According to the Auburn Reporter, “the latest evidence points to the worrisome possibility of actual erosion in the abutment (of Howard Hanson Dam).” Why is the city seemingly only content to plan for flooding of the city instead of dealing with the issue and resolving it?
The Corp of Engineers wants to lay down a gout curtain over the abutment. Even it states it does not know whether this curtain will solve the problem. The problem obviously is that water is leaking through the abutment, and this water is eroding the abutment. If it is not addressed, sooner or later, the abutment will be eroded so badly, it will give way. There are many examples of this happening in the past to other dams, and it would be foolish of the Corps not to address the issue more aggressively.
The answer should be obvious. The construction of a cofferdam extending from the dam itself to the shore, built high enough to allow use of the dam under most normal conditions, should be implemented as soon as possible. No matter what is done to the abutment ultimately from sealing it to replacing the entire structure, it will require the construction of a cofferdam behind it in order to do it.
A cofferdam usually consists of a series of pilings, or large tubular structures driven down into the bedrock and then filled with concrete or other material. The pilings are designed to interlock, forming a very strong structure. Any water that leaks through is handled by pumps installed for the purpose.
The cofferdam’s purpose is to allow the water behind the abutment to be pumped dry so that work can then proceed to repair the abutment.
By constructing a cofferdam now, the Corps accomplishes two obvious goals: one, it eliminates the present problem because there will be no water against the abutment, and therefore any danger of erosion is immediately ceased; and two, any permanent repairs are advanced as the cofferdam necessary to accomplish them already is in place. The repairs can then be effected more rapidly.
Cofferdams can be put in place fairly quickly once construction begins. They are a lot cheaper than the flooding that can be caused should the dam not effectively hold back the flood waters.
– Bill Walker
