Geoscience Reference
In-Depth Information
periods between runoff events, and are termed
'dry weather' processes (Fig. 6.8). Many of these
are similar to those taking place during dia-
genesis in receiving water bodies (section 6.2.4),
although at a smaller scale. Chemical studies
of gully pot liquor have shown that the water
reached negative Eh during dry period events,
with major impacts upon redox-sensitive chem-
ical species (e.g. Morrison et al. 1988, 1995).
The major changes are the consumption of
oxygen in liquor, changes in chemical oxygen
demand (COD) and ammonium, and the release
of contaminants and nutrients adsorbed onto
redox-sensitive species into the pot liquor (Wei
& Morrison 1994b; Memon & Butler 2002b;
Fig. 6.8). Dry-weather processes overall increase
the pollutant level of pot liquor, with subsequent
negative impacts on runoff into water ways
during wet-weather processes. The longer the
period of dry weather the greater these changes
(Memon & Butler 2002b).
Gully pots are the entry points into the urban
drainage system, but sedimentation within the
more extensive sewer system itself has marked
physical and chemical impacts on urban drain-
age through the reduction of capacity, leading
to sewer flooding, and the build-up of pollut-
ants. Although many sewer systems date from
the mid-1800s, there have been few advances
in sewer design with respect to sediment move-
ment. Attention upon sewer design is directed
towards predicting the movement and accumu-
lation of sediment. Using hydrological models
of sediment movement, predictive models for
sediment build-up in sewer networks have been
developed, with some limited success (e.g.
Gerard & Chocat 1998).
rivers commonly possess unique physical and
hydrological properties. In addition to this phys-
ical modification, urban rivers became the major
vectors of domestic and industrial waste removal.
A direct result of this was high pollution levels,
through both organic and inorganic contamin-
ants. It is only with increased discharge legislation,
and investment in sewer networks and treatment
plants that urban rivers are now improving in
water quality in developed nations. As high-
lighted in Chapter 3, however, the majority of
the contaminant load in rivers is associated with
the sediment fraction. The residence time of sedi-
ment in rivers is greater than that of water, and
in spite of water quality improvement many urban
rivers still possess low sediment quality. This
legacy of pollution is one of the largest problems
facing urban catchments.
Sediment within urban rivers is itself considered
to be a major non-point source pollutant, as fine
sediment causes a range of problems in rivers.
An increase in fine sediment has impacts upon
river turbidity, affecting biological processes
and ecology. In particular, the impacts of high
suspended sediment concentrations upon fish
are marked. Impacts include effects on behavi-
our and health of fish through gill damage and
damage to spawning grounds, through filling in
spaces between sand grains, and reducing oxygen
delivery to the fish eggs (Watts et al. 2003). In
addition, fine suspended material contains the
dominant load of metals, nutrients and other con-
taminants. An increase in coarse sediment has
a physical impact causing channel aggradation,
which may lead to channel volume reductions
and flooding. Urbanization can also decrease the
erosion rate of the land surface through its cover
of concrete, thereby reducing sediment load in
rivers. Furthermore, the changes in the flow prop-
erties of the river may lead to the downstream
erosion of in-channel sediment due to increased
flow events. This has major impacts upon the
ecology and biodiversity of urban rivers.
Many of the processes that operate upon
sediments, and their interaction with water, in
urban rivers are similar to those for other river
systems, and the reader is directed to Chapter 3
for full details. The specific study of heavily
6.3.3 Urban rivers
Rivers are natural features of catchments, and
are a major component of urban environments,
many of which grew up around existing rivers.
During the process of urban development rivers
were modified to enable navigation, culverted
to minimize flooding, and diverted to allow
development. These changes led to the physical
modification of rivers, to the extent that urban
