The sea level is actually rising. But the actual temp of the rise is essentially negligible: It is 3 mm per year, that means, 30 cm per century. This would not be a problem even for Bangladesh.
Unfortunately, it is predicted that the temp of the rising increases. Fortunately, in the next century this will nonetheless not be very much. The predicted range is from 0.3 m to 1.2 m for 2100. Worst case scenarios give 2.4 m. This would be already some problem in the delta of the Ganges and Bramapthra. Let's look at the size of the problem. The light blue defines the land which would be below sea level after a rise of 3 m:
So, in the most serious case, the worst case scenario, there would be some problems to be solved in Bangladesh already during the next century.
What would be the possibilities to solve this problem, taking into account that Bangladesh is a rather poor country now, so that one cannot assume very large investments?
Fortunately, simple methods to solve this problem are well-known. They have been applied since medieval times by the Dutch.
The most important thing is to build high enough dikes. What is "high enough" depends on the danger of large floods. In principle, to establish this would be the job of insurance companies. Their specialists would look at the dikes, and then estimate how probable it is that they fail, and, in dependence of this estimate, define the price of the insurance.
Then, once we talk about land below the water line, there have to be pumps. Some amount of water will be there, by rain, and one cannot be sure that it will not be too much. So, there have to be pumps to get rid of the unwanted water. And, of course, there has to be sufficient drainage so that the unwanted water flows to the place where the pump is installed to get rid of it.
One should not forget about another possibility: to pour soil onto the land. If one buys good soil, this is also a good method to improve the quality of the land.
The point which makes this a reasonable, efficient way to solve the problem with the rise of the sea is that the temp of the rise is so small that all one has to do is to pour more soil onto the land than is lost by erosion: Actually one needs only 3 mm per year. If this speed heavily increases, giving the worst case scenario, one needs 3 cm per year. That means, for a piece of land of 100 \(m^2\) (or 1 a) one would need only 5 t of earth. Even in Germany this would be affordable for less than 250 €. Thus, to compensate in such a way the 3 mm per year of the rate today, one would need 25 € per year and per 100 \(m^2\).
Near the river, that means, on the pieces of land most endangered by floods, one could use techniques similar to beach nourishment. One simply takes out mud and sludge from the river to the pieces of land near the riverbanks. Taking out sludge from the river is something which is anyway done regularly to clear the shipping lane. A lot of sludge comes down with the river. If there is no heavy contamination, sludge will not be bad quality as soil, possibly even good quality.
What could one do if there would be enough money for a really large investment to secure the territory of Bangladesh from possible sea level rises? Let's take a rough look at this. Let's assume we want not only protect existing land, but also, like the Dutch, gain new land. So, let's try to estimate the size of the investment.
The main part would be a large dike which separates the river as well as the sea from the area we want to protect. This may look like on the following picture:
Such project proposals, if made by laymen, are usually complete nonsense, usually because the costs would be beyond anything reasonable. So, let's try to estimate at least the order of magnitude of some key costs of such a project.
The length of that dike would be of order 500 km. According to floodmap.net the depth seems to be almost everywhere less than 10 m, I have found only a few places where it is between 10 m but less than 20 m. Let's assume an average height of 16 m. The most trivial way to construct the dam would be to throw the material down from the water surface from ships or, for the parts on the top, from the border of the already constructed part. The material falling, under water, on the top of the dam, would slide down until the angle of slope is slow enough. The angle where it no longer slides down is the angle of repose. For most of materials like stones, sand or so it is larger than \(\arctan(0.5)\approx 27^o\). For that angle, the cross section would have an area of \(16 m \cdot 32 m \approx 500 m^2\). This gives a volume of \( 2.5 \cdot 10^8 m^3\) for the whole dam. The density of such materials is usually less than 2 tons per \(m^3\) so that this will be less than \(5 \cdot 10^8 t\). How expensive would it be to get so much material and transport it to the place? I prefer to make a guess from above, looking at something plausibly much more expensive. So, let's look at the costs in a high price land like Germany, where I have found an offer for fill material for around 5 € per t and transport for 6 € per t. This would be costs for the material and the transport for the 500 km of 5.5 billion €. Thus, this part of the wall would be affordable even for Bangladesh.
There would be, of course, something more necessary for such a project. A lot more small dikes on various parts of the river. Streets for the transport of the material, in particular on the top of the dikes. But these are things which are already done anyway even in Bangladesh. There are a lot of subtleties to ensure that the dam is stable (means there have to be a sufficient number of big stones) as well as sufficiently impermeable for water (which requires material of small size, like sand, clay, mud), all this sufficiently stable (so that sand will not be washed out, here medium sized stones may be helpful). Whatever, it is known that such things can be done because they already have been done, in the Netherlands, with the Afsluitdijk, a dike with a length of 32 km finished 1932. (See a video about the construction.)
Fortunately, the more complicate and expensive things, in particular at first gates for the river itself which one could close on high tide and open on low tide, and later even pumping the whole river into the sea, would not be necessary initially. High enough dikes along the river would do the job initially as well. Then, of course, prices in Bangladesh will be different - but with high probability lower than in Germany. So, taking everything which is really necessary into account, this will hardly change the order of magnitude of the investment, thus, it would remain affordable for Bangladesh.
Once initially one would not have to care about managing all the water of the river itself, it would not be a problem to allow for some controlled amount of inflow of river water into the protected area as well as some outflow out of it into the sea. This would replace the salt water inside the protected area with freshwater, as it has happened in the IJsselmeer, which became a freshwater lake after the Afsluitdijk was closed. (The process of replacement would start already earlier, if one starts to build the dike on the sea side first.)
It would make sense to preserve some sufficiently large area as a freshwater reservoir which will be filled during the rainy season from the flood of the river and used during the dry season for agriculture. But as well one would like to gain more land for agriculture. So, in a next step, one could get rid of some part of the water inside the land protected by the outer dikes. Here one would not need much power for pumping. Probably evaporation alone would be already sufficient to get rid of most of the remaining water after all the inflow of water is stopped. But some pumping would be useful anyway, given that some forms of incoming water cannot be prevented (rain, groundwater). If not, all what matters is that there is pumped out more than what is incoming. See this video about some of the problems which may arise on the way toward using the land for agriculture.
To summarize, even a really big project that protects not only existing land but adds even a lot of new land actually covered by water - the Bay of Bengal - would be affordable by Bangladesh in principle even today.
With increasing sea level, one would have to increase the heigth of the dikes, not only of the part which protects the land from sea side, but also along the river inside the land.
After some time, it would make sense to build gates on the river which could be closed at high tide and open at low tide. With even more increase, one would finally have to pump the water of the river into the sea. Once this is done, one has no longer to increase the height of the dikes along the river, but only those of the big dike toward the sea. But this will be hardly necessary, not even in far future. Only the worst case scenario, where the whole Antarctica ice shield melts, would require this, and even in this case not in short times, but after millenia.
Disclaimer: Of course, such a big project would be something horrible from the point of view of the green movement. One can be sure that it would endanger and destroy some ecosystems and endanger some particular species and so on. I do not care here about such things - I think such a project would be justified if it helps the people living in Bangladesh.
But, in fact, this was only an excersize in evaluating the possibility of doing such things. The point is that this could be done in principle, and that it would be affordable in principle. It is not a proposal for a real project. Why?
One could think that Bangladesh has, nonetheless, not enough money. No, this is not the point. Even if Bangladesh would not have the necessary money for such a project now, they could easily gain them from China. Imagine the following offer made by Bangladesh: You do all the investment into the dikes and as the payment some outer part of the new territory gained from the Bay of Bengal would be given to China, as Chinese territory. So, China could built there a big port on the Indian ocean on its own newly created territory. Would China reject such an offer? I doubt. One can be also sure that this new Chinese port would be connected by very good connections to the land, with railroad and street on the top of the dike and further improved connections with China. So, Bangladesh could all this gain for free, by selling to China ground which is covered today by the sea and of no use for Bangladesh.
So, what is missed in the calculations above? Very simple, the gains which are coming from that project. I have not computed them. It seems unnecessary: If the climate alarmists were correct, then such a project would prevent the starvation to death of millions of people of Bangladesh or so, who would have to die of starvation once all the arable land would be destroyed. So, the gains would be astronomical. But in reality, the situation is different. The gains would be simply land which could be used for agriculture. But this is not worth that much today. There is actually enough of such land in the world to feed the world population, and no need at all for doing expensive projects to gain simply more land.
The situation of the Dutch people was different at the time when they build Afsluitdijk. At that time, there was, indeed, a necessity to gain land for agriculture. And those gains have helped the Netherlands to become one of the world leaders in agriculture. But with modern technology, especially with fertilizers, farm land was no longer necessary for the Netherlands. So, one polder was used for more urgent needs, namely for building houses instead of farm land. A last one was simply left full of water just for tourism and ecology and so on (see this video).
So, the chances that this project will be realized are, actually, zero. But this does not change the point: If the horror scenarios of the alarmists would become true, the value of new farmland would rise astronomically. And the project would become a good investment, and would be realized.