While looking for a railroad bridge
on a satellite map, I noticed a big, long lake. Whenever I see a long lake, I check to see if one of the ends is a straight line. Sure enough, Lake Meredith is straight on the east end. Zooming in, I noticed that the spillway structure was rather unique. So I decided to research this dam.
|US Bureau of Reclamation|
"The Sanford Dam is a zoned earthfill structure with a crest width of 40 feet, a crest length of 6,380 feet, and a structural height of 228 feet. The spillway has an ungated morning-glory entrance structure, a 22-foot-diameter concrete conduit, and a chute and stilling basin. The reservoir formed by Sanford Dam, named Lake Meredith, has a surface area of 30,466 acres at maximum water surface and a total capacity of 1,407,572 acre-feet."
[Actually, that maximum is the "controlled storage capacity" when the water level is at the spillway crest. It can hold an additional 1,206,643 acre-feet to control extreme floods. [33:34]]
I'm learning that water supply is an important function for dams in Texas. The Denison Dam
is contracted to supply 125,000 acre-feet to local communities. And this dam is owned by a water authority
The outlet pipe is a normal design. It is interesting to note that no water is flowing out of it. That is because the building at the base is a pump house that sends water south to the communities that are helping to pay for the dam.
And the 22' diameter morning-glory emergency spillway
with chute and stilling basing is not too surprising. Although, in practice, I don't see morning-glory spillways too often.
But what was surprising is the spillway that goes around the regular emergency spillway. I'm going to call it the trench spillway. Obviously, it can pass water long before the water reaches the flood pool at the morning-glory spillway elevation.
Before discussing the trench spillway, let me note that the morning-glory spillway has three "prongs." How do they help the flow? By adding some air to the water flowing through the 22' concrete conduit?
Back to the trench spillway. A key is that the watershed of this dam is the desert in New Mexico. In fact, the panhandle of Texas is also desert like. There is less than 20" of rainfall in a good year. [1:30
] That means it very seldom rains, but when it does rain, it pours and much of the water ends up in the rivers because desert soil does not absorb water very well. I had already noticed that the Canadian River has carved a well defined valley when I studied the BNSF/SantaFe Viaduct
. To verify the bursty nature of the river, I verified that the first road downstream of the dam is normally a long bridge over dirt.
Some lake levels of interest: [WaterDataForTexas-water
|Level||Elevation||Depth||Distance from Top||Acre-Feet||Acres|
The flood level is the crest of the spillway. I think the dead zone level is the height of the outlet pipe. Below, this level, the pumps would be sucking air. There was a drought during 2012 and 2013 when the pumps were dry. The water authority did buy water rights and drill a bunch of water wells in 2001. Even if the pumps are not dry, they blend in well water to reduce the salt level. [crmwa-facts
] During the first 40 years, the dam provided over 3 million acre-feet of water. [33:50
] But water has still been removed from the Ogallala aquifer. It recharges at a rate of only a quarter-inch/year, and it takes thousands of years for today's rain to get to the Ogallala. [crmwa-overview] Before the dam was built, about 400,000 acre-feet of rain water was lost down the river every year. [4:14
The record high of 101.85' was set in April 1973, and the record low of 26.14' was set on Aug 7, 2013. [crmwa-overview
] So neither spillway has been tested, but the pumps have gone dry.
The boundaries of the Lake Meredith National Recreation Area should at least surround the flood level of the lake. How much of the design level of the lake is protected from development, I don't know.
The fact that a 228' tall dam has no hydroelectric plant is another indication that the normal flow in this river is negligible. Even though it does not have a hydroelectric plant, it does have a conservation pool elevation of 2,936.5 (74.5' below the top). I believe that is the bottom of the trench spillway. So normally, a hydroelectric dam can pass water through its turbines when the level of the reservoir is above the conservation level. But in this case, the dam can pass water downstream when the level of the reservoir is above the conservation level. In fact, it is obligated to. Agreements between New Mexico, Texas, Oklahoma and the Federal governments control how much water can be retained for conservation before water has to be released to the next government entity. But the 1950 agreement below specifies 500,000 acre-feet instead of 817,970, so I'm still a little confused.
The correct term for the "trench spillway" is the Flood Control Outlet Works. [9:18
The narration says each of the three concrete conduits are 15.5' in diameter. But they look rectangular to me.
Downstream of the control gates, the conduits are described as horseshoe shaped with a height of 17' and a width of 17'. Recall that the emergency spillway on the left in this image is fed by a 22' diameter conduit. I was not able to find any cubic-feet/sec figures for the outlet pipe nor these spillways.
As we have seen with the BNSF/SantaFe Bridge
and the road bridge above, the Canadian River flows through in a natural valley. I presume the flow rate of the three conduit structures is easily handled by that valley. The additional flow rate added by the emergency spillway would be a tradeoff between causing flood damage downstream and risking water flowing over the crest of the dam. The spillway level for the Denison dam was designed for a 500-year flood. Unfortunately, it has already seen five "500-year" floods since it was built in 1944. In contrast, Oklahoma has never received a drop of water from this river because the water level has yet to reach the conservation level. In fact, looking at the above graph, it would not have received any water since 1988 if the dam had been built to the 500,000 acre-feet specification.
I included the towns of Sanford and Fritch so that you can get a sense of scale as to how small the reservoir was during the drought.
|Global Earth, Dec 2013|
Texas has Global Earth images for every December since 1984. That is the first time I have seen so many images for the 21st Century. Until the turn of the Millennium, the reach of the lake stayed close to the green line with a tree. And then it began to shrink.
|Global Earth, Dec 1984|
This survey is up to just the conservation level. The Texas Water Development Board is concerned only about the water that they can keep.
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