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agreement with Nelson & Booth (2002), who
found that as well as landslides and channel bank
erosion, 15% of sediment in an urbanized catch-
ment was from road surface erosion. Further
such studies are required before a full appre-
ciation of the relative contribution of sediment
and contaminant sources to urban rivers can be
gained. As well as contaminant input from road
runoff, increased levels of nutrients (especially
phosphorus) and micro-organic pollutants (e.g.
pharmaceutical products) are sourced from
sewage treatment works (Owens & Walling
2002; Warren et al. 2003). Industrial processes
also source metal contaminants to urban rivers
(Walling et al. 2003).
Compared with rivers, which receive sediment
from a wide area, canal sediment is commonly
dominated by material that is more locally
derived, as a result of the limited transport of
sediment in canals. This may be derived from
industrial sources or sewage, as well as natural
material eroded from nearby land and road sur-
faces. Major canal and dock systems that have
significant water inputs from rivers, however,
can have a significant sediment source from out-
side the system. For example, Qu & Kelderman
(2001) showed that sediment, and associated con-
taminants, in the Delft canals, The Netherlands,
have been derived predominantly from the River
Rhine, with the remainder coming from urban and
industrial sources. Urban docks and canals also
commonly receive high levels of organic matter,
discharged from combined sewer overflows,
and contaminants derived from boat traffic, for
example hydrocarbons and tributyl tin (e.g.
Wetzel & Van Vleet 2003). Within urban lakes,
sediment sources are generally a combination
of both eroded soil material from the surround-
ing catchment and anthropogenic material from
the urban environment. Atmospheric deposition
may also be an important source of particulates
and associated contaminants, especially for lakes
with no direct river input (Charlesworth &
Foster 1999). Sediments deposited within lake
systems are probably highly catchment specific
and observations made cannot readily be applied
to other urban catchments (Charlesworth &
Foster 1999).
6.2.2 Sediment transport processes
The transport of sediment in urban environments
is complex. There is a relatively poor under-
standing of the pathways that sediments take
from their source to receiving water bodies,
the rate of sediment transport, the location of
short-term and long-term sinks, and how these
pathways have an impact upon the longer term
fate and distribution of contaminants in the
urban environment. The movement of sediment
through the urban environment can be repres-
ented in what has been termed the urban sedi-
ment cascade (e.g. Charlesworth & Lees 1999;
Fig. 6.4). This cascade recognizes the relationship
between sediment sources, transport mechan-
isms and deposition (storage) of sediment. The
urban sediment cascade is a highly dynamic sys-
tem. As in many depositional systems there are
stages of temporary deposition, or storage, prior
to further transport. The top of the cascade is
considered to be the sources of RDS. The deposi-
tion of sediment upon street surfaces is highly
transient in nature, with remobilization taking
place down the cascade.
The bulk of sediment transport in the urban
environment takes place through the action of
water, but local redistribution of sediment upon
street surfaces may also take place by wind.
Storm drains carry urban runoff from the street
surfaces (as well as other impervious surfaces)
to receiving water bodies (rivers, canals or docks;
Fig. 6.5). In addition to this surface water runoff,
the urban drainage system also has to deal with
industrial and domestic wastewaters and sewage.
In general, there are two main types of urban
drainage: combined systems and separate systems.
In combined systems road runoff is transferred
to a common sewer, and from then onwards to a
sewage treatment works. In this case, these com-
bined sewers rarely have sufficient hydraulic
capacity during storm events, and the water is
discharged directly (including sewage) through
a combined sewage overflow outlet (CSO) into
receiving water bodies (Fig. 6.5). In more recently
built drainage systems, a separate sewer sys-
tem transports runoff directly to receiving water
bodies, without involvement in the domestic
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