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Most of the lands of the southern coastal zone of Bangladesh are protected from tidal flooding and storm surges by embankments constructed during the 1960s and 1970s, creating polders. Sluice gates in the embankments connect the canals (former river distributaries) inside the polders to the surrounding rivers, providing the opportunity for water intake from, or drainage to the rivers by judicious management of the gates. Yet, unlike northern and central Bangladesh, the polders have not benefited from the genetic breakthroughs which underpinned the Green Revolution during the 1960s. Flooding during the wet season followed by severe drought and high levels of salinity during the dry season plague the coastal zone, limiting the application of improved technologies that increase agricultural productivity (amount of harvestable produce per unit of land). While the potential of these technologies has been clearly demonstrated, the lack of an integrated landscape approach to water and crop management precludes agricultural intensification in the polders. Therefore, the ability of community-led water management and land-use planning at the watershed level was tested to improve agricultural productivity in a polder in southwest Bangladesh during 2015–2017. Engaging with communities to employ more pragmatic land-use planning (based on topography/hydrology) and scheduling of sluice gate operations has shown the potential to intensify and diversify cropping systems in the polders, and increasing production by twofold to threefold. This was achieved using improved rice varieties and non-rice crops in wet and dry seasons, respectively, enabled by strategic management of the sluice gates to drain the lands as needed during the wet season and to facilitate timely establishment of non-rice crops after rice harvest. The community-based water management approach has been the key entry point for making inroads toward transforming the agricultural production systems in the polders.

Prior to implication of mathematical model as a tool for any kind of policy making or decision making, the model must be calibrated against appropriate observed data. The two-dimensional sediment transport model of the Bay of Bengal usually being calibrated against suspended sediment concentration at different locations comparing the model results with field measurements to make the model performance to a satisfactory level. Since field data of sediment concentration is rarely available for model calibration, an attempt has been made in the present study, to generate sediment concentration data by using satellite images to calibrate the sediment transport model in a better way. Calibration of sediment transport model result against image generated sediment concentration at different locations of Bay of Bengal show acceptable agreement.

The Meghna Estuary is formed by the combined flows from three great rivers in the south-east Asia; the Ganges, the Brahmaputra and the Meghna. This estuary experiences dynamic hydro-morphological changes due to very high discharge of water and sediment from these three mighty rivers. Erosion and accretion occur simultaneously at very high rates; such that, the flow and sediment discharge through the estuary are the third highest and the highest, respectively, in the world. On the other hand, the tidal current itself is also strongly affected by the dynamic morphology changes. It is thus essential to understand the hydraulic behavior and interactive features of tides and morphology change in the estuary. With this end in view a two dimensional general model of Bay of Bengal was developed under cyclone shelter preparatory study (csps) undertaken by Bangladesh Government and the model was updated later during other projects. The present article focuses on development of model, its set-up, boundary conditions and few calibration results of this two dimensional hydrodynamic and morphology model. The model applications clearly show the variation of the flow structure; their speed and direction separately for monsoon and dry season around the model area which covers the northern part of Bay of Bengal more specifically the Meghna Estuary.

Mathematical models have advancement to a point where they are considered to be an effective tool for simulating natural phenomena in coastal regions. This paper discusses the development of Bay of Bengal Model (BoBM), updating of the model with the recent bathymetry and shore line of islands and coastline and upgrading from rectangular grid to finer size of mesh grids by using latest version of MIKE21 FM modeling system. This model is very useful for the hydrodynamic study in the coastal region of Bangladesh. The article also presents the model set-up, boundary conditions and few calibration results of the model. The model applications clearly show the variation of the flow structure, their speed and direction separately for monsoon and dry season around the model area which covers the northern part of Bay of Bengal.

Langat River is located in a financial strategic place in Malaysia where a number of industries have been developed over the time. All of them are completely depended on Langat River for water supply and navigation. Besides, the river water is also used for different purposes such as domestic use, agriculture, fisheries etc. Therefore,the quality (salinity level)and quantity ofthe river water is very important for the country from financial point of view and that is why the assessment of salinity level in the river system is essential for severe low flow event such as extreme drought condition. The assessment of salinity intrusion during extreme drought condition requires use of scientifically based and tested state-of-the-art mathematical modelling tools. This paper presents the selection of extreme drought event through Stream Flow Drought Index (SDI) and the assessment of salinity intrusion through Langat River system using such tested modeling tools. Three different models namely Hydrological, Hydrodynamic and Salinity models were developed and calibrated under this study to assess the salinity intrusion through Langat River. The study determines the extreme drought year as 1998 and 2010. All the models were simulated for2009 (an average condition), 1998 (extreme drought), 2010(extreme drought) and no flow from upstream. The results show that the Langat River is not vulnerable for salinity intrusion for any kind of extreme drought event but if there is no flow from upstream 1ppt saline line may intrude more 9km towards upstream. Keyword:Langat River, Stream Flow Drought Index, Mathematical Modelling, Salinity Intrusion

The coastal embankment system has been gradually built during the last 40 years. The embankments were originally designed to increase agricultural production by preventing salt water intrusion not to protect against cyclonic storms. The alignment of the embankments did not consider the changing conditions in bathymetry of the sea and thalweg migration of the rivers and therefore many embankments are located under tidal water level and have severe toe and slope erosion problems during the monsoon season. The crest level and embankment cross sections have not optimized the protection of hinterland and the embankment itself and therefore the embankments typically only provide protection for the cyclones with 5-12 year return periods and the designed crest level of the sea facing coastal polder equal to the sum of normal maximum recorded water stage plus 1.50m. In this study to estimate the design crest level and side slope for sea facing embankment have been established based on maximum storm surge level, wave run-up for cyclonic wave, freeboard allowing 5 l/m/s overtopping ,potential climate change impact and land subsidence. Statistical analysis of surge level and wave run-up is carried out using Extreme Value Analysis (EVA) in MIKE Zero.

Bangladesh is the largest delta of the world formed mainly by alluvial deposit of the Ganges, the Brahmaputra and the Meghna river systems. These three mighty rivers carry annually more than a billion tons of total sediment. The sediment load is then conveyed by Lower Meghna River and discharged into the Bay of Bengal. The river borne sediment inflow from the three major rivers the Ganges, the Brahmaputra and the Meghna influences the morphological development of the Meghna Estuary. The knowledge on sediment deposition volume and its distribution in the Meghna estuary is limited. It is important to know how much sediment is retained in the Meghna estuary and distribution of sediment deposition and erosion for understanding the morphological developments and planning of land reclamation and erosion control. Analysis of time series bathymetric chart collected under different project over the years has been done to analyze the spatial distribution of sediment deposition and erosion. The Natural Neighbour method is used for interpolation of surveyed data to develop sea bed topography using Mike 21 FM. The entire area has been divided into seven sub areas and erosion/ deposition volumes have been calculated based on interpolated depth values of the maps using 50x50m grid. The analysis of bathymetric data shows that accretion process exceeds the erosion process in the Meghna Estuary. Erosion process is dominant in the upper region of the estuary part i.e. from Chandpur to north of Bhola island, north coast of Hatiya island and northeast coast of Bhola and along the Boyer char (Ramgati). The highest rate of deposition is seen in the Sandwip, Jahajerchar and Urirchar area. The net deposition of sediment over the last ten years (1999 to 20098) in the entire estuary is about 2,760 Mm3 and annual average is about 276 Mm3.