Flood Risk Assessment Due to Urbanisation and Climatic Factors using Geospatial Approach: A Case Study of Ghaggar River Basin

Abstract

Flooding, among other natural disasters that can also be called anthropogenic, is the most frequent and devastating. Flooding over an area depends on rainfall and geomorphological, climatic, and social factors. From 1900 – 2021, approximately 5593 flood events occurred worldwide, out of which 66% occurred after 2000. In the Asian continent, 2324 flood events occurred during 1900-2021. In the Asian continent, 13.5% of the total Asian flood occurred in India, followed by China (13.3%), Indonesia (10.6%), Philippines (6.7%), Pakistan (4.6%), Afghanistan (4.2%), Bangladesh (4.1%) and other countries. The decadal economic losses in India found to be $0.1billion during 1960-1969, $1.6 billion during 1970-1979, $4.0 billion during 1980-1989, $9.5 billion during 1990-1999, $16.8 billion during 2000-2009, $41.2 billion during 2010-2019 and 13.6 billion during 2020-2021. Floods in India have majorly occurred in the well-known river basins of India such as Ganga, Brahmaputra, Yamuna, Mahanadi, Testa, Kosi, Cauveri, Narmada, Tapi, Krishna, etc. Studies of flood hazard and vulnerability has been conducted widely in large river basins or at the regional level. Less or no attention has been given to the small river basins. Ghaggar River Basin is one of the small basins located in northern India and sub-basins of the Indus River basin. Several flood events of different intensities have occurred in this river basin. However, the 1993 flood event was the most devasting flood in the basin. This study aims to determine the rainfall trend analysis and extreme rainfall events in the study area. This study focussed on analyzing flood zones in the Ghaggar River basin using rainfall trends and extreme rainfall as flood factors. The impact of change in land use land cover in urbanisation on flooding over an area has been analyzed, and future flood maps have been projected. The dataset used for this study was collected from different sources. Software such as ArcGIS, Terrset Idrisi, SPSS, R, and Climdex were used to analyze these datasets. The descriptive statistics of monthly, seasonal, and annual rainfall were analyzed. The average rainfall ranges from 1.8 – 551.6 mm monthly, 7.6 – 1421mm seasonal, and 266.7 – 1703.2mm annually. The rainfall trend analysis was analyzed using the Mann-Kendall (MK) test and Innovative Trend Analysis (ITA) method and compared with the results. The serial correlation from the data was removed before the trend was analyzed using the MK test. 90% of the monthly, 85% of the seasonal, and 91% of the annual results were found to have similar trend direction on both MK test and ITA method. The trend slope, i.e., change in the amount of rainfall, lies in the range of (-)9.4 – 5.65mm/year monthly, (-)17.02 – 7.21mm/year seasonal, and (-)23.09 – 4.93 mm/year annually. The annual rainfall trend is increasing at the highest rate at Lehal, whereas the decreasing rate at the highest rate at Banur region. The change point of rainfall trend was analyzed using Pettit’s test, Standard Normality Homogeneity test, and Buishand’s test. The change point was assumed where the two or more method shows a similar changepoint. The average rainfall lies in the 379.4 – 2223.2mm range before the change point, decreasing to 53.4 – 1470.9mm after the change point. After the change point, the average rainfall decreases at all stations except the Lehal. The trend magnitude of rainfall was (-)13.62 – 94.95mm/year before the change point, which decreased to (-) 43.9 – 23.35 mm/year after the change point. The rainfall trend has been observed to decrease after the change point. Extreme rainfall was analyzed using Expert Team on Climate Change Detection Indices (ETCCDI), and ten indices were analyzed using Climdex software in R. The trend analysis of these indices was analyzed using the MK test and ITA method. The average CDD (Consecutive Dry Days) was found to be in the range of 69.5 – 223.2 days, whereas the average CWD (Consecutive Wet Days: wet days are when the rainfall for the day is 1mm or more) was found to be 3.4 – 9.6 days a year. The trend magnitude of CDD and CWD varies in the range of (-)2.97 – 24.97 days/year and (-)0.714 – 0.093 days/year, respectively. The total precipitation on wet days (PRCPTOT) varies in the range of 69.5 – 1828.6mm, and the trend magnitude changes in the range of (-)35.53 – 21.45 mm/year. The simple daily precipitation intensity index (SDII) lies in the range of 16.3 – 26.9 mm, and the trend magnitude of SDII lies in the range of (-)1.391 – 1.276 mm/year. R10mm, R20mm, and R30mm are the days received 10mm, 20mm, and 30mm of rainfall, respectively. The number of days for R10mm, R20mm, and R30mm lies in the range of 9.8 – 42.4, 5.9 – 27.5, and 3.6 – 19.2, respectively, and their trend magnitude lies in the range of (-)1.944 – 0.023 days/year, (-)0.805 – 0.116 days/year and (-)0.611 – 0.280 days/year respectively. The average rx1day, rx3day, and rx5day in the Ghaggar River basin lie in the range of 66.4 – 160.3 mm, 100.2 – 252.4 mm, and 114.2 – 311.3 mm, respectively, whereas the trend magnitude lies in the range of (-)2.803 – 6.011 mm/year, (-)4.73 – 9.25 mm/year and (-)3.898 – 9.688 mm/year respectively. Flood risk was analyzed using Graph Theory and Analytical Hierarchy Process (AHP). The flood risk was analyzed by integrating flood hazard and flood vulnerability analysis. The factors used for flood hazard analysis are Elevation, Land Use Land Cover (LULC), Hydrologic Soil Groups (HSGs), Rx3day, and Distance from rivers and slopes. The factors used for flood vulnerability analysis are Population density, LULC, Household Density, Distance from roads, literacy rate, Efficient drainage system, Distance from flood shelters, and Rainfall trend. The flood zones have been classified into five categories: very low, low, moderate, high, and very high, based on the score obtained from Graph Theory and the AHP method. It is observed from Graph theory results that around 29.8% of the basin area is prone to very high flood hazard, whereas 70.1% of the basin area is prone to high flood hazard. Similarly, around 17.4% of the basin area is prone to highly vulnerable to flood, whereas 82.5% of the basin area is prone to moderately vulnerable to flood. The flood hazard analysis using AHP methods shows that around 14.54% (2033.5 km2) and 85.2% (11922.7 km2) of the basin area lie under the high and very high categories of flood hazard, respectively. Around 0.15 % of the basin area (21.3 km2) is observed under the moderate flood hazard category. 4.4% (620.7 km2), 93.25% (13035.2 km2), and 2.3% (322 km2) of the basin area lying under the moderate, high and very high categories of flood vulnerability, respectively. The flood risk of the Ghaggar River basin varies from moderate to very high category. But, most of the basin, i.e., 94.1% (13165.2 km2), is at high risk of flood, whereas only 5.52% (771.6 km2) is at very high risk of flood. The basin areas under moderate risk of flooding were observed to be 0.2% (41.09 km2). Both methods' results are compared, and high flood risk zone was obtained through graph theory and AHP. Flood risk was also analyzed considering dynamic factors of urbanisation and population density changes and projection flood map based on the urbanisation and population density change map. Urbanization was analyzed using the LULC change analysis using the Land Change model. The LULC map of 2001 and 2010 was used to analyze LULC and predicted for 2019 to validate with the actual 2019 LULC map. The LULC map for 2030 is projected from the model. The urban built-up category of LULC has gained 8.59 sq. km of its area from Savannas (82.51%), grasslands (2.45%), and croplands (15.03%) during the 2001-2010 transition. The urban built-up of the study area was 450.72 sq. km in 2010 and increased by 8.68 sq. km in 2010, 14.97 sq. km in 2019, and 10.42 sq. km in both 2030 and 2040. The population density changes are analyzed directly using the rate of changes in density as per Macrotrends change rate. Ghaggar River basin’s actual and predicted average population density is 457, 499, 549, and 582 per sq. km for 2011, 2019, 2030, and 2040, respectively. The flood hazard and vulnerability zone of the Ghaggar River basin, considering Urbanisation and population density changes, was analyzed using the AHP method and flood maps were projected for 2019, 2030, and 2040. The flood hazard analysis using urbanisation and population density changes shows that the high flood hazard zone increases with time from low and moderate hazard zone. The area under high and very high flood-vulnerable zones is increasing with respect to time, and the area under moderate zones is decreasing. The moderate flood risk decreases, and the high flood risk zone increases with time. The area of flood hazard, vulnerability, and risk from the lower category to the upper category has been directly correlated with increased urbanization.