Local topography and erosion rate control regolith thickness along a ridgeline in the Sierra Nevada,California

The ridgelines of mountain ranges are a source of geomorphic information unadulterated by the arrival of sediment from upslope. Studies along ridgecrests, therefore, can help identify and isolate the controls on important regolith properties such as thickness and texture. A 1.5 km section of ridgeline in the Sierra Nevada (CA) with a tenfold decrease in erosion rate (inferred from ridgetop convexity) provided an opportunity to conduct a high‐resolution survey of regolith properties and investigate their controls. We found that regolith along the most quickly eroding section of the ridge was the rockiest and had the lowest clay concentrations. Furthermore, a general increase in regolith thickness with a slowing of erosion rate was accompanied by an increase in biomass, changes in vegetation community, broader ridgeline profiles, and an apparent increase in total available moisture. The greatest source of variation in regolith thickness at the 10–100 m scale, however, was the local topography along the ridgeline, with the deepest regolith in the saddles and the thinnest on the knobs. Because regolith in the saddles had higher surface soil moisture than the knobs, we conclude that the hydrological conditions primarily driven by local topography (i.e. rapid vs. slow drainage and water‐storage potential) provide the fundamental controls on regolith thickness through feedbacks incorporating physical weathering by the biota and chemical weathering. Moreover, because the ridgeline saddles are the uppermost extensions of first‐order valleys, we propose that the fluvial network affects regolith properties in the furthest reaches of the watershed. Copyright © 2015 John Wiley & Sons, Ltd.     

Stable calcium isotopic composition of meteorites and rocky planets

New measurements of mass-dependent calcium isotope effects in meteorites, lunar and terrestrial samples show that Earth, Moon, Mars, and differentiated asteroids (e.g., 4-Vesta and the angrite and aubrite parent bodies) are indistinguishable from primitive ordinary chondritic meteorites at our current analytical resolution (± 0.07‰ SD for the ⁴⁴Ca/⁴⁰Ca ratio). In contrast, enstatite chondritic meteorites are slightly enriched in heavier calcium isotopes (ca. + 0.5‰) and primitive carbonaceous chondritic meteorites are depleted in heavier calcium isotopes (ca. ? 0.5‰). The calcium isotope effects cannot be easily ascribed to evaporation or intraplanetary differentiation processes. The isotopic variations probably survive from the earliest stages of nebular condensation, and indicate that condensation occurred under non-equilibrium (undercooled nebular gas) conditions. Some of this early high-temperature calcium isotope heterogeneity is recorded by refractory inclusions (Niederer and Papanastassiou, 1984) and survived in planetesimals, but virtually none of it survived through terrestrial planet accretion. The new calcium isotope data suggest that ordinary chondrites are representative of the bulk of the refractory materials that formed the terrestrial planets; enstatite and carbonaceous chondrites are not. The enrichment of light calcium isotopes in bulk carbonaceous chondrites implies that their compositions are not fully representative of the solar nebula condensable fraction.     

Measuring streambank erosion due to ground water seepage: correlation to bank pore water pressure,precipitation and stream stage

Limited information exists on one of the mechanisms governing sediment input to streams: streambank erosion by ground water seepage. The objective of this research was to demonstrate the importance of streambank composition and stratigraphy in controlling seepage flow and to quantify correlation of seepage flow/erosion with precipitation, stream stage and soil pore water pressure. The streambank site was located in Northern Mississippi in the Goodwin Creek watershed. Soil samples from layers on the streambank face suggested less than an order of magnitude difference in vertical hydraulic conductivity (K_s) with depth, but differences between lateral K_s of a concretion layer and the vertical K_s of the underlying layers contributed to the propensity for lateral flow. Goodwin Creek seeps were not similar to other seeps reported in the literature, in that eroded sediment originated from layers underneath the primary seepage layer. Subsurface flow and sediment load, quantified using 50 cm wide collection pans, were dependent on the type of seep: intermittent low‐flow (LF) seeps (flow rates typically less than 0·05 L min^?1), persistent high‐flow (HF) seeps (average flow rate of 0·39 L min^?1) and buried seeps, which eroded unconsolidated bank material from previous bank failures. The timing of LF seeps correlated to river stage and precipitation. The HF seeps at Goodwin Creek began after rainfall events resulted in the adjacent streambank reaching near saturation (i.e. soil pore water pressures greater than ?5 kPa). Seep discharge from HF seeps reached a maximum of 1·0 L min^?1 and sediment concentrations commonly approached 100 g L^?1. Buried seeps were intermittent but exhibited the most significant erosion rates (738 g min^?1) and sediment concentrations (989 g L^?1). In cases where perched water table conditions exist and persistent HF seeps occur, seepage erosion and bank collapse of streambank sediment may be significant. Copyright © 2007 John Wiley & Sons, Ltd.     

Determining hydraulic resistance in gravel‐bed rivers from the dynamics of their water surfaces

Traditionally, approaches to account for the effect of the boundary roughness of a gravel‐bed river have used a grain‐size index of the bed surface as a surrogate for hydraulic resistance. The use of a single grain‐size does not take into account the spatial heterogeneity in the bed surface and how this heterogeneity imparts resistance on the flow, nor the way in which this relationship changes with variables such as flow stage. A new technique to remotely quantify hydraulic resistance is proposed. It is based on measuring the dynamics of a river's water surface and relating this to the actual hydraulic resistance created by a rough sediment boundary. The water surface dynamics are measured using a new acoustic technique, grazing angle sound propagation (GRASP). This proposed method to measure hydraulic resistance is based on a greater degree of physical reasoning, and this is discussed in the letter. By measuring acoustically the temporal dynamics of turbulent water surfaces over a water‐worked gravel bed in a laboratory flume, a dependency is demonstrated between the temporal variation in the reflected acoustic pressure and measured hydraulic resistance. It is shown that the standard deviation in acoustic pressure decreases with increasing hydraulic resistance. This is shown to apply for a range of relative submergences and bed slopes that are typical of gravel‐bed rivers. This remote sensing technique is both rapid and inexpensive, and has the potential to be applied to natural river channels and to other environmental turbulent flows, such as overland flows. A whole new class of low‐cost, remote and non‐intrusive instruments could be developed as a result and used in a wide range of hydraulic and hydrological applications. Copyright © 2006 John Wiley & Sons, Ltd.     

A model of inter-annual variability in beach levels

Beach profile data, collected twice per year at 19 stations over a 25 km length of coastline in Tremadoc Bay, have been analysed to quantify the inter-annual variability in beach levels over a 7 year period and the results compared against the output of a numerical model. Using hourly wind data as forcing, the morphological development of northern Tremadoc Bay was simulated by wave, tidal, longshore transport, total transport and bed level change models. The modelling methodology was efficient and innovative, allowing realistic simulations of long duration with a time step of 1 h, hence capturing the high frequency nature of wind events. The model was run for each of the 7 autumn/winter periods (generally November–April) and the modelled net change in beach levels compared with the data from all 19 stations. The model results had reasonable agreement with the beach profile surveys. However, the observed magnitude of bed level change in the bay lagged the model output by 1 year, indicating that sediment processes acting over a larger area are important in a relatively localised study of inter-annual variability.     

Contrasting pyroclast density spectra from subaerial and submarine silicic eruptions in the Kermadec arc: implications for eruption processes and dredge sampling

Pyroclastic deposits from four caldera volcanoes in the Kermadec arc have been sampled from subaerial sections (Raoul and Macauley) and by dredging from the submerged volcano flanks (Macauley, Healy, and the newly discovered Raoul SW). Suites of 16–32?mm sized clasts have been analyzed for density and shape, and larger clasts have been analyzed for major element compositions. Density spectra for subaerial dry-type eruptions on Raoul Island have narrow unimodal distributions peaking at vesicularities of 80–85%, whereas ingress of external water (wet-type eruption) or extended timescales for degassing generate broader distributions, including denser clasts. Submarine-erupted pyroclasts show two different patterns. Healy and Raoul SW dredge samples and Macauley Island subaerial-emplaced samples are dominated by modes at ~80–85%, implying that submarine explosive volcanism at high eruption rates can generate clasts with similar vesicularities to their subaerial counterparts. A minor proportion of Healy and Raoul SW clasts also show a pink oxidation color, suggesting that hot clasts met air despite 0.5 to >1?km of intervening water. In contrast, Macauley dredged samples have a bimodal density spectrum dominated by clasts formed in a submarine-eruptive style that is not highly explosive. Macauley dredged pyroclasts are also the mixed products of multiple eruptions, as shown by pumice major-element chemistry, and the sea-floor deposits reflect complex volcanic and sedimentation histories. The Kermadec calderas are composite features, and wide dispersal of pumice does not require large single eruptions. When coupled with chemical constraints and textural observations, density spectra are useful for interpreting both eruptive style and the diversity of samples collected from the submarine environment.     

Estimation and propagation of volcanic source parameter uncertainty in an ash transport and dispersal model: application to the Eyjafjallajokull plume of 14–16 April 2010

Data on source conditions for the 14 April 2010 paroxysmal phase of the Eyjafjallaj?kull eruption, Iceland, have been used as inputs to a trajectory-based eruption column model, bent. This model has in turn been adapted to generate output suitable as input to the volcanic ash transport and dispersal model, puff, which was used to propagate the paroxysmal ash cloud toward and over Europe over the following days. Some of the source parameters, specifically vent radius, vent source velocity, mean grain size of ejecta, and standard deviation of ejecta grain size have been assigned probability distributions based on our lack of knowledge of exact conditions at the source. These probability distributions for the input variables have been sampled in a Monte Carlo fashion using a technique that yields what we herein call the polynomial chaos quadrature weighted estimate (PCQWE) of output parameters from the ash transport and dispersal model. The advantage of PCQWE over Monte Carlo is that since it intelligently samples the input parameter space, fewer model runs are needed to yield estimates of moments and probabilities for the output variables. At each of these sample points for the input variables, a model run is performed. Output moments and probabilities are then computed by properly summing the weighted values of the output parameters of interest. Use of a computational eruption column model coupled with known weather conditions as given by radiosonde data gathered near the vent allows us to estimate that initial mass eruption rate on 14 April 2010 may have been as high as 10⁸?kg/s and was almost certainly above 10⁷?kg/s. This estimate is consistent with the probabilistic envelope computed by PCQWE for the downwind plume. The results furthermore show that statistical moments and probabilities can be computed in a reasonable time by using 9⁴?=?6,561 PCQWE model runs as opposed to millions of model runs that might be required by standard Monte Carlo techniques. The output mean ash cloud height plus three standard deviations??encompassing c. 99.7?% of the probability mass??compares well with four-dimensional ash cloud position as retrieved from Meteosat-9 SEVIRI data for 16 April 2010 as the ash cloud drifted over north-central Europe. Finally, the ability to compute statistical moments and probabilities may allow for the better separation of science and decision-making, by making it possible for scientists to better focus on error reduction and decision makers to focus on ??drawing the line?? for risk assessment.     

Apparent dynamic stability of the southeast African coast despite sea level rise

The coast of southeast Africa is dominated by sandy beaches that tend to be confined within log‐spiral or headland‐bound embayments. Investigations using serendipitous air imagery data set have been previously undertaken and conclusions drawn about the stability of the coast. We show that conclusions drawn from this data, with respect to the high water mark (HWM) position are fraught with errors, which include tidal state, pressure regime, beach slope, high‐swell erosion, seasonal and multi‐annual changes. We highlight and discuss these sources of error, together with their magnitudes. The most significant of these are the high‐swell, seasonal and multi‐annual variations. From case studies we show that the seasonal beach rotation and long‐term beach width variation are responsible for tens of metres of unaccounted HWM variation, 30 to 50 m is common, with maximums reaching 60 to 100 m. Overall the southeast African coastline appears to be in a state of long‐term dynamic equilibrium. There is no evidence of any sea‐level rise‐forced transgression in the coastal sediment budget, despite sea‐level rise (SLR). If such a signal is, in fact present, it is lost within the beach width variation. Some southeast African coastal reaches are suffering chronic erosion, but these are related to anthropogenic impacts. The extreme difficulty of placing a HWM, with any temporal validity on this coast precludes the routine use of the Bruun Rule. Although no transgressive signature is found, there is evidence of a decreasing coastal sand budget as a result of anthropogenic or natural climate change, or both. This decrease in the coastal sand volume is likely to result in increased future erosion. Copyright © 2016 John Wiley & Sons, Ltd.     

Comparing expert judgement and numerical criteria for hydrograph evaluation

Abstract

This paper investigates the relationship between expert judgement and numerical criteria when evaluating hydrological model performance by comparing simulated and observed hydrographs. Using a web-based survey, we collected the visual evaluations of 150 experts on a set of high- and low-flow hydrographs. We then compared these answers with results from 60 numerical criteria. Agreement between experts was found to be more frequent in absolute terms (when rating models) than in relative terms (when comparing models), and better for high flows than for low flows. When comparing the set of 150 expert judgements with numerical criteria, we found that most expert judgements were loosely correlated with a numerical criterion, and that the criterion that best reflects expert judgement varies from expert to expert. Overall, we identified two groups of 10 criteria yielding an equivalent match with the expertise of the 150 participants in low and high flows, respectively. A single criterion common to both groups (the Hydrograph Matching Algorithm with mean absolute error) may represent a good indicator for the overall evaluation of models based on hydrographs. We conclude that none of the numerical criteria examined here can fully replace expert judgement when rating hydrographs, and that both relative and absolute evaluations should be based on the judgement of multiple experts.

Editor D. Koutsoyiannis     

Influence of grass soil cover on water runoff and soil detachment under rainfall simulation in a sub‐humid South African degraded rangeland

In most regions of the world overgrazing plays a major role in land degradation and thus creates a major threat to natural ecosystems. Several feedbacks exist between overgrazing, vegetation, soil infiltration by water and soil erosion that need to be better understood. In this study of a sub‐humid overgrazed rangeland in South Africa, the main objective was to evaluate the impact of grass cover on soil infiltration by water and soil detachment. Artificial rains of 30 and 60 mm h^?1 were applied for 30 min on 1 m² micro‐plots showing similar sandy‐loam Acrisols with different proportions of soil surface coverage by grass (Class A: 75–100%; B: 75–50%; C: 50–25%; D: 25–5%; E: 5–0% with an outcropping A horizon; F: 0% with an outcropping B horizon) to evaluate pre‐runoff rainfall (Pr), steady state water infiltration (I), sediment concentration (SC) and soil losses (SL). Whatever the class of vegetal cover and the rainfall intensity, with the exception of two plots probably affected by biological activity, I decreased regularly to a steady rate <2 mm h^?1 after 15 min rain. There was no significant correlation between I and Pr with vegetal cover. The average SC computed from the two rains increased from 0·16 g L^?1 (class A) to 48·5 g L^?1 (class F) while SL was varied between 4 g m^?2 h^?1 for A and 1883 g m^?2 h^?1 for F. SL increased significantly with decreasing vegetal cover with an exponential increase while the removal of the A horizon increased SC and SL by a factor of 4. The results support the belief that soil vegetation cover and overgrazing plays a major role in soil infiltration by water but also suggest that the interrill erosion process is self‐increasing. Abandoned cultivated lands and animal preferred pathways are more vulnerable to erosive processes than simply overgrazed rangelands. Copyright © 2011 John Wiley & Sons, Ltd.