3.1.2. Maximum Scour Depth

Scour depth increases with time for Darrieus tidal turbine at various tip-bed clearances. After equilibrium, the maximum scour depths are 2.2*D*, 2.2*D*, 1.7*D*, and 1.6*D* for *C*/*H* = 0.25, 0.5, 0.75, and 1.0, respectively. It can be shown that maximum scour depths in cases of *C*/*H* = 1.0 and *C*/*H* = 0.75 are much smaller than cases of *C*/*H* = 0.5 and *C*/*H* = 0.25. The difference is ~23%. The reason for this is when the turbine is installed higher than 0.5H distance from bed, the scour process is clear water scour. The rotating rotor shows weak influence on scour around the turbine's foundation. The slipstream velocity between seabed and turbine is not grea<sup>t</sup> enough to move sediment around scour hole. The scour hole developed mainly due to the action of a horseshoe vortex system. However, when the turbine is installed near the bed (for cases of *C*/*H* = 0.5 or 0.25 in the experiment), the rotor's effect is notable. The near bed velocity is grea<sup>t</sup> enough to move sediment around hole. The scour process is combined with digging by the downflow and slant bed erosion until the dynamic equilibrium is reached.

#### 3.1.3. Position of Maximum Scour Depth

The position of maximum scour occurs at both back sides of the monopile regardless of the clearance distances. The specific location is x/*D* = −0.5, y/*<sup>D</sup>* = 0.5 in each case. This position is also the maximum bed shear stress occurs under the action of horseshoe vortex system. These results also prove that the horseshoe vortex system still the main function for scour around tidal current turbine.
