Locks and Barges
How do they decide how big to make a lock?
We would suppose that many factors must be considered. Like, how much width of the project can you devote to the lock, versus spillways and powerhouse? Does the amount of water per lockage have anything to do with it? We doubt it, because smaller locks require more lockages to move a big tow through. On the other hand, do you want to move that much water downstream for one small fishing boat? So maybe they do consider the mix of traffic they expect.
Of course, a one factor has to be, how big are the barges? We found some interesting info while reading about the problems with the lock at Chickamauga. See a summary of that report here... this is a pdf file, and requires that you have a pdf reader installed.
When Chickamauga was built, the barges were mostly the so-called "standard" size, measuring 26 x 175 feet. Thus, a 60 x 360 foot lock could accommodate 4 at a time. Now, however the standard barges are all but phased out, being replaced by the "jumbo" barge, measuring 35 x 195 feet. Do the math, and you will see that only one at a time can go. Ouch! The average processing time of a tow at Chickamauga is 8 hours, the worst in the entire Ohio River System.
The average tow is 15 barges, arranged three across and five deep. The barges are all lashed together with steel cables, and a "tug boat" is at the rear, pushing the whole assembly. Locking through a small lock like Chickamauga, Watts Bar, or Fort Loudon, all 60 x 360 feet, requires disassembling the entire tow, and requires two tug boats, one at each end of the lock. We suppose you could do it with one boat, since there are mooring points to keep a tow in place as it is disassembled or reassembled, but locking the boat back and forth would just take that much longer. We have seen big tows locking through at Chickamauga and at Watts Bar, and there were two boats in use both times.
Downstream from Chickamauga, all of the locks are newer, being 110 x 600 feet at Nickajack, Guntersville, Wheeler, Wilson, and Kentucky, and 110 x 1000 feet at Pickwick. A new 110 x 1200 foot lock is being built at Kentucky. Most of these dams also have the original smaller locks, kept in service as auxiliary locks (a double lock at Wilson), an exception being Nickajack... it looked like they had an auxiliary at one time, but it was permanently plugged. The 110 x 600 foot locks only require that the barges be separated into a 3 x 3 and a 3 x 2 arrangement, and requires only two lockages. Plus, using the "mule" to pull the first 3 x 3 assembly out of the lock, only one boat is required. The boat locks through with the 3 x 2 assembly, the barge hands lash the two assemblies back together, and off they go in a couple of hours. We got to watch this process at Wilson. The 1000 foot lock at Pickwick will accommodate the whole tow, but the tug boat has to wait, so two lockages are still required. At least you don't have to disassemble the tow, you just separate it from the tug. Pickwick also has a 110 x 600 auxiliary lock, and both can be used at the same time. If conditions permit, they can lock the boat through the auxiliary while the tow goes through the main lock, to make things even faster (this is called a "Fast Double" as opposed to a "Straight Double" lockage). The 1200 foot lock at Kentucky will obviously accommodate the whole smash! The one boat we have seen that we remember the name of is the "Francis R. Keegan", which we saw at Wilson and at Chickamauga. According to the Coast Guard page, she is 127.5 feet long and 38.2 feet wide... shorter but just a little wider than a jumbo barge.
Long story short, the Chickamauga Lock study indicates that closing the lock is not a good idea, nor is "replacement in kind", i.e. building a new 60 x 360 foot lock. The most cost effective option is a 75 x 400 foot lock, which will accommodate four of the jumbo barges in a 2 x 2 arrangement. However, the study is clearly pushing the 110 x 600 foot option that "has advantages that cannot be captured in the economic analyses but are valid nonetheless". The primary focus is obviously reduced lockage time for a 15 barge tow.
So maybe barge and tow size IS the primary factor! It will be an interesting process to watch...
UPDATE! While we weren't paying attention, apparently the decision was made! Check out this site for more info... (long story short: look for a new 110 x 600 lock in the not too distant future!)