Shoal Creek flooding a north Austin park this week actually wasn't a bad thing; here's why

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2 days ago

AUSTIN (KXAN) — Shoal Creek flooded on Wednesday evening when Austin was hit by severe storms that brought damaging winds, large hail, and localized flooding to an approximately 10-mile stretch of town.
During the storm, part of Beverly Sheffield Northwest District Park was overtaken by floodwaters from nearby Shoal Creek. But that's not a bad thing; it's actually decades-old flood mitigation plans at work.
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According to the city of Austin, Beverly Sheffield Northwest District Park serves a 'dual purpose.'
'During dry weather, the area is a district park. During wet weather, the area holds stormwater to reduce flood risk to properties along Shoal Creek,' the city's website explained.
When there's heavy rainfall, once the creek's flow hits a certain threshold, water will enter the park near the tennis courts. As water levels rise in the creek, they rise in the park, and as the amount of water in the creek decreases, the water in the park starts draining through two outflow pipes. One is near the duck pond and the other is near the south parking lot by the baseball field, per the city's website.
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The dam and detention pond at Beverly Sheffield Northwest District Park were built in 1986 following what Henry Price, a supervising engineer with the Austin Watershed Department, called a devastating flood on Memorial Day in 1981.
He explained how the park's flood mitigation works.
'If we were just going to make it simple, after a response to a storm event, you know, the spillway starts engaging when we have about 4,000 cubic feet per second of flow in the creek. And so when you think about a cubic foot per second, if you thought about a basketball, and you had 4,000 basketballs flowing down Shoal Creek every second, that's when water would start coming into the park,' Price explained. 'And we anticipate that that happens, you know, like during the 10-year event, or there's a 10% chance that that happens every year.'
That detention pond captures and holds water that would otherwise stay in Shoal Creek and likely flood homes and other structures immediately downstream of the park, Price explained.
Price said the park sometimes suffers as a result of mitigating flooding, but the trade-off is that residential areas nearby are spared.
'We had a pretty significant flooding event in 2015 on Memorial Day,' Price said. 'Water came into the park, similar to what we saw this week. There were some of the park facilities that were damaged, but it was pretty minor, and the Parks Department was able to kind of get the park back up online relatively soon after that, but it did help spare the downstream neighborhood [from] having water in their houses and things like that.'
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As far as any damage from this week's flash flooding, Price said the city and the watershed department are still assessing the direct impacts.
Ivey Kaiser, executive director of the Shoal Creek Conservancy, said via email that conservancy staff are also still patrolling the creek and assessing the damage. Shoal Creek Conservancy is also hosting volunteer opportunities for anyone wanting to help with cleanup efforts.
The city's website notes that the dam and detention pond need 'heavy maintenance' and modernization. Price said that's simply because of its age.
'It's been almost 40 years since the city constructed it, as with any type of civil work project asset, like a road or, you know, storm drain pipe or a sewer pipe, we have to maintain and update those from time to time to make sure that they're as effective as they were when they were first constructed,' he said.
Price also talked about an ongoing project that the Watershed department is coordinating with some other city departments to modernize the pond and 'harden the infrastructure [and] address some ongoing issues that we've been having there.'
According to the city's website, that project includes the following:
Adding overtopping protection to the top of the embankment
Repairing erosion that has occurred on the creek side of the embankment
Removal of woody vegetation on the embankment
Protecting the embankment from erosive foot traffic within the park
Reinstalling drainage behind the retaining walls
Sealing cracks and joints within all retaining walls
Reestablishing positive drainage near the tennis courts
Reestablishing positive drainage near the duck pond
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