Salinity hazard assessment in coastal area of Bangladesh

Abstract

Salinity hazard has become one of the major concerning problem in Bangladesh coast. It has several adverse impacts on ecosystem, agriculture and livelihood of people. Saline water intrusion is caused by a complex interaction between freshwater flow coming from upstream river systems and saltwater flow coming from tidal forcing of the ocean. Several parameters like upstream river discharge, sea water level, tidal storage, salinity magnitude, tidal excursion, residence time, etc. play the vital role in determining the intrusion process of salinity. Salinity hazard is generally assessed by salinity magnitude. In this research, a new indicator named as Non-Dimensional Index for Salinity (NDIS) is developed to assess salinity hazard that incorporates not only salinity magnitude, but also other governing parameters associated with saltwater intrusion process. Residence time of salinity, named as RT, is identified as one of the governing parameters in NDIS. RT represents residing time of saline water in an estuary. An equation is developed to compute the residence time for a specific salinity magnitude in an estuary. NDIS is developed through non-dimensional analysis. To calculate the spatial and temporal variability of the governing parameters of NDIS and RT, a 2-Dimensional salinity model, Delft3D is applied. Validated model simulations from Delft3D salinity model are used to compute NDI, RT and Tidal Excursion in base condition and in two future scenarios. Two future scenarios considered in this study are – sea level rise and cyclone superimposed on sea level rise condition. It is found that governing parameters of salinity intrusion are non-linearly interrelated. Residence time of salinity follows the general spatial variation trend of salinity magnitude along the coast but not maintaining exactly one to one relation. It is also found that in most of the estuaries, residence time and upstream discharge are the most dominant parameters of salinity hazard. In base condition, the south-central region has the lowest salinity hazard (NDIS<30) whereas southwest region has the highest salinity hazard (NDIS>700). It is also found that Residence Time is lowest (<15 hours) in the middle part of the south-central region and gradually increases toward the south west region to exceed 80 hours for a 12 hour tidal cycle. Hence, salinity of similar magnitude resides for a longer time in south-west region than in south-central region. Sea level rise mostly affects the mouth and upper part of south central coast. For a SIDR-like cyclone in sea level rise condition, salinity hazard is highest along the cyclone track and, also affects upper part of south central and southwestern coasts. It is found that assessment of salinity hazard with the new method realistically represent the salinity condition in the coast in a changing climate. This assessment method of salinity hazard can be used in Coastal Zone Management or in Delta Plan as it takes into account all the physical processes involve in salinity intrusion.