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Abt, Wittler, Taylor, and Love "'-, ~ -., Figure 3. Safety Mechanism for Human Subjects. helmet and was offered the use of a flotation preserver. Whenever the subject was in the flume, an attendant was assigned to observe and assist the subject. A second tether rope was linked from the subject directly to the attendant to facilitate subject extraction from the flume. Each subject was individually tested, The monolith was placed into the flume upon the appropriate footing condition, The flow was initiated with a flow depth of approximately 2,0 ft (0,61 m) and velocity of 0,5 feet per second (0,15 mps) (P,N. :I: 1), Instrumentation was started and the discharge was slowly increased in 5 percent increments to minimize surging and wave gen- eration, The discharge was increased and data recorded until the monolith toppled. The discharge was then lowered to pre-test conditions and the flume and in- struments were prepared to conduct another test. The testing procedure for the human subjects was modified from that of the monolith. The flume flow con- ditions were pre-set where the product number was ap- proximately 6, An initial flow depth was established at approximately 2 to 3 ft (0.61 to 0,91 m), Upon entering the flow, subjects were allowed to acclimate and gain perspective and experience in maneuvering in the flow. The data acquisition equipment was then activated. The flow was slowly increased in increments of approx- imately 5 percent, Subjects were periodically asked to walk into the flow, walk perpendicular to the flow and WATER RESOURCES BULLETIN face downstream of the flow. The test continued until subject maneuverability was lost and the subject could no longer remain stable in a standing or walking pos- ture, Once the subject indicated his or her instability, the flow was immediately reduced to the pre-test con- dition. RESULTS AND DISCUSSION The results of the 71-test program are presented in Table 2 and Table 3 for the 0.005 and 0.015 channel slopes, respectively. Each table presents the average velocity, depth of flow, and product number at which instability was determined for human subjects and top- pling occurred for the monolith subject, Subjects were subjected to flow velocities ranging from 1.18 to 10,01 fps (0.36-3 .05 mps) and flow depths of 1,60 to 3.95 ft (0.49-1.2 m). Monolith Subject The monolith subject was tested a minimum of three times for each surface condition, The product numbers reported in Table 2 and Table 3 are the average value of the multiple tests, It is observed in Table 2 that the 884