Large-Scale Coherent Flow Structures in a Natural Braided Reach Section of a Gravel-Bed River
Abstract
Stream-wise length scales of coherent flow structures were examined to test the hypothesized 2 to 6 times the depth scale in relatively deeper (~1 m) and faster (~1.5 m/s) flows in a braided reach of the Kootenai River, Idaho. Velocities were measured using a custom Acoustic Doppler Current Profiler, an array of six electromagnetic current meters, and an Acoustic Doppler Velocimeter in a variety of river depths (0.6 1.7 m) and velocities (0.3 1.6 m/s). Energetic (50% of total energy), coherent (along the array), low-frequency (< 0.05 Hz) motions were found for all deployment locations. Coherent times and lengths were 5 s and 10 m in the ~1.5 m/s flows and 45 s and 22 m in the ~0.5 m/s flows. Multi-resolution decomposition provided coarse low-frequency limit for the coherent motions and suggests the temporal scales range from 10 to 1000 s. Length scales of the low-frequency motions determined by frequency-wavenumber spectra indicate that the motions are longer than hypothesized. The coherent times and lengths were consistently less than the computed time and length scales, suggesting the energetic low-frequency motions evolve as they propagate downstream.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jun 01, 2011
- Accession Number
- ADA548065
Entities
People
- William F. Ashley
Organizations
- Naval Postgraduate School