D7.5. Report on Sewer System Hydraulic Performance in Networks with Defects and Deteriorating Assets

Defects and deterioration are common in sewer networks. Often these are ignored as they are difficult to quantify with current inspection methods. It is known that defects have an impact on sewer hydraulic performance, but this impact is rarely quantified. This deliverable reports on modelling studies that aimed to investigate whether it would be possible to quantify the impact of deterioration or a defect in a single pipe within a network. Previous studies have focused on the random insertion of defects into an existing sewer network hydraulic model and then examining the overall impact on network performance. In this study we automatically placed a defect into individual pipes and examined the local and overall network impact. Two types of defect/deterioration were examined; the first was an accumulation of sediment in a pipe invert and the second was a defect such as a poorly installed lateral connection or roots that intruded into a pipe from its soffit. These two defects were selected as they are common and were thought to be able to influence system performance in different ways.

An initial case study was carried out in a small UK catchment, which had 195 nodes, 193 conduits, 3 outfalls and drained around 55 hectares. The sewer network was combined. A second case study catchment was selected which had over 4000 nodes and over 4000 conduits, over 30 outfalls and over 60 weirs. The sewer network was mainly combined and drained an area of around 925 hectares.

Defect/deterioration code was developed to automatically incorporate a defect into a SWMM sewer network model so that its impact on network hydraulic performance can be quantified. This enhanced sewer network model was used to examine the impact of in-pipe defects. In the initial case study blockages formed on the invert of the pipe were examined. Short-time rainfall time series was used to estimate the impact on flood severity. This work clearly showed that in-pipe blockages had a significant impact on local flood severity. It also showed that for some locations an in-pipe blockage at that location could impact on system performance at remote locations, but these were small in number.

In the second case study both types of the defects described above were used – invert blockage and soffit blockage. The second case study area was a much larger network. Because of the computational load in the second case study the simulations were carried out using a single measured rainfall event. The rainfall intensity of this event was scaled to understand the impact of individual defects for events with different rainfall volumes and to understand the impact on system performance of differently sized defects. The measure of system performance used was the overall flood volume leaving the system through manholes.

The results showed that in-pipe defects do impact on the flood risk performance of networks. Different types of blockages create different impacts. There is some evidence that blockages can create flooding impacts remote from their location, but this aspect requires more study. Only flood risk impact has been examined, however other system performance indicators, such as overflow spill frequency or overall overflow spill volume could be used in such modelling studies.