Micro-modeling key to Cedar Rapids flash flooding fight

CEDAR RAPIDS - Flood-prone Cedar Rapids heads into 2017 with a new game plan to manage stormwater problems, particularly those due to bottlenecks in old sewer pipes during severe rains, such as in 2014 when 4 feet of standing water flooded some streets.

Since formally adopting a Stormwater Master Plan and Sanitary Sewer Master Plan last month, city officials have been working on one of its foundational elements: modeling the micro-watershed basins around the city.

'The core of the plan is the models, and there's different levels of modeling,” said Sandy Pumphrey, a Cedar Rapids flood mitigation project engineer. 'We have an umbrella view of the entire city, but then in order to make smart decisions about changes to infrastructure, we need very detailed models to help us prioritize.”

The master plan is designed to tackle the backlog of infrastructure needs in the stormwater and sanitary sewer systems, either directly by addressing inadequate infrastructure or indirectly through mitigation efforts, over the next 10 to 20 years.

The document identifies and prioritizes work over time but is seen as a living document to be reviewed and updated yearly based on new information.

'Our objective really is to prioritize our capital improvement projects in a cost-efficient manner and use modeling efforts to help us do that,” said Dave Wallace, a sewer utility engineering manager, '(So) we have an objective prioritization and criteria plan that we follow and look at some policy issues that might affect our capital improvement plan going forward.”

One of the first priorities is modeling 12 'micro basins” - two per year, over a six-year period. The first two models have been completed: the Kenwood Basin, which includes older areas just north of downtown, and the O Avenue NW Basin.

Work is underway for the next two micro basin models of E Avenue NW and Indian Creek, Pumphrey said.

Upcoming models include the basins at Rockford Road SW, Czech Village, Morgan Creek, Ushers Ferry, McLoud Run and Cedar River SE and NE.

The modeling is already being incorporated into other projects. Engineers have been using the E Avenue model to assess where water is coming from along that street to identify flood mitigation options before building a water pump station for the city's eventual flood control system.

'If we did not do any mitigation, the pump station would be huge,” Pumphrey said. 'The model helps identify areas to hold back water, so when water gets to the river, it doesn't have to pump as quickly.”

As another example, modeling data shows where to place infiltration structures - such as permeable pavers for parking lanes, bioswales and tree trenches - for upcoming street repair projects on 29th and 32nd streets NE.

Stormwater sewers have a design specification to handle a five-year rain event - a major rain that has a 20 percent chance of happening in any given year.

Whenever the storms are more severe than a five-year event, the pipes can bottleneck, causing water to flow over land, Pumphrey said.

'We know already our pipes cannot handle that, and it will go over ground,” Pumphrey said. 'The idea is not to get rid of all the bottlenecks; we expect some street ponding - but ... to get rid of street flooding that causes property damage or compromises public safety.”

After the 2014 flash flood, the city compiled a list of projects and determined significant needs. The master plan approved last month attached preliminary dollar figures to those needs totaling $75 million to $100 million. Those costs are to be refined over time under the plan, Pumphrey said.

'The needs are substantially greater than the funding we currently have available,” said Scott Olson, a Cedar Rapids City Council member who helped create the plan as chairman of the council's infrastructure committee. 'So you can see it is more important we have a prioritization system so our limited resources are put to good use, and those projects have an impact.”

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