Image of the Danube Delta, acquired on 28 September 2003 with Envisat's MERIS
Description of study area
With a surface of 580 000 ha, the Danube Delta is the 3rd largest delta of Europe, after the Volga and Kuban deltas. Due to the great biodiversity found there, the Danube Delta was declared a Biosphere Reserve in 1990. It is the only delta in the world with this status, and one of the biggest areas of compact reeds on the planet (ARBDD). The largest part of the Danube Delta is in Romania, the rest is in the Ukraine. Three branches make up the main delta system: Chilia (north), Sulina (centre) and Sf. Gheorghe (south).
A kml file, Danube Delta.kml, with all the points discussed in this case study is found in Vectors on the right of this page.
The first bifurcation of the Danube is at the point called Ceatal Ismail. Here the Danube divides into two branches: Chilia to the North and Tulcea to the South. A second bifurcation takes place at Ceatal Sf. Gheorghe, where Tulcea is divided in two other branches: Sulina and Sf. Gheorghe. The word Ceatal means pitchfork in Turkish. In the process of evolution, the Chilia arm formed two secondary deltas, a third is currently being formed and leads into the Black Sea. At the Sf. Gheorghe mouth is a little secondary delta and a barrier island, called Sacalin Island.
Sf. Gheorghe coastal zone showing different types of environment
A barrier island represents an accumulation of sedimentary material (usually from sand and mud), commonly found at the mouths of rivers. Their elongated shapes often have the appearance of an arc. Their occurrence is closely related to the existence of rich sedimentary input brought to the river mouth. The combined action of waves and currents define the general appearance of these islands.
At a first glance of the above image we can see different types of aquatic environments:
- Black Sea
- The system of delta lakes
- The system of channels cut into the delta (almost straight lines)
Between these are wetland areas with levees (Saraturile strand plain). The most dominant types of vegetation are reeds and trees (poplar, water willow and other sand or salty species). The sand appears very distinctive on the image, having a white colour.
Change detection technique
Change detection is a technique used in remote sensing for estimating the changes that occur in an environment, using two or more scenes over the same geographic area, at different time scales. There are many applications in urban growth, land cover analysis, coastal zone monitoring, forest management, etc. Change detection is a general concept that can be undertaken in different ways: by looking at the differences between images, thematic classification, looking at the ratio of images, change detection using vector layers. The basic principle is that the digital number (DN) of the first image acquired is different from the DN of the second image that was acquired.
The nature of changes to an environment can be interpreted in a spatial and a temporal dimension, using statistical quantification, and a final graphical representation, usually in the form of a map of changes detected. The first step is to obtain the images and to choose the best resolution for your purpose. Another important aspect is to know the land cover associated with your images. Photographs of the terrain can sometimes aid image interpretation.
|Path 181, Row 29||1984-09-07|
|Path 181, Row 29||2000-07-02|
For our study we will use two Landsat images from 1984 (7 September) and 2000 (2 July), downloaded using the Earth Explorer interface from the USGS website. The images are freely available if you have registered an account (username and password required). Another option is the USGS GLOVIS website.
The time between these image acquisitions, 16 years, is enough to understand the evolution of the shoreline. Because the purpose of this tutorial is to analyse the evolution of the shoreline, the difference between the images (July-September) is relevant only in terms of vegetation.