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Willenbring J.K.,University of Pennsylvania | Willenbring J.K.,National Center for Earth Surface Dynamics | Codilean A.T.,German Research Center for Geosciences | McElroy B.,University of Wyoming
Geology | Year: 2013

We use a new compilation of global denudation estimates from cosmogenic nuclides to calculate the apportionment and the sum of all sediment produced on Earth by extrapolation of a statistically significant correlation between denudation rates and basin slopes to watersheds without denudation rate data. This robust relationship can explain approximately half of the variance in denudation from quartz-bearing topography drained by rivers using only mean slopes as the predictive tool and matches a similar fit for large river basins. At slopes >200 m/ km, topography controls denudation rates. Controls on denudation in landscapes where average slopes are <~200 m/km are unclear, but sediment production rates in these areas average ~45 mm/k.y., 75% of the denudation rates being >10 mm/k.y. We use global topographic data to show that the vast majority of the Earth's surface consists of these gently sloping surfaces with modest, but positive, gross denudation rates, and that these areas contribute the most sediment to the oceans. Because of the links between silicate weathering rates and denudation rates, the predominance of low sloping areas on the Earth's surface compared to areas of steep mountainous topography implies that mountain uplift contributes little to drawdown of CO2 at cosmogenic nuclide time scales of 103-106 yr. The poorly understood environmental controls that set the pace of denudation for the largest portion of Earth's surface hold the key to understanding the feedbacks between erosion and climate. © 2013 Geological Society of America.


Zakeri A.,Geotechnical Engineering Group | Zakeri A.,International Center for Geohazards | Si G.,University of Oslo | Marr J.D.G.,National Center for Earth Surface Dynamics | And 2 more authors.
Submarine Mass Movements and Their Consequences - 4th International Symposium | Year: 2010

The characteristics of submarine debris flows and the generated turbidity as well as their relationship with the deposit thickness are discussed herein. There is a gap in our understanding of the processes in which a submarine debris flow and the overriding turbidity form seabed deposits and how the deposits relate to the parent landslide. The experimental program reported here studied subaqueous gravity flows of different clay-rich slurries in a flume. The flume results provide insight into the thickness of the slurry flows with the overriding turbidity clouds and the deposited sediments and lays groundwork for future studies. The thickness of the slurry head tends to decrease with increasing slurry clay content whereas the thickness of the turbidity overriding the slurry head tends to decrease with increasing clay content. Further, the thickness of the deposited layer measured a few seconds after termination of the slurry flow increases with clay content. Geometrically, the flume experiments represented flowing debris of a landslide from 50 m to 120 m water depths with a 600 m travelling distance and downstream velocities between 5 and 13.5 m/s. © Springer Science + Business Media B.V. 2010.


Foufoula-Georgiou E.,University of Minnesota | Syvitski J.,University of Colorado at Boulder | Paola C.,National Center for Earth surface Dynamics | Hoanh C.T.,International Water Management Institute | And 6 more authors.
Eos | Year: 2011

Marine and lacustrine deltas around the world are economic and environmental hot spots. They occupy approximately 1% of the Earth's land area but are home to more than 500 million peoplea population density more than 10 times the world average [Ericson et al., 2006]all within 5 meters of sea level [Overeem and Syvitski, 2009]. This high density is supported by high productivity, rich biodiversity, and transport along a network of waterways. Yet deltaic systems are some of the world's most delicate and vulnerable natural systems, residing at the boundary between land and water, and are subject to upstream human control, local resource exploration, and climatic impacts.


Willenbring J.K.,National Center for Earth Surface Dynamics | Willenbring J.K.,University of Pennsylvania | Gasparini N.M.,Tulane University | Crosby B.T.,Idaho State University | Brocard G.,University of Pennsylvania
Geology | Year: 2013

In equilibrium landscapes, 10Be concentrations within detrital quartz grains are expected to quantitatively reflect basin-wide denudation rates. In transient landscapes, though detrital quartz is derived from both the incising, adjusting lowland and the unadjusted, relict upland, the integrated 10Be concentrations still provide a denudation rate averaged across the two domains. Because field samples can provide only a snapshot of the current upstream-averaged erosion rate, we employ a numerical landscape evolution model to explore how 10Be-derived denudation rates vary over time and space during transient adjustment. Model results suggest that the longitudinal pattern of mean denudation rates is generated by the river's progressive dilution of low-volume, high-concentration detritus from relict uplands by the integration of high-volume, low-concentration detritus from adjusting lowlands. The proportion of these materials in any detrital sample depends on what fraction of the upstream area remains unadjusted. Because the boundary of the adjusting part of the landscape changes over time, the longitudinal trend in cosmogenic nuclide-derived erosion rates changes over time. These insights are then used to guide our interpretation of geomorphic and longitudinal cosmogenic nuclide data from the South Fork Eel River (SFER) in the California Coast Range (United States). The northward-propagating crustal thickening and rock uplift associated with the passage of the Mendocino triple junction generates a mobile wave of uplift that progressively sweeps longitudinally down the SFER. The consequences of this forcing can be both replicated in the model environment and observed in the field. The SFER contains transient landforms including knickpoints and river terraces along mainstem and tributary channels that define a clear boundary between an incised, adjusting lowland and an unadjusted, relict upland. We report nine nested, basin-wide denudation rates in the mainstem of the SFER using terrestrial cosmogenic 10Be in river-borne sediment. We find that denudation rates increase in the downstream direction from ~0.2 mm/yr in the upper catchment to ~0.5 mm/yr at the outlet. Using comparisons to the modeled landscape, we show that this pattern of denudation rates, paired with the distribution of relict topography throughout the watershed, reflect the immaturity of the landscape's transient adjustment. Later in this modeled transient, the predicted erosion rates decrease downstream before they become uniform. This interpretation of our data has potentially far-reaching implications for quantifying the uplift history and response time of transient landscapes using cosmogenic nuclides. © 2013 Geological Society of America.


Green M.B.,University of Minnesota | Green M.B.,National Center for Earth Surface Dynamics | Green M.B.,Plymouth State University | Green M.B.,U.S. Department of Agriculture | And 2 more authors.
Biogeochemistry | Year: 2010

Many ecologists and biogeochemists explore the interaction of the nitrogen (N) and phosphorus (P) cycles by addressing N:P ratios. While N:P ratios are recognized as broadly important to the composition and functioning of lotic ecosystems, the fundamental controls on stream water N:P ratio variation remains poorly understood. Low N:P ratio (less than 16) streams appear more likely in arid climates than in mesic climates, suggesting possible hydrologic or landscape controls. We explored the importance of watershed hydrology to the variation of total N to total P (TN:TP) ratios in stream water, and whether such variation is characteristically different across watershed classes based on mean annual precipitation and median observed TN:TP ratio. Nonparametric scatter plot analysis was applied to normalized TN:TP ratios and associated discharge (Q) measurements from 57 minimally-impacted watersheds from the contiguous United States. At the seasonal scale, TN:TP ratios showed a negative relationship with Q in semiarid climates and a positive relationship with Q in humid climates. Over storm event scales, TN:TP ratios decline with increasing Q across all watershed classes. The results broadly indicate hydrology is an important driver of TN:TP ratio variation over multiple time scales. We hypothesize that the broad differences across watershed classes are driven by variation in the nature of connectivity (frequency and magnitude of connections) of the landscape to streams. A strong physical control of N:P ratios in stream water is in stark contrast to the biological control of N:P ratios in the oceans, suggesting that application of stoichiometric theory-developed using marine systems-to lotic systems requires a broader consideration of controlling factors. © 2009 Springer Science+Business Media B.V.


Singh A.,University of Minnesota | Singh A.,National Center for Earth Surface Dynamics | Foufoula-Georgiou E.,University of Minnesota | Foufoula-Georgiou E.,National Center for Earth Surface Dynamics | And 4 more authors.
Journal of Geophysical Research: Earth Surface | Year: 2012

A series of flume experiments were conducted in a large experimental channel at the St. Anthony Falls Laboratory to understand the coupled dynamics of flow and bed forms above the sediment-water interface. Simultaneous high resolution measurements of velocity fluctuations, bed elevations and sediment flux at the downstream end of the channel, were made for a range of discharges. The probability density functions (pdfs) of bed elevation increments and instantaneous Reynolds stress reveal a power law tail behavior and a wavelet cross-correlation analysis depicts a strong dependence of these series across a range of scales, indicating a feedback between bed form dynamics and near-bed turbulence. These results complement our previous findings in which the signature of bed form evolution on the near-bed velocity fluctuations was confirmed via the presence of a spectral gap and two distinct power law scaling regimes in the spectral density of velocity fluctuations. We report herein a strong asymmetry in the probability distribution of bed elevation increments and instantaneous Reynolds stresses, the latter being further analyzed and interpreted via a quadrant analysis of velocity fluctuations in the longitudinal and vertical directions. We also report the presence of intermittency (multifractality) in bed elevation increments and interpret it, in view of the asymmetric nature of the pdfs, as the result of scale coupling. In other words, the geometric asymmetry at the bed form scale gets transferred down to a probabilistic asymmetry at all smaller scales indicating a local anisotropy in the energy transfer. Finally, we propose a predictive relationship between bed form averaged sediment transport rates and bed form averaged instantaneous Reynolds stress and validate it using our experimental data. © 2012. American Geophysical Union. All Rights Reserved.


Gran K.B.,University of Minnesota | Gran K.B.,National Center for Earth Surface Dynamics | Finnegan N.,National Center for Earth Surface Dynamics | Finnegan N.,University of California at Santa Cruz | And 5 more authors.
Bulletin of the Geological Society of America | Year: 2013

Many high-latitude fluvial systems are adjusting to base-level changes since the last glaciation. Channels that experienced baselevel fall may still be incising, often through glacial diamictons (tills). These tills can be quite competent, behaving more like weak bedrock than unconsolidated sediment, and erode at a fast pace, thus providing a unique opportunity to test models of channel incision and knickpoint migration in transient systems. Here, we integrate light detection and ranging (LiDAR) topography, strath terrace chronology, and numerical modeling to determine knickpoint migration and incision history of the Le Sueur River in central Minnesota, USA. Results indicate that the Le Sueur River is best modeled as a detachment-limited channel, with downstream coarsening related to lag clasts from tills playing a critical factor in longitudinal profile development. The Le Sueur River meanders as it incises, so we coupled the best-fit incision model to a meander model to determine valley excavation history. The excavation history was used to determine a natural background erosion rate, prior to land-use changes associated with settlement and agricultural expansion in the mid-1800s. We compared background fine sediment (silt and clay) erosion rates with historic decadal-average annual suspended loads. Results show that modern fine sediment contributions from sources associated with valley excavation are three times higher than modeled presettlement loads. Recent changes in hydrology associated with land use and climate change have increased flows in rivers, leading to higher sediment loads, not just from field erosion, but from increased bank and bluff erosion in the deeply incised valleys. © 2013 Geological Society of America.


Singh A.,University of Minnesota | Singh A.,National Center for Earth surface Dynamics | Howard K.B.,University of Minnesota | Guala M.,University of Minnesota | Guala M.,National Center for Earth surface Dynamics
Physics of Fluids | Year: 2014

The structure of flow turbulence, measured experimentally, in the wake of a model wind turbine is investigated here through higher order scale-dependent statistics of the velocity increments and compared to the smooth wall turbulent boundary layer (base flow) case. The wind turbine wake flow is observed to possess higher turbulent kinetic energy,when compared to the base flow, though it contains more homogenized scale-dependent velocity increments, as confirmed via magnitude cumulant analysis of the streamwise velocities. Along with a reduction in intermittency (a measure of inhomogeneity) in the wake of the wind turbine, the asymmetry of the probability density functions of the velocity increments is also observed to be reduced. This is interpreted in terms of scale decoupling mechanisms and attenuating interactions and non-local energy transfer. In other words, wind turbines reduce the intermittency and asymmetry in the wake flow by breaking and/or deflecting the large-scale flow structures of the incoming flow, thus rendering the structure of the velocity fluctuations more homogenized as compared to the base flow. Experiments were conducted in a large scale, boundary layer wind tunnel at the St. Anthony Falls Laboratory. © 2014 AIP Publishing LLC.

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