Field measurement of the velocity of overland flow using dye tracing

Abstract A new method of computing the mean velocity of overland flow using dye tracing is proposed in which a specified cross‐section is divided into zones of relatively uniform flow characteristics, termed partial sections. The mean surface velocity for each partial section is determined by timing the passage of injected dye, and this figure is multiplied by 0.67, 0.70, or 0.80, depending on whether the flow is laminar, transitional, or turbulent, to give mean velocity. The mean velocity for the entire cross‐section is calculated by multiplying the mean velocity of each partial section by its cross‐sectional area, summing the products for all the partial sections, and dividing by the total area of the cross‐section. A field test shows that mean velocity derived in this manner differs from mean velocity derived by the discharge method (i.e. by dividing discharge measured volumetrically by cross‐sectional area) by an average of only 7.07 per cent. Thus the partial‐section technique appears to provide a reliable method of estimating mean velocity of overland flow.