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Peró coastal plain vulnerability271plain and the last on the flank of the active dunefield. At the other traps, sediment capture was nil orvery small. Therefore the few areas of significantwind transport are associated to places withoutvegetation cover at the inner or lateral flank of thedunes resulting in remobilization of their ownsediment stock, suggesting that there is nocontinuous transport from the foredunes to thedeflation plain. This becomes clearer when the trapsare grouped by location. Trap 7 presented significantsediment capture while at traps 5 and 4 located,respectively, outside the trailing ridge of theparabolic dune and at the deflation plain upwindfrom the dune, sediment trapping varied from nil tovery small. Trap 11 was also located on thewindward side of a reactivated parabolic dune. Trap14 was located on the flank of the active dune fieldwhere sediment transport is active because of acomplete absence of vegetation. Trap 12 was placedin a similar position, though closer to the beach, inorder to assess the flow of sediment which fed thisdune field from the beach. Surprisingly, this trapcaptured no sediment, suggesting that the main dunefield is also largely being fed by its own sedimentreservoir without receiving any significant contributionfrom the beach.Our results strongly suggest that themaintenance of the vegetation cover is fundamentalto inhibit the transport of sand, which is extremelysensitive to the water balance as illustrated by thesediment mobility that occurred in the areas with novegetation cover (traps 7 and 14). The observedpattern reflects a strong seasonality with expansionof vegetation cover and decrease of eolian sedimenttransport (Fig. 6).During the period of investigation the rainyseason, with monthly rainfall above 60 mm,extended from November 2007 to April 2008 withthe highest levels of precipitation in January andMarch (Fig. 7) while, according to Barbiére (1975),there is normally a decrease in precipitation afterDecember. The expected relationship between highprecipitation and low eolian sediment transport wasnot found in November, January or August. Asshown in figure 7, the number of rainy days in eachmonth was fewer than five, mainly associated withthe passage of cold fronts. After the rain the sandsdeprived of vegetation became once againsusceptible to transport by winds. Thus, for theeolian sediment transport, frequency of rainfall ismore important than the amount, whereas for themaintenance of vegetation the amount of monthlyprecipitation is still important.A better explanation for the observed patternof eolian sediment transport was found when themonthly duration of the highest wind speeds wastaken into consideration. During the period ofinvestigation mean monthly wind speed variedbetween 7 m/s and 9 m/s, with a constant standarddeviation of ±2 m/s, while the highest values rangedbetween 9 m/s and 14 m/s. When considering onlywinds from 50º to 70º, which represent the maindirection of transport, and velocities ≥ than 9 m/s therelation with eolian sediment transport issignificantly higher (Fig. 8). The 9 m/s windvelocity threshold was chosen as being the highestmean monthly wind velocity.Figure 5. Graphic correlation between median grain sizeand standard deviation of the beach, foredune anddeflation plain sediments. The samples inside the ellipserepresent the grain size characteristics of the sourcesediments that feed the parabolic dune and the activedune field; these sediments are characterized by lowstandard deviation (well to very well sorted) and mediangrain size finer than 2.3 phi (0.2 mm).Figure 6. Sediment accumulated in the traps reflectsalternating humid and dry weather conditions.In the light of the preliminary results theimportance of the vegetation cover is clear, both ininhibiting sediment mobility and in producing theopposite effect when removed. During the monitoringperiod it became apparent that the activityof off-road vehicles (buggies and motorcycles) alsotriggered the remobilization of sediments by dama-Pan-American Journal of Aquatic Sciences (2010), 5(2): 267-276