Network Rail scour protection programme

Location: South Wales                                                                              Client: Network Rail

We have been and still are heavily involved with the Network Rail scour assessment programme, in which Network Rail structures over water are assessed for their scour risk. Any structures that fall within the high priority rating are then put forward for scour protection to reduce the scour risk.

As part of an ongoing UK-wide scour protection programme, we were commissioned by Network Rail to complete the scour protection design at several structures that fell within the high priority rating category. The structures were in south Wales around the Bridgend area on a stretch of railway that connected mid-Wales to southern Wales. The scour protection project required a unique collaboration between our environmental and engineering teams to design the solution for our client.

Challenge

The design of scour protection is based on several factors, primarily around hydraulic velocities, flows at the structure as well as bed levels and foundation depths. The foundation depths are particularly important as the protection must be designed so as not to extend beyond the base, therefore, maintaining the structural integrity of the bridge. Several of the structures were cored to confirm the foundation depths, with the results showing shallow depths. This meant the protection would likely be at, or slightly above, existing bed levels.

As part of any works with a watercourse, a Flood Risk Activity Permit must be submitted to the relevant consenting body. In this case, this was Natural Resources Wales (NRW). We had initial discussions with NRW regarding the design and scour protection levels. NRW raised concerns with the type of protection and the levels at which it was to be installed, stating that the concrete mattress levels were too shallow and would potentially cause velocities to increase through the structure and disturb the migration of fish and other aquatic animals.

NRW recommendations, based on the JBA design, were to bury the mattress approximately 1m below existing bed levels to allow natural deposition of gravels, over time, and significantly reduce the risk of the mattress being exposed, limiting the impact on the natural ecological cycle of the watercourse.

Solution

With assistance from our ecology and hydro-morphology teams, JBA engineering designs do take into consideration the ecological and environmental impacts the protection may have on the watercourse. Having considered several different options, the NRW request for a 1m installation of the protection would not be possible at this location. This was mainly due to the shallow foundations, the depth of water and the excavation likely required to install the mattress to a 1m depth making construction unsafe and impractical.

Our environmental team took on the task of both modelling the protection hydraulically and assessing the impact of the mattress on the wider water body environment at various depths within the channel. By doing this, we could assess at what depth the mattress could be installed to meet both the NRW concerns and those outlined through the Water Framework Directive (WFD). Through a detailed WFD assessment report, using the results from several hydraulic models, it was found that at a depth of 600mm below existing bed levels the impact of the mattress on the wider watercourse, both ecologically and hydraulically, would be negligible. We presented the data, in the form of flood water levels, channel velocities and gravel migration heat maps, to NRW in pre-works and post-works tabulated results. These tables showed that at 600mm below bed level the mattress did not increase any of the parameters above and as such was agreed and consented to by NRW.

Benefit

By using expertise from both our environmental and engineering teams, and by working collaboratively with our client and stakeholders, our solution was consented to by NRW and allowed Network Rail to protect an important asset from scour. The 600mm depth meant that the mattress could be installed safely and within the client’s budget.



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