Risk Analysis and Management of Urban Floods with Geomorphological Approach (Karaj Metropolis)

 The water flow that is higher than the river level and penetrates the surrounding lowlands is called flood. In urban environments, due to high human density and its economic importance, the occurrence of floods is important and can lead to a lot of human and financial losses. In the metropolis of Karaj, in the floods that occurred in 2018, in addition to the damage to urban infrastructure, 8 individuals were dead, 10 injured and 12 missing and this phenomenon damaged 40 vehicles as well. Karaj metropolis from a geomorphological point of view, according to the physical characteristics and form of waterways, is characterized by hydrological coefficients of sub-basins, high rock outcrop, and high intensity of storms. Due to the importance of preserving lands and gardens along rivers, and its agricultural status and existence of gardens within its basins, and most importantly, the establishment of Karaj metropolis at the river outlet from the mountains, conducting flood estimation and management and water storage in this basin is necessary.
Therefore, due to the importance of floods in urban issues, a lot of research has been done based on data from 1: 50000 topographic maps and 1: 100000 geological maps, lithology, digital land model, land use, aerial photographs, satellite images and Google Earth in order to obtain flood-prone areas. Therefore, in this study, the hydro-geomorphological features of Karaj metropolitan basins with emphasis on flooding are investigated by estimating and studying the physiographic characteristics of watersheds leading to Karaj metropolis and calculating the probability of floods in different time periods and spatial analysis. In order to manage possible floods, appropriate scenarios and locations should be identified according to the landform characteristics of the basin for storing and maintaining excessive water and floods.

In this research, the research steps are as follows:Required data collection:
Preparation of remote sensing data including: Digital terrestrial model data (altitude, slope and waterway data),
Landsat 8 OLI sensors (land cover data, vegetation density index and FCD index),
Ground data preparation including: Karaj Municipality data (absorption wells, drainage canals and urban lands),
Geological Survey (fault and lithology) data,
Regional water organization data (hydrometry and groundwater),
Meteorological organization data (climate data),
Data from ground observations

 The upstream basins of Karaj metropolis consist of 5 sub-basins named Klak and Hesar (in the east of Karaj metropolis), Azimiyeh, Taleghani, Siah Kalan and Delmbar (in the north of Karaj metropolis). The flow of surface water in the Klak and Hesar basins, which is directed to the Karaj River, and due to the construction of the Karaj Dam upstream of this river, practically possible floods in this basin are directed out of the urban environment by using the mentioned river. Therefore, there are no flood hazards in this basin. However, the northern basins of the city, due to the diversion of all surface water into the urban environment of Karaj metropolis, and the location of this city in its waterway bed, such as Taleghani Street, has faced high flood risks. Therefore, the northern basins of the city have been studied in detail. In Azimiyeh, Delmbar and Taleghani basins, protection measures including diversion canals and water storage ponds have been constructed to control floods, but due to lack of protection and proper management, they have lost their function.
Drainage networks located in the city of Karaj are transferred using two canals and a metro collector, which are located in the bed of the old streams of Kamalabad (Beheshti canal) and Hosseinabad (metro collector). These two old streams have irrigated the lands of Kamalabad and Hosseinabad using the water of Karaj River, so the slope of these canals is from east to west, which collects surface water from the city and through Haftjoye leads to the Shore River.
 The results of flood simulation in different return periods using HecRAS model show that in a 20-year return period, 0.27 cubic meters per second flood is produced in the largest waterway located in Azima, which affects 14 hectares of surrounding land. Also in Taleghani, Siah Kalan and Delmbar waterways, 0.3, 0.58 and 1.91 cubic meters per second of floods are produced, respectively, which affect the area equivalent to 15.82, 22.65, 85.51 hectares of the surrounding lands, respectively. In addition, by using the TOPSIS model and hierarchical weighting systems, the areas with flooding potential were identified. For this purpose, 9 criteria were used and normalized using fuzzy logic model and straight and inverse linear functions, and then using the AHP model, the effective weight of each of them was obtained. The results of zoning show that 56.8% of the area of ​​Karaj metropolis and its upstream basins are exposed to flooding and the potential for flood production, to the extent that north of Karaj and the slopes overlooking the city of Karaj due to he highest slope, height, rainfall and poor in terms of vegetation, are categorized with high and very high flood potential. On the other hand, the southern part of the city, due to the slight slope and rainfall compared to the northern part and the slopes overlooking the city of Karaj, and the density of gardens and vegetation and agricultural lands, has the lowest risk of flooding potential.
With the aim of flood management and by using 14 criteria and a combination of weight assessment (WASPAS) locations to store flood water were identified:  4 locations (end of North Taleghani Boulevard with water storage volume of 4620 meters3, northern part of Azimiyeh slope with 8400 meters3 of water storage, northern part of the Atomic energy street with a storage volume of 8400 cubic meters of water and the northern part of the Epsino Valley with a storage volume of 3000 meters3 of water. Assuming a depth of 2 meters in each of the proposed zones, a total of 20 thousand cubic meters of flood water is self-sufficient through which flood water can be managed. Location of deep and semi-deep well drilling site is another way of managing surface runoff and flood that was located using 10 criteria and MCE model and the result is the identification of 16 sites for construction and drilling of absorption wells. Another measure for surface runoff and flood management is to increase vegetation density. The result of examining the relationship between surface runoff and vegetation density percentage obtained from the FCD distance sensing algorithm, shows that zones with maximum runoff have lower vegetation density. 25% of the area has the most runoff, So, according to the cooficit correlation (%98) between runoff and vegetation density, increasing the area of ​​green space and vegetation density from 25% to 75% reduces the runoff to less than 25 mm which prevents the area from causing floods.

The results show that despite the construction of transmission canals and absorption wells in the city of Karaj, in some northern streets such as Tarbiat Moallem Boulevard, Bouali Gharbi and Nawab Safavi Street located in District 8 of the municipality, as well as Goodarzi and Nedaye Shomali streets in the south, no measures have been taken to control the flood, and due to the impermeability level, heavy rainfall compared to the southern part of Karaj and a steep slope (more than 15%), there are conditions to increase runoff on the street. Also, despite the construction of Taleghani, Moazen and Delmbar canals, which direct the surface water from the Taleghani, Siah Kalan and Delmbar sub-basins to the Beheshti collector, due to the lack of transmission capacity of the artificial canals implanted in at the metropolitan area, excessive surface water enters urban streets and generates floods. In order to manage floods and surface runoff in the city of Karaj, the identification of suitable locations for digging absorption wells and water storage pools has been considred, according to which 16 suitable locations for drilling wells and 4 suitable locations for constructing artificial pools had been identified for storing a total of 36,800 cubic meters of water. Also, increasing the surface infrastructure and vegetation from 25% of its density to 75% will reduce the amount of surface runoff by 75%, which leads to flood management.