A method for evaluating local scour depth at bridge piers due to debris accumulation

A novel method for evaluating the effect of debris accumulation on local scour depth at bridge piers is introduced. The concept of a ‘debris factor’ is proposed to replace the current effective and equivalent pier width approaches that have been shown to overestimate debris-induced scour in many instances. The concept enables a simpler, more direct and realistic estimation of the change in local scour depth due to debris since it accounts for debris length (streamwise), width (spanwise) and thickness (depth wise), along with the influence of debris elevation in the flow; that is, it is applicable for free-surface debris, submerged debris or debris resting on the stream bed. The concept works with all existing local scour equations alongside other factors that influence scour depth such as flow angle of attack and pier shape. The mathematical model underpinning the proposed concept was derived through multiple linear regression of experimental data obtained at Exeter University and elsewhere. The proposed method was found to improve accuracy by at least 24% and 5% in comparison with the effective and equivalent pier width approaches, respectively. More importantly, the method is robust, providing highly consistent results with significantly less uncertainty.