The following paper, authored by Wessex Water’s Helen Isaacs, summarises the findings of a case study into the effectiveness of CIPP to improve hydraulic capacity to reduce flood risk.
by Helen Isaacs, Critical Sewer Engineer, Wessex Water, Bristol, UK
The use of cured-in-place pipe (CIPP) lining has been proven to have many benefits to the sewer networks, including improved structural integrity, reduced groundwater ingress and the prolonged service life of key assets. Given the reduced costs associated with lining a sewer as opposed to a traditional open cut replacement, it is surprising that in some instances sewer lining is not encouraged.
This can be due to the industry perception that rehabilitation of a sewer through lining decreases the hydraulic capacity of the pipeline due to the reduction in cross sectional area. This can lead to conflicts: is it better to line an infiltrating sewer downstream of a flooding location, removing some of the groundwater ingress that might be contributing to the flooding, or is it better to maintain the status quo and not risk increased frequency of flooding at a sensitive location?
It is possible that there is not a wider appreciation within the industry of the effect of lining a pipe upon the roughness values and hence pipe full capacity of a pipe. The finish to a lined pipe is in general smoother than the original pipe wall through the material characteristics of the resin, as well as the ‘smoothing out’ of joints and cracks in the pipes. Table 1 gives the standard roughness (ks) values for a number of different pipe materials, with the values taken from Urban Drainage Third Edition.
For comparison, the Wessex Water hydraulic model design standards gives 1.5 mm as soffit roughness and 3 mm as invert roughness for all foul pipes unless flow survey data indicates otherwise.
The ks value of CIPP lining is believed to be between 0.3 and 1 mm, with Insituform reporting a manning roughness coefficient of 0.01 on average for uncleaned, in-service pipe, which is equivalent to a ks value of approximately 0.6 mm. From comparing this to the ks values in Table 1 it is clear the largest improvement in pipe roughness would come from lining a brick sewer; however, more common modern pipe materials, like clay or concrete, are also rougher than the liner material.
When it comes to proposing lining in hydraulically sensitive areas the real question for design engineers is how much of a difference does this reduction in roughness make? A significant number of flooding properties are located in areas served by relatively small clay or concrete pipes, where we would expect the roughness gains to be less.
The following case study was located in such an area, served by 225 mm diameter clay pipes; however, due to sensitive customer relations in the area the location has not been explicitly described.
Figure 1: Smooth interior of a lined pipe. Image courtesy of Wessex Water.Case study
Five lengths of 225 mm diameter clay sewer, totalling 360 m, in a town in Wiltshire, England was identified for epoxy lining following an infiltration identification exercise. The sections of sewer served a high-profile flooding location, which had recently been the focus of flooding alleviation scheme.
Figure 2 shows the five selected lengths, with the flooding location located approximately 100 m upstream.
An InfoWorks CS v14.5 hydraulic model of the catchment built and verified as part of the flooding scheme suggested that the reported flooding at the property was largely due to infiltration and potentially fluvial ingress. Lining the sewer would reduce the volume of flow infiltrating to the sewer locally; however, it would also reduce the cross sectional area of the pipe, reducing the capacity of the asset to cope with the storm events and the continued infiltration in the upstream catchment.
This was considered an unacceptable risk and therefore further hydraulic analysis was recommended using the hydraulic model to provide an assessment of the impact of lining. Best and worst case measures of liner thickness were provided by the lining team (4 mm and 6 mm) and 0.6 mm selected as a suitable ks value for the liner material.
For comparison the design standards for ks values for clay pipes at Wessex Water are 3 mm for the invert (bottom third of the pipe) and 1 mm for the remainder, assuming the pipe is clean. Pipe full capacity values for the five lengths of interest have been taken from the InfoWorks model and are presented in Table 2.
The modelling results actually predict a modest increase in pipe full capacity for the lined pipes. This is only a marginal increase; however, it is contrary to the accepted wisdom that liners reduce hydraulic capacity in small sewers.
A decrease in pipe full capacity is predicted if the pipe roughness reduction is not taken into account. Table 3 shows the results of scenarios run without the reduction in pipe roughness. To further understand the effect of lining of the five lengths, a number of design storms were run in order to assess the risk of increased flooding upstream. The design storms were a typical suite of 10, 20 and 30 year events of 600 minutes in duration (the critical duration for the catchment).
Very low volumes of flooding are predicted by the model for the design ‘unlined’ scenario and this was reduced to no predicted flooding for the two lined scenarios. If the reduced roughness is not taken into account the flood risk is increased.
The hydraulic modelling results are significant in suggesting that if the pipe is lined to a high standard we may expect to see a reduced or at least equivalent flood risk upstream as a result of the reduced pipe roughness. Additional benefits in terms of infiltration reduction and structural improvements may also contribute to
reduced flood risk.
In conclusion, the use of hydraulic modelling software can increase confidence in carrying out lining works in areas with limited hydraulic capacity. When carrying out flood alleviation schemes consideration should be given to using, hydraulic models to confirm the effect of lining on the sewer before discounting lining works as an option.
There are a number of areas for further research, including investigation into the effect of lining different sizes and thickness of liner. Work in this area could potentially lead to the production of a matrix detailing the relationship between pipe size, liner thickness and pipe full capacity, enabling quick assessments of the effect of lining, which would be particularly useful for areas with no existing hydraulic models.
Further work in confirming the roughness values of lined sewers over their lifetimes would also be beneficial in increasing confidence in the modelling results.
This article was featured in the Summer edition of Trenchless International. To view the magazine on your PC, Mac, tablet, or mobile device, click here.
For more information visit the Wessex Water website.
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