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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
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.
i
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.),
K 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):
k
Q(t )  q00 (t )   qi j (t  t j )
j 1
0, roca t  t j  j

1

 (t  t j )  1  (t  t j ), when t j   j  t  t j
 j
exp   (t  t ) when t  t
j
j
j



Q
H
30.12.1968
23.12.1968
16.12.1968
09.12.1968
02.12.1968
25.11.1968
18.11.1968
11.11.1968
04.11.1968
28.10.1968
21.10.1968
14.10.1968
07.10.1968
30.09.1968
23.09.1968
16.09.1968
09.09.1968
02.09.1968
26.08.1968
19.08.1968
12.08.1968
05.08.1968
29.07.1968
22.07.1968
15.07.1968
08.07.1968
01.07.1968
24.06.1968
17.06.1968
10.06.1968
03.06.1968
27.05.1968
20.05.1968
13.05.1968
06.05.1968
29.04.1968
22.04.1968
15.04.1968
08.04.1968
01.04.1968
25.03.1968
18.03.1968
11.03.1968
04.03.1968
26.02.1968
19.02.1968
12.02.1968
05.02.1968
29.01.1968
22.01.1968
15.01.1968
08.01.1968
01.01.1968
md . nat anebi - s. nat anebi 1968 w.
300
250
200
150
100
50
0
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
2
3
4
5
6
7
8
9
59
106
254
256
292
295
297
338
346
112
85.4
113
143
97.9
106
98.6
91.6
127
2
3
2
1
1
1
2
3
2
0.13
0.20
0.16
0.13
0.20
0.11
0.17
0.25
0.09
1
tj
qj
τj
αj
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.
Thank yo very much for attention !