Jetty is the only ‘poor’ bathing water site in the Wessex Water
region as categorised by the EU Revised Bathing Water Directive.
Wessex Water has been working with the Environment Agency and
committed £39 million to 12 schemes to improve this rating.
Bristol Road Sewer Pumping Station (SPS) and Combined Sewer
Overflow (CSO) rebuild is one of the major aspects of these
works at £6.6m.
The overflow from Bristol
Road SPS discharges directly to the River Parrett, which in turn
flows into the Bristol Channel at Burnham-on-Sea. The hydraulic
model predicted that the site spills approximately 21 times per
bathing season, with a volume of combined flow equating to
20,254m3. To improve the bathing water it was agreed
with the Environment Agency to reduce this number of spills. By
increasing the pass forward flow to 165l/s (from 100l/s) and
providing 3000m3 of storage, the frequency of spills
was reduced to 1 per bathing season (1801m3).
The existing pumping
station had legacy operational issues due to the very flat
catchment in Bridgwater. With deep, flat sewers, the network
discharged directly into the pump station at the invert level of
the wet well. There was no operational volume within the SPS and
hence the whole network was relied upon to fill before pumps
Consequently, the pumps
were operating three times a day, but running for a significant
amount of time. Whenever it rained, a large amount of sediment
and rags throughout the flat system would be flushed into the
wet well, causing regular blockages and performance issues on
the duty pumps.
There was no separate
stormwater chamber; instead the storm pumps worked on a level
sensor, within the same wet well, just above a gravity overflow
to the surge tower. A coarse screen was located in a surge tower
with a gravity outfall to the River Parrett.
The existing Bristol Road
Site needed to remain live throughout any construction and
operations need continuous access to the site. The upstream
population is 4180, equating to a domestic dry weather flow of
7.5l/s and a trade dry weather flow equating to 34.7l/s. The
timescale was just under three years from the start of concept
design to operational site.
Bristol Road SPS during construction, with flow arrows -
Courtesy of Wessex Water
Click to enlarge
It was necessary to
design an integrated solution comprising a 15m diameter, 23m
deep shaft pumping station, capable of storing 3000m3
of screened stormwater. By incorporating the storage requirement
and pumping station, the team was able to utilise significantly
more of the volume and save on building two structures, in a
very restricted space. Additional monitoring has been
incorporated in the design to ensure it is known when spills
occur to the River Parrett and their duration.
Gravity flows enter the
pumping station by a new connection on the existing line. The
sewage flows through the inlet channel before dropping into the
high level duty well. When storm flows beat the pass forward
pumps of 165l/s, the duty well and inlet channel levels rise and
overflow through 2 (No.) Huber Rok 2 mechanical screens and into
the 3000m3 of storage. The storage space continues to
If the storm subsides,
flows will be returned back to the inlet channel. If the storm
continues and the storage fills, this will lead to the storm
pumps operating, lifting the storm flows into the newly extended
surge tower and gravitating out to the River Parrett. The storm
pumps have been sized to pump a 1 in 50 year storm event at high
Model of 3D survey of the excavated secant piled shaft -
Courtesy Sedimentation Skanska
well with drop pipe curved de-aeration cowl and corner
Courtesy of Hydrotec and Wessex Water
Challenges that had to be
Bristol Road SPS is located just off the busy A38, it is
situated in between a tyre company and a residential building.
Behind the site is a flood bank and footpath immediately
adjacent to the River Parrett.
► Ground Conditions:
Boreholes suggested the upper regions of ground was made of
alluvial clays and tidal flat deposits, followed by a narrow
band of gravels (Burtle formation) until approximately 23m.
Below 23m, Merica mudstone was prominent until at least a depth
of 45m. The alluvium clay had a significant water content with
Dewatering with usual caisson construction methods was not going
to be acceptable, due to the risk of settlement. Skanska
designed and built a secant piled outer shaft with 66 secant
piles each approximately 1.2m diameter, 30m deep. Half of the
piles incorporated rebar, which allowed the piles to interlock,
creating a watertight structure. This secant piled shaft was the
largest in the UK without the need for ringbeams. Once the
secant piles were complete, the whole shaft was excavated,
enabling a bottom up segmental shaft and base plug to be
Drilling rig with scalloped guide - Courtesy of Wessex
► Power: A
new power supply was required. The storm pumps have been sized
to protect against a 50 year storm, allowing for an astronomical
high tide. Using a new power supply has allowed the whole site
to be built and commissioned off line, before turning any of the
flows from the existing pump station.
Wessex Water felt a backup generator was still required due to
there being no gravity outfall for the storm overflow. If a
gravity overflow pipe was installed it was believed that the
risk of a flap valve (situated in a difficult location to
maintain) failing and allowing river water into the storage
shaft), was higher than that of the power failing.
► Wet well design:
The wet well design is a unique shape, as it is a curved
backwall and sides. Wessex Water initially considered CFD
modelling to test the structure, but instead chose for Hydrotec
to test the wet well with a physical model. This allowed the
incorporation of a number of simple innovations, which made a
significant impact on the efficiency of the pumps.
These included vanes below the pumps, which encouraged laminar
flow to enter the pump, the inclusion of a cowl at the bottom of
a drop pipe to defuse energy and direct flows and screenings,
and a bespoke benching design to minimise any settlement of
solids or screenings.
Construction of inlet channel and duty well - Courtesy
of Wessex Water
It was a priority that Wessex Water operatives would not have to
enter this site under any normal operation. It has been designed
so that all equipment can be accessed from the surface. However,
if absolutely necessary, access can be gained using a planned
Wessex Water standards recommend slopes of 1:2 for benching in
detention tanks, this would have meant a significant amount of
mass concrete, that would not be needed for flotation. Several
designs were looked at including flush mechanisms to clean the
shaft. By working with Flygt (Xylem), Mouchel created a design
that reduced the benching to 1:67 slope and would still provide
cleansing, using a mixer pump.
The role of the mixer pump is purely to mix the bottom sediment
15 minutes prior to the return pumps returning the stored
stormwater back into the inlet channel and ultimately pumped
back to the treatment works. It is not designed to mix all the
flows within the shaft or to create aeration etc.
Keeping the system live:
Being able to construct the whole system off line was
invaluable. A separate power supply allowed the commissioning to
be carried out before final turning of the flows, Operations
were able to witness the commissioning and see how the site will
work, and gave greater confidence in the final successful
operation of the site. The back-up generator will be permanently
housed in the location of Bristol Road existing site allowing
for the old pump station site to be demolished post a trial
period of the new system.
The best accolade that
was received came from operations recently, asking why a Bristol
Road type solution had not been created on a different scheme
that is likely to need updating in the next AMP. In a short
period, the concerns for the innovative design have been allayed
and the positive learnings spread to other areas. We challenged
standards and in turn created new ones. Having restrictions in
the project enables innovation.
Bristol Rd SPS - whole site showing constraints -
Courtesy of Wessex Water
The hydraulic model
anticipated that the site will now (on average) spill once per
bathing season. The new integrated pumping station and storage
structure was commissioned at the end of April 2017.
It can be seen from the
telemetry and additional monitoring that has been installed that
it is currently working well and has already been used in anger;
storing flows that previously would have spilled into the River
As wastewater assets
begin to reach the end of their lives and usable space for new
assets are at a premium the need to challenge standards and use
joined up thinking is going to be paramount to providing
buildable, cost effective solutions that are relevant and
Bristol Rd SPS - whole site
of Wessex Water
Bristol Rd SPS shaft
of Wessex Water
and publishers would like to thank Lucy George, Assistant
Project Manager and Implementation Engineers with Wessex
Water, for providing the above article for publication.