Environmental Engineering Reference
In-Depth Information
SOD in the MSC is spatially variable and changes according to sediment
mobilisation during high flows and dredging activities, but peak values of over
1000mgm
2
hr
1
(in situ) have been recorded (HR Wallingford
1999
). This is far
in excess of what would be expected in unimpacted sites and is representative
of heavily organically polluted sediments. Further research by Teesdale (
2002
)
examining the spatial variation of SOD between Pomona Docks and Mode
Wheel Locks revealed that SOD was highest within the Turning Basin and
attributed it to greater particulate deposition due to the reduction in flow as
the Irwell enters the MSC. The effect of newly deposited particulates on the
sediment composition with depth was also highlighted in sediment core analy-
sis in the upper MSC (APEM
1996
). Surface sediment BOD averaged 1200mg kg
1
and reached peaks of almost 3 000mg kg
1
in the Turning Basin, whilst at the
majority of sites bottom sediment BODs averaged 630mg kg
1
. The differences
with depth were ascribed to the presence of recently deposited easily digested
sewage-derived particulates at the surface whilst more inert carbon compounds,
such as oil, would be prevalent in the lower and older (
100 years old) sedi-
ments. Surface carbon content was found to be around 20% in the Turning Basin
(APEM
1996
), twice that expected in an uncontaminated sediment (Donze
1990
).
Bottom sediments from areas near dock entrances at Pomona and Salford had
carbon content in excess of 40%, which probably reflects past shipping activity.
The docks and channels upstream of Mode Wheel Locks provide an efficient
settling area for sediment load brought down by the River Irwell. Low flow
velocities are ensured by the maintenance of a standard operational water
level in the dock area and by the need for deep water for navigation. Dredging
is undertaken for flood defence and navigation purposes. Dredging used to be
undertaken to maintain a water depth of 8.5m, although in the past decade
this has been reduced to 7m. The location of the dredging varies from year
to year and is partly dependent upon the flow conditions of that year, indeed
some areas are not dredged for several years whilst other areas receive
deposits of over 0.5m each year. Dredging usually takes place between January
and May with an estimated 60 000m
3
to 90 000m
3
of sediment being removed
each year.
In addition to contributing to water column anoxia, the high organic content
of the sediment is thought to play a role in sediment rafting, noxious gas
generation and metal mobilisation (White et al.
1993
; Boult & Hendry
1995
)as
well as providing a continual source of nutrients for potential algal growth.
Sediment dredging is undertaken in the Turning Basin in response to flood
defence requirements and navigational needs, a process that inevitably re-
suspends sediment-bound nutrients. Recent total phosphorus concentrations
in the water column (530
m
gL
1
, 2003 2007 average) are over an order of
magnitude greater than required for algal proliferation. According to Reynolds
2
<
2
Professor Colin S. Reynolds - Institute of Freshwater Ecology.
