Robert T. Pack, Research Associate Professor, Civil and
Tarboton, Professor, Utah Water Research Laboratory,
Erkan Istanbulluoglu, PhD Student, Civil and Environmental Engineering, Utah State University. (Now at MIT. firstname.lastname@example.org)
Lee Benda, Earth Systems Institute,
Dan Miller, Earth Systems Institute,
Charlie Luce, USFS Rocky Mountain Research Station,
The objective of this project was to develop and test a method for determining the risk of increased sediment introduction to streams from upland processes during and following forest road construction, timber harvesting, and wildfire in forested watersheds. This work emphasizes the characterization and integration of the frequency-magnitude character of processes that combine to contribute sediment to stream channels. This work focused on the role of gullies, their initiation and extent and the processes involved in the removal of sediment from hillslopes and the delivery of sediment to streams.
The specific study areas selected for this research are Trapper and Robert E. Lee Creeks within the North Fork of the Boise River in southwestern Idaho. Trapper Creek was intensely burned by a wildfire in 1994 and extreme gullying was initiated by a convective summer storm in 1996, possibly due to water repellent conditions of the surface soil. The gullies generated by this storm are probably ephemeral channels, but nevertheless resulted in considerable erosion. Robert E. Lee (REL) Creek was partially burned to a light to moderate degree. Although this area was presumably exposed to the same thunderstorm, intense gullying did not occur. In these watersheds channel head locations were surveyed. Local slopes at the channel heads were measured in the field and a GPS was used to record locations. Sediment size samples were collected just above the headcuts to approximate the sediment size in transport when the incisions occurred. These samples were sieved and median size of each sample was determined. Contributing areas were derived from the 30m DEM of the study site using the D¥ algorithm (Tarboton, 1997).
The first contribution from this work was the development of a new probabilistic approach for the initiation of channels. The channel head represents an important transition point in hillslope erosion processes. There is a nonlinear threshold transition across the channel head with soil loss much larger in channels than on hillslopes. Based on analysis of our field data we suggest that the channel initiation problem be viewed probabilistically with a spatially variable probability of channel initiation that depends on slope, specific catchment area and the probability distributions of median grain size, surface roughness and excess rainfall rate. The channel initiation threshold is cast as a random variable to characterize the variability of aSa at channel heads. Here a is specific catchment area and S is slope. Considerable erosion research literature has focused on the channel initiation threshold expressed this way. We show that, for our field sites, median grain size measurements at each channel head explain a significant part of the observed variability of aSa. We then characterize the variability of model inputs (median grain size, roughness and excess rainfall) using probability distributions and show that the probability distribution of area-slope threshold derived from these inputs matches the probability distribution of area-slope thresholds measured at field channel head locations. A gamma probability distribution provides a reasonable match to the distributions of area-slope threshold measured and modeled at channel heads in our study area and in other published channel head data. This work has been reported in a PhD Dissertation and paper published in Water Resources Research (Istanbulluoglu, 2002; Istanbulluoglu et al., 2002).
The second contribution from this work was the development of a sediment transport model for the incision of gullies on steep topography. In our field areas, measured gully length and cross sections were used to estimate the volumes of sediment loss due to gully formation. These volume estimates are assumed to provide an estimate of sediment transport capacity at each survey cross section from the single gully-forming thunderstorm. Sediment transport models commonly relate transport capacity to overland flow shear stress, which is related to runoff rate, slope and drainage area. We have estimated the runoff rate and duration associated with the gully-forming event and used the sediment volume measurements to calibrate a general physically based sediment transport equation in this steep high shear stress environment. We find that a shear stress exponent of 3, corresponding to drainage area and slope exponents of M=2.1 and N=2.25, match our data. This shear stress exponent of 3 is approximately two times higher than those for bedload transport in alluvial rivers, but is in the range of shear stress exponents derived from flume experiments on steep slopes and with total load equations. Our results, although preliminary due to the uncertainty associated with the sediment volume estimates, suggest that for steep hillslopes such as those in our study area, a greater nonlinearity in the sediment transport function exists than that assumed in some existing hillslope erosion models which calculate sediment transport capacity using the bedload equations developed for rivers. This work has been reported in a PhD Dissertation and paper under review for publication in Water Resources Research (Istanbulluoglu, 2002; Istanbulluoglu et al., 2003).
The third contribution from this work was the development of a model of the interactions between forest vegetation disturbances and sediment yields. The model simulates soil development based on continuous bedrock weathering and the divergence of diffusive sediment transport on hillslopes. Soil removal is due to episodic gully erosion, shallow landsliding and debris flow generation. In the model, forest vegetation provides root cohesion and surface resistance to channel initiation. Forest fires and harvests reduce the vegetation. Vegetation loss leaves the land susceptible to erosion and landsliding until the vegetation cover reestablishes in time. When vegetation is not disturbed by wildfires over thousands of years, sediment delivery is modeled to be less frequent but with larger event magnitudes. Increased values of root cohesion (representing denser forests) lead to higher event magnitudes. Wildfires appear to control the timing of sediment delivery. Compared to undisturbed forests, erosion is concentrated during the periods with low erosion thresholds often called “Accelerated Erosion Periods” following wildfires. Our modeling suggests that drainage density is inversely proportional to root cohesion and that reduced forest cover due to wildfires increases the drainage density. We compare the sediment yields under anthropogenic (harvest) and natural (wildfire) disturbances. Disturbances due to forest harvesting appear to increase the frequency of sediment delivery however the sediment delivery following wildfires seems to be more severe. These modeling based findings have implications for engineering design and environmental management where sediment inputs to streams and the fluctuations and episodicity of these inputs are of concern. This work has been reported in a PhD Dissertation and paper (Istanbulluoglu, 2002; Istanbulluoglu et al., 2004).
Istanbulluoglu, E., (2002), "Quantification of Stream Sediment Inputs from Steep Forested Mountains," PhD Thesis, Civil and Environmental Engineering, Utah State University. [PDF 1.8 MB]
Istanbulluoglu, E., D. G. Tarboton, R. T. Pack and C. Luce, (2002), "A Probabilistic Approach for Channel Initiation," Water Resources Research, 38(12): 1325, doi:10.1029/2001WR000782. [PDF (481KB)]
Istanbulluoglu, E., D. G. Tarboton, R. T. Pack and C. Luce, (2003), "A Sediment Transport Model for Incising Gullies on Steep Topography," Water Resources Research, 39(4): 1103, doi:10.1029/2002WR001467.
Istanbulluoglu, E., D. G. Tarboton, R. T.Pack and C. H. Luce, (2004), "Modeling of the Interactions between Forest Vegetation, Disturbances and Sediment Yields," JGR - Earth Surface, 109(F1): F01009, doi: 10.1029/2003JF000041. [PDF (450K)]
Tarboton, D. G., (1997), "A New Method for the Determination of Flow Directions and Contributing Areas in Grid Digital Elevation Models," Water Resources Research, 33(2): 309-319. [PDF (9.1Mb), PDF preprint (257K), TAUDEM computer programs.]