How to figure out a fifty-year storm (and other storms too)

December 22, 2010 § 4 Comments

Blue lines indicate 50-year rainfall predictions for Los Angeles over a 24-hr period. See below for how to use this map. Source: Los Angeles County Hydrology Manual Appendix B

Rain always makes for good filler stories in the news. I got my dose of it this morning, video coverage of the storm damage running in the background while I’m finalizing restoration concept plans for a couple of local waterways.

Ohmygod! Lytle Creek’s a RIVER!

And sure enough, there’s a shot of the creek pumping. It would have been cool, if someone hadn’t driven past the barriers with flood warning signs on the bridge over it and nearly killed themselves. Note to self: don’t drive into a floodway during a flood.

It worries me a little, this panicfearhyperbole, as it can keep us from seeing the sense in restoration. We need to be more like students of natural processes and less helpless about them. Learn from them, prepare for them, make new decisions based on what they teach us. And we have the tools.

I loved seeing Lytle Creek pumping on tv. I’d have loved it more if the reporter could have estimated the flow and velocity in the creek, and told us something useful about what those numbers meant, followed with a healthy “stay out of here in the rains.” Oh, and maybe something about why the bridge was flooding? Just think, if we had rivers and streams with natural connections to the ocean, the reporters could be out there looking for steelhead returning, much like is happening up on Lagunitas Creek in Marin with Coho Salmon right now. How’d that be for a change?

So here’s a contribution to the principle of learning about rain and disaster for the day: the 50-year, 24-hr isohyet developed by the County of Los Angeles. With this handy map, the precipitation map I linked to a few days ago, and a calculator, you can assess just how much panic you need to go into when you hear the rain updates.

How bad is the rain, really, truly, in your area (or perhaps more importantly, upstream of you)? First you need an isohyet – a rainfall map.  Go to the LA County Hydrology Manual’s Appendix B to download a USGS map of your area, like the cropped one at the top of this page. As you can see in my example above, Lincoln Park, the USC Medical Center complex and Dodger Stadium all fall near the blue 6.2″ line. If 6.2″ of rain falls in 24 hours, you’ve got a “50-year” storm in that area. A “50-year” storm, technically, is a storm with a 1-in-50 or 2% chance of occuring.

So how does that compare with our recent rainfall? I trundle over to the handy precip map link and see that between downtown LA and Eagle Rock, there’s been 1.18″ and 1.57″ of rain in the past 24 hrs (as of this writing).  Because Dodger Stadium, Lincoln Park etc is closer to downtown, I’m going to guestimate its rainfall around 1.25″ for the past 24 hrs.

That’s a looong way from the 50 year storm number.

So what storm even is our current rainfall closest to? The County has multipliers to help us figure this out:

2-year storm (50% chance of occurring): multiply by 0.387

5-year storm (20% chance): multiply by 0.584

10-year storm (10% chance): multiply by 0.714

25-year storm (4% chance): multiply by 0.878

100-year storm(1% chance): multiply by 1.122

500-year storm(.2% chance): multiply by 1.402

(Table 5.3.1, Page 43, Hydrology Manual)

When I multiplied 6.2″ by 0.387, I got 2.4″ – in other words, the last 24 hrs rainfall (1.25″) doesn’t even meet the standard of a 2-year storm at this location! (It could be much different upstream, however)

A 2-year storm, by the way, generates rainfall roughly equivalent what is often called “bankfull” flows – flows that shape and maintain a stream’s main, active channel. When the flows go above that, you have flooding in your stream’s floodplain. That’s what it’s supposed to do. So if we knew this information, our reporters could be telling us, Folks this is how high the water gets in a X-type storm. Imagine how much higher it would get in a Y-type storm!

Now this doesn’t mean one should dismiss this potential for flooding with this rain. We can however put in context just how much rainfall it would take to create a big bad. Following on Joe’s post yesterday, here’s another snippet from the County Hydrology Manual about the conditions that create the worst storms:

“…most major precipitation events in the county are the result of extratropical winter storms. Significant runoff tends to occur when these storms last several days and are comprised of several individual bands of intense precipitation. In the case of a multiple day storm, the most intense rainfall tends to occur on the last day. These observations form the basis for Public Works’ 4-day design storm.” (Page 42, County of Los Angeles Hydrology Manual)

In other words, with our watersheds nicely saturated by several days of rainfall, today’s predicted storm, if its rainfall is as intense as described on our isohyets, is like the powerhouse batter that can sweep when bases are loaded. Since we’ve built in our floodplains, we probably won’t like that particular home run.

But it will take a lot of rain to get to that point too. I can hear it starting up right now. Be safe, be smart, and if you live in a debris flow area, take extra care and caution – as with any rainstorm.

And don’t drive over barriered floodways.

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§ 4 Responses to How to figure out a fifty-year storm (and other storms too)

  • Jessica Hall says:

    It may also be worth noting that rainfall (which we measure in inches) and river/stream discharge (which we measure in cubic feet/second) do not necessarily have a 1-to-1 relationship: as we all know, paved areas have higher runoff rates, as do denuded slopes, compacted soils, etc…

  • Emily Green says:

    I’m new to the foothills but we had more than three inches of rain today and the day before and the day before that… almost 12 inches in 4 days. By contrast, where I used to live in mid-city / West Adams, it was a good season if we got that in a year. Anyway, what I don’t understand (besides everything when it comes to the fear-mongering on the news) is why flooding would put people off stream restoration. If anything, if communicated properly, I’d think it would win converts. Watching rivers run down Fair Oaks Avenue into uncapped storm drains would make me a believer in stream restoration if I hadn’t become a convert when I learned that Las Vegas Valley floods from even light monsoonal showers because it’s so over-paved….Am loving your blog in the rain, 50 year or otherwise.

  • [...] Since we’ve built in our floodplains, we probably won’t like that particular home run. — How to figure out a fifty-year storm (and other storms too), LA Creek Freak, December 22, [...]

  • This was a nicely informative post. Thanks!

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