As flooding events become more prevalent, railways and highways bridges over rivers have become more vulnerable to collapse through the scouring away of their foundations. This leads to instability of the superstructure and the outcome is the type of failure we saw at Feltham Railway Bridge in 2009, six separate road bridges in Cumbria also in 2009 and Tadcaster in 2015. Thankfully there have no associated fatalities since Glanryhd in 1987.
Scour at bridge sites is typically classified as constriction scour and local scour. The former occurs across the cross-section of the bed on account of increased flow velocities associated with narrowing of the channel at the bridge site. Local scour is brought about by the increased velocities and vortices at abutments and piers. Debris struck at a crossing can exacerbate the effect by further constricting the channel.
Scour Assessment Services
Level 1 Scour Assessments
HRS can provide level 1 scour assessments to BD97/12 by surveying the riverbed, foundations and any existing protection measures and undertaking a preliminary risk rating.
Level 2 Scour Assessments
Depending on the outcome of the level 1 assessment, HRS can also proceed to a level 2 assessment involving the calculation of flow depths and velocities under flood conditions resulting in the assignment of a Scour Risk Rating and recommendations for management measures. Our requirements from the client include:
- Topographic survey of the bridge showing its key features (abutments, piers, arches, decks) and including cross-sections through the river channel upstream and downstream of the structure. The survey should include details of the bridge foundations (level) and any existing scour protection
- Geotechnical information/boreholes/trial pitting of the bridge foundations to determine whether undercutting of abutments or piers is likely. This would also include obtaining the grading of granular materials forming the river bed
- A hydrological analysis to determine extreme flows within the channel (usually based on Flood Estimation Handbook techniques) and a hydraulic model of the watercourse to determine the velocity of water running through the bridge structure during peak flow. We are able to undertake the hydrological analysis and hydraulic modelling if an existing hydraulic model is not available from the Environment Agency
HRS can also remove the debris deposited during flood events, including when the river is in spate
HRS has undertaken inspections on trunk road structures for clients the UK’s leading contractors and consultants and numerous local authorities.
Since 1990 HRS has inspected over 1,000 structures on live railways and 3,000 disused rail structures.