Assessment of Turbulence Parameters of Slope Watercourses

Abstract

The turbulence of flows is known [3, 6, 8, 13, 17] to play a decisive role in the processes of sediment transport, energy dissipation, formation of flow friction, diffusion, and so on. The turbulence characteristics were measured under natural conditions mainly on large rivers (Dnester and its tributary Turunchuk [3‐6, 16], Kirzhach, Polomet, Volga [10]). The measurements on small rivers in Moldavia were made by V.I. Nikora [18]. The turbulence characteristics of slope watercourses were not measured. The overland flow is, as a rule, divided into sheet, channelized, depression, and other types of flows. The channelized and especially hollow flows on slopes can be regarded as easily deformable channel flows and can be studied with the use of the apparatus applied to channel flows. To assess the extent of turbulence in overland flows a series of turbulence characteristics was calculated with the use of known relationships involving averaged flow parameters. The observations were conducted with common chernozem, overlaying loess, in the “Udarnik” Experimental Farm, Institute of Soil Protection, Ukrainian Academy of Agrarian Sciences, with chernozem overlaying loess, sand, and marl, in Krasnodon Region, and on gray forest soil, overlaying loess, in Obukhovskii Region, Kiev Province. The object under observation was the streamlet network that forms on the slope and is flattened during soil treatment by agricultural machinery (that is, primary streamlet network). Several gage sections (from four to eight, depending on the streamlet length) were established in each streamlet. In each section, water discharge and turbidity were measured using procedures accepted in the hydrometeorological service, with several exceptions owing to the specific features of the examined watercourses (in particular, water surface slope was measured by a proprietary device [2], based on the action of geodesic levels). The observations of snowmelt runoff included 259 measurements in different agronomic situations and soil types. The measurements were made in years with different water abundance (1996‐2000) and at different types of snow melting. Within the period of study, water discharge varied from 0.0000043 to 0.021 m 3 /s. The probability that water discharge exceeds 0.0016 m 3 /s is 10%. The section-averaged turbidity of slope watercourses varied from 0 to 99.99, whereas near the bed it was as high as 139.15 kg/m 3 . No sediment transport was observed in 7.7% of cases (predominantly in the morning, when flow starts moving over winter crops or permanent grasses on the soil that has not melted yet). The probability of the turbidity exceeding 50.0 kg/m 3 is 2.5% (the turbidity exceeded 14.4 kg/m 3 in 13% of cases). The flow velocity in the watercourses varied from 0.0182 to 0.85 m/s (the velocity was higher than 0.17 m/s in 55% of cases). The maximum flow depths were 0.004‐0.07 m (the probability of the maximum depth exceeding 0.05 m was 2%). The Froude number Fr was 0.00193‐6.19 at the variation coefficient C v = 1.39. The probability of Fr > 1 was 5.7%. Within the observation period, the Reynolds number Re varied from 55.71 to 13975.3 (Re 5000 in 7.3% of cases). Transient flow mode was observe for almost all conditions of snowmelt runoff formation (in 59.2‐93.7% of cases at different formation conditions). The turbulent flow conditions predominated only on common chernozems overlaying sands.