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."

Launching the party in Sydney, founder Diaa Mohamed told Fairfax he wanted a louder voice for Muslims.
"There are a lot of parties out there to specifically oppose Islam and Muslims, yet Muslims don't have any official representation," he said.
Australia has a number of anti-Islam parties, including one backed by Dutch MP Geert Wilders.
Australia is not short of political parties. From today you can add another to the list: the Australian Muslim Party.
But its chances of winning a senate seat are slim. Even in the parliamentary constituency with the highest proportion of Muslims, just over 20% of people follow the Islamic faith.
"Anyone can set up a political party," ABC's political analyst Antony Green told me.
"It doesn't mean you're going to get elected and it's far from clear how much community support he (Mr Mohammed) has."
Read more from Jon Donnison: Australian Muslim Party's tough road to representation
Mr Mohamed, a 34-year-old businessman, also defended launching the party immediately after terror attacks in Paris this week.
"There are going to be a lot of questions raised in the coming days of the events recently, and this is the whole reason we created this party," he told the ABC's AM program.
"So it is as good a time as any to launch it."
Mr Mohamed condemned the attacks in Paris and said Islam strictly forbade the killing of innocent people.
But he said his party would never support the invasion of a Muslim country in response to an incident such as the attacks in Paris.
"Let's look at how well that's worked in the past. We've invaded Afghanistan â€¦ we've invaded Iraq, and we're in the mess we're in right now," he said.
"So would I support something that's never worked in the past? No."
A £7.5m scheme to protect an area devastated by floods almost four years ago is set to get underway.
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Australia's first party representing Muslims intends to contest senate seats at the next federal election.