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To alleviate the scour depth around a series of existing spur dikes (SDs), a protective SD located upstream of the first was experimentally investigated. To determine the optimal design parameters, various values, lengths, angles and the spacing of a protective SDs were experimentally tested under different clear-water conditions in a straight river reach and a fixed flow depth. To investigate the behaviour of the flow field around the SDs, acoustic Doppler velocimeter was used to measure instantaneous velocity components on a rough rigid bed. The results indicate that a properly designed protective SD is able to decrease the average of the maximum scour depth around the main SDs. The optimum design parameters of a protective SD are recommended for each length for a particular experiment.