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Sand dunes are accumulations of wind blown sand found at the rear of a beach, above the usual maximum reach of the waves. They range in size from ridges less than a meter in height and width, to massive dune fields that extend inland for many kilometers. The word “dune” derives from a medieval Germanic or Norse word – “dun”, a hill.
Uses of Coastal Sand Dunes
Coastal sand dunes serve many functions.
1. Act as an essential store of sediment, protecting the land behind them from storm erosion and potential sea level rise.
2. With vegetation, trap wind blown sand and prevent sand being blown further inland.
3. Act as a filter for rainwater and groundwater and in some situations provide aquatic habitats (e.g. dune lakes)
4. Provide habitats for specially adapted plants, birds, and animals – several of which are now rare or endangered.
Sand Dune Formation and Life Cycle
Shaped by wind into curving ridges, coastal sand dunes are among the most dynamic and fragile natural formations. Their contours shift over time until hardy dune pioneer plants take hold in drifting sand and create a stable landform. Even then, dunes can change form rapidly under the stress of storm waves and wind, or the traffic of human activity.
Offshore sand bars and sediment deposited at the mouths of rivers are the most important sources of material for dune building. Sediment is carried by long shore currents until a projection landform traps the particles and they are deposited on the beach by wave action. Dune formation begins when wind blows dry sand particles landward from the beach. All it takes is a bit of a breeze (16 km/hr) to put fine sand in motion (Figure 1). The finest grains may be suspended in the air and carried along (suspension). Heavier grains tend to bounce along as they are lifted into the air, fall back to the ground, then bounce back up again (saltation). The heaviest grains the wind can move are usually nudged along by impact from bouncing, saltating grains (impact creep).
Dunes develop at the back of beaches where there is sufficient sand supply and wind energy to transport it landwards. Drifts accumulate around physical barriers (vegetation, driftwood, rocks) that interrupt the wind flow. With steady winds, the sand drift acts as a barrier to moving sand, and the drift gradually grows into a sizeable mound, forming rows at right angles to the prevailing wind direction (transverse ridges). The wind affects the mound of sand by eroding sand particles from the windward side and depositing them on the leeward side (Figure 2). Gradually this action causes the dune to “migrate” inland; as it does so it accumulates more and more sand. All dune fields consist of 2 or 3 sets of parallel dunes, with the most recently formed foredunes nearest the beach, and the older, usually vegetated and stabilized dunes farthest inland. The inland dunes may be thousands of years old.
Young dunes are called yellow dunes while older dunes which have a higher humus content are called grey dunes. Leaching occurs on the dunes, washing humus into the slacks (“valley” or trough between dunes), and the slacks may be much more developed than the exposed tops of the dunes.
Sand dunes have an ever-changing life cycle, forever moving landward. Beaches and foredunes (those closest to the beach) are in a constant state of change in response to wind conditions, sea levels, and sediment supply. Erosion and accretion (rebuilding) are the results of this readjustment. For example, a beach may take on a very different form after a storm (Figure 3). During a storm the beach may be eroded and the face of the foredune severely cut back (scarped) as a response to the greater waves imposed upon it. These changes can be rapid – a matter of hours, but the recovery of the beach may be much more gradual – days or weeks, as sand is again returned to the beach and blown up against the eroded dune face. In some cases erosion or accretion is permanent; but more often it is just temporary changes and part of a natural cycle of dune erosion, rebuilding and stabilization. However, until the dune is completely veiled and stabilized by plant cover, sand may be borne away by the wind.
Foredune Flora and Fauna
Sand dunes can be important ecosystems supporting unique plant life and a healthy population of small animals, birds and insects.
As a dune forms, plant succession occurs. Critical to the formation, stabilization and post-storm recovery process is the presence of specialized dune plants, in particular, Marram grass (Ammophilia breviligulata). This grass covers much of the surface of the dunes. It is a coarse, pale green grass, and even a single shoot can trap windborne sand. It thrives on being buried in the sand, an action that stimulates the growth of rhizomes (horizontal runners just below the surface of the sand) and the growth of the shoots and roots. The fine mat of roots further traps sand grains (the roots have a small fungus growing inside them which puts out sticky threads that help to glue the sand grains in place). The grasses also add nitrogen to the soil, meaning other less hardy plants can then colonize the dunes. Closer to the beach you will find purple beach pea (Lathyrus japonicus) while beach heather or woolly hudsonia (Hudsonia tomentosa) forms grey woolly mats on the leeside of dunes and bayberry bushes (Myrica pensylvanica) populate the sheltered hollows.
Threats to Sand Dunes
Coastal sand dunes are one of Prince Edward Island’s most endangered habitats. These dunes are very vulnerable to erosion by natural processes (storm waves, wind), and by human activity (beach vehicles, foot traffic, ‘dune surfing’, construction of seawalls and other structures that interfere with natural coastal processes, and dams that trap river sediments, depleting the sand supply). Other threats include pests (e.g. rabbits), grazing farm animals, which eat native dune grasses, and the introduction of exotic plants and weeds, which invade and displace native dune plants.
To claim this Earthcache:
1. Take and post a picture of you and your GPSr at the base of the dune (Figure 4) and e-mail me the answers to the following:
2. Estimate the height of the sand dune at the posted coordinates (do NOT climb the dune).
Do NOT post your answer on your log, encrypted or otherwise.
DO NOT LOG AS A FIND UNTIL YOU HAVE A PICTURE READY TO POST AT THE TIME OF LOGGING A FIND FOR THIS EC. Logs with no photo of the actual cacher logging the find or failure to answer required questions through e-mail will result in a log deletion without notice. Exceptions will be considered if you contact me first.
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