Assessing “Urban Karst” Effects from Groundwater–Storm Sewer System Interaction in a Till Aquitard

Storm sewer systems and their associated utility trenches may strongly influence the effects of urbanization on a groundwater system. This study was undertaken to identify the causes of district-wide basement infiltration in an aquitard system. It comprised widespread continuous monitoring of utility trench wells and dye tracing from storm sewer system exfiltration tests. The results indicate that a major effect of urbanization on shallow groundwater is related to storm sewer system exfiltration, which is marked by a characteristic pattern of head variations in the aquitard unrelated to distributed surface infiltration. The aquitard constrains flow from storm sewer system exfiltration to the utility trench, creating an urban flow path for groundwater discharge. Temporary buildup of water levels in the utility trench drives relatively high-velocity flow through the permeable sewer bedding material of the utility trench to a separate foundation drainage collector system, ultimately causing a severe “urban karst” effect that produces system surcharging and widespread basement water infiltration. The main conditions causing the “urban karst” are the large hydraulic conductivity ratio between the utility trench material and the aquitard, and the shallow depth and low gradient of the storm sewer system imposed by a very flat drainage basin.     

Linking the recruitment and survivorship of a freshwater stream-specialist fish species to flow metrics in Mediterranean climate temporary streams

Novel modelling was utilised in the present study to reveal significant relationships between the abundance of the Australian freshwater stream-specialist fish Galaxias olidus and metrics defining flow regimes across a region dominated by temporary streams. It was revealed that increases in total abundance were linked to metrics (both 1- and 3-year periods) that indicate greater water availability and the persistence of water in pools across the year, namely the average duration of zero-flow days over the low-flow season (negatively) and total duration of bankfull flows across the year (positively). The analysis identified 3-year metrics as being more important to the abundance of 0+ fish rather than annual ones. Taken together, these findings describing the flow requirements of a stream specialist will help to guide implementation of environmental flows, but will also highlight the need for continued exploration of flow–ecology relationships.     

Assessing the significance of wet-canopy evaporation from forests during extreme rainfall events for flood mitigation in mountainous regions of the United Kingdom

There is increased interest in the potential of tree planting to help mitigate flooding using nature-based solutions or natural flood management. However, many publications based upon catchment studies conclude that, as flood magnitude increases, benefit from forest cover declines and is insignificant for extreme flood events. These conclusions conflict with estimates of evaporation loss from forest plot observations of gross rainfall, through fall and stem flow. This study explores data from existing studies to assess the magnitudes of evaporation and attempts to identify the meteorological conditions under which they would be supported. This is achieved using rainfall event data collated from publications and data archives from studies undertaken in temperate environments around the world. The meteorological conditions required to drive the observed evaporation losses are explored theoretically using the Penman–Monteith equation. The results of this theoretical analysis are compared with the prevailing meteorological conditions during large and extreme rainfall events in mountainous regions of the United Kingdom to assess the likely significance of wet canopy evaporation loss. The collated dataset showed that event Ewc losses between approximately 2 and 38% of gross rainfall (1.5 to 39.4 mm day⁻¹) have been observed during large rainfall events (up to 118 mm day⁻¹) and that there are few data for extreme events (>150 mm day⁻¹). Event data greater than 150 mm (reported separately) included similarly high percentage evaporation losses. Theoretical estimates of wet-canopy evaporation indicated that, to reproduce the losses towards the high end of these observations, relative humidity and the aerodynamic resistance for vapour transport needed to be lower than approximately 97.5% and 0.5 to 2 s m⁻¹ respectively. Surface meteorological data during large and extreme rainfall events in the United Kingdom suggest that conditions favourable for high wet-canopy evaporation are not uncommon and indicate that significant evaporation losses during large and extreme events are possible but not for all events and not at all locations. Thus the disparity with the results from catchment studies remains.     

Improving sub-salt imaging using 3D basin model derived velocities

The quality of depth imaging is directly related to the accuracy of the underlying velocity model. In most sub-salt settings, lack of angular illumination severely degrades the resolution and accuracy of velocity information derived from the seismic data itself. A standard approach for building a starting velocity model uses more reliable velocity information outboard of salt which is subsequently extrapolated to populate the sub-salt regions. The shortcoming of this method lies in the assumption that the effective stress observed outboard of salt can be extrapolated beneath salt solely as a function of depth below mudline.     

Geostatistical Simulation of Acoustic Log Data for Seismic Depth Conversion

Seismic reflection methods measure the time a seismic wave takes to travel through the ground, from the user defined source to a series of signal monitoring sensors known as geophones. The measured times need to be depth converted to allow for integration with other geological data. In order to convert from time to depth, an estimate of the rock volume velocity field must be made. The velocity field estimate can be made by assignment of velocity estimates to a geological model independent of the seismic processing. This article presents the results of using the acoustic geophysical log data extrapolated via sequential Gaussian simulation to derive the velocity field. The uncertainties associated with the velocity estimates were significant and provided the means to assess confidence limits for the actual depth determination. The technique is assessed by application to a major coal deposit, approximately 2.1 m thick and 210 m deep. Considering only the uncertainty associated with estimating the velocity field, half of the confidence interval values showed approximately 1 m of uncertainty in depth. The application of sequential Gaussian simulation to model the 3D distribution of acoustic velocity can be extended to other geophysical log parameters or derived estimates.     

Calculation of magnetic anomalies caused by 2D bodies of arbitrary shape with consideration of demagnetization

Forward calculations of magnetic anomalies caused by two-dimensional bodies of any shape and magnetic properties may be performed either without considering demagnetization as in the equivalent source technique or taking demagnetization into account as in the volume integral equation (VIE) approach, in which, for this purpose, magnetized bodies are divided into a set of rectangular prismatic cells. Ignoring demagnetization may result in distortion of the shape and the amplitude of an anomaly, whereas rectangular cells may not be an optimal representation of the source. Moreover, an inaccurate form approximation in the VIE technique may lead to inconsistent results in the near-body region. In this paper, a method is proposed, based on the VIE approach but differing by applying triangular elementary cells. The method largely overcomes the above-mentioned limitations of the VIE technique. It allows us to delineate large and complex structures exactly and only requires the source to be divided into a few elementary cells to take demagnetization into account satisfactorily. These improvements have been attained through analytical calculation of the Green's function in the complex plane, using the theory of the Cauchy-type integral. Comparing numerical solutions with analytical solutions for homogeneous elliptic cylinders without remanence, the method is found to be consistent with the theory in the range of relative magnetic permeability of 2–20, not only far from but also at subcell distances from the body. The method is appropriate for modelling highly and inhomogeneously magnetized 2D bodies of any shape. It may be of value in interpreting underground measurements or topographic effects, as well as in modelling regional geomagnetic profiles, and it is also a convenient tool for testing questionable geological hypotheses. In the framework of the method, the gravitational anomaly for the same causative bodies can be easily calculated. However, at higher and geologically uncommon values of relative magnetic permeability, the algorithm may become unstable but may be stabilized with SVD regularization. The fact that discrepancies were found with the method employed is a basis for further research.     

Geoarchaeological evidence for dredging in Tyre's ancient harbour, Levant

Chrono-stratigraphic data from Tyre's ancient northern harbour delineate extensive dredging practices during the Greco-Roman and Byzantine periods. Radiocarbon dates from four cores consistently cluster between ca. 500 B.C. and 1000 A.D. and indicate rapid rates of sedimentation in the basin, namely ∼10 mm/yr during the Greco-Roman and Byzantine periods, compared to 0.5-1 mm/yr for the period 6000-4000 B.C. Absence of strata between 4000 B.C. and 500 B.C. is not consistent with a natural base-level sediment sink and cannot be interpreted as a depositional hiatus in the high-stand systems tract. Ancient dredging is further corroborated by persistent age-depth inversions within the fine-grained harbour facies. These data support removal of Middle Bronze Age to Persian period sediment strata, with deliberate overdeepening of the harbour bottom by Greco-Roman and Byzantine societies.     

Situating local experience of risk: Peripherality, marginality and place identity in the UK foot and mouth disease crisis

Research into exposure to, and experience of, environmental risk that has an explicitly spatial focus can be broadly differentiated into two strands. The first strand focuses on the responses of communities of exposure (or the threat of exposure) to some form of environmental hazard and to the policies put in place by institutional actors to manage the hazard. The second strand addresses social inequalities in exposure to environmental hazards and seeks to correlate uneven spatial distributions of risk across different social groups. It is argued that both strands are limited by their respective understandings of space - and that the way in which vulnerable communities experience environmental risk and its management will be shaped significantly by the complex interactions of different spatialisations or constructions of space. We explore this process by examining accounts of local experience of the UK’s 2001 foot and mouth disease crisis and its management in terms of the interplay of two different spatialisations: socio-cultural marginality and political-economic peripherality. We trace the relationship between these cultural and political-economic spatialisations through an analysis of the discursive mobilisation of contrasting place rhetorics. We conclude that focusing on these rhetorics can enhance our understanding of the spatial processes which are constitutive of place identity and in turn mediate the experience of environmental risk and its management.     

Utility of the Hadley Centre sea ice and sea surface temperature data set (HadISST1) in two widely contrasting coral reef areas

The HadISST1 sea surface temperature data set is examined for two contrasting areas: the Chagos Archipelago, central Indian Ocean which has a small (approximately 3 degrees C) annual temperature fluctuation, and Abu Dhabi in the southern Arabian Gulf whose annual air temperature fluctuation of approximately 24 degrees C is the largest known for coral reef habitats. The HadISST1 data are shown to match air temperature records closely, both in terms of annual moving averages and residual analysis. Temperatures in 1998 caused massive mortality of corals in the Indian Ocean: sea surface temperature (SST) values causing this were 33.8 degrees C in the Arabian Gulf at a time when average daily air temperature was over 40 degrees C, while in Chagos the SST lethal to corals was 29.8-29.9 degrees C, when air temperatures peaked at about 31 degrees C. The HadISST1 record was searched back to 1870 for previous abnormal peaks: one of 29.7 degrees C was found for Chagos SST in 1972, though this did not cause coral mortality. Analysis of 12-month running means of the residuals from the annual cycle show that, between 1870 and 1999, the largest SST deviations occurred between October 1997 and May 1998 in Chagos and between August 1998 and July 1999 near Abu Dhabi. The event of 1998-1999 was the largest in these regions for at least 130 years. SSTs have risen over the last three decades at rates of about 0.22 degrees or 0.23 degrees per decade in both locations.