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Current projects: Geochemistry and geomorphology of soil mantled landscapes, Salt marsh stability
Past research: Hillslopes, soil, and chemical weathering, Ecogeomorphology, Arid region hydrology, Debris Flows Glaciers

Current Projects

The Coupled geochemical and geomorphic evolution of hillslope soils

Along with Kyungsoo Yoo at the University of Delaware, I am involved in two projects to investigate the coupled geochemical and geomorphic evolution of hillslope soils. We have carried out extensive field measurements of soil geochemistry and mineralogy at Tennessee Valley, CA, specifically choosing a location where geomorphic processes are well understood. We are working on linking the geomorphic evolution, which drives hydrologic flow paths and particle residence times, to soil chemical weathering rates. We have already developed the mathematical framework to address this problem (see here and here), and we are currently preparing manuscripts describing our findings on soil chemistry. We are further exploring the link between erosion rates and soil chemistry in the Feather River basin in the Sierra Nevada of California. This study site has homogenous bedrock and a reasonably uniform climate, yet deep river canyons are incising into a relict, low relief surface and erosion rates in the area vary over an order of magnitude.

 

Salt marsh stability in the face of rising sea level

I am working with a number of collaborators to create ensemble simulations vegetated salt marshes and how they respond to changes in sea level rise. My component of this effort is using the OIMAS-N marsh accretion model, described here. In addition, Andrea D’Alpaos (University of Padua) and I are exploring the role of different sedimentation processes on marsh surface accretion.

Past Work

Hillslopes, soil, and chemical weathering

How chemical weathering influences topography

In Mudd and Furbish (2004) we posed the simple question: how does chemical weathering affect the shape of hillslopes? We considered hillslopes that were steady state: that the supply of sediment to a hillslope was in balance with denudation caused by a combination of chemical and mechanical processes. We found that if chemical weathering varies in space (e.g., divides weather more quickly than toeslopes), then chemical denudation can exert a first order control on topography. Further we found that if chemical denudation is equal to or greater than mechanical denudation, hillslopes could have a convex-concave profile. This profile was previously though only to occur in landscapes that are not in steady state or landscapes where overland flow processes are important.

Citation:

Mudd, S.M. and Furbish, D.J., 2004. The Influence of chemical denudation on hillslope morphology. Journal of Geophysical Research, 109(F02001): doi:10.1029/2003JF000087. pdf

 

Divide migration

In Mudd and Furbish (2005) we examined how divides move when adjacent rivers incise at different rates. In this study we calculated the rates at which divides migrate as a function of hillslope properties, and found that stream capture due to differential channel incision would take hundreds of thousands to millions of years. We also examined the response of hillslopes to time varying incision and found that incision rates that vary over long periods are more likely to affect the divide than high frequency oscillations, and that as one moves up slope one averages erosion rates over longer and longer time periods.

Citation:

Mudd, S.M. and Furbish, D.J., 2005. Lateral migration of hillcrests in response to channel incision in soil mantled landscapes. Journal of Geophysical Research, 110(12): F04026, doi:10.1029/2005JF000313. pdf

 

Using chemical tracers to quantify chemical weathering rates

In Mudd and Furbish (2006) we examined how the concentration of a weathering resistant mineral, Zircon, would change in space if chemical weathering is spatially heterogeneous. We found that in order to use Zircon grains to quantify chemical weathering rates, it is essential that sediment transport is taken into account. In this paper we also showed that in steadily eroding landscapes the time soil particles reside in the soil is spatially homogenous, and also that these times are exponentially distributed.

Citation:

Mudd, S.M. and Furbish, D.J., 2006. Using chemical tracers in hillslope soils to estimate the importance of chemical denudation under conditions of downslope sediment transport. Journal of Geophysical Research, 111: F02021, doi:10.1029/2005JF000343. pdf

 

The time is takes hillslopes to respond to changes in channel incision rates

Changes in climate or tectonics are communicated through channel networks and then onto hillslopes. Because on eroding hillslopes some record of erosion is retained (in the form of soil topography, thickness, and chemistry), it is important to understand how long changes in channel incision rates may be preserved upon the landscape. In Mudd and Furbish (2007) we derived the response timescale for two sediment flux laws, confirming prior numerical results that the response time is proportional to the square of the length of the hillslope divided by the so-called diffusivity, which is a measure of the efficiency of sediment transport on the hillslope. We also derived the time it takes a signal to move from the channel to a hillslope divide: it is approximately one tenth of the response timescale. The response timescale of typical hillslopes is between ~10ka-10Ma, and on most hillslopes the response timescale is much longer than the timescale of climatic fluctuation in the quaternary. Thus we find that almost all natural hillslopes are experiencing transient, rather than steady, erosion rates.

Citation:

Mudd, S.M. and Furbish, D.J., 2007. Responses of soil-mantled hillslopes to transient channel incision rates. Journal of Geophysical Research-Earth Surface, 112(F3): F03S18 doi:10.1029/2006JF000516. pdf

 

High speed imaging of rainsplash

In arid regions with little soil cover, rainsplash is one of the principal causes of soil erosion. Furbish et al (2007) used high speed cameras to quantify how raindrop impacts eject loose sediment from the ground surface during rainstorms. From our experiments, we were able to derive an equation that describes the probability of particles landing some distance form a raindrop impact point; this equation was then used to quantify the rate of splash transport as a function of slope angle.

Citation:

Furbish, D.J., Hamner, K.K., Schmeeckle, M., Borosund, M.N. and Mudd, S.M., 2007. Rain splash of dry sand revealed by high-speed imaging and sticky paper splash targets. Journal of Geophysical Research-Earth Surface, 112, F01001, doi:10.1029/2006JF000498. pdf

The importance of quantifying time particles spend in the soil and how this affects estimates of chemical weathering rate

In many studies that seek to quantify long term chemical weathering rates from soils, a soil ‘age’ is typically defined as the time since the soil surface has become stable. In Yoo and Mudd (2008a) we demonstrated that this approach can lead to misleading estimates of the rate at which minerals weather. Further, we demonstrated that in order to understand weathering rates from soils one must understand the distribution of times that individual minerals have speant in the near surface zone that experiences chemical weathering.

Citation:

Yoo, K. and Mudd, S.M., 2008a. Discrepancy between mineral residence time and soil age: Implications for the interpretation of chemical weathering rates. Geology, 36(1): 35-38. pdf

 

A Mathematical framework for the coupled geochemical and geomorphic evolution of soil

In Yoo and Mudd (2008b) we derived a series of equations for quantifying soil evolution in a wide array of landscapes. Our mathematical model is capable of accommodating soils that are in depositional, non-eroding, or eroding settings. We demonstrate how sediment transport, erosion, and the downward propagation of weathering fronts affect soil chemistry and provide a framework for future quantitative studies linking soil science and geomorphology.

Citation:

Yoo, K. and Mudd, S.M., 2008b. Toward process-based modeling of geochemical soil formation across diverse landforms: A new mathematical framework. Geoderma, 146(1-2): 248-260. pdf

 

A simple model linking chemical and physical erosion

In Gabet and Mudd (2009) we develop a simple model that traces particles as they enter the weathering zone, are chemically weathered, and then are eroded from a landscape in order to quantify how physical denudation and chemical denudation are linked. The simple model provides a unified theory from what previous authors (notably West et al., 2005, ESPL v235, 211-228) had described using two end member models. We are able to fit data reported by West et al. (2005) using this new model, and explain why at low rates of denudation chemical weathering rates are linearly proportional to the denudation rate but there are diminishing returns at higher erosion rates and the relationship between total denudation and chemical denudation becomes less than linear.

Citation:

Gabet, E.J. and Mudd, S.M., 2009. A theoretical model coupling chemical weathering rates with denudation rates. Geology, 37(2): 151-154. pdf

 

Ecogeomorphology

Marsh plants, flow, and sedimentation

Mudd et al. (2004) was the first paper to quantitatively couple a model of biomass growth with physics-based models of flow and sedimentation on vegetated salt marshes. Using a long term (18 year) record of stand characteristics of Spartina alterniflora, we were able to quantify the characteristics of a marsh plant community that affected the physical processes of flow and sedimentation; sedimentation then affected the elevation of the salt marsh platform which in turn affected the growth of marsh plants. Thus in this paper we quantified the direct feedback between biological and physical systems.

Citation:

Mudd, S.M., Fagherazzi, S., Morris, J.T. and Furbish, D.J., 2004. Flow, sedimentation, and biomass production on a vegetated salt marsh in South Carolina: toward a predictive model of marsh morphologic and ecologic evolution. In: S. Fagherazzi, A. Marani and L.K. Blum (Editors), The Ecogeomorphology of Tidal Marshes. Coastal and Estuarine Monograph Series. American Geophysical Union, Washington, D.C., pp. 165-187. pdf

 

Marsh plants, sedimentation, and tidal creeks

In D’Alpaos et al. (2006) we demonstrated how marsh plants affect the shape and widening of salt marsh creeks. The study determined that the presence of marsh plants increases drag and results in shallower channels than would be expected in mudflats. We found that the infilling of the channel due to establishment of vegetation on the marsh was a more important process in microtidal marshes than in meso- and macrotidal marshes.

Citation:

D'Alpaos, A., Lanzoni, S., Mudd, S.M. and Fagherazzi, S., 2006. Modeling the influence of hydroperiod and vegetation on the cross-sectional formation of tidal channels. Estuarine Coastal and Shelf Science, 69(3-4): 311-324. pdf

 

Marsh plants, sedimentation, and carbon

In Mudd et al. (2009) we constructed both analytical and numerical models of marsh accretion that included, in addition to aboveground processes common in prior studies, a number of belowground processes such as root growth and carbon dynamics. These models were used to show how carbon decay can affect the interpretation of 210Pb and 137Cs profiles used for sedimentation studies. The models were also used to show how changes in sea level rise could affect carbon accumulation in salt marsh sediments, and demonstrated that the carbon balance marshes with low inorganic sediment supply are extremely sensitive to changes in sediment supply (due to, e.g., human perturbation of upstream sediment sources).

Citation:

Mudd, S.M., Howell, S.M. and Morris, J.T., 2009. Impact of dynamic feedbacks between sedimentation, sea-level rise, and biomass production on near surface marsh stratigraphy and carbon accumulation. Estuarine Coastal and Shelf Science, 82(3): 377-389, doi:10.1016/j.ecss.2009.01.028. pdf

 

Arid region hydrology

Influence of infiltration on the velocity and runout distance of flash floods in ephemeral channels

In Mudd (2006), I demonstrated that momentum losses due to infiltration during flash floods could significantly affect the speed at which flash floods travel through a channel network. An important finding of this study was that hydrograph shape could have a significant impact on how far downstream flash floods propagate: floods with sharper peak flow will reach further downstream than floods with a longer flood with a lower peak flow. A comment by Cao et al. (2007) highlighted the issue of slope parallel momentum of flood waters near the sediment surface. Cao (2007) argued it was nil whereas I had included such a momentum term in the original paper. Using a 2D finite element model of groundwater infiltration I showed in response to Cao (2007) that the slope parallel velocity of water at the base of the flood can affect the momentum balance near the flood bore, but not in the tail of the flood. Thus the term in the momentum balance was less than I had originally proposed but more that nil as argued by Cao et al. (2007). The original term that I had overestimated in the 2006 paper was in any case a negligible part of the momentum balance and the correction had an minimal change in the flood behaviour (5m difference in flow propagation in a 6.5km flood runout, and error of <0.1%).

Citation:

Mudd, S.M., 2006. Investigation of the hydrodynamics of flash floods in ephemeral channels: Scaling analysis and simulation using a shock-capturing flow model incorporating the effects of transmission losses. Journal of Hydrology, 324(1-4): 65-79. pdf

Cao, Z.X. and Yue, Z.Y., 2007. Comment on "Investigation of the hydrodynamics of flash floods in ephemeral channels: Scaling analysis and simulation using a shock-capturing flow model incorporating the effects of transmission losses" by S.M. Mudd, 2006. Journal of Hydrology 324, 65-79. Journal of Hydrology, 336(1-2): 222-225. pdf

Mudd, S.M., 2007. Reply to "Comment on 'Investigation of the hydrodynamics of flash floods in ephemeral channels: Scaling analysis and simulation using a shock-capturing flow model incorporating the effects of transmission losses' by S.M. Mudd, 2006 (Journal of Hydrology) 324, 65-79" by Cao and Yue. Journal of Hydrology, 336(1-2): 226-230. pdf

 

Debris Flows

The Mobilization of Debris Flows from Shallow Landslides

The prevailing theory explaining how rigid landslides transform into debris flows assumes that soils are loose and collapse during failure; as the soil collapses, pore pressures shoot up and debris flow behaviour initiates. In Gabet and Mudd (2006) we tested this theory by analyzing soils from areas that were the sources of debris flows and slumps (i.e., failures that did not liquefy). Surprisingly, we found that all of the soils dilated during shear, even the ones that produced debris flows. This observation, coupled with the observations of others who have suggested that most natural soils are dilational, prompted us to re-examine the standard model of debris flow initiation via a numerical model. We concluded that a soil's potential for liquefaction is independent of porosity (i.e., how loose it is) but sensitive to sand content because of its effect on hydraulic conductivity.

Citation:

Gabet, E.J. and Mudd, S.M., 2006. The mobilization of debris flows from shallow landslides. Geomorphology, 74(1-4): 207-218. pdf

 

Glaciers

The Origin of the Antarctic ice sheet

Coming soon to Nature magazine.