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Analysis of the cause and the mechanism of the flood in the Apollonia (North ) basin on October 2006

Conference Paper · September 2007

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Dimitrios Stathis Panagiotis Stefanidis Aristotle University of Aristotle University of Thessaloniki

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ANALYSIS OF THE CAUSES AND THE MECHANISM OF THE FLOOD IN THE APPOLONIA (NORTH GREECE) BASIN ON 8-9 OCTOBER 2006

Dimitrios J. STATHIS, Panagiotis STEFANIDIS Fani TZIAFTANI Institute of Mountainous Water Management and Control, Dept. of Forestry and Natural Environment, Aristotle University of Thessaloniki, P.O. Box: 268, 54124, Thessaloniki, Greece [email protected]

Abstract The lake located in Prefecture Thessaloniki in (north Greece) and its drainage basin occupies extent of 1177 km2. To the lake lead 29 torrential streams which supply this with water and sedi- ments. Due to the Mediterranean climate of Greece the precipitation regime appears non-uniform distribution in space and in time. The result of the unequal distribution of rainfalls is the occurrence of intense floods but and droughts. Another characteristic in Mediterranean areas is the high intensity of rains. On 8-9 October 2006 ex- cessive rain of high intensity fell over in the area of east Thessaloniki Prefecture that resulted in catastrophic floods. The aim of this paper is to survey the causes and to determine the mechanism of the flood and the ex- treme sedimentation which took place in the watershed of Apollonia torrent. The natural factors of flooding as well as the features of the particular rainfall are examined. Furthermore, man-made interference in the water- sheds and in the beds of the torrential streams is examined. Finally, a general plan is given in order, the area to be able to confront the intense flooding hazard in similar incidents. The plan is based on the effects of the par- ticular flooding incident

Key words: flood, rainfall, Appolonia

INTRODUCTION

In many areas of the Globe floods have last years become more destructive, causing lots material damages as well as thousands fatalities annually. Floods constitute a substantial category of natural disasters originated from weather. In Greece many areas often experience flood phenomena(Stefanidis, P., 1995, Baloutsos et al., 2000, Papamichail et al., 2001). The distribution of precipitation in Greece is uneven in space and in time. This characteristic of Mediterranean climate creates great flood problems not only in regions with high average annual precipitation but in drier areas as is the area of Macedonia. A catastrophic flood oc- curred in the east area of Thessaloniki Prefecture and north Chalkidiki Prefecture. There lo- cated the Lake Volvi which drainage basin occupies extent of 1177 km2. In lake let out 29 torrential streams which supply the lake with water and sediments. Among these torrential is the Appolonia torrent that flooded on 8–9 October 2006. These dates after a rain of high in- tensity in the area flood phenomena occurred. Noteworthy was the catastrophes that hap- pened to the area. The damages conclude houses, roads and bridges, infrastructure works (electricity and water supply), agricultural machines, and agricultural crops. Also many peo- ple have put their lives in great risk. The aim of this paper is to survey the causes and to determine the mechanism of the flood and the extreme sedimentation which took place in the watershed of Apollonia torrent. The natural factors of flooding as well as the features of the particular rainfall are examined. Furthermore, man-made interference in the watersheds and in the beds of the torrential streams is examined. Finally, a general plan is given in order, the area to be able to confront the intense flooding hazard in similar incidents.

1 of 7 International Conference Erosion and Torrent Control as a Factor in Sustainable River Basin Management 25-28 September 2007, Belgrade – Serbia

DESCRIPTION OF THE WATERSHED

The research was conducted in the area of the Municipality of Apollonia, located south- east in the Prefecture of Thessaloniki. It deals with the watershed of the torrent “Apollonia” which occupies surface of 186.56 Km2. More of the drainage area come from the neighbour- hood Prefecture Chalkidiki (see Fig. 1). It is a triangular shape (Figure 2) and its altitude is between 80-1060 m with a mean altitude of 468.40 m and mean slope 27.6%. The drainage network is of the dendritic form. The mean annual rainfall of the area is 597 mm.

Figure 1. Location of the research area

Figure 2. The watershed and the hydrological network of the torrent “Apollonia

In Table 1 and the figure 3 (left), the geological composition of the watershed is given. From that Table it can be seen that the major part of the watershed is covered with gneiss and igneous rocks, which are mainly found in higher altitudes (78.25%). The low values of infil-

2 of 7 International Conference Erosion and Torrent Control as a Factor in Sustainable River Basin Management 25-28 September 2007, Belgrade – Serbia tration in the area mean more of the rainfall flow over-land. In the plain areas neogenic and alluvial formations are found (16.59%).

Table 1. Geological composition of the watershed Rock Surface (km2) Percentage (%) Gneiss and igneous 145.99 78.25 Schist 2.07 1.11 Neogenic and alluvial formations 31.09 16.66 Limestone 7.41 3.97 Total 186.56 100

In Table 2 and figure 3 (right), the categories of land use and vegetation cover in the wa- tershed are presented. From that Table it can be seen that the area is dominated by forests (evergreen and deciduous broadleaved species) which offer protection to the forest soil to erosion.

Figure 3. Geological composition (left) and vegetation cover (right) in the watershed

Table 2. Categories of land use and vegetation cover in the watershed Land use Surface (km2) Percentage (%) Forest 70.10 37.57 Brushwood and grass 84.08 45.07 Agricultural land 32.38 17.36 Total 186.56 100

WEATHER CONDITIONS

Weather disturbances passing over Greece peninsula due to the complex topography cause orographic precipitation (Stathis et al., 2005). So the west part of the country receives higher amounts of mean annual precipitation. While mean annual rainfall amount in the study area is 597 mm the heavy prolonged rainfalls result in extensive flooding (Stathis, D., Ste- fanidis, P., 2000). On 8–9 October 2006 major flooding phenomena took place in other areas within Greece as well. Unfortunately in the area of the watershed doesn’t operate a meteorological station so we use the data from the nearest station that is in a distance 30 km from the centre of water- shed. This locates in the village Megali Panagia where a tilting siphon autographic raingauge operate. Daily rainfall and rainfall intensity are shown in Fig. 4.

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16 180

14 160 140 12 120 10 100 8 80 6

rainfall (mm) rainfall 60 4 40 2 20 0 0 1 3 5 7 9 111315171921232527293133353739414345 h o u r s

Figure 4. Daily rainfall and rainfall intensity

The maximum hourly rainfall intensity was 15 mm. The rain totals recorded in 24 hours in Megali Panagia station it was 150.0 mm. We conclude that the centre of the storm was northward the site of the station because there were the great flooding problems such as damages in houses (photo 1), fields, agricultural machines (photo 2), roads, bridges etc.

Photo 1. Flood damages showing the flood traces

4 of 7 International Conference Erosion and Torrent Control as a Factor in Sustainable River Basin Management 25-28 September 2007, Belgrade – Serbia

Figure 2. Damages caused from the 8–9 October flood in area

HYDROLOGICAL CONDITIONS – FLOOD PHENOMENON

. The peak-discharge was estimated by using the method of flood-water traces, based on the geometrical features of the cross-section of the main stream, the slope and the roughness of the torrent- bed, by applying the Manning-Strickler equation (Chow, V.T., 1964; Linsley et al., 1988; Kotoulas 2001). The peak-discharge (Q) that passes a cross-section estimated from the formula: Q = F⋅ u (1) Q = max discharge (m3·s-1) F = watered area (m2) u = average velocity of water (m·s-1)

The average velocity of water estimated by using the Manning-Strickler formula: u = k ⋅ R2/3 ⋅ J1/2 (2) u = average velocity of water (m/sec) k = coarseness coefficient R = hydraulic radius(m) R = F / U (3) F = watered area (m2) U = watered perimeter (m) J = mean slope of steam’s bed

The next day following the flood incident, after a local visit to the area, the reasons that caused the flood searched. During the flood a lake formed and covered the lower part of Mellisourgos village (Fig. 2). This happened because the torrent’s bed close to the village forms a narrow canyon. Data were collected in order to find out the channeling capacity in this site. In table 4, below, are shown the hydraulic characteristics of the cross section and in Fig. 5 the shape of the cross section.

5 of 7 International Conference Erosion and Torrent Control as a Factor in Sustainable River Basin Management 25-28 September 2007, Belgrade – Serbia

Figure 5. Cross-section D, at the mainstream of the torrent Apolonia

Table 4. Hydrological characteristics and maximum peak-discharge Cross- Coarseness Watered Watered pe- Hydraulic Stream’s velocity max section coefficient area rimeter radius slope u discharge R = F / U Qmax k F (m2) U (m) (m) (%) (m/sec) (m3/sec) D 19 206.5 38.0 5.43 1.0 5.8 1197.7

From the above it concluded that peak-discharge was 1197.7 m3⋅s-1 or 6.42 m3⋅s-1·km-2. Stream’s gentle slope at the upper start of the narrow canyon resulted to couldn’t channelling all the water flow. Also sedimentation problem was intense. Material of various diameters (trunks), taken away from the mainstream and banks of the torrent, were transported through the bed downhill. The above materials were deposited in upper start of the narrow canyon and resulted in the reduction the channelling capacity of the stream and overflow.

CONCLUSIONS

The initial reason that caused the flood of the torrent “Appolonia” was the rainfall oc- curred on 8–9 October 2006. The used rainfall data from close meteorological station wasn’t sufficient to explain the volumes of water that flooded the flood-plain of torrent. The second reason that caused the flood was the geomorphology of the study area. As mentioned above the narrow bed close to village resulted to the flood. The channeling capacity wasn’t suffi- cient to remove the over-land flow. The problem of sedimentation intensified catastrophic flood results. As it concluded the causes of flood were natural as anthropogenic interferences in torrent bed and watershed there aren’t. To effectively deal with the flooding phenomena in the area can be achieved by using the Forest Techniques System of torrent control. This system includes technical and rural technical works and also technical works with the use of plants. Works on purpose to regulate the discharge which arrive to the rock canyon will confront the flood problem. The protection of the Melissourgos village construction of dikes in connections to stonework constructions of sills will face the problem.

REFERENCES

Baloutsos, G., Koutsoyiannis, P., Economou, A., Kalliris, P. (2000): Investigation of the hydrologic response of the Xerias torrent watershed to the rainstorm of January 1997 using the S.C.S. method. Geotechnical scien- tific issues, 11/1; pp. 77-90. (in Greek, English summary)

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Chow, V.T. (1964): Handbook of applied Hydrology, section 14. Mc Graw – Hill Book Company. New York; pp 1-54. Kotoulas, D. (2001): Management of torrent streams. Part I. Thessaloniki; pp. 151-152 (in Greek) Linsley, R. K., Kohler, M.A., Paulhus, H.L., (1988): Hydrology for Engineers. McGrow- Hill Book Company. London, Second Edition; pp. 94-125. Papamichail, D., Georgiou, P., Karamouzis, D. (2001): Estimation of flood hydrographs for the 7-8 October high rainfall in Megali Panagia Chalkidikis area. Hydrotechnika, 11; pp 47-60 (in Greek). Stathis, D., Stefanidis P. (2000): Analysis of the conditions of flood formation in torrents in the area of north Halkidiki (Greece) in October 2000. Proceeding 3th Balkan Scientific Conference Study, Conservation and Utilisation of Forest Resources, Sofia. III; pp. 213-222. Stathis, D., Ivanova, D., Balafoutis, Ch., Makrogiannis, T. (2005): Orographic effect on heavy rainfall in Chalkidiki peninsula (Greece) induced by a mediterranean cold front: a case study on 7 to 8 of October 2000. Croatian Meteorological Journal, 40; pp. 490-493. Stefanidis, P. (1995): The cause and the mechanism of the debris flow in Brasna and Asprovalta, North Greece. Scientific Conference with Participation of Foreign Specialists 90 Years of soil Erosion Control in Bulgaria, Sofia, 3; pp. 30-36.

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