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Assessing Flash Flood Risk

Discussion in 'General Discussion' started by AlohaChris, Jul 18, 2015.

  1. AlohaChris

    AlohaChris

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    Scott P.'s post about the sudden increase in flow around the San Rafel Swell overnight got my attention. I had plans to be down there this morning, but they fell through. Had I gone, I would have been just looking at a forecast, the sky and the amount/speed of water in the canyon I was in. I could have been in for trouble.

    Can somebody point me to sources of information about how to "read" what area a canyon drains and how to assess flood risk by checking sources of data? Is there a list of Utah canyons and sites to check flow rates?
  2. Bootboy

    Bootboy Atwood Gear

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    Like what?

    There really is only so much data you can gather, its nature.

    Topographic maps are a good place to start, but far from what your looking for (which doesn't exist).

    Way too many canyons to monitor flows.

    The usgs water site water.usgs.gov provides realtime data for major streams and rivers but not really any slots, with the exception of the Narrows and the San Rafael through the black boxes.

    If it might rain, don't go. There is so much more to risk management than simply looking at maps and radar.
  3. gajslk

    gajslk

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    Some slots have a very small catch basin and the time between rain falling and water running will be short. An eye on the sky can help you assess the risk. Some slots have very large, and more importantly, long, catch basins. Think Buckskin or White Canyon. It's possible to get caught by water that fell 12 hours or more before in one of those. The time we got caught in the Black Hole, the rain had fallen about 10pm the night before, 50 miles upstream. While we were stuck on our ledge, we measured the water speed. About 4mph. Just about right. We even saw the storm the night before but figured that flash floods moved so fast that it wouldn't be an issue. We figured wrong and were lucky to escape. My rule of thumb is to stick to short drainages when the monsoon is rolling.

    Gordon
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  4. AlohaChris

    AlohaChris

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    How does one determine the catch basin for a given canyon? I've read that you can look at the ridge lines around the canyon and make a guesstimate. Is there a better way? Long vs short, is this the overall length of the canyon itself?
  5. Bootboy

    Bootboy Atwood Gear

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    There is no magic radar vision that gives you the ability to clairvoyantly assess an entire drainage.


    Contour lines on topographic maps will tell you pretty accurately the size of the catch basin. What it won't tell you is how efficiently the drainage catches water. For example: how much of the drainage is slick rock that has no ability to absorb water vs how much soil there is which can absorb water to varying degrees.

    Many places have small drainages that can be seen in their entirety from various points in the approaches or exits, e.g. :Some north wash slots.

    Map reading is truly becoming a lost art.

    I Think you seem to placing a lot of weight in the decision making process on the relative drainage size, when the truth is that there are so many other factors to consider. Even tiny drainages can kill you in the right circumstances.

    Like I said, if there's a good chance of rain, don't go.
    Last edited: Jul 19, 2015
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  6. gajslk

    gajslk

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    Look at a map. The important length is the distance from where you plan to go to the farthest point in the catch basin. But all staying out of those does is cover your butt for the blue sky overhead flash flood. Like Bootboy said, you can get killed in even a small drainage if a storm moves into your area. All sticking to a small drainage does is reduce your risk. A tiny drainage won't be safe if you're in a slot and a T storm moves in. I avoid going into any slots during monsoon season. Were I to go, I'd only do something with a small catch basin that saw me done by noon.

    Gordon
  7. Jolly Green

    Jolly Green

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    If nothing else, most beta sites make an effort to mention how large a drainage area is or how worrisome the flash flood risk is. Certainly not for every canyon but they do try. If I'm going to the north wash and the weather might be iffy, I always have some beta ready for canyons with small drainages to create other options.
  8. ratagonia

    ratagonia

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    Did Kolob yesterday, Saturday July 18, 2015, with a threatening forecast. Let me break out the reasoning for you:

    1. Last canyoneering day with the Mayor of South African kloofing. Wanted to show him one of our best. Imlay was the plan. The group consisted of 5 people total, of which only I was experienced with Zion canyons and weather. So it was my call, my responsibility.

    2. Forecast (Friday at 4 pm) was clear until noon, 20% 1 to 3 pm, 40% 4 pm to 8 pm. With a different group, WE might have chosen to go to Imlay, but probably not. ("The canyon will be there next week; your job is to make sure you are.")

    3. Kolob has a short approach hike (1 hour) and a fairly short technical section (3-4 hours). While the long hike out and up the MIA might be inconvenienced by rain and flash flood, it would not be life threatening. Most likely we would be done with the tech part by noon or 1 pm.

    4. Kolob is a canyon often done while flowing 3-5 cfs, with a current flow of 0-1 cfs. A small rise in flow would not be consequential.

    5. The basin of Kolob is limited by the dam. The catch basin above the dam does not need to be considered. The catch basin is small, and for the most part forested. It contains virtually no slickrock. (Thus, even fairly strong rain would not produce much flow into the streambed).

    On the downside, if someone got hurt and immobilized in Kolob, and had to spend the night in the canyon, there is virtually no place safely out of the way of rising waters. We did have a doctor along, which might have helped in this kind of circumstance. We brought an extra rope (despite the MIA exit) in case we got one stuck.

    Imlay on the other hand: small catchbasin, but not REAL small, and almost entirely slickrock. Long approach and lengthy narrows, not expected to exit the Imlay narrows until 5 pm, even with an early start. A lot of short rappels directly in the watercourse = even moderate flow could make the canyon very hard and dangerous.

    Had a great time in Kolob. Finished the tech part about 12:30. Started raining on us about a half-hour short of the MIA. Up the MIA in the rain was OK - certainly kept the temps down, and was therefore comfortable. Road was only a little greasy driving out.

    Tom
    Last edited: Jul 19, 2015
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  9. ratagonia

    ratagonia

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    Here are two examples of Catchbasin analysis. Birch Hollow and Englestead Hollow. But, most of these catchbasins are heavily wooded, so they barely count in terms of flashfloods.

    Catchbasins.
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  10. Cameron

    Cameron Long Tall Texan by birth

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    I had plans to run Kolob today but we bagged it Friday based on the weather.

    We figured we SHOULD be okay today no problem but we didn't have a number 1. from above...it wasn't our last day. We all lived here and while we really wanted to get together the canyon was going to be there and besides we went climbing instead. Also when you are a local and don't have the pressure of visiting.....you become canyon spoiled and like to do canyons when its ideal weather because I can! Speaking for myself though only!

    Thanks for the topo Tom. It always good to visualize. I assume its a lot like Spry that needs a direct and heavy hit to flash
  11. ratagonia

    ratagonia

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    Spry is different. The catchbasin is small, but it is 100% slickrock. Spry has a history of flashing hard. And it has a few places where a party would be in non-escapable narrows for a significant period of time.

    Steepness has a big effect on flashfloodiness. Steep = water goes into the streambed quickly. Flattish = water takes its merry time getting into the streambed. Water moves quickly down a steep streambed, slowly down a flattish one. For example, Pine Creek above the technical section has a big catchbasin with a lot of steep slickrock. But the streambed itself is very flat, AND has a lot of water holding capacity. So it takes a long time for rainfall to create a flash flood into the technical section. (YMMV)

    Tom
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  12. Cameron

    Cameron Long Tall Texan by birth

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    The slick rock vs wooded makes sense.

    Keep the good knowledge coming, I love learning! :pompus:
  13. AlohaChris

    AlohaChris

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    YES! Thank you, Tom for the map, drawing out the catchbasins, and posting your decision making thought process.

    Wasn't Kolob the sight of a disaster for a scout group? IIRC, it was because they did not know about the scheduled water release from the dam you mentioned?
  14. ratagonia

    ratagonia

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    It was the sight of an unfortunate event involving an LDS youth group in 1993. I am editing a book on the subject, that we hope to have to print in a couple weeks. I was looking for an excerpt that would make sense, but nothing short is really available.

    But let me use the Ham and Eggs Breakfast Analogy. The water release from the dam was like the chicken, rather than like the pig. Involved, but not too-involved.

    Tom
  15. Mountaineer

    Mountaineer Is that an X slot?

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    Had a permit for Checkerboard, bailed Saturday due to the forecast. Didn't want to chance an afternoon flood in the narrows. Just completed Boundary today, and followed a well defined MIA trail. Much has changed in three years.

    Found a lot of muddy footprints the last half mile out. Looks like Tom and crew did a bit of mud. :)
  16. ratagonia

    ratagonia

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    Did I show you this??

    image1 (2).JPG
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  17. Mountaineer

    Mountaineer Is that an X slot?

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    We estimated 4-5 in your party? Wow, yes you found the mud. Glad your vehicle made it back up to the terrace road.
  18. ratagonia

    ratagonia

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    Excellent tracking, sir Mountaineer. Five in our party.

    T
  19. Mike Zampino

    Mike Zampino Canyon season never ends.

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    Spry flashed Saturday. we saw the water gushing out. Pine creek also flashed and was raging. Even lodge was flowing some 3 hours after we finished. These were taken at 5:55p I can't find one of Spry. It might be on my GF phone.

    I was amazed that this spill-off was flowing and looks to have a very tiny catch basin.
    P1360674.JPG

    Yes that is pine creek below. P1360675.JPG
    Last edited: Jul 20, 2015
  20. Mountaineer

    Mountaineer Is that an X slot?

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    Heavy rain and lightening Saturday afternoon at Zion Ponderosa, resulting in them losing power.

    Earlier in the day it was sunny, and the warnings looked like they were all wrong. Don't take the risk, as I'm glad we opted out.
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