How do geomorphic effects of rainfall vary with storm type and spatial scale in a post-fire landscape?

Abstract

In post-fire landscapes, increased runoff and soil erosion can cause rapid geomorphic change. We examined how different types of rainfall events in affected hillslope-scale erosion and watershed-scale channel change in two 14-16 km 2 watersheds within the 2012 High Park Fire burn area in northern Colorado, USA. The first set of rainfall events was a sequence of 12 short, spatially variable summer convective rain storms, and the second was a >200 mm week-long storm in September. We compared rainfall characteristics, hillslope sediment yield, stream stage, and channel geometry changes from the summer storms to those from the September storm. The summer storms had a wide range of rainfall intensities, and each storm produced erosion primarily in one study watershed. The September storm rainfall had less spatial variability, covered both watersheds, and its total rainfall depth was 1.5 to 2.5 times greater than the total summer rainfall. Because rainfall intensities were highest during some summer storms, average hillslope sediment yields were higher for summer storms (6 Mg ha-1) than for the September storm (3 Mg ha-1). Maximum storm rainfall intensities were good predictors of hillslope sediment yield, but sediment yield correlated most strongly with total depths of rainfall exceeding 10-30 mm h-1 intensity thresholds. The combined summer storms produced relatively small changes in mean channel bed elevation and cross section area, with no clear pattern of incision or aggradation. In contrast, the sustained rain across the entire study area during the September storm led to extensive upstream incision and downstream aggradation. Because of different spatial coverage of storms, summer storms produced more total hillslope erosion, whereas the September storm produced the greatest total channel changes. At both scales, high intensity rainfall above a threshold was responsible for inducing most of the geomorphic change.

Reconstructing extreme post‐wildfire floods: a comparison of convective and mesoscale events

“…In contrast to the depositional nature of the convective flood, the primary geomorphic changes from the long duration mesoscale flood were incision and widening (Table ; Figure supplementary Figures S1–S10). Only XS1 experienced deposition, and the 2.3 m of aggradation was due to the backing up of a downstream culvert (Kampf et al , ). The ultrasonic sensor at XS1 showed that high flows lasted for ~60 h beginning about 1845 MDT on 11 September 2013.…”

“…In our analyses, we estimate an uncertainty in the elevation of the HWMs of 0.10 m, which reflects potential undulations in the free surface at peak discharge, and uncertainties of this magnitude have been used for indirect discharge analyses of the 2013 flood at other locations along the Colorado Front Range (Moody, ). Stage data were collected near XS1 during the mesoscale flood by an ultrasonic water level sensor, but these could not be used to calculate discharge because the bed aggraded by more than 2 m during this flood (Kampf et al , ). Maximum inundated area and mean flow depth were calculated for each cross‐section that had a nearby HWM by projecting a horizontal water surface elevation across the channel from the measured HWM.…”

“…The mesoscale flood in September 2013 affected the entire central and northern Colorado Front Range, resulting in widespread flooding that washed out numerous major highways (Gochis et al , ; Yochum, ) and triggered over 1100 landslides and debris flows (Anderson et al , ). This flood caused extensive geomorphic change in SG (Figure ), and for this flood we had rainfall data from seven tipping bucket rain gages and detailed pre‐ and post‐flood topographic data (Kampf et al , ).…”

Interaction of wind and cold‐season hydrologic processes on erosion from complex topography following wildfire in sagebrush steppe

“…Reduced sediment concentrations in streamflow in the second year post fire suggest that the supply of available sediment has declined, but watershed sediment yield remains elevated over that of similar adjacent unburned sagebrushdominated watersheds in the RCEW (Pierson et al, 2000(Pierson et al, , 2001a. Collectively, visible ample sediment in swales and persistent low ground cover and high watershed-scale sediment yield indicate that the study site remains susceptible to amplified erosion in swales during either a rare high-intensity summer rainfall event, long-duration rainfall on frozen or snow-covered soils, or over another year with high runoff from cold-season hydrologic processes (Benavides-Solorio and Mac-Donald, 2005;Kampf et al, 2016). Sediment yields associated with combined aeolian and coldseason hydrology and erosion processes in this study are consistent with post-fire erosion levels documented in the literature (see Shakesby and Doerr, 2006;Moody and Martin, 2009;Shakesby, 2011).…”

Transience of channel head locations following disturbance

“…Precipitation in mountainous regions is highly spatially variable, but colleagues at Colorado State University installed four rain gauges at the Skin Gulch watershed in July 2012 and two rain gauges at Woodpecker Woods in May 2013. These storms were followed by a large, unusual rainstorm during September 9-17 that produced a mean rainfall of 257 mm in Skin Gulch but created sediment yields of only 3 Mg/ha because of lower intensity rainfalls than the preceding summer 2013 convective rainfalls (Kampf et al, 2016). Mean total rainfall for summer 2013 averaged 167 mm in Skin Gulch, which experienced 12 convective storms that resulted in average hillslope sediment yields of 6 Mg/ha.…”