Climate change is increasing the frequency and severity of extreme weather events such as storms and floods. This presents an increasing threat to structures susceptible to scour during periods of high water flow. Transport Scotland therefore conducted a trial to determine whether access to remote water level measurements could improve the protection of critical transport infrastructure.
The test was carried out on a bridge over Vales Burn near Hawick in Scotland using water level monitoring equipment supplied by OTT HydroMet.
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“The system was configured to issue alarms at pre-set water levels, which improved the timing and efficiency of site inspections,” says Wendy Strain of OTT.
The stability and integrity of structures such as bridges, culverts and walls can be compromised by “bridge scour” which is caused by the rapid flow of water which removes materials around the abutment flanges bridge or pillars.
There are three main types of scour. Local scour is caused by the faster flow of water around piers and abutments. Constriction scour is created by the acceleration of water through a narrow opening, and degradation scour can lower the bed of a river over a longer period, both upstream and downstream d ‘a structure.
The effects of bridge washouts can be disastrous. For example, during a flood in 1987, the Glanrhyd Bridge in Carmarthenshire partially collapsed, causing a train to fall into the River Towy, killing the driver and three of the passengers. In 2009, the failure of the Malahide Viaduct in Dublin, Ireland, was caused by scouring of the foundations. Two commuter trains packed with hundreds of passengers were seconds away from disaster when a 20-metre section of the viaduct collapsed after they passed. No one was killed, but travel chaos ensued.
Today, road and rail authorities carry out regular inspections to assess the risk of scour.
Bridge washouts in Scotland
Transport Scotland is divided into four operational regions and Jim Brown is the South East Unit Bridge Manager. Prior to this role, Jim spent 42 years as a civil engineer with Network Rail, including 13 years leading the geotechnical discipline in Scotland. So it’s fair to say that Jim knows a thing or two about bridge washouts. “The most important goal is early detection of increased risk,” says Jim. “Once we have established which structures are most vulnerable during flooding, we are able to implement different levels of mitigation.
“A lot of research has been undertaken to try to monitor scour but this is obviously problematic as potential scour occurs at times when it is not possible to see the river bed and certainly not possible to deploy divers,” adds Jim. “For this reason, continuous water level monitoring is not the perfect solution, but may be the best and most practical.”
The water level is already used as a risk factor by the inspection teams where the flood level of 1 in 200 is materialized on the structures by a fixed mark. However, Transport Scotland has to monitor 700 structures over or near water, and site inspections are costly; both financially and in resources. The inspection system could therefore be more effective if it could be informed by remotely available water level data, particularly if the data allows faster and more effective implementation of mitigation measures.
If a structure is deemed high risk, a hydrological assessment is undertaken. This may require reinforcement of the structure itself, as well as measures to protect or fill in potential scour areas and/or to alter water flow and disrupt eddies. In extreme cases, when a potentially vulnerable structure is exposed to flooding, it may be necessary to close highways or railways until the level of risk can be reduced.
Vales Burn Trial
Given the potential for actual water level data to inform the bridge inspection process, OTT HydroMet was asked to propose a monitoring system that could be assessed at a culvert on the Vales Burn. “Our solution included an OTT ecoLog 1000 water level logger in a stilling well, with a cellular data connection to Hydromet Cloud; a web-based platform for processing, displaying and storing hydrometeorological data,” says Wendy Strain. “Data has been set to be transmitted regularly every six hours, but two alarm levels have been set to trigger SMS alerts, if necessary.”
The monitoring system was installed in September 2021, and Jim Brown says: “The trial is a great success; we use the Scottish Environmental Protection Agency (SEPA) flood warnings to monitor flooding and in our experience the highest water levels usually occur around a day or two after a storm. One of the main objectives of the trial was therefore to determine the best time to send inspection personnel.
“The monitors issued alerts during the trial, but that was when levels reached the lower alarm level, and in those cases further intervention was not required. water, so we will be able to optimize the deployment of inspection teams and mitigate risk,” adds Jim.
Bridge scour is a long-standing problem for transportation infrastructure, so the deployment of remote water level monitors/alarms accessible via PCs or a mobile app offers rail and road authorities the ability to optimize their scour assessment activities, prioritize countermeasures, improve transport resilience and protect safety.
Reflecting on the trial, Jim says, “This project showed that a network of water level monitors could notify bridge engineers of potentially affected structures. I understand that the Hydromet Cloud platform is capable of connecting to an almost infinite number of water level monitors, so we may be able to deploy this on a larger scale in the future.
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