How to curb losses during annual heavy rainfalls
Flood control measures largely depend on:

The amount of money available to create flood control measures
The availability of raw materials that could be used to create such
measures
The locality of the potentially flood affected region
The source of potential flooding  and its relation to the region that
could be flooded.

One extreme example is Bangladesh.
Most of Bangladesh is located within the delta region of the Ganges and Brahmaputra Rivers; that is where the two rivers meet the ocean.
The region is flat and devoid of easily accessible sources of natural rock. Elsewhere, competent rock, such a basalt, can be located, extracted and crushed to form aggregate  that can be used as a construction material. Because Bangladesh lacks such rock sources it needs to bake clay to create a form of aggregate.
Being positioned at the tail end of two major rivers, the government of Bangladesh does not have control of significant reaches of the rivers to create flood control structures further upstream.
Politically and religiously, flood control measures on either of these rivers could cause a lot Hindus to become very upset.
Financially, Bangladesh is a poor country with a population of 162 million people. If it were able to construct flood control devices on the two rivers, it would also need to construct flood prevention devices along its coast as the combination of flood waters from the rivers and high tides from the ocean can exacerbate flooding and its effects on the country.
Finally, rising sea levels from climate change are only going to make matters worse for Bangladesh, because it is a very low lying country.

Edit 2 August 2020
Concerning successful flood control measures, two quickly come to mind, the Thames Barrier in London, England and the Aswan Dam in Egypt, particularly the Aswan High Dam.

The Thames Barrier is a movable barrier system that is designed to prevent the floodplain of most of Greater London from being flooded by exceptionally high tides and storm surges moving up from the North Sea. It has been operational since 1982. When needed, it is closed (raised) during high tide; at low tide, it can be opened to restore the river's flow towards the sea.


Based on the success of the Low Dam, then at its maximum utilization, construction of the High Dam became a key objective of the government following the Egyptian Revolution of 1952; with its ability to better control flooding, provide increased water storage for irrigation and generate hydroelectricity the dam was seen as pivotal to Egypt's planned industrialization. Like the earlier implementation, the High Dam has had a significant effect on the economy and culture of Egypt.