An experimental study of spur dike construction impacts on bed scour under bidirectional tidal currents

Spur dikes are used for bank protection in tidal rivers. During construction, the dike is extended segment by segment, and its interaction with the reversing tidal flow directly reshapes the adjacent riverbed morphology. Flume experiments were conducted to simulate this process and capture short-term bed deformation. The results revealed that tidal-induced scour holes developed in alignment with flood tide direction under flood dominance, and the secondary scour holes emerged in the ebb direction near the dike head. Under unidirectional flow, the scour hole forms along the flow direction and extends downstream in a narrow, elongated shape. For inclusion of construction-process effects (ICE), the pre-existing scour hole by the increasing dike length and newly formed scour holes after dike completion on the upstream side of the spur dike head exhibit interwoven extension patterns, while the scour holes of exclusion of construction-process effects (ECE) include a stronger primary scour hole in the flood tide direction and a smaller secondary scour hole in the opposite ebb tide direction. Scour depth of the ECE exceeds that of the ICE in the mid-to-downstream region in front of the dike head, whereas in the mid-to-upstream region of the dike head, scour depth of the ICE may surpass that of the ECE, which requires significant attention in practical construction. An empirical scour depth prediction formula for ICE was established and a parameter $\eta$ was proposed to determine the maximum scour zone. The parameter $\eta$ is related to a percentage $\beta$, which defines the contour of the scour area. Smaller $\beta$ values indicate a wider scour zone, while larger β values indicate a more concentrated zone. A value of β = 60 % was ultimately selected as a representative threshold. This research provides scientific support for dynamic construction and protection design of tidal estuarine spur dikes.