Breaching is far easier than originally planned, making it possible to move from a decision to breach
to breaching in a matter of months, not years
Given the relative ease of hydraulically breaching an earthen embankment, there is no need for lengthy modeling, engineering, design or complicated/lengthy contracting. New dam overtopping modeling software has been developed since the 2002 EIS was drafted which allows a safe breach plan to be created quickly. The breach itself is done in two phases. First, as drawdown of the reservoir is taking place, earth moving equipment, likely two D8 bulldozers and an excavator, will be cutting a notch in the earthen portion of the dam. When drawdown is below spillway crest and
the notch cut to that depth, controlled hydraulic breaching will begin which uses the turbine gates to control flow. This takes approximately 8 hours with maximum flows not exceeding high flows normally encountered during spring runoff. Armoring protection and other channelization work can
also be accomplished with several pieces of heavy equipment. The entire “construction” effort can easily be accomplished through “Time and Materials” or rental contracts. Details to the breach approach can be found in the 18 Feb 2016 Supplement (unofficial) to Appendix D Natural River
Drawdown Engineers of the 2002 EIS. In short, what the Corps’ Walla Walla District originally estimated would take several years in modeling, engineering, design and contracting and well over $70 million, can be done in a matter of months for around $1 million.
© John Twa / Jim Waddell
Hydraulic breaching has been used before. The cofferdam on the Sandy River in Oregon was hydraulically breached
with surprising success.