Lincoln's defenses against Salt Creek flooding will face a stiffer test against heavier rains in the future, a study commissioned by the city found.
The city installed the Salt Creek levee system in the 1960s to withstand a 50-year flood, or a flow that has a 2% annual chance of occurring, the study said.
Since then, the city has had three floods equal to or exceeding a 50-year flood level and one, in May 2015, that exceeded the 100-year flood level.
"It's protected us pretty well so far," Lincoln's Assistant Director for Utilities Donna Garden said in an interview last week.
But the city remains vulnerable, she said, and the May 2015 storm highlighted the risk when floodwaters overtopped the levees, swamped the South Bottoms neighborhood and prompted evacuations.
Successive floods in 2014 and 2015, updated rain models from the National Oceanic and Atmospheric Administration and the looming concerns of future climate impacts prompted the city in late 2018 to seek guidance on how to mitigate flood potential in future years.
The models and projected flood risk anticipate Lincoln would see an 0.8-foot rise in floodwaters on Salt Creek during a 100-year event, said Brian Dunnigan of Olsson, which did the analysis for the $262,000 study.
Armed with that information, Olsson recommended a host of strategies to bolster the city's flood defenses, but just what proposals from the Salt Creek Floodplain Resiliency Study should be undertaken became a point of contention during the City Council's recent consideration of the Climate Action Plan.
Olsson estimated building 16 dams or reservoirs in and along Salt Creek could drop flood elevations 2.6 feet for a 100-year flood event under current conditions and would cost about $140 million.
Those structural improvements were not part of the Climate Action Plan. Lincoln Transportation and Utilities instead began by focusing on measures such as changing policies that affect new housing construction and subdivision development.
"We’re dealing with things we can do right now to protect the public," Garden said.
But Mike Eckert of Civil Design Group, which helps design new housing in and around Lincoln, questions the city's immediate focus on development regulations amid concern over housing affordability.
Lincoln is expected to grow by about 50 square miles in the future, and new regulations designed to ensure increased flood protection would affect properties citywide, Eckert said.
"They were putting 100% of the solution on what was only 3% of the area in the entire (Salt Creek) Basin," he said.
Lincoln Transportation and Utilities staff continue to review possible changes to the regulations governing drainage on new developments. Among the changes recommended by the study is a requirement for builders to make the base floor of newly constructed homes an additional foot higher.
After Eckert and his colleagues learned the Climate Action Plan included adopting those requirements, he testified in opposition to the section of the plan that would implement the Salt Creek study's recommendations.
Eckert believes the city has not given enough attention to the physical flood protections such as the 16 dams or fully explored how much changing regulations might drive up the price of new housing, he said.
Further analysis may prove that building dams would bring a level of protection that's not worth the investment, he said.
Since the action plan was amended, the city has agreed to further explore the cost of those regulations, which would ultimately be voted on by the City Council, and will perform a full cost-benefit examination of building new dams or reservoirs.
Building dams would likely hinge on federal funding to pair with local contributions.
Another recommendation likely to take off from the Salt Creek study will be a pilot program allowing homeowners in the floodplain to voluntarily sell their properties to the city, which would likely use unspent stormwater bond dollars to buy properties that can be otherwise hard to sell, Garden said.
The Olsson study did not change the floodplain maps, but based on the NOAA changes, the city will look to redraw the floodplain maps in a few years, she said.
Creating new floodplain maps would take time and additional funding that likely would factor into a stormwater bond in 2023 or 2024, and the resulting maps could add homes and properties to the floodplain based on the higher rainfall intensity anticipated in future floods, she said.
That makes non-structural changes such as policy revisions more important because they can help the city quickly improve its flood ratings with the federal government and in turn help homeowners in floodplains secure discounted flood insurance premiums, she said.
That may not comfort everyone, but the city and its partners will do what they can to protect residents and their property, Garden said.
"Our main goal is to keep people safe and dry," she said.
CLEANING UP FROM THE 2015 FLOOD:
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