The delineation of high flood hazard zones within a flood plain is usually independent of the parameters that constitute a life threatening situation. In order to define human instability in high flow hazard areas, a study was conducted to identify when an adult human could not stand or maneuver in a simulated flood flow. A series of human subjects (90-201 lbs) were placed in a recirculating flume and tested to determine the velocity and depth of flow which caused instability. 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.