The Utility of Morphometric Parameters Extracted from SAR Radar Images in the Monitoring of the Dynamics of the Danube Island System, Giurgiu-Călăraşi Sector, Romania

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2.2 Geological, geomorphologic and climatic characteristics of the study area

The geological and geomorphological features of the islands are similar to the Danube floodplain and the Romanian plain. They are genetically related and tectonically influenced [3]. The relative altitude of the islands does not exceed or rarely exceed 4 or 5 m compared to the average level of the Danube. Small islands, with short altitudes, are often flooded. Alluvial deposits on their surfaces belong to the Upper Quaternary (Upper and Present Holocene). They are formed of pebbles, sand, loess deposits with varied grain size according to their positions, near the major tributaries of the Danube or the banks. The natural vegetation on the islands is partially replaced. Chernozem soils are found on the terraces and in the plain and alluvial soil are found in the major river bed [4].

This study area is characterized by a very contrasting climate between summer and winter. Its relatively flat topography promotes rapid movement of air masses, very high wind speeds, snowstorms and excessive increases in summer temperatures [5].

As major driver of island dynamics, the sediment load of the Lower Danube River is dominated by coarse sediment in suspension. The sediment load is decreasing along the Lower Danube River especially since the construction of the dams of Iron Gates I and II in 1972 and 1984 [6].

4.1 Morphometric parameters of the islands from 1995 and 2009

Using amplitude radar images, we were able to extract the morphometric parameters such as perimeters (P), areas (S) and UTM coordinates from the center (X, Y) of each island and sub-island at different dates, July and December 1995 and May 2009 (Tables 2, 3 and 4). The sub-islands marked with (*) are not in one of the two dates. The abbreviations Bg and Ro denote Bulgaria and Romania respectively.

Table 2. Morphometric parameters of the islands and sub-islands from the Giurgiu-Călărași sector, of July 6, 1995

Table 3. Morphometric parameters of the islands and sub-islands from the Giurgiu-Călărași sector, of December 28, 1995

Table 4. Morphometric parameters of the islands and sub-islands from the Giurgiu-Călărași sector, of May 28, 2009

4.2 Comparison of the islands system between 1995 and 2009

To observe the evolution of the island system between 1995 and 2009, we made a comparison of the previously extracted parameters by dates. The results obtained are summarized in Table 5. For each island and sub-island, the procedure consists in calculating the average of the subtraction between the same parameter measured at different dates using the following formulas:

 $\Delta X=\frac{\sum\limits_{1}^{n}{\left( {{X}_{f}}-{{X}_{i}} \right)}}{n}$       (1)

$\Delta Y=\frac{\sum_{l}^{n}\left(Y_{f}-Y_{i}\right)}{n}$      (2)

$\Delta P=\frac{\sum_{l}^{n}\left(P_{f}-P_{i}\right)}{n}$       (3)

$\Delta S=\frac{\sum_{l}^{n}\left(S_{f}-S_{i}\right)}{n}$        (4)

where, X and Y: UTM coordinates of the center of the island; P: Perimeter of the island; S: Area of the island; n: number of dates used; f: final date; i: initial date.

Table 5. Shows the names and averages of the parameters variations, ΔP, ΔS, ΔX and ΔY of each island and sub-island

4.3 Evolution of the islands system between 1995 and 2009

Analysis of the parameters extracted from the SAR radar images made it possible to detect and follow the evolution of the Danube island system, sector Giurgiu-Călăraşi, with great precision. The positive values of the calculated averages, the UTM coordinates of the center (ΔX, ΔY), show the very important role of the wind factor or other major phenomena in the displacements of the sediments of the islands and sub-islands, as shown in Figure 7. We estimated an average island system speed of about 1 m/year in the Southeast direction. This displacement is accompanied by the elongation of the islands [7].

The evaluation of the sedimentary accumulations was determined from Figures 8 and 9, showing the morphometric parameter variations according to their measurement dates, and the results of the perimeters and area means (ΔP and ΔS), obtained using formulas (3) and (4).

The speed of the accumulations of the surfaces is given by the mean growth speeds of the islands and sub-islands perimeters and areas. For perimeters, we estimated an average speed of about 18 m/year and for areas of about 5621 m²/year. These results must be distributed over the number of islands and sub-islands listed, which gives average speeds respectively of 0.78 m/year and 244.39 m²/year.

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Figure 7. Map of displacement of the centre of the islands between 1995 and 2009

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Figure 8. Evolution of the perimeter of the islands and sub-islands between 1995 and 2009

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Figure 9. Evolution of the area of the islands and sub-islands between 1995 and 2009

Positive values show an increase in sediment during the 14-year period (1995 and 2009) with an estimated average accumulation of approximately 2972.5 m²/year, a radius of 30.77 m each year. This gives an estimate of an accumulation of 129.24 m²/year and radius 1.34 m/year for each island and sub-island.

On the other hand, the few minimal negative values can be explained by errors due to the treatments carried out and the phenomena of compensations.

Figures 8 and 9 show the evolution of the perimeters and the surfaces of the islands and sub-islands between 1995 and 2009. The higher values in December 1995 when compared to July 1995 are probably due to low water levels of the river in winter.

During floods, rivers in the region carry large quantities of solid materials, causing changes in the transverse and vertical profiles of the canals [8]. As example, if the average annual suspended sediment load of the Vedea River – a tributary of the Danube upstream of the study area – at Alexandria is only 12.6 kg/s, during major floods (1972, 1975, 1995 and 2005) it exceeded 5000 kg/s. Its maximum value was reached during the flood of July 2005, with 5838 kg/s [9]. The flooding of the Danube River on 24 April 2006 resulted in volumes of sediments corresponding to a maximum liquid flow ranging from 16100 to 16500 m³/s between Giurgiu and Călăraşi [10-12].

This evolution corresponds to the general dynamics of the Danube River islands along the studied sector: slight area increase since 1980 [13]. Along the Lower Danube, lateral (side) erosion is still common as many of the banks and islands are unprotected, which determines the formation of sand bars [14]. The process is probably more intense post- flood events.