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.
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•Bridges likely to accumulate debris assessed by direct and indirect observations.•Direct observations include satellite imagery and inspection reports.•Indirect observations are based on the location of the bridge.•Direct observations are robust and provide a more solid evidence than indirect.•Applications include river and bridge assessment and management.
The accumulation of large wood debris around bridge piers obstructs the flow, producing increased upstream water levels, large horizontal structural loadings, and flow field modifications that can ...considerably exacerbate scour. These effects have frequently been held responsible for the failure of a large number of bridges around the world, as well as for increased risk of flooding of adjacent areas. Yet little is currently known about the time evolution and processes responsible for the formation and growth of these debris piles. This paper is aimed at deciphering the whole life of debris accumulations through an exhaustive set of 570 experiments in which debris elements were individually introduced into a flume and accumulated at a pier model downstream. Our findings show that in all experiments, the growth of accumulations is halted at a critical stage, after which the jam is removed from the pier by the flow. This condition typically coincides with the time when the dimensions of the accumulations are maxima. The values of the accumulation maximum size display a clear dependence on flow characteristics and debris length distribution. On the other hand, other variables have shown much weaker effects on the geometry of the accumulations. For a given debris length, accumulations are wide, shallow, and long at low flow velocities but become narrower, deeper, and shorter with increasing velocities. A comparison of results of accumulations formed with debris of uniform and nonuniform size distributions has revealed that the former can be up to 2.5 times wider than the latter.
Key Points
Experimental results show that the growth of debris jams at piers is halted at a critical stage
New relations are proposed between the size of debris jams and flow and debris variables
Accumulations formed with uniform are considerably larger than with nonuniform debris
This paper compares the application of two recently published guidance documents for risk-based assessment of hydraulic actions on bridges, namely the UK Design Manual for Roads and Bridges and the ...Italian Ministry of Infrastructure and Transport's Guidelines, to two case study bridges (Staverton Bridge, UK; Borgoforte Bridge, Italy). This work is one of the first to illustrate how to apply these guidelines. Both documents present risk-based methods for the assessment of hydraulic actions, while exhibiting fundamental differences. For example, the UK method prescribes calculations for local and constriction scour, water depth, and velocity at several cross-sections; by comparison, the Italian method does not prescribe calculations to assess the risk level. For the case studies in this paper, the hydraulic risk obtained for Staverton Bridge resulted as 'High' using both methods. The scour score for the Borgoforte Bridge resulted higher using the Italian method (Medium-High), as compared to the UK approach (Medium). This difference is due to how the guidelines assess the vulnerability associated with the minimum clearance. The comparison of these two risk-based approaches and the resulting discussion may serve as a useful resource for those wishing to develop new risk-based methods for assessing hydraulic actions on bridges.
Alluvial river channels respond to changes in sediment supply by adjusting their geometry. Landslide sediment delivery and geomorphic response of river channels during floods are poorly understood ...and rarely examined in tropical settings. We investigate the impact of landslides on channel geomorphic changes during an extreme typhoon‐induced flood event in the Philippines, specifically the complex geomorphic response of the Antamok River to Typhoon Mangkhut in September 2018, which triggered >500 landslides in the Ambalanga catchment. The catchment has a legacy of anthropogenic modifications, such as extensive small‐scale (artisanal) mining and tailings storage facilities (TSFs) from large‐scale mining activities.
We use a novel mix of mapping and computational modelling approaches to test the hypothesis that landslide sediment delivery is a major control on channel geomorphic change. Pre‐ and post‐event imagery show that the overall active channel area increased by 35.9% and the mean active channel width increased by 9.1 m. Spatially, we find no clear relationship between landslide sediment input or unit stream power and channel width geomorphic change, with longitudinal changes in active channel width complicated by TSFs. Multi‐phase modelling using r.avaflow revealed how landslide sediment delivery and TSFs interacted with the flow to generate the observed patterns of channel change. The model simulated channel incision in the upper parts of the catchment (up to 0.78 m) and deposition in the TSFs (up to 1.73 m).
Our findings demonstrate that well‐established methods (e.g., stream power threshold) fail to fully explain channel width geomorphic changes, particularly for anthropogenically altered catchments. Integrating techniques, such as landslide mapping and multi‐phase computational modelling improves understanding of sediment supply's role in channel width change during extreme events. Numerical simulations also demonstrate that conventional assumptions of increased erosion and deposition with rising flow discharge are inaccurate with large sediment input, highlighting instead the effectiveness of multi‐phase models.
The Antamok River (Philippines) was affected by more than 500 landslides during Typhoon Mangkhut in September 2018. Landslide mapping using satellite imagery and remote sensing, together with multi‐phase computational modelling, highlighted key areas of erosion and deposition in the catchment, which could not be represented by traditional approaches such as unit stream power threshold.
The accumulation of large wood debris at bridge piers obstructs the flow, producing increased upstream water levels, large horizontal structural loadings, and exacerbated scour. These effects have ...frequently been held responsible for the failure of a large number of bridges around the world, as well as for increased risk of flooding of adjacent areas. Yet, little is known about the formation and growth of these debris piles. This thesis is aimed at deciphering the whole life of debris accumulations through an exhaustive set of 732 experiments in which debris elements were individually introduced into a flume and accumulated at a pier model downstream. In all experiments the growth of debris accumulations was observed to stop at a critical stage, after which the jam is removed from the pier by the flow. This condition typically coincides with the time when the dimensions of the accumulations are maxima. The values of the jam maximum size display a clear dependence on flow characteristics and debris length distribution, whilst other variables (such as pier diameter, debris diameter, debris density, water depth, pier shape) have shown much weaker effects. For a given debris length, accumulations are wide, shallow, and long at low flow velocities but become narrower, deeper, and shorter with increasing velocities. A comparison of accumulations formed with debris of uniform and non-uniform size distributions has revealed that the former can be up to 2.5 times wider than the latter. The effect of the shape of debris pieces was also studied by using cylindrical dowels, unbranched sticks and single-branched sticks. The maximum size of debris piles formed by idealised cylindrical debris is smaller than that of jams formed by natural wood of irregular shape. Experiments with branched debris resulted in jams significantly smaller and less stable than those with nonbranched sticks. On the base of the experimental results, a mechanistic theoretical model of idealised jam geometry and a reduced set of dynamic actions was developed through conservation of angular momentum. The resulting system of ODEs was studied in the phase plane, which revealed that the failure of the accumulation depends on both planar asymmetry and ratio between the length of the jam and the extension downstream of the pier, defined as tail. The former is necessary for any jam to fail, and higher asymmetries lead to less stable jams; the latter provides stability for large tails and small lengths, but yields instability when the ratio is reduced. Results from this thesis will pave the way for practical applications in bridge engineering and flood risk assessments, and inform future research about debris jams at bridge piers.
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