ASSESSMENT OF BANK FAILURE IN THE BRAHMANI RIVER, ODISHA USING RIVER MORPHODYNAMICS, FLOOD ASSESSMENT AND NUMERICAL MODELING APPROACHES

Abstract

Floods and river bank failures are a major concern in Odisha, India, as many perennial rivers passes through the state. The intensity of floods has been relatively high in the past decades, resulting in an unprecedented loss of life and property. Rapid urbanization, anthropogenic activities, and an increase in population density proximal to riverbanks impact river morphodynamics. Riverbank failure results from various natural and anthropogenic activities. The construction of dams affects the velocity and sediment-carrying capacity of the river. The occurrence of major floods, cyclones, and amalgamation of both resulted in major impacts on the stability of riverbanks. The acceleration of natural and anthropogenic activities pushes the river towards embankment failure. Consequently, the failure of embankments resulted in the loss of agricultural land, infrastructure and human lives. The erosion of riverbanks affects the sediment budget of the river, which in turn adversely affects the river ecosystem. Among the various rivers in Odisha, the Brahmani River is the second largest river in Odisha and acts as a major lifeline for irrigation, water, and transportation. Multiple events of breaching of embankments and riverine floods result in higher vulnerability of the river. Demarcation of vulnerable zones of river migration, flood-susceptible zones, and critical bank failure locations are the major prerequisites for attaining a safe environment for future disasters. The process of insitu migration of river channels involves the modification of morphological parameters, which influence river morphodynamics at both temporal and spatial scales. Over the past two decades, the Brahmani River has experienced five significant flood events and the impact of Cyclone Phailin. In recent years, there have been numerous instances of embankment failure along the Brahmani River, leading to substantial loss of life and property. The current study aimed to understand the mechanism of insitu migration of river channels by analyzing the temporal scale change from 2000 to 2019. The entire length of the Brahmani River, spanning approximately 799 km, was examined using Landsat data and Bhuvan Cartosat DEM. Planform, erosion, accretion, and stability parameters were analyzed using Landsat data, whereas slope-forming parameters were derived using the Cartosat DEM. The factors of stability, erosion, accretion, sinuosity, and channel area exert considerable influence on directing the river toward in situ migration and embankment failure. The insitu migration zone model was derived by integrating planform, slope-forming, erosion, accretion, and stability parameters using the Analytical Hierarchy Process (AHP). The lower reaches of the insitu migration model map exhibit numerous reported riverbank erosion locations, which are situated in areas of very high to high vulnerability zones. More vulnerable zones were also identified along the entire stretch of the river. This research will greatly aid in gaining a thorough comprehension of the temporal evolution of rivers in relation to in-situ migration areas, thereby enabling the development of effective strategies by governmental and non-governmental organizations for the management of hazard-prone zones and provision of emergency relief in the region. Floods are one of the natural disasters in the Brahmani River basin, which is exacerbated by anthropogenic activities, climate change, unpredictable weather, and heavy rainfall. This leads to significant losses in life, property, and resources. The goal of this study was to evaluate the performance of various bivariate statistical and hybrid models, including the frequency ratio, Weight of Evidence (WoE), Evidential Belief Function (EBF), Index of Entropy Analytical Hierarchy Process (AHP), AHP-TOPSIS, and AHP VIKOR, to create flood susceptibility maps for the Brahmani River basin. Flood susceptibility maps will aid flood management and decisionmaking by identifying high-risk areas. This study considers a range of causative factors, such as slope, aspect, elevation, curvature, geology, geomorphology, topographic wetness index, topographic ruggedness index, distance from streams and roads, rainfall, normalized difference vegetation index (NDVI), stream power index, soil, drainage density, and land-use land cover. The flood inventory was compiled from various sources including atlases and news reports. The models were validated using 70% of the flood training points and 30% of the testing points. The accuracy of the flood susceptibility maps was assessed using the Area Under Receiver Operating Characteristic (AUROC) curves and Seed Cell Area Index (SCAI) values. The results indicate that the Frequency ratio, EBF, and WoE models achieved the highest accuracy in AUROC, with AUC values of 0.91, 0.90, and 0.81, respectively. These models also provided reliable results in the SCAI validation process. The susceptibility maps will help in identifying hazard-prone areas, which will help policy makers better assessment of natural hazards. The study examines the mechanism of riverbank stability in hazard-prone locations along the Brahmani River in Odisha. The morphology of riverbanks is influenced by several factors, such as pore water pressure, riparian vegetation, seepage flow, bank stratification, and fluvial erosion. It is crucial to evaluate the stability of riverbanks to understand fluvial dynamics and identify areas where erosion is most severe, which can then inform the implementation of riverbank stabilization techniques. The study conducted field and geotechnical investigations of vulnerable riverbanks in critical zones, and employed slope stability analysis in PLAXIS 2D using finite element modeling of vulnerable slopes. The analysis incorporated data on water levels of rivers to assess variations that could lead to slope failure. The simulation of 107 events from 2000 to 2013 revealed that significant floods between 2011 and 2013 had a severe impact on the Brahmani River. The peak stage of the river had the highest pore pressure among all the river stages, and the factor of safety decreased to less than 1 during the rapid drawdown of the river stage. Location 15,19 and 23 were identified as the most critical riverbank locations in terms of stability. It is recommended that river stabilization methods be implemented at these critical locations to protect the riverbank from failure. Additionally, it is recommended that similar investigations be conducted in other vulnerable rivers of India in which bank failure occurs.