Anisotropy and depth‐related heterogeneity of hydraulic conductivity in a bog peat. I: laboratory measurements

Anisotropy and heterogeneity of hydraulic conductivity (K) are suspected of greatly affecting rates and patterns of ground‐water seepage in peats. A new laboratory method, termed here the modified cube method, was used to measure horizontal and vertical hydraulic conductivity (K_h and K_v) of 400 samples of bog peat. The new method avoids many of the problems associated with existing field and laboratory methods, and is shown to give relatively precise measurements of K. In the majority of samples tested, K_h was much greater than K_v, indicating that the bog peat was strongly anisotropic. Log₁₀K_h, log₁₀K_v, and log₁₀ (K_h/K_v) were found to vary significantly with depth, although none of the relationships was simple. We comment on the scale dependency of our measurements. Copyright © 2002 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     

Simulating damage evolution and fracture propagation in sandstone containing a preexisting 3-D surface flaw under uniaxial compression

Preexisting flaws and rock heterogeneity have important ramifications on the process of rock fracturing and on rock stability in many applications. Therefore, there is great interest in numerical modelling of rock fracture and the underlying mechanisms. We simulated damage evolution and fracture propagation in sandstone specimens containing a preexisting 3-D surface flaw under uniaxial compression. We applied the linear elastic damage model based on the unified strength theory following the rock failure process analysis code. However, in contrast to the rock failure process analysis code, we used the finite element method with tetrahedron elements on unstructured meshes. It provided higher geometrical flexibility and allowed for a more accurate representation of the disk-shaped flaw with various flaw depths, angles, and lengths through locally adapted meshes. The rock heterogeneity was modelled by sampling the initial local Young's modulus from a Weibull distribution over a cubic grid. The values were then interpolated to the computational finite element method mesh. This method introduced an additional length scale for the rock heterogeneity represented by the cell size in the sampling grid. The generation of three typical surface cracking patterns, called wing cracks, anti-wing cracks, and far-field cracks, were identified in the simulation results. These depend on the geometry of the preexisting surface flaw. The simulated fracture propagation, coalescence types, and failure modes for the specimens with preexisting surface flaw show good agreement with recent experimental studies.     

To what extent have changes in channel capacity contributed to flood hazard trends in England and Wales?

The frequency of floods has been projected to increase across Europe in the coming decades due to extreme weather events. However, our understanding of how flood frequency is affected by geomorphic changes in river channel capacity remains limited. This paper seeks to quantify the influence of trends in channel capacity on flood hazards. Measuring and predicting the effect of geomorphic changes on freshwater flooding is essential to mitigate the potential effects of major floods through informed planning and response. Hydrometric records from 41 stream gauging stations were used to measure trends in the flood stage (i.e. water surface elevation) frequency above the 1% annual exceedance threshold. The hydrologic and geomorphic components of flood hazard were quantified separately to determine their contribution to the total trend in flood stage frequency. Trends in cross‐sectional flow area and mean flow velocity were also investigated at the same flood stage threshold. Results showed that a 10% decrease (or increase) in the channel capacity would result in an increase (or decrease) in the flood frequency of approximately 1.5 days per year on average across these 41 sites. Widespread increases in the flood hazard frequency were amplified through both hydrologic and geomorphic effects. These findings suggest that overlooking the potential influence of changing channel capacity on flooding may be hazardous. Better understanding and quantifying the influence of geomorphic trends on flood hazard will provide key insight for managers and engineers into the driving mechanisms of fluvial flooding over relatively short timescales. Copyright © 2016 John Wiley & Sons, Ltd.     

Clay slides in the Målselv valley, northern Norway: Characteristics, occurrence, and triggering mechanisms

The distribution of a large number of clay slides in the Målselv valley, northern Norway, is analysed and put into context with the stratigraphic organization of the valley-fill sediments. About 32% of the landslides larger than 10⁴ m³ occur close to the valley margins, where mud is either exposed or at shallow depth. About 57% of the landslides occur mid-valley, where relatively thin (< 15 m) coarse-grained deltaic sediments overlay fine-grained marine and glaciomarine sediments, and about 11% of the landslides occur in front of ice-contact deposits. The slide-prone areas are all characterized by the occurrence of heterogeneous sediments (interbedded clay, silt and sand), in addition to the presence of steep slopes eroded by rivers. The heterogeneous nature of the sediments probably enhanced groundwater drainage and leaching of salts from the clay, increasing sensitivity. Thus, the distribution and organization of the valley-fill sediments and groundwater drainage probably controlled the position of the slide scars and sliding planes. Since deglaciation of the valley (11,500 BP–present), isostatic rebound has enhanced fluvial incision and the creation of steep slopes due to a fall in relative sea level of up to 80 m.Arcuate-shaped, ‘bottleneck’ landslide scars ranging from c. 10⁴ to 10⁷ m³ in size is the dominant morphological signature of the slides, typical for quick clay slides or earth flows involving fluid mud. Their most common triggering mechanism is probably erosion at the toe slopes by the river Målselv or its tributaries. River erosion close to the valley margin, where glaciomarine and marine sediments are present, seems to give the most severe slides. The overall valley-fill organization controls the distribution of clay slides, which may apply to other fjord valleys having similar sediment distribution.     

Snow avalanche regime and climatic conditions in the Chic-Choc Range, eastern Canada

Because snow avalanches occur at altitudes close to the 0°C isotherm in mountain environments, they should respond quickly to climatic variations. This study provides tree-ring-based high-magnitude avalanche chronologies for 12 subalpine avalanche paths in the Chic-Choc Range of Québec (eastern Canada). For the period covered by the chronologies, i.e., between 1895 and 1999, high-magnitude avalanches occurred with an average return interval of 5.3 years, which represents an average annual probability of 21% for all paths. A regional avalanche activity index (RAAI) was developed to help differentiating widespread regional avalanche activity from avalanche events resulting from local factors. Nineteen years of high-magnitude avalanche occurrence were identified (1898, 1936, 1939, 1941, 1965, 1969, 1970, 1971, 1977, 1981, 1984, 1987, 1988, 1989, 1990, 1991, 1992, 1994 and 1995). Of these, 15 occurred after 1950 and 10 after 1980. Propitious climatic conditions that account for widespread avalanche activity in the study region were identified as follows: (1) above-average total snowfall, (2) high-frequency of snowstorms, (3) major rain events and facet–crust development, (4) sequences of freezing rain and strong winds, and (5) early-season weak layers of faceted crystals and depth hoar. The number of days with air temperature above 0°C has strong implications on the internal structure and stratigraphy of the snowpack, and consequently on avalanche release. Land managers should thus consider more closely the impact of climatic conditions and warming on avalanche activity.     

Automated cleaning and uncertainty attribution of archival bathymetry based on a priori knowledge

Hydrographic offices hold large valuable historic bathymetric data sets, many of which were collected using older generation survey systems that contain little or no metadata and/or uncertainty estimates. These bathymetric data sets generally contain large outlier (errant) data points to clean, yet standard practice does not include rigorous automated procedures for systematic cleaning of these historical data sets and their subsequent conversion into reusable data formats. In this paper, we propose an automated method for this task. We utilize statistically diverse threshold tests, including a robust least trimmed squared method, to clean the data. We use LOESS weighted regression residuals together with a Student-t distribution to attribute uncertainty for each retained sounding; the resulting uncertainty values compare favorably with native estimates of uncertainty from co-located data sets which we use to estimate a point-wise goodness-of-fit measure. Storing a cleansed validated data set augmented with uncertainty in a re-usable format provides the details of this analysis for subsequent users. Our test results indicate that the method significantly improves the quality of the data set while concurrently providing confidence interval estimates and point-wise goodness-of-fit estimates as referenced to current hydrographic practices.     

Ecological implications of possible enteropneust faecal casts in tertiary deposits near castlepoint,New Zealand

Unusual “spiral” faecal casts from mid‐Tertiary beds of the Castlepoint area of North Island, New Zealand, are described and compared with modern enteropneust casts photographed at abyssal depths in the south‐west Pacific. Evidence suggests that the fossil casts originated in relatively shallow depths, and this discrepancy emphasises the caution required in the use of trace fossils for palaeoecological interpretations.