Levee Breach Analyses Support an Emergency Action Plan and Tabletop Exercise for Miles City, Montana

By Tony Krause from USACE Omaha District

Miles City (population 8400) at the confluence of the Tongue and Yellowstone Rivers in Montana has seven miles of unmanaged and unengineered levees that date back to the 1930s. A recent floodplain modeling project indicated that the levees only provided adequate defense for a 25 year flood. As a result, two-thirds of the city (3200 structures) is located within the 100 year floodplain.

The Montana Silver Jackets team identified the need for an emergency action plan (EAP) to assist the community in preparing for a disaster. This EAP would also assist in meeting programmatic objectives for the Community Rating System (CRS) and State Dam Safety Program for the Tongue River Reservoir upstream.

Before developing an EAP for the community, the USACE performed a breach analyses using an unsteady 2D HEC-RAS model that evaluated inundation scenarios for 6 breach locations. The analyses provided flood inundation mapping for possible levee overtopping scenarios for time increments from 1 to 24 hours. Data generated included flood development times, flood depths, and flow velocities, as well as a count of insured and uninsured flooded structures.

The data provided the community and state officials with a context to identify the flood risk in preparation of a local EAP. The USACE and the state provided a template for an EAP and coached the community through the various steps and data needs for developing a comprehensive EAP. The plan addressed a variety of issues that needed input from local and state government agencies, such as flood impacts on critical infrastructure, evacuation protocols, key decision points, communication protocols, inspection schedules, levee security plans, flood fight roles, location of resources and supplies, training, shelters, and an exercise schedule.

Miles City tested and refined the EAP while conducting a tabletop exercise in November 2015. During the exercise, the local officials were able to identify impacted resources. For example, after a short time into one of the flood scenarios, the community's flood fight resources (soil, sand, and machinery) were inundated. Additionally, the secondary lines of defense inside the community provided by non-levee features (highway and railroad) were recognized as critical. In the end, the EAP, as a living document, will provide a clear and effective structure with which to manage a future flood emergency so that practical considerations and physical limitations are not overlooked.

A meeting was held in April 2016 to provide critical information to the public. At the meeting a video was used to demonstrate the impact of the potential flooding, which allowed the public to better understand and address their risks. This understanding and acceptance will be essential in building support for the community’s future flood risk management efforts.

As an outcome of this project, the city is determining the best course of action, including developing a levee task force, hiring a contractor to propose improvement options, actively pursuing reduced flood insurance rates through CRS, and initiating a public health assessment of vulnerable populations. The city has joined with Custer County to form a regional flood protection committee and will use the products from this project to support a public outreach program.