Environmental Engineering Reference
In-Depth Information
Nivation The erosion of surface depressions by the combined processes of rock
weathering and mass wasting associated with the growth and decay of snowpack.
Nordenskjöld line The limit of the Arctic and Antarctic as defined by both the
temperature of the warmest month and the temperature of the coldest month.
Normal stress The portion of rock or soil strength dependent on the anchoring effect of
the mass of a particle or intact block, normal (at right-angles) to a surface on which it
rests; this is at a maximum if the surface is horizontal but diminishes as slope angle
increases.
Nuée ardente An incandescent (fiery) cloud of ash and volcanic gas developing as near-
surface pyroclastic gravity flow after volcanic eruption, capable of destroying
anything in its path.
Nunatak An isolated mountain or hill protruding through, and completely surrounded by,
glacier ice.
Obligate symbiotic relationships Relationships which are restricted to specified
conditions only.
Obliquity of the ecliptic The tilt of Earth's axis of rotation relative to the plane of its
orbit. It varies between 21·8° and 24·4° over a period of about 40,000 years.
Occupational fire A fire started by humans in order to manage an ecosystem for
economic gain (e.g. grazing, land clearance or herding wild animals).
Offshore A zone of deeper water lying on the inner margins of the continental shelf,
beyond the nearshore zone.
Ooze Fine-grained, marine sediment comprised of more than 30 per cent skeletal remains
of pelagic organisms and clay minerals.
Open systems Systems which are characterized by the exchange of both matter and
energy with their surroundings. The majority of natural systems are open.
Ophiolite A sliver of oceanic crust caught up in an accretionary prism and found out of
place in a subsequent orogen .
Organismic community The concept which regards the plant community as a 'super-
organism', with properties not present in its individual constituent organisms.
Orogen A linear continental mountain range elevated mechanically or thermally by plate
collision, crustal shortening and uplift.
Orogenesis The formative processes of an orogen.
Oroshi A cold-air drainage current blowing from the mountains of central Japan.
Orthogonal Said of a system with right-angle relationships between its components.
Outlet glacier A steep, fast-flowing glacier discharging large ice volumes from inland
portions of ice sheets through a confining rock-walled channel.
Output The flow of energy and matter out of a system.
Outwash plain An extensive land surface covered by glaciofluvial sediments and
braided meltwater streams released from a glacier terminus, especially in unconfined
piedmont zones.
Overbank discharge That portion of stream discharge not confined to the channel during
a flood.
Overburden pressure A compressive stress exerted on Earth material by the mass of
overlying rock, soil, water or ice.
Overland flow Non-channelled surface water flow where precipitation intensity exceeds
infiltration capacity, taking two forms: Horton overland flow develops as sheet flow on
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