Laboratory Experiments on Wide Pier Scouring

Troubles related with the in-river piers, like potential clogging of limited spans, scour around each single pier, costs of provisional works suggest to reduce their number increasing the dimension of each single pier. This leads to piers of width comparable with the water depth, being the latter less than 2-3 time the former, defined as wide piers in the literature. Scouring phenomena around piers are widely discussed in literature, but the wide pier condition is still covered by grey areas. Moving from narrow to wide piers, the coherent structures causing the localized erosion change, and the prediction of the maximum scour depth using relationships available in the literature may lead to incorrect results. An accurate prediction of the maximum scour depth is fundamental in the design of bridge foundations. The underestimate of this value may cause the pier foundations exposure leading to serious damage or failure of bridge, while its overestimation causes unnecessary extra-construction costs. In the present work, results from specifically developed laboratory experiments are discussed and compared with similar evidences from literature. The experiments have been developed in a rectangular flume 1 m wide, using quite uniform bed sediments characterized by median grain size d50=0.34mm and a gradation coefficient, defined as the square root of the ratio between d84 and d16, of 1.60. Four circular pier widths (D) have been combined with seven different water depths (y) to cover the range y/D=0.25-3.20 for a total number of 22 experiments carried out to minimize the causes of uncertainties recognized from literature analysis. All tests were run in steady state clear water conditions adopting a constant ratio between flow and sediment critical velocity, maintaining the same temporal duration for all the tests and limiting the ratio between pier width and the grain size (D/d50) in the range 140-470. Moving from the narrow to the wide pier case, the maximum depth as well as the shape of the erosion change, revealing a reduction of the scour depth upstream and a lateral increase of both depth and erosion footprint. The results of experiments in term of maximum scour depth are critically compared with available literature data and fitted by different predictive formulas. The analysis of the results shows that the formulas usually adopted lead to an overestimation of the maximum scour affecting wide piers for y/D<2.