Threshold for oscillatory-flow ripple initiation on cohesive and non-cohesive mixtures of sand and mud

The threshold hydraulic condition of wave ripple initiation has been essential in a wide range of disciplines, including coastal engineering and geology, but remains unclear for the case of sand-mud mixtures as substrates. Scale experiments were conducted using an oscillating-bed to study wave ripple initiation in sand-mud mixtures, considering bed material, wave period and maximum orbital velocity. Four kinds of sand-mud mixtures were employed as bed material with different muds (cohesive kaolin and non-cohesive silt) and mixing ratios. Ripple initiation in the cases with sand-mud mixtures tended to require larger orbital velocity than pure sand, indicating that mud mixed in bed material hinders ripple formation and spreading of the ripple field. The results of the threshold for pure sand and 10% kaolin mixture show that cohesive mixed sand-mud hindered ripple formation and spreading. In contrast, in the case of the non-cohesive sand-silt mixture beds, the results differed depending on the proportion of sand and silt, which implied the effect of the strength of network structure. The inhibition of ripple initiation due to mud mixing observed in the present experiments is considered to have a significant effect, particularly in environments with limited wave action duration, such as intertidal zone.<p></p>