The Many Faces (And Facets) of Flooding

October 23, 2016  | By Shoal Creek Conservancy

This blog post was written by Andy Sobchak. To learn more about the author, please visit this site.

I find it hard to describe my emotions when I see flooding like we saw in 2015, but the term awesome comes to mind.

I type that word hesitantly, knowing our community has suffered great loss over the years, but also intentionally, because awesome is one of those strange English adjectives with potentially opposite meanings depending on use. It can mean stunning or staggering, wonderful or daunting, inspiring of admiration or  inspiring of fear.

Flooding here, as everywhere, is a process as old as the Earth, a process which in the natural context – like a forest fire – possesses a certain beauty, regenerative quality and purpose. But flooding within the human context is nothing short of devastating. When life and property are consumed by it, flooding assumes that menacing persona with which most of us are familiar. It’s Mother Nature at her angry worst.

Because of this duality, awesome seems like the perfect word to use.

Not Just A Rainfall Issue

Flooding along 10th Street at Duncan Park – Memorial Day 2015 Flood.

The City of Austin Watershed Protection Department (WPD) estimates ~275 structures are in danger of flooding, lying within Shoal Creek’s 100-year storm floodplain. Floodplains – areas outside the typical banks of a watercourse – which are demarcated based on flooding probability: The 100-year storm floodplain will be inundated by a storm which has a 1% chance of occurring in any given year, while the 5-year storm floodplain will be inundated by a storm which has a 20% chance of occurring in any given year. Austin’s 100-year storm drops 10 inches of rain in 24 hours and the last time we experienced one was only six years ago when Tropical Storm Hermine hit the Bull Creek watershed especially hard. The storm which produced Shoal Creek’s Memorial Day 2015 flooding wasn’t even close to 100-year status.

But the flooding of Shoal Creek is not solely an issue of rainfall depth or ferocity of storm. How the watershed reacts and generates runoff as well as the creek’s efficiency in draining runoff downstream greatly impact the extent of flooding.

Timing Can Mean Everything

I am a self-professed water geek, and one aspect of stormwater management I always like to highlight is how runoff timing can be manipulated to reduce flooding or eliminate it altogether. The best analogy for this is rush-hour traffic:

At least twice a day (and seemingly now every day) in Austin, automobiles course out of driveways, parking lots and garages and flood our streets, trying to make their way from home to work or vice versa. The moment when this “flood” exceeds the limited capacity of our transportation infrastructure (i.e., highways and roads), we experience gridlock, the worst occurring at constrictions, such as lane reductions on I-35, or intersections like 6th and Lamar with long traffic lights. (Water and traffic “flooding” are so similar that water and transportation engineers use similar modeling equations and software programs.)

Urban mobility advocate Metropia recently released a video which depicts this congestion (and flooding) problem well. You may even remember the rice and funnel demonstration here from middle school.

When a jar of rice is emptied quickly into a funnel, rice grains clog the funnel stem and flood out of the funnel mouth. When the rice is poured at a moderated rate, all the grains make it through the constriction. Timing may be everything in comedy, but it is critical in flood and traffic management, too.

Capetown, South Africa, is one of the latest global cities looking to battle gridlock by aggressively promoting staggered workdays where employees’ days end at 3pm, 4pm, 5pm, 6pm and so on. The staggered release means a greater chance of everyone getting home quickly without having to increase existing levels of infrastructure. (Mayor Adler in recent speeches has encouraged Austin business and civic leaders to follow this formula.)

Engineers, scientists, land use planners and others who manage watersheds can similarly manipulate the timing of urban runoff through the implementation of specific stormwater management and planning practices. Even in the Shoal Creek watershed, which is considered to be “fully developed,” opportunities may exist to retrofit neighborhoods with green stormwater infrastructure (GSI) or low-impact development (LID) features. These alone are likely not silver bullet solutions, but locating them strategically in the watershed’s upland areas may help alleviate flooding in the urban core because they slow runoff down and delay its release into the Shoal Creek system.

Implementing GSI and LID with new developments and retroactively for older ones is like pouring the rice slowly into the funnel. It is the equivalent of a staggered work day to combat rush hour. Dredging the Lower Shoal Creek channel or building an underground mega storm sewer, like Waller Creek’s or the Shoal Creek Tunnel proposed in 1991, is like trying to widen the stem of the funnel to increase drainage capacity.

Many Facets = Many (Possible) Solutions

The flooding of Shoal Creek may be natural, but we’ve certainly made it worse by hardening the watershed without providing suitable controls for the corresponding increase in runoff which follows. And we’ve elevated our risk of loss by building and living too close to the creek. Simply, our city grew up in a time when water management practices were not as advanced as they are today.

  • 53% of the Shoal Creek watershed has a hard/impermeable surface.
  • 56% of development in the watershed occurred before the adoption of drainage regulations in 1974, and 71% was constructed before the adoption of water quality regulations in 1991.
  • Currently, only 21% of the watershed’s hard/impermeable surfaces have water quality controls for runoff.

-City of Austin

Shoal Creek fundamentally floods because it does not have capacity to convey the runoff generated from large storms, but many factors affect Shoal Creek’s flooding equation. Runoff timing is a major factor along with channel geometry and other watershed characteristics. Because there are many factors in the equation, there may be many possible solutions or combinations of solutions to investigate. In 2017, WPD will be commissioning the $900,000 Shoal Creek Flood Mitigation Study which will examine the feasibility of these possible solutions.

In their May 2016 final report, the City of Austin’s Flood Mitigation Task Force stated: “There is no practical way to build systems that can take on water bomb levels of rainfall.” It’s true. Shoal Creek has always flooded and will continue to flood, and Mother Nature will always be able to conjure up storms capable of ‘bombing’ more water on our little creek than it can handle.

Our job now, in the human context, is to think ahead and mitigate the risk.