Cosecha de follaje de yuca

Ballistically rippled pyramidal sand accumulations form behind grass tussocks in the backshore and foredune area and in the lee of tidal debris along the strand line, during offshore or onshore winds (Plate 7.5). The tapering edge of the structure points downwind. Similar structures have been described in literature as 'sand shadows' (Fig. 7.3). If the sand surface is dry the sand shadow becomes characteristically ballistically

COASTAL AEOLIAN SEDIMENTARY STRUCTURES & FACIES ANALYSIS 7.6

rippled during moderate to strong winds (Plate 7.5, width of scale 13cm). The ripple crests are almost rectilinear and strike more or less directly across the crest of the shadow. Their length/width (30/24, 27/21, 57/46) ratio (the shadow length is partly dependent on the width of the obstacle) ranges from 1.2 - 1.3; the wavelength of the ballistic ripples ranges from 3.5-4 cm. The observed maximum height of the tapering sand shadow in the upwind region ranges from 6-8 cm.

Fig. 7.3 A ballistically rippled sand deposit in the lee of a tussock of vegetation. Wind towards observer (diagram from Allen, 1984, after a photograph by Stone, 1967).

Interpretation

The term sand shadow was used by Bagnold (1954, ppl89 -90) to describe a tapering accumulation of sand formed in the lee of an obstacle where the wind velocity is locally reduced. 'Current shadow' is a term used by Allen (1984) for such features in both aeolian and aqueous environments. The term sand drift was used by Reineck & Singh (1980, p221), as an alternative to Bagnold's (1954) sand shadow, for any kind of sand deposit associated with obstacles in the way of sand laden wind, which have well developed internal foreset laminae. The wind-flow over sand deposits is largely indicated by the patterns of superimposed ballistic ripples (Bradley, 1957; Gripp & Martens, 1963; Beheiry, 1967). Where the vegetation is more dense, the ripples mostly face inwards slightly towards the axis of the shadow, suggesting an inwards and upwards but still largely downstream flow of air in the mid - wake (Allen, 1984, pl99).

While investigating the modification of airflow by a discrete, semi-circular roughness element (e.g. vegetation) in order to understand the formation of 'shadow dunes’ Hesp (1988) concluded that these dunes are formed to the lee of discrete roughness elements by reverse flows that occur within a horizontally separated wake region. The height of the shadow dunes is determined by the roughness element width and the repose angle of the sand (Hesp, 1988 ).

7.2.2.2 Lee dunes/Current crescent Description

Ridge shaped sand deposit tapering downwind in the lee of tree stumps, brought ashore by the spring tides along the backshore, are observed at Tentsmuir (Plate 7.6). The obstacle (tree stump) is partly embedded in an extensively deflated backshore environment. Small-scale deflation ridges on either side of the ridge are indicative of the scouring action of the wind in assimilating sediments from the side of the obstacle towards the centreline (Plate 7.6).

(a)

(b)

(c)

(d)

Fig. 7.4 Development of four types of lee

Interpretation BX.ld>l954)rf °f “ °bS“C'e As the airflow is deflected around and

over an obstacle, a horseshoe shaped vortex is created and sand is initially deposited both in front of the obstacle and on either side as two tapering

wings (Bagnold, 1954; Greeley Ct al, 1974 b) Fig. 7.5 Schematic diagram showing the

development of a horseshoe vortex around an

(Fig. 7.4a, 7.5). The two wings eventually obstacle (from Pye & Tsoar, 1990).

coalesce (Fig. 7.4d) as the arms of the horseshoe

vortex gradually transfers sand towards the centreline of the obstacle, and over time the lee dunes become higher, longer and narrower (Pye & Tsoar, 1990). Such structures originating from coalesced horseshoe shaped sand accumulations have been referred to as 'current crescents' by Allen (1984, pi89-91).

7.2.2.3 Scour remnant ridges Description

The surface of the damp beach becomes deflated during strong winds leaving behind scattered ridges of sand, 5-10 millimetres high. (Plate 7.7, wind blowing from right to left). The surface of the beach then becomes irregular. The wind appears to

COASTAL AEOLIAN SEDIMENTARY STRUCTURES & FACIES ANALYSIS 7.8

erode sand from behind and around the ridges. Some sand is accumulated behind the ridges in the downwind direction and the structure is a couple of centimetres long. Sand strips are seen to move across and over these remnant ridges.

Interpretation

Various hypotheses have been put forward for the development of these ridges. 'Scour remnant ridges' a term coined by Allen (1965a), and described as 'sand tails' by Reineck & Singh (1980, p79) and also described by Wunderlich (1972) are believed to be small simple ridges of either snow, sand or mud preserved to leeward of resistant objects such as shells, stones or the remnant crusts of hardened snow, or due to the breaching of the salt hardened crust (McKee, 1957; Gripp & Martens, 1963). The downstream slope of the ridge crests (5-12°) are thought to compare with the angle of impact of the saltation (Allen, 1984, p203). However, Berry (1973) observed the development of ridges, along the Baja California beach, which was devoid of resistant objects as shells or stones and has attributed the formation of these structures to the non-uniform compaction of sand grains.

In all cases the erosion is accomplished by saltating grains. The presence of resistant objects does not appear to be a prerequisite for the formation of the ridges. The abrasive action of a strong sediment laden wind on a moist beach surface will lead to the development of the ridges. The spacing and slope of the ridges is a function of the saltation trajectory.

7.2.2.4 Pyramidal wind shadow dunes

Strong offshore winds produce pyramidal sand accumulations 2-3 m high in the lee of the primary dune ridge (semi-vegetated by Ammophila sp.) along West sands. These pyramidal wind shadow dunes (Hesp, 1981) (Plate 7.8) ('echo' dunes of Tsoar, 1983) are formed by slip face accretion within a three dimensional flow structure, during offshore wind, in the lee of the primary dune ridge (>1.5m). Strong offshore winds (maximum speed 20 ms1) cause eddy currents in the lee of the pyramidal dunes to pick up sand from the surface downwind from the dune and transport it to the lee

slope. Massive aeolian sand accumulation in the lee of the an eroded dune cliff has also been observed. Often a scarp-fill sequence is observed on eroding dunes at Tentsmuir.

Interpretation

A pyramidal wind shadow dune is formed in the wake region by the eddies and vortices which flow from the edges of the plant cluster to the ’centreline' (Hesp, 1988). The centreline is the zone of maximum deposition where opposing vortices meet. Along the centreline a ridge is formed with linear slopes extending outwards from the ridge crest to the bed (Hesp, 1988).

This pyramidal sand shadow accumulation during high velocity offshore winds in the lee of a primary dune ridge, is a

phase in the beach-dune recovery cycle

(Carter, 1980; Carter et al, 1990) where

sediments which have previously been lost from the dune by marine undercutting and scarping are returned to the dune slope (Fig. 7.6). Once the scarp slope becomes stable (below the angle of repose) it is stabilised by vegetation. As an agent of the recovery cycle, the offshore winds are conducive to the formation of a positive

Fig. 7.6 Three phases of dune slope undercutting, slumping and recovery (after Carter, et al, 1990).

dune budget where net shoreline retreat is less than the gross retreat.