The report, released by Lincolnshire County Council today (Friday) outlines how over 150ml fell in two days, leading to the overtopping and erosion of the relief channel to the River Steeping on June 12.
Nearly 600 homes were evacuated, with some people still not expecting to be able to return to their homes until later this year.
Lincolnshire County Council, which is the Lead Local Flood Authority, launched the review after a £96,000 underspend was revealed on its flood management budget.
Officials said this was because the budget is for response spending, and just 21 flood incidents were recorded in 2018/19.
However, 40 separate investigations were started in the two weeks following the floods.
Other factors highlighted in the report which played a part in the flooding included the design of the banks, amount of vegetation on the banks, silt levels, the level of water in the relief channel, cattle grazing on the banks, overflowing surface water and foul drainage systems caused by the significant rainfall and water flow.
Coun Colin Davie, executive member for Environment at Lincolnshire County Council, commented: "This was some of the worst flooding we've ever seen in the county – an absolutely awful experience for the community. We commissioned Norfolk County Council to find out what caused it, and to find out how we can reduce the risk of it happening again.
"We're pleased to see this independent report, but I would have liked it to be more focused with more solid recommendations.
"I do hope this report will go some way to answering some of the questions on why homes were flooded in this unprecedented event. I also hope it gives residents reassurance that we, as the lead local flood authority, will work together with all the agencies involved to do our very best to reduce the risk of this happening again."
Along with the causes of the flooding, the report highlights 18 key things agencies and locals could do to try to protect the town in the future.
These include, amongst others, assessing the conditions of the banks along the Steeping, monitor silt levels and vegetation growth, reviewing maintenance programmes, improve land management practices e.g. controlling cattle and managing the effects of burrowing animals, implement flood mitigation measures, look at ways to better manage surface water, and to develop community plans.
Colin added: "I'm pleased to see that over the past few months, in anticipation of the report, there has been much going on in the Steeping community. We have been supporting our partners, including the Environment Agency and Internal Drainage Boards, with their enhanced maintenance programmes along the river banks and have been looking at how water in the wider catchment area can be better managed in the future."
The report is available in full on www.lincolnshire.gov.uk/wainfleetThe report will be heard and discussed at the Flood and Water Manager Scrutiny meeting at the Storehouse in Skegness on Monday, February 24.