Performance of a sonar altimeter in the nearshore

A 1 MHz sonar altimeter with automatic gain control is shown to provide accurate estimates of the distance between the instrument and the seafloor. Laboratory experiments indicate that distance estimates degrade slightly when the bottom is rough or sloped and when sediment is suspended in the water column. Results from field tests, both within and seaward of the surf zone, show some degradation owing to a combination of suspended sediment and bubbles, bed undulations, and perhaps the dynamic nature of the sand bottom under waves. Seaward of the surf zone the bottom can be located within +/2 cm nearly continuously, whereas inside the surf zone the bottom can be located only intermittently and the accuracy decreases to +/3 cm. A 300 m long cross-shore transect of 16 altimeters was deployed from the shoreline to about 4 m depth for 3 months in summer-fall 1994 near Duck, NC. Results show that the altimeters are robust and can usually provide estimates of the seafloor position every few minutes even in the surf zone during large storms.