International Journal of Geosciences, 2011, 2, 68-74
doi:10.4236/ijg.2011.21007 Published Online February 2011 (
Copyright © 2011 SciRes. IJG
Inundation Maps for Extreme Flood Events at the Mouth
of the Danube River
Daniela E. Nistoran Gogoase1, Iuliana Armaş2, Cristina S. Ionescu1
1Hydraulics Department, University Politehnica of Bucharest, Buchare s t , Rom ania
2Department of Ge ography, University of Bucharest, Bucharest, Romania
Received December 6, 2010; revised January 20, 2011; accepted January 26, 2011
Hydrodynamic modeling is used to analyse the inundation behavior of St. George village during extreme
flood events, in particular for a flood happened in spring 2006. The study reach, 4 km in length, is situated in
the Danube Delta, at the mouth of St. George distributary and includes St. George village. Land and bathy-
metric surveys were used to create a digital terrain model (DTM) of the river channel and the village. By
coupling the geometry with hydrologic data, a 2D hydrodynamic model was built up with the help of the
CCHE2D code (University of Mississippi). The model is based on integrating Saint-Venant shallow waters
(depth averaged) equations through finite-difference implicit numerical scheme. It was calibrated in terms of
roughness coefficients on measured values of water surface elevation registered in the St. George port. Flood
maps obtained from computations were compared to satellite images from the same days of the spring 2006
extreme event. Inundation behaviour of the St. George village was analysed for different scenarios of river
hydrological and sea level (variable because of wind waves) conditions. Findings were compared with high
water marks and inhabitants testimonials. The model proved that sea level has a higher influence upon the
inundability of the area than the river flood events.
Keywords: Hydraulic Modeling, Finite Difference Method, Flood, Inundation Maps
1. Introduction
Considerable advance has been made the last decades in
modeling the hydrodynamic behavior of rivers during the
flood periods for predicting flow variable variation in
time and space and obtaining flood maps. According to
the variation in space of flow variables, models range in
increasing order of complexity: from cross-section aver-
aged-1D (used for long, straight reaches), to shallow
water-2D [1], which are capable of reproducing more
realistically spatially-distributed phenomena, such as the
cross-stream component of flow, to 3D, capable of pre-
dicting secondary flows in meander bends and treat mo-
mentum fluxes varying in the vertical direction.
1D step-backwater models have intensively been used
so far due to their advantages such as: computational
simplicity and ease of parameterization, calibration facil-
ity (in terms of method and necessary data), accuracy
when coupled with detailed topographic data [2], re-
quirement of small computation times. However, when
dealing with cross-section variation of hydraulic pa-
rameters (such as deltas or flood inundation prediction,
estuaries, confluences/diffluences, braiding, recirculation
zones, riffle-pool sequences, meanders, etc.) 2D models
offer a better representation of the flow field and sedi-
ment fate. At the same time these models tend to require
more computational time, be more data intensive, require
distributed topographic and friction data and work with a
much more comple x gri d (triangular or rectangular mesh
with variable density most of the time). The 2D models
also need distributed calibration and validation data ac-
quired by using modern measurement techniques as well
as remote sensing images. In terms of coupled 2D mod-
els, MOBED2, TELEMAC 2D (with its SISYPHE sedi-
ment module), CCHE2D, RMA2, River2D [3] etc. are
some of the very widely used codes.
The aim of this study is to analyze the flooding be-
havior of a study area situated at the mouth of the south-
*sponsor–AMTRANS–CEE X 2006-Romania
ern distributary of Danube Delta, St. George, for flood
events having return periods between 20 and 1000 years.
For this purpose, a 2D hydraulic model was set up with
the help of CCHE-2D finite difference code (University
of Mississippi, USA). In this context, the objectives of
present study are: 1) to extend the sparsely available
measured values of hydrodynamic parameters over the
entire study reach for discharge values in the (800-1600)
m3/s range; 2) to draw the flood maps and compare them
with corresponding satellite images [4]; 3) to analyze the
flooding behavior of St. George village and assess the
risk factors under various scenarios such as: river flood
events, sea level raise or sea storms (which lead to high
waves and raised sea level at the downstream boundary
of the study reach). The findings are helpful for local
authorities in order to inform th e population and take the
appropriate defense measures in the future.
2. Site and Data
The Danube Delta is located in the north-western part of
the Black Sea, between 44˚25’N and 45˚30’N and be-
tween 28˚45’E and 29˚46’E. The Romanian delta plain
covers an area of about 5,800 km2 (including water)
(Figure 1). It has three distributaries (main branches),
named from N to S: Kilia, Sulina and St. George. Delta
apex is known as Ceatal Izmail.
About 20% of the Danube delta represents areas with
negative relief (i.e. with an average level below the
Black Sea-Sulina gauging system), about 54.5% of the
Danube delta plain consists of areas having altitudes be-
tween 0 and 1 m above the sea-level and 18% with alti-
tudes between 1 and 2 m. The reed plot swamp vegeta-
tion is predominant and it covers about 78% of the total
area, while the salting vegetatio n covers about 6% of the
total area. These two factors: flat terrain and compact
vegetation (generally up to few meters in height) makes
it almost impossible for the topographic surveys to be
performed, and erroneous for the remote sensing topog-
raphic data acquisition (such as LIDAR, [5]).
Average multiannual (1960-2006) river flow near the
Danube apex (Isaccea gauging station) is 6638 m3/s with
a maximum value of 16500 m3/s (registered in April
2006) and a minimum value of 1970 m3/s (registered in
September 2003). The monthly multiannual average sea
level has the same trend as the corresponding Danube
flow, with amplitudes of 14 cm between high levels in
spring and low levels in autumn [6].
Predominant winds are from the N and NE, and the
most frequent induced wind waves recorded are from NE
corresponding to the prevailing wind direction [7]. The
mean maximum heights of wind induced sea waves in
front of the Danube Delta reached even 7.0 m. The storm
Figure 1. Map of the Romanian Danube Delta with the
three main distributaries (from N to S): Kilia (Romania-
Ukraine border), Sulina and St. George.
surges from N, NE, E and SE directions calls water level
rises of 1.2-1.5 m. Therefor e, water level at the mouth of
the three distributaries has important variations, its aver-
age annual amplitude being of about 0.70–0.80 m. The
Black Sea tide has small amplitudes of only 7 ÷ 11 cm
In such conditions, flooding events in the Danube
Delta occur when the water flow at the apex exceeds
10,000 m3/s [8] and/or when waves from the sea are high.
Most recent historic event (having a return period of ap-
proximately 200 years) took place in spring 2006. This
paper focuses on St. George distributary, which is the
most meandered (local meandering coefficient = 2.35)
and conveys approximately 20-25% of the total Danube
discharge. Top width varies between 150m and 600m,
whereas the depth varies between 3 and 27 m beneath
water level corresponding to the low flow regime. At its
mouth, a secondary delta with conic entangled branches
has been form ed.
A study site was chosen at the mouth of this distribu-
tary. Its length is 4.3 km, minimum top width of 200 m
in front of the St. George port and maximum top width
of 600 m at km 1.5 from the mouth (Figure 2). Maxi-
mum depth along the study reach is of about 15 m (cor-
responding to an approximate average multiannual flow)
near the port.
At km 8 from distributary mouth there is a gauging
station (where flow is measured), and at km 4.3, in the St.
George port, there is a level gauge (where water surface
elevations are systematically recorded and flow data is
obtained only through correlation). The flow hydrographs
Copyright © 2011 SciRes. IJG
Km 4.3 port
Km 5.6
Km 1.1
0 m 250 m 500 m 750 m 1000 m 1500 m
Figure 2. Study site. Arial photograph of Danube St. George
distributary mouth and village surrounded by the flood
protection dike.
recorded during the spring flood are shown in Figure 3;
at the Delta apex (a) and for the study site (b). The study
period is delimited (April 25th - May 3rd 2006). A rating
curve has been derived at km 1.1 since sea level at the
mouth is dependent of river inflow.
Bathymetric surveys were performed (using a Garmin
188 echo sounder) on a yearly basis (in July 2005, 2006
and 2007) along closely-spaced cross-sections (about 50
m). For the terrain part of the study area, recent topog-
raphic survey data (acquired with a Leica TPS 407 total
station) were coupled with existent data (from detailed
topo-hydrographical maps 1:25000) in order to obtain the
digital elevation model (with a 20 × 20 m grid cell). The
domain area was chosen to cover all inundated area in
case of a 1000 year-flood event (Figure 4).
3. Method
In order to analyze the flooding behavior of the area the
CCHE-2D software (Center for Computational Hydro-
science and Engineering, University of Mississippi, USA),
was used. The program integrates the shallow water equ -
ations by using the fi ni t e di fference me t hod.
Several meshes were created and tested for computa-
tional stability and accuracy. They were obtained by tri-
angular interpolation of a plane, rectangular mesh over
the topo-bathymetrical data. Figure 5(a) shows one of
the meshes (with 25,000 = 250 100 nodes) used to
represent the geometry of the domain in the hydrody-
namic computations.
Different values of the Manning non-homogeneous
roughness coefficient were tested in the model calibra-
tion process: 0.015 ÷ 0.02 s/m1/3 for the St. George dis-
tributary channel, 0.2 ÷ 0.025 s/m1/3 for the floodplain
vegetated areas and 0.025 ÷ 0.03 s/m1/3 for the village
areas with houses (Figure 5(b)) [9].
Steady flow regime computations were firstly run for
Figure 3. Hydrographs of the 2006 spring flood; (a) Flow
and stage at the St. George study site; (b) flow and stage at
Ceatal Izmail (delta apex); the period between April 25th
and May 3rd 2006 (with a maximum flow of 16500 m3/s at
delta apex) represents the studied flood event.
10 increasing inflow values covering the entire consid-
ered range. The model was calibrated in terms of rough-
ness coefficient on registered water stages at the port
gauge. Absolute maximum differences were less than
few cm.
Unsteady flow computations were performed after-
wards, for the spring 2006 flood event. As upstream and
downstream boundary conditions (Figure 5(b)) were
used the flow hydrograph and the derived rating curve at
the mouth of St. George distributary, respectively.
Computed water surface elevation in the domain ob-
tained from the steady flow computations were used as
initial condition for the unsteady flow computations.
Velocity, shear stress, water surface elevation, unit flow,
Froude no. and eddy viscosity fields were inspected in
the flow domain. Care was taken in the runs for the out-
flow to be equal to the in flow in the computatio n domain
(no flow accumulations).
Computations were performed for different time steps,
t (1 s ÷ 40 s) and a total no. of steps of 2,000, com-
plying with the 2D Courant criterion (even though the
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Copyright © 2011 SciRes. IJG
Figure 4. DTM of the study area (with a 20 × 20 m grid cell) displayed over the ortophotoplan.
numerical scheme is implicit). A trade-off value of 20 s
per time step was chosen for the flow to pass a computa-
tion cell (of about 16 m 17 m), very close to the water
flow physical time computed with the average distance
and velocity. About 10,000-15,000 s were necessary to
pass the warming-up period and for the instabilities to
settle. The hydrodynamic parameters were computed
with the k-
turbulence model under two scenarios: with
calm sea and with sea waves (of 30 cm and 70 cm in
4. Results and Discussion
Figure 6(a) and Figure 6(b) show two inundation maps
obtained from computations performed under steady
flow conditions for inflow discharge values of 823 m3/s
(the 20-year flood) and 1517 m3/s (the 1000-year flood).
In the first scenario, the absolute average downstream
sea levels were of 61 cm (a) and 96 cm (b) respectively
(according to the derived rating curve).
In another scenario, for which wind coming from the
sea produces high waves, sea level increases with 30 cm
in the case of the first discharge value ( Figure 6(c)), and
with 70 cm in the case of the higher discharge value
(Figure 6(d)), worst case scenario, with river flood and
maximum sea waves occurring simultaneously).
White areas in Figure 6 represent dry, uninundated
areas. One can see they are obviously smaller in case b)
than in case a), whereas vectors and the legend colors
indicate velocity distribution and its magnitude. By su-
perposition of the inundation maps over the villag e maps
one may see inundated houses. This result of the model
is very useful for local authorities in case of such a flat
Figure 5. Examples of: (a) meshes; (b) roughness coefficient
values and upstream and downstream boundary conditions-
used in the unsteady computations.
Figure 6. Inundation maps (white color means dry areas)
obtained from computations performed under steady flow
conditions. (a) Q = 823 m3/s; (b) Q = 1517 m3/s; c) Q = 823
m3/s and increased sea level with 0.3 m; d) Q = 1517 m3/s
and increased sea level with 0.7 m high.
terrain, in order to draw flood risk maps and inform the
Maps of water depth may also be obtained from the
model for the entire domain, in order to see the damage
extent. Case d) actually has never been recorded; most of
the river floods occur during spring (after defrost) or
during summer, whilst most important icy north wind
blows during wi nt er.
Inundation maps have been recorded every 12 hours
during the 9 days period of the 2006 spring flood event
for which numerical simulations were performed, in or-
der to get water boundaries. Figure 7 and Figure 8 show
a comparison between satellite images (a) and computed
inundation maps (b) from the same days. Satellite images
are available from Romanian Space Agency, ROSA
_2006.htm) for April 25, 26, 27, 30 and for May 02 and
03, 2006 (for the sake of simplicity, only two days are
shown in this study). The flow hydrograph at km 4 from
the river mouth is also shown, with highlighted instanta-
neous discharge values. Computations with a 20 s time
step, for these 9 days, took about 8 hours on a standard
PC with 2 GHz and 2 GB of RAM.
One can see in Figure 7 and Figures 8(a) and (b) the
same inundated areas from the E, N-E and N-W parts of
the St. George village on the 30th of April, 2006, when
the hydrograph reached its peak.
Measured high water marks found on site matched
within few cm corresponding stages from computations.
5. Conclusions
2D hydraulic modeling is used to analyze the flooding
behavior of St. George distributary mouth (estuary) and
village from Danube Delta. St. George village has a his-
tory of frequent floods which endangered local fisherman
community and the village touristic and cultural attrac-
tions. The 1.8 m in height dyke proved to be too low for
the water level attained during several past important
flood events and needs to be enlarged.
The site is very flat (maximum difference in terrain
elevation is about 2 m) and covered with compact, tall
vegetation which makes a challenge for topographic
surveys to be performed. This is the first attempt to set
up a 2D model for the area. Modeling such flat terrain is
difficult, as results are very susceptible to small errors in
measured land elevation or computed water level.
Present model was built with the help of CCHE-2D
code (University of Mississippi) and calibrated in term of
roughness parameter on water level recordings in the St.
George port. Steady runs were performed for different
upstream hydrological scenarios and downstream water
levels (due to varying Black Sea wind conditions).
Copyright © 2011 SciRes. IJG
April 2006
Figure 7. Situation on the 27th of April 2006 (a) satellite
image of the study site (flooded areas in blue); (b) inunda-
tion map obtained the from the computations; (c) Inflow
hydrograph with current day value (current flow value, Q =
1170 m3/s on hydrograph).
The hydraulic model was set up and run under un-
steady flow conditions too, for an extreme flood event
happened in spring 2006. Satellite images from that pe-
riod, showing inundation extent, were used as a qualita-
tive comparison with the computed flood maps. Quanti-
tatively, high water marks matched within few cm cor-
responding stages from computations.
April 2006
Figure 8. Situation on the 30th of April 2006 (a) satellite
image of the study site (flooded areas in blue); (b) inunda-
tion map obtained the from the computations; (c) Inflow
hydrograph with current day value (current flow value, Q =
1481 m3/s on hydrograph).
Computed hydrod ynamic parameter values (water lev-
el, velocity in the grid cells) are very useful to draw
flood risk maps and inform the population.
Computed water stage values and flood maps led to
the conclusion that sea level has a higher influence upon
the inundability of the study area than the intensity of the
river flood events. Therefore sea storms (waves) and
Copyright © 2011 SciRes. IJG
Copyright © 2011 SciRes. IJG
black sea level constant raise (due to climate variability
and North Atlantic Oscillation–NOA, [10]) have a
stronger influence on the flooding behavior of the St.
George village. Worst case scenario of simultaneous
1000-year river flood event and maximum sea waves
proved to be devastating for th e village. For the first time,
local authorities may use such a model as a prognosis
tool in developing contingency and flood emergency
plans and take the appropriate defense measures (such as
enlarging the villag e enclosure dyke).
6. Acknowledgements
The authors would like to thank National Institute of
Hydrology and Water Management (INHGA, Bucharest)
of Romania, for providing the hydrologic data in the
study and to Dr. Stefan Constantinescu, Associate Pro-
fessor at the University of Bucharest, Department of
Geography for acquiring and providing the topo-hydro-
graphic. The work was sponsored by AMTRANS, Ro-
mania under the DANUBERES research grant “The im-
pact of climatic variability and anthropic interventions
upon the hydrologic regime of Danube and coast sedi-
ment dynamics” (developed between 2006 and 2008).
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