Mudslides test LA’s debris basins – RCHS proposes a new solution

Flash floods can send walls of water, mud and rocks powering down the mountains, into debris basins, destroying life and property in their way. Rios Clementi Hale Studios suggests a "micro basin" concept for breaking the speed and catching the debris higher up the slopes.

The Dunsmir Sediment Basin in Southern California (image courtesy FEMA).

In 1988, writer John McPhee wrote a lengthy two-part essay in the New Yorker entitled “Los Angeles Against the Mountains.” In it he described in fascinating detail the 150 debris basins that abut the San Gabriel mountains. They are meant to capture the mud and rocks that power down the mountains when dislodged by a flash flood.

“They were quarries, in a sense,” he writes, “but exceedingly bizarre quarries, in that the rock was meant to come to them. They are known as debris basins. Blocked at their downstream ends with earthfill or concrete constructions, they are also known as debris dams. With clean spillways and empty reservoirs, they stand ready to capture rivers of boulders—these deep dry craters, lying close above the properties they protect. . . For fifty miles, they mark the wild boundary like bulbs beside a mirror. Behind chain links, their idle ovate forms more than suggest defense. They are separated, on the average, by seven hundred yards. In aggregate, they are worth hundreds of millions of dollars. All this to keep the mountains from falling on Johnny Carson.”

A debris flow destroyed this Los Angeles home in 2010 (image courtesy Geo Space)

These debris basins form a line of defense for the millions of people who live below the mountains on the urban coastal plain of Los Angeles. An engineer named Donald Nichols told McPhee: “To make the area inhabitable, you had to put in lined channels on the plain and halt the debris at the front. If you don’t take it out at the front, it will come out in the plain, filling up channels. A filled channel won’t carry diddly-boo.”

Now there are 162 debris basins and according to architect Greg Kochanowski, principal at Rios Clementi Hale Studios, designers of many buildings and parks including Grand Park, they are coming to the end of a 50 year lifespan. In his view the large debris basins are not very successful because they don’t protect structures along the sides of mountains and hills, and, once filled, they have to be emptied and the materials trucked away, creating added waste, energy and maintenance costs.

So he and his team wondered if there could there be another way to protect against the terrifying walls of water, mud, cars and anything else picked up en route that are euphemistically called debris flows.

They proposed a concept that has won them accolades within the landscape design community, entitled SLIDE, and its goal is to absorb the power and the mud and rocks through a design that blends more harmoniously with the landscape while still permitting people to live in mountain zones at risk of mud flows.

Kochanowski explained their thinking to DnA: “With mudslides it’s all about velocity and volume. And so as things come down the hill they pick up more debris and speed up and really create that devastating impact. If there is a way to capture that or slow that much further up the hill, you can reduce that overall velocity and volume and then mitigate the amount of damage that would happen down below. So the thought is to truly capture before it it can come up to these kind of breakneck speeds.”

“We asked, could there be another way that we could rethink the singular infrastructural elements into maybe more of a kind of network of micro basins that would not sit at the end of the flow but would actually sit within the flow?”

What the team came up with was a system of micro debris basins placed at many points up the slope of the mountain, instead of one large one at the bottom. These would allow debris to accumulate but let water pass through, like large sieves.

Diagrams here and below illustrate the Slide concept of micro basins designed by Greg Kochanowski and team at Rios Clementi Hale Studios

The project proposes little V-shaped gradients that capture debris as it comes down the slope.

Then, the rocks and mud and other materials caught in these micro basins could form the basis of “restructuring the urban fabric… in a productive way” in areas like new linear, urban parks in the historic flow patterns.

“What we’re looking to do is create a kind of resilient infrastructure, one that performs the way that it should to defend property and people, but also allows for a kind of rebuilding or reconstituting of the mountainside so that habitat can resurge much more quickly,” he said.

Kochanowski says that longterm we need to consider a resilient symbiotic posture rather than a “defensive posture.” By defensive he’s referring to our tendency to put up walls, walls that eventually fail.

While SLIDE is a proposed solution to catching debris within the mountains, he says the same philosophy can be applied to roads and highways. After all, sections of coastal freeways like the 1 and 101 have been obliterated by mudslides.

Kochanowski says we need to soften the edges of our roads and highways. Right now for example you’ll see Jersey barriers or K rails — those portable concrete barriers that are like upside down Ts — put up to divert water and mud from people’s properties. These turn streets into chutes of mud and water.

He says instead we should be adding swales — a kind of ditch with planting in it that filters water back to aquifers and stabilizes the soil — along perimeters of sidewalks or streets that can capture rainwater and mud and serve as a natural interface between traffic and hillside. You can see more and more swales around the edges of parking lots.

Rios Clementi Hale Studios’ SLIDE concept is speculative and longterm.

Right now, there are several things people can do to protect their homes and neighborhoods from mudslides, including structural changes like building flow areas, channels, retaining walls or deflection walls to direct the flow around buildings (being sure not to direct it onto a neighbor’s property!); and planting deep-rooted plants that hold topsoil in place and are fire resistant. It’s possible to get mudslide insurance, and FEMA suggests getting a geological survey of your property to better know the area’s likelihood of mudslides.

But in the long term, says Kochanowski, we have to think more holistically about how we live in the wildland interface zone.

The story of California since it’s been developed is that we pay a price for living in paradise. It is a glorious but unstable landscape and people are spreading further into areas that are prone to danger.

Short of ceasing to build in risky terrain, Kochanowski says we have to consider how we can cohabitate with an ecology whose cycles include fire, flood and mud.

The challenge is to act when the moment of crisis concentrates the mind.

If not, the crisis passes, the sun comes out, we move on and the cycle begins again.