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Evaluation of the risks of flooding associated with the climate change at the eastern coastal zone of the Black Sea (Georgia). Davit Kereselidze, Kakhaber Bilashvili, Vazha Trapaidze, Giorgi Bregvadze.

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Evaluation of the risks of flooding associated with the climate change

at the eastern coastal zone of the Black Sea (Georgia)

Davit Kereselidze, Kakhaber Bilashvili, Vazha Trapaidze,

Giorgi Bregvadze

BS-OUTLOOK Conference, Odessa, Ukraine, 31 Oct - 4 Nov, 2011

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Introduction

According to the forecast of the 4-th Report of the Intergovernmental Panel of Climate Change (IPPC, 2007), the changes of the average temperature field of the atmosphere and ocean, intense melting of ice and snow cover and increased average sea level are an objective evidence of the global warming cycle. Global warming and process of climate change in the Black Sea area of Georgia are accompanied by several phenomena, provoked by the mentioned processes, such as increased storm intensity and frequency, displaced line of breaking waves deeper into the land, increased floodplains at the river mouths, more frequent catastrophic floods and abrupt increase of water discharge . According to the observational statistical data available in Georgia and preliminary scientific evaluations, one of the most sensitive areas to the given phenomena is the section of the coastal zone of Black Sea between the rivers Supsa and Natanebi. The accumulation processes along the given section are intense and catastrophic freshets are very frequent, what on its turn drastically increases the flooding areas at the mouths. The flooding areas are also increased due to the more frequent stormy events and increased strength of storms. Therefore, the risk of land flooding at the mouth areas of the rivers in the coastal zone is drastically increased. This causes the washout and flooding of the coastal line and threatens the operability of existing and planned, extremely important, economic objects and infrastructure of the coastal zone.

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Methodology

The work was based on the statistical analysis of the runoff of the rivers of the Black Sea basin of Georgia. The river Natanebi was investigated and studied in details. The length of the river is 60 km, its catch basin is 657 km2 and the average height of the basin is 830 m. The river network is well developed in the basin, particularly along its left bank and in its upper reaches.

The level regime of the river is mostly characterized by strong and intense floods during the year. In the upper reaches of the river, at 1000-1500 m altitude and above, spring flood is formed, which lasts from March to April and is followed by frequent rain freshets. The floods are more intense in April and May and cause high river levels along the whole river length.

The methodology of specifying the zones of freshets and flooding in the river basin is based on the use of the modern software products (MIKE 11). MIKE 11 is a unique instrument to create one-dimensional runoff models in the natural and artificial watercourses. The hydrological module (NAM) of MIKE 11 is a homogenous deterministic model simulating a simplified overland hydrological cycle. On its turn, it includes a unit hydrograph sub-module (UHM) modeling individual downpour runoff. Our aim was to change it with a stochastic model meaning the following:

One of the most important properties of the freshet period is the water peak discharge . Further, daily maximums will be considered, as shifting from daily maximums to immediate maximums is possible by well-proven standard methods. If in the observations of i-th range (i=1,2,…,n.),Ki freshets with Qij peak discharges are fixed j=1,…Ki, then Qmax peak discharge of water is the greatest of Ki values. In years, we will gain the peaks of freshets of peak discharges of ∑Ki=Kn values (K is the mean value of peaks in the period of freshet).

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In order to draft a curve of annual peak runoff, many scientists at present are working to use the increased number of the observed annual data. The given approach is used in practice by the scientists of the USA, Great Britain, Russia and other countries. This approach is based on the following considerations: all local maximums are Qij=(i=1,…,Ki; j=1,…,n) subject to the same distribution function; in the final account Qij, values are independent; the annual number of freshet peaks is subject to Poisson equation.

The model considers the periods of peak discharges for the river. t=0 is taken as the starting point of the freshet period, and t=T is the end of it, with T as its duration. Therefore, the process of variation of the measured water discharges Q(t) is considered as T›[0,T]. The model is given as the following function based on the approximation for each year (season):

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The maximum period of freshet for the river Natanebi lasts all the year, from January 1 through December 31, i.e. T is 365 days. K=9 peak of freshets was fixed in the year under review, where the basic runoff is an insignificant value if compared to the freshet runoff, and therefore, a form of hydrograph of the basic runoff may be considered a constant value: φ(t)=1, when q0=9,8m3. The parameters of a hydrograph are given in table 1.

Table 1. Hydrograph parameters of the river Natanebi in 1998

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In the HEC-FDA system, by using MIKE 11 GIS-FAT software, we have created a thorough picture of peak discharge passages, which allows identifying the potentially flood-prone areas (individual plots of the area adjacent to the river), which after modeling is assessed by an experimental function “depth-loss”, in possible relative values, e.g. in percentage.

Based on the gained results, the flooding zones corresponding to 0.1%, 1%, 5% and 10% provision levels were identified and plotted on the digital maps.

The hydrograph parameters are more precise and reliable, while the creation of the model maximally similar to the real situation is one of the most important preconditions of intense use of the river floods.