*2.3. Flow Velocity Determination*

Figure 4a shows the flow velocity curve measured by the ADV probe at the position of 10 cm below water surface with a water depth of 50 cm and a flow velocity of 0.35 m/s. The sampling frequency of the ADV is 100 Hz. At each height, the mean value of three 20 s time histories of the flow velocity was calculated and used to remove the effect of the noise of the ADV velocity data.

**Figure 4.** The flow velocity measurement by ADV: (**a**) determination of underwater 10 cm flow velocity; and (**b**) velocity distribution at different heights.

The flow velocity at different heights was measured by the above method with height increment of 2 cm. The velocity distribution along the height z is shown in Figure 4b. The height of the boundary layer is about 30 cm. When the distance between the ADV probe and water surface is less than 20 cm, the measured velocity is basically stable near the maximum value. This phenomenon has also been found in previous similar studies [37].
