In recent years, flooding has become an increasing concern across many parts
of the world of both the general public and their governments. The climate
change inducing more intense rainfall events occurring in short period of
time lead flooding in rural and urban areas. In this study the flood
modelling in an urbanized area, namely Samsun-Terme in Blacksea region of
Turkey is performed. MIKE21 with flexible grid is used in 2-dimensional
shallow water flow modelling. 1
Flooding has the potential to cause significant impacts to economic activities as well as to disrupt or displace populations. Changing climate regimes such as extreme precipitation events increase flood vulnerability and put additional stresses on infrastructure. Knowing that flood risk is a function of flood vulnerability and flood hazard, the studies performed so far for obtaining flood hazard maps depend on the numerical solution of shallow depth flow equations using DEM, cross-sections along the channel and the discharges having different return periods obtained from meteorological forcing. The hydrodynamic modelling approach is considered to be the most suitable method for generating comprehensive flood hazard maps at high spatial and temporal resolutions. The hydrodynamic modelling approach is used to simulate flood inundation in floodplains using both one-dimensional and two-dimensional modelling schemes. Especially for urban floods, the upstream and/or downstream precautions must be clarified to decrease the floods' adverse effects.
The peak values of the hydrographs having different return period for four subbasins and the area of the sub basins.
The flood problem is not a recent issue neither for Turkey nor for other countries. Therefore, the need for the flood protection and flood management are not new too. There are many studies about flood management around the world. Recent researches suggest a risk-based approach in flood management (Hooijer et al., 2004; Petrow et al., 2006; van Alphen and van Beek, 2006). The necessity to move towards a risk based approach has also been recognized by the European Parliament (de Moel et al., 2009), which adopted a new Flood Directive (2007/60/EC) on 23 October 2007. According to the EU Flood Directive, the member states must prepare the flood hazard and risk maps for their territory and then these maps will be used for flood risk management plans. Structural management measures play also important role among various mitigation facilities and flood management strategies.
Various flood mitigation facilities were constructed and some flood management strategies were established in Turkey following the severe floods; some of which are 25–26 August 1982 (Ankara), 18–20 June 1990 (Trabzon), 16–17 May 1991 (Eastern Anatolia), 4 November 1995 (İzmir), 21 May 1998 (Western Black Sea), 28 May 1998 (Hatay), 2 November 2006 (Batman), and 9 October 2011 (Antalya) (Şahin, 2013).
The aim of this study is to analyse the possible upstream structural measures for an urban area flooding. The effect of meanders to the flood peak discharge attenuation at the downstream part of the stream was also analysed.
Study area is selected from Black Sea region of Turkey. The mean rainfall is
720 mm per year and the topography is tough due to the mountains lying
parallel to the sea and they cause flash floods in the urban areas located on
the coast line. The Terme River passes through Terme city centre and
separates city into two parts. The project area is beginning from the Black
Sea and going through 32 km upstream of Terme. First 6 km of the study area
is settlement area of city. The Terme River and upstream part with four
branches contribute the study area. In July 2012 Terme City Centre was
exposed a small flood event. Approximately 510 m
1
The river with upstream branches, 1
In this study, hydraulic modelling works were conducted with Danish Hydraulic
Institute (DHI) MIKE11 (one dimensional) (DHI, 2009) and MIKE21 (two
dimensional) (DHI, 2010) models. The hydrographs having different return
periods were obtained from a previous study in which classical statistical
techniques were used to estimate the hydrographs at the discharge observation
station (DSI2245) close to SalıpazarıBridge. Hydrographs for the
sub-basins were obtained by simple area-ratio based technique (Table 1). The
location of the sub-basins are presented in Fig. 1b. The upstream precautions
were analysed using the hydrographs for each sub-basin as different
scenarios. The bed resistance (1 n
Scenario1 model results.
Scenario2 model results.
DTM
There is a dam project under construction at the downstream part of Basin 1.
It is a multi-purpose dam for irrigation, water supply and flood control
purposes. The reservoir volume of the dam at the normal water level
(134.00 m) is 15.90 hm
The length of the stream is long in the study area, therefore it is preferred to use flexible mesh and the DTM of the study area (Fig. 2a) is converted into triangular mesh. One of the advantages of the flexible mesh is creating different size of elements for different parts of the maps. These different sizes of the elements give advantages for modelling (Fig. 2b).
Scenario3 model results.
Scenario4 model results.
Scenario5 model results.
The model scenarios were created for three different situations. The first one existing situation includes the today's conditions of the study area. The second one is about the application of SalıpazarıDam Project which is under final planning stage. The last one is hypothetical structures which are proposed as the structural management measures at the upstream of the basin.
The input hydrographs for different return periods to be used in these scenarios were obtained by using the observed discharge values at gauging station DSI2245 through statistical analyses. The input hydrographs are numbered as follows:
Hydrograph 1: these Hydrographs were obtained from discharge observations at gauge DSI2245 and represent the discharge contributions from Basin 1, Basin 2 and Basin3.
Hydrograph 2: the hydrographs for Basin1 were obtained by using area-ratio method where discharge observations at gauge DSI2245 were used in the calculation.
Hydrograph 3: these Hydrographs for Basin2 were obtained by using area-ratio method where discharge observations at gauge DSI2245 were used in the calculation.
Hydrograph 4: these hydrographs for Basin3 were obtained by using area-ratio method where discharge observations at gauge DSI2245 were used in the calculation.
Hydrograph 5: these hydrographs for Basin4 were obtained by using area-ratio method where discharge observations at gauge DSI2245 were used in the calculation.
Hydrograph 6: this model hydrograph includes summation of the
Hydrograph 7: this model hydrograph includes summation of the
Hydrograph 8: this model hydrograph includes summation of the
The upstream structural management measures were studied on scenario basis.
Scenario 1: the aim of the scenario is to see the input hydrograph peak
discharge and output hydrograph peak discharge differences due to the
meanders effect. The input point was selected as the Salıpazarı(DSI2245)
and output point was selected as the Terme City centre (Terme Bridge) (Fig. 1a). The DSI report at the date of 11 July 2012 says; the flood event at the
day of 9 July 2012 was measured as 990 m
This scenario represents the existing situation of the river and the basins. Table 2 gives the peak discharges of input and output hydrographs for different return periods and the percentage of difference in the input and output hydrograph peak values giving an information about the attenuation in the hydrographs due to meanderings.
The model results state that the meandering effect between SalıpazarıCity
and the Terme City has a major role on the downstream stream discharge value.
The routing capacity of the meanders at the study area can be seen from the
discharge differences. Figure 3a shows that at some parts of the river water
leaves the river bank and spreads over the open field. The river capacity at
the Terme City centre is approximately 500 m
Scenario 2: the aim of the study is to show the effect of Basin4
contribution on the downstream hydrograph. The model studies were carried out
only for
The model results show that Basin 4 participation to the Terme River has a
major effect on Terme City flood. Even if the peak discharges are not
overlapping, Basin 4 has the highest
Scenario 3: this scenario represents the SalıpazarıDam project
constructed situation. SalıpazarıDam project includes flood capacity so
it has an effect on the Basin 1 output discharge. Since the studies were
based on the interim project of the dam, these models can be called as
projected situation. Since the location of SalıpazarıDam is at the
downstream part of Basin 1, hydrological studies were changed only for
Basin1. The other Basins were remained the same with existing situation. The
model studies were carried on for
Hydrograph 6 was used as input hydrograph to represent the Basin 1, Basin 2 and Basin 3. Addition to that, Hydrograph 5 was used as input to represent the Basin 4. Both of the Hydrographs reach the peak discharges at the same time individually. However, since the hydrograph input points are not the same, peak discharges do not overlap. Table 4 gives the peak discharges of input and output hydrographs for different return periods and the percentage of difference in the input and output hydrograph peak values
The model result shows that even if the SalıpazarıDam is constructed with the planed flood capacity, it is not sufficient for the Terme City safety for the condition of the whole basins are affected from the flood at the same time.
Scenario 4: this scenario represents the SalıpazarıDam design project constructed situation and possible future projects for remaining sub-basins. Since the Scenario 3 shows that the SalıpazarıDam flood capacity is not sufficient at the time of the other three sub-basins are also affected from the flood, the aim of this scenario is controlling the whole flood discharges of the Basin 2 and 3 in addition to SalıpazarıDam flood capacity for model calculations.
This scenario represents the possible solutions at the basins for the
upstream part of the SalıpazarıCity. Basin 1 could be controlled with
SalıpazarıDam and only bottom outlet discharge (62 m
The model studies were carried on for
The model results show that if upstream precautions are applied before the
SalıpazarıBridge, the
Scenario 5: this scenario represents the SalıpazarıDam Design project
constructed situation and possible future projects for remaining basins. The
Scenario 4 model results show that
This scenario represents the possible upstream solutions for Basin 2, Basin 3 and 4. The base hydrological input for this scenario is Basin 1 controlling with SalıpazarıDam. In addition to that two of the three basins are controlled. So that only one basin remains uncontrolled. The study also aims to show which basin has important role for flood condition. Table 6 gives the peak discharges of input and output hydrographs for different return periods and the percentage of difference in the input and output hydrograph peak values.
In total, five different scenarios were studied for four upstream sub-basins. The existing circumstance of the Terme River states that the meanders of the river have a major effect on the flood situation. The discharge measurements between SalıpazarıBridge and the Terme Bridge have approximately 35 % reduction of the peak discharge. The model studies with and without Basin 4 state that, Basin 4 has the important role on the Terme City flood. The flood discharge of the Basin 4 is higher than the other three basins' flood discharges. Since the Basin 4 connection is closer to the urbanized area, risk factor is increasing. SalıpazarıDam flood capacity is not sufficient individually to protect Terme City against flooding. However other basins do not have any flood protection structures yet and additional control structures would also be needed for other sub-basins.
All model studies were based on the assumption of the peak discharges overlapping at basins. The hydrological model has an important role on flood modelling studies. Well calibrated hydrological model is needed to be used in calculations of the model input discharges with rainfall-runoff relation. Early warning systems for the sub-basins can be also considered since the flood peak discharge reaches from SalıpazarıBridge to the Terme City approximately in 4 h.
The authors thank DSI staff for providing the data and support during this study.