Environmental Engineering Reference
In-Depth Information
fore-dunes. It is tolerant of physiological drought, nutrient-poor and saline-rich
embryonic soils, and wind shear. Ammophila arenaria (marram grass) and Agropyron
spp. (couch grass) are common in northern hemisphere mid-latitude foredunes and have
the additional ability to grow through and anchor frequent sand burial. Progressive
enlargement and colonization establish a fore- or yellow dune barrier which modifies the
air stream and allows a more diverse succession to develop in its lee. Shell fragments
deflated from the backshore add calcareous nutrients. The dune is not yet stable, and
parabolic blow-outs occur which advance the dune inland by 1-20 m a −1 . In the
progressively sheltered conditions of the dune slack and meadow , soil develops and
succession advances towards shrubs and woodland. Hind or grey dunes are more stable,
although still vulnerable to climatic change and human disturbance (Figure 16.3). Coastal
dunes may individually reach over 100 m high and dune landsystems extend for 10 1-3 km
inland.
CONCLUSION
Wind is more dependent on the prior operation of other geomorphic processes than any
other agent and is, ironically, not dominant in Earth's storm belts. Aridity enhances wind
power and the absence of vegetation is a decisive factor in determining the distribution of
aeolian landsystems. Wind lacks extensive rock-quarrying power, and its erosive impact
is confined largely to the scouring and ornamentation of bedrock surfaces. Dune bed
forms reflect the interactive effects of air flow and landforms on each other. Accelerated
deflation is one agent of desertification, removing topsoil and burying vegetation
incapable of matching sand aggradation rates. However, plant communities tolerant of
the harsh aeolian environment form distinctive biogeomorphic dune systems in suitable
locations. Aeolian deposits are also common in the geological record. Continental
siliciclastic sediments of Devonian (Welsh borderland, Scottish midland valley) and
Permo-Triassic (English Midland basin) age show clear dune bed forms and mark
Britain's northward passage across the subtropical divergence belts 400-200 Ma ago.
They are important sources of long-term palaeoclimate and palaeocurrent information,
and modern ergs and loess sheets retain clear evidence of Quaternary climatic change.
Figure 16.3 The form and some environmental gradients of a
coastal sand dune system, showing the direction in which
parameters increase (+) or decrease ( - ).
Source: Partly after Carter (1993).
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