Geeking out on gages in the rain

December 20, 2010 § 6 Comments

Curious about the rain? Los Angeles County Department of Public Works has a fun online site for monitoring rainfall.

Screen shot of County's Precipitation map. Click image to get to live updates.

You can also go to the USGS website to view real-time stream gage data. Here’s a couple of examples:

Malibu Creek stream gage. Note high so far is about 3000 CFS.


Los Angeles River at Sepulveda Basin. High flow so far - +/- 8000 CFS (cubic feet/second).

Neither of these highs strike me as particularly high flows, despite all the storm-of-the-decade hyperbole. But it is interesting and useful to monitor the changes.  If you are seriously geeking out on this stuff and have a mac computer, you can also download stream gage widgets here.


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§ 6 Responses to Geeking out on gages in the rain

  • […] to break rainfall records. To watch those records falling, go to Jessica Hall’s post on LA Creek Freak for the rain geek’s best links to rainfall […]

  • I don’t know if I agree…I went to the Arroyo Seco page

    and based on 98 years of records, the median flow is 2.3 cubic feet per second (cfs). The lowest recorded was 0.1 cfs in 1930 and highest value was 251 cfs in 1922 but it topped 1,000 cfs each of the last three days and may have topped 2,000 today (at my reading at 14:28, the Most Recent Instantaneous Value Dec 22 was 1440). Unless I am seriously misreading this data, that’s a lot of water.

  • Jessica Hall says:

    Hi David,
    Here’s some additional perspective. In the Arroyo Seco Restoration Feasibility Study Technical Appendix there’s a table in Appendix H (Table 11) that shows the Discharge-Frequency at Devil’s Gate Dam, the location where your stream gage above is located.

    A 2-year interval storm has a discharge of 684 CFS.
    5-year interval storm: 3,570 CFS.
    25-year interval storm: 10,800 CFS
    50-year interval storm: 14,300 CFS
    100-year interval storm: 19,300 CFS
    Capital Storm: 20,100 CFS

    You are correct that over the past three days, discharges at Devil’s Gate have spiked between 1 and 2,000 CFS (and is now exceeding that). More than bankfull flows, definitely would be overbank if it were a natural stream, but still not a huge flow compared to what it could be. The concrete channel’s designed capacity is mostly 100-year or greater, but some reaches are less than that (can’t tell you how much less).

    • Jessica Hall says:

      I am also trying to figure out how to think about the comparative data presented at the linked gage. As it seems to be a comparison based on the same day over many years, it doesn’t strike me as particularly useful for understanding flood risks, comparisons based on historical seasonal highs and lows strike me as potentially more useful – the sort of data that goes into formulating Discharge Frequency charts (like the one referenced above).

  • Jessica Hall says:

    Aha! Here’s peak discharges from a Phil Williams study for the USGS stream gage (2000):

    2-year storm: 556 CFS
    5-year storm: 1,670 CFS
    10-year storm: 2,860 CFS
    20-year storm: 4,401 CFS
    50-year storm: 7,040 CFS
    100-year storm: 9,550 CFS
    500-year storm: 17,300 CFS

    Looks like Arroyo Seco discharges are exceeding the 5-year and creeping towards the 10-year storm interval. No doubt it would be an impressive flow. But I’d like our region to treat these events as not so much sporadic but part of our regular cycle. The natural rivers and stream’s own channel forms, bed material and vegetation tell us to expect it.

  • charlie says:

    It seems like this storm is a very impressive one not for its 24 hour total but for its duration of rain. I remember in 2005 during the somewhat similar storms, the creeks ran so long at bankfull (which they usually only reach for a few hours during/after storms) that the water was totally clear and sediment-free before it receded. In particular, Solstice Creek, usually either a trickle or a muddy flood, ran for weeks at a very substantial flow rate, and resembled one of the creeks coming out of the Eastern Sierras, despite the tiny watershed. These sorts of storms may cause problems for some, but in a lot of places, they are just great groundwater rechargers.

    That being said, in the San Bernardinos where it rained 20 inches, the storms seem a bit more destructive.

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