Environmental Engineering Reference
In-Depth Information
Denitrification
Bacterial processes occurring in soils, in the absence of free oxygen, to
break down nitrates and nitrites with the evolution of free nitrogen.
Denudation
The combined processes of
weathering
, mass wasting and erosion which
cause the disaggregation of rock mass and its removal to lower-lying ground;
denudation may proceed far enough to cause the virtual destruction and levelling of
continental crust down to
sea level
.
Denudation chronology
A scheme of land surface development which saw many
variations developed over almost a century up to the 1960s, based on belief in a
denudation cycle
; modern
plate tectonics
provides more realistic mechanisms but
more complex geomorphological histories.
Denudation cycle
A simple scheme of continental
denudation
which involved the
cyclical stimulation of uplift of a new land surface, followed by progressive
denudation down to a low residual plain and its renewed uplift or 'rejuvenation'.
Depletion curve
The expression on a
hydrograph
of the gradual decline in the
baseflow
component of stream discharge due to dwindling groundwater discharge.
Deposition
Any means by which sediments may accumulate from a condition of motion
or activity such as mechanical fall-out from suspension, chemical precipitation or the
assemblage of biogenic debris.
Depression storage
The short-term storage of precipitation in small surface depressions,
from which it may evaporate or percolate underground, and its capacity to delay the
start of
overland flow
.
Desert pavement
A loose, gravelly surface layer protecting underlying rock or fine
sediments and probably representing a residual material from which fines have been
deflated or washed out.
Desertification
'Land degradation in arid, semi-arid and dry subhumid areas resulting
mainly from adverse human impacts' (UN Environment Programme, 1992).
Destructive margin
The boundary between two convergent crustal plates, where ocean
crust is consumed by subduction; however, it also triggers igneous activity which
forms new crust at volcanic arcs.
Detrivores
Organisms that feed on dead or decaying organic matter.
Devensian stage
The most recent global
cold stage
, known as the Weichselian in Europe
and Wisconsinan in North America, extending from
c
. 115,000 years to 10,000 years
BP.
Diagenesis
Minor, non-destructive changes in the mechanical or chemical properties of
rock shortly after initial emplacement, associated with the final stages of
lithification
.
Diamicton
A non-sorted, terrigenous sediment containing a very wide range of particle
sizes; the term is descriptive, not generic, and a prefix would identify its origin, e.g. a
glacial diamicton.
Diapir
A bulbous intrusion of less dense igneous or sedimentary rock which forms a
dome or broad fold in denser surrounding rock through which it rises.
Diffuse double layer
A model to explain the distribution of ions concentrated near
colloid surfaces in soils; the inner layer at the colloid surface consists of positively
charged cations, whereas the outer layer in the soil solution contains equal amounts of
cations and anions.
