Traceless Quantitative Imaging Alternatives for Free-Surface Measurements in Natural Streams

Abstract

Flow measurements provided by the extensive streamgage networks worldwide are crucial for river monitoring in hydrology, fluvial geomorphology, and riverine ecology. However, the current measurements methods are technically outdated and require equipment deployment at the measurement location. An extensive search of new, non-contact technologies, with special emphasis on those pertaining to the new generation of digital instruments, has been pioneered by United States by US Geological Survey that potentially might change the paradigm in stream data collection program. Preliminary tests in field conditions illustrated that the ground penetrating radar (for bathymetry measurements) combined with microwave mono- or bi-static radar (for free-surface velocity measurements) have the potential to yield remotely, non-contact, river discharges in real-time. Another potential candidate for free-surface measurements in the attention of the committee is the Large-Scale Particle Image Velocimetry (LSPIV). LSPIV is a relatively new approach for remote, real-time flow visualization and measurement of the flow free surface based on digital imagery. Among the distinct advantages of LSPIV over other means for river flow estimation are its capabilities to provide instantaneous whole-field velocity vectors and to measure in extreme river flow conditions, i.e., floods, partially icecovered flows, and low velocity flows. Visualization and quantitative measurement of river flow features using LSPIV can be made quickly, with little site preparation or pre-planning, and for extended time spans. The major shortcoming of the LSPIV currently applied in natural settings is the need for seeding of the stream free surface in order to detect its motion. The present paper discusses alternatives for implementation of unseeded LSPIV. Two proof-of-concept tests for implementation of unseeded LSPIV are presented herein. The tests refer to two different flow situations: - fast velocity flows, where a texture of image brightness on the free-surface is present, and - low velocity flows, where there is no texture on the free surface (mirror-like appearance). For the first flow category, the texture is formed by small water waves (5mm to 3 cm wavelenght) naturally occurring on the free surface generated by either wind or the large-scale internal river turbulence. Wind waves appear to dominate for slow rivers while turbulencegenerated waves dominate for fast ones. These patterns are convected with the free surface velocity, maintaining their appearance for short time intervals. The texture of image brightness in the free surface recordings is due to specular reflection produced by small deformations of the water free surface.