Topographic response to the transition from snowmelt- to rainfall- triggered extremes

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In September of 2013, a "1000-yr" storm occurred in the foothills of the mountains near Boulder, CO, triggering widespread landslides and floods. However, the causal chain from extreme rainfall to flooding to river erosion and landslides is complex. For example, this event produced "25- to 200- yr" floods and landslides locally removed 100's to 1000's of years of sediment production. This project focuses on how the two main modes of runoff generation (snowmelt and rainfall runoff) in mountain landscapes can lead to diagnostic differences in the rates and patterns of erosional processes. We use the Colorado Front Range, USA as a case study where mean runoff systematically increases with elevation due to increased snowmelt, while peak runoff is highest at intermediate elevations caused by large, intense rainfall events. Better accounting for elevation-dependent runoff generation has implications for water resource management, flood and erosion hazard assessment, and landscape evolution modeling. To this end, this project fosters collaboration among three institutes at the University of Colorado, Boulder (the Boulder Creek Critical Zone Observatory, Earth Lab, and the Community Surface Dynamics Modeling System) to better integrate data collection, analysis, and model development via open, reproducible workflows made available to researchers, decision-makers, and citizens.The main goals of this project are to develop a quantitative framework for river incision that accounts for two well-documented attributes of the Colorado Front Range: (1) an orographic transition from snowmelt- to rainfall- triggered extreme events, and (2) the formation and persistence of high elevation, low relief, erosion surfaces, into which bedrock river canyons have incised. While the Colorado Front Range is the focus, the approach is transferable to other settings with strong orographic gradients in event-scale runoff variability (e.g., Andes; Himalayas). To accomplish objectives, research tasks consist of three main components: (A) Hydroclimatic data analysis; (B) Topographic data analysis; (C) Numerical modeling of river incision. Hydroclimatic data analysis focuses on how sub-daily to daily (i.e., intensity, duration) and spatial (i.e., storm size) attributes of rainfall and snowmelt events imprint into flood frequencies. Topographic data analysis focuses on quantifying how channel steepness, channel width, and grain size distributions vary in the landscape based on data from field surveys and photogrammetry, high-resolution airborne lidar topography along Boulder Creek and the Rampart Range, and moderate-resolution regional topography. Numerical modeling of river incision builds on a 1-D stochastic-threshold model of bedrock incision, that currently only includes stochastic runoff intensity, by adding more stochastic parameters (i.e., the location and spatial footprint of runoff generation). Explicit accounting for orographic gradients in rainfall and snowmelt statistics and their role in setting the timescale of river response to base level fall provides a first test of a previously unrecognized feedback between climate, topography, and erosion.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.