# Delineation of the Flooded Areas in Urban Environments Based on a Simplified Approach
## 3 Case Study and Mathematical Model  
The municipality of Tulcea is located in the south-eastern part of Romania in Dobrogea County, on the right bank of the Danube River just prior to the initiation of the Danube Delta (Figure 1). It is on a promontorty of the north-Dobrogean slope (Figure 2). The town is protected against the floods of the Danube River by a dike designed at 1% annual exceedance probability of the Danube's maximum discharge. Between 1988-2013 the flooding level was exceeded twice, and the dike effectively protected the town.  
However, the lower area of the town is frequently flooded (two to three times per year on average) by torrential rainfalls. The slopes of the surrounding hills (Figure 2) are  
Figure 1: Location of the Tulcea municipality: (**a**) country level; (**b**) county level.  
Figure 2: Administrative limits of the Tulcea municipality and the corresponding DEM.  
characterized by high runoff coefficients, and torrential rains generate flows exceeding the drainage capacity of the sewage system. The excess water is accumulated in the former meadow of the Danube River, which has become an urban area protected by dikes along the Danube. In the downtown area, the highest water level was 1.5 m above the ground, with flooding of commercial areas, hotels, and houses.  
One of the highest torrential rains occurred on 12-13 September 2013, characterized by a duration of 1 h 25 min and a total precipitation of 67.8 mm. Another pluvial event took place on 14 June 2016. A series of images regarding the flooded areas in Tulcea were collected for these two urban flood events, utilising the internet as a source (Figure 3). After exceeding the carrying capacity of the sewage network, the surface runoff along the streets (Figure 3a) accumulated in the low areas of the municipality, mainly in the area of Isaccei street from its intersection with Pacii street (Delta hotel roundabout area) to the intersection with Podgorililor street (Gara roundabout area), causing serious damages to trade and social activities (Figure 3b). Exceeding the designed capacity can be caused by the overlapping of several factors, such as excess precipitation, change of slope (transition from a high gradient slope to a low gradient slope), and sudden changes in the direction of the main collectors. Thus, in the N-S direction, the slope of the land is between 6-10%, while in the meadow area, the slope varies in the range 0.5-3%. Because the sewage system generally follows the natural slope of the land, these slopes often generate unsatisfactory operation of the sewage system. The torrential precipitation produced on this rugged relief lead to the erosion of the slopes and solid transport downstream, reducing the active area of the grate inlets and the transport capacity of existing collectors.  
Partially, the existing sewage network is a combined system and separate system, respectively (Figure 4). Images of the sewage network, in which the sensitive areas are marked with yellow, are presented in Figures 5 and 6.  
Figure 5: Most sensitive areas (yellow) and the pluvial drainage system.  
Figure 6: Main sensitive areas in case of pluvial floods and the corresponding main streets.  
Figure 7: Longitudinal profile along: (**a**) Pacii street; (**b**) Babadag street; (**c**) Unirii street; and (**d**) Isaceae street.  
It can be estimated that surface runoff accumulates either in the intersection of Isaccei street with Unirii and Pacii street or in the lower part of Babadag street (Civic Centre area), where the city elevations register the lowest level (around 5 m.a.s.l.). On streets such as Babadag or M. Kogalniceanu, surface runoff can originate from both the eastern slope and the western slope. At the same time, these streets are being flooded with pluvial waters sourced from the southern slope by [PERSON], Grivitei, and Pacii streets.