Global Occurrence of Sandy Shorelines

Coastal classifications have been widely employed in the field of geomorphology to characterise the diversity of coastal landforms and the contexts within which they emerge, but hitherto no single system of classification has been comprehensive in scope or coverage21,22. Criteria in these classifications typically include tectonic23 and hydrodynamic controls, as well as the sedimentological response. Hydrodynamics controls considered include classifications of wave parameters24, tidal range24,25 and a combination of both26. A ternary classification presented by Boyd et al.27, which considers the relative importance of fluvial inputs, wave energy, and tidal forcing provided a useful analysis of siliciclastic sedimentary coasts. The combination of tectonic and hydrodynamic controls led to the proposition of coastal morphogenetic classifications28, which are probably the most widely used classification schemes.

Sediment texture and composition29 are additionally useful to classify and describe coastal sedimentary environments. However, previously reported values of the global occurrence of sandy shorelines vary between 10% and 75% (see Table 1). The methods used to arrive at these values remain, in most cases, unclear or qualitative (as also indicated in Table 1).

Table 1 Reported values of global and regional occurrences of sandy shorelines and percentages of chronic erosion and accretion. Full size table

In our analysis, we applied supervised (human-guided) classification to global cloud-free satellite images (see Section 3.2) to identify sandy shorelines. One of the main reasons for our focus here on sandy beaches is that detecting shoreline dynamics for non-sandy shores like muddy coasts can be complex. Mild foreshore slopes, resulting in large horizontal tidal excursions, and high water content hampers correct shoreline detection. In the case of mangroves, seasonal growth cycles can impede correct shoreline detection. Moreover, it should be noted that as the reflectance signatures of sand and gravel beaches cannot be differentiated in the satellite imagery, as both materials originate from the same granular composites of finely divided rock, our references to sandy beaches herein also includes gravel beaches.

Our analysis showed that 31% of the ice-free world shoreline is sandy. The continent with the highest presence of sandy beaches is Africa (66%), while in Europe only 22% of the shoreline is sandy (see inserted table in Fig. 1). The percentage of sandy shorelines obtained from this analysis for USA and Australia compare well with the more recently reported regional scale values (see Table 1). The larger deviation in percentage found for Europe is significantly influenced by the smaller total length of shoreline used in the Eurosion10 data base. It should be noted that the sandy beach classification also includes the gravel beaches in the world. The reflectance signatures of sand and gravel beaches cannot be differentiated in the satellite imagery as both materials originate from the same granular composites of finely divided rock.

Figure 1 Global distribution of sandy shorelines; the coloured dots along the world’s shoreline represent the local percentage of sandy shorelines (yellow is sand, dark brown is non-sand). The subplot to the right presents the relative occurrence of sandy shorelines per degree latitude, where the dashed line shows the latitudinal distribution of sandy shorelines reported by Hayes31. The lower subplot presents the relative occurrence of sandy shorelines per degree longitude. The curved, dashed grey lines in the main plot represent the boundaries of the ice-free shorelines considered in our analysis. The underlined percentages indicate the percentages of sandy shorelines averaged per continent. Map is created with Python 2.7.12 (https://www.python.org) using Cartopy (v0.15.1. Met Office UK. https://pypi.python.org/pypi/Cartopy/0.15.1) and Matplotlib58. Full size image

The global latitudinal distribution of sandy shorelines shows a distinct relation with latitude and hence with climate; no relation is found with longitude. The relative occurrence of sandy shorelines increases in the subtropics and lower mid-latitudes (20°–40°) with maxima around the horse latitudes (near 30°S and 25°N; see Fig. 1). In contrast, they are relatively less common (<20%) in the humid tropics where mud and mangroves30 are most abundant as a result of high temperatures and rainfall. The percentage of sandy shorelines decreases beyond the 50° parallel. This latitudinal distribution of sandy shorelines is in line with the latitudinal variation of the common sediments in the inner continental shelf reported by Hayes31, based on ~2000 transects from 131 coastal areas (see right subplot in Fig. 1).

Global sandy beach erosion

Worldwide beach erosion became apparent during the 1980s following the studies of the International Geographical Union working group on the Dynamics of Coastal Erosion (1972–1976) and the Commission on the Coastal Environment (1976–1984). In these studies, two hundred participants representing 127 countries contributed to a survey which indicated that 70% (10%) of the world’s sandy beaches experienced net erosion (accretion) while 20% were stable32. However, as these estimates were primarily a result of interviews, they are necessarily qualitative, at best. Furthermore, the estimates likely did not take into account changes occurring along undeveloped and uninhabited coasts due to the subjective methodology adopted.

The quantitative global distribution of sandy shorelines presented herein, for the first time, allows the derivation of objective and up to date global scale assessment of chronic shoreline changes (i.e. beach erosion/accretion). Beach erosion can occur at a range of timescales33. Individual storms will generally result in rapid short-term erosion, followed by short-term accretion, leading to negligible net change over time scales of a few weeks-months. If sediment deficiencies persist for long periods of time (e.g. due to longshore gradients in sediment transport, reduction of fluvial sediment supply to the coast), chronic erosion can result. The analysis presented here focusses on such chronic erosion and accretion. However, there are no common standards for the classification of rates of chronic beach change34 which is generally quantified through some statistical treatment of erosion rates and/or volumetric losses (e.g. ref.35.

The accuracy of the SDS data of ~0.5 pixel (see Section 1) and the study period of ~30 years allows for a classification of beach change rates with class boundaries of 0.5 m/yr. Hence, we adopted the chronic beach erosion classification scheme proposed by Esteves and Finkl36 and extended it with a classification for extreme erosion resulting in the below scheme:

Accretion>0.5 m/yr

Stable−0.5 to 0.5 m/yr

Erosion−1 to −0.5 m/yr

Intense erosion−3 to −1 m/yr

Severe erosion−5 to −3 m/yr

Extreme erosion<−5 m/yr

Our assessment shows that 24% of the world’s sandy beaches are persistently eroding at a rate exceeding 0.5 m/yr over the study period (1984–2016), while 27% are accreting (see Table 1). About 16% (18%) of sandy beaches are experiencing erosion (accretion) rates exceeding 1 m/yr.

Chronic erosion of beaches (<−0.5 m/yr) is shown across the globe with relatively low latitudinal variation (see Fig. 2). Generally, between 30% and 40% of sandy beaches per degree latitude are eroding with relatively high eroding values up to 50% just south of the equator associated with large-scale land losses adjacent to the Amazon River mouth.

Figure 2 Global hotspots of beach erosion and accretion; the red (green) circles indicate erosion (accretion) for the four relevant shoreline dynamic classifications (see legend). The bar plots to the right and at the bottom present the relative occurrence of eroding (accreting) sandy shorelines per degree latitude and longitude, respectively. The numbers presented in the main plot represent the average change rate for all sandy shorelines per continent. Map is created with Python 2.7.12 (https://www.python.org) using Cartopy (v0.15.1. Met Office UK. https://pypi.python.org/pypi/Cartopy/0.15.1) and Matplotlib58. Full size image

More severe erosion rates are found at various locations across the globe. About 7% of the world’s sandy beaches experience erosion rates classified as severe. Erosion rates exceed 5 m/yr along 4% of the sandy shoreline and are greater than 10 m/yr for 2% of the global sandy shoreline. On the other hand, about 8% of the world’s sandy beaches experience significant accretion (>3 m/yr), while 6% (3%) are accreting more than 5 m/yr (10 m/yr).

Taking a continental perspective, Australia and Africa are the only continents for which net erosion (−0.20 m/yr and −0.07 m/yr respectively) is found, with all other continents showing net accretion. The continent with the largest accretion rate (1.27 m/yr; see table in Fig. 2) is Asia, likely due to the artificial development of the Chinese coast and large land reclamations in, for example, Singapore, Hong Kong, Bahrain and UAE. On a global scale, the world’s beaches have accreted on average 0.33 m/yr over the past three decades, i.e. a total gain of 3,663 km2 over this period.

Using the SDS data we then focussed on coastlines that are internationally recognised as nature protected areas by the World Database on Protected Areas (WDPA), which is the most comprehensive global database on terrestrial and marine protected areas, produced by UNEP-WCMC and IUCN37. Compared to the global average, a relatively high percentage of sandy shorelines in the WDPA-identified areas are experiencing erosion. Our analysis indicates that 32% of all marine protected shorelines are sandy of which 37% are eroding at a rate larger than 0.5 m/yr, while 32% are accreting.

Quantifying local scale erosion/accretion due to human interventions

No single explanation can easily account for the observed erosion/accretion trends along the global sandy shoreline, or for the acceleration of erosion/accretion on any particular beach38. However, analysis of local trends derived from the global scale shoreline assessment presented herein can help identify natural and human drivers of shoreline change. To illustrate this, we present two highlights of erosive behaviour and two of accretive behaviour. Another four highlights are presented in the Supplementary Material (S3).

a) Sand mining and subsidence

The Mekong Delta in Vietnam, the third largest delta in the world, is increasingly affected by human activities and exposed to subsidence and coastal erosion. The large-scale shoreline erosion is attributed to excessive sand mining in the river and delta channels, and subsidence due to unregulated groundwater extraction39. Analysis of the SDS data (Fig. 3a) reveals slight erosion between 1984 and 1990, after which higher, but steady erosion rates are found. Erosion rates in the considered area typically range between 25–30 m/yr over the last three decades. Based on the strong linear trend, the SDS data may be used for projections of land loss and displacement strategies, as it is not expected that erosion rates will decrease in the near future unless mitigating measures are implemented.

Figure 3 Examples of the satellite derived shorelines for four selected cases of beach erosion and accretion due to human interventions. The left column presents two erosive cases while the right column shows two accretive cases. In each figure, the blue line indicates the oldest SDS shoreline while the red line is the most recent SDS shoreline. The graphs below indicate the shoreline positions over time at the white dashed transect for each case; the upper graphs correspond to the images on the upper row. The indicated change rates (m/yr) are obtained from fitting a line-of-best fit to the shoreline position data for each transect. Figure is created with Python 2.7.12 (https://www.python.org) using Matplotlib58. Maps are created with QGIS version 2.18.3 (Open Source Geospatial Foundation Project, http://qgis.osgeo.org) using satellite images provided by Google Maps. Map data: Google, Terrametrics, CNES/Airbus, IGP/DGRF, and DigitalGlobe. Full size image

b) Coastal structures

The harbour structures at Nouakchott, Mauritania, blocked the large unidirectional north-south longshore transport of sand since 1986, causing areas of beach erosion that has impacted the local social and urban developments. The shoreline evolution rates observed after the harbour construction are 10 times larger than the values that would have been observed in the natural state40. The harbour breakwaters induced severe erosion over a distance of more than 10 km in the downdrift zone where accretion was likely to occur in the absence of the harbour. The SDS data (Fig. 3b) shows erosion rates of 20 m/yr.

c) Sand Nourishments

A large-scale bypass system became operational in 2001 at the Tweed River, New South Wales, Australia, to mitigate erosion of the beaches to the north of jetties constructed at the river entrance41. The bypass system pumps sand from south of the river mouth to three beach compartments located north of the river through buried pipelines. The SDS data (Fig. 3c) depicts a beach widening of ~250 m at Coolangatta Bay in the four years after the bypass system was commissioned.

d) Interception of longshore drift by coastal structures

The construction of two training breakwaters at Praia da Barra near the Aveiro Lagoon, Portugal interrupted the high southward ambient alongshore transport estimated at about 1 million m3/yr42. This resulted in erosion at the south of the trained inlet affecting the shoreline over about 30 km downdrift, but also strong accretion updrift. The SDS data reveals the continuous and ongoing accretion of the northern beach at a rate of about 10 m/yr (Fig. 3d).

Global hot spots of erosive and accretive beaches

Here we present the top eroding and accreting coastal stretches (i.e. hot spots) in the world (Table 2). The largest erosive hot spot is just south of Freeport in Texas where a 17 km stretch the beach has eroded on average more than 15 m/yr over the last three decades. The world’s longest coastal stretch suffering severe erosion is located farther to the east in Texas where we observed a 29 km stretch of sandy beach with a mean erosion rate of 5.3 m/yr. Interestingly, four of the seven largest hot spots are located in the USA, consistent with the widespread concern and reports of erosion in the USA11,35,43,44.

Table 2 World’s largest erosive and accretive sandy beach hot spots. Full size table

The largest accretive hot spot is in Namibia at a location where a mining company has built unprotected sandy bunds in the sea to facilitate the diamond prospecting. The area landward of the bunds is dried out to enable more convenient diamond prospecting. Naturally accreting beaches of lengths exceeding 20 km and change rates larger than 7 m/yr are found at a migrating barrier island (Schiermonnikoog, The Netherlands) and at locations where sand dunes migrate into the sea (Madagascar and Mauritania). It is noteworthy that four of the seven largest accretive hot spots are in fact human-induced.

Outlook

In the near future we foresee great potential for remote sensing techniques and big data analysis in operational monitoring of the World’s coast and beaches. The global sandy shoreline change analysis presented herein is primarily based on Landsat imagery with 30 m resolution and a revisit time of 16 days. In recent years new satellites (Sentinel-2a,b) that will significantly enrich the satellite imagery data both in temporal (revisit time of a few days) and spatial resolution (<10 m) have been launched. At present, private institutions already provide satellite images at approx. 1 m resolution with a daily revisit and global coverage. We expect that this trend will demand more emphasis on big data statistics in the near future to closely and better monitor how the planet is changing.