A practical method to assess risks from large wood debris accumulations at bridge piers

Abstract

Accumulations of large woody debris can worsen scour at a bridge pier and thereby lead to structural damage. Accumulations can also increase the flood risk in adjacent areas. These consequences can cause disruption to local communities and even pose a risk to human life. Current methodologies acknowledge the existence of these effects of debris but do not provide a practical method, usable by engineers and practitioners, to assess the potential for debris accumulation at a bridge structure based on readily available data. This work aims to address this practical need by proposing a methodology based on direct and indirect observations. Using this methodology, a desk-based analysis can be performed to assess whether a bridge is prone to the formation of debris accumulations. Direct observations may include information from inspection reports, satellite imagery and tree removal works, while indirect observations may use information related to the geographical location of the bridge such as on other structures that share the watercourse or the presence of forested areas in its proximity. This methodology has been applied to local authority-owned bridges in Devon, UK. Results show that a large number of the structures (100 out of over 3000 bridges) are liable to debris accumulations. Direct observations served as primary evidence for over 80% of the bridges liable to debris accumulations. For many cases, direct observations existed to corroborate indirect observations suggesting that indirect observations can also be relied upon. The proposed methodology has also been applied to the prioritisation of bridge inspections for scour assessment. Results showed that many of the bridges prone to debris accumulations would need to be prioritised for scour inspections over other bridges in the aftermath of floods due to their significantly higher risk to scour in the presence of debris.