THALWEG VARIABILITY AT BRIDGES ALONG A LARGE KARST RIVER - THE SUWANNEE RIVER, FLORIDA

Geomorphologists and engineers have different perspectives and approac hes for examining river channels and the changes that occur during flo ods. The field-oriented approach typically adopted by geomorphologists has little predictive ability and design usefulness. In contrast, the empirical approach adopted by engineers is based on predictive equati ons or models that often differ greatly from reality. Such equations a re not based on comprehensive field data and often fail to consider a number of site conditions, especially geology and geomorphology. Yet, in order for geomorphic techniques to be useful to the design and plan ning of engineering structures such as bridges, it is important that s ufficient observations exist in order to characterize long-term and sh ort-term changes in bottom topography and scour potential. Six gaging stations on the Suwannee River, a large river draining karst terrain i n the southeastern US, were used to examine the temporal variability i n thalweg elevation, the deepest point in a given cross-section. The c ross-sections have maximum thalweg variability of just a few meters, d espite the occurrence of several large floods. suggesting that the bot toms are fairly stable. Historical approaches can be applied to design the length and depth placement of pilings by providing information on site conditions not considered in engineering equations, such as resp onse of bottom materials to various flow conditions, and thus have pot ential benefits to public safety and cost effectiveness. (C) 1998 Else vier Science B.V.