## 1 Introduction  
In the UK, there is over 600,000 km of sewer pipes and over 70% of these are combined, in that they carry both wastewater and rainfall run-off from urban surfaces [1]. Due to climate change and increasing capacity demand because of increased population and urbanization, this ageing infrastructure is progressively under pressure and deteriorating, causing a steady increase in maintenance budgets [2, 3]. Water companies are regulated by the Office of Water Services (OFWAT), which is a non-ministerial government department that is responsible for ensuring that water companies offer adequate service to the customers at a fair price [2]. With these stringent regulations, water companies have invested more than \(\mathcal{EB}\) billion in 2019-20 to improve their level of services. These include customers' sewer repair, replacement and rehabilitation, one of the major areas of spend [3]. This has opened an unprecedented opportunity for instrument makers and researchers to develop partnerships to offer novel approaches to proactively reduce the risk of failure and allow better understanding of the behaviour of the sewer network under both normal and extreme operating conditions [4, 5, 6, 7, 8, 9, 10, 11, 12]. There are approximately around 31,000 combined sewer overflows (CSOs) in the UK [13]. CSOs play an important role as emergency discharge structures in a sewer network. Their primary function is to limit the discharge of wastewater to wastewater treatment plants, without causing local flooding when the sewerage system is overloaded by intense rainfall. When the capacity of the sewer system comes close to its maximum, excess flow is discharged directly into a local watercourse. These discharges are regulated by the Environment Agency, and water companies are required to comply with aesthetic control standards, which ensure that no solids having dimension greater than 6 mm in two orthogonal directions can be released [14]. Water companies have therefore installed screens within the CSO chamber to meet the aesthetic control standards. The condition of the CSO screen determines the ability of a CSO to operate to its legal discharge consent. Blocked screens can lead to CSO discharges being bypassed and CSO spills containing solids that do not meet the aesthetic control standards and leave them liable to substantial financial penalties. In order to reduce the risk of screens failing, they need to be regularly inspected and maintained (cleaned). This is costly for water utilities as planned maintenance requires engineers to visit a large number of remote sites, to potentially institute traffic management, enter a confined space (CSO chamber) and then manually inspect the screen conditions. Screens get blocked after spill events, so a preplanned maintenance program often results in many unnecessary visits. A more cost-efficient and safe approach would be to acquire the real-time information about the screen condition if it is required to be cleaned. The key physical assets that constitute a CSO are the inlet and outlet pipes, which convey wastewater downstream. The diameter of these pipes, their slope and wastewater discharge determine the water level in the CSO chamber. If the wastewater level exceeds a certain threshold, e.g., in the case of a heavy rainstorm event or downstream blockage, then wastewater water spills through the CSO screen into the outlet pipe, into a receiving watercourse, e.g., local river or natural stream. The screen is installed so that spill flow can be screened for large solids. If the screen gets blocked, then an emergency overflow can bypass the screen (marked A in Figure 1) to allow unscreened wastewater to be directly discharged to the receiving watercourse. In the latter case, the CSO is failing its discharge consent. To monitor the conditions of the inlet and outlet pipes of the CSO, several pipe inspection methods have had either destructive or non-destructive testing approaches proposed and implemented in the literature [16; 17; 18; 19; 20; 21; 22; 23].