Geoscience Reference
In-Depth Information
Sediment inputs
- see Fig. 6.3
Suspended
sediment
Surface ru
noff
Storm
overflow
Flow
down
drains
Accumulation
in gully pot
Fig. 6.5
Schematic diagram of the
pathways of sediment transport in
the urban environment.
To sewage
treatment
works
Sewer pipe
In addition to street surfaces, the major sites
of sediment deposition in the urban sediment
cascade are gully pots and storm sewers, rivers,
canals and docks, and lakes. Deposition (storage)
on street surfaces, gully pots and storm sewers
is short-term, as is the storage of sediment in
urban rivers. In all of these systems sediment is
moved during high-flow events associated with
rainfall. Sediment storage in canals, docks and
lakes is longer term and removal is generally by
engineering activities (e.g. dredging). These are
often termed receiving water bodies and sedi-
ment accumulation in these systems can lead
to long-term physical and chemical impacts
upon water and ecology, as well as providing a
historical record of urbanization.
As well as physical transport down the urban
cascade, chemical changes may take place in the
sediment from source to final deposition. These
take place as a result of chemical and biochem-
ical interactions between water and sediment,
including weathering, adsorption, desorption,
mineralogical transformations and diagenesis. A
study by Charlesworth & Lees (1999) concluded
that metal levels in sediments changed during
transport from street surfaces, through gully pots
and sewers and into longer term sinks (rivers,
lakes). Some elements (e.g. Cu) increased in con-
centration during transport through the cascade,
probably as a result of their strong affinity for
solids, and the dominantly fine-grained nature
of transported sediments. In contrast, some ele-
ments (e.g. Cd) decreased in concentration in
deposited sediments, probably reflecting their
greater affinity for the aqueous phase.
In contrast to other sedimentary environ-
ments there has been little attempt to determine
sediment budgets in urban sedimentary environ-
ments or model sediment movement. Exceptions
are small-scale modelling of sediment transport
in sewer systems from an engineering perspec-
tive (section 6.3.2), and temporal measures of
suspended sediment transport in urban rivers
(section 6.3.3). In particular, transport pathways
of contaminants in urban sedimentary environ-
ments are complex and much work is needed
to fully understand the longer term impacts of
sediment-borne contaminants to atmospheric
and aquatic systems. Furthermore, site specificity
limits generalizations that can be made between
different urban catchments.
6.3
SEDIMENT ACCUMULATION
6.3.1 Road-deposited sediment
Road-deposited sediments have probably received
more attention than other urban sediments in
recent years as a consequence of their potential
impact upon urban air quality and urban run-
off. They are also easy to sample and have the
potential to act as a very good proxy for urban
pollution levels. Road-deposited sediment is com-
posed of a wide range of sediment grains, which
are dominated by quartz, clay and carbonates.
