Concerning the clustering of spur dikes on river systems, the spacing thresholds of twin spur dikes are important parameters to influence the estimations on the impact scales of spur dike groups and the overall responses of river systems. In this study, both numerical investigations and experimental measurements are proceeded to quantify the influence of the spacing threshold of non-submerged twin spur dikes with ipsilateral and orthogonal layout in a straight rectangular channel. Through dimensional analysis, three normalized indices, i.e., Froude number
Fr, ratios of channel width to dike length
B/b, and ratios of channel width to water depth
B/h are identified as the main influencing factors of the relative spacing threshold
Sc/b, i.e., dike spacing threshold to dike length. The simulation results indicate that the similarity of mean velocity along the water depth nearby the tips of twin spur dikes is determined by the criterion of the spacing threshold of non-submerged twin spur dikes with ipsilateral and orthogonal layout in straight rectangular channel. The results also show that:
Fr plays the least impact among the three influencing factors; with the fixed values of
Fr and
B/h, the relative threshold
Sc/b sharply increases first and then decreases slightly as
B/b factor increases, with which the relationship presents approximately convex quadratic function; while both
Fr and
B/b are fixing, the
Sc/b changes oppositely, i.e., slightly increasing first and then sharply decreasing as
B/h increases, which, again presents a convex quadratic function. Hence, the normalized empirical formula of spacing threshold can be deduced by multivariate regressions and verified by the corresponding measurements in good agreements. Such empirical formula further suggests that the reasonable spacing threshold ranges from 24
b to 130
b, which is wider than the recovery area scales found in literature. The outputs of this study provide foundation for the characterization of impact scales of spur dike groups.
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