Urban environments - Environmental Sedimentology - page 209

urbanized rivers, and the physical and chemical
characteristics of the sediments therein, has been
limited. It is only recently with the undertaking
of integrated projects on urban catchments
(e.g. the Natural Environment Research Council
funded URGENT and LOIS programmes in
the UK) that detailed information has become
available. Two major differences can be recog-
nized between urban rivers and those from other
environments: the spatial and temporal scales
of sediment movement, and the level of con-
tamination of this sediment.
Sediment in urban rivers can be considered to
be in two major forms. Channel-bed sediment
is stored in the river channel and transported
only rarely by traction and saltation, thus mov-
ing downstream slowly. Suspended sediment
is carried downstream in suspension during
regular flow, with high suspended sediment
transport under higher energy flow conditions.
The former acts as a major storage of sediments
and contaminants in urban rivers, but the latter
is the most important for sediment and con-
taminant flux through the river, especially on
short time-scales.
Studies of urbanized rivers have shown the
clear increased levels of contaminants associ-
ated with the suspended sediment fraction in
urbanized river basins. Walling et al. (2003)
showed that metals (Cr, Cu, Pb, Zn) and PCBs
increased significantly in urbanized sections of
the rivers Aire and Calder (north-east England).
For example, Cr in suspended sediment in-
creased from around 100

to organic phosphorus ratio from < 2 upstream

to 4 downstream of urban centres. This is

significant in that inorganic phosphorus is more
bioavailable than organic phosphorus.
Owing to both the impervious nature of urban
land surfaces, and the engineered and culverted
nature of urban rivers, the flow in such rivers
responds rapidly to rainfall events. This results
in a rapid rise and fall in the river level, and
the river is said to display a flashy response to
rainfall. This leads to high rates of fine-sediment
transport in suspension during these flow events,
often several magnitudes more than during low-
flow periods. Suspended sediment concentrations
in urban rivers can be very high. Gromaire-
Mertz et al. (1999) compiled data showing mean
suspended sediment concentration for high-
flow events to be in the range 49- 498 mg L −1 .
Gromaire et al. (2001) also reported sewer outlet
suspended sediment concentrations in the range
152- 670 mg L −1 , illustrating the importance of
sewer outfalls in urban river sediments. Sediment
yields to rivers in urban catchments, calculated
from suspended sediment measurement, are in the
range of 93.6 - 479 t km −2 yr −1 (Goodwin et al.
2003). A study of the Bradford Beck, Yorkshire,
UK (Old et al. 2003) documented the extreme
levels of suspended sediment transport during a
single storm event. Suspended sediment concen-
trations increased from 14 to 1360 mg L −1 over
a period of 15 minutes. A peak sediment flux
of 47.2 kg s −1 was recorded, illustrating the high
levels of sediment that are transported by urban
rivers during high flow, and that it is these short-
lived events that dominate sediment movement
(Case Study 6.2 and Case Fig. 6.2).
The role of suspended sediment in contamin-
ant flux is further indicated in Fig. 6.9 for a small
urban river. At low flow, suspended sediment
concentrations are low, with suspended sediment
loads of only 20 kg h −1 . As a result these low-flow
stages contribute only low levels of contaminant
flux. At high-flow events suspended sediment
loads of over 12,000 kg h −1 are observed, with
resulting high levels of contaminant flux (e.g.
over 3 kg h −1 of zinc). High-flow events, there-
fore, have a significant impact on contaminant
input into receiving water bodies.

>

gg −1

μ in non-urban

sections to around 400 gg −1 in urbanized

sections, and PCBs showed a similar fourfold
increase. Owens & Walling (2002) documented
a clear increase in sediment-bound phosphorus
(a major contributor to river eutrophication)
as a result of urbanization in the same rivers.
They documented changes in total phosphorus
from < 2000

μ

gg −1 in upstream sections to over

μ

7000 gg −1 in sections downstream of urban-

ization. They concluded that this increase repre-
sented point inputs of phosphorus from sewage
treatment works and combined sewer overflows.
The input of these point sources was further sup-
ported by a change in the inorganic phosphorus

μ