Mill Creek Storage Dam
Interim Risk Reduction Measures
U.S. ARMY CORPS OF ENGINEERS BUILDING STRONG®
The U.S. Army Corps of Engineers
owns and operates 635 dams serv-
ing a variety of purposes including
navigation, ﬂood risk management,
water supply, irrigation, hydropow-
er, recreation, and environmental
To ensure acceptable public safety
levels for its dams, the Corps is
using a risk informed process to
prioritize addressing dam safety
deﬁciencies on a nationwide basis.
As part of this process, the Corps
is screening its dams and assign-
ing safety classiﬁcation ratings.
Mill Creek Flood Control Project, which contains two dams, was completed in 1942. The Storage Dam works
in concert with the Diversion Dam to divert ﬂood water from the Blue Mountains to a storage reservoir at Ben-
nington Lake. Diverting ﬂood water helps reduce potential ﬂood damage to the city of Walla Walla and adjacent
downstream areas bordering Mill Creek, Yellowhawk Creek, and Garrison Creek.
The storage dam is an earth-ﬁlled structure 120-feet high, 3,050 feet long and 800 feet wide at its base. The
reservoir was last ﬁlled in 1996. At that time, ﬂows were about 6,000 cubic feet per second (cfs). There were
two other occasions in the past 100 years with similar ﬂows. In 1906, ﬂows were 5,200 cfs, and in 1931, ﬂows
were about 6,000 cfs. (for more information on the Diversion Dam see Mill Creek IRRM at http://www.nww.usace.
Dam Safety Screening & Interim Risk Reduction Measures
On May 31, 2007, the Corps released Engineer Circular (EC) 1110-2-6064, Interim Risk Reduction Measures
(IRRM) for Dam Safety. This circular includes a Dam Safety Action Classiﬁcation (DSAC) rating table. The
screening for all dams is on-going, and is scheduled to be completed in the Fall of 2009.
Based upon the assigned DSAC values, we develop, prepare, and implement interim risk reduction measures to
reduce risk of damage to property and prevent loss of life. Interim risk reduction measures are our short-term ef-
forts to reduce dam safety risks while we pursue long-term solutions
In 2008, Mill Creek Storage Dam initially received a Dam Safety Action DSAC rating of I primarily because
there are potential dam seepage and piping issues with the foundation of the dam when Bennington Lake is
more than 17 percent full for an extended period of time. The ﬁndings create an urgent and compelling require-
ment for interim risk reduction measures and system modiﬁcations. In October 2009, the dam status was up-
graded to a DSAC II after additional data gathering and analysis showed the risk of dam failure under normal
operations was not as high as originally estimated.
Mill Creek Storage Dam Status
Because there are potential issues with the dam’s integrity, the Corps limits the time and amount of water
stored in the reservoir. The reservoir’s water level is normally between 5-10 percent of its capacity. About once
every three years, it is necessary to divert some water to the reservoir during ﬂood season. When the Corps
diverts water into the reservoir, we face challenges in meeting our high standards for public safety – our top pri-
ority. To address these concerns, the Corps closely monitors the project performance and modiﬁes operations
if there is unreasonable risk to the public. The Corps is implementing interim risk reduction measures to reduce
risk to the Walla Walla Community.
Storage Dam Speciﬁc Deﬁcencies
1. Gravel and silt materials in the dam are possibly washing away
2. The dam has inadequate protective measures to prevent seepage
3. Dam materials are not sufﬁciently compacted (dense)
4. Gravels in the dam’s foundation may contribute to foundation failure
5. The dam has no emergency spillway or secondary outlet
6. The inlet and outlet works were not designed to withstand an earthquake
7. The outlet pipe does not meet new seismic design criteria
8. An analysis on the current Probable Maximum Flood was not conducted
Storage Dam - Potential Failure Modes
1. Seepage and movement of soil in the embankment, abutments, foundation and around the outlet pipe.
2. Intake tower may not be stable during an earthquake.
3. Embankment and foundation seismic instability.
4. Water could overtop and erode the dam.
5. Full lake level could cause the dam to be unstable.
What the Corps is Doing Now
The Walla Walla District is pursuing funding to implement the following Interim Risk Reduction Measures
to minimize risk to public safety:
1. Modiﬁying operating procedures to include ﬂood control, conservation pool constraints and diversion
2. Coordinating with Walla Walla County on pre-positioning emergency supplies and contracts.
3. Increasing monitoring and surveillance.
4. Working with Walla Walla County to maximize an emergency alert system.
5. Installing lighting to assist with emergency operations at night.
6. Perform a potential failure modes analysis.
7. Conducting a comprehensive dam seepage and stability analysis.
8. Revising and test the Emergency Action
9. Installing a seepage ﬁlter and pressure
test the outlet conduit.
10. Verifying dam and project elevations.
11. Evaluating intake tower stability.
12. Evaluating additional options for diverting
13. Evaluating increased channel capacity.
14. Completing a hydrologic re-evaluation.
15. Working with Walla Walla County to
develop a ﬂood response plan.
Starting in 2010, the Corps will develop
long-term alternatives and solutions that will
require funding to implement from 2012 to
Throughout this process, the Walla Walla
District will pursue permanent solutions to
determine required repairs/modiﬁcations.
The Walla Walla District will continue to keep the public informed of safety issues related to Mill Creek
Storage Dam and progress made in implementing risk reduction measures.
updated: June 2009
U.S. ARMY CORPS OF ENGINEERS – WALLA WALLA DISTRICT
201 NORTH 3RD AVE - WALLA WALLA, WA 99362