Study determines dam-raise alternative won’t meet cost-benefit requirements; Water Board to consider future study options for flood-risk, water-supply concerns (Joint Agency Release)

Published May 18, 2016

BOISE, Idaho – The U.S. Army Corps of Engineers, Walla Walla District, provided a status update for the Boise River Feasibility Study to the Board’s Water Storage Committee members during a special meeting in Boise, today.

ver the past several years, the Corps, in partnership with the Idaho Water Board worked together on the feasibility study to evaluate an array of alternatives to reduce flood risks and increase the amount of water available for use in the Boise River Basin.

The Boise River Feasibility Study team conducted detailed hydrologic and economic modeling and analysis and prepared cost estimates for the alternatives. Based on preliminary results, the solution for both flood risk and water supply needs appeared to be a raise of Arrowrock Dam. Several potential dam height raises were considered. However, the results of more detailed economic benefits-and-cost analyses indicate that the dam-raise alternatives do not generate enough economic benefits to exceed the project costs over a 50-year period of analysis. For the Corps to recommend a solution for this study, the benefit-to-cost ratio must exceed 1.0 -- the benefit-to-cost ratio for a dam-raise alternative was a 0.7.

“The Corps and the Board are partners on this study, and it is important we are able to make a joint and informed decision regarding how we will proceed,” said Corps Project Manager Karen Zelch.

Corps and Board study team members provided Water Storage Committee members with a detailed technical briefing of their findings thus far in the feasibility study. At this point, the options for the future of the study are to terminate the study and finalize work products, or request approval to formulate additional alternatives that may meet the study objectives, Zelch explained.

“It’s important to remember that the results of this study do not mean that the flood-risk within Boise and surrounding communities is low. The Boise River is the highest flood-risk priority in the Corps’ Walla Walla District area of operations,” said District Commander Lt. Col. Timothy Vail. “Evidence shows that, while capable of lessening damages for most minor flood events, the Boise River system of dams does not have the capacity to provide enough protection during less frequent, but more intense, flood events. A single large event could cause up to billions of dollars in damages to Boise infrastructure. Though rare, events of this magnitude are likely to happen in the future."

“If the Board is interested in developing additional alternatives for flood-risk and water-supply options, the Walla Walla District will seek approval from Corps Headquarters to reformulate the study. This will include setting a new schedule and budget,” said Zelch. The feasibility study was authorized to spend $3.5 million, with a 50/50 cost share between the Board and the Corps. Work on the study to date has cost $2.6 million (approximately $1.4 million from the Corps and $1.2 million from the Board).

“There is a significant amount of information from the study and feedback from the community to be considered before any final decisions are made,” said Chuck Cuddy, Idaho Water Resource member and committee chair. “IDWR has many other ongoing and anticipated water-resource studies and projects which may having a bearing on the Board’s decision regarding the Boise GI study’s future.”


USACE-Walla Walla District
Public Affairs Office
IDWR Public Relations
Steve Stuebner

Release no. 16-028