Flushing times for the Providence River based on tracer experiments

The flushing time of the Providence River was estimated using three different data sets and three different methodologies. Dye concentrations were measured following instantaneous dye releases during wet weather experiments performed by the Narragansett Bay Project between October 1988 and June 1989. These data were analyzed to obtain flushing time estimates. Salinity measurements collected during the Sinbadd (Sampling In Narragansett Bay All During the Day) cruises, Spray (Sampling the Providence River All Year) cruises and wet weather experiments were used with the fraction of fresh water method and box model to calculate flushing time. The Sinbadd cruises performed 4 seasonal surveys at 22 stations in Narragansett Bay during 1986 to obtain a view of the whole Narragansett Bay with respect to the concentrations of nutrients and trace metals. The Spray cruises collected data in the Providence River at 10 stations to determine the relationship of nutrients and trace metals concentrations in the Seekonk and Providence rivers as a function of point source inputs. Based on the flushing time estimates, an exponential relationship between freshwater inflow and flushing time was developed (correlation coefficient of 0.826). The flushing time ranged from 0.8 d at high (90 m³ m^?1) freshwater inflows to 4.4 d at low (20 m³ s^?1) freshwater inflows. The average flushing time of the Providence River was estimated as 2.5 d for the mean freshwater inflow of 42.3 m³ s^?1.     

GeoWeb and crisis management: issues and perspectives of volunteered geographic information

Mapping, and more generally geopositioning, has become ubiquitous on the Internet. This democratization of geomatics through the GeoWeb results in the emergence of a new form of mapping based on Web 2.0 technologies. Described as Web-mapping 2.0, it is especially characterized by high interactivity and geolocation-based contents generated by users. A series of recent events (hurricanes, earthquakes, pandemics) have urged the development of numerous mapping Web applications intended to provide information to the public, and encourage their contribution to support crisis management. This new way to produce and spread geographic information in times of crisis brings up many questions and new potentials with regard to urgency services, Non Governmental Organisations (NGO), as well as individuals. This paper aims at putting into perspective the development of GeoWeb, both in terms of technologies and applications, against crisis management processes.     

Deep circulation driven by strong vertical mixing in the Timor Basin

The importance of deep mixing in driving the deep part of the overturning circulation has been a long debated question at the global scale. Our observations provide an illustration of this process at the Timor Basin scale of ～1000 km. Long-term averaged moored velocity data at the Timor western sill suggest that a deep circulation is present in the Timor Basin. An inflow transport of ～0.15 Sv is observed between 1600 m and the bottom at 1890 m. Since the basin is closed on its eastern side below 1250 m depth, a return flow must be generated above 1600 m with a ～0.15 Sv outflow. The vertical turbulent diffusivity is inferred from a heat and transport balance at the basin scale and from Thorpe scale analysis. Basin averaged vertical diffusivity is as large as 1 × 10^?3 m² s^?1. Observations are compared with regional low-resolution numerical simulations, and the deep observed circulation is only recovered when a strong vertical diffusivity resulting from the parameterization of internal tidal mixing is considered. Furthermore, the deep vertical mixing appears to be strongly dependent on the choice of the internal tide mixing parameterization and also on the prescribed value of the mixing efficiency.     

Experimental analysis of groundwater flow through a landslide slip surface using natural and artificial water chemical tracers

Artificial and natural tracer tests combined with high accurate electronic distancemeter measurements are conducted on a small landslide with a well known slip surface geometry. Outflow yields and chemical contents are monitored for all the experiment duration and they analyzed to estimate the slip surface hydraulic parameters. The main result is that the slip surface acts as a drain for groundwater flows that evacuates interstitial pressures in the slope and brings the sliding mass to be more stable one. Copyright © 2007 John Wiley & Sons, Ltd.     

Land subsidence due to gas/oil production in inhomogeneous transversally anisotropic half-space by a boundary element method

In a previous paper¹ the authors have developed and implemented a new boundary element (BE) model to simulate and predict land subsidence occurring over three-dimensional gas/oil fields in a homogeneous and isotropic half-space. The approach relies on Betti's reciprocal theorem and makes use of the classical fundamental solution of Boussinesq in the framework of the theory of linear poroelasticity. The BE method is here extended to inhomogeneous, transversally anisotropic soils by the aid of a two-dimensional finite element (FE) model which provides a fundamental numerical solution for the actual multi-layer setting of the subsurface system. The new FE–BE approach is then used to simulate the subsidence caused by gas production over the deep reservoir of Campo Ravenna Terra, Ravenna (Italy) from 1950 to 1980. The results compare very favourably with the outcome from a full more expensive three-dimensional FE model of the same occurrence.     

Seafloor mapping from high-density satellite altimetry

In this paper, we publish the results of a bathymetry survey based on the processing of satellite altimetry data. Data gathered from GEOSAT (Geodetic Mission), SEASAT, ERS-1 and TOPEX/POSEIDON satellites were processed to recover the seafloor topography over new seamounts in a test area located in the south central Pacific. We show that by processing high-density satellite altimetry data, alone or in combination with shiptrack bathymetric data, it is possible to produce full coverage bathymetric maps.     

A boundary element solution to land subsidence above 3-D gas/oil reservoirs

A linear boundary element (BE) model is proposed for the uncoupied simulation of land subsidence due to gas, oil and hot water production over three-dimensional (3-D) arbitrarily shaped reservoirs. The pore pressure decline is assumed to be specified in advance, e.g. via a numerical model of flow. Use is made of the fundamental solution derived in 1885 by Boussinesq for a vertical load acting upon the traction-free surface of a semi-infinite medium. A straightforward application of Betti's (1872) reciprocal theorem allows for the development of a boundary integral whose numerical execution yields directly the downward settlement over the point of interest. The new procedure is applied to assess land sinking caused by an uniform pore pressure decline occurring within fields of elliptical shape and to explore the influence of the assumption of small reservoir thickness which underlies the ‘tension center’ or ‘strain nucleus’ approach previously developed by Geertsma in 1966. The results emphasize the numerical efficiency of the solution and the promising features of the BE method for the evaluation of ground subsidence in 3-D problems. The present model is based on the theory of the linear poroelasticity and is implemented for a mechanically homogeneous and isotropic half-space. It allows for any arbitrary geometry of the reservoir and for a non-uniform distribution of the pore pressure decline. It may easily be extended to other physical settings for which a vertical surface point load solution is available.     

An opportunity to connect the morphodynamics of alternate bars with their sedimentary products

Field investigations that help clarify local sedimentary processes involved in the migration of alternate bars as a consequence of flood events are lacking. A simple approach combining scour chains, stratigraphy and frequent bathymetric surveys is proposed to connect the dynamics of free migrating alternate bars present in disconnected channels of large sandy‐gravelly rivers with their sedimentary products and vice versa. The results show that the spatial distribution of bars before a flood partly governs the scour and fill processes and that the sediment transport rates vary significantly on a single cross‐section. This can be due to preferential axes of the migration of the bars determined by their location on the cross‐section, the bank direction and the discharge. The approach allows the reconstruction of local sedimentary processes involved in alternate bar migration by combining maximum scour depths reached during a flood with frequent channel bed topography surveys and post‐flood stratigraphy. It is also possible to distinguish deposited and preserved sediments compared with sediments by‐passed during the flood. Copyright © 2011 John Wiley & Sons, Ltd.