Glossary - Fundamentals of the Physical Environment - page 874

Available water The amount of water in a soil available for plant growth after excess
water has drained away under the influence of gravity.
Average surface lowering A rate of denudation extrapolated from sediment yields to
give the average thickness of a land surface layer removed per unit area per unit time.
Avulsion An abrupt rerouting of stream flow into a new or abandoned channel due to
aggradation of the flood plain.
Azonal Soils still in a raw, immature state (young soils).
B-subduction See Wadati-Benioff subduction or Benioff subduction .
Back arc A basin created by crustal extension or stretching on the further side of a
volcanic arc from a B-subduction zone, floored by oceanic crust and usually flooded
by the sea.
Backshore The upper part of a beach , lying between the ordinary high-tide mark and the
coastline , which is wave-swept only at exceptionally high tides and may be a source of
landward aeolian sand transport.
Backwash A seaward return pulse of water from breaking waves.
Bajada A continuous alluvial apron in an arid environment, composed of a coalescence
of Piedmont fans .
Bank caving The slumping, sliding or toppling of fluvial sediments into an active river
channel by current turbulence or at low flow stages when lateral support is absent.
Bankfull discharge River discharge through a channel at maximum capacity.
Bar (1) A ridge of coarse, granular fluvial sediment deposited where and when stream
velocity falls, especially in mid-stream and on the inside of meanders. (2) The unit of
pressure under the metric (cgs) system; one bar = 0·987 atmosphere.
Barchan An individual sand dune with a crescentic plan form, pointing downwind, with
a gentle windward slope and a steep leeward slope.
Barchanoid Said of the coalescence of individual barchan dunes into a transverse dune.
Barrier island A coast-parallel low ridge of coarse granular debris (sand, gravel)
sheltering a lagoon on its land-ward side; common on wide-shelf coasts and may
eventually accrete on to the land through storm washover and migration.
Basal shear stress The shear stress exerted by Earth materials, particularly glacier ice,
moving over their bed.
Basal sliding The sliding of a glacier past its bed and sides, greatly facilitated by the
presence of subglacial water.
Basalt A basic fine-grained extrusive igneous rock.
Base cations Metallic cations (e.g. potassium, calcium, magnesium, sodium) that are
plant nutrients and take part in cation exchange reactions.
Base exchange The process whereby a basic ion in the soil solution exchanges with a
basic cation adsorbed on a soil colloid.
Base flow A more enduring component of stream flow contributed by groundwater
transfers.
Base saturation The condition when the entire cation exchange capacity is occupied by
base cations.
Basin and range A zone of continental crustal extension marked by elevated mountain
blocks ( horsts ) separated by graben fault basins.