Some insight into the vertical layout of Lanier
Using years of historical data from Lake Lanier, AWS reader Ryan Kolter has put together a line chart showing the speed with which the lake was filled, foot by foot, based on time.
Assuming consistent inflow, outflow and precipitation, this chart shows how quickly the lake rose in elevation. This chart can now be used to track how quickly the lake will drop. To quote Ryan from a post in our forums:
Now the lake itself is really spiny - it has all sorts of inlets and the like, whereas the chart can only really assume a bowl shape. so when there is a very slow depth growth, we would have to assume that the lake surface is really branching out, and where we have fast depth growth, we have to assume it’s really adding depth.
Looking at the chart, we can see a few things:
- The rate of drop will slowly increase until we get to about 1040 feet, at which time it will slow way down. This is good timing, as it’ll make the final decent to the “dead pool” a bit slower.
- That slower pace will continue until about 1030 feet, at which point the rate of drop will greatly speed up.
- It will slow down once again inside of about 1010 feet.
Again, this data isn’t completely accurate, as variations in inflow/outflow aren’t factored in. Still, it’s a neat way to view the lake. Thanks Ryan!
November 12th, 2007 at 2:24 pm
Hi,
Could you please explain what this graph is showing? What is the horizontal axis for instance (I’m assuming based on the numbers involved the vertical axis is elevation). Something like this could be very useful to those of us that would like to be prepared, but it needs to be explained a little. Even the author of the graph didn’t explain what is being presented in his/her forum post.
I really appreciate your work to try to inform the public in the face of the often conflicting reports that we see in the media, so with that goal in mind, could you or the author of this chart please do your best to diffuse what is being plotted.
For instance, you say “the rate of drop will slowly increase until we get to about 1040 feet, at which time it will slow way down.” This is very ambiguous. Do you actually mean the rate of drop that will slow down, or do you mean the increase in the rate of drop will slow down (i.e. the rate at which the water level drops will start to stabilize). There is a subtle difference in language, but big implications in meaning.
November 12th, 2007 at 2:30 pm
Good points. I’ll try to get a revised chart from him that has more detail.
In regards to your final paragraph, I mean that the INCREASE in the rate of drop will go down.
As the lake gets lower, it will drop faster and faster (for example, 0.2 feet per day, then 0.3 feet per day, then 0.4 feet per day, etc). When we get to about 1040 feet the rate of drop will be more consistent (0.4, 0.4, 0.4) due to the steep angle of the lake at that point.
November 12th, 2007 at 2:39 pm
Right, that’s what I thought. Then it’s slightly misleading to say “it’ll make the final decent to the ‘dead pool’ a bit slower,” since the rate of loss in feet will not slow unless we get rain, but I guess as this data indicates, it may become more constant per day depending on the topology of the lake.
November 12th, 2007 at 2:42 pm
Glad to explain the chart -
The x-axis (horizontal) is TIME. The y-axis (vertical) is DEPTH. The chart shows that as time progresses (from left to right horizonatally) the depth increased (from bottom to top vertically).
I didn’t label the x-axis because it just has dates from 2/1957 through 7/1958 as the lake was filling up - what’s important is that the dates progress forward in time left to right.
What the chart shows you, is how the lake reacted to being filled. Where the line is steep (like at the bottom left) the lake was primarily rising in depth and not expanding outwards. When the line is shallow, it means that the lake was rapidly expanding it’s surface area - filling into all those little inlets and shallow depressions that make it a beautiful lake.
The same amount of water poured into a steep cup will cause the depth to rise rapidly. If poured into a shallow bowl, the depth won’t increase so much, but the surface area will.
So, by looking at this chart, we can get a feel for how the surface area will affect how fast the lake drains. We’re at the 1053 foot mark (or thereabouts). Where the line is shallow, the lake will drain more slowly - instead of dropping in depth, it’s losing surface area. Where the line is steep, the lake will drain more quickly - it is droppin in depth but not losing as much surface area.
By following where we are on the line, we can get an idea of how much the lake will drop over that period of time - assuming the inflow and outflow remain basically constant.
I’m waiting on data that will let me produce a more accurate chart that takes into account the fickle inflow and outflow, but given that a quick review of the data doesn’t show any monster outflows or inflows, it’s reasonable to suspect the later chart will be similar in shape.
November 12th, 2007 at 2:49 pm
Hi My name is Steven Moore. I am a student in Spalding High school In the distrect of spalding County In Griffin Georgia….my i think it is a bad thatn that we are running out of water…sombody needs to do somthing before we do cuz many ppl will suffer though this tragady..my family is already stocked up on water to be prepared just incare it happens
November 12th, 2007 at 3:26 pm
In terms of surface area we have lost 720.09 acres as of midnight this morning for the month of November so far. And with the numbers the way they are for today so far that number is growing fast. In fact this should be the high day for the month thus far for total. surface area
November 12th, 2007 at 8:01 pm
I eyeballed a lot of segments of topo maps of the lake bottom over the weekend. Very unscientific; I didn’t post anything because I was very unsure of what I was seeing. However I was amazed and delighted to see the flat area on your graph just before the dead pool. That confirms what it was looking like to me. Counterintuitive as it is I think your graph is, very generally, correct.
November 12th, 2007 at 8:13 pm
I was curious about something.
When the lake was filled for the first time, the area it gradually covered was dry dirt and land. Thus, i am sure that much of the water was seeping into the ground as it hit dry dirt until that area became saturated enough to hold water. That would make the filling up process very slow.
However, losing water from the current saturated area it seems would happen much quicker.
For example, when filling the lake water would be seeming into the ground, but when draining, water would not come up from the ground.
Anyway, i guess it doesn’t matter weather it’s 80 days or 180 days, this will be a long term problem
November 13th, 2007 at 12:14 pm
Sorry folks, I know a lot of people are distressed at the water situation, so I don’t mean to add to the woes. However, reality, no matter how unpleasant, should be preferable over blissful ignorance, as it allows you to make more timely decsisions that must be made.
Therefore, I disagree with Ryan Kolter’s conclusions. I think the approach used by Ryan is interesting, but I am certain that the primary assumption (”Assuming consistent inflow, outflow and precipitation”), as well as the reliance upon the absolute time to achieve initial resevoir elevations is fatal to the final conclusion, which is: “The rate of drop will slowly increase until we get to about 1040 feet, at which time it will slow way down. This is good timing, as it’ll make the final decent to the “dead pool” a bit slower.”
I researched the inflows and outflows from the resevoir filling years 1957- 1958, as well as reviewed the midnight pool data from 1957-1959, and found that the plateau at approx lake El. 1040 was due to filling in the period Oct-Dec 1958, historically part of the dry season in Georgia, and 1958 was no exception to that climate pattern.
Furthermore, as the lake level drops, the acre-feet surface are of the lake (and thus volume of the lake) must continue to decrease at an accelerating rate as well. Given a continuation of the status quo (continued stable drought conditions, continued water discharges to the river and continued water consumption by Atlanta) this means only one thing: the rate at which the level of the lake is decreasing must continue to accelerate as well. Due to lake bottom topography, there will be periods when the rate of acceleration slows, and certainly any rainfall in the upper river basin will help mightily as well. But without significant rains soon, the depletion trend will only increasingly accelerate towards the deadpool, and thus an increasingly dire situation.
November 13th, 2007 at 1:16 pm
Interesting observations Tim! Some comments -
(1) The chart was made assuming consistent inflow, outflow, and precipitation - I did not at any point state this was the best method, only that this was the method used for this chart. The ACE has no records of inflow or outflow during the Feb 1956 - June 1957 period, the entire period covering the deadpool range of the lake.
(2) The quote you attribute to me, was made by Mickey (owner of the site), based on the chart.
(3) The plateau you describe occurred in 1957, not 1958. By July of 1958, the lake hit it’s full point (or very nearly so).
(4) Historically, August, September, and October have less than average rainfall, but that is only 0.2, 0.4, and 0.7 inches, respectively. You actually have fairly regular precipitation throughout the year.
http://countrystudies.us/united-states/weather/georgia/atlanta.htm
While I agree entirely that SOME of that shallow slope both in 1956 and 1957’s fall months are attributable to slackened rainfall, I don’t think it’s reasonable to disregard the entire change. Especially in light of the fact that there isn’t a record of a drought during those years - so we anticipate you got about the average rainfall then.
(5) It is not correct to say “Furthermore, as the lake level drops, the acre-feet surface area of the lake (and thus volume of the lake) must continue to decrease at an accelerating rate as well.”
The volume of the lake will decrease daily exactly as much as is being taken out of the lake. If this rate does not increase, the volume’s decrease will not accelerate. This can be shown mathematically - take any shape, and reduce it’s volume by X. It’s surface area will change such that the volume remaining is equal to the original volume, minus X.
For a given loss of volume X, the surface area and depth will decrease such that the volume is now Original - X. If the depth drops tremendously, the surface area will drop very little. If the depth drops very little, the surface area will be reduced significantly. (we are assuming that “X” is a big number).
I think what you meant to say was that because the surface area is dropping, each subsequent outflow from the lake will cause the depth to drop a little more than before. And this is entirely true - likewise my chart does not take the growing surface area into account as the lake filled. Worse, because it’s being drained lots faster than it’s being refilled, what took 30 months to fill will NOT take 30 months to drain.
I guess, finally:
(6) I don’t disagree at all with your overall assessment - things are bad, and they’re going to get worse - I don’t see how you could get a rosy picture out of the chart - I certainly don’t see good times ahead. The goal of the chart was just to give a general feel of how much, at different levels, the loss of surface area would affect the loss of depth from the lake. The plateaus probably do exist, but if you’re sucking a quarter foot a day from the lake, those plateaus won’t provide much comfort.
November 13th, 2007 at 1:25 pm
Heh, I made two mistakes in my above statement -
From (4): The reduced rates of rainfall for August, September, and October - those are the amounts less than average, not the rainfall amounts. So August of .02 is not .02 inches of rain, but .02 less than normal.
From (5): And second was that the chart DOES take into account the rising surface area of the lake as the lake filled. *facepalm* That’s kind of the whole point of the chart.
November 13th, 2007 at 2:43 pm
Guys, give it up - its not the LAKE! The lake is gone for years. This is serious.
November 13th, 2007 at 3:25 pm
DoSomethingSonny! - I understand you consider the problem to be finding a new reliable water source. And, of course, you are correct. However, a new reliable 500+ million gallon resource isn’t simply laying around.
The clear solutions of desalination or deep wells for municipal use take a lot of time to develop, even if the money appeared overnight. In the meantime, as the water ratchets down in lake Lanier, it becomes a very effective clock - when it runs out, you need a solution in place.
Thus, speculating about the lake is more than just thumb twiddling.