The trial was conducted at a bridge over Vales Burn near Hawick in Scotland, and the monitoring equipment was provided by OTT HydroMet. The system was set to deliver alarms at pre-set water levels.
Climate change is increasing the frequency and severity of extreme weather events such as storms and floods. This presents a growing threat to structures that could be susceptible to scour during periods of high water flow. The trial was therefore established to determine whether access to remote water level measurements could enhance the protection of critical transport infrastructure.
The stability and integrity of structures such as bridges, culverts and walls can be compromised by ‘bridge scour’ which is caused by fast-flowing water that removes material from around the footings of bridge abutments or piers.
There are three main types of scour. Local scour is caused by the faster water flow that occurs around piers and abutments. Constriction scour is created by water accelerating through a narrow opening, and degradation scour can lower a river bed over a longer period, both upstream and downstream of a structure. The effects of bridge scour can be disastrous. For example, during a flood in 1987, the Glanrhyd Bridge in Carmarthenshire partially collapsed, causing a train to plunge into the River Towy, killing the driver and three of the passengers.
Today, highways and rail authorities conduct regular inspections in order to asses scour risk.
Transport Scotland South East Unit bridge manager Jim Brown said: “Much research has been undertaken in an attempt to be able to monitor scour, but this is obviously problematic because potential scour takes place at times when it is not possible to see the river bed and certainly not possible to deploy divers,” Jim adds. “For this reason, the continuous monitoring of water level is not the perfect solution, but may be the best and most practicable.”
Water level is already used as a risk factor by the inspection teams, where the 1 in 200 flood level is shown on structures by a fixed marker. However, Transport Scotland have to monitor 700 structures which are over or adjacent to water, and site inspections are costly; both financially and in resources. The inspection system could therefore be more efficient if it could be informed by remotely available water level data, especially if the data enables faster, more efficient implementation of mitigation measures.
In light of the potential for live water level data to inform the bridge inspection process, OTT HydroMet was invited to propose a monitoring system that could be evaluated at a culvert on the Vales Burn. “Our solution was comprised of an OTT ecoLog 1000 water level logger in a stilling well, with cellular data connection to Hydromet Cloud; a web-enabled platform for processing, displaying and storing hydrometeorological data,” explains Wendy Strain. “Data were set to be transmitted routinely every six hours, but two alarm levels were set to prompt SMS alerts, when necessary.”
The monitoring system was installed in September 2021, and Jim Brown says: “The trial is a great success; we utilise the Scottish Environmental Protection Agency (SEPA) flood alert warnings to monitor flooding and it has been our experience that the highest water levels generally occur about one or two days after a storm, so one of the main objectives of the trial was to determine the best time to despatch inspection staff.
“The monitors did issue alerts during the trial, but these were when levels met the lower alarm level, and in these cases, further interventions were not required. With the benefit of water level data, we will therefore be able to optimise the deployment of the inspection teams and mitigate risk,” Brown added.
He concluded: “This project has shown that a network of water level monitors could inform bridge engineers of potentially affected structures. I understand that the Hydromet Cloud platform is able to connect with an almost infinite number of water level monitors, so we may be able to roll this out on a larger scale in the future.”