The delineation of high flood hazard zones within a flood plain is usually independent of the hydraulic parameters that constitute a life threatening situation. In order to defme human instability in high hazard areas, a study was conducted to identif' when an adult human could not stand or maneuver in a simulated flood flow. An analysis was performed on a rigid body monolith resulting in a toppling hazard envelope curve (velocity vs. depth). A 120 lb monolith was then constructed and tested to relate the actual flow velocity and depth at toppling to theory. A series of human subjects (90-20 1 ibs) were placed in a recirculating flume and tested to determine the velocity and depth of flow that caused their instability.The test results determined that the product number, which is the product of the velocity and depth at toppling of the monolith, closely compared to the theoretical envelope curve. The monolith results represent the lower limit of human stability. Also, the product number appeared to be a predictor of human instability in flood flow. A relationship was developed to estimate the product number at which a human subject becomes unstable as a function of the height and weight of the subject. (KEY TERMS: flood hazard; human stability; product number.) 1Paper No. 89010 of the Water Resources Bulletin. Discussions are open until April 1, 1990. 2Respectively, Professor,