The last year the Boise River overflowed its banks and engulfed residences and streets was 1943. The river spilled over the top of Arrowrock dam and more than 14,000 cfs raged through town. That event lead to the construction of Lucky Peak dam around 1955. According to the US Army Corps of Engineers, the flood began on April 1 and was “the result of rapid snowmelt augmented by light rainfall”.
Snowpack levels are only one variable in water management. Natural runoff is ultimately dictated by mother nature, and is difficult to predict. The Natural Resources Conservation Service (NRCS) Idaho Water Supply Outlook Report for May 1, 2011 shows that the Boise River basin snowpack is only slightly above normal. This is surprising since many areas of the state are 150-200% of normal, such as the Upper Snake and Payette River basins. So while Boise residents have that in their favor, it is still snowing at moderate elevations and raining in the valleys.
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| Boise River above East ParkCenter Bridge - May 16, 2011 |
Snowpack levels are only one variable in water management. Natural runoff is ultimately dictated by mother nature, and is difficult to predict. The Natural Resources Conservation Service (NRCS) Idaho Water Supply Outlook Report for May 1, 2011 shows that the Boise River basin snowpack is only slightly above normal. This is surprising since many areas of the state are 150-200% of normal, such as the Upper Snake and Payette River basins. So while Boise residents have that in their favor, it is still snowing at moderate elevations and raining in the valleys.
The Bureau of Reclamation and US Corps of Engineers have been releasing additional water since late March to provide reservoir capacity. Today, Arrowrock is 56% full, Anderson Ranch is 81%, and Lucky Peak is 69%. In all, this means there is 280,000 acre ft of remaining storage. With the recent warm weather and rain, the natural inflow has suddenly shot up to 18,800 cfs. The comfortable Boise River flow to prevent flooding is 7,000 cfs, where it is today. The New York canal is running at 2,250 cfs. So the net inflow to the reservoirs is 9550 cfs. If those conditions persist at roughly the same levels, the reservoirs will reach capacity in 15 days, or on May 31.
[(280,000 acre ft)(43,560 cubic ft/acre ft)]/[(9550 cfs)(86,400 seconds/day)] = 15 days
According to the National Oceanic and Atmospheric Administration (NOAA), water may begin flowing down ParkCenter Blvd at 9,500 cfs, but should remain below most residential areas. At 10,500 cfs, flooding of residences near the river is likely. At 12,000 cfs, ParkCenter pond is now connected to the river. And at 16,000 cfs, we have major flooding.
Since May 3, the Middle Fork of the Boise River has gone from 2,000 cfs to nearly 10,000 cfs. If and when the reservoirs upstream reach capacity, what goes in must come out.
6 comments:
Nice analysis, Steve. Lets hope we don't have to face flooding.
-Adam
Steve,
the date that you came up with is incorrect. I crunched the current numbers and actually found that the dam will actually lose about a half foot a day.
You cant add all the inflows and out flows. There is three dams in place to hold water.
Middle fork and south fork both go into the arrowrock. were the most storage is actually available. then an out flow from the dam is pumped into lucky peak. then out of lucky peak the boise river is pushing out about 9150 CFS with new york cannal.
The upper south fork comes in at 2300 cfs and leaves in the south fork at around 3500 cfs. actually creating room in the Anderson Resevoir.
So if inputs of 3500 and 4250 cfs flow into Arrow Rock and the same amount is then turned over to lucky peak then a total of 7750 flows into lucky peak and a total of 9150 flows out in the form of the Boise river.
Just to make it clear.
I used the current stream flows on the usgs site for flows as of May 19.
just your average college student
dan
Dan-
The date I came up with was correct given the snapshot of flows on May 17. I am well aware of all three dams. I am defining the net inflow as the difference between what is naturally flowing in and what is being released below Lucky Peak.
Since the post, the M. Fork at Twin Springs has dropped from nearly 10,000cfs to 4,000cfs. Keep in mind that More's Creek was also kicking in nearly 2000cfs three days ago.
So yes, of course if you run the numbers today, the date will push out. The net inflow has dropped considerably. It is only about 1600cfs today.
Don't be so hard on yourself, you sound like an above average college student.
Correction...my snapshot was on May 16, not May 17 as I commented.
Keep in mind also that there are other tributaries contributing to the inflow. While the SF Boise below Anderson is flowing less than the SF Boise above, the reservoir has actually gone from 81% full on 5/16 to 83% today.
Steve,
I would agree in the fact that there is a lot of water that has to come down from the mountains. But the amount of water is not in fact enough.
When you calculate the numbers i don't think you should add the numbers of the Upper south fork along with the flows of the lower south fork. since they are both don't compound on each other going into Arrowrock.
Also we don't know how much water they are letting out from Arrowrock to Luckypeak
I don't mean to prove you wrong or anything but i just don't think that the Boise river will flood, at least not this year.
dan
The amount of water coming into the system on May 16 was enough to fill the reservoirs in 15 days, but only if that amount of inflow was constant for that amount of time. The math does not lie.
To be clear, I did not add both SF Boise flows above and below Anderson Ranch dam. What matters is what is coming into the reservoirs vs. what is released through town and into the New York Canal. The outflow of ~9000cfs will be fairly constant until this moderate flood threat subsides.
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