FLOW COMPETENCE RESTRICTION OF THE HEC-RAS MODEL FOR PALEOFLOOD DISCHARGES

Paleoflood models were developed, using the HEC-RAS computer program, to address a long-standing belief that catastrophic outburst flooding from proglacial lakes created Slippery Rock Creek Gorge in western Pennsylvania. A benefit of this research was the practicality of using flow competence to reduce the erroneously large discharge from an unrestricted model to a more realistic restricted peak discharge. Models were classified as Unrestricted, Semi-restricted, and Restricted. Unrestricted paleoflood models of three streams and three proglacial lake spillways suggested that the Laurentide deglaciation was characterized by high-magnitude flooding. The erosional and sedimentological evidence, however, suggested that high magnitude flooding was not realized. The restricted model, which was constrained by the velocity required to transport the largest clasts in outwash deposited downstream of the glacial margin, described a discharge orders of magnitude lower and comparable to the discharge of the historic flood of record. The results of this study suggest that the proglacial lakes drained more slowly than previously believed by eroding their cols in step with the lowering of their water surface, rather than draining by catastrophic dam bursts. Additionally, the results of this study suggest that Laurentide deglaciation in western Pennsylvania was of some duration. Finally, the technique of restricting the flow velocity in the HEC-RAS program based on flow-competence data may be beneficial for any paleoflood that deposited coarser-grained sediments.