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Last week I happened to be in Southern Malawi looking at issues in relation to the planned Shire Valley Irrigation Project and saw (at a distance) some of the land covered in silt and the tents occupied by people displaced by the floods. Your question happened to arrive when I was in a meeting with World Bank and Government people and I took the opportunity to ask them about any plans in relation to the effected land.
While the World Bank is putting a lot of money into rebuilding damaged infrastructure (https://www.worldbank.org/en/news/press-release/2015/05/07/major-support-from-world-bank-to-rehabilitate-malawis-flood-damaged-infrastructure-and-livelihoods) there is little that they or Government can do in relation to clearing land buried under a great depth of sand. The cost of moving it would be huge and where would you move it to?
If it is really sand then it may be difficult to start farming on top of it as there will be no nutrients in the soil. River silts in some places are very fertile and a prized agricultural resource but not sure if this is the case here. The best approach in the medium to long term would be to get organic matter into the soil through compost, crop residues or animal manure. Artificial fertilisers will help and Malawi does benefit from the fertiliser subsidy programme but the land will hold little water and will quickly drain after rain carrying away fertilisers.
Some advice on farming in sandy soil can be found at the following links and there is much more on the internet on the subject; ftp://ftp.fao.org/docrep/fao/010/ag125e/ag125e03.pdf https://msfp.org.au/wp-content/uploads/2013/06/Farmtalk-no37-Success-with-Sandy-Soils.pdf https://permaculturenews.org/2014/08/26/permanently-improve-sandy-soil/ https://www.grow-it-organically.com/gardening-in-sandy-soil.html
The problem derives from extensive deforestation, often for charcoal burning, in the catchment and the Malawian habit of planting on ridges that do not necessarily follow contours. The disturbed soil is left bare and easily eroded when it rains. Reforestation, providing alternative fuel sources and Conservation Agriculture in the catchment may partly resolve flooding in the future. Also the on-going Shire River Basin Management Plan with the current raising of the Liwonde Barrage by 40cm and the management of the various structures to include a flood defence function as well as the optimisation of the hydro-electric schemes should reduce floods in the future. I'm afraid that none of this is of much help to those affected today.
Hope that some of this helps.