Spatial mapping of flood prone areas and risk assessment of Chalakudy river basin using HEC-HMS and HEC-RAS models

Abstract

Floods are one among the most devastating natural disasters that affects life on the globe. For the planning and design of water resources projects in the preferred area, planners and engineers usually require reliable estimates of flood magnitude and frequency. Kerala state in the Indian sub continent received a catastrophic flood in the year 2018. The present study attempts to model the flood flows and map the flood prone areas of a river basin in Kerala. The Chalakudy river basin, one of the worst-affected river basins due to heavy rains and floods was selected for the present study. This is the fifth largest river in Kerala. The basin is predominant with agricultural land and falls under the humid tropical zone, where water resources planning and management is necessary for irrigation scheduling, flood control and design of various engineering structures. In order to address the above issues, an attempt was made to calibrate and validate HEC-HMS model for simulating the flood hydrograph for the Chalakudy river basin. Flood frequency analysis was carried out to estimate the flood peak values using frequency distributions in HEC-SSP software. The results were compared with the estimated flood peak values for different return periods obtained from the HEC-HMS model. Hydraulic routing was done in HEC-RAS model and the flood inundation maps were prepared. The cadastral level risk areas were identified based on water surface profiles of velocity and depth of flood extent and its characteristics. Food vulnerability maps based on land use patterns were developed in order to identify the severely affected land uses. The HEC-HMS model for the basin was developed using SCS-UH, SCS- CN, Recession and Muskingum methods to find out the loss rate, runoff transformation and routing of flood respectively. Statistical performance indices of the model, Nash-Sutcliffe efficiency (NSE) and Coefficient of correlation (R2) values were obtained above 0.7, Error in Peak Flow (%) and Error in Volume (%) were figured below 20% and Root Mean Square Error-Standard Deviation Ratio (RSR) was acquired as 0.5 and below. These values indicated that HEC-HMS model simulation performed well in both calibration and validation. Thefrequency discharge values calculated using Log Pearson type-III distribution indicated a high degree of similarity to the HEC-HMS generated values with an R 2 value of 0.862. The results of the Log Normal and Gumbel distributions are significantly lower than those of the HEC-HMS model values. The assessment of the vulnerability due to the flooding was made with regard to the land use pattern and cadastral level risk map of Chalakudy river basin was developed for different return periods. Kadukutty Panchayat located in the downstream of Chalakudy river basin was found to be the maximum flood inundated area for 10 year return period ( 557 ha) and for 200 year return period (681 ha). Manjapra Panchayat located in upstream was found to be the least flood inundated area for 10 year return period (6 ha) and for 200 year return period (9 ha). Annamanada, Kadukutty, Melur and Pariyaram panchayats were under high risk areas, with depths greater than 20 m. Ayyampuzha, Chalakudy, Mala, Kuzhur, Parakkadavu and Puthenvelikara panchayats were under medium risk areas with depths varying from 10 to 20 m. Athirappilly, Manjapra and Karukutty panchayats were under low risk areas with depths less than 10 m. The flood vulnerability maps were generated by intersecting the flood plain land use map with the flooded area polygons. Paddy land near to the river banks was found to be the highest inundated by different return period floods, followed by forest and other vegetation, barren land and other land use classes.