Correction factor to dye-measured flow velocity under varying water and sediment discharges

Authors: ZHANG, G - Beijing Normal University; LUO, R - Beijing Normal University; CAO, Y - Beijing Normal University; SHEN, R - Beijing Normal University; Zhang, Xunchang

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2010
Publication Date: 7/1/2010
Citation: Zhang, G.H., Luo, R.T., Cao, Y., Shen, R.C., Zhang, X.J. 2010. Correction factor to dye-measured flow velocity under varying water and sediment discharges. Journal of Hydrology. 389:205-213.

Interpretive Summary: The dye-tracing method is widely used to measure flow velocity for soil erosion and hydrologic studies under both laboratory and field conditions. Few studies have been performed to quantify the effects of sediment concentration on measurement accuracy of mean flow velocity on steep slopes. The goal was to quantify correction factor to dye-measured mean flow velocity as affected by sediment concentration. The study was conducted in a laboratory flume under a wide range of flow discharge, slope steepness, and sediment load. Mean velocity was estimated from known total flow discharge and uniform flow depth measured by a digital level probe. This mean velocity was considered true value, and was compared to the dye-measured velocity to develop a correction ratio (1 means no correction). The results showed that correction factor decreased as slope increased and increased as flow turbulence increased for clear water flow with a mean value of 0.659. For sediment-laden flow, correction factor varied from 0.224 to 0.685 with a mean value of 0.464. The correction factor decreased as sediment load increased and increased as flow turbulence increased. The overall results indicated that caution must be exercised and proper correction be made when using the dye method to measure mean flow velocity in surface hydrology and erosion studies. These results would provide useful information to erosion scientists and hydraulic engineers on obtaining more accurate flow velocity estimates from dye-measured velocity.

Technical Abstract: Dye-tracing technique was a widely used method to measure velocity of overland flow in soil erosion studies under both laboratory and field conditions. Few studies were performed to quantify the effects of sediment load on correction factor on steep slopes. The objective was to investigate the potential effects of sediment load on correction factor of overland flow to determine mean velocity in a glued hydraulic flume under a wide range of hydraulic conditions and sediment load. Slope gradient (S) varied from 8.7 to 34.2%, unit flow rate (q) from 0.0007 to 0.005 m2/s, and sediment load (Qs) from 0 to 6.95 kg/m/s. The Reynolds number (Re) ranged from 350 to 5899. Mean velocity was calculated from the volumetric relation using measured flow depth by a digital level probe and flow discharge. The results showed that correction factor decreased as S increased and increased as Re increased for sediment-free flow with a mean value of 0.659. Correction factor could be estimated with a logarithmic function of S and Re (r square=0.88). For the sediment-laden flow, correction factor varied from 0.224 to 0.685 with a mean value of 0.464. The correction factor decreased as sediment load (Qs) increased and increased as Re increased. But in the most cases (80%), the correction factor was dominated by Qs. It could be estimated with a logarithmic function of Re and Qs (r square=0.80). For the combination of sediment-free and sediment-laden flows, the correction factor varied from 0.224 to 0.783 with a mean of 0.503. The correction factor was inversely related to slope gradient and sediment load, and directly to Reynolds number (r square=0.86). The overall results indicated that caution must be exercised and proper correction be made when using the dye tracer method to measure flow velocity in surface hydrology and erosion studies.