Migrating rivers, consequent paleochannels: The unlikely partners and hotspots of flooding

Flood vulnerability of a few areas in the foothills of the Western Ghats: a comparison of AHP and F-AHP models

Flooding is one of the most destructive natural catastrophes that can strike anywhere in the world. With the recent, but frequent catastrophic flood events that occurred in the narrow stretch of land in southern India, sandwiched between the Western Ghats and the Arabian Sea, this study was initiated. The goal of this research is to identify flood-vulnerable zones in this area by making the local self governing bodies as the mapping unit. This study also assessed the predictive accuracy of analytical hierarchy process (AHP) and fuzzy-analytical hierarchy process (F-AHP) models. A total of 20 indicators (nine physical-environmental variables and 11 socio-economic variables) have been considered for the vulnerability modelling. Flood-vulnerability maps, created using remotely sensed satellite data and geographic information systems, was divided into five zones. AHP and F-AHP flood vulnerability models identified 12.29% and 11.81% of the area as very high-vulnerable zones, respectively. The receiver operating characteristic (ROC) curve is used to validate these flood vulnerability maps. The flood vulnerable maps, created using the AHP and F-AHP methods, were found to be outstanding based on the area under the ROC curve (AUC) values. This demonstrates the effectiveness of these two models. The results of AUC for the AHP and F-AHP models were 0.946 and 0.943, respectively, articulating that the AHP model is more efficient than its chosen counterpart in demarcating the flood vulnerable zones. Decision-makers and land-use planners will find the generated vulnerable zone maps useful, particularly in implementing flood mitigation plans.

Assessment of the impact of flood on groundwater hydrochemistry and its suitability for drinking and irrigation in the River Periyar Lower Basin, India

Owing to the consequences of Anthropocene climate changes, extremes of weather and climate disasters such as heavy precipitation, tropical cyclones, and floods, have strengthened in every region across the globe and are badly affecting the earth system processes. The climate change has a tremendous impact in the Western Ghats, the UNESCO heritage site in India, and has to be studied in detail. Floods are the most common (and among the most deadly) natural disasters bringing heavy destruction in the Western Ghat river basins of India, and the Periyar basin is one among them. The low-lying areas of Periyar river basin (RPLB) is one of the worst affected river basins in the 2018 Kerala floods. Since the basin was continuously monitoring, the groundwater quality has been assessed in Periyar basin during three periods (pre-flood, flood, and post-flood-April, August, and October 2018 respectively) and is described in this paper. Since the water table is shallow in RPLB, floodwaters quickly reach the groundwater table, thus influencing the groundwater quality. So, the groundwater samples were collected from 26 sites in RPLB, and analyzed for quality determining parameters. Parameters like pH, TDS, EC, TH, NH₃^-, NO₂^-, HCO₃^-, SO₄^2-, SiO₄^4-, Mg²⁺, and K⁺ decreased during flood compared to pre-flood times. On the other hand, Ca²⁺, Na, and Cl^-, turbidity values were increased during flood times. The values of pH, TDS, EC, turbidity, NH₃^-, HCO₃^-, SO₄^2-, SiO₄^4-, Mg²⁺, Na⁺, and K⁺ were slightly increased in post-flood period compared to flood period. When compared to the pre-flood values, turbidity, NH₃^-, SO₄^2-, Na⁺, and K⁺ were slightly increased in post-flood period. It was also observed that pH, TDS, EC, TH, NO₂^-, Cl^-, HCO₃^-, SiO₄^4-, PO₄^3-, Ca²⁺, Mg²⁺, and E. coli values were higher in pre-flood periods compared to the post-flood scenario. Except pH, turbidity, and E. coli, the averages of all other parameters are falling under the recommended drinking quality values of WHO. The pH belongs to acidic nature throughout the study period. The Piper plot indicates the dominance of Ca-Mg-HCO₃ type in pre-flood, and mixing of Ca-Mg-HCO₃ type in both flood and post-flood periods. The Gibbs plot reveals that the concentration of elements is mainly depending on rock weathering and sub-surface water flow. The USSL plot depicted that the 92-96% of samples are noted as lower salinity and low sodium hazard at the source of C2S1 and C1S1; however, the 4-11% of samples are noted as high salinity but low sodium hazard at a source of C4S1 and C3S1. The Wilcox diagram indicates that the 88-96% of groundwater was found as higher suitability for irrigation during the study periods. Based on GIS based WQI model, out of the 26 samples studied, 69% of wells showed improvement in water quality after flood; meanwhile, 19% well water samples in flood were observed for lower quality compared to pre-flood times and 12% of samples remain unchanged during flood. Though floods are having positive and negative impacts, from this study, it is clear that quality of the groundwater in the RPLB is not severely affected, but they became diluted to permissible limits during flood and post-flood periods except some locations. Since the flood impact studies on groundwater systems are meager, this data from Periyar basin can be used as baseline groundwater reference data for all future flood-related river basin studies and will be very beneficial for the policy and planning needs in the context of climate change. It is high time to establish the baseline data of all river basins of Western Ghats since the normal earth system processes are worse affected by the recurrence of floods which are reporting every year.