Stability of Short-span Bridges Subject to Overtopping during Floods: Case Studies from UK and the USA

Donal Ryan, Robert Ettema, Brian Solan, Gerard Hamill, John McRobert

Tuesday 30 june 2015

9:15 - 9:30h at South America (level 0)

Themes: (T) Water engineering, (ST) River and coastal engineering

Parallel session: 4J. Floodrisk – Flooding

As a consequence of increased levels of flooding, largely attributable to urbanization of watersheds and perhaps climate change, more frequent extreme rainfall events are occurring and threatening existing critical infrastructure, many of which are short-span bridges over relatively small waterways (e.g., small rivers, streams and canals). Whilst these short-span bridges were designed, often many years ago, to pass the then standard return-period floods, in recent times the failure incidence of such short-span bridges has been noticeably increasing. This paper presents, and draws lessons, from bridge failures in Ireland and the USA. For example, in November 2009, the UK and Ireland were subjected to extraordinarily severe weather conditions for several days. The resulting flooding led to the collapse of three UK bridges that were generally 19th century masonry arch bridges, with relatively shallow foundations. Parallel failure events have been observed in the USA. To date, knowledge of the combined effect of waterway erosion, bridge submergence, and geotechnical collapse has not been not been adequately studied. Recent research carried out considered the hydraulic analysis of short span bridges under flood conditions, but no consideration was given towards the likely damage to these structures due to erosive coupling of hydraulic and geotechnical factors. Some work has been done to predict the discharge downstream of an inundated arch, focusing on predicting afflux, as opposed to bridge scour, under both pressurised and free-surface flows, but no predictive equation for scour under pressurised conditions was ever considered. The case studies this paper presents will augment the initial findings from the laboratory experiments investigating the effects of surcharged flow and subsequent scour within the vicinity of single span arch bridges. Velocities profiles will be shown within the vicinity of the arch, in addition to the depth of consequent scour, for a series of flows and model spans. The data will be presented and correlated to the most recent predictive equations for contraction and abutment scour. The accuracy of these equations is examined, and the findings used as a basis for developing further studies in relation to short span bridges.