A floodplain study by the Austin Watershed Protection Department was initiated in 2014 after the Halloween flooding in 2013. The department recently completed Phase 1 of its Onion Creek Floodplain and Flood Mitigation Study, which involved updating the floodplain maps laid out in the 1980s. There are 139 homes that are considered to be in the floodplain.
Phase 2 of the study, led by engineering firm Halff Associates, is underway and involves evaluating which of the various options is the most effective to mitigate flood risk in Onion Creek. The watershed department will take results from the Halff Study into consideration when evaluating options.
“We really hoped there would be a magical solution to come out of this study,” said Pam Kearfott, supervising engineer in the watershed engineering division of the city’s watershed protection department. “The study’s science and modeling has gotten better, but the results are not that different. The reality is it’s a really big watershed, and it's really expensive to try and control it.”
District 5 Council Member Ann Kitchen, who represents the area, attended the meeting to assure residents this was on her priority list.
“Flood mitigation is the key to the game,” Kitchen said. “There’s a lot of work to be done in trying to determine what solution we will take, and my office is here to help us get to a flood-mitigation solution for this neighborhood.”
Pam Kearfott, supervising engineer in the watershed engineering division of the city’s watershed protection department, meets with Onion Creek residents following a flood-mitigation update May 16.[/caption]Stand-alone options
- Regional detention ponds: The option, which involves building detention ponds upstream designed to temporarily hold floodwater before slowly draining once flood conditions have passed, does offer some benefits but does not completely eliminate the risk for residents living within the floodplain, Cindy Engelhardt, project engineer and director of water resources in Austin for Halff Associates, said. The detention ponds would not have provided much protection in the 2013 and 2015 floods, she said. The option does have potential benefits, Engelhardt said, including the removal of 80 homes from the floodplain, which leaves about 60 homes still at risk.
- Cost: $51 million
- Time frame: 10 years or more to implement
- Channel clearing: The project would include removing the underbrush and trees from the creek. Engelhardt said this option is not highest on the list for several reasons, including the impact on the environment. Clearing the area is equal to 126 acres of land and would require extensive annual maintenance. This particular option would require removal of 52 homes from the floodplain, leaving 87 homes still at risk.
- Cost: $36 million
- Time frame: two to five years to implement and ongoing maintenance annually
- Voluntary buyout program: Engelhardt said this is the most effective means of flood mitigation, although there are some downfalls, including making perpetual improvements to the structures. This process is strictly voluntary but would remove all 139 structures from the floodplain.
- Cost: $99 million
- Time frame: two years to implement
Combination options:
- CenTex West Detention Pond with channel modifications: This option would relieve flooding in the Wild Dunes neighborhood but would only protect 111 structures in the floodplain, and would also require the reconstruction of the River Plantation Bridge on I-35 and Bradshaw Lane.
- Cost: $72 million
- Time frame: over 10 years to implement
- Pinehurst flood protection wall with voluntary buyouts: The proposed option involves building a flood wall along the driving range of the Onion Creek Golf Course that would be about 5.5 feet tall and includes interior drainage that would protect the 48 homes along the wall in the Pinehurst neighborhood. Similarly, since the flood wall does not provide protection for the Wild Dunes neighborhood, Engelhardt said Halff is proposing voluntary buyouts. This solution protects all 139 homes in the floodplain.
- Cost: $62 million
- Time frame: five to seven years for construction of the floodwall and to conduct the buyouts
- Pinehurst flood wall with channel modifications: This solution would call for improvements to the River Plantation Bridge to help improve the flow of water downstream as well as a 5.5-foot wall along the golf course in the Pinehurst neighborhood. This option would also remove all 139 homes from the floodplain
- Cost: $88 million
- Time frame: seven to 10 years to implement
What’s next
Halff Associates is working to finalize the feasibility study. The firm plans to make a presentation to the city’s environmental commission on June 7. Once results are finalized, they will be presented to City Council for review.
The city will also need to research avenues of funding for these projects. Kearfott said the projects would not be eligible for federal or state funding. The watershed protection department does have a drainage utility fund it can utilize to help pay for some of the options, but it would not fund the project entirely.
“Funding constraints are something we have to consider,” Kearfott said. “None of these solutions are cheap. Even the cheapest of the options is more expensive than the entire capital improvement project budget for the watershed department in a year."
