shoreline extraction on very high-resolution multispectral imagery

Comprehension of vulnerability to coastal erosion in dynamic coastal environments strongly depends on accurate and frequent detection of shoreline position. The monitoring of such environments could benefit from the semi-automatic shoreline delineation method, especially in terms of time, cost, and labour-intensiveness. This article explores the potential of using a semi-automatic approach in delineating a proxy-based shoreline by processing high-resolution multispectral WorldView-2 satellite imagery. We studied the potential and differences of basic and easily accessible standard classification methods for shoreline detection. In particular we explored the use of high spatial and spectral resolution satellite imagery for shoreline extraction. The case study was carried out on a 40 km coastal stretch facing the Northern Adriatic Sea (Italy) and belonging to the Municipality of Ravenna. In this area a frequent monitoring of shoreline position is required because of the extreme vulnerability to erosion phenomena that have resulted in a general trend of coastal retreat over recent decades. The wet/dry shorelines were delineated between the classes of wet and dry sand, resulting from different supervised (Parallelepiped, Gaussian Maximum Likelihood, Minimum-Distance-to-Means, and Mahalanobis distance) image classification techniques and the unsupervised Iterative Self-Organizing Data Analysis Technique (ISODATA). In order to assign reliability to outcomes, the extrapolated shorelines were compared to reference shorelines visually identified by an expert, by assessing the average mean distance between them. In addition, the correlation between offset rates and different types of coast was investigated to examine the influence of specific coastal features on shoreline extraction capability. The results highlighted a high level of compatibility. The average median distance between reference shorelines and those resulting from the classification methods was less than 5.6 m (Maximum likelihood), whereas a valuable distance of just 2.2 m was detected from ISODATA and Mahalanobis. Heterogeneous coastal stretches exhibited a larger offset between extracted and reference shorelines than the homogeneous ones. To finally evaluate the coastal evolution of the area, results from Mahalanobis classification were compared to a shoreline derived from airborne light detection and ranging (lidar) data. The fine spatial resolution provided by both methodologies allowed a detailed Digital Shoreline Analysis System (DSAS) comparison, detecting an erosive trend within a wide portion of the study area.

Impact of coastal storms in a sandy barrier (Sancti Petri, Spain)

ABSTRACT Benavente, J., Del Río, L. Plomaritis, T.A, Menapace, W. 2013. Impact of coastal storm in a sandy barrier (Sancti Petri, Spain). Behaviour of sandy geomorphological units under the impact of coastal storms is of great importance when studying coastal evolution in a medium- to short-term scale. In this paper we analyze the effect of storms in determining the recent evolution of Camposoto beach, a sector of Sancti Petri sandspit (Cadiz, SW Spain). Morphological and volumetric changes of washover deposits were evaluated under the effect of coastal storm series recorded during the winter 2009–2010. Several pre- and post-storm topographic surveys of the beach and foredune were performed using an RTK-DGPS, together with a topographic survey of the dune system and the washovers after that winter. The results were compared with dating from the winter 2008–2009, and with a DTM extracted from LIDAR from 2008. Moreover, changes in the area covered by the washover deposits and in the foredune morphology were evaluated by means of GIS tools on aerial photographs from 2008 and 2010. Results showed clear shoreline retreat in the study area, as well as an important impact of the 2009–2010 storm seasons over the foredunes. Changes in the morphology and dimensions of the washovers gave information about the patterns of storm-related medium-term evolution of the system. The above data were used for the calculation of thresholds of the minimum wave height capable of either activating former washover deposits or creating new washovers and foredune retreat. Based on these results we established thresholds for storms generating coastal retreat in the long-term evolution of the sandspit, as a consequence of dune erosion and overwash processes.

GIS-based approach to the assessment of coastal vulnerability to storms. Case study in the Bay of Cádiz (Andalusia, Spain)

ABSTRACT Santos, M., Del Río, L., Benavente, J., 2013. GIS-based approach to the assessment of coastal vulnerability to storms. Case study in the Bay of Cádiz (Andalusia, Spain) Storminess is considered one of the most significant natural threats to coastal areas, especially on sandy environments. When storm-related processes occur in developed areas, changes in coastal morphology are often accompanied by socioeconomic losses due to damage to infrastructure caused by erosion and flooding. The aim of this study is to assess coastal vulnerability related to storm events along a mesotidal, low-energy sandy coast in SW Spain, by analysing the main factors responsible for storm impact. This impact is considered in terms of vulnerability and coastal exposure, by the integration of physical variables (e.g. dune height or recent shoreline changes) and socioeconomic variables (e.g. land use type or population density) into a GIS. These variables are weighted and combined into two different indexes, a Physical Vulnerability Index (PhVI) and a Social Vulnerability Index (SoVI), which are then merged into a single normalized Coastal Vulnerability to Storm Index (CVSI). Results show significant spatial differences in vulnerability to storms along the study area, with the highest vulnerability level being found in the northernmost sector of the study area (Santa María del Mar urban beach), and the lowest level corresponding to the southernmost sector (El Castillo natural beach). Through the generation of colour-coded vulnerability maps, the index can be used as a guideline contributing to the determination of causes, processes and consequences of vulnerability related to storm events. Overall, the CSVI provides a rapid identification of potential risk areas, where specific intervention strategies should be adopted to prevent storm impacts.

Chapter 6 The rock coast of continental Europe in the Atlantic

Abstract Rocky coasts occur along more than one-third (37%) of the Atlantic continental European coastline, approximately 3666 km, often forming vertical cliffs and characteristically gently sloping shore platforms. The continental European Atlantic rocky coasts show a great variability of rock types and structural contexts, as well as different wave climates and tidal ranges. Through a review of previously published data on cliff retreat rates and shore platform erosion measured on monthly, seasonal, annual and decadal timescales, this paper highlights the different processes and agents, their magnitude and frequency in shaping rocky coasts. In particular, the links between cliff retreat, shore platform evolution, present dynamics and inheritance (understood as whether platform and other rock coast features were shaped by a higher sea level than the present) comprise one of the major contributions from continental European Atlantic rocky coasts to a global understanding of rock coast coastal geomorphology.

Morphological and evolutionary classification of sandy beaches in Cadiz coast (SW Spain)

ABSTRACT Del Río, L., Gracia, F.J. and Benavente, J., 2013. Morphological and evolutionary classification of sandy beaches in Cadiz coast (SW Spain). Sandy beaches are extremely dynamic systems, so gaining insight about decadal patterns of beach change is essential for the adequate management of risks affecting coastal zones. This work aims at improving understanding of the factors that control erosion-accretion processes and evolution of sandy beaches at the intermediate time scale. For this purpose, recent evolution of beaches along the 150 km long Atlantic coast of Cadiz (SW Spain) is investigated in relation to their morphology and dynamics. Dune toe and high water line changes are assessed based on georectified aerial photographs from 1956–2008, using GIS tools. Results show considerable spatial and temporal variability of recent shoreline changes along the study area, with mostly eroding trends along the northern sector and a predominantly stable southern sector. Important exceptions at certain points are related to the heterogeneity of the coast and the diverse natural and anthropogenic factors contributing to shoreline change in the area. A classification of the studied beaches is proposed, based on beach morphology and dynamics, thus helping to understand the way coastal morphological characteristics influence erosion-accretion trends. Rectilinear beaches are predominantly stable or accreting, whereas reef-supported beaches are mostly erosive. Z-bays generally experience erosion at one end and accretion at the opposite end, greatly influenced by local conditions. Enclosed beaches are stable where sediment budget remains unchanged, but rapidly erode or accrete if human interventions alter the balance. The classification allows identifying those beaches which are most sensitive to variations in controlling factors, such as sediment supply.

Influence of storms on coastal retreat in SW Spain

ABSTRACT Puig, M., Del Río, L., Plomaritis, T.A., Benavente, J., 2014. Influence of storms on coastal retreat in SW Spain. Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 193–198, ISSN 0749-0208. Natural and anthropogenic controls contribute to coastal change at diverse temporal and spatial scales. This study presents an analysis of storminess and a correlation with the medium-term shoreline erosion rates in two sand spits of the Gulf of Cadiz (SW Spain). Recession rates were assessed by means of aerial photographs and orthophotographs considering the dune foot as an appropriate proxy. Storm events were obtained from a combination of coastal wave buoy data and the hindcast database of the HIPOCAS project. The characteristics of the storms were compared with shoreline changes determined using the aerial photographs, and the correlation between them was estimated and discussed. The best agreement between storms and shoreline change was observed in section 1 of Sancti Petri sand spit, while Valdelagrana sand spit showed a lower dependence between shoreline erosion and storminess. This is related to the contrasting exposure of both sites, with Sancti Petri being more exposed and thus more sensitive to storm impact. It is suggested that the main causes of retreat in Valdelagrana are human interventions performed on the coast and in the nearby rivers (breakwaters, jetties and dams). Nevertheless, storminess contributes to modulate recession rate, so that erosion rates increase when the number of storms is higher.

Dunes in the Gibraltar Strait Realm

This chapter examines the origin, evolution and present state of the coastal dunes along the South Atlantic Spanish coast around the Strait of Gibraltar. Wind regime in the zone is strongly affected by the North Atlantic Oscillation (NAO). Low or negative NAO index values give rise to the prevalence of westerly humid winds, while during periods with a positive NAO index easterly, dry winds prevail. Several Holocene and historical aeolian phases have been identified in the zone, represented by different aeolian deposits, many of them containing archaeological remains. The oldest one dates to shortly after the mid-Holocene eustatic maximum. During historical times dune-building episodes were associated with a higher frequency and persistence of easterly winds. During the 20th century, and especially in the last few decades, the prevalence of positive values of the NAO index has favoured the growth and advance of mobile dunes in the vicinity of the Strait of Gibraltar. In contrast, dune ridges associated with westerly winds are much less mobile or stable and form shore-parallel ridges covered by vegetation. In the Atlantic side of the Strait, environmental health of coastal dunes is satisfactory, especially those included in the Strait of Gibraltar Natural Park, where more effective protection measures are applied. The Mediterranean side is characterized by a dense population and intense human interventions, leading to erosion problems. Climate change predictions for the next decades suggest an increase in the frequency and intensity of easterly-Levante winds in the Strait. This situation would favour the reinforcement and growth of the Levante-derived dunes around the Strait, and perhaps recovery of the small dunes existing in the Mediterranean side.

Beaches of Cadiz

The coast of Cadiz province (SW Spain) constitutes a complex system with a wide variety of hydrodynamic and geological-geomorphological conditions. Morphodynamic behaviour of beaches in this area shows more dissipative states in the Atlantic coast and more reflective states in the Mediterranean coast. Recent shoreline trends indicate a relatively stable behaviour along most part of Cadiz coast over the last decades, although there are numerous sectors with a clearly erosive trend, mainly in the Northern half of the province. The great spatial and temporal variability of shoreline changes observed is related to the heterogeneity of the coast, as well as to the diversity of factors contributing to erosion processes on each sector. Several coastal hazard hotspots appear along the study zone, mostly related to extreme meteorological events and short-term trends, namely coastal erosion and flooding. The frequency and severity of the risks derived from these hazards have increased over the last decades, mainly due to the increased anthropogenic pressure and urban development on the coast. The coast of Cadiz province (SW Spain) constitutes a complex system with a wide variety of hydrodynamic and geological-geomorphological conditions. Morphodynamic behaviour of beaches in this area shows more dissipative states in the Atlantic coast and more reflective states in the Mediterranean coast. Recent shoreline trends indicate a relatively stable behaviour along most part of Cadiz coast over the last decades, although there are numerous sectors with a clearly erosive trend, mainly in the Northern half of the province. The great spatial and temporal variability of shoreline changes observed is related to the heterogeneity of the coast, as well as to the diversity of factors contributing to erosion processes on each sector. Several coastal hazard hotspots appear along the study zone, mostly related to extreme meteorological events and short-term trends, namely coastal erosion and flooding. The frequency and severity of the risks derived from these hazards have increased over the last decades, mainly due to the increased anthropogenic pressure and urban development on the coast.

Multiple-Source Cliff Erosion in Southern Spain: Current Risk and Future Perspectives

ABSTRACT Del Río, L.; Gracia, F.J., and Benavente, J., 2016. Multiple-source cliff erosion in Southern Spain: current risk and future perspectives. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 1072 - 1076. Coconut Creek (Florida), ISSN 0749-0208. This work analyses coastal erosion causes, impacts and management strategies in two sandstone cliffs in SW Spain: El Almirante (EA) and Fuente del Gallo (FG). Five sets of aerial photographs spanning the last 50 years were used, combined with detailed field inspection. Retreat rate of the cliff top in EA is 0.8–1.6 m/yr, increasing over the last decade to 4 m/yr at some points. This poses a serious risk for infrastructure located on the cliff top, and cliff recession leaves an outstanding palaeontological site exposed to wave attack. Erosion rates in FG are lower, but there is a significant risk to beach visitors due to mass movements involved in cliff retreat. Interventions to mitigate erosion impacts in EA have consisted in tetrapod deployment and concrete fencing, while in FG stabilization measures include removal of unstable blocks, soil nailing, draining tubes, seeding, a riprap and beach nourishments. However, these measures have only had a limited success, as the main causes for cliff erosion are related to marine and subaerial processes enhanced by rainfall and cliff fractures. Undermining of the cliff base by waves in FG and in eastern EA triggers rock falls, which are easily dismantled by waves, resulting in a rapidly retreating cliff face. In central EA rotational slides are responsible for cliff top recession, favoured by a plastic substratum of marls. Future projections of shoreline position indicate a likely increase in potential damage generated by erosion in both sites, leading to the need for adopting alternative mitigation measures.