Progradation and retrogradation of the Medjerda delta during the 20th century (Tunisia, western Mediterranean)

et al. 2021

Sci Rep

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The arid coasts of North Africa, extending over 4633 km from the Gulf of Tunis to the Nile Delta, are undergoing pronounced shoreline retreats and coastal floodings that are reported as a consequence of the ongoing sea level rise resulting from global warming. Of particular interest are the abnormal shoreline dynamics for deltaic and sandy beaches, which are severely impacted by abrupt decadal variabilities in both climatic and anthropogenic drivers resulting in their increased vulnerability to disturbances from coastal hazards. Unfortunately, the evolution, distribution and impacts of these drivers remain largely unquantified, let alone understood, for these extensive arid coasts that harbor the major portion of North Africa’s population as well as unique and fragile marine ecosystems. To address this deficiency, we use GIS-based multi-criteria approaches combined with analytic hierarchy process to map the Coastal Vulnerability Index and the Socioeconomic Vulnerability Index along these coasts to investigate the amplitude and extent of shoreline deterioration resulting from sudden fluctuations in sediment transport to the coastline. We use the western bay of the Gulf of Tunis, the coasts of Tripoli and the Nile Delta as three validation sites for our vulnerability assessment. The statistical Integrated Coastal Vulnerability Index map reveals that 47% of arid North African coasts are characterized by high to very high vulnerability. In particular, we observe that the densely populated deltaic coasts in both Tunisia and Egypt are 70% more vulnerable than any others coast in the eastern Mediterranean Basin. These abnormally high-vulnerability extensive areas are also correlated with significant deterioration of coastal aquifers and hence in crop production, compromising local food security and resulting in increasing outflow migration trends. Both Tunisia and Egypt observed dramatic increases in the net population outflow migration by respectively 62% and 248% between 2000 and 2016, mostly from coastal areas. Our source analysis of the amplitude and extent of these high coastal vulnerabilities suggests that they result from the anthropogenic drivers of damming and rapid urban growth over the last few decades rather than the effects of global warming.

mentioning

confidence: 99%

Alarming coastal vulnerability of the deltaic and sandy beaches of North Africa

Hzami

Heggy

et al. 2021

Sci Rep

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“…As coastal sediments for both headland-coast and embayed beaches are provided by uvial sedimentation from several local drainage networks (Supplementary Data, Fig. S1 (a) (b)), the build-up of dams since the early to mid-century has considerably reduced natural sediment discharge to the coast, resulting in a sediment budget imbalance that causes the observed shoreline retreat [37].…”

Section: Total Observed Shoreline Retreatmentioning

confidence: 99%

Deconvolving The Effects of Coastal Erosion and Sea-Level Rise in Assessing Shoreline Retreat Along Semi-Arid Urban Coasts

Heggy

Hzami

2022

Preprint

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The alarming vulnerability of low-lying sandy beaches to the acceleration of global sea level rise has been confirmed in the recent IPCC AR6 report. The situation is worsened by increasing coastal erosion, resulting in additional shoreline retreat of sandy beaches along several semi-arid urban coastal areas around the globe. The additional shoreline retreats from erosion are indicative of the rising imbalance in coastal sedimentary processes, which are a direct consequence of changes in precipitation patterns, urban growth, and change in land use. To quantify the magnitude and timescale of both coastal erosion and sea-level rise (SLR) in generating shoreline retreat of sandy beaches in semi-arid urban areas, we combine photogrammetric and statistical methods to measure and forecast the decadal evolution of these coastlines using two well-characterized sites that are hypothesized herein to be globally representative of these types of coasts undergoing rapid urban growth. We use multi-decadal shoreline positioning and land use classification surveys of the Southern California (SC, USA) and the Hammamet-North (HAM, Tunisia) beaches from aerial and orbital photogrammetric images, combined with the Digital Shoreline Analysis System, for the period from 1985 to 2018. Our results suggest that the current average shoreline retreat rates of sandy beaches range from -0.75 to -1.24 m/yr in SC and from -0.21 to -4.49 m/yr in HAM under similar aridity, land coverage and precipitation patterns. The observed decadal changes in shoreline positions along these semi-arid urban coastal areas are found to be accentuated by anthropogenic drivers associated with extensive urbanization, causing sediment imbalance at the coastline, adding up to the effect of the accelerating SLR. We assess that ~81% and 57% of the observed shoreline retreat was due to SLR, and 19% to 43% due to coastal erosion from urban growth along SC and HAM beaches, respectively. Using these measured rates, we establish a semi-empirical numerical model that combines urban growth and the observed shoreline retreat rate to forecast retreat rates through 2100 for both of our study areas, inferred herein to be representative of other global semi-arid urban coasts. Our model suggests that future average total shoreline retreat rates, accounting for both urban growth and SLR, range from -2 to -4 m/yr for SC and HAM sandy beaches, respectively, through 2100. The above suggests that if no mitigation is made, by 2100 the cumulative shoreline retreat in these urban areas could significantly exceed the Global Scale Assessment Model’s [46] cumulative projected average retreat of -30 m, confirming the alarming vulnerability of the semi-arid coastal urban areas that would need intensive and costly beach nourishment to control increasing shoreline erosion.

“…After the dam construction, by the 1980th both the deltaic plain and the nearshore seabed are marked by decreasing of the grain size distribution with a clay deposit facies (D50 = 60 µm). The temporal evolution shows a rapid recession of the delta plain and shoreline level (Kotti et al, 2018b;Benmoussa et al, 2019). The dominance of clay deposits more than coarser fraction results from the impact of the construction of several large dams on the basin.…”

Section: Sediment Discharge Evolution From River To Coasts After Damsmentioning

confidence: 99%

“…Sediment discharge responds both to the impact of human activities and to changes in climate. Since the mid-century, human influences on the sediment flux to the coast have a significant effect on floodplain (Syvitski and Kettner, 2011), a large coastal land loss and delta-plain retrogradation (Benmoussa et al, 2018(Benmoussa et al, , 2019. Dams construction and maritime infrastructures have both intercepted the sediment supply and disturbed the sediment movements along the longshore path-ways.…”

Section: Introductionmentioning

confidence: 99%

Multi-criteria approaches to identify the shoreline retreat downstream of dams: the North African case

Mahé

2021

Proc. IAHS

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Abstract. Northern African beaches are among the most vulnerable areas under the extreme climate change hazard. Mainly sedimentary low-lying platform, the coasts are supplied by terrestrial yields, which are increasingly interrupted by dams. Unfortunately, the sediment fluxes are rarely measured and monitored, so that it is quite impossible today to assess the contribution of continental sediments to the coast and its variability. The aim of our study is to determine the sampling protocol of delta sedimentation plain and nearshore seabed for better understanding of the anthropogenic driver in contrast to climate change. We adopt a multi-criteria analysis based both on the geomorphologic feature and the historic evolution from the River to the littoral plain. The shoreline evolution reveals an alarming retreat trend reaching −20 m ± 0.15 m yr−1 after the human-induced change where ∼50 % of sediment discharge has been trapped upstream the dam, including quite all the coarse material, like sand. The shoreline retreat and the decreasing sediment rate of fluvial flow are all due to the dam construction.