IT, SCADA AND TELEMETRY
of the trunk manholes or from the CSOs that serve it, into the To ensure they did not force-fit the model simply to replicate what
surrounding networks. This is particularly evident in the area being was observed during the flow survey, the modellers had to calibrate the
investigated, where even a relatively minor storm event causes raw head-discharge curves for each manhole against historical flooding
sewage to flood from the downstream manhole. This then ponds and events within the area to ensure the trunk sewer had sufficient relief
stagnates behind the sewer when water levels recede, as visible in within the system to guarantee that flooding was not created within
the bottom left of the photo. nearby residential areas.
At another manhole, video footage taken during a more extreme
storm event with higher river levels showed spill flows being forced out Limitations
of a gap between the cover slab and the main manhole chamber, as There were some limitations with the approach taken for this scheme.
well as out around the cover. Firstly, the head-discharge curve replicating the river interaction was
Three main challenges in modeling the scheme were the slow assumed, though it was calibrated as far as possible. This means there
response of infiltration and ingress from the catchment, the transient was an element of uncertainty within the design events, and as the
sediment within the sewer itself, and the key issue of how to replicate decision was made to err on the side of caution the results were
the manhole flooding within the model. conservative. Additionally, the use of slow response sub-catchments
meant the model had a tendency to over-predict spill volumes from the
Slow response infiltration CSOs and trunk manholes for design events. This was not deemed
Observed flow survey data for one of the trunk sewer monitoring points significant as the consultancy was already considering firm options that
indicated that it could take seven or eight days for the trunk sewer to did not involve providing storage.
return to its normal dry weather flow level after major rainfall. This very
delayed response was challenging to replicate within the model. Atkins Further work
chose to model the response using standard sub-catchments with slow Option development is still ongoing. Atkins is working to rationalise the
response runoff areas and routing factors, for ease of use. four assets that have to be addressed into one discharge location. As part
of the Dwr Cymru Welsh Water PR09 submission the consultancy
Transient sediment produced a strategy for the entire catchment that involves removing the
Transient sediment within the trunk main was predominantly due to the
interaction with the river. When river levels rose, water entering the sewer
deposited silt. In some places larger detritus such as river cobbles were
found, possibly washed in through manholes or CSOs.
In some instances, up to 450mm of sediment was found in a
900mm pipe, most of which would be flushed downstream when
storm levels began to rise again. Sediment was only considered as an
issue during the dry weather flow verification. Ultimately the modelers
decided to use the average storm level of sediment for modelling
and optioneering purposes.
Manhole flooding
It was not possible to replicate flooding in the model using standard node
types within InfoWorks CS, as the floodwater was not stored on the
surface as it would be with a standard flood cone. Neither was the flow
instantaneously lost from the system as with a ‘Lost’ flood type, due to it
‘mushrooming’ above the manholes. The head exerted by that pressure Contaminated water stagnantes behind the sewers when flooding recedes
forced more flow through the system, something that could not be
replicated with a standard node. Therefore the gully modelling tool was trunk sewer from the river wherever possible with a view to reducing the
adapted within InfoWorks CS to try to reproduce the head-discharge infiltration and potential for pollution, rather than the current ’sticking-
relationship it was encountering. plaster’ approach of addressing known issues.
Each of the two manholes had unique spill mechanisms, so it was It was estimated that around 40% of the dry weather flow within the
necessary to arrive at individual head-discharge relationships for each. As a trunk sewer is actually river infiltration. Eliminating this would have a
result, an excellent fit was achieved for both manholes. significant effect on the pollution not just from the problematic assets but
It was found that the downstream manhole could be represented fairly also at the CSOs across the entire catchment. It was necessary to ensure
simply as an over-sized road gully, in which as more head was created that the plans proposed for the downstream end of the catchment were
within the system more flow was forced out of the top. The upstream coordinated with the proposals for the area as a whole to avoid the
manhole proved more complicated to model because it had two possibility of any asset becoming redundant in future years.
methods of spilling: when the pressure head increased and the flow level At the moment, the company is in negotiations with the Environment
rose, an initial discharge occured through the gap between the cover slab Agency to ensure that the proposed works do not cause any detriment
and the main manhole chamber; however, in a more extreme event, in to the river environment and that any works in the river do not have a
which water levels rose further, the sewer would pressurize to a greater negative impact on predicted flood levels and flood frequency.
extent and eventually the manhole cover would be forced off, creating a Discussions are still ongoing with the client and the other stakeholders
major release of flow from the system. to identify the best way forward. ■■■
The other advantage of using the gully modeling technique was that the
modelers were able to replicate the river interaction for the more extreme This article is based on a paper presented at the Wallingford Software
design events when the river level over-topped the manhole covers. International User Conference in September 2008 by Will Wormald of Atkins.
November 2009 Water & Wastewater Treatment 21
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