Modelling the impact of climate change and sea level rise on the morphology of lower Meghna River

Abstract

The impact of climate change on a river may be viewed as a complex interaction between climate, hydrology, hydraulics and morphology of the river system. The anticipated changes in temperature, precipitation and sea level is likely to have a profound impact on the morphology of a river. Altered basin water balance due to precipitation changes and rising levels of sea affects the discharge and water level of the river. The combined effect of such changes disrupts the existing equilibrium of water and sediment transport through the channel and affects various morphologic processes of the river such as change in siltation rate and consequent rise of river bed, progradation of delta along the river etc. In order to assess such complex hydraulic and morphologic response of a river to climate change, application of mathematical modelling is essential. The present study is an effort to investigate various hydro-morphological changes of Lower Meghna River due to climate change and sea level rise with the application of different mathematical models. The GCM precipitation projections along with the sea level rise scenarios given by IPCC have been used to construct different climate change scenarios namely A1FI, A1B and B1 for the periods of 2020s, 2050s and 2080s. Based on these scenarios, a hydrological Artificial Neural Network (ANN) model and a hydromorphological MIKE 21 FM model have been developed to assess various hydraulic and morphologic changes of the river. To verify the projections, the ANN model has been calibrated and validated with the available observed data from the year 1975 to 1994. Considering the base period as year 2008, the MIKE 21 FM model has been calibrated for the year 2006 and validated for year 2007. Then the models have used to evaluate the various hydro-morphological changes. In addition, a numerical morphological model has been developed and applied to assess the delta progradation along the river for various climate change scenarios. The study reveals that the Lower Meghna River exhibits high seasonality with higher discharge during wet season and less discharge in the dry season. For scenario A1FI, maximum monthly discharge has been found as 95539 m3/s, 132835 m3/s and 111730 m3/s for the periods of 2020s, 2050s and 2080s respectively. The backwater effect will be more pronounced during dry seasons and will elevate the water level upto 20.4 cm, 16.9 cm and 13.5 cm for scenarios A1FI, A1B and B1 respectively at the end of 2080s. Such effects will initiate heavy deposition along the river. Siltation increases xviii progressively at a rate of 1.02 cm/year, 2.29 cm/year and 2.96 cm/year for scenario A1B upto 2080s and the consequent bed level rise has been found as 1.86 m. Due to excess deposition and higher sediment transport the delta front of the river moves seaward. The maximum progradation of delta front towards sea has been found as 26.56 km during 2080s for scenario A1FI.