Flood Review

Re: Flood Review

Postby andrea-lake » Apr 4th, 2018, 9:47 pm

Re: Post by JoeS – It’s not an over-reaction to draw down the lake this year. The 1974 rules, based on historical data and reconfirmed in the 1998 flood review, reflect 80 years of actual experience. There are specific levels to which the lake is to be lowered to accommodate the amount of water expected depending on the size of the snowpack in a given year. Last year they were surprised by a late snowfall and did not have time to draw down the lake in advance. Instead, they were following the rules that apply when a drought is expected, i.e., “leave the lake as high as reasonably possible” and unfortunately the result was a major flood. In hindsight, maybe last year they should have left a wider safety margin given the uncertainties in the forecast, etc. but this year is there is no uncertainty about the very high snowpack, high ground water and more rain forecast for April. The only uncertainty is about how much larger the snowpack will get. Accordingly, the right approach is to draw down the lake to as close to the legal maximum as allowed. After it’s drawn down, the lake will not remain a “puddle” for long. It will quickly fill back up to full pool in May and June. See further detail below in my response to the post by Jhunter99. The graph below illustrates that when the lake has been drawn down to very low levels it fills back up quickly. If anything, the problem has been not drawing the lake down far enough and permitting a flood to occur.

Re: Post by Jhunter99 – I agree there is uncertainty about how quickly the upper snowpack melts and this can cause the freshet to stretch out a little longer. However, the total amount of water in the snowpack comes down regardless and is available to fill the lake back up to full pool. It really doesn’t matter whether it takes 2 months or 3 months. Given the size of the snowpack the lake will fill back up to full pool as long as they just draw it down according to the 1974 rules. If they see the snowpack melting slower than expected, they can simply slow the outflow at Penticton. That will prevent the lake from being drawn down too much before the rest of the snowpack melts and flows into the lake. This is clearly illustrated in the graphs below that shows the variation in lake levels from 1990 to 1997.

Seasonal Variation in Okanagan Lake Levels 1990-1997

Water levels 1990-97 (2).JPG


Note: in 1997 the line spiked well above the top axis, indicating flooding that year. They could have avoided that flooding by following the 1974 rules and drawing the lake down 25 cm below the level they did, Similar graphs over the last 30 years show that the mistake has always been not drawing the lake down as far as the 1974 rules directed and the result was a flood in high snowpack years. This year we have an opportunity to learn from that history and avoid that result.
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Re: Flood Review

Postby swamp1967 » Apr 4th, 2018, 10:12 pm

19490407 - Kelowna Courier Newspaper - Mission Creek.jpg


Almost 70 years ago, flood preparations in Mission Creek were underway because of concern about the snowpack at the time. Now we hear nothing but silence regarding flood preparations for Okanagan Lake's largest tributary.
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Re: Flood Review

Postby alanjh595 » Apr 5th, 2018, 5:30 am

Blown up from the above article. I haven't been following this thread closely, so I have highlighted some facts and maybe Glac could compare them to this years stat? Just out of curiosity. Great find on the newspaper article, swamp1967.

Capture.JPG


Capture2.JPG
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Re: Flood Review

Postby Glacier » Apr 5th, 2018, 9:36 am

Notice that at this time last year the snowpack was still BELOW normal! Now it's the 2nd highest it's been since 1974!

missionsnowpack.png


Bonus map (because I have it available):

MosleySnowpack.png
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Last edited by Glacier on Apr 5th, 2018, 12:13 pm, edited 1 time in total.
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Re: Flood Review

Postby andrea-lake » Apr 5th, 2018, 10:12 am

https://www.castanet.net/edition/news-s ... htm#222816
Mr. Reimer reported that the lake was 99 cm below full pool on April 4th. (See link above). This is a good start, but we must not forget that last year the lake level rose 150 cm from its low in April to its high in June, with a much smaller snowpack than we have this year. We must also build in a contingency for both predicted and unexpected rain, which has been the cause of flooding roughly 50% of the time in past years. The 1974 rules direct that the lake be drawn down to 341.01 meters in a year with a MAJOR FRESHET. The full pool level is 342.48 meters, which is 1.47 meters above the maximum draw down level recommended in the rules. With this year's snowpack and rain the lake is expected to rise a minimum of 1.5 meters (150 cm), and there is a reasonable probability it will rise an additional 20 to 40 cm. That would result in a maximum elevation of 342.88 meters compared to 343.25 meters last year at the peak of the flood. Therefore, the 1974 rules are in no way extreme but simply prudent.

Note: the maximum draw down level of 341.01 meters = 0.74 on the left axis of the graph below.

Levels to full pool.JPG
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Re: Flood Review

Postby Glacier » Apr 5th, 2018, 10:23 am

And what if we get a hot, dry May and June? The whiners will be saying they shouldn't have let out so much water in April!
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Re: Flood Review

Postby Fancy » Apr 5th, 2018, 10:26 am

Guess that will be proven if boat launches and docks aren't usable.
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Re: Flood Review

Postby LTD » Apr 5th, 2018, 10:57 am

Glacier wrote:And what if we get a hot, dry May and June? The whiners will be saying they shouldn't have let out so much water in April!

lol if we get a hot may and june a lack of water wont be the issue there will be lots of it all at once
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Re: Flood Review

Postby Glacier » Apr 5th, 2018, 11:08 am

LTD wrote:
Glacier wrote:And what if we get a hot, dry May and June? The whiners will be saying they shouldn't have let out so much water in April!

lol if we get a hot may and june a lack of water wont be the issue there will be lots of it all at once

Yes, but after it all flows out, what will be left in August?
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Re: Flood Review

Postby Frisk » Apr 5th, 2018, 11:14 am

Glacier wrote:Yes, but after it all flows out, what will be left in August?


Muddy, goose poop beaches.
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Re: Flood Review

Postby LTD » Apr 5th, 2018, 11:18 am

theres a ton of snow up high this year i have been on the sled in average years near the end of june this year I'm thinking i might get my first july ride lack of water wont be an issue this year
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Re: Flood Review

Postby Glacier » Apr 5th, 2018, 12:23 pm

Interesting facts from graphs I posted above:

Earliest peak snowpack at Mission Creek (elevation 1780m) = March 29th (1990).
Latest peak snowpack at Mission Creek = May 16th (1999).
Average peak snowpack (mm of Snow-Water Equivalent) = 578 mm
Highest Peak = 859 mm (May 16, 1999)
Lowest Peak = 383 mm (April 16, 1992)

Earliest peak snowpack at Mosley Creek (elevation 1655m) = January 31st (2000).
Latest peak snowpack at Mosley Creek = May 20th (2003).
Average peak snowpack (mm of Snow-Water Equivalent) = 311 mm
Highest Peak = 573 mm (March 24, 1991)
Lowest Peak = 383 mm (April 20, 1993)

Amazing what a difference 130m rise in elevation gets you, eh! On average, it's a 85% increase in snowpack!
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Re: Flood Review

Postby andrea-lake » Apr 5th, 2018, 12:27 pm

AccuWeather forecast for Kelowna:

May –temperatures ranging from 13c to 24c, following historical norms, with rain expected on 8 days

May 2018 temps.JPG


June –temperatures ranging from 20c to 29c, following historical norms, with rain expected on 4 days

June 2018 temps.JPG
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Re: Flood Review

Postby andrea-lake » Apr 5th, 2018, 9:51 pm

When you draw the lake down to a low level because of a high snowpack there’s no worry about a lack of water in late summer or early fall because the large freshet from the melting snowpack is guaranteed to fill the lake up to full pool or more by late June or early July. At that point, with a full lake, you simply let the water out at a controlled rate that ensures there is sufficient water for the fall and winter months.

As an example, refer to the 1948 flood year in the graph below. This clearly shows that, despite the drawdown to 341.43 meters (1.18 meters on the left axis) in March, the lake quickly filled back up beyond full pool, in fact, it caused a flood and left more water than needed for the rest of the year. The 1974 rules would have required them to draw it down by another 40 cm to an elevation of 341.05 meters (roughly 0.8 meters on the left axis). Had they done that, the lake would have peaked at 342.65 meters (2.4 meters on the left axis), which is roughly full pool, instead of 343.07 meters (2.82 meters on the left axis), which is the flood level. Targeting the lake level at 342.65 meters (2.4 meters on the left axis) would still have left plenty of water for the rest of the year.

The main point is that we will make the same mistake this year as they did in 1948 if the lake managers limit the drawdown to 341.43 meters, which is the typical level they target. To be certain that we avoid a flood given this year’s snowpack and ground water, the lake should be drawn down to 341.05 meters at a minimum. Rest assured this will not result in a lack of water later in the year!

1948 graph.JPG
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Re: Flood Review

Postby Glacier » Apr 6th, 2018, 7:57 am

The 1948 flood had nothing to do with the snowpack. The snowpack that winter was like last year, not that great, but then it rained starting in April, and didn't stop until September! For example, Penticton had 70mm of rain April, 57mm in May, 67 mm in June, and 51 mm in July, and 72 mm in August! It was by FAR the wettest April to August on record!

PentictonAtoA.png
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