A new approach to rapid,desktop-level,environmental flow assessments for rivers in South Africa

Abstract

An approach is presented for desktop-level environmental flow requirement (EFR) determination that is aligned with the Habitat Flow–Stressor Response (HFSR) method which evolved in South Africa over recent years. The HFSR method integrates hydrological, hydraulic and ecological habitat data, involves ecological and hydraulic specialists and is data-intensive and time-consuming. The revised desktop method integrates hydrological information with estimates of channel hydraulic cross-sectional characteristics to generate habitat-type frequencies under changing flow conditions. This information is used with the expected natural habitat requirements to determine acceptable habitat availability under different levels of ecological protection, which is then used with the hydraulic data to define flow regime characteristics that meet the ecological objectives. The paper describes the model components, discusses the assumptions, data requirements and limitations and presents some example results. The revised desktop approach uses approaches that are aligned with the more complex methods and generates results that are similar.

Editor D. Koutsoyiannis; Guest editor M. Acreman

Citation Hughes, D.A., Desai, A.Y., Birkhead, A.L., and Louw, D., 2014. A new approach to rapid, desktop-level, environmental flow assessments for rivers in South Africa. Hydrological Sciences Journal, 59 (3–4), 673–687.     

Observation of the seasonal evolution of the Yellow Sea Cold Water Mass in 1996–1998

We use the hydrographic data obtained during the joint survey of the Yellow Sea by the First Institute of Oceanography, China and the Korea Ocean Research and Development Institute, Korea, to quantify the spatial structures and temporal evolution of the southern Yellow Sea Cold Water Mass (YSCWM). It is indicated that the southern YSCWM is a water mass that develops in summer and decays in fall. In winter, due to the intrusion of the Yellow Sea Warm Current (YSWC), the central area (approximately between 34°N and 35°N, 122°E and 124°E) of the Yellow Sea is mainly occupied by relatively high temperature water (T>10 °C). By contrast, from early summer to fall, under the seasonal thermocline, the central area of Yellow Sea is occupied by cold water (T<10 °C). In summer, the southern YSCWM has two cold cores. One is formed locally southeast of Shandong Peninsula, and the other one has a tongue-like feature occupying the area approximately between 34°N and 37°N, 123°E and 126°E. The bottom layer temperature anomalies from February to July in the cold tongue region, along with the trajectories of the bottom floaters, suggest that the cold water mass in the northeast region has a displacement from the north to the central area of the Yellow Sea during the summer.     

Flow based oversampling technique for multiscale finite element methods

Oversampling techniques are often used in porous media simulations to achieve high accuracy in multiscale simulations. These methods reduce the effect of artificial boundary conditions that are imposed in computing local quantities, such as upscaled permeabilities or basis functions. In the problems without scale separation and strong non-local effects, the oversampling region is taken to be the entire domain. The basis functions are computed using single-phase flow solutions which are further used in dynamic two-phase simulations. The standard oversampling approaches employ generic global boundary conditions which are not associated with actual flow boundary conditions. In this paper, we propose a flow based oversampling method where the actual two-phase flow boundary conditions are used in constructing oversampling auxiliary functions. Our numerical results show that the flow based oversampling approach is several times more accurate than the standard oversampling method. We provide partial theoretical explanation for these numerical observations.     

Application of implicit sub-time stepping to simulate flow and transport in fractured porous media

In general, the accuracy of numerical simulations is determined by spatial and temporal discretization levels. In fractured porous media, the time step size is a key factor in controlling the solution accuracy for a given spatial discretization. If the time step size is restricted by the relatively rapid responses in the fracture domain to maintain an acceptable level of accuracy in the entire simulation domain, the matrix tends to be temporally over-discretized. Implicit sub-time stepping applies smaller sub-time steps only to the sub-domain where the accuracy requirements are less tolerant and is most suitable for problems where the response is high in only a small portion of the domain, such as within and near the fractures in fractured porous media. It is demonstrated with illustrative examples that implicit sub-time stepping can significantly improve the simulation efficiency with minimal loss in accuracy when simulating flow and transport in fractured porous media. The methodology is successfully applied to density-dependent flow and transport simulations in a Canadian Shield environment, where the flow and transport is dominated by discrete, highly conductive fracture zones.     

Reconstruction and analysis of sub-plinian tephra dispersal during the 1530 A.D. Soufrière (Guadeloupe) eruption: Implications for scenario definition and hazards assessment

The last magmatic eruption of Soufrière of Guadeloupe dated at 1530 A.D. (Soufrière eruption) is characterized by an onset with a partial flank-collapse and emplacement of a debris-avalanche that was followed by a sub-plinian VEI 2–3 explosive short-lived eruption (Phase-1) with a column that reached a height between 9 and 12 km producing about 3.9 × 10⁶ m³ DRE (16.3 × 10⁶ m³ bulk) of juvenile products. The column recurrently collapsed generating scoriaceous pyroclastic flows in radiating valleys up to a distance of 5–6 km with a maximum interpolated bulk deposit volume of 11.7 × 10⁶ m³ (5 × 10⁶ m³ DRE). We have used HAZMAP, a numerical simple first-order model of tephra dispersal [Macedonio, G., Costa, A., Longo, A., 2005. A computer model for volcanic ash fallout and assessment of subsequent hazard. Comput. Geosci. 31, 837–845] to reconstruct to a first approximation the potential dispersal of tephra and associated tephra mass loadings generated by the sub-plinian Phase 1 of the 1530 A.D. eruption. We have tested our model on a deterministic average dry season wind profile that best-fits the available data as well as on a set of randomly selected wind profiles over a 5 year interval that allows the elaboration of probabilistic maps for the exceedance of specific tephra mass load thresholds. Results show that in the hypothesis of a future 1530 A.D. scenario, populated areas to a distance of 3–4 km west–southwest of the vent could be subjected to a static load pressure between 2 and 10 kPa in case of wet tephra, susceptible to cause variable degrees of roof damage. Our results provide volcanological input parameters for scenario and event-tree definition, for assessing volcanic risks and evaluating their impact in case of a future sub-plinian eruption which could affect up to 70 000 people in southern Basse-Terre island and the region. They also provide a framework to aid decision-making concerning land management and development. A sub-plinian eruption is the most likely magmatic scenario in case of a future eruption of this volcano which has shown, since 1992, increasing signs of low-energy seismic, thermal, and acid degassing unrest without significant deformation.     

Hydrochemical dynamics of the “lake–spring” system in the crater of El Chichón volcano (Chiapas,Mexico)

El Chichón volcano (Chiapas, Mexico) erupted violently in March–April 1982, breaching through the former volcano–hydrothermal system. Since then, the 1982 crater has hosted a shallow (1–3.3 m, acidic (pH ∼ 2.2) and warm (∼ 30 °C) crater lake with a strongly varying chemistry (Cl/SO₄ = 0–79 molar ratio). The changes in crater lake chemistry and volume are not systematically related to the seasonal variation of rainfall, but rather to the activity of near-neutral geyser-like springs in the crater (Soap Pool). These Soap Pool springs are the only sources of Cl for the lake. Their geyser-like behaviour with a long-term (months to years) periodicity is due to a specific geometry of the shallow boiling aquifer beneath the lake, which is the remnant of the 1983 Cl-rich (24,000 mg/l) crater lake water. The Soap Pool springs decreased in Cl content over time. The zero-time extrapolation (1982, year of the eruption) approaches the Cl content in the initial crater lake, meanwhile the extrapolation towards the future indicates a zero-Cl content by 2009 ± 1. This particular situation offers the opportunity to calculate mass balance and Cl budget to quantify the lake–spring system in the El Chichón crater. These calculations show that the water balance without the input of SP springs is negative, implying that the lake should disappear during the dry season. The isotopic composition of lake waters (δD and δ¹⁸O) coincide with this crater lake-SP dynamics, reflecting evaporation processes and mixing with SP geyser and meteoric water. Future dome growth, not observed yet in the post-1982 El Chichón crater, may be anticipated by changes in lake chemistry and dynamics.     

Models of sub-grid variability in numerical simulations of solute transport in heterogeneous porous formations: three-dimensional flow and effect of pore-scale dispersion

Modeling transport of contaminants in the earths subsurface relies on numerical solutions over grids with blocks larger than Darcys scale. The hydraulic conductivity is homogenized over the grid blocks and the plumes spreading is reduced as a consequence of the wiped-out variability. To compensate for this loss Rubin et al. (1999) proposed to augment mixing by block-effective dispersion coefficients, and Rubin et al. (2003) showed, by means of two dimensional simulations, how this concept can be applied in practice. In this paper, we present new solutions of the block-effective dispersion tensor for an axisymmetric exponential covariance model. In addition, we discuss the influence of pore-scale dispersion in both two- and three-dimensional applications.     

Estimation of Site Effects in Beijing City

— For the realistic modeling of the seismic ground motion in lateral heterogeneous anelastic media, the database of 3-D geophysical structures for Beijing City has been built up to model the seismic ground motion in the City, caused by the 1976 Tangshan and the 1998 Zhangbei earthquakes. The hybrid method, which combines the modal summation and the finite-difference algorithms, is used in the simulation. The modeling of the seismic ground motion, for both the Tangshan and the Zhangbei earthquakes, shows that the thick Quaternary sedimentary cover amplifies the peak values and increases the duration of the seismic ground motion in the northwestern part of the City. Therefore the thickness of the Quaternary sediments in Beijing City is the key factor controling the local ground effects. Four zones are defined on the base of the different thickness of the Quaternary sediments. The response spectra for each zone are computed, indicating that peak spectral values as high as 0.1 g are compatible with past seismicity and can be well exceeded if an event similar to the 1697 Sanhe-Pinggu occurs.     

Coastal coal pollution increases Cd concentrations in the predatory gastropod Hexaplex trunculus and is detrimental to its health

Parameters of environmental health, including paracellular permeability of external epithelia, functional state of lysosomes and the level of metallothioneins (MTs), were examined using fluorescent markers and vital microfluorometry in different tissues of the marine gastropod, Hexaplex trunculus, from a coal-polluted and coal-free site. Vital microfluorometrical examinations exhibited enhanced paracellular permeability of external epithelia to the anionic marker, fluorescein (FLU), lower lysosomal accumulation of neutral red (NR) as well as higher levels of MTs, when compared with epithelia of gastropods from the coal-free site. Those differences were particularly marked in the foot epithelium, which is in direct contact with the substrate. In addition, cadmium was measured by ICP-AES in the hepatopancreas of gastropods sampled from the coal-polluted site and two coal-free sites. Significantly higher levels of Cd were found in gastropod hepatopancreas from the coal-polluted site. In addition, two months feeding experiments conducted in aquaria containing: (a) coal pieces covered by barnacles; (b) natural rocks covered by barnacles; and (c) natural rocks with barnacles + bare coal pieces, demonstrated significant increase of Cd concentration in the hepatopancreas of the gastropods exposed to coal. We suggest that coal in the marine environment has detrimental effects on marine gastropods, both directly through contact with the organisms and indirectly through the food web.     

Simulation of oasis breeze circulation in the arid region of the Northwestern China

In this paper, the process of oasis-desert circulation (ODC) is simulated by MM5V3.5 model through designing an ideal oasis-desert scheme and assuming that initial atmosphere is at rest (V = 0). The findings showed that the key of forming special oasis boundary structure is the difference of energy and water between oasis and desert. The evaporation of oasis surface consumes heat energy, and the low temperature of oasis causes an oasis breeze circulation (OBC), which drives an ODC with a downdraft over the oasis and an updraft over the desert. Later, the cold, dry and stable boundary over oasis is gradually formed, on the contrary,the atmospheric boundary over desert on the edge of oasis is hot, humid and unstable and its height is about 600 hPa. The updraft over the desert forms a wet ring that acts as a vertical wall weakening the low-level moisture exchange between the oasis and desert. The downdraft of OBC increases the atmospheric stability that reduces the oasis evaporation. The low-level outflow from the oasis (into the desert) prevents the dry, hot air flowing from the desert into the oasis.Thus an oasis self-preservation mechanism may be formed due to OBC. The horizontal area influenced by oasis is twice as oasis area and the vertical range is four times as oasis. The ODC is strong in the daytime and reaches the strongest at 17:00, and the influenced area is the largest at 20:00.