An official website of the United States government
Debris flows are powered by sediment supplied from steep hillslopes where soils are often patchy and interrupted by bare‐bedrock cliffs. The role of patchy soils and cliffs in supplying sediment to channels remains unclear, particularly surrounding wildfire disturbances that heighten debris‐flow hazards by increasing sediment supply to channels. Here, we examine how variation in soil cover on hillslopes affects sediment sizes in channels surrounding the 2020 El Dorado wildfire, which burned debris‐flow prone slopes in the San Bernardino Mountains, California. We focus on six headwater catchments (<0.1 km2) where hillslope sources ranged from a continuous soil mantle to 95% bare‐bedrock cliffs. At each site, we measured sediment grain size distributions at the same channel locations before and immediately following the wildfire. We compared results to a mixing model that accounts for three distinct hillslope sediment sources distinguished by local slope thresholds. We find that channel sediment in fully soil‐mantled catchments reflects hillslope soils (D50 = 0.1–0.2 cm) both before and after the wildfire. In steeper catchments with cliffs, channel sediment is consistently coarse prior to fire (D50 = 6–32 cm) and reflects bedrock fracture spacing, despite cliffs representing anywhere from 5% to 95% of the sediment source area. Following the fire, channel sediment size reduces most (5‐ to 20‐fold) in catchments where hillslope sources are predominantly soil covered but with patches of cliffs. The abrupt fining of channel sediment is thought to facilitate postfire debris‐flow initiation, and our results imply that this effect is greatest where bare‐bedrock cliffs are present but not dominant. A patchwork of bare‐bedrock cliffs is common in steeplands where hillslopes respond to channel incision by landsliding. We show how local slope thresholds applied to such terrain aid in estimating sediment supply conditions before two destructive debris flows that eventually nucleated in these study catchments in 2022.
more »
« less
Sediment size on talus slopes correlates with fracture spacing on bedrock cliffs: implications for predicting initial sediment size distributions on hillslopes
Abstract. The detachment of rock fragments from fractured bedrock on hillslopes creates sediment with an initial size distribution that sets the upper limitson particle size for all subsequent stages in the evolution of sediment in landscapes. We hypothesize that the initial size distribution shoulddepend on the size distribution of latent sediment (i.e., fracture-bound blocks in unweathered bedrock) and weathering of blocks both before andduring detachment (e.g., disintegration along crystal grain boundaries). However, the initial size distribution is difficult to measure because theinterface across which sediment is produced is often shielded from view by overlying soil. Here we overcome this limitation by comparing fracturespacings measured from exposed bedrock on cliff faces with particle size distributions in adjacent talus deposits at 15 talus–cliff pairs spanning awide range of climates and lithologies in California. Median fracture spacing and particle size vary by more than 10-fold and correlate stronglywith lithology. Fracture spacing and talus size distributions are also closely correlated in central tendency, spread, and shape, with b-axisdiameters showing the closest correspondence with fracture spacing at most sites. This suggests that weathering has not modified latent sedimenteither before or during detachment from the cliff face. In addition, talus at our sites has not undergone much weathering after deposition and isslightly coarser than the latent sizes because it contains unexploited fractures inherited from bedrock. We introduce a new conceptual frameworkfor understanding the relative importance of latent size and weathering in setting initial sediment size distributions in mountain landscapes. Inthis framework, hillslopes exist on a spectrum defined by the ratio of two characteristic timescales: the residence time in saprolite and weatheredbedrock and the time required to detach a particle of a characteristic size. At one end of the spectrum, where weathering residence times arenegligible, the latent size distribution can be used to predict the initial size distribution. At the other end of the spectrum, where weatheringresidence times are long, the latent size distribution can be erased by weathering in the critical zone.
more »
« less
- Award ID(s):
- 2012353
- PAR ID:
- 10342454
- Date Published:
- Journal Name:
- Earth Surface Dynamics
- Volume:
- 9
- Issue:
- 4
- ISSN:
- 2196-632X
- Page Range / eLocation ID:
- 1073 to 1090
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
null (Ed.)Bedrock weathering regulates nutrient mobilization, water storage, and soil production. Relative to the mobile soil layer, little is known about the relationship between topography and bedrock weathering. Here, we identify a common pattern of weathering and water storage across a sequence of three ridges and valleys in the sedimentary Great Valley Sequence in Northern California that share a tectonic and climate history. Deep drilling, downhole logging, and characterization of chemistry and porosity reveal two weathering fronts. The shallower front is ∼7 m deep at the ridge of all three hillslopes, and marks the extent of pervasive fracturing and oxidation of pyrite and organic carbon. A deeper weathering front marks the extent of open fractures and discoloration. This front is 11 m deep under two ridges of similar ridge-valley spacing, but 17.5 m deep under a ridge with nearly twice the ridge-valley spacing. Hence, at ridge tops, the fraction of the hillslope relief that is weathered scales with hillslope length. In all three hillslopes, below this second weathering front, closed fractures and unweathered bedrock extend about one-half the hilltop elevation above the adjacent channels. Neutron probe surveys reveal that seasonally dynamic moisture is stored to approximately the same depth as the shallow weathering front. Under the channels that bound our study hillslopes, the two weathering fronts coincide and occur within centimeters of the ground surface. Our findings provide evidence for feedbacks between erosion and weathering in mountainous landscapes that result in systematic subsurface structuring and water routing.more » « less
-
Abstract Hillslope topographic change in response to climate and climate change is a key aspect of landscape evolution. The impact of short‐duration rainstorms on hillslope evolution in arid regions is persistently questioned but often not directly examined in landscape evolution studies, which are commonly based on mean climate proxies. This study focuses on hillslope surface processes responding to rainstorms in the driest regions of Earth. We present a numerical model for arid, rocky hillslopes with lithology of a softer rock layer capped by a cliff‐forming resistant layer. By representing the combined action of bedrock and clast weathering, cliff‐debris ravel, and runoff‐driven erosion, the model can reproduce commonly observed cliff‐profile morphology. Numerical experiments with a fixed base level were used to test hillslope response to cliff‐debris grain size, rainstorm intensities, and alternation between rainstorm patterns. The persistence of vertical cliffs and the pattern of sediment sorting depend on rainstorm intensities and the size of cliff debris. Numerical experiments confirm that these two variables could have driven the landscape in the Negev Desert (Israel) toward an observed spatial contrast in topographic form over the past 105–106 years. For a given total storm rain depth, short‐duration higher‐intensity rainstorms are more erosive, resulting in greater cliff retreat distances relative to longer, low‐intensity storms. Temporal alternation between rainstorm regimes produces hillslope profiles similar to those previously attributed to Quaternary oscillations in the mean climate. We suggest that arid hillslopes may undergo considerable geomorphic transitions solely by alternating intra‐storm patterns regardless of rainfall amounts.more » « less
-
Abstract Landscapes following wildfire commonly have significant increases in sediment yield and debris flows that pose major hazards and are difficult to predict. Ultimately, post-wildfire sediment yield is governed by processes that deliver sediment from hillslopes to channels, but it is commonly unclear the degree to which hillslope sediment delivery is driven by wet versus dry processes, which limits the ability to predict debris-flow occurrence and response to climate change. Here we use repeat airborne lidar topography to track sediment movement following the 2009 CE Station Fire in southern California, USA, and show that post-wildfire debris flows initiated in channels filled by dry sediment transport, rather than on hillsides during rainfall as typically assumed. We found widespread patterns of 1–3 m of dry sediment loading in headwater channels immediately following wildfire and before rainfall, followed by sediment excavation during subsequent storms. In catchments where post-wildfire dry sediment loading was absent, possibly due to differences in lithology, channel scour during storms did not occur. Our results support a fire-flood model in bedrock landscapes whereby debris-flow occurrence depends on dry sediment loading rather than hillslope-runoff erosion, shallow landslides, or burn severity, indicating that sediment supply can limit debris-flow occurrence in bedrock landscapes with more-frequent fires.more » « less
-
Abstract Valley width is largely controlled by lithology and upstream drainage area, but little work has focused on identifying the processes through which valleys widen. Bedrock valleys widen by first laterally eroding bedrock valley walls, followed by the collapse of overlying bedrock material that must then be transported away from the valley wall before the valley can continue widening. We hypothesize that talus piles that cannot be transported by the river protect the valley wall and slow valley widening, while talus piles that are rapidly transported allow for uninterrupted valley widening. We used field measurements from 40 locations in both wide and narrow valleys along the Buffalo River, AR to test this hypothesis. Our data show that wide valleys tend to have fewer talus piles and smaller talus grain sizes, whereas talus in narrow valleys is larger in size and more continuous along valley walls. We calculated potential talus block entrainment at each site location and found that talus blocks in wide valleys are potentially entrained and moved away from valley walls during moderate and large flood events, whereas talus blocks in narrow valleys are very rarely moved. Our results show that the potential transport of talus piles protecting bedrock valley walls from widening is controlled by the block size of collapsed bedrock wall material relative to stream competency. Our results also suggest that persistence versus mobility of collapsed talus piles is an important process in the development of wide bedrock valleys.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.5194/esurf-9-1073-2021
