Engineers vs Nature

Christmas mockingbird
Christmas mockingbird

The Back Story

Willowbrook Reach was a certified Wildlife Habitat when the city engineers came along a few years ago with a plan to address possible flooding issues. Flood prevention sounds like a great idea; nobody wants her house to go under water in the event of a hundred year flood. But when the community looked closely at the details people started to realize that the city’s vision of a functioning creek didn’t quite mesh with our aspirations of nurturing a wildlife habitat.

Austin’s creeks have suffered over the years. The 1950s were a particularly bad decade. Trees and shrubs were removed. Turf was installed and many creeks even had their banks covered in concrete. Everyone lost: not only did the canals destroy habitat but the impervious surfaces caused faster and more dangerous storm-water flow.

Our creek was one of very few spared the indignity of concrete.

While people here don’t always agree about the meaning of the creek our neighbourhood does tend toward green inklings.

The city’s plan to replace ageing waste-water lines and stabilize the creek bank called for the removal of many mature trees and the installation of riprap. Three departments were involved: Public Works, Watershed Protection and Water Utility. I think when these departments presented their plan they were surprised to find an interested public.

And so negotiations began meaning many tedious meetings for those involved. Some trees were saved but the use of riprap near the culvert at 38 1/2 Street was not open to negotiation. Yet, in less than a year since its installation it is already failing. The community was wise to oppose it.


If you don’t know what riprap is you are fortunate. I don’t think I saw it used much before I moved to Austin. The engineers here must really love this stuff because I see it used all over the place. Riprap is sometimes called rock armour. In theory it is used for erosion control.

Basically, it is a stack of rock or concrete placed between soil and water. As you can see below, it is sometimes encased in metal. Ugly stuff. And kind of dangerous. I know of one older woman who tripped when her shoe got caught in the mesh. I hate seeing kids playing near it because I always imagine terrible disasters.

On the north bank of the creek it has been placed terrace fashion.

riprap on the south bank of the creek
riprap on the north bank of the creek as seen from above
riprap north bank
riprap on the north bank as seen from across the creek

On the south side it was placed in a way that resembles scree on a mountainside. I honestly don’t understand why it was placed there in that particular strip.

riprap south bank
riprap on the creek’s south bank

Stones are already tumbling down and the soil between the rocks is being washed away. There is a word for that: it is called erosion. Oops. In this case it seems the solution is causing the problem.

Maybe our civil engineers never read the recommendations put out by FEMA and the EPA. Both agencies recommend against using rock to stabilize steep creek banks. Rock is impermeable of course and so it has a tendency to speed water flow. Faster water leads to greater erosion further down the creek which can lead to the installation of even more riprap. Plus, rock has a tendency to reflect heat and adds to the urban heat island effect. A lot of case studies have proven that wood and plants are much more effective for protecting and stabilizing creek banks.

Riprap is more expensive than using wood, too. Makes me wonder if the local quarries did some lobbying recently.

Bad Feng Shui

Anyone could probably tell at a glance that the riprap installed at Willowbrook Reach would fail — simply because it looks so ugly. Ugliness can be a symptom that something about a situation is wrong.

It seems arrogant to me that engineers might think they can improve on nature when nature has had millions of years to get its systems right.

Water flow has increased over time because of all the paved surfaces in the neighbourhood. Yet the creek adapted. More water meant the survival of more plants. Living plants — especially trees — are ideal because they take that excess water up into their tissues. Many of the plants growing along the creek are deciduous. The leaves fall and become leaf mould which has an amazing ability to retain water. And the timing is right because we get much of our rain during the winter months. Dead wood (like the pecan tree that recently fell) also retains water. As it rots the soil underneath becomes more porous which provides a base for deep roots and opportunities for even more plants to grow. Nature really does know how to repair itself. We just need to follow its lead.

To fix the problem of excess storm-water going into the creek nature is telling us that if we want to help we need to add more plants. Any banks that need stabilizing are probably better terraced with wood instead of stone. Between the terraces we could add native plants suitable for rain gardens.

The ancient Chinese Wu Xing system shows an understanding of this relationship between  water, soil and wood. As I understand it, this tradition uses five elements (fire, earth, metal, water, wood) to describe natural processes. When elements interact, the flow of energy can be affected. Sometimes energy is slowed; sometimes it is sped up.

image from Wikipedia

The city engineers wouldn’t have made the riprap mistake if they knew anything about feng shui.

The red arrows show how interactions slow energy. For example, water slows fire. The grey arrows show how the flow of energy can be increased. Using the example of fire again we see that wood feeds fire.

I love how this chart shows the relationships between wood, earth and water. it is like a recipe for flood control. As FEMA has come to understand, wood is superior at holding or overcoming earth because it is expansive. So when water energies come into a creek system they tend to be drawn into the wood. The wood (through roots or through sheer contact) can then grasp and protect the soil.

What the engineers did instead was use rock and metal to tame water. Adding stone and metal powered up the water energies. At the same time it slowed the wood energies that were the key to solving the problem. In other words the engineering solution did everything wrong.

Nature, local knowledge and ancient traditions don’t always get it right but in this case they seemed in agreement and to have had a better intuition of how to solve the problem of excess storm-water. I can only hope that when the city engineers notice the problem that they will take a moment to reconsider using riprap in the future.


20 thoughts on “Engineers vs Nature

  1. havent heard the term riprap but gabion baskets are definitely a big idea in engineering here – seen here alongside roads holding back mountains where they have sliced and diced to widen the narrow and winding road and then with tree cover gone how to hold those banks in check. and yes as a solution in places they are a bigger part of the problem. good point that ugly can be a symptom about something wrong. very astute debra .


  2. Debra what an interesting post and well thought out. I have some flooding problems I made worse and I will be using nature to solve it now.


  3. Rip rap – loose rock. Gabion basket – rock inside the metal cage. Gabion baskets used to be used all the time, not so much anymore – now it’s the giant limestone boulders. Also, from your picture, it looks like maybe the rip rap was laid down to be of use as an access point to the creek for project installation and/or creek maintenance purposes (the alternative would have been pouring a concrete driveway). In any case, projects done in creeks nowadays are more in line with trying to be as natural as possible to enhance the function of the creek while still providing something substantial enough to not get ‘blown out’ during a flood – a good example of the new project way of thinking is in lower Blunn Creek (by Monroe) where there are a few giant limestone blocks at the toe of the slope for stability with plants on the upper 2/3 of the slope.

    FEMA has a community rating system where each participating community can gain points for various activities which help reduce the impact of flooding to buildings. One of the categories is preserving the natural function and characteristics of the floodplains (i.e. have green space along the creeks instead of buildings). Other categories include doing outreach about flooding, other projects to reduce the impact of flooding (like floodwalls, or even the Waller Creek Tunnel), requiring higher standards for buildings which are built in the floodplain, etc.. The points are then used to get a reduction in flood insurance premiums for everyone in the city.

    One of the major hurdles with stream restoration is that there are federal and City requirements which mandate that flooding potential cannot be made worse by a project – which basically means that any stream restoration project in a floodplain needs to be engineered in a way so as not to cause an increase of flood depth at a building or on non-city-owned land. Very doable in places where the floodplain is contained in the creek or if there’s a park along the creek. Very difficult in locations where a concrete lined channel was installed for the express purpose of reducing the floodplain (like Tannehill Branch creek) if only because there are usually houses built right up to the new bank.


    1. Wow. Thanks for all the info. I have go look at Blunn Creek now. I haven’t been down there in ages. That sounds really interesting. I am relieved to hear that riprap is being used less. As bad as our little strip is what they did to Shoal Creek was a crime.


  4. An excellent post. I have not heard the name riprap. They look like what I would call a Gabion, I have a Pinterest board on them because they interest me.
    While they can be used to slow the flow of water as you have noted it is not the right solution for a creek or river side. I like them as walls, furniture and architectural elements. When used as a retaining walls they will let the water seep through and that might have been what mislead the engineers. I would not use them on a slope to a water source.
    We have lost several jobs becasue we will not use hard scape solutions like retaining walls on the edge of bayous (creeks) as a solution for erosion. There are so many plants that do the job better as you suggest. Here is an interesting article . Virginia Sweetspire (Itea virginica) is a favorite of mine to add to a mix of natives to help stabilize a slope.
    Sad when humans think they can do it better than nature! We try to help our clients understand that we want to work with nature and learn from nature. We encourage using organic compost and organic fertilizers and creating a living ecosystem that is sustainable. This even goes into the whole drainage issue. If we have more water permeable surfaces on our property we don’t burden the city filtration systems and let the rain filtrate down through the soil as it is suppose to do. We try and teach them that if you get your soil healthy with good bacteria and fungi that natural aerate the soil and break down the minerals in an available source for the plants the plants (and trees) can get their roots deeper as the air and water can go deeper. And this also makes them more frost and drought tolerant (big buzz words these days). I know Austin has a thin soil profile and hills so not the same as Houston but I am sure it still works there.
    Unfortunately as Houston grows into a megatropolis it has been a common practice here of putting in loads of concrete and then need to add drainage to the street because their yards flood. Now the city is charging a drainage fee based on the covered surfaces on your property so this helps our case! The new builds are the worst because they put in too many houses compared to open space and create a drainage nightmare. Streets that never flooded do now! And the city spends millions on drain pipes. We suggest our clients work on soil health before putting in drainage to the street. A rain garden in a particularly boggy spot can be a beautiful solution.
    I hope the City of Austin removes all the concrete from the creek sides and works with nature to create a riparian green belt zone instead.

    Liked by 1 person

    1. Great links! Thank you.

      Austin actually holds two ecosystems that literally and figuratively mark a divide of social inequality. The soil of the east side is geographically connected to the prairie soil of Houston (deep and fertile); the soil on the west side looks more like the thin caliche soil you describe. The thin soil is on higher ground (the top of the Edwards plateau) where wealthy people like to build their embarrassing McMansions. When rain comes the water cascades off those impermeable surfaces and heads in a rush to lower ground sometimes resulting in floods like the one at Halloween in 2013.

      The City does have a riparian restoration project that is ongoing thank goodness. Change takes time and it all starts with eco-literacy.

      Liked by 1 person

  5. Humans interacting with living water systems will always involve our making mistakes as we go past coexistence and move towards attempts at control. Running water can be a very patient teacher.

    It is great that concerned residents are involved and paying attention in your situation. Perhaps when we get past relearning the ways our conventional approach is failing us, the community will continue to be part of crafting a solution that works with the creek, rather than trying to overcome its natural tendencies.


    1. I am not -too- worried because we have some competent people here who know how to work with city departments. The next step in the flood control plan is habitat restoration. Those gaps created as the riprap has failed look like planting opportunities. We can build a consensus about what needs to happen next.


  6. We’ve got some of that stuff installed near us in 3 or 4 places. It doesn’t have the metal mesh. I didn’t see why they did it. Erosion didn’t seem to be a problem. Some good plants were growing there and need encouragement and more plants.


    1. The best science always starts with careful observation. I think a lot of these installations were knee-jerk responses to the Katrina disaster and some new rules put out by the corps of engineers. Suddenly floodplain issues came to everyone’s attention resulting in some quick ‘fixes’ to satisfy insurance companies.


  7. An excellent post, Debra. The use of riprap is surely a very short-sighted “solution” to creek problems caused by urbanization. I agree with you that I don’t understand why it’s used so much here in Austin. Is the neighborhood still involved with decisions about the care/protection of the creeks?


    1. Thanks, Tina. I feel fortunate to live in a community that includes activist neighbours — people who not only know about urban planning but have the social skills to work with city departments to get things done. I am confident that we can work past the mistakes made by the city. The riprap failure could be a teachable moment.


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