DETERMINING THE RECIPE FOR A FLASH FLOOD: AN EAST-CENTRAL COLORADO ANALYSIS OF HIGH-RESOLUTION RAINFALL, FLOW, AND INFILTRATION IN A SEMI-ARID GRASSLAND

Flash floods commonly cause rapid gully erosion, creating headwalls that erode into previously stable surfaces, thereby reducing arable land and releasing sediment that can contaminate water supplies. We are concerned with flash floods associated with gully networks because they occur with little warning. In this study, we analyzed the necessary and sufficient conditions for flash flood generation in grassland gully networks. An ideal example of a dryland zone impacted by gully erosion is West Bijou Creek, located in the Denver Basin of east-central Colorado. To determine the conditions that promote flash flood generation, we examined six years of data from a monitoring site in a 7.9 ha gully basin. The monitoring station consisted of a tipping-bucket rain gauge and a sonic water-level sensor positioned above the channel. During rainstorms, rainfall and water level were recorded at 20-second intervals, providing a high-resolution picture of rainfall intensity and flood hydrographs. Analysis of rainfall and stage records revealed a clear intensity-duration threshold for flood generation. Rainstorms with a peak 15-minute intensity greater than 30 mm/hour always produced flash floods. Less intense but longer-lasting events were also capable of producing flash floods; for example, storms lasting 5 hours or more produced flash floods when the 15-minute intensity exceeded 5 mm/hour. In addition, to estimate the soil infiltration capacity, we conducted a sprinkler experiment on two 0.6 m² – one on a grass-mantled surface and one on a bare surface. The sprinkler experiments revealed a dramatic contrast in soil infiltration capacity between bare and vegetated surfaces. The vegetated plot was able to sustain infiltration rates as high as 43 mm/hour, whereas the bare plot exhibited an infiltration rate of only about 6 mm/hr. These findings imply that high-intensity rainstorms associated with summertime moist convective systems drive gully incision, and that a self-enhancing feedback may exist in which initial incision creates steep and relatively bare slopes that tend to generate more runoff, leading to more aggressive gully incision. Therefore, gullying should be more active generally in monsoonal climates that provide episodic, intense rainfall.