Retrospective Evaluation of Cured-in-Place Pipe - (NEPIS)(EPA ...

water industry conferences. A senior STW manager, George Hedley, chaired the Department of the
Environment’s Committee responsible for materials used in sewer and water mains networks. In 1980, it
conducted a survey of sewers in 240 locations distributed across its territory from Shrewsbury in the west
to Scunthorpe in the east and Cheltenham in the south. The resulting analysis relating age, size,
construction material, depth and condition led to a comprehensive estimate of rehabilitation needs and
costs which had an influence on the National Water Council and the Department of the Environment and
the direction of its Water Research Centre in developing sewer rehabilitation strategies.
The reputation developed by STW in the 1980s for its expertise in the practice of rehabilitation was such
that it became respected consultants providing engineers and managers to guide overseas water agencies
such as the Delhi Jal Board and the Bombay Municipal Corporation in sewer cleaning and maintenance,
condition assessment and rehabilitation. The expertise continued after privatization when STW
purchased Haswell and Partners, a specialist consultant firm, and transfered operations engineers and
managers into the consultancy firm during a period of reorganization. Haswell Consulting Engineers
ceased to trade in 2005 and residual staff were transfered back to Severn Trent, some finding
opportunities in Severn Trent Services, Severn Trent Water’s sister company.
STW has experienced significant difficulties with flooding. In 2007, flooding cost the company U.S. $50
million and affected more than 140,000 customers in and around the Gloucestershire city of Tewkesbury.
It is involved in extensive network monitoring and drainage studies, which will have future network
maintenance implications.
In the period since privatization, AMP1 through AMP4 (1991-2010), Severn Trent undertook a
significant program of sewer renewal (Table C-4). According to OFWAT statistics, it renovated 630 km
(391 mi) of critical sewer and replaced 414 km (247 mi). OFWAT commenced reporting volumes of
renovation work on non-critical sewers in 2001. Recorded volumes of non-critical sewer work within
STW are surprisingly low, a little over 25 km (16 mi). Follow-up discussions with contractors suggest
that more work has been undertaken in this area and so, as discussed in the Thames Water report, the
under and over reporting to OFWAT may be misleading and it may be reasonable to suppose that the
overall volumes of non-critical sewers lined may be similar to the volumes of critical sewers lined.
Conflicting reports were received from STW about its rehabiliation and replacment volumes in AMP4,
which perhaps confirms the difficulties of classification and interpretation described earlier. According to
STW, in AMP4 it rehabilitated 31 km (19.3 mi) of critical sewer and 10 km (6.2 mi) of non-critical sewer;
it replaced 25 km (15.5 mi) of critical sewer and 97 km (60.3 mi) of non-critical sewer.
Table C-4. STW and U.K. Sewer Renovation Rates for 1990 to 2010
Asset
Management
Period
Severn Trent Water All U.K. Water Companies
Critical
Sewers
Renovated
km
Non-Critical
Sewers
Renovated
C-7
Critical
Sewers
Renovated
km
Non-Critical
Sewers
Renovated
Water
Mains
Relined
km
AMP1 90-95 79 NA 509 NA 10,639
AMP2 95-00 221 NA 711 NA 9,670
AMP3 00-05 63 13 505.4 350.5 9,255
AMP4 05-10 271 11 1162.5 1356.7 6,238
Sub Total 634 >24 2887.9 >1707.2 35,802
Proposed AMP5 10-05 96.1 161 769.4 3358.9 1,126