Creek Restoration Turns Flood Plain into Park-Like Amenity

Restored Area Survives Massive Storm Without Major Flooding

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Posted: Wednesday, November 28, 2012 6:03 pm

In the spring of 1999, residents of a neighborhood on the north side of Dubuque, Iowa, faced a serious dilemma: With tornado sirens blaring outside, they could either evacuate their homes in the middle of a major thunderstorm, or they could risk drowning in their flooded basements as the water continued to rise around them.

To make matters worse, it’s doubtful that many who occupied the 1,150 homes in the path of the flooding even knew they were at risk prior to the storm. At that time, not many had even heard of Bee Branch Creek or were aware their homes sat within its watershed.

After all, years earlier the portion of the creek that ran through their neighborhood had been converted into an underground storm sewer spilling into a detention pond near the Mississippi River, some 4,500 feet downstream.

For most residents, the creek was out of sight, out of mind … until the night of that unprecedented storm.

Following the flood, then-President Bill Clinton declared the neighborhood a disaster area. But, unfortunately, that was only the beginning of the story. As it turns out, the volume of runoff created by that “unprecedented” storm has been equaled or exceeded six times since 1999, said Deron Muehring, a civil engineer for the city of Dubuque.

It eventually became clear that maintaining the status quo would be unsustainable. Something had to be done.

“We hired a consultant to prepare a stormwater management plan to really look at how stormwater management was occurring in our city and how we could make it better,” Muehring told a group of workshop attendees at the recent Growing Sustainable Communities Conference in Dubuque. “We had businesses and citizens more and more frequently mentioning that they had issues with stormwater.”

City officials decided the solution wasn’t to “modernize” the Bee Branch storm sewer system. Instead, they would take the creek back in time, restoring it to something far closer to its natural state: An open, meandering creek with grassy banks and room to ebb and flow. In doing so, the city had to remove more than 80 homes and commercial buildings that had been repeatedly victimized by the flooding.

Another phase of the project corrected a bottleneck in the creek upstream from the flood zone along West 32nd Street. Over the years, as the town had slowly expanded north, development played a major role in constricting and straightening the Bee Branch Creek sub-watershed, turning it into a shallow trench that fed into an open mud plain before feeding into the underground stormwater system through an existing dam. The area had a basin sediment capture efficiency of 41 percent and could hold approximately 1,100 cubic yards of sediment a year. With approximately 2,600 cubic yards of erosion mass and 80 cubic yards of urban runoff sediment every year, the city was forced to excavate the area every other year to remove the excess sediment.

Eric Thompson of MSA Professional Services, the engineering firm spearheading the project, explained how building a second dam upstream to create a two-celled storage system significantly increased flood storage capacity. The system was designed to result in a 50 percent reduction in runoff rates during 100-year storm events.

“When you’re managing stormwater, there are really three things you can do,” Thompson said. “You can either make a lot of capacity to get the running water out of there; you can make a lot of (storage) capacity to keep that water back; or you can start looking into treating the symptoms. In this instance we were limited with the 10-foot pipe going into the existing system … so we needed to add more storage,” he said.

This two-cell system approach consisted of four major phases including construction of the cells, developing the sedimentation basin, wetland and upland restoration, and the channel re-meandering.

The new sedimentation basin is located upstream from the new dam in the first cell and is designed to mitigate the amount of sediment that reaches the second cell and the stormwater system. Equipped to handle 6,432 cubic yards per year of runoff with a five year cleaning cycle, the city will cut cleaning costs and time by more than 50 percent. The higher concentration of sediment also has extra benefits when it comes to wetland and upland restoration.

Careful attention was paid during the construction phase to include natural grading around the sedimentation basin and in the lower cell to give a more pleasing aesthetic feel to the project, as well as to promote a varying range of wildlife. Closer to the channel, more aquatic plants take root while native prairie grasses and flowers flourish higher up on the banks.

“The thing we are really pleased about with the retention pond is that we were able to retain some of the trees around it and keep it from looking like an entirely newly graded project,” said Thompson. “We had a little bit of tug-of-war because from a flood prevention perspective we wanted as much capacity as we possibly could and all the natural scallops cut into that capacity. But with the variations of the slope we now have different plants and different habitats coming up.”

The final phase of the project consisted of creating a more natural flow of the channel through the second cell. While urban development had straightened the channel’s banks, the new channel has more natural curves to slow the flow of water and to spread ground saturation. By allowing the channel to naturally meander through the second cell, this allows better ground saturation which further encourages vegetative growth through the area, and coupled with sediment retention farther upstream, prevents the area from having the look of a washed-out flood plain.

The result has the newly restored Bee Branch Creek looking more like a park than a drainage system. And, city officials say, the restoration project had lower upfront construction costs and is projected to have lower long-term maintenance costs when compared to conventional stormwater management techniques.

Added to the tangible benefits are a variety of assets that enhance the quality of life in the surrounding neighborhoods downstream. Officials expect the $48 million project to serve as a catalyst for economic development. The open waterway includes amenities such as bike/hike trails, benches, bridges, lighting and landscaping.

While the aesthetic pleasures and lower maintenance costs are benefits, ultimately the project will be judged on performance. Prior to the project, the area saw water flows of 1,760 cubic feet per second. The 2001 Master Plan Concept estimated that water flows could be slowed to 900 cfs and, upon completion, flows are now averaging 380 cfs.

In July 2011, the Dubuque area was hit with the largest storm in the city’s history. Previously the rainfall record in 12 hours stood at 5.5 inches. That July night, the media reported 10.6 inches of rain had fallen at the Dubuque Municipal Airport, located on the south end of the city. Individual accounts elsewhere in the city reported as many as 14 inches. Though areas north of town did experience flooding, the Bee Branch area that the West 32nd Street Stormwater Basin was built to protect was generally unaffected.

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