US Army Corps of Engineers
Walla Walla District

The scope of the GI study on Mill Creek will include the entirety of the Mill Creek Basin, from its headwaters in the Blue Mountains, through 7 miles of the existing flood-risk-management system, to its confluence with the Walla Walla River.
Background – Mill Creek is a tributary of the Walla Walla River, draining a 96-square-mile watershed. It drops about 430 feet in elevation from its headwaters in the Blue Mountains, through 7 miles of the existing flood-risk-management system, on its way to the river. Mill Creek flooded the City of Walla Walla, Washington, 15 times between 1878 and 1931, including the disastrous flood of 1931. The constructed channel reaches are part of the Mill Creek Flood Control Project, which was authorized in 1938. The city’s economic security and the safety of its citizens depend on the system’s four main components: (1) Storage dam, which impounds Bennington Lake, an off-channel flood storage reservoir. (2) Mill Creek diversion dam, which provides the head required to divert water through a channel into Bennington Lake. (3) Leveed channel (two sections), upstream and down-stream of the concrete-channel section of the system. (4) The concrete channel, which runs through and under the City of Walla Walla, beneath businesses, city streets and state roadways, parking lots and historic buildings. It was constructed by the Works Progress Administration in the 1930s using unknown materials and methods, and was later incorporated into the Corps project upon completion in 1948. Vulnerabilities – The age and deterioration of the levees, concrete channel and infrastructure located above or alongside the channel are of great concern to the community of Walla Walla. The channel was designed in the 1930s to handle flows of up to 5,400 cubic feet per second (cfs). However, a 1996 flood of 4,100 cfs (75 percent of the design flow) damaged the leveed portions of the system, flooded upstream and downstream of the levees and damaged parts of the concrete channel, sometimes nearly overtopping the channel.
Left Bank (south side of levee system) -- Population at Risk (PAR): 19,000; Number of Structures 8,500; Estimated Damages $800 million. Right Bank (north side of levee system) -- Population at Risk (PAR) 2,700; Number of Structures 1,100; Estimated Damages: $400 million. Total at Risk -- Population at Risk (PAR) 21,700; Number of Structures 9,600; Estimated Damages $1.2 billion. Most Importantly -- There is potential life loss associated with a potential breech of the Mill Creek Levee System.
Feasibility Study Process: (1) Alternatives Milestone, (2) Tentatively Selected Plan Milestone, (3) Agency Decision Milestone, (4) State & Agency Review, (5) Chief of Engineer’s Report with Final NEPA Documentation. NEPA Process During Feasibility: (1) Identify Need for Action, (2) Begin Public Scoping, (3) Begin drafting EA or EIS, (4) Release Draft EA or EIS for Public, Technical & Policy Review, (5) Publish & Distribute Final EA or EIS. [SIDEBAR TEXT] Your Input Helps Us Decide -- Website - www.nww.usace.army.mil/MillCreekGI; Email -NEPANWW@usace.army.mil; U.S. Postal Service - USACE - Walla Walla District, ATTN: PPL-C, Mill Creek GI, 201 North 3rd Ave, Walla Walla, WA, 99362-1876. [QR CODE GRAPHIC]
The U.S. Army Corps of Engineers (USACE) is conducting a General Investigation Study in partnership with Walla Walla County to assess the overall flood risks Mill Creek presents to the Walla Walla, Washington, community. The Mill Creek flood-risk-management system begins about 1 mile east of Walla Walla and runs through downtown to Gose Street. The system, consisting of federally and county-maintained elements, has performed well for more than 70 years, but needs improvements. The channel was designed to handle flows of up to 5,400 cubic feet per second (cfs). However, a 1996 flood of 4,100 cfs (75 percent of the design flow) caused significant damage to the levee system, flooding upstream and downstream of the channel, and damaging and nearly overtopping parts of the concrete channel. The flood also exposed scour issues and raised embankment erosion concerns. Some components of the system have degraded, resulting in decreased capacity and reliability: the concrete channel may have a lower capacity than originally believed; development has decreased flows that could be diverted through Yellow Hawk and Garrison Creeks; infrastructure, including some that is more than100 years old, exists adjacent to and over the channel, presenting a collapse risk; an assessment determined that the levee is likely to breach if overtopped. Other factors, such as climate change, increased frequency of wildfires and changes in precipitation could increase surface water runoff, landslides and risk of debris flows in Mill Creek. The reduced channel capacity and increasing potential for aging infrastructure to cause an obstruction within the concrete channel combine to increase risks to levels that have not been fully assessed. The goal of the current flood-risk-management system is to keep flows through the city below 3,500 cfs.
Q. What is happening? A. The U.S. Army Corps of Engineers (USACE) and Walla Walla County (county) are partnering on a General Investigation Study to assess the overall flood risks Mill Creek poses to Walla Walla, Washington, and surrounding communities. Q. Why does the Mill Creek Channel need to be studied? A. The Mill Creek system of levees, dams, reservoir and concrete channel in Walla Walla is more than 70 years old and may need improvements to continue to provide flood-risk-management benefits to the community. The system’s concrete channel runs through the heart of the City of Walla Walla, with a significant portion directly underneath the award­winning, historic downtown commercial core. Walla Walla County continues an active and effective maintenance program, but project age is increasing the frequency, scope and magnitude of repairs and rehabilitations. Some system components have degraded, decreasing capacity and reliability. Deterioration in some infrastructure over the channel has resulted in load restrictions, parking lot closures and the rerouting of emergency vehicles. The channel was designed to handle flows of up to 5,400 cubic feet per second (cfs). However, a 1996 flood of 4,100 cfs (75 percent of the design flow) caused significant damage to the levee system, flooding upstream and downstream of the channel, and damaging and nearly overtopping parts of the concrete channel. The goal of current flood-risk-management-system operations is to keep flows through the city below 3,500 cfs. Q. What else are you concerned about? A. Many buildings and other structures located adjacent to and over the concrete channel are at least 100 years old.
NEPA tells all federal agencies to consider environmental impacts while accomplishing their mission. The National Environmental Policy Act of 1969: Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That this Act may be cited as the “National Environmental Policy Act of 1969.” Purpose Sec. 2 [42 USC § 4321]: The purposes of this Act are: To declare a national policy which will encourage productive and enjoyable harmony between man and his environment; to promote efforts which will prevent or eliminate damage to the environment and biosphere and stimulate the health and welfare of man. [MAIN TEXT] To fulfill NEPA requirements, agencies must: analyze the environmental impacts of planned actions, document the impacts, seek public input, consider the impacts and public feedback. Environmental resources include, biological (wildlife), physical (air, water, land and noise), cultural (archaeological and historic properties), socioeconomic (traffic, jobs, community and quality of life). The NEPA process will result in one of these documents: Categorical Exclusion (CATX), Environmental Assessment (EA), Environmental Impact Statement (EIS). Public Input Opportunities: initial suggestions on the scope of the EA/EIS, comments on the Draft EA/EIS, comments on the Final EA/EIS. Walla Walla District, USACE; Environmental Compliance Section; NEPAnww@usace.army.mil
Map of the U.S. Army Corps of Engineers, Walla Walla District, Mill Creek Dam and Bennington Lake federal civil works project.
The U.S. Army Corps of Engineers is a partner in flood risk management on Mill Creek. The most obvious aspect is the operation of the Mill Creek diversion dam and Bennington Lake to reduce flood flows through the City of Walla Walla and vicinity. USACE staff estimate Mill Creek Basin runoff by monitoring snowpack, and forecasting storm development and temperatures. Reservoir operations reduce peak flows downstream of the diversion dam by beginning to divert water to Bennington Lake when flows exceed 1,400 cubic feet per second (cfs). This reduces erosion and scour damages downstream of the designed channel, which ends at Gose Street, near the City of College Place. The critical operations are for very large floods, when water is diverted to maintain flows through Walla Walla at 3,500 cfs – the design capacity of the levee system. Hydrology and Hydraulics staff conduct evaluations of the levees and dams for breach risks, determining the probabilities of specific flows reaching the project and where the flood waters would go in the event of a failure of a levee or a dam. Staff also evaluates the probability of a flow that would exceed the capacity of a levee or a dam. USACE staff also support the National Flood Insurance program, conducting studies leading to levee certification and inundation mapping. Flood Risk Management is at the heart of USACE’s engineering considerations. Staff look at a broad range of approaches to solve flood problems and work together with partner agencies sharing the responsibility for flood risk management.
[MAIN TEXT] The U.S. Army Corps of Engineers considers environmental issues in all aspects of its corporate enterprise. In particular, a strong emphasis on compliance with environmental laws and improving sustainability is translated into everyday actions that have an effect on today’s environmental conditions, as well as the uncertainties and risks of the future. These challenges are complex, ranging from global trends such as increasing and competing demands for water and energy, climate change and declining biodiversity; to localized manifestations of these issues in extreme weather events, the spread of invasive species and demographic shifts. The Mill Creek watershed contains valuable fish and wildlife habitat and offers recreational opportunities. Ongoing fish passage improvements for Endangered Species Act-listed steelhead and bull trout present an important consideration in flood-risk-management alternative development. Therefore, developing environmentally compatible solutions for flood risk management is integral to the Mill Creek GI Study. A variety of environmental and cultural considerations will inform the feasibility study, including the following: Aesthetics & Visual Resources, Aquatic Resources & Water Quality, Threatened & Endangered Species, Fish & Wildlife Habitat, Vegetation, Climate Change, Recreation, Environmental Justice, Geology & Soils, Socioeconomics, Historical & Cultural Resources, Land Use Practices, Utilities & Public Services. [SIDEBAR TEXT] Considering the Environment in Everything We Do -- In 2002, USACE adopted its Environmental Operating Principles, which continue to guide our environmental and sustainability work today. Learn more at www.usace.army.mil/Missions/Environmental/Environmental-Operating-Principles
Cultural Resources is one of the resource areas that factors into the planning process for the Mill Creek GI Study. Cultural resources can include archaeological sites, significant components of the built environment, and sites with religious or cultural significance to Indian Tribes. Both the National Environmental Policy Act process and Section 106 of the National Historic Preservation Act incorporate and consider effects to cultural resources. One example of a cultural resource is the Mill Creek Flood Control Project (MCFCP), which has been determined to be eligible for listing on the National Register of Historic Places. The MCFCP was authorized by Congress in 1938 and cost about $1.5 million to build. It was constructed by the Works Progress Administration (WPA) and the U.S. Army Corps of Engineers. The contractors were Parker-Shram of Portland, Oregon, and Eaton & Smith of San Francisco, California. Numerous local firms and residents worked on the construction, and nine landowners had their properties condemned for the 750-acre Project. All workmen were provided by the American Federation of Labor union. Three shifts worked six days a week. Wages for unskilled workers were 65 cents per hour. Power-shovel operators received $1.65 per hour. About 200 workmen were employed at maximum, and construction was completed by 1944. The MCFCP is just one of a multitude of historic properties within the study area that must be considered in planning for the Mill Creek GI. The City of Walla Walla and surrounding communities contain historic buildings and structures that are listed on the National Register of Historic Places. The study area was also the central homeland of the ethnographic Liksiyu (Cayuse) people and was used by the Wallulapam (Walla Walla) and the Imataláma (Umatilla).
Public Scoping Period: October 15 through November 16. Please return your comment card in the comment box by theregistration table before leaving or mail comments to: USACE - Walla Walla District, ATTN: PPL-C, Mill Creek GI, 201 N. 3rd Ave, Walla Walla, WA 99362; or email comments to NEPANWW@usace.army.mil; or visit the project webpage www.nww.usace.army.mil, click on Mill Creek Project GI Feasibility Study under Current Projects.

Mill Creek General Investigation Feasibility Study

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The U.S. Army Corps of Engineers, Walla Walla District (Corps), in partnership with its non-federal sponsor, Walla Walla County, proposes to conduct a general investigation study on the Mill Creek Flood Control Project (Project) in Walla Walla, Washington.  The study includes a modernized reassessment of the overall flood risks Mill Creek presents to the Walla Walla community.  The study will consider Project capacity, performance, and reliability of several components through a range of alternatives that could be implemented to reduce risks to the community looking forward. 

The Project was authorized in 1938 with the single purpose of reducing flood risk to the City of Walla Walla and surrounding communities.  

The existing Project consists of four main components:  

1) Bennington Dam, which impounds Bennington Lake, an off-channel reservoir 
2) Diversion Dam, which diverts flood water through a channel into Bennington Lake
3) Leveed Channel (upstream and downstream of Walla Walla), 
4) The concrete channel through downtown Walla Walla.  Since the disastrous 1931 flood, life safety risks have increased.  

The Project used to be able to divert floodwaters to Yellowhawk and Garrison Greeks. 

The flood risk reduction provided by the Project may be insufficient to address current risks, and the number of buildings in the flood zone has tripled and now includes five elementary schools, two middle schools, two high schools, two colleges, six nursing and assisted-living homes, and three hospitals.

A public scoping period was conducted Oct. 15 through Nov. 16, 2018, including a public meeting held Nov. 1, 2018, in Walla Walla, Washington. Information and comments submitted during the scoping period will be included in the permanent public record, and used to identify issues to address in the study, exploring alternatives and identifying potential environmental effects of possible changes to the system. The study process will include additional outreach activities with citizens and stakeholders across the community to gather their thoughts and ideas related to the way we could manage flood risks and other important water resources considerations.

The Mill Creek GI study will also include coordination and consultation with other federal and state agencies to find ways to minimize potential effects to valuable environmental resources, such as ESA-listed fish, water quality, and cultural and historic resources.

The Corps is committed to working with our non-federal sponsor, Walla Walla County, and the larger community to provide safe and sustainable solutions to reduce flood risk in the City of Walla Walla and surrounding communities.

We will keep the public informed by updating this webpage and distributing information to area news media as the study progresses.