Wind velocity and sand transport on a barchan dune

We present measurements of wind velocity and sand flux performed on the windward side of a large barchan dune in Jericoacoara, northeastern Brazil. From the measured profile, we calculate the air shear stress using an analytical approximation and treat the problem of flow separation by an heuristic model. We find that the results from this approach agree well with our field data. Moreover, using the calculated shear velocity, we predict the sand flux according to well-known equilibrium relations and with a phenomenological continuum saltation model that includes saturation transients and thus allows for nonequilibrium conditions. Based on the field data and theoretical predicted results, we indicate the principal differences between saturated and nonsaturated sand flux models. Finally, we show that the measured dune moves with invariant shape and predict its velocity from our data and calculations.     

Geomorphology, Stratigraphy, and Radiocarbon Chronology of LlanquihueDrift in the Area of the Southern Lake District, Seno Reloncaví, and Isla Grande de Chiloé, Chile

Glacial geomorphologic features composed of (or cut into) Llanquihue drift delineate former Andean piedmont glaciers in the region of the southern Chilean Lake District, Seno Reloncav', Golfo de Ancud, and northern Golfo Corcovado during the last glaciation. These landforms include extensive moraine belts, main and subsidiary outwash plains, kame terraces, and meltwater spillways. Numerous radiocarbon dates document Andean ice advances into the moraine belts during the last glacial maximum (LGM) at 29,363–29,385 ¹⁴C yr BP , 26,797 ¹⁴C yr BP , 22,295–22,570 ¹⁴C yr BP , and 14,805–14,869 ¹⁴C yr BP . Advances may also have culminated at close to 21,000 ¹⁴C yr BP , shortly before 17,800 ¹⁴C yr BP , and shortly before 15,730 ¹⁴C yr BP . The maximum at 22,295–22,567 ¹⁴C yr BP was probably the most extensive of the LGM in the northern part of the field area, whereas that at 14,805–14,869 ¹⁴C yr BP was the most extensive in the southern part. Snowline depression during these maxima was about 1000 m. Andean piedmont glaciers did not advance into the outer Llanquihue moraine belts during the portion of middle Llanquihue time between 29,385 ¹⁴C yr BP and more than 39,660 ¹⁴C yr BP . In the southern part of the field area, the Golfo de Ancud lobe, as well as the Golfo Corcovado lobe, achieved a maximum at the outermost Llanquihue moraine prior to 49,892 ¹⁴C yr BP . Pollen analysis of the Taiquemmire, which is located on this moraine, suggests that the old Llanquihue advance probably corresponds to the time of marine isotope stage 4. The implication is that the Andean snowline was then depressed as much as during the LGM. A Llanquihue-age glacier expansion into the outer moraine belts also occurred more than about 40,000 ¹⁴C yr BP for the Lago Llanquihue piedmont glacier.