L'activité solaire et les hivers extrêmes

Résumé Des intervalles de grande variabilité dans les températures des hivers en Europe occidentale coincident avec une période de l'activité solaire d'environ 80 ou 90 années. Le coefficient de corrélation pour la serie 1831–1937 est +0.74 à Prague, +0.82 à De Bilt.

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Summary Intervals of large variability in the winter temperatures of Western Europe coincide with a period of solar activity of about 80 or 90 years. The correlation coefficient for the year 1831–1937 amounts to +0.74 at Prague, +82 at De Bilt.

     

Participatory slum upgrading as a disjunctive process in Recife,Brazil: Urban coproduction and the absent ground of the city

This article engages with the coproduction of urban space by focusing on a slum upgrading project in Recife, Brazil. It argues that the urban situation is essentially inconsistent, unpredictable and unstable. It documents the history of urban planning in Recife, paying special attention to the coexistence of two different planning traditions, one aimed at what city planners call the informal city, which is participatory, bottom up and democratic, yet susceptible to be corrupted by political clientelism, and another aimed at the formal city, which is ‘strategic’, top down, technocratic and neoliberal. It argues that the informal/formal binary operates as a disjunctive synthesis that separates social actors rather than connecting them and provides the coordinates within which processes of coproduction take place. The disjunctive synthesis renders possible all sorts of fantastic imaginations that both disavow and reveal the missing ground of the city. Community leaders play a central role in the coproduction of urban space and function as the symptom of this absent ground. The article concludes that participatory urban development interventions aiming to curtail the role of community leaders end up as veritable tyrannies of participation, which should be seen as evidence of the disjointed character of planning rather than as forms of effective governmentality.     

Embryo dune development drivers: beach morphology,growing season precipitation,and storms

For development of embryo dunes on the highly dynamic land–sea boundary, summer growth and the absence of winter erosion are essential. Other than that, however, we know little about the specific conditions that favour embryo dune development. This study explores the boundary conditions for early dune development to enable better predictions of natural dune expansion. Using a 30 year time series of aerial photographs of 33 sites along the Dutch coast, we assessed the influence of beach morphology (beach width and tidal range), meteorological conditions (storm characteristics, wind speed, growing season precipitation, and temperature), and sand nourishment on early dune development. We examined the presence and area of embryo dunes in relation to beach width and tidal range, and compared changes in embryo dune area to meteorological conditions and whether sand nourishment had been applied. We found that the presence and area of embryo dunes increased with increasing beach width. Over time, embryo dune area was negatively correlated with storm intensity and frequency. Embryo dune area was positively correlated with precipitation in the growing season and sand nourishment. Embryo dune area increased in periods of low storm frequency and in wet summers, and decreased in periods of high storm frequency or intensity. We conclude that beach morphology is highly influential in determining the potential for new dune development, and wide beaches enable development of larger embryo dune fields. Sand nourishment stimulates dune development by increasing beach width. Finally, weather conditions and non‐interrupted sequences of years without high‐intensity storms determine whether progressive dune development will take place. Copyright © 2017 John Wiley & Sons, Ltd.     

Beach-fill equilibration and dune growth at two large-scale nourishment sites

Natural dune growth after nourishment is often observed, and such growth plays an important role in beach management for coastal communities. Nourishment sand equilibration after construction is another important topic for project planning and design. Large-scale nourishment projects at Nags Head (NC, USA) (completed in 2011) and Bridgehampton–Sagaponack (NY, USA) (completed in 2014) are under comparatively high-wave energy conditions and offer new insight regarding these topics. After nourishment, a natural beach and inshore morphology were produced with high rates of dune growth by eolian transport. At Nags Head, volumetric dune growth averaged 8 m³/m/yr over the first 5 years following project completion, while Bridgehampton–Sagaponack averaged 9 m³/m/yr over the first 3 post-project years. Results are compared with the Bagnold (1941) analytical model predictions of dune growth and are shown to correlate and decay closely with dry-sand beach width as the nourished profile equilibrates. The extra volume and elevation in the dunes have provided a higher level of storm protection and have helped the sites avoid any major damage to oceanfront properties during hurricanes or numerous severe winter storms.     

Saltwater Upconing Due to Cyclic Pumping by Horizontal Wells in Freshwater Lenses

This article deals with the quantification of saltwater upconing below horizontal wells in freshwater lenses using analytical solutions as a computationally fast alternative to numerical simulations. Comparisons between analytical calculations and numerical simulations are presented regarding three aspects: (1) cyclic pumping; (2) dispersion; and (3) finite horizontal wells in a finite domain (a freshwater lens). Various hydrogeological conditions and pumping regimes within a dry half year are considered. The results show that the influence of elastic and phreatic storage (which are not taken into account in the analytical solutions) on the upconing of the interface is minimal. Furthermore, the analytical calculations based on the interface approach compare well with numerical simulations as long as the dimensionless interface upconing is below 1/3, which is in line with previous studies on steady pumping. Superimposing an analytical solution for mixing by dispersion below the well over an analytical solution based on the interface approach is appropriate in case the vertical flow velocity around the interface is nearly constant but should not be used for estimating the salinity of the pumped groundwater. The analytical calculations of interface upconing below a finite horizontal well compare well with the numerical simulations in case the distance between the horizontal well and the initial interface does not vary significantly along the well and in case the natural fluctuation of the freshwater lens is small. In order to maintain a low level of salinity in the well during a dry half year, the dimensionless analytically calculated interface upconing should stay below 0.25.     

Coastal flood risks and seasonal tourism: analysing the effects of tourism dynamics on casualty calculations

Since coastal tourism is one of the fastest growing sectors of tourism industry, coastal areas have become increasingly vulnerable in the case of flooding. While in recent years a number of different methods have been put forward to map coastal flood risks, the implications of tourism dynamics for the assessment of human casualties has remained largely overlooked in these models. This chapter examines to what extent the ignorance of (residential) coastal tourism may bias the calculations of human casualties. To this end, a case study has been conducted on the Belgian coast. Both the dynamic nature of coastal tourism and the behaviour of residential tourists in storm surge scenarios are considered. The results of this study show that including tourism dynamics in flood risk management is justified and appropriate, depending on the tourist attractiveness of the flood-prone area and its temporal fluctuations.     

Impact of mega-scale sand extraction on tidal dynamics in semi-enclosed basins: An idealized model study with application to the Southern North Sea

Long-term considerations of repeated and increasing sand extraction on the Netherlands Continental Shelf (North Sea) may lead to the creation of a mega-scale extraction trench in front of the Dutch coast (length hundreds of km, width over 10 km, depth several m). We investigate the impact of such a huge topographic intervention on tidal dynamics, which is a key aspect in hydrodynamics, and indirectly also affecting morphodynamics and ecology.     

Iron-hydroxide clogging of public supply wells receiving artificial recharge: near-well and in-well hydrological and hydrochemical observations

Clogging of water wells by iron-hydroxide incrustations due to mixing of anoxic and oxic groundwater is a common well-ageing problem. The relation between well operation (on and off), the spatial and temporal variations in hydrochemistry outside and inside a supply well, and the distribution of clogging iron-hydroxides were studied in an artificial recharge well field in the Netherlands. Camera inspection, high-resolution multi-level water sampling outside the well and detailed in-well pH/EC/O₂ profiles revealed remarkable patterns. During pumping, the top of the upper well screen abstracted oxic filtrate, although the larger part of the in-well water column was anoxic. The column rapidly turned oxic after shutdown due to a downward short-circuiting of oxic water via the well. Within 15 d it became anoxic due to the slow advance of anoxic lake filtrate created by local changes in flow direction as the neighboring wells continued to pump. Severe clogging occurred where the oxic filtrate entered the well, while half-clogging of the upper well screen occurred due to less inflow of oxic filtrate on the lake side. Transport of iron flocs and bacterial slimes after shutdown seemed to clog the lower part of the well screen. Frequent on/off switching should be avoided in iron-clogged wells.     

Identification of potential sites for aquifer storage and recovery (ASR) in coastal areas using ASR performance estimation methods

Performance of freshwater aquifer storage and recovery (ASR) systems in brackish or saline aquifers is negatively affected by lateral flow, density effects, and/or dispersive mixing, causing ambient groundwater to enter ASR wells during recovery. Two recently published ASR performance estimation methods are applied in a Dutch coastal area, characterized by brackish-to-saline groundwater and locally high lateral-flow velocities. ASR performance of existing systems in the study area show good agreement with the predicted performance using the two methods, provided that local vertical anisotropy ratios are limited (<3). Deviations between actual and predicted ASR performance may originate from simplifications in the conceptual model and uncertainties in the hydrogeological and hydrochemical input. As the estimation methods prove suitable to predict ASR performance, feasibility maps are generated for different scales of ASR to identify favorable ASR sites. Successful small-to-medium-scale ASR varies spatially in the study area, emphasizing the relevance of reliable a priori spatial mapping.