Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Satellite images give a better picture of sediment transport

05.04.2005


Erosion and human activities are inducing large amounts of terrigenous sediment input to the southwest lagoon of New Caledonia. Such deposits can pose a threat to the lagoon’s ecological balance and biological richness. Scientists from the IRD’s Noumea centre have for several years been applying modelling techniques in order to unravel the system of current circulation and sediment transport (1). Satellite remote sensing provides reliable quantified data on the concentration of suspended matter in water. The researchers compared the data transmitted by the Landsat satellite with figures from in situ measurements made in the lagoon. The results show the possibility of making a detailed map of the water turbidity using optical remote sensing, with uncertainty rate less than 20%. The satellite images therefore provide key data for calibration, refining and validation of the numerical sediment-transport models elaborated. This dual approach furthermore arrives at a spatial distribution of the potential for sea-floor sediments to return into suspension. This advance opens up new prospects for research.



The lagoon which surrounds New Caledonia is known for its high biodiversity and for the fragility of its coral reefs. What effects do human activities, stemming mainly from urbanization and the mining industry, coupled with sediment input from rivers resulting from natural soil erosion, have on the lagoon ecosystems ? Scientists from the Camélia Unit of IRD’s centre at Noumea took on this question. Their work is focused mainly on the southwest lagoon, off the capital Noumea, where half the country’s population live. Investigations involve several aspects, including notably the accumulation of metals in marine organisms, currents responsible for dispersing materials brought by the rivers, and also quality analysis on the sea water (dissolved elements, suspensions). Turbidity is a factor that limits the development of corals because it reduces light penetration into the water. Moreover, the fine suspended particles carry in metals that can pollute aquatic ecosystems.

In research on particular environments, numerical models can give an accurate picture of events and situations on a range of scales and offer predictions of their impact. The effectiveness of modelling requires validation of the method’s results, by means of comparison against sets of data determined in situ. It is also possible to compare, at the ocean surface, digital simulations with satellite images. Combination of these data and in-situ measurements with model-derived information results in a three-dimensional picture and description of the processes under investigation.


Such a procedure was adopted to study the sediment transport regime operating in the southwest lagoon of New Caledonia. The physics of sediment transport calls for hydrodynamics methods. Researchers use a numerical model that reproduces the currents and material transport that occur, taking into account the constraints exerted by the wind field and tidal effects (1). The model can determine the erosion fluxes and pinpoint the areas of particle deposition. Validation and refinement of the models are achieved by means of in situ measurement of turbidity, and of sediments, in order to determine in particular the degree of heterogeneity of the sea floor characteristics: white or grey sand, with or without marine plant cover, sandy-clay and silty floor, presence of a river mouth, and so on. As New Caledonia’s lagoon covers an area of about 2000 km2, calibration of a numerical model solely on the basis of in situ measurements would be a difficult and complex operation.

The scientists of IRD’s Camélia Research Unit recently showed that satellite images could help considerably in modelling sediment transport. Effectively, the water turbidity results from the presence of fine suspended particles, therefore variations in the type and concentration of these particles alter the optical properties of marine waters. Remote sensing, which shows up colour variations of sea water, can therefore be used to draw up a map of this turbidity. This map, put together using images transmitted by the Landsat satellite, was studied alongside quantified data derived from in situ measurements, and against figures gleaned from the numerical model. Estimation of concentrations of suspended matter by remote sensing proved to be reliable, with a low average uncertainty rate of 17.5 %. However, satellite images are accessible only in clear weather without strong wind (i.e. at < 10 m/s wind speed or < 20 knots), which would generate too much foam and make it difficult to determine the optical properties of the waters. Remote sensing is nevertheless a powerful tool, which when coupled with numerical models provides a means of indirectly determining the spatial distribution of the erosion rate over the whole of the southwest lagoon. The data provided thus help to refine and validate the numerical models, the only type capable of rendering three-dimensional information on the dynamics and movement of sedimentary particles with temporal predictions.

Furthermore, the combined use of remote sensing and numerical models opens up new prospects for research. Seeing that it is possible to plot the spatial distribution of the sea-floor erosion rate, future research will attempt to analyse this parameter taking into consideration the origin of the sediment particles that have been put back into suspension. These can be the result of soil erosion (terrigenous origin) and subsequent transport by rivers. In this case, they are more easily put back into suspension in the lagoon. Other sedimentary particles can come from the activities of marine organisms (biogenic origin). These muds of marine origin, more compact, are more difficult to put into suspension. Accurate indications of the sedimentary particles’ biogenic or terrigenous nature should therefore lead to improve further the precision of predictions provided by the models.

(1) see the Scientific news sheet n°179 of July 2003, “New Caledonia lagoon: sediment transport tracking and prediction”, which can be viewed on: http://www.ird.fr/fr/actualites/fiches/2003/fiche179.htm

Mina Vilayleck – DIC

Translation : Nicholas Flay

Hélène Deval | EurekAlert!
Further information:
http://www.ird.fr

More articles from Ecology, The Environment and Conservation:

nachricht Upcycling of PET Bottles: New Ideas for Resource Cycles in Germany
25.06.2018 | Fraunhofer-Institut für Betriebsfestigkeit und Systemzuverlässigkeit LBF

nachricht Dry landscapes can increase disease transmission
20.06.2018 | Forschungsverbund Berlin e.V.

All articles from Ecology, The Environment and Conservation >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: First evidence on the source of extragalactic particles

For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.

To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...

Im Focus: Magnetic vortices: Two independent magnetic skyrmion phases discovered in a single material

For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.

Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...

Im Focus: Breaking the bond: To take part or not?

Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.

A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...

Im Focus: New 2D Spectroscopy Methods

Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.

"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....

Im Focus: Chemical reactions in the light of ultrashort X-ray pulses from free-electron lasers

Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.

Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Leading experts in Diabetes, Metabolism and Biomedical Engineering discuss Precision Medicine

13.07.2018 | Event News

Conference on Laser Polishing – LaP: Fine Tuning for Surfaces

12.07.2018 | Event News

11th European Wood-based Panel Symposium 2018: Meeting point for the wood-based materials industry

03.07.2018 | Event News

 
Latest News

Metal too 'gummy' to cut? Draw on it with a Sharpie or glue stick, science says

19.07.2018 | Materials Sciences

NSF-supported researchers to present new results on hurricanes and other extreme events

19.07.2018 | Earth Sciences

Scientists uncover the role of a protein in production & survival of myelin-forming cells

19.07.2018 | Life Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>