Posts Tagged: debris basins

In preparation for my recent presentation at Harvard GSD’s panel on Landscape Infrastructure, I sat down with Steve Dwyer of the Army Corps of Engineers to discuss Los Angeles’ concrete rivers. The channels initially met a need, but now that we understand other ways of working with runoff and how these systems –in their natural, or naturalized form— can be an amenity for our city we are faced with the challenging realities of renovating them. In this interview, Steve lays out the opportunities and obstacles facing our rivers.

Ying-Yu Hung: How many miles of concrete channels are there in the city and county of Los Angeles?

Charles (Steve) Dwyer: We built 571 miles in channels. That includes the LA River, the Rio Honda and all the tributaries.

YH: Within the watershed, are there natural surface flows that feed into the channels from those channels out into the ocean?

CD: We call that nuisance flow, from people watering their lawns. Just water that happens to fall, not rainfall exactly – obviously the system collects rainfall. There is another smaller system that links these two storm drains that you see on the side of the road. These are maintained by the county of L.A. They bring the water to us.

YH: If we start at this point, where the concrete channel begins, this natural flow coming down the mountain will feed into these channels. How does that work?

CD: These are called debris basins. There is one of them at the mouth of every canyon. The water comes out of the canyon come into the debris basin which is like a bathtub. It has a drain in the bottom, a small one. The water comes in, fills up, slows down, drops its debris, sand, that kind of thing into the basin and flows out. The water that actually gets into the channels is purified from having rocks and things that could abrade the channel. Every year or so the county comes in and digs out the accumulative sediment, which is a useful commodity, it’s saleable for building purposes.

YH: Are there techniques to filter some of the surface contaminant from that water?

CD: Yes, there are. They’re pretty rudimentary at this point. If you look at storm drains you’ll often see a grate – a perforated grate – and that keeps leaves and twigs and stuff from going into the channel. It filters mostly, big stuff but there are many storm drains that are not protected that way. These communities find after a rain that all this trash is washed down from that huge watershed.

YH: With the funding right now, what projects do you see moving forward? What do you see happening in the next 10-15 years?

CD: I see more funding for the L.A. River pilot programs. We’ve got several areas that have been identified like the bowtie parcels or the Taylor Yards as one that has a lot of potential and that’s probably where we will start. We’ll do a small pilot project that may be half a mile long, maybe experimenting as the students are suggesting, like taking a wall out, using the empty land to create an engineered wetland. But all of those proposals have to go through environmental and engineering review through us. For instance, Santa Monica Bay Restoration Commission is proposing to make some changes to Ballona Creek, where you’ve got the two levees. Removing this levee or actually moving it over here so this whole area can become engineered wet lands.

YH: So it’s widened?

CD: Partial to that is that you can put it into this area, but you have to get it back into the channel somehow. In this case it would go right into the sea, so this is an ideal location for this, you don’t have to capture it again as you would if were up here (further inland). There a lot of technical problems that are solvable but we would have to do a thorough engineering departmental review of the Santa Monica Bay Restoration plans to make sure that flood waters can get through here to here. That’s the bottom line – we have to be able to get storm water from the mountains to the ocean as quickly and safely as possible.

YH: What’s your vision for LA with regard to waterways and your vision for the next 50 years? Although the population is still growing and we have certain parks, do you still see this as part of that system as a way for people to have recreational areas?

CD: Yes, and I would see us over the next 25-50 years closely looking at the individual channels and finding opportunities to build pocket parks or to modify the channel to give up an engineered wetlands. I see that will happen someday.