The Competition Between Frequent And Rare Flood Events: The Impact On Erosion Rates And Landscape Form
Adams, Jordan M 1 ; Gasparini, Nicole M 2
1 INSTAAR, Â鶹Ãâ·Ñ°æÏÂÔØ
2 Department of Earth and Environmental Sciences, Tulane University
Runoff variability, defined as a function of the frequency and magnitude of flood events plays an important role in landscape evolution. Previous work has shown that under highly variable climate conditions, the increased likelihood of large, geomorphically-effective flood events can sculpt watershed morphologies, driving characteristics such as relief and drainage density. Numerical models have been applied to explore these patterns, but often assuming hydrologic steady-state, which neglects the dynamics and erosional impact of an event hydrograph. This study uses a novel overland flow model that drives a flood wave across a gridded domain, generating hydrographs at each grid location. To initiate surface water flow, this nonsteady discharge model is driven by a stochastic storm generator that generates rainfall time series for two climate variabilities: a high variability climate characterized by short-duration, high-intensity storms, and a low variability climate characterized by longer duration, lower-intensity storms. Using steady and nonsteady hydrology methods, detachment-limited erosion rates are calculated and tested with a range of critical shear stresses. Erosion results illustrate that when no critical shear stress is considered, storms in the lower variability climate driver greater overall erosion than the higher variability case. However, as the critical shear stress threshold is increased, a trade-off occurs, and the higher variability climate is able to do more erosive work. While landscape sensitivity to climate is similar between the steady and nonsteady discharge methods, the erosional patterns are distinct, as erosion rates calculated using the nonsteady discharge method are more sensitive to changes in storm characteristics. This sensitivity varies spatially, as the event hydrograph duration changes with drainage area, driving significant downstream increases in erosion rates and depth. Results from this work indicate that the erosional influence of a hydrograph may be important to consider when exploring climatic variability in landscape evolution studies.
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