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.
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.
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.
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.