Stream Bathymetry Retrievals with the PinPoint Bathymetry Lidar

Direct determination of stream discharge is possible from non-contact stage and velocity measurements provided that the stream cross-section is known. In the case of an unstable streambed, updating the cross-section regularly is critical to maintaining the validity of discharge estimates. The PinPoint bathymetry lidar is a low-cost, low-power, water-penetrating lidar designed to be fixed above a clear, small stream for periodic cross-section updates. It may also be attached to a vertical pole for use as a surveying instrument. The PinPoint comprises a laser rangefinder module on a vertical turntable, allowing it to sweep bank-to-bank (nadir ±45°), penetrating the water and detecting the bottom at each angle. Post-processing is applied to compensate for refraction and the speed of light in water, producing a plot of depth versus cross-stream distance. When compared to a manual in-situ survey, the majority of depth measurements are within 20 mm of the surveyed depth. Overall cross-sectional area estimates are typically within 2-6% of that estimated from the survey, depending on the complexity of the bottom and number of surveyed points.

Retrieving depth by optical means (e.g. lidar or hyperspectral imaging) presents several critical challenges: reflections from the water surface, interference from ambient light, scattering in turbid water, and variations in the returned signal due to streambed reflectance or roughness. Absorption of light within the water column is, in principle, another challenge for optical methods; however it is not applicable in practice for the depth of streams targeted by the PinPoint. In addition to a system overview, recent work is presented herein quantifying the impacts of ambient light and turbidity and demonstrating how bathymetry estimates may be further improved by applying clustering techniques to consecutive sweeps in post-processing.