Development and testing of a surface roughness measurement device based on aerial ultrasonic reflections

Repairs of concrete irrigation channels in Japan are guided to a large extent by the degree to which their walls have degraded over decades of use. Current methods of estimating Manning roughness coefficients from the roughness of the channel wall are based on linear profiles of arithmetic mean roughness. This study developed a method to estimate Manning roughness coefficients for a planar region directly from the peak-to-peak value of a reflected aerial ultrasonic wave, using a transceiverbased ultrasonic device. Model channel experiments using manufactured materials of known roughness were undertaken to confirm the strong correlation between peak-to-peak values and Manning roughness coefficients and to derive a conversion equation that was incorporated in the ultrasonic device. The device yielded Manning roughness coefficients for handmade concrete panels that matched estimates made by a conventional moulage gauge technique. It also matched the results of estimates by the commonly used Manning-Strickler equation. A field experiment in an operating irrigation channel confirmed these results, showing that aerial ultrasonic measurements of surface roughness are as accurate as those by existing techniques. Keywords Manning roughness coefficient • Aerial ultrasonic sensor • Model channel experiment • Open channel • Wear * Kenji Okajima