*Article* **Local Scour at Complex Bridge Piers in Close Proximity under Clear-Water and Live-Bed Flow Regime**

#### **Yifan Yang \*, Bruce W. Melville, Graham H. Macky and Asaad Y. Shamseldin**

Department of Civil and Environmental Engineering, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand

**\*** Correspondence: yyan749@aucklanduni.ac.nz

Received: 24 June 2019; Accepted: 19 July 2019; Published: 24 July 2019

**Abstract:** In this study, we investigated the characteristics of scour at complex bridge piers in close proximity. The experiments were performed under both clear-water and live-bed flow regimes. We compare our results with those for a single complex pier. Further, the performance of existing predictors is discussed. In this study, four typical pier arrangements were adopted, including side-by-side with aligned or 30◦ skewed flow, staggered, and tandem. The results show that the skew angle for a side-by-side arrangemen<sup>t</sup> significantly accelerates the clear-water scour development at all the vertical piles as well as between the piers, and the most scoured pile shifts from the upstream end to the downstream end of the upstream pier flank. The staggered and tandem pier arrangemen<sup>t</sup> show significant protection to the downstream pier for both the developing rate and the equilibrium scour depth. When the flow velocity exceeds the threshold for general bed motion, the clear-water scour pattern for all the pier arrangements may be altered significantly due to the upstream sediment supply, the weakened protection e ffect, and the enhanced flow contraction. The bed-forms migrate via the bridge opening and are damped gradually by the flow, and thus the response of the bed morphology under live-bed conditions is quite unsteady.

**Keywords:** scour; complex bridge pier; pier-pier proximity; temporal evolution; equilibrium scour pattern; bed-form migration; scour predictor evaluation
