Frequency Analysis of Flash Floods for Establishing New Danger Levels for the Rivers in the Northeast Haor Region of Bangladesh

A danger level at a river location is the level above which a flood may cause loss and damage to nearby lives, crops, and homesteads. Currently, in Bangladesh, the danger level is designed and updated considering the riverine flood during the monsoon season (June-September). However, during the premonsoon season (March-May), flash floods, almost every year, damage crops in some parts of the northeast Haor (saucer- or bowl-shaped large tectonic depression) region. Damage to Boro rice, the only crop that is grown in this region, is catastrophic to the local population and threatens the food security of the country. At present, there is no warning or danger level to inform the community to save the Boro crops from flash floods in that region. Hence, this study attempts to establish a flash flood danger level at 40 locations in the northeast region of Bangladesh based on flood frequency analysis and elevations of the river floodplains. The homogeneity and adequacy of the data series was tested by performing the run test, lag 1 auto correlation, Mann-Kendall test, and Mann-Whitney test. After passing the data quality tests, flood frequency analysis was conducted using six widely used distributions: lognormal, lognormal Type III, Pearson Type III, log-Pearson Type III, and Gumbel and generalized extreme value (GEV) distributions. Best regional distribution was selected from the probability plot correlations coefficient (PPCC), normalized root mean square error (NRMSE), and mean absolute error (MAE) results. Among the candidate distributions, GEV distribution was found to provide the best results. Then, the danger level was fixed on the basis of the flooding inundation scenarios of different return periods at which the stage is at or near the floodplain level. The results showed that for most of the stations, the 2.33-year flood level coincides with the existing floodplain level. For the rest of the stations, flash flood danger levels were fixed at the 5-year return period's flood level. The recommended flash flood danger levels were found much lower than the existing monsoon flood danger levels. For example, at Sunamganj station, the existing flood danger level is 8.25 mPWD (Public Works Datum) whereas the recommended flash flood danger level is only 6.41 mPWD. At some stations, the difference is too prominent, even more than 5 m, which can cause severe destruction for the Boro rice cultivation during a flash flood. It is expected that flash flood danger levels estimated in this study may assist in flash flood forecasting in the northeast Haor region and provide an effective warning at the time of necessity, thus minimizing the loss and damage by the flash flood to some extent. (c) 2019 American Society of Civil Engineers.