September 2009, Vol. 21, No.9

Problem Solvers

Problem Solvers - Modeling Software Enables Atlanta Officials To Build a Stormwater Retention Pond

Problem: Insufficient runoff and sewer modeling data hampered plans to build a stormwater retention pond in a low-lying area of Atlanta.
Solution: Two-dimensional modeling software simulates overland runoff and its interaction with underground systems.

Flooding from stormwater runoff in low-lying areas of Atlanta, especially around its City Hall East and a nearby shopping center, discouraged development in the area, made it unusable during flooding, and led to costly recovery efforts after flooding.

One of the worst-known cases of flooding at City Hall East resulted in 1.2 m (4 ft) of water in a loading-dock area, according to Kai Iaukea, hydraulic modeler task manager for Clean Water Atlanta.

To prepare for development in the area, city officials wanted to address the flooding issues by building a retention pond. According to the Old Fourth Ward neighborhood’s Web site, Atlanta allocated $30 million to create a 2-ha (5-ac) retention pond as the centerpiece of the Historic Fourth Ward Park.

City Needs a New Model
Using the pond for capacity relief for the combined sewers allowed the city to use funds earmarked for sewer capital improvements. However, the city needed a method to model stormwater runoff to size the pond accurately and design conveyance systems to maximize its use. In addition to sewer models, an accurate model of aboveground runoff was necessary to plan installation of the pond, a necessary component of City Hall East’s redevelopment.

Previous attempts to estimate the necessary size of the pond calculated runoff using the U.S. Soil Conservation Service method and factoring in naturally occurring detention areas. This resulted in a conservative estimate in which assumptions were made, according to Iaukea.

New Modeling Module Solves Problem
The city previously used InfoWorks CS from Wallingford Software (Fort Worth, Texas) for system modeling and another model for runoff modeling. The solution to their modeling problems came in October 2007, when Wallingford released a new extension, InfoWorks 2D.

“The 2D module is unique in that it allows modelers to see what happens when collection and/or storm networks become full,” said John Howell, account manager at Wallingford. “During peak weather events, the network can become surcharged and overflow. Where does that overflow go? That’s what InfoWorks 2D models — the movement of fluid over the land until it ultimately re-enters the sewer system, reservoir, or other retaining feature.”

The new module simulates how runoff travels over land, as well as how it interacts with the underground system model, unifying the two previous software programs in a “linked solution that saves hundreds of man-hours,” Howell explained.

In a case study from Clean Water Atlanta, Iaukea said that “InfoWorks 2D works with the existing sewer system model by bringing in topographic data and modeling how runoff travels above- ground and its interaction with the enclosed sewer system, either by depositing runoff into a manhole with capacity or by taking overflows from a manhole and routing the overland flow.”

“The ability to view the flow of stormwater runoff aboveground and how it interacts with the combined-sewer system is incredibly powerful,” Iaukea said.

The module can define naturally occurring detention areas, as well as indicate where water flows from them, the path of runoff, and time of overland flow before runoff enters the sewer system.

Some issues were encountered with the new modeling software, according to Wallingford’s presentation “InfoWorks 2D — Finding an Above Ground Solution for an Underground Problem.” For example, the models were calibrated to a 1-year, 24-hour design storm, but the city planners wanted the pond to be built to hold runoff for a 100-year design, 24-hour storm, so the data had to be interpolated to account for this.

“Many of the initial shortcomings have been solved by recent updates that have sped up runtimes and now allow for rainfall to interact directly with the ground-model mesh,” Iaukea said.

Future Looks Bright (and Dry)
Clean Water Atlanta estimates that millions of dollars were saved by planning the retention pond. Original capital improvement projects were to include upsizing of capacity for combined sewers “in an already congested underground system,” according to Iaukea.

The city held a groundbreaking ceremony for construction of the Old Fourth Ward Park in October 2008. Project construction began in July 2009 with a projected duration of 1 year, according to Iaukea.

In addition to cost savings and more-accurate models for design, the new models enabled planners to identify other potential flooding problems in the area. The software also revealed where additional drainage inlets could be placed to alleviate flooding.

According to Iaukea, InfoWorks CS and the 2D module are now used by the entire city of Atlanta to determine system capacity and to aid in master planning.

Flooding issues in Atlanta’s Old Fourth Ward neighborhood will be alleviated with the help of the new retention pond.