Investigation of the Hydraulic Patterns in a Riffle Sequence using 3-D Velocity Characteristics

Three-dimensional flow characteristics are vital in understanding the development, maintenance and dynamics of pool-riffle sequences. The focus of this study is on riffles, looking at potential controls on velocity and turbulence. The first objective is to evaluate consistent relationships between three-dimensional velocity characteristics and site-specific variables, such as stage and relative roughness. The second objective is to evaluate turbulence within thalweg velocity profiles, by calculating coefficient of variation and periodicity within the time series. The study reach is a riffle section on the North Fork of the Cache La Poudre River, Colorado with a drainage area of 919 km2, approach gradient of 0.03, and D50=12.5cm. The reach is 6.0m long with five cross-sections, approximately 16m wide, and stations every 0.5 m along each cross-section. An acoustic doppler velocimeter (FlowTracker Handheld ADV) is used to measure 180 seconds of flow velocity in the downstream (Vx), cross-stream (Vy), and vertical (Vz) direction at each station. Flow velocities were measured at 0.6*h for depths less than 45 cm, and 0.2*h and 0.8*h at depths greater than 45 cm. Detailed thalweg velocity profiles were measured at each cross-section. Velocity was measured at discharges of approximately 1.1 m3/s, 2.4 m3/s and 3.0 m3/s. In order to evaluate the independent variables such as relative roughness and stage, a detailed survey of the stream bottom was measured, as well as a Wolman pebble count. A mixed model ANOVA has been performed, using SAS, to test the hypothesis of variables controlling velocity. Preliminary results indicate that velocity is significantly affected by stage (p=0.0286), roughness zones (p<0.0001) and an interaction between stage, roughness zone and cross section (p=0.0192). Thalweg velocity profiles have been compared for the three different discharges. Downstream (Vx) profiles have similar logarithmic shape, with the mean velocities greater at the higher discharge level. The cross-stream (Vy) profiles have higher velocities in the middle of the water column at higher discharges. The vertical (Vz) profiles have less distinguishable trends. Turbulence intensity was quantified using the coefficient of variation for each measurement point in the thalweg velocity profiles. Preliminary results indicate the highest turbulence intensity on the bed, with CV increasing with larger discharges. Time-series analysis will be used to define periodicity or trends within the velocity structure.