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<br />w <br />~ <br />w <br />o <br /> <br />25 <br /> <br />The velocity dropped off very quickly with distance from the plate, <br /> <br />The greatest mass of dye substantially trailed the leading edge <br /> <br />which was just below the plate. This large mass was assumed to be <br /> <br />representative of the average velocity. <br /> <br />Measurements were made with varying disc speeds, Measurements <br /> <br />at medium speeds (30-60 rpm) were most accurate. At high speeds, the <br /> <br />dye moved across the viewing window too quickly for accurate timing. <br /> <br />At low speeds the dye diffused and clouded the viewing window before <br /> <br />the leading edge could be detected. <br /> <br />It was assumed that the velocity of flow over the small size <br /> <br />range of unweathered shale would not differ significantly from the <br /> <br />velocity of flow over the weathered shale. A curve (Sunday, 1979) was <br /> <br />used to determine the average velocity for both the 'U' runs and the <br /> <br />'W' runs. Separate dye measurements were made for the rock bed. The <br /> <br />average velocity was slightly lower (0.47 ft/sec, Figure 2.2) than for <br /> <br /> <br />the small size range of particles (0.50 oft/see, Figure 2.3), This <br /> <br /> <br />resulted from the higher roughness in the rock bed, <br /> <br />2.1.1d Calculation of Average EC <br /> <br />The measured EC values were weighted according to the time <br /> <br />interval of sampling and averaged, <br /> <br />EC =E(tjECi) <br />E ti <br /> <br />(3.1) <br /> <br />where ECi denotes the average of the EC values measured at ti_l, and <br /> <br />t. is the time interval of sampling, <br />1 <br />