Phase 3. Assessment guidance development

This phase focuses on the development of a new model for assessing debris-induced scour at bridges. This method integrates with current approaches to improve scour estimations at bridge piers. RAMB project also provides insights for evaluating hydrodynamic pressure at arch bridges.

Effect of debris on scour 

To include effect of debris accumulation on local scour depth, we propose a new model instead of “equivalent pier width” approach. This will enable a more accurate and realistic estimation of the depth of local scour with debris accumulation. The proposed model is developed using not only data from our in-house flume experiments, but also most of available experimental data in literature. Also, in contrast to the “equivalent pier width” method, the model is not restricted to floating debris but is also applicable to cases where debris is submerged or resting on the bed.

We introduce a new factor Φdebris, derived via multivariate regression analysis, to be incorporated into any scour estimation equation. For instance, equation in CIRIA C742 will be modified as

Ys/Bs = Φshape.Φdepth.Φvelocity.Φangle.Φdebris 

where Ys = scour depth; Bs = pier width; and Φdebris  is a debris factor identified based on debris dimensions and elevation.

Hydrodynamic pressure on the arch sofit

We have studied hydrodynamic pressures at a single span arch bridge under inundation conditions using a 1:10 scaled model (following schematic). We particularly measured hydrodynamic pressures at the arch soffit surface (enclosed by green circle in the following figure) using miniature pressure transducers.

We detected large negative pressure (suction in order of upto ~ 3 times the hydrostatic pressure) at the surface of arch soffit. This was a rather surprising finding since negative pressure was not detactable by underwater visual inspection. We identified this to be matching with the observations by practitioners who have diagnosed arch soffit surface often prone to suction and mortar loss. Based on this, we are proposing to do closer post-flood inspection of this location to detect any likely mortar loss and repair it before affecting the structural integrity of the bridge.

Project description


RAMB project is funded by EPSRC as shown in here. It is aimed at studying hydrodynamics and scour around masonry bridges due to debris blockage, which has been identified as a major cause of bridge failure during flooding. Debris can reduce conveyance capacity, enhance scour and increase hydrodynamic forces, which in turn may detrimentally affect bridge stability even under moderate flow rates. Debris build-up can occur around any obstruction to water flow such as bridge piers. Masonry bridges, which constitute over 40% of the nation’s bridge stock, are particularly susceptible to debris blockage due to their short spans and low clearances over water levels. While hydrodynamic effects due to debris are known to significantly heighten the chances of bridge failure by worsening pier scour and constricting flow, current guidance for design and assessment of bridges are highly inadequate for evaluating effect of debris on scour and also hydrodynamic loading at bridges.

This research will directly address this urgent and practical need for an approach to evaluate the risks from debris accumulation at bridges. It will investigate the scour-enhancing effects of debris accumulation in the watercourse, and devise a systematic methodology to assess scour enhancing effect of debris blockage on bridge piers. The methodology, which will be built into existing CIRIA guidance [1] for assessment of bridges under hydraulic action, will enable optimal planning of interventions to effectively target bridges at risk to debris blockage and thereby improve resilience of the transport network and the rate of post-flood recovery. RAMB project also investigate hydrodynamic pressures at arch bridges which can be of practical importance for maintenance of masonry arch bridges.

This research comprises of the following three key elements

  1. Froude-scale hydraulic experiments to depict effects of debris blockage on flow parameters and pier scour;
  2. CFD modelling to simulate flow under masonry bridges and around bridge piers with floating debris blockage; and
  3. Formulation of a risk-based strategy to assess the hydrodynamic effects of debris accumulation at bridges.

[1] Kirby et al. (2015), Manual on scour at bridges and other hydraulic structures, second edition