Miriam Burch

DATA 150: Data Science for Human Development

Professor Brewer

21 November 2021

Word Count: 2,894

METHODOLOGICAL INVESTIGATION: COASTAL EROSION IN CATALONIA

I. INTRODUCTION

Over the past decade, technology has evolved rapidly. The techniques of data science, from machine learning, artificial intelligence and geotargeting to data visualization have significantly helped analyze regions most impacted by climate change. By studying the effects and patterns of the change via the use of various methodologies, scientific methods, datasets and predictive analytics models, data scientists have helped our human development process. Data scientists are continuously exploring and finding new ways to cope with climate change issues as they assimilate and compare data in order to predict future trends. With these advancements, climate change is now viewed as a more complex human development process and adaptive solutions can be developed to help solve problems by locating and pinpointing areas in which changes need to be made that will impact climate targets.

Catalonia, the northeastern most region of Spain, is an ideal part of the world to study climate change because of its diverse geography including sea, coastline, mountains, rivers, cropland, and urban and rural areas. This region has experienced temperature increases, drying climates, shortage of rainfall and sea temperature and sea level rises, especially in the summer months, all of which have been occurring at an accelerating rate in recent decades. In particular, Barcelona, the capital of Catalonia, has undergone some of the worst impacts of climate change such as temperature rises, a significant decrease in precipitation and an increase in droughts, heatwaves and coastal erosions. This methodological investigation focuses on two of the most important areas of climate change within Catalonia: (i) coastal erosions in Catalonia and (ii) oceanographic and meteorological observations at a coastal station in Catalonia (sea level and sea temperatures rising). Research surrounding these two data science methods have uncovered tremendous economic impacts. Maintenance and development (i.e., streets, railroads, houses) of eroded beach promenades have cost the Spanish government over EUR 50 million in the last twenty years. Additionally, during the late 80s and early 90s, more than 10Mm³ of sand was added to replenish the beaches. In Barcelona beaches alone, over 1Mm³ of sand has been replenished over the past six years and is now totally gone.

II. DATA SCIENCE METHODS

A. SOURCES

1. Coastal Erosion in Catalonia

In Managing Coastal Environments under Climate Change: Pathways to Adaptation, coastlines in Ireland, Spain and Romania are analyzed. The Spanish coastline is approximately 7900km long and according to the article, “50% consists of hard and soft cliffs (especially present at the Atlantic border) and about 25% consists of sandy beaches.” (1) Furthermore, the Catalan coast is around 800km long, located along the Spanish Mediterranean. The north is mostly composed of cliffs (about 280km long) and the south consists of low-lying beaches. The central region is shaped around the metropolitan area of Barcelona and includes various marinas and urban settlements. As stated in this article “urban sandy beaches are typically bounded in their backside by a seafront promenade and infrastructures like streets, roads, railways and houses.” (1) In the last years, the maintenance and development of promenades have been one of the major investments undertaken by the Spanish government, with more than 50 million euros only in Catalonia during the last two decades.” (1) A study was conducted to compare the erosion and shoreline changes of beaches from 1995-2004. It showed that “more than 70% of Catalan beaches eroded, with average rates of about −2.1 m/yr and of about −3.3 m/yr when the Ebro delta is considered. (1) The authors then compare low and high sustainability practices, describing what one should and should not do. In the low sustainability practices, local politicians and environmentalists don’t try to prevent climate change but rather jump straight to the solution as actions are taken after the impact (e.g. beach erosion). An example of this is the Maresme coastal stretch in the Central part of the Catalan coast. This area is where eroding beaches and locally accretive beaches are mostly found due to a net littoral current drift from north to south and the interruption of that drift by various marinas along the coast, resulting in narrower beaches (more erosion) the further south one travels along the Catalan coastline. This coastal area faces the most energetic wave events from the East, resulting in no sand replacement on the beach once it’s washed away because of the infrastructure (i.e., buildings, apartments, roads, railways) directly abutting the beach. To prevent further erosion, and without a comprehensive plan in place for the entire coastline, local authorities have undertaken the construction of riprap revetments to defend certain beach areas. In addition to the building of more marinas, construction behind the beach (e.g., streets, houses, buildings) has formed a barrier, causing less sand to wash down from hilly areas during heavy rainfalls to replenish beach sand and also interrupting the north-south movement of the current and sediment. One comprehensive policy solution to prevent beach erosion under these circumstances would be to limit the construction of new marinas, prevent new construction along the coastline that “breaks” the natural flow of sand from hilly areas to the shore, and also construct an ephemeral small dune of 1.5 m to prevent additional beach erosion.” (1) This dune would also serve as a sediment buffer to naturally incorporate more sand onto the beach over a period of time. Potential long-term downsides of the dune pointed out by the authors is that it will eventually collapse because of sea level rise as a response to climate change. The policy solution would also incorporate incremental interventions with regular monitoring and local community involvement. (1)

Figure 1a: Coastal archetypes from Spanish coast

2. Oceanographic and Meteorological Observations at a Coastal Station in Catalonia

This article, Forty-five Years of Oceanographic and Meteorological Observations at a Coastal Station in the NW Mediterranean: A Ground Truth for Satellite Observations, measures the air temperature and sea surface temperature in L’Estartit (a small town on the north-eastern coast of Catalonia) over many years. Local precipitation is trending downward, meaning that it has been raining less, and sea level temperatures are trending upward, or getting warmer. In fact, for every ten years, the sea level is rising three centimeters and sea levels are expected to rise one foot every 100 years. (2)

An environmental dimension of human development is being addressed by the authors’ research as climate change is the main factor in causing erosion along the coast. The development goal that can be considered in relation to this article is the analysis of the sea-level rise and temperature increase. One solution would be to not rake the beach so often, preventing erosion from occurring.

Figure 1b: Observational setting. Reference map for the position of station AA (a). Location of meteorological box (magenta circle), sea level gauge (red triangle), wind station (blue diamond) on a map (b), and on a picture (c), and bathymetric chart of the area (d)

B. COMPUTATIONAL DESCRIPTION

1. Coastal Erosion in Catalonia

Local interventions for coastal sustainability are gathered into two different groups: 1) working with nature (reed beds at the Danube Delta, restoration of dunes at Inch, overwash enhancement at Ebro Delta) and 2) Transient Defence Measures (TDM), such as the fuse dune system at Llobregat Delta. This TDM method consists of coastal protection based on specific actions that should be deployed before extreme events occur. The interest in these interventions will grow in a changing climate because of enhanced flooding and erosion due to sea-level rise and increases in population, which will also raise social and economic vulnerability. Eventually, TDM will become more competitive because they present lower costs and impacts than conventional engineering solutions and offer added flexibility based on local resources. Such an approach, however, requires the support of an early warning system to predict the storm impact with enough advance time to deploy the TDM for the more vulnerable coastal sectors. (1)

Figure 5: Erosional hotspots southward Catalan harbours

2. Oceanographic and Meteorological Observations at a Coastal Station in Catalonia

For climate trend analyses, “the primary measurements (all temperatures and sea level) have been averaged monthly. To remove the seasonal cycle, monthly anomalies are calculated by subtracting the monthly-averaged climatology (calculated over 45 years of data). Therefore, the total number of data points for each trend estimation is 540. Trends are obtained by linear regression of the anomalies and converted into values-per-year (per annum). The trend estimate is evaluated using the “t test” compared against a “null hypothesis” of no trend (zero trend value). The confidence interval is computed based on the significance level α = 0.01 (99% confidence) and the associated “p value”, stating the probability of observing the given trend under the null hypothesis, quantifies how rare it is to observe such a trend only by chance.” (2)

According to the article, “during the last years, the dataset has been used in many scientific papers related to climate research and to document extreme events such as cold or heatwaves, storms and their consequences on neritic ecosystems.” (2) Robust estimates of annual trends of sea temperature from SST (0.03 °C/year), S80T (0.02 °C/year), AT (0.05 °C/year) and sea level (3.1 mm/year) are presented.” (2) Two graphs in particular (“a” and “b”) illustrate climate satellites that focus on sea surface temperature (heat) and both serve to cross-check the data set of the other. Graph “b” is a close-up of graph “a” from March 2016 to December 2018 and to pinpoint this data, the authors use “multi-scale Ultra-high Resolution (MUR) analysis from the NASA Jet Propulsion Laboratory.” (2) Various satellite-based sea surface temperature (SST) analysis data sets were utilized to compare statistics from previous years. The authors are investigating the pattern of which sea levels rise and coastal flooding carries sand along the coast. How these patterns occurred from the start and how they can be prevented offshore L’Estartit is the scientific question the authors are seeking to answer.

Both of these data methods are complimentary towards one another because looking at ports a and b in the maps below relates to the sea level rise and helps us further understand how and why sea level temperatures are rising and coasts are eroding more rapidly.

Figure 2: Example of daily logs

C. COMPARISON OF METHODS

1. Coastal Erosion in Catalonia

The Gava municipality has proposed a maintenance plan where the main target is to safeguard a minimum emerged width. “Based on this an initial plan has been designed taking the sediment from the emerging and shallow neighbouring beaches instead of looking for sources outside the littoral cell. Sediment placement has also been designed to have a smooth coastline shape in order to reduce longshore sediment transport gradients. At the back beach, an ephemeral small dune of 1.5 m has been proposed to protect infrastructures from storm events of 15 years return period. The dune also acts as a sediment buffer which will naturally feed the area, thus enhancing the presence of an emerged beach. Fig. 7 below shows the dune/beach profile response at different locations selected for testing the design using a modified XBEACH model. The parameters of the model have been calibrated with similar dissipative beaches in the neighbourhood.” (1)

Figure 6: Shoreline changes in south Barcelona harbour during the period 2004-2010

2. Oceanographic and Meteorological Observations at a Coastal Station in Catalonia

In terms of the data analyzed vs the current satellite producers for the oceanographic and meteorological observations, “The L’Estartit sea surface temperature (SST) time series agrees quite well with the SST time series derived from the various satellite-based data sets down below, with an RMS difference of consistently just over 0.5 °C. This is comparable with the typical minimum uncertainty in satellite SST measurement of 0.3–0.5 °C. The satellite SST is cooler than the L’Estartit SST by 0.1 to 0.2 °C on average. A potential cause for this bias is the diurnal warming of sea temperature, meaning that while the satellite data sets are aiming to report the pre-dawn temperature, the L’Estartit measurements are performed mostly in the late morning to noon. The feature resolution (smoothness of the SST field) of the satellite data sets used are noted and vary vastly from around 1 up to 100 km, with the OSTIA and NCEI data sets having a similar feature resolution of around 100 km despite having fairly different grid spacing. The comparison statistics (RMS and bias) are remarkably consistent across feature resolutions up to 25 km, and degrade only by 0.1 to 0.2 °C for the OSTIA and NCEI data sets. This indicates that the temporal variability of the L’Estartit time series is representative of the areal average SST from the surrounding sea.” (2) The graph below represents the time series of sea surface temperatures recorded from the L’Estartit site and from Multi-scale Ultra-high Resolution analysis.

Figure 11: Time series of sea surface temperature recorded from L’Estartit site and from the Multi-scale Ultra-high Resolution (MUR, in grey) analysis. a 2002-2018. b Zoom from 2016-2018

D. FINDINGS

1. Coastal Erosion in Catalonia

In the first data method, the averaged loss of sand with the transient dune during storms (spanning from 5 to 15 years return periods) is around 15,000 m3 . Water depths from 0 to 2 m present an accretive behaviour (around 7000 m3 ). Longshore currents also play an important role (around 0.4 m/s in average), with littoral drift mobilising significant sand volumes southwards. (1)

In fact, the emerged zone flattens when the dune collapses. In Fig. 7 below, the dune starts to fail not under a collision regime (erosion at the offshore side) but because of overtopping volumes from the storm peak. The dune can withstand increases of 40 cm in mean sea levels that would correspond to a present storm surge or a future climatic rise. Beyond that point, the dune entirely collapses and sea level has a more prominent effect in the morphodynamic signature. This is because as waves increase, more sediment can be advected onshorewards, reaching a maximum for elevations between 1.5 and 2 m. I think that placing a dune and following the plan which they have been analyzing is necessary in the case of coastal erosion because human industrialization and development is being incorporated more and more along coasts nowadays and such infrastructure is preventing sediments from coming down to replenish the beaches. Additionally, creating these dunes are simply more economically efficient. (1)

Figure 7: Transient dune alternatives and storm response at Gavà beach (NW Mediterranean)

2. Oceanographic and Meteorological Observations at a Coastal Station in Catalonia

“The average annual cycle of air and SST (Fig. 4a) shows that from late winter-early spring to mid-summer, SST is lower than air temperature while during the rest of the year, the situation is reversed. The initial and ending days for the period when SST are colder than air temperature is presented in Fig. 4b. There is a successive advancement of the starting point of the period (p < 0.0001) while the delay in ending point is less significant (p < 0.01, i.e., still within 99%). Such behaviour has a relevant role in the air-sea interaction, in particular concerning sea water evaporation/condensation, and thus in the local water cycle.” (2)

Figure 4: a: Mean annual cycle (1974-2018) of monthly air and sea surface temperature. The period when air temperature is higher than sea surface temperature is marked. b: Time evolution of the period when air temperature is higher than sea surface temperature. Blue points indicate the initial day of this period and pink points the end. Lines show the linear fitting

III. CONCLUSION

Dealing with the effects of climate change in Catalonia has not been easy for local municipalities or the regional government. Through a critical analysis of the investigation relating to climate change in three main areas—inland water resources, fires and anthropogenic warming, and oceanographic and coastal areas—we see that each presents a unique challenge and, consequently, each requires a unique solution. In one case (inland water resources) the challenge involves more of an environmental and economic approach, while in the others (fires/anthropogenic warming and oceanographic/coastal areas) a multifactor approach dealing with the social, economic and environmental impacts of climate change must be addressed simultaneously to overcome the problems at hand.

One of the largest gaps in climate erosion is tourism. Erosion will have a greater effect on foreign tourism in the northern municipalities. The results of both these data methods and analysis underline the need to apply an articulated erosion risk assessment focusing on specific targets depending on the site in question. In a more structured and long-lasting approach, this procedure can further help coastal managers to acquire measured actions to deal with the impacts of erosion. Because there are larger population concentrations and developing human activities are more attracted to these areas, coastal zones are among one of the most highly threatened areas which increases the potential effects of damage by natural and human-induced hazards. Beach erosion not only poses a risk to existing assets, but also causes a significant setback to recreation and tourism and, consequently, threatens one of the most important sources of the economy in coastal regions.

No matter which data set or data science method one looks at across the investigation, one overarching theme presents itself: In order to overcome the effects of climate change, an integrated, long-term and adaptive policy framework is required at the level of regional government in close cooperation with local municipalities, informed by robust and predictive modelling with incremental interventions ready to be undertaken. As tourism is extremely important to the regional economy in Catalonia (Barcelona having the largest cruise ship port in the Mediterranean and one of the busiest airports in Europe), this begs the important question: Why is there no research on the effects of tourism on climate change in Catalonia? Even though regional and local politicians might not like the outcome of these studies, this gap in the investigation needs to be addressed immediately and constitutes an important focus for future research.

REFERENCES

(1) Sánchez-Arcilla, A., et al., Managing coastal environments under climate change: Pathways to adaptation, Sci Total Environ (2016), http://dx.doi.org/10.1016/j.scitotenv.2016.01.124

(2) Alan Buis, NASA’s Jet Propulsion Laboratory. “Forty-five years of oceanographic and meteorological observations at a coastal station in the NW Mediterranean: a ground truth for satellite observations” June 23, 2020.