Emmanuel J. Gabet

Gopher bioturbation: field evidence for non-linear hillslope diffusion

It has generally been assumed that diffusive sediment transport on soil-mantled hillslopes is linearly dependent on hillslope gradient. Fieldwork was done near Santa Barbara, California, to develop a sediment transport equation for bioturbation by the pocket gopher (Thomomys bottae) and to determine whether it supports linear diffusion. The route taken by the sediment is divided into two parts, a subsurface path followed by a surface path. The first is the transport of soil through the burrow to the burrow opening. The second is the discharge of sediment from the burrow opening onto the hillslope surface. The total volumetric sediment flux, as a function of hillslope gradient, is found to be: qs (cm3 cm−1 a−1) = 176(dz/dx)3 − 189(dz/dx)2 + 68(dz/dx) + 34(dz/dx)0·4. This result does not support the use of linear diffusion for hillslopes where gopher bioturbation is the dominant mode of sediment transport. A one-dimensional hillslope evolution program was used to evolve hillslope profiles according to non-linear and linear diffusion and to compare them to a typical hillslope. The non-linear case more closely resembles the actual profile with a convex cap at the divide leading into a straight midslope section. Copyright

A theoretical model coupling chemical weathering rates with denudation rates

Uplift of the Himalayas has been proposed to have locally accelerated chemical weathering, thus leading to enhanced CO2 sequestration and global cooling. This hypothesis assumes that rapid erosion exposes fresh, highly reactive minerals at Earths surface. Empirical studies quantifying the relationship between erosion and weathering have produced apparently conflicting results, where the nature of the relationship is dependent on the weathering regime of the sampled landscapes. We derive a quantitative model that defines this relationship across the range of weathering regimes, from supply-limited to kinetically limited conditions. The model matches trends in field data collected by others and reconciles apparently conflicting results. The model also demonstrates that, as erosion rates increase, potential increases in weathering rate from the exposure of fresher materials are offset by the decrease in the total volume of minerals exposed due to thinner regolith. We conclude that the relationship between weathering and erosion is one of diminishing returns, in which increases in erosion rate lead to progressively smaller increases in weathering rate; indeed, at the highest erosion rates, weathering rates may decline. The ability, therefore, of accelerated uplift and erosion to stimulate greater CO2 sequestration may be significant in landscapes eroding at rates of 10–102 t km−2 yr−1. However, where erosion rates are greater than 102 t km−2 yr−1, increases in denudation may not be matched by increases in chemical weathering. Finally, our results suggest that watersheds with regolith thicknesses of ~0.5 m will yield the greatest solute fluxes.

Landslides on coastal sage-scrub and grassland hillslopes in a severe El Nino winter: The effects of vegetation conversion on sediment delivery

During the 1997-1998 El Nino, record rainfall triggered.150 shallow landslides within a 9.5 km2 area near Santa Barbara, Cal- ifornia. They were studied to analyze the sediment delivery to val- ley floors from landslides in coastal sage scrub and converted grasslands. The conversion of coastal sage to grasslands, primarily to provide pasturage for cattle, is common in the region, and the landscapes response may affect water quality, reservoir infilling, and debris flow hazards. We explore the relationship between lateral- root reinforcement and landslide volume by developing a slope- stability analysis that incorporates root cohesion along the sides of the failure. The stability analysis correctly predicts an inverse re- lationship between landslide volume and hillslope angle in the sage. The volumes of failures in the grasslands do not vary systemati- cally with slope and are generally smaller than those in the sage. From aerial-photograph analysis and field mapping, we find that there are 22.9 failures per square kilometer in the grasslands com- pared to 13.2 failures per square kilometer in the sage. Despite the lower failure density in the coastal sage, greater failure volumes and longer transport distances delivered more sediment to valley floors, with a specific volumetric flux of 2.8 3 10 -2 m 3 ·m -1 for this El Nino compared to 1.73 10 -2 m 3 ·m -1 in the grasslands. We con- clude that the conversion from vegetation with stronger and deeper roots (coastal sage) to vegetation with weaker and shallower roots (grass) has caused a pulse of increased landsliding in the grasslands because the soils are currently too thick for the prevailing root reinforcement. We suggest that, over time, soils in the grassland hollows will become thinner as the evacuation by landslides is re- peated until the landsliding rate declines to balance the soil sup- plied from local colluvium production and diffusive processes upslope.

Climatic controls on hillslope angle and relief in the Himalayas

Comparison of rainfall data and mean hillslope angles in the Himalayas of central Nepal shows that mean hillslope angles decrease with increasing mean annual rainfall. Higher pore pressures and higher rates of chemical weathering in the wetter regions may decrease the threshold angle of hillslopes prone to landsliding. When valley spacing is held constant, the sensitivity of mean hillslope angle to climate implies that relief, in the absence of limits due to rock strength, is also dependent on climate. These results suggest that wet-to-dry climatic changes increase relief in regions with incising bedrock channels and that dry-to-wet climatic changes reduce relief.

Bedrock channel geometry along an orographic rainfall gradient in the upper Marsyandi River valley in central Nepal

channel width. We focus on small catchments (0.6–12.4 km 2 ) along a more than tenfold gradient in monsoon rainfall. Rainfall data are gathered from a dense weather network and calibrated satellite observations, the pattern of Quaternary exhumation is inferred from apatite fission track cooling ages, and rock compressive strength is measured in the field. Bedrock channel widths, surveyed at high scour indicators, scale as a power law function of discharge (w a Qw ) that is estimated by combining rainfall data with 90-m digital topography. The results suggest that power law width scaling models apply (1) to regions with pronounced rainfall gradients, (2) to tributary catchments distributed across a climatically diverse region, and (3) to large, rapidly denuding orogens. An analysis of rainfall data indicates that the regional gradient of rainfall during storms that drive erosive discharge events is about half as large as the gradient of seasonal rainfall across the same area. Finally, numerical models in which the maximum rainfall is displaced significantly downstream from the headwaters predict a midcatchment zone of relatively rapid decreases in channel gradient and increases in channel concavity that are driven by locally enhanced discharge. Because differential rock uplift can produce analogous changes in gradients, the influence of rainfall gradients should be assessed before tectonic inferences are drawn.

Effects of sediment pulses on channel morphology in a gravel-bed river

Sediment delivery to stream channels in mountainous basins is strongly episodic, with large pulses of sediment typically delivered by infrequent landslides and debris flows. Identifying the role of large but rare sediment delivery events in the evolution of channel morphology and fluvial sediment transport is crucial to an understanding of the development of mountain basins. In July 2001, intense rainfall triggered numerous debris flows in a severely burnt watershed in the Sapphire Mountains of Montana. Ten large debris flow fans were deposited on the valley floor, and investigations focused on the channel response to these sediment pulses. The channel has aggraded immediately upstream of each fan, and braided in reaches immediately downstream. Channel incision through the fans has created sets of coarse-grained terraces. The deposition upstream of the pulses consists almost exclusively of fine material, resulting in a median bed material size ( D 50 ) 1–2 orders of magnitude lower than the ambient channel material. The volume of sand being transported is so great that these aggrading reaches can extend hundreds of meters upstream of the fans, with 1–2 m of sand deposited across the entire valley floor. Along a 10 km study reach, cross section surveys, longitudinal profiles, and pebble counts chronicle channel response to a punctuated increase in sediment supply and provide insight on the processes of sediment wave dispersal.

Valley asymmetry and glacial versus nonglacial erosion in the Bitterroot Range, Montana, USA

Theories that propose feedbacks among climate, tectonics, and surface processes commonly assume that erosion is enhanced by glacial activity. Indeed, studies have shown that glaciers appear to limit the elevation of mountain ranges; however, comparisons between rates of glacial and nonglacial erosion are difficult to make. Ideally, such comparisons must hold precipitation and lithology constant, while only varying the erosional regime. Located in a climatic transition zone during the Pleistocene, the east-west–trending valleys of the Bitterroot Range present an opportunity for a quantitative analysis of glacial and nonglacial erosion because the north-facing sides of the valleys were glaciated, whereas the south-facing slopes were not. The different erosional regimes operating on either side of the valleys created strongly asymmetric ridges. Ridgelines separating the east-west–trending valleys have been pushed southward by glacial headwall retreat such that ridge-to-valley distances are ∼50% greater on the north-facing slopes than on the south-facing slopes. In addition, mean hillslope angles are 6° lower on the glaciated slopes than on the unglaciated slopes, and calculations of geophysical relief suggest that, on average, glaciers have removed nearly twice as much rock as nonglacial processes. Finally, we conclude that, although rates of vertical incision by glacial processes in the Bitterroot Range were more rapid than nonglacial processes, the dominant geomorphological impact of glaciers was lateral erosion by headwall retreat.

Physical, chemical and hydrological properties of Ponderosa pine ash.

In this study, ash is analysed as a geological material; in particular, we focus on ash produced by the burning of Ponderosa pine, a conifer that is widespread throughout mountainous landscapes of western North America. One set of ash samples used in the analyses was collected from a wildfire site and another set was created in the laboratory. We found that the median particle size of the ash was in the fine sand to silt range with at least 25–50% of the particle size distribution in the appropriate range for maintaining debris flow behaviour. Measurements of the infiltration capacity of ash found values similar to fine sands, indicating that a layer of ash can reduce the infiltration capacity of coarse soils. The elemental composition of ashes analysed through inductively coupled plasma emission spectrometry was dominated by Ca, K, Mg, P, Mn, Fe and Al. X-ray diffraction analysis revealed the presence of calcite, quartz and feldspars in ashes created from a variety of fuels; fuel type and combustion temperature were found to have a dominant control on ash mineralogy. The results suggest that the elemental and mineral composition of ash could be used to identify dominant fuel sources and combustion temperatures.

Hydrological controls on chemical weathering rates at the soil-bedrock interface

Chemical weathering of bedrock is critical to maintaining terrestrial life, and climate, typically as manifested by precipitation, is often identified as having a first-order control on rates of chemical weathering. The ability of precipitation to dissolve rock, however, is modulated by the properties of the overlying soil that influence the contact time between water and minerals. Flume experiments were conducted to investigate the hydrological controls on rates of chemical weathering. Solute concentrations of runoff flowing across synthetic bedrock overlain by nonreactive pseudosoils of differing hydraulic conductivities were measured to examine the role of seepage velocity in influencing weathering rates on steep slopes. The results suggest that, where weathering is not limited by the supply of fresh minerals, weathering rates should increase with decreasing hydraulic conductivity. In addition, a mathematical relationship between hydraulic conductivity and chemical weathering on hillslopes is introduced to explore the hydrological controls on feldspar and calcite dissolution rates. The mathematical model supports the results from the experiments.

Particle transport over rough hillslope surfaces by dry ravel: Experiments and simulations with implications for nonlocal sediment flux

[1] Past studies of hillslope evolution have typically assumed that soil creep processes are governed by a linear relationship between local hillslope angle and transport distance. The assumption of “linear diffusion” has fallen out of favor because, when coupled with an expression of mass continuity, it yields unrealistic hillslope profiles. As a consequence, a better understanding of the mechanics of sediment transport is needed. Here we report results from a series of flume experiments performed to investigate sediment transport by dry ravel, a common soil creep process in arid and semiarid environments. We find that, at gentle slopes, transport distances follow distributions characteristic of local transport. As gradients steepen, a fraction of the particles begins to exhibit nonlocal transport, and that fraction increases rapidly with slope. A stochastic discrete element model that couples an effective friction term with a shock term reproduces the results from the flume experiments, suggesting that it can be used to explore the nature of particle transport on rough surfaces. The model predicts that exponential distributions of transport distances on gentle slopes evolve into quasi-uniform distributions on steep slopes, and the transition occurs as slopes approach the angle of repose. Our results support previous findings that the angle of repose represents a threshold between friction and inertial regimes. In addition, we propose that the angle of repose represents a fuzzy boundary between local and nonlocal transport.