NEAR-SUBSTRATE HYDRAULIC CONDITIONS UNDER ARTIFICIAL FLOODS FROM PEAKING HYDROPOWER OPERATION - A PRELIMINARY-ANALYSIS OF DISTURBANCE INTENSITY AND DURATION

FST hemispheres of identical size but varying density were used to eva luate the intensity and duration of near-bottom shear stresses during peaking hydropower floods of various magnitudes (maximum 345 m3 s-1; b aseflow 2.75 m3 s-1). The FST values at single points were recorded at rapid intervals along previously surveyed transects during the rising limb of flood events at various distances from the hydropower facilit y. Whole transect evaluations were accomplished after the 'flood' leve l had stabilized. Measurements of over 1300 FST values were significan tly correlated with mean water column velocity and complex hydraulic c onditions such as turbulence, shear velocity and viscous sublayer. FST hemispheres are a rapid means of characterizing hydraulic conditions in medium order rivers. Changes in near-substrate hydraulic conditions were patchy and could be classified in terms of risk to fauna of eith er dislodgement or physical damage. Low risk (class 1) changes occurre d when FST values increased rapidly but returned to initial values aft er a short period of time (< 90 minutes) from the arrival of the peaki ng wave. Medium risk (class 2) changes occurred when FST values increa sed rapidly and decreased after a short period of time, but remained a t levels higher than before the arrival of the flood wave. High risk ( class 3) changes occurred when FST values increased rapidly and remain ed at high levels during the entire flood event. It is suggested that floods that do not initiate significant bed movement create a mosaic o f patches of variable disturbance risk to benthic fauna. Rapid evaluat ion by FST hemispheres could be used to characterize the overall distu rbance potential for the channel by the construction of a weighted ind ex based on the frequency of the various risk classes occurring in the reach being evaluated. A more appropriate analysis of impacts of peak ing hydropower and other flood events must include the existence of hy draulic refugia and the movement patterns of both benthic species and those species that occupy the water column.