Trends in the Frequency and Intensity of Compound Coastal Flooding Events along the Indian coastline during 1980-2020

The Bay of Bengal and the Arabian Sea adjoining the coastline of India are breeding grounds for depressions and tropical cyclones, with 2 to 3 cyclones making landfall every year on average. The frequency and intensity of compound coastal flooding events are expected to increase as the world continues to warm. The impact of these events will also be more due to the growing exposure and vulnerability of human settlements in the coastal areas of India. The compound coastal flooding events are primarily driven by extreme sea levels and heavy rainfall during tropical storms and depressions making landfall near the coast. However, there is no comprehensive study on the trends in compound flooding scenarios with reference to Indian coastline. This study presents results from an analysis of compound extreme flood events in the Indian coastal region and assesses the change in frequency and intensity of these events based on in-situ data for the period 1980-2020. The hourly sea-level data was obtained from 9 Tide Gauge stations (TGs) operated by the Survey of India. The daily rainfall data at these stations are extracted from 0.25° resolution gridded rainfall product of the India Meteorological Department (IMD). Harmonic analysis is carried out on the detrended sea-level data to separate the astronomical tide component and obtain skew surge time series at predicted high tide timesteps. The extremes corresponding to 90^th, 95^th and 98^th percentile thresholds are identified for both skew surge and rainfall time series, and the co-occurrence probability of the two extreme events is analysed for the historical data. The evolution of frequency and intensity of the potential compound flood days over the historical period is also investigated.