CONCLUSIONES Y RECOMENDACIONES

As the root cause of the CSO pollution problem is surface water combined with foul sewage flows, separating the two is an obvious potential option for consideration. This could be achieved by either having the existing sewers deal only with surface water and installing a new foul system or by installing a new storm water collection system with foul flows only in the existing sewers. However, there would be significant disruption to nearly all communities in the Beckton and Crossness catchments as a result of construction work in potentially every road in London and the

modification of the drainage system for virtually every property26_.

The TTSS found that the construction of a separate system for the catchment served by the combined collecting system connected to Beckton and Crossness STWs would only be possible at very high cost, unlikely to be less than £12,000 million, and would entail construction over a very

long timescale27. An example, for instance, is the recent and ongoing disruption caused to

Londoners by the Victorian Mains Replacement scheme.

Lengths of sewer, split into use and size categories, are recorded in Thames Water’s assets database. For the Beckton and Crossness catchments, the data for sewers classified as combined is given in Table 5.4. The TTSS figure of £12bn for sewer separation equated to a cost per metre of sewer length of £2,500 (at a December 2006 cost base), which is realistic given the nature of the area served.

Table 5.4 TW asset data: Sewers categorised as combined Size <300 (Length in metres) Size 300-600 (Length in metres) Size >600 (Length in metres) Total length of combined sewer in metres Beckton catchment 985,000 517,000 1,203,000 2,705,000 Crossness catchment 1,190,000 413,000 493,000 2,097,000 Total 2,175,000 930,000 1,696,000 4,801,000

NB: Based on figures and sewer categorisation provided by Thames Water, July 2009. Please note that the combined sewer lengths shown above and the catchment areas and percentage of combined sewer by area (as in Sections 4.1.2 and 4.1.3) cannot be correlated. It was pertinent to update the TTSS work previously undertaken on sewer separation. Separation of the combined system could be achieved by establishing:

 a separate storm water collection network, or

 a separate foul network (involving smaller diameter pipework) with the existing combined

network converted to a storm water only carrier, or

 a collection system for storm water from roads and hardstandings only.

These various approaches were investigated in a Sewer Separation Feasibility Study undertaken by MWH; see Appendix D. This study developed hydraulic models, outline designs and cost estimates for five representative sub-catchments as follows (the LTT CSO reference in brackets):

 Frogmore (Buckhold Road) (LTT CSO ref. CS07B) sub-catchment (454 ha) in the Crossness

catchment. This area is 40% - 70% combined and is largely residential with some light industrial premises.

26 _{TTSS Supplementary Report to Government (2006), Summary Section 2.2}

 South West Storm Relief (LTT CSO ref. CS17X) sub-catchment, eastern branch (1,404 ha), in the Crossness catchment. This area is 40% - 70% combined and is largely residential with some commercial and light industrial premises.

 Lots Road Pumping Station (LTT CSO ref. CS10X) sub-catchment, southern area (3,276

ha), in the Beckton catchment. This area is 70% - 100% combined and is largely residential with some commercial properties.

 Regent Street (LTT CSO ref. CS22X) sub-catchment, southern area (1,015 ha), in the

Beckton catchment. This area is 70% - 100% combined and is largely residential with significant areas of business and commercial premises.

 Northumberland Street (LTT CSO ref. CS23X) sub-catchment (93 ha) in the Beckton

catchment. This area is 70% - 100% combined and is largely commercial and includes government premises.

These sub-catchments were selected as they are located in the west of the Beckton and Crossness catchments where there is a broad spectrum of different land uses and sewer types. Additionally, these areas have fewer cross connections and interactions with the interceptor sewers and hence there is greater potential to find robust separation opportunities to achieve no more than four spills per typical year into the River Thames or its tributaries.

The study established that the collection of storm water solely from roads and hardstandings would not achieve the required reduction in discharges to four spills per typical year. Consequently a significant proportion of roof drainage would also need to be captured. The study further

established that if the required reduction in spills could be achieved by capturing the drainage from road-facing roofs only (up to 50% of the total roof area) then construction of a separate storm water system would be the appropriate less disruptive solution. However, where more than 50% of roof area drainage needed to be captured to meet the requisite spill target, then construction of a separate foul network would be more appropriate. This is because establishing a separate storm water system in these circumstances would require access to the rear of properties for further roof rainfall collection. A more comprehensive re-arrangement of all household and connection pipework would then be required meaning greater disruption to individual properties than would be necessary to establish a new foul water collection system.

Outline design of the new separate foul and storm water systems was carried out to Thames Water specifications. These include minimum velocities for new foul sewers and 1 in 30 year design rainfall capacity for the new storm water networks. This resulted in the need for very large diameter storm water sewers and very large storm water pumping stations to discharge the peak flows into the River Thames at high tides. The pipe and pumping station sizes were much greater than in the existing, albeit under capacity, interceptor sewer networks. This apparent anomaly was

investigated and the reason found to be the lack of attenuation in the numerous and discrete new networks. The Thames Tunnel, on the other hand, will have the capability to attenuate and store the inflows over a comparatively large geographical area, easily accommodating the 1 in 30 year storms until nearly full, after which one of the four spills per typical year would occur.

Estimating the construction costs of the outline sewer separation designs followed a methodology consisting of:

 Property/premises costing for property type, land use, density, pipe size, typical depths and

lengths, inspection chambers and buildability/difficulty assessments;

 Local and street level costing for pipe sizes, depths, lengths and manhole numbers;

 Spine system costing based on the hydraulic modelling including new pumping stations, new

outfalls, appropriate construction methods (trenchless technology), existing foul and storm water system modifications and buildability/difficulty assessments.

To establish the total costs for each of the study areas the uplift factors, risks and non-construction costs were taken into account. These indicative allowances, which are based on Thames Water’s experience on similar types of contract (Victorian Mains Replacement, London Water Ring Main and extensions, West Ham Flood Alleviation Scheme) and are common to all the alternative options costed (tunnels, sewer separation and SUDS), are listed in Table 5.5 below.

Table 5.5 Allowances for non-construction costs

Category Allowance applied

to construction costs

Comments

Design and Planning

consents 10% Higher than for Thames Tunnel (8.1%) due to complexity of multiple contracts

Preliminaries 25% Higher than for Thames Tunnel (18.7%) _{due to complexity of multiple contracts}

Fees 8.5% As Thames Tunnel estimates

Insurances 5% As Thames Tunnel estimates

Price contingency 10% As Thames Tunnel estimates

Risk allowance 35% As Thames Tunnel estimates

Management and overhead

costs 18% As Thames Tunnel estimates

Contingency for external

costs 11.5%

As Thames Tunnel estimates Based on 80% probability using the industry standard analysis of assumed likelihood and three point cost estimates of each identified risk.

Optimism bias 27%

As Thames Tunnel estimates From HM Treasury optimism bias calculator for non-standard civil

engineering projects (on civil and MEICA construction costs)

Enabling works, including utilities diversions and investigations, were taken into account by establishing the degree of complexity of each sub-catchment studied and attributing an uplift bias. Similarly the social impacts and compensation costs were assessed for each sub-catchment. The sewer separation feasibility study took the non-construction costs into account and developed the following cost estimates for each of the study sub-catchments, as given in Table 5.6 below.

Table 5.6 Total costs for each sewer separation study area

Sub-catchment Area (ha) Total costs

(£ millions)1

Frogmore (Buckhold Road) 454 267

South West Storm Relief 1,404 542

Lots Road Pumping Station 3,276 1,494

Regent Street 1,015 1,175

Northumberland Street 93 110

Sub-totals 6,242 3,588

1

December 2008 cost base

This estimate is for the works required to reduce spills at the ten CSOs within the five sub- catchments studied to four spills per typical year. The sewer separation proposals were then extrapolated to cover the whole of the Beckton and Crossness catchments served by the Thames Tunnel scheme to reduce spills at the 34 unsatisfactory CSOs. The upscaling exercise estimated the costs of foul sewer separation only (the lower cost separation option) by sub-catchment and then made adjustments for the double counting created by numerous interconnections across sub- catchment boundaries. This resulted in a total estimated cost of £14,000 million, with a variance

range of +50% to -30% due to the estimate being based on a planning level of detail: See Appendix D, Annex 1, Sewer Separation Total Cost.

This extrapolated cost estimate of sewer separation is significantly more than the preferred tunnel options. Furthermore such a foul sewer separation arrangement has the following limitations which should be noted:

 The capacity of the existing combined sewer network to convey storm water flows would

only marginally increase with the removal of foul flows. This is due to the large volume of infiltration that enters the existing sewer network from the ‘lost rivers’ of London. Thus the conversion of the existing combined sewers to convey the surface water flows will only provide marginal relief from sewer flooding.

 Full separation would not remove all pollution from entering the River Thames due to the

build up of hydrocarbons and debris on the roads and pavements that would be flushed into the surface water system during times of rainfall. Misconnections, which occur when errors in domestic plumbing introduce connections between systems, will often mean a system will never be completely separate and there would always be the opportunity for foul flows to be in a separate surface water system.

The desk-based qualitative study indicated that high negative social impacts could be expected during the construction of separate foul sewer systems in London. High levels of transport

disruption could be anticipated with potential financial impacts as a consequence of the disruption. Social and economic impacts identified also included loss of amenity due to access limitation of parks, public and tourist facilities and attractions. In addition there is a limit to how much

construction could be carried out in any given year both in terms of resource to carry out the work and also an acceptable level of funding from water bills. The annual spend for the Victorian water mains replacement project is approximately £200 million and if this spend rate were to be doubled the sewer separation project would still be estimated to take some 35 years to complete.