Assessment high dune activity Navarro et al 2011

Geomorphology, 2011

Dune activity or stability has usually been studied over long time periods; however, this may not reflect changes that occur in the short term, especially for highly active dunefields. Extreme wind conditions that are generated near the Strait of Gibraltar (SW Spain) have given rise to the transgressive Valdevaqueros dunefield. The current work focuses on analyzing the sand drift potential and the evolution of the dune profile in the medium term (months), the short term (days) and the very short term (hours). Topographic data, which were collected with a differential GPS, were interpreted from reconstructed empirical orthogonal functions (EOF). The results showed that generally the dune profile presented shifting morphologies, especially around the crest and brink, and a trend towards migration to a gentler steady state. As a result, the leeward side adopted continuous slope variations during the different survey periods, whereas the windward slope did not undergo any significant change. Lateral and vertical displacements were analyzed during a severe easterly sandstorm, when the dune brink experienced an advance migration rate of 1.75 m in 24 h. Sand transport rates of 25.5-36.5 m 3 m − 1 month − 1 , 22.52 m 3 m − 1 day − 1 and 0.93 m 3 m − 1 h − 1 were measured for the medium term, short term and very short term, respectively. These values were compared to the theoretical sand transport rate for Valdevaqueros dune, based on the classic Bagnold equation as well other more recent formulae, to obtain a ratio between the real and the theoretical rates for each study period. These results together with the sand drift potential (up to 10,000 vector units) demonstrate that Valdevaqueros (Tarifa) is a dunefield with one of the highest sand transport capacities in Europe.

The Valdevaqueros dune is located at one of the windiest points of Europe, where the frequent occurrence of strong easterly winds has generated a highly mobile dune. Several rotating cup anemometers in vertical array and a self-designed vertical sand trap, were placed to retain the drift sands at different heights over the surface in order to determine theoretical and actual sand transport rates in the Valdevaqueros dune system. General results show that 90% of the wind-blown sand is transported within the first 20cm above the dune crest surface. Theoretical transport rates based on different empirical formulae were 0.33 to 0.78 times the in-situ sand transport rate detected, which was 2.08·10-2kgm-1s-1 under moderate wind power (mean speed ranging from 8.4 to 17.9ms-1). Analysis of different statistical grain-size parameters helped to understand sand transport distribution at different heights.

Geologica Acta, 2015

The Valdevaqueros dune is located at one of the windiest points of Europe, where the frequent occurrence ofstrong easterly winds has generated a highly mobile dune. Several rotating cup anemometers in vertical array anda self-designed vertical sand trap, were placed to retain the drift sands at different heights over the surface in orderto determine theoretical and actual sand transport rates in the Valdevaqueros dune system. General results showthat 90% of the wind-blown sand is transported within the first 20cm above the dune crest surface. Theoreticaltransport rates based on different empirical formulae were 0.33 to 0.78 times the in-situ sand transport rate detected,which was 2.08·10-2kgm-1s-1 under moderate wind power (mean speed ranging from 8.4 to 17.9ms-1). Analysis ofdifferent statistical grain-size parameters helped to understand sand transport distribution at different heights.

Given the risk of two roads being buried by the sand of two highly mobile coastal dunes (Valdevaqueros and Bolonia, SW Spain), several measures have been taken over the last half century in order to stabilize them or at least slow their progress: installation of several rows of 1-m-high concrete structures, planting of species such as Pinus pinea, Retama monosperma and Ammophila arenaria, wicker and wooden fencing, and sand mining (only performed on the Valdevaqueros dunefield). The evolution of the two dunes was investigated through the interpretation of intensive topographical monitoring and aerial photography. Average migration rates of approximately 10 m yr -1 were detected and the burial of anthropic structures has eventually occurred. This process has caused a serious regional problem, making dune management of these areas a difficult challenge. The comparison of the evolution of these two dunes has been of great importance in assessing the effect of historical human intervention and has provided a new perspective for future dune management strategies. The results obtained show that management measures based on sand removal have been proven to be unsustainable. Moreover, the monitoring methodology presented herein has proven very useful in predicting dune advance rates. For instance, the Bolonia dune could reach the nearest road in approximately 12 years.

We performed a computer simulation on patterns and dynamics of desert dunes. Dune patterns observed in deserts were reproduced. From the initial random state, barchans and linear dunes are produced, depending on the variability of the wind direction. The efficiency in sand transport is calculated through the course of development. We found that the sand transport is the most efficient in the linear transverse dune, barchans next, and the least when no pattern is formed. The efficiency in sand transport always increased through the evolution, and the way it increase was stepwise. We also found that the shadow zone, the region where the sand wastes the chance to move, shrinks through the course of evolution.

2007

Located in Tarifa, near the Strait of Gibraltar, the migration of Valdevaqueros dune was controlled by sand extractions for building purpose. As the Spanish Shore Act was approved in 1988, the sale of sand was forbidden and the dune started to move forward, burying the leeward pine wood and the only road to Maspalomas village. Consequently, six topographic levellings have been taken during the last eleven years. Sedimentary granulometries and meteorological features were taken into account. D 50 values have been calculated for dune foot, windward and dune top. Wind shear speed is estimated above 15 km/h. East winds are responsible of dune movement, whose advance rate has reached very high values (over 18 m/year). Slope and volumetric changes have been determined, providing a clearer explanation of the dune evolution.

Journal of Coastal Research, 2013

The Corralejo dune system in Fuerteventura in the Canary Islands (Spain) is an aeolian landscape whose genesis and evolution has commonly been associated with a traditional input-output model by which sediments entered from northern beaches and accumulation occurred in the southernmost (land-locked) sections of the dune field. Satellite images as well as aerial photographic evidence over the last 60 years, however, shows that the evolution of the dunes may reflect a more complex scenario in which under current settings and orientation of the coast, a sediment leakage back into the southern coastal environment is occurring. Under these circumstances, longshore currents can then transport this sediment back along the coast toward its initial landfall location, thus re-joining a sediment-recycling pathway. Human occupation of such a system can sometimes interrupt this pathway, seriously impinging its functionality. Better understanding of its morphodynamics at this scale will help negate these impacts.

New Frontiers in Engineering Geology and the Environment, 2012

Dune systems have a complex dynamics which is difficult to model purely physical due to the no linearity of the wind shear stress estimation, the complexity of the dune systems geomorphology and the difficult to obtain real world field data to test them. Cellular models offer an interesting alternative approach with simple data entry requirements and very good reproduction of dynamic processes observed in the real world. These models can be an useful tool in the study of the dynamics of the El Fangar spit dune system. The application of these models to the dune system data will help to predict morphological evolution, identify sediment sources and sinks, study the importance of formation factors and analyze possible future scenarios focused on climate change risk assessment. The actual cellular models had not been widely applied to real world dune systems so some improvements must to be taken into account in an attempt to approximate them more closely to complex real systems: integration of wind data as the driving force of the sand saltation process, introduce the possibility of study the system dynamics under a direction and velocity changing wind regime and couple a sand supply model to the dune dynamic model.

Geomorphology, 2009

The advance of the Valdevaqueros transgressive dune has resulted in gradual invasion of the adjacent road and ecosystem. Despite restoration and control activities performed within the last decades, strong winds have favoured dune destabilisation. Topographic surveys of five profiles were carried out in the research area from 1995 to 2006 to study the dune evolution. It was found that the average migration rate is 17.5 m/year, with a maximum of 38.0 m/year in the most active profile. The Empirical Orthogonal Functions method (EOF) is applied herein to identify tendencies in the aforementioned profile, separating spatial from temporal variations. Four spatial and temporal eigenfunctions account for 99.94% of the total data variability. The analysis of the individual eigenmode contributions allows us to identify particular and independent physical interpretations. The first eigenfunction represents the equilibrium dune profile. The second individual eigenmode shows pivot points separating different behaviours, which indicate a positive sand balance from the windward side towards the crest and the dune slip face. Finally, the third and fourth spatial and temporal modes reveal a slope increase and identify parallel dune advances, respectively. Although the dune sand budget (138 m 3 /m•year) is similar to the sand extraction rate (147 m 3 /m•year) over the research period, sand is still extending inland. Some different sustainable and less costly steps should, therefore, be taken to mitigate the dune invasion.

The main constraints for sustainable agricultural development in Sinai are essentially the migration of sand dunes that occupy about 5000 Km2 of the coastal zone. This adversely affects the cultivated lands and the newlyreclaimed areas. The study area is located at the NW corner of Sinai Peninsula and it .is subjected to sand duneencroachment. Movement of sand dunes causes severe damage to the human settlements, roads, irrigation anddrainage constructions. Migration of aeolian sands results in the migration of longitudinal dunes at a rate of 2.25 m/y for those south of Bir El Abd and 13 m/y at Wadi El Gady. The rate of barchan dunes movement at Wadi El Massaged is 3.5 m/y. Also, a lateral movement of the segments of longitudinal dunes is recorded south of Bir El Abd and at Wadi El Gady. The morphodynamic model of the longitudinal dunes shows that deposition (percent) on the longitudinal dunes south of Bir El Abd is low relative to erosion. However, deposition on the longitudinal dunes at Wadi El Gady is relatively high comparing to erosion.