Salt Plug Formation Caused by Decreased River Discharge in a Multi-channel Estuary

Abstract

Freshwater input to estuaries may be greatly altered by the river barrages required to meet human needs for drinking water and irrigation and prevent salt water intrusion. Prior studies have examined the salt plugs associated with evaporation and salt outwelling from tidal salt flats in single-channel estuaries. In this work, we discovered a new type of salt plug formation in the multi-channel Pasur River Estuary (PRE) caused by decreasing river discharges resulting from an upstream barrage. The formation of a salt plug in response to changes in river discharge was investigated using a conductivity-temperature-depth (CTD) recorder during spring and neap tides in the dry and wet seasons in 2014. An exportation of saline water from the Shibsa River Estuary (SRE) to the PRE through the Chunkhuri Channel occurred during the dry season, and a salt plug was created and persisted from December to June near Chalna in the PRE. A discharge-induced, relatively high water level in the PRE during the wet season exerted hydrostatic pressure towards the SRE from the PRE and thereby prevented the intrusion of salt water from the SRE to the PRE.

Spatiotemporal variation of Van der Burgh's coefficient in a salt plug estuary

“…Approximately 68 km upstream from Akram Point, the Chunkhuri Channel connects the Pasur River to the Shibsa River at Chalna. The interconnecting channel contributes to complex water circulation between the Pasur and Shibsa estuarine systems (Shaha and Cho, 2016). There is no direct link between the Shibsa River upstream and the major freshwater source, the Ganges River.…”

“…Salt water intrusion into tropical estuaries has received substantial attention in recent years due to changes in rainfall frequency and intensity levels. In addition, salt water intrusion can be aggravated by decreasing river discharges that result from barrages being built upstream to provide water for drinking and irrigation (Shaha and Cho, 2016). Changes in river discharge levels alter estuarine circulation, stratification, flushing times, salt water intrusion, as well as the transport of biota and dissolved and particulate materials such as salt, pollutants, nutrients, and organic matter (Azevedo et al, 2010;Lee and An, 2015;Savenije, 2012;Shaha and Cho, 2016;Valle-Levinson, 2010).…”

“…In addition, salt water intrusion can be aggravated by decreasing river discharges that result from barrages being built upstream to provide water for drinking and irrigation (Shaha and Cho, 2016). Changes in river discharge levels alter estuarine circulation, stratification, flushing times, salt water intrusion, as well as the transport of biota and dissolved and particulate materials such as salt, pollutants, nutrients, and organic matter (Azevedo et al, 2010;Lee and An, 2015;Savenije, 2012;Shaha and Cho, 2016;Valle-Levinson, 2010). Therefore, it is particularly important to understand the responses of estuarine salt transport mechanisms to temporal changes in river discharge levels be-cause salt water intrusion may lead to shortages of drinking and irrigation water (Khan et al, 2011), decreased rice production (Mirza, 2004), reduced freshwater fish habitat (Dasgupta et al, 2014), and inadequate industrial freshwater supplies (Mirza, 1998).…”

“…Salt water intrusion in the PRE has received substantial attention in recent years due to increases in the magnitude and frequency of salt water intrusion upstream as a result of climate change -from which there is a predicted sea-level rise of 30 cm by the year 2050 (Intergovernmental Panel on Climate Change (IPCC), 2007) -and decreases in river discharge levels resulting from an upstream barrage (Shaha and Cho, 2016). Most previous studies focused primarily on analyzing the relationship between discharge and salinity in the PRE (Mirza, 1998(Mirza, , 2004Rahman et al, 2000;Uddin and Haque, 2010).…”

“…Recently, a new type of salt plug formation was discovered in the multi-channel PRE. This was found to have been caused by decreasing river discharge levels resulting from an upstream barrage (Shaha and Cho, 2016). However, earlier work typically omitted details of the salt transport mechanisms in the PRE, and these details are necessary for a complete understanding of the hydrodynamics and causes of salt water intrusion upstream.…”