El Niño Doesn’t Correspond to L.A. River Flooding, La Niña Does

February 29, 2016 § 1 Comment

The interview as it appeared in Friends of the L.A. River's Current News nearly 20 years ago

The interview as it appeared in Friends of the L.A. River’s Current News nearly 20 years ago. Click to enlarge.

Something came up in a recent discussion I was having about current spate of U.S. Army Corps of Engineers make-work projects to degrade the L.A. River in the name of El Niño. If you haven’t seen it, the cutting vegetation and installing dirt-fill barriers along the edges of parts of the river, resulting in nutty bike path detours.

What makes me sad is that the L.A. River generally hasn’t flooded during El Niño years, but instead mostly during La Niña years.

I know this from an excellent interview that FoLAR bird expert Dan Cooper did with climatology professor Richard Minnich back in 1998. I ran excerpts from this in 2010 – a drier La Niña year with some big storms. Below is the whole article.

Talkin’ El Niño
An interview with Dr. Richard Minnich of University of California Riverside, by Dan Cooper

Richard Minnich is a professor of biogeography and climatology in the Department of Earth Sciences at UC Riverside. He has been studying weather patterns and landscape ecology in Southern California and Baja for the past two decades, and recently spoke with FoLAR’s Technical Advisory Board chair, Dan Cooper, in Riverside on March 6, 1998

Dan: Dr. Minnich, let’s begin with the basics – what causes flooding in L.A.?

Rich: Two components are involved, long-term and short-term causes. In the long-term, the ground has to get completely saturated by rain; water hitting dry ground won’t do a thing. Now, in the short term, it’s the hourly rates throughout the day that are important. These rates are what cause catastrophic flooding like we had in 1938.

Dan: What kind of rain are we talking about?

Rich: Ballpark rates, maybe 20 inches in a day in the Transverse Ranges (incl. the San Gabriel and San Bernardino Mtns.).

Dan: Twenty inches in one day? That’s typically what we get in a year.

Rich: In January ’43, it rained 20″ in the mountains, but it was on dry ground so nothing happened. Now downing the coastal plain where everyone lives, all that concrete has led to the potential for flash flood conditions – the water has nowhere to go but into the channels. But even without concrete, major floods are possible – the floods in ’38 occurred before the whole plain was concrete and the rivers were completely channelized.

Dan: So 1938 must have been a big El Niño year…

Rich: Pretty neutral, actually. Neither El Niño nor La Niña conditions were recorded that year. Another neutral year was the winter of 1966-67 – the Transverse Range got 30 inches in December of ’66. The Transverse Range got 30 inches in December of ’66.

Dan: So El Niños don’t coincide with flooding in the L.A. Basin?

Rich: The three spectacular El Niños we’ve seen this century have been 1940-1, 1982-3, and again in the past season [1997-8]. Not one of them caused extensive flooding in the basin.

Dan: So what does cause it? Have our rivers ever seriously spilled over?

Rich: Two years to remember: ’38 and ’69. The storm in ’38 was so bad it was used as a model for subsequent flood control projects in California. In ’69 we survived by a whisker – needed about another 1/2 day of rain and that would have been it. The rain rates were exceeding an inch an hour for several hours, and that’s when you start worrying.

Dan: So back to El Niño…

Rich: It’s actually the La Niñas (the periods when abnormally cold ocean currents form near the equator) that are more devastating. The so-called “Pineapple Expresses” that sweep across and hit north of here are usually more destructive than what we get. Northern California saw it in ’55 and again in ’64. The storms that characterize El Niños are strong but brief. There’s rain, break, rain, break, rain – that’s not what causes flooding.

Dan: So weak El Niños might cause more flooding than strong ones?

Rich: Generally. These result in a “standing pattern” of precipitation – the storms just sit over us and dump rain. With strong El Niños, all that energy is constantly available and being renewed, and the storms just peel off quickly and zip across us, one after the other.

Dan: So describe this past season. How did the flooding compare with past years?

Rich: This February [1998] was the wettest on record – no doubt a major event occurred. But in terms of flooding, we’re only talking about maybe a 5-10 year flood event. It just doesn’t rival past years like during the ’60s, or even what happened two years ago in ’95 when the Ventura River flooded that trailer park.

Dan: Do what can you say about the rush to clear out the vegetation in the channels?

Rich: The engineers have their models telling them how to move water the fastest, and any vegetation slows down water. The question is, is it really significant? They do the same thing our here every year along the Santa Ana River — usually bulldoze it down to dirt, but now they leave some willows here and there. But does anyone have any evidence that it’s saving us from flooding? The Santa Ana has only flowed bank-to-bank once, in ’69.

Dan: So leaving vegetation in parts of the channel may not be incompatible with flood control?

Rich: As long as you don’t have giant cottonwoods or palms right down in the middle of the channel, the mulefat, willows — even Arrundo — is going to be torn out. I wouldn’t leave a forest, but a willow thicket’s just going to bow over or get uprooted and carried downstream.

Dan: How about all that debris clogging up the system? A lot of people might say that it was the channel clearing that saved us this year.

Rich: Plenty of vegetation was carried down those channels; just take a walk on the beach in Long Beach. Look nobody could claim the system worked or didn’t work – it was[n’t] even tested this year. This was no big deal.

Cover of FoLAR Current News 1998

Cover of FoLAR Current News 1998

Last page of interview

Conclusion of 1998 interview - after the jump

Conclusion of 1998 interview – after the jump

(Thanks to Diane Ward for tracking down the digital copies of these pages.)



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§ One Response to El Niño Doesn’t Correspond to L.A. River Flooding, La Niña Does

  • The bike path closures are really poorly done, why not open the path to cyclists only? I was also looking at the barriers on the east side of the river near Atwater and they abruptly end, so suddenly it appears there is a place for the water to just spill past them. In any case I wish the closures were better documented.

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