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USGS Mendenhall Postdoctoral  Research Fellowship Program

14-29. High-resolution modeling of runoff and sediment transport in recently burned steeplands

Wildfire dramatically alters the hydrologic response of steep watersheds making normally benign, 1-2 year recurrence interval storms capable of producing dangerous flash floods and debris flows. These hazards threaten water and wildlife resources downstream and human populations and infrastructure nearby. For example, debris flows from burned areas above San Bernardino, CA killed 16 people on Christmas Day 2003 [Chong et al., 2004]. More recently in 2010, debris flows from the largest fire in L.A. county history damaged or destroyed over 40 homes near Pasadena, CA [Lin et al., 2010]. Unfortunately, the threat of post-fire debris flows in the Western U.S. is likely to increase due to both climate-related increases in fire frequency and magnitude and population growth along the urban-wildland interface [Cannon and DeGraff, 2009]. Thus, there is an urgent need to better understand debris-flow generation processes and improve hazard predictions.

Runoff from intense rainfall in burned areas plays a major role in generating debris flows. The magnitude of runoff is greatly enhanced relative to unburned conditions due to a fire-induced reduction in the infiltration capacity of the soil and a lack of rainfall interception from vegetation [Shakesby and Doerr, 2006]. The runoff mobilizes loose sediment stored on hillslopes and channels, and this sediment can progressively bulk into a debris flow having much greater depth and destructive power than a flood [Cannon, 2001].

Recent measurements of post-fire debris flow show this process occurs extremely rapidly within minutes after the start of intense rainfall [Kean et al., 2011, 2012]. In addition, terrestrial laser scanning [Staley et al., 2012] indicates that small, meter-scale variations in channel topography play an important role in the transition of water runoff to debris flow. These results highlight the need for accurate predictions of runoff and sediment transport at very high spatial (≤ 1 m) and temporal (≤ 1 min) resolutions. Such high-resolution modeling has yet to be conducted for burned areas susceptible to debris flows.

We seek a postdoctoral fellow to explore ways in which distributed modeling of runoff and sediment transport on high-resolution topography can be used to improve predictions of the timing, magnitude, and duration of post-fire debris flows. The research is expected to contribute to a more generalized understanding of post-fire debris-flow generation processes and to help better predict the response of future burn areas in different geologic settings to storms of variable intensity and duration.

Research under this Opportunity can capitalize on several new data sets that contain both high-spatial resolution topography and high-temporal resolution hydrologic data for burned areas. The data span a range of scales, from small (0.01 km2) headwater basins with ultra-high 2.5-cm topography to larger-scale (2 km2) watersheds with 1-m resolution topography. At all scales, corresponding time series of rainfall, soil water content, and flood and debris-flow stage are available to constrain models. Additional data on eroded sediment volumes are available from sediment retention basins and repeat topographic measurements of headwater basins. These data sets not only contain the basic inputs to develop models at extraordinary resolution, they also include rare constraints on post-fire flow timing and sediment volume that can be used to test predictions at multiple sites and scales.

A variety of research directions can be taken with this project. Research could focus on testing and modification of existing flow and sediment transport algorithms, or develop new models to characterize infiltration, surface flow, sediment transport, and/or erosion in burned areas. Research could concentrate primarily on the flow component of the problem, such as identifying the critical level of discharge required to generate observed debris flows at multiple sites; or the research could place more emphasis on predicting rates and patterns of sediment transport and erosion.

Different techniques may be required to make predictions at different resolutions and basin scales. As such, the Mendenhall Fellow may choose to concentrate on modeling small areas at ultra-high resolution (cm), larger areas at meter resolution, or develop techniques to upscale or downscale model results. Research may also include a field work component to better constrain model parameters.

A further avenue available to this project is to link it with ongoing USGS debris-flow monitoring at Chalk Cliffs, Colorado, where similar high-resolution data sets have been collected. Debris flows at this unburned but sparsely vegetated site are generated by similar processes as in burned areas. This provides a unique opportunity to apply methods developed during the course of the project to an unburned setting.

REFERENCES

Cannon, S.H., 2001. Debris-flow generation from recently burned watersheds. Environmental Engineering Geoscience, 7(4), 321-341.

Cannon, S.H., DeGraff, J., 2009. The Increasing Wildfire and Post-Fire Debris-Flow Threat in Western USA, and Implications for Consequences of Climate Change. In: K. Sassa, P. Canuti (Eds.), Landslides Disaster Risk Reduction. Springer Berlin Heidelberg, pp. 177-190.

Chong, J., Renaud, J. and Ailsworth E., 2004, Flash floods wash away lives dreams, Los Angeles Times, p. B.1. 3 Jan.

Kean, J.W., Staley, D.M., Cannon, S.H., 2011. In situ measurements of post-fire debris flows in southern California: Comparisons of the timing and magnitude of 24 debris-flow events with rainfall and soil moisture conditions. Journal of Geophysical Research, 116(F4), F04019..

Kean, J.W., Staley, D.M., Leeper, R.J., Schmidt, K.M., 2012. A low-cost method to measure the timing of post-fire flash floods and debris flows relative to rainfall. Water Resources Research, 48(5), W05516.

Lin, R.G., Kim V., and Vives, R., 2010, 'Niagara' of mud hits homes: Dozens of houses in La Canada Flintridge are damaged or destroyed in a storm that defied forecast, Los Angeles Times, p. A.1., 7 Feb.

Shakesby, R.A., Doerr, S.H., 2006, Wildfire as a hydrologic and geomorphic agent, Earth Science Reviews, 74, 269-307.

Staley, D.M., Wasklewicz, T.A., and Kean, J.W., 2012, Influence of surface form and sediment transport in recently burned watershed derived from multi-temporal terrestrial laser scanning data, Abstract EGU2012-11542, 2012 EGU General Assembly.

Proposed Duty Station: Golden, CO

Areas of Ph.D.: Earth science (geology, hydrology, geography, geophysics), engineering, physics, applied mathematics (candidates holding a Ph.D. in other disciplines but with knowledge and skills relevant to the Research Opportunity may be considered).

Qualifications: Applicants must meet one of the following qualifications - Research Geologist, Research Hydrologist, Research Physical Scientist, Research Mathematician, Research Physicist, Research Civil Engineer.

(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of theposition will be made by the Human Resources specialist).

Research Advisors: Jason Kean, (303) 273-8608, jwkean@usgs.gov.; Dennis Staley, (303) 273-8568, dstaley@usgs.gov; Susan Cannon, (303) 273-8604; cannon@usgs.gov.

Human Resources Office Contact: Robert Hosinski, (916) 278-9397, rhosinski@usgs.gov.


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U.S. Department of the Interior, U.S. Geological Survey
URL: http://geology.usgs.gov/postdoc/opps/2014/14-29 Kean.htm
Direct inquiries to Rama K. Kotra at rkotra@usgs.gov
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Last modified: 15:54:26 Tue 23 Jul 2013
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