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On 31 October 2018, we will have a workshop at the University of Exeter on Assessment of Debris Related Scour Risks to Bridges. The workshop will present a new approach for assessing the scour risks due to debris blockage at bridge piers. This approach is in the process of becoming embedded as part of (i) C742: CIRIA manual for scour at bridges and hydraulic structures findings and (ii) BD97: the Highways England guidance for assessing scour risks for bridge structures. The approach and other findings to be presented at this workshop are outputs from an EPSRC-funded Project titled “Risk Assessment of Masonry Bridges under Flood Conditions: Hydrodynamic Effects of Debris Blockage and Scour”.
Followings are details and directions.
- Wednesday 31 October 2018 (10:00 – 16:00)
- University of Exeter (UoE), Reed Hall (EX4 4QR) – Ibrahim Ahmed Room (http://www.reedhall.co.uk/contact-us/find-us/)
- University of Exeter (UoE), Harrison Building (for optional tour to the Fluids Lab at the end of the workshop).
The research team is composed of the following institutions and individuals:
* University of Exeter, Centre for Water Systems
Prof Slobodan Djordjevic: principal investigator and leading the experimental phase
Prof Gavin Tabor: leading the CFD modelling phase of the project
Dr Prakash Kripakaran: expert on bridge management and leading the third phase of the project
Dr Mohsen Ebrahimi: former research fellow & physical (experimental) modeller; currently hydraulic engineer at Mott MacDonald
Dr Recep Kahraman: research fellow & CFD modeller
Matthew Riella: PhD researcher in CFD
Bernardas Jankauskas: PhD researcher in CFD
* Heriot-Watt University
Prof Scott Arthur: leading expert in debris modelling
* University of Belgrade
Prof Dušan Prodanović: expert and advisor in experimental hydraulics
This phase will focus on the application of CFD to predict hydrodynamic effects of debris blockage for a broad range of bridges. We will evaluate computationally the two main hydrodynamic effects on a bridge, namely scour and pressures. Two distinct approaches to modelling scour will be investigated and later compared for their effectiveness. Results from the flume experiments in Phase 1 will be used to test and validate the CFD models. Accuracy and computational costs of the two distinct approaches for scour prediction and modelling will be assessed to determine the most suitable approach for the next stage of the project.
Scour depths will be empirically related to flow velocities near piers or abutments based on results from the scour modelling approach identified as the most suitable for scour prediction. We will then use full scale CFD simulations to predict debris effects for a diverse set of debris and bridge scenarios. This research will is interested in mainly evaluating the interactions between model parameters such as bridge span and pier width that largely determine the two quantities of interest – flow velocities and pressures. Results from the CFD models, which will include flow velocities around piers and abutments, and pressure distributions on the bridge superstructure, will be compiled for use in Phase 3.
In the following results from a sample CFD simulation (using OpenFOAM) is shown.
In the following simulated scour maps for a two-span arch bridge is shown.
And a looking upstream view of flow streamlines:
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.
Scour maps in single-pier experiments with a log debris at free surface (top) and on bed (bottom). Flow is from left to right.
Scour maps in single span-arch experiments with and without log debris . Flow is from right to left.
Followings are a few photos showing our recent activities in flume experiments (flow is from right to left).
Following photo illustrates pressure sensors embedded in a pier model for measuring hydrodynamic pressure (acoustic Doppler velocimeter (ADV) can be also seen on the left side).
Velocity vector field measured around a pier using ADV.
Power spectral density plot at a point adjacent to the bridge pier.
Scour protection using riprap around a pier blocked by a log debris (not shown in the image). Flow is from left to right.