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Dune morphology and activity

/(with contributions from R. E. Vance, S. Wolfe and N. Lancaster.
Revised February 2005)/

Geoindicator


NAME: Dune morphology and activity

BRIEF DESCRIPTION: Dunes and sand sheets develop under a range of climatic and environmental controls, including wind speed and direction, and moisture and sediment availability. In the case of coastal dunes, sea-level change and beach and near shore conditions are important factors. Organized dune systems and sheets in continental environments form from sediment transported or remobilized by wind action. New generations of dunes may form from sediment remobilized by climatic change and/or human disturbances.
Dune formation and movement is well documented from the margins of many deserts, as well as from temperate regions and along sandy coasts [see shoreline position]. Sand movement is inhibited by moisture and vegetation cover, so that dunes can also be used as an indicator of near-surface moisture conditions. Changes in dune morphology or position may indicate variations in aridity, wind velocity and direction [see wind erosion], or disturbance by humans. Dune changes can be correlated with climatic variables using aridity indices and the dune mobility index, which is the ratio between available wind energy and the precipitation-potential evapotranspiration ratio.

SIGNIFICANCE: Dunes can migrate as fast as 10-20m per year, and may engulf houses, fields, settlements and transportation corridors. Active dunes in sub-humid to semi-arid regions decrease arable land for grazing and agriculture. They also provide a good index of changes in aridity. Coastal dunes are important determinants of coastal stability, supplying, storing and receiving sand blown from adjacent beaches. Dunes play an important role in many ecosystems (boreal, semi-arid, desert, coastal) by providing morphological and hydrological controls on biological gradients. Tracking movement of dunes, for example in coastal zones, has helped to assess the environmental impact of human activities: the lack of monitoring in the 1960s and 1970s led to significant degradation of coastal dune systems in the Great Lakes and Atlantic coastal regions.

HUMAN OR NATURAL CAUSE: Changes in dune morphology and movements can result from variations in aridity (drought cycles). Widespread changes can also be induced by changes in wind patterns and by human disturbance, such as alteration of beach processes and sediment budgets, destruction of vegetation cover by trampling or vehicle use, overgrazing, and the introduction of exotic species.

ENVIRONMENT WHERE APPLICABLE: Sand dunes occur widely - in deserts, in tropical and sub-tropical latitudes, semi-arid continental mid-latitude regions. They also occur along sandy ocean beaches, estuaries and lake shorelines from the Arctic to the Equator. Relict Quaternary sand dunes occur in most continental interior regions.

TYPES OF MONITORING SITES: Margins of active dune areas. Sand hills and vegetation-stabilized dunes in mid-continental areas, ideally located along climatic transects.

SPATIAL SCALE: patch to landscape / regional

METHOD OF MEASUREMENT: Changes in size, shape and position of individual dunes, dune fields and sand sheets can be monitored by repeated ground, aerial, or satellite surveys. Measurement of active and dormant/relict areas and associated vegetation/surface cover may be monitored by aerial photographs or satellite imagery.

FREQUENCY OF MEASUREMENT: Dune systems should be monitored every 5-10 years to observe changes associated with drought cycles, more frequently when movement is detected.

LIMITATIONS OF DATA AND MONITORING: Climatic records, especially wind data, are commonly lacking.

APPLICATIONS TO PAST AND FUTURE: A record of dune activity for the last 50 years can be constructed for many semi-arid areas and correlated with temperature and precipitation records. Paleorecords, including paleowind directions, exist for relict Quaternary dunes, which are widespread in Africa, Australia, India and North America. The potential impact of future climatic variability on aeolian systems may also be assessed. An understanding of aeolian system sediment state, and the combined role of transport capacity, sediment supply and sediment availability, may provide a new conceptual framework for viewing dune fields as geoindicators.

POSSIBLE THRESHOLDS: Dune mobility index M>50, where M is the ratio between (1) the percentage of the time the wind blows above about 5 m/sec (the threshold velocity for sand transport), and (2) annual rainfall divided by potential transpiration. Other thresholds could be based on acceptable limits for active dune areas on agricultural land, as well as on associated groundwater levels.

KEY REFERENCES:

Cooke, R., A.Warren & A.Goudie 1993. Desert geomorphology. London, UCL Press.

Forman, S.L., Oglesby, R. and Webb, R.S. 2001. Temporal and spatial patterns of Holocene dune activity on the Great Plains of North America: megadroughts and climate links. Global and Planetary Change 29: 1-29.

Kocurek, G. and Lancaster, N. 1999. Aeolian system sediment state: theory and Mojave Desert Kelso dune field example. Sedimentology 46: 505-515.

Lancaster N. 1995. Geomorphology of desert dunes. Routledge 290p.

Muhs, D.R. & V.T.Holliday 1995. Active dune sand on the Great Plains in the 19th Century: evidence from accounts of early explorers. Quaternary Research 43: 118-124.

Nordstrom, K.F., N.Psuty & B.Carter 1990. Coastal dunes: form and process. Chichester, John Wiley and Sons.

Rogers, S. and D. Nash 2003. The dune book. North Carolina Sea Grant, 28p.

OTHER SOURCES OF INFORMATION: Agricultural and environmental agencies, geological surveys, desert research institutes, International Union for Quaternary Research, International Association of Geomorphologists.

RELATED ENVIRONMENTAL AND GEOLOGICAL ISSUES: Mobile dunes may invade and destroy productive agricultural land and affect transportation routes. Human efforts to stabilize dunes commonly fail because they introduce disequilibrium structures that run counter to natural trends. Many efforts have been made to stabilize dunes, especially in coastal complexes by planting sand-binding vegetation. Dune migration may affect shallow water table levels by reducing surface evaporation. Reduction of vegetation cover by over-grazing and deforestation can add to the problem of dune activation and desertification in semi-arid and arid regions.

OVERALL ASSESSMENT: Dunes are very important indicators of environmental change in arid and semi-arid regions and coastal zones.

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