Question 7 of 7: Coastal & Aeolian (Wind) Processes

Question 7: Coastal & Aeolian (Wind) Processes (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) (i) Beach drift – the net zig-zag transport of sediment along a shoreline caused by waves swashing obliquely up the beach (at the angle of wave approach) and then draining straight back down-slope under gravity (backwash), producing a saw-tooth net movement parallel to the coast. (ii) Baymouth bar – a sand spit/bar built by longshore drift that extends completely across the mouth of a bay, sealing it off from the open sea and often enclosing a lagoon. (iii) Jetty – a built structure (typically a pair) extending perpendicular to shore at a harbour or river mouth, designed to stabilize the channel and block longshore drift from infilling it. (iv) Beach nourishment – a "soft" shoreline- protection method that artificially adds sand (dredged or trucked in) to a beach to replace material lost to erosion and maintain beach width.

(b) Sand-sized grains form the bed load carried by wind (i), moving by saltation – skipping and bouncing along the surface (ii). Fine silt makes up the suspended load (iii). Where prevailing winds blow steadily from one direction, transverse dunes form as long ridges oriented perpendicular to the wind (iv). Extensive windblown-silt deposits are called loess (v). The sloping internal layers within a dune, oriented along the direction of migration, are cross-beds (cross-bedding) (vi).

(c) (i) Slump – slide (a rotational slide along a curved failure surface). (ii) Lahar – flow (a volcanic debris/mudflow). (iii) Solifluction – flow (slow, water-saturated soil creep over an impermeable, often permafrost, layer). (iv) Avalanche – flow (a very rapid, gravity-driven mass flow of snow/ice/rock/debris). (v) Talus slope – fall (an apron of accumulated rock fragments delivered by repeated rockfall). (vi) Alluvial fan – strictly a fluvial (stream-deposited) landform rather than a mass-wasting process in its own right; where a fan is built primarily by debris flows off a steep catchment it is best classed with the flow family, so it is included here as a flow-dominated depositional feature, with the caveat that ordinary water-laid alluvial fans are not mass wasting at all.