Flooding from a 100-year hurricane would cause an estimated $1.6 billion in damage to Ascension Parish and its three municipalities, a new parish worst-case estimate says.

Likewise, multiple levee failures on the Mississippi River and along internal parish levees would cause a similar level of damage across a wide swath of the parish.

While Ascension has a 36 percent chance of being hit by a hurricane in a given year — basically once every three years — the parish has a 20 percent chance of a tornado strike in any year.

These and other dire contemplations on natural disasters are contained in parish government’s draft hazard mitigation plan that has been under development since August.

The planning document takes a look at the possible natural hazards in Ascension and lays out steps parish, city and town officials believe can help buttress the parish against those risks, parish officials said.

“What it does is it creates a framework for risk-based decision-making for all the executive leadership in the parish,” said Rick Webre, director of the Ascension Parish Office of Homeland Security and Emergency Preparedness.

Hazard mitigation plans, which are required in every county and parish in the nation, are updated on a five-year cycle. Ascension’s plan was last updated in 2010.

The 276 pages of reports and appendices in Ascension’s plan were written with help from LSU’s Stephenson Disaster Management Institute. Parish officials recently completed the public comment period on Feb. 17.

The plans are required for communities to be eligible for hazard mitigation dollars through the Federal Emergency Management Agency.

Ascension’s plan contains a list of projects leaders would like to see funded. Projects cover a wide variety of topics: drainage improvements on Bayou Conway near Sorrento and in key bayous near Donaldsonville, new sewer lift stations, backup generators for sewer services and key community buildings, storm surge modeling for lakes Pontchartrain and Maurepas and more radios for emergency responders.

According to the plan, the parish has received almost $12.4 million for hazard mitigation projects in the past five years, including finished housing elevation projects and a major planned upgrade to the 4-H Building at Lamar-Dixon Expo Center near Gonzales.

The draft plan incorporates new data from recent significant events, including unprecedented storm surge from Hurricane Isaac in 2012 and flooding from a severe rain storm in late May that unexpectedly inundated Astroland and other parts of Ascension. But Webre said the general risks have not changed much over time, including flooding, thunderstorms, lightning, hurricanes and tornados.

Under the plan, the highest risk events are floods and hurricanes while levee failure is considered a medium risk.

Webre said the new plan also incorporates more input from neighboring parishes, including St. James and Iberville.

“These hazards don’t know parish boundaries,” Webre said.

He has hopes the better coordination will allow the parishes to collect post-storm data regionally and make a stronger case to FEMA for federal funding.

The plan also contains a wealth of data about the parish. Broad sections of unincorporated areas in the parish and its municipalities are in the 100-year floodplain, an area where flood insurance can be required. In all, $2.5 billion worth of homes and businesses carry flood insurance and pay $7 million annually in premiums. The policyholders have had total loss claims of $27.7 million, the report says.

Meredith Conger, parish homeland security planning and intelligence section chief, said the plan is due to FEMA by mid-March, though the plan will be turned in much sooner. After FEMA approval, the Parish Council has a year to vote on the final document.

Follow David J. Mitchell on Twitter, @NewsieDave.