A first classification scheme of flow-bed interaction for clay-laden density currents and soft substrates

Many aquatic environments exhibit soft, muddy substrates, but this important property has largely been ignored in process-based models of Earth-surface flow. Novel laboratory experiments were carried out to shed light on the feedback processes that occur when particulate density currents (turbidity currents) move over a soft mud substrate. These experiments revealed multiple types of flow-bed interaction and large variations in bed deformation and bed erosion, which are interpreted to be related to the interplay between the shear forces of the current and the stabilising forces in the bed. Changes in this force balance were simulated by varying the clay concentrations in the flow and in the bed. Five different interaction types are described, and dimensional and non-dimensional phase diagrams for flow-bed interaction are presented.     

Computing Earthquake Probabilities on Global Scales

Large devastating events in systems such as earthquakes, typhoons, market crashes, electricity grid blackouts, floods, droughts, wars and conflicts, and landslides can be unexpected and devastating. Events in many of these systems display frequency-size statistics that are power laws. Previously, we presented a new method for calculating probabilities for large events in systems such as these. This method counts the number of small events since the last large event and then converts this count into a probability by using a Weibull probability law. We applied this method to the calculation of large earthquake probabilities in California-Nevada, USA. In that study, we considered a fixed geographic region and assumed that all earthquakes within that region, large magnitudes as well as small, were perfectly correlated. In the present article, we extend this model to systems in which the events have a finite correlation length. We modify our previous results by employing the correlation function for near mean field systems having long-range interactions, an example of which is earthquakes and elastic interactions. We then construct an application of the method and show examples of computed earthquake probabilities.     

Effects of urbanization and stormwater control measures on streamflows in the vicinity of Clarksburg,Maryland, USA

Understanding the efficacy of revised watershed management methods is important to mitigating the impacts of urbanization on streamflow. We evaluated the influence of land use change, primarily as urbanization, and stormwater control measures on the relationship between precipitation and stream discharge over an 8‐year period for five catchments near Clarksburg, Montgomery County, Maryland, USA. A unit‐hydrograph model based on a temporal transfer function was employed to account for and standardize temporal variation in rainfall pattern, and properly apportion rainfall to streamflow at different time lags. From these lagged relationships, we quantified a correction to the precipitation time series to achieve a hydrograph that showed good agreement between precipitation and discharge records. Positive corrections appeared to include precipitation events that were of limited areal extent and therefore not captured by our rain gages. Negative corrections were analysed for potential causal relationships. We used mixed‐model statistical techniques to isolate different sources of variance as drivers that mediate the rainfall–runoff dynamic before and after management. Seasonal periodicity mediated rainfall–runoff relationships, and land uses (i.e. agriculture, natural lands, wetlands and stormwater control measures) were statistically significant predictors of precipitation apportionment to stream discharge. Our approach is one way to evaluate actual effectiveness of management efforts in the face of complicating circumstances and could be paired with cost data to understand economic efficiency or life cycle aspects of watershed management. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.     

Clearcutting and pine planting effects on nutrient concentrations and export in two mixed use headwater streams: Upper Coastal Plain,Southeastern USA

Timber harvest temporarily increases water yield; however, relationships between hydrologic and nutrient chemistry changes have not been consistent. This study quantified the effects of forest harvesting and site preparation without fertilization and with modern best management practices on nutrient concentrations and yields in small headwater streams of the Southeastern Coastal Plain. We monitored two watershed pairs for 2 years prior to and 1 year following timber harvest and for 2 more years following site preparation and planting. Treatment watersheds were clearcut, and downstream portions of streamside management zones were thinned in Fall 2003. Site preparation (herbicide application and burning) and planting followed a year later. All operations followed 1999 Georgia forestry best management practices. Previously published research revealed a large increase in water yield following harvest. Nutrient concentrations varied significantly within and between monitoring periods, even in reference watersheds. Silvicultural activities had no discernible effect on phosphorus and ammonium concentrations; however, statistically significant increases in nitrate/nitrite (67–340 µg L⁻¹) and total nitrogen concentrations (100–400 µg L⁻¹) in treatment watersheds followed stand re‐establishment. Nutrient yields increased after timber harvest largely as a result of increased water yields, although increased nutrient yields were small relative to inter‐annual and inter‐watershed variability and variability. Annual water yield largely explained the variability in annual nitrogen and phosphorus export from reference and treatment streams (r² values from 0.65 to 0.98). High NO_x concentrations coming from an upstream agricultural area decreased 1600–1800 µg L⁻¹ over several hundred metres in the treatment streams by dilution, uptake or denitrification. Copyright © 2013 John Wiley & Sons, Ltd.     

Ice storms,tree throw,and hillslope sediment transport in northern hardwood forests

In December 2008, 694 trees uprooted within a 108 ha (1·08 km²) watershed in central Massachusetts due to a severe ice storm, resulting in the displacement of ~1300 m³ of root material, unconsolidated sediment, and fractured bedrock. Overall, we find that uprooting and tree throw is often grouped in clusters and cascades; conifers displace more material than deciduous trees; areas with abundant mature hemlock and steep slopes are more susceptible to tree throw, with clusters as dense as 125 per hectare; and failure is predominantly downhill, suggesting that ice storms promote efficient downslope hillslope sediment transport in northern hardwood forests. Combining the recurrence interval of severe storms in New England (20–75 years) with the forest response presented here, we calculate a sediment transport rate of 2–5 × 10^?5 m³ m^?1 a^?1 averaged over the entire watershed. Forest susceptibility to tree throw differed based on location in the watershed; some areas experienced up to ~30× higher than average sediment transport rates, while others experienced no tree throw. Two severe storms following the 2008 ice storm (hurricane in 2011; snow storm in October 2012) did not result in significant tree throw within the study area, highlighting that the coupling of storm severity and forest susceptibility controls the amount of tree throw during a given forest disturbance. In addition to recent tree throw from the 2008 ice storm, widespread pit and mound microtopography in the study area indicates that tree throw is a recurrent process in this landscape. Two factors emerge that will influence future ice storms related hillslope sediment transport in the steep forested hillslopes of New England: regional climate gradients and changing climate determine the size, intensity and recurrence of ice storms; forest management practices and health control the tree age and type. Copyright © 2015 John Wiley & Sons, Ltd.     

From experimental plots to experimental landscapes: topography,erosion and deposition in sub‐humid badlands from Structure‐from‐Motion photogrammetry

In the last decade advances in surveying technology have opened up the possibility of representing topography and monitoring surface changes over experimental plots (<10 m²) in high resolution (~10³ points m^‐1). Yet the representativeness of these small plots is limited. With ‘Structure‐from‐Motion’ (SfM) and ‘Multi‐View Stereo’ (MVS) techniques now becoming part of the geomorphologist's toolkit, there is potential to expand further the scale at which we characterise topography and monitor geomorphic change morphometrically. Moving beyond previous plot‐scale work using Terrestrial Laser Scanning (TLS) surveys, this paper validates robustly a number of SfM‐MVS surveys against total station and extensive TLS data at three nested scales: plots (<30 m²) within a small catchment (4710 m²) within an eroding marl badland landscape (~1 km²). SfM surveys from a number of platforms are evaluated based on: (i) topography; (ii) sub‐grid roughness; and (iii) change‐detection capabilities at an annual scale. Oblique ground‐based images can provide a high‐quality surface equivalent to TLS at the plot scale, but become unreliable over larger areas of complex terrain. Degradation of surface quality with range is observed clearly for SfM models derived from aerial imagery. Recently modelled ‘doming’ effects from the use of vertical imagery are proven empirically as a piloted gyrocopter survey at 50m altitude with convergent off‐nadir imagery provided higher quality data than an Unmanned Aerial Vehicle (UAV) flying at the same height and collecting vertical imagery. For soil erosion monitoring, SfM can provide data comparable with TLS only from small survey ranges (~5 m) and is best limited to survey ranges ~10–20 m. Synthesis of these results with existing validation studies shows a clear degradation of root‐mean squared error (RMSE) with survey range, with a median ratio between RMSE and survey range of 1:639, and highlights the effect of the validation method (e.g. point‐cloud or raster‐based) on the estimated quality. Copyright © 2015 John Wiley & Sons, Ltd.     

Plagioclase in the Skaergaard intrusion. Part 1: Core and rim compositions in the layered series

The anorthite content of plagioclase grains (X_An) in 12 rocks from the layered series of the Skaergaard intrusion has been studied by electron microprobe (typically ∼30 core and ∼70 rim analyses per thin section). Mean core compositions vary continuously from An₆₆ at the base of the layered series (LZa) to An_32–30 at the top. On the other hand, crystal rims are of approximately constant composition (An_{50 ± 1}) from the LZa to the lower Middle Zone (MZ). Above the MZ, core and rim compositions generally overlap. Profiles across individual plagioclase grains from the lower zone show that most crystals have an external zone buffered at X_An ∼50 ± 1. The simplest explanation for these features is that during postcumulus crystallization in the lower zone, interstitial liquids passed through a density maximum. This interpretation is consistent with proposed liquid lines of descent that predict silica enrichment of the liquid associated with the appearance of cumulus magnetite.