Margaret Hughes, 91, died and 400 homes were overrun by water when the River Elwy burst its banks in St Asaph, Denbighshire in November 2012.
Work to protect the city from future flooding starts on 10 October.
A drop-in session on Wednesday outlines plans which include removing the Spring Gardens Bridge and replacing it with a higher and wider structure.
Natural Resources Wales (NRW) hopes this will improve flow capacity downstream and reduce flood water levels in the city.
The first phase will cost around £3m and is being funded by the Welsh Government with work scheduled to finish in May 2017.
The second phase  - set to start in January 2017 - will see flood defences raised and improved in the city and at other locations.
Planning permission and funding for the second phase are yet to be finalised.
NRW's Tim Jones said: "Building a flood risk scheme of this scale involves a vast amount of preparatory work - from modelling the flood risk, finding options for a scheme, deciding on a preferred option, working on detailed design and securing all permissions and funding.
"This takes time but we are delighted to be in a position to start building now.
"While we can't always prevent flooding from happening, we believe we have a robust scheme for St Asaph that will significantly reduce the risk and provide effective, long-term peace of mind for people in the city."
NRW estimates the entire scheme will be completed within 12 to 18 months. Until then, short-term measures to reduce flood risk will continue on the River Elwy.
Councillor Denise Hodgkinson said: "It's very welcome. People who were flooded still can't sleep at night when it rains.
" Unfortunately it's taken a long time to raise the funds and put the right permissions and plans in place. Hopefully they will get it right first time."
A £7.5m scheme to protect an area devastated by floods almost four years ago is set to get underway.