ROLE OF FLOW DURATION, HYDROGRAPH SHAPE, AND ENERGY EXPENDITURE IN CONTROLLING GEOMORPHIC RESPONSES TO CATASTROPHIC FLOODING

Geomorphologists have long studied the impacts of extreme floods, yet the association between the magnitude of flow parameters (Q, velocity, shear stress, or stream power) and resulting geomorphic effectiveness remains vague and non-deterministic. Taking advantage of Tropical Storm Irene’s rapid track through eastern Vermont, this research presents the array of geomorphic responses to a short duration (time to peak of < 12 hrs) but high magnitude flood that was the 20^th century flood of record for numerous watersheds. We present the geomorphic imprint of Irene flooding within a larger context of fluvial theory concerning the role of, and trade-off between, the magnitude of energy expenditure during a flood and its duration. Focusing on detailed field sampling within the 187 km² Saxtons River basin, we elucidate: (1) the geomorphic effects of a short duration flood in a humid, well-vegetated landscape, (2) the relationship between (a) peak stream power, (b) total stream power, and (c) flow duration of stream power above a critical threshold and the types of associated geomorphic responses, and (3) the spatial variation of geomorphic effects relative to reach-scale geologic and geomorphic controls. Flooding associated with Irene ranged from the 1000 yr flood (Gumbel flood frequency analysis) to the 300 yr RI flood (log Pearson Type III). At the Saxtons River gage, this discharge spawned a peak unit stream power of 712 W/m² with total energy expenditure throughout the event of ~ 16,000 x 10³ joules. Despite these exceptional hydraulic values, channel widening was spatially infrequent and limited in magnitude; however, other geomorphic effects were profound; including (1) the entrainment, transport, and deposition of extremely coarse material, (2) stripping of floodplain surfaces, (3) channel avulsions and incision into Pleistocene-aged material, (4) and deposition of coarse material across floodplains. Based on our extensive field data and hydrologic/hydraulic analyses, we contend that short duration, high energy flows can have profound sedimentological effects but have limited erosive, channel widening impacts. Gravel entrainment and deposition of a catastrophic nature can certainly occur under these flow regimes, but the manifestation of these extreme flows to channel geometry may have limited expression.