Satellite images give a better picture of sediment transport

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