AUV-based observations of rough bed hydrodynamics

For highly irregular boundaries such as coral reefs, the choice of a characteristic roughness scale for use in hy-drodynamic modeling is not straightforward. As an initial step in relating measurable physical scales to hydrodynamic roughness, we explore the use of a combination of sidescan, high-resolution altimetry, and water velocity measurements collected with a REMUS-100 (Remote Environmental Monitoring UnitS, Hydroid, Inc) AUV in the vicinity of the Kilo Nalu Nearshore Reef Observatory (Oahu, HI). Different substrate classes (i.e. sand, coral, mixed) are identified using principal component analysis of variables derived from sidescan backscatter. Each bottom class is then connected to physical roughness measurements made with a high-resolution altimeter. Within the sampled region, sandy areas show a spectral root mean square (RMS) height of less than 3cm, while coral patches show RMS heights between 8cm to 14cm. To gain insight into the hydrodynamic response to broad-band roughness, we conducted a series of tests over the reef using the AUV mounted DVLs. In order to validate these hydrodynamic measurements, along-shore water velocities are sampled with spatial averaging of 50-100m and compared with two nearby bottom-fixed ADCPs. The results show that the REMUS DVLs can be effective in resolving steady boundary layer structure, which integrate the effects of roughness and implicitly reflect the response of wave motion to the boundary. Ongoing efforts are aimed to correlate bottom type, roughness RMS and hydrodynamic response at reef scales.