IB Geography B.2.4 Sand Dune Development Notes

Dunes Require Sand, Wind and a Place to Accumulate

1. A wide sandy beach provides loose grains that can dry between tides.
2. Onshore winds move dry sand from the upper beach towards the backshore.
3. A gentle beach gradient and a large tidal range can expose a broad source area at low tide.
4. Driftwood, shingle, plants or small changes in relief slow near-surface wind.
5. When wind speed falls below the threshold needed for transport, sand is deposited around the obstacle.
6. Continued sediment supply allows the deposit to grow into an embryo dune.

Wind Transports Grains in Different Ways

Grain size controls the transport mode

1. Surface creep rolls or slides the largest sand grains along the ground.
2. Saltation lifts medium grains into short bouncing paths close to the surface.
3. Suspension carries the finest particles higher and potentially farther downwind.
4. Saltating grains can strike the surface and dislodge other grains, sustaining transport.

Wind speed changes deposition

1. Strong gusts can entrain more grains and move them farther inland.
2. Surface roughness reduces wind speed close to the ground.
3. Moisture increases cohesion between grains and makes sand harder to entrain.
4. Rain, high humidity or tidal wetting can therefore interrupt aeolian transport.
5. Shelter behind an obstacle creates a zone of lower velocity where grains accumulate.

Embryo Dunes Begin at the Strandline

Pioneer plants tolerate disturbance

1. The strandline is the upper-beach zone where waves leave seaweed, driftwood and other debris.
2. Debris traps the first wind-blown sand and creates small mounds.
3. Salt-tolerant pioneer plants such as sea rocket and sand couch colonize some mounds.
4. Their stems slow the wind, while roots and rhizomes bind the loose sediment.
5. Further deposition raises the mound and reduces the frequency of tidal flooding.

Positive feedback builds relief

1. A small plant traps sand and creates a slightly higher surface.
2. The higher surface offers more space for plant growth above saline flooding.
3. More vegetation increases roughness and traps additional sand.
4. This reinforcing interaction allows a low mound to develop into a larger foredune.

Mobile Foredunes Are Dominated by Sand Burial

Marram grass engineers the dune

1. Foredunes, often called yellow or white dunes, receive frequent additions of fresh sand.
2. Marram grass tolerates drought, salt spray and burial that exclude many competitors.
3. Rapid vertical and lateral growth keeps shoots above accumulating sand.
4. Dense roots and rhizomes bind sediment below the surface.
5. Leaves and stems increase roughness and reduce wind velocity.

Dune ridges can migrate

1. Erosion on the windward face and deposition on the sheltered face can shift a dune inland.
2. A break in vegetation may focus wind and excavate a blowout.
3. Sand from the blowout can bury vegetation farther inland and create new mobile ridges.
4. Storm waves can cut a steep scarp into the seaward dune face.
5. Later onshore winds may return some beach sand and rebuild the ridge.

Succession Changes Conditions Inland

Fixed dunes gain soil and species

1. Farther from the shore, sand deposition and salt spray usually decline.
2. Plant litter decomposes and increases humus in the developing soil.
3. Humus improves water and nutrient retention compared with bare sand.
4. Leaching can reduce carbonate content and lower soil pH.
5. Greater stability allows grasses, herbs, shrubs and eventually woodland to establish where climate permits.

Dune slacks create a wet interruption

1. A dune slack is a low depression between ridges where the water table lies at or near the surface.
2. Wind erosion may excavate a hollow down to the water table, or ridges may enclose a naturally low area.
3. Seasonal flooding and moist soil support plants different from those on dry dune crests.
4. Changes in groundwater abstraction or drainage can alter slack hydrology and biodiversity.

Wind, Vegetation and Coastal Processes Form One System

The beach supplies the dune

1. Waves and littoral drift deliver sand to the beach.
2. Low tide exposes the sand so that sunshine and wind can dry its surface.
3. Onshore wind transfers grains above the normal swash limit.
4. Vegetation traps the grains and stores them in a dune ridge.
5. Storm erosion can return dune sand to the beach and nearshore zone.

Sediment balance determines advance or erosion

1. A positive sediment budget allows embryo dunes to form seaward and the dune system to prograde.
2. A negative budget narrows the beach and allows waves to erode the dune toe more often.
3. Rising relative sea level can force the beach-dune system landward if space is available.
4. Fixed sea defences or development can block landward migration and produce coastal squeeze.

Studland Shows a Clear Dune Sequence

Age and exposure change inland

1. Studland Dunes in Dorset contain embryo dunes near the active beach and older fixed dunes farther inland.
2. Sand couch and lyme grass occur in early exposed zones, while marram dominates mobile dune-building areas.
3. Reduced sand mobility inland permits more varied grassland, heath and wet slack communities.
4. The sequence makes it possible to compare vegetation, soil and microclimate across increasing surface age.

Field measurements test the model

1. A belt transect records plant species and percentage cover from the beach inland.
2. Quadrats provide repeatable samples of vegetation within each dune zone.
3. Soil measurements can compare pH, moisture, organic content and depth.
4. Slope profiles and sand accumulation indicate dune form and recent mobility.
5. Wind speed readings on exposed and sheltered sides test how vegetation changes airflow.

Human Actions Can Alter Each Link

1. Trampling breaks stems and exposes loose sand to deflation.
2. Beach cleaning can remove strandline debris that would have trapped the first sand.
3. Sand fencing can slow wind and encourage deposition where dune rebuilding is desired.
4. Boardwalks concentrate access and reduce repeated disturbance across vegetation.
5. Planting native sand-binding species can restore roughness, but it cannot replace an absent sediment supply.

A Process Chain Explains Dune Development

1. Marine processes supply and expose dry sand on the upper beach.
2. Onshore wind transports grains inland by creep, saltation and suspension.
3. Obstacles and pioneer plants reduce wind speed and initiate deposition.
4. Sand burial favours adapted grasses that trap and bind more sediment.
5. Declining disturbance inland permits soil development and vegetation succession.
6. Storms, groundwater and people interrupt or redirect this sequence.