Levee Breach Modeling- Erosion Rate Calculation

The USACE Modeling Mapping and Consequences Production Center (MMC) provides hydraulic modeling, mapping, and consequence analysis in support of the USACE Dam & Levee Safety and Critical Infrastructure Protection and Resilience (CIPR) Programs. The MMC has developed processes, tools, and standards for creating hydraulic models in support of dam and levee breach models for use in Risk Assessments, emergency action plans (EAP) and watershed modeling in support of the Corps Water Management System (CWMS) program. One of these products is a Levee Breach Erosion Rate Calculator.

The simplified physical breach method within the Hydrologic Engineering Center-River Analysis System (HEC-RAS) allows for the calculation of breach parameters based on physical conditions. This method can be applied to any type of breach and is recommended for MMC Levee Breach studies. The method requires the input of a breach widening and downcutting versus velocity relationship (erosion rate). As there are currently no widely accepted erosion rates for use with this method, the MMC has developed a method to consistently produce site specific relationships.

The paper Calculation of Levee-Breach Widening Rates, April 2022, by Bryant Robbins and Maureen Corcoran, is primarily used to support the MMC application of this breach method. This paper builds upon an ERDC Report by Johannes Wibowo. An equation set from the 2022 Paper was used to develop a calculator spreadsheet. A single relationship is developed for each breach location for both widening and downcutting parameters. The spreadsheet has the option to adjust 4 parameters, k_d, τ_c, Manning’s N, and Levee Height.

For the purposes of MMC modeling, and at the recommendation of (Risk Management Center) RMC and MMC leadership, it is recommended that Shear Stress (τ_c) be set to 0. This assumption is a conservative estimate to ensure breach initiation and progression in areas of low velocity gradients. The lack of a modeled breach initiation was a programmatic modeling issue for levees that have properties that would align with a less erodible erosion rate.

It is recommended that the Manning's value through the breach be set at 0.034 unless site specific data for the roughness condition during a breach is available. This value approximates a relatively rough earthen channel.

It is recommended that levee height be estimated to the nearest foot at the levee breach initiation location. If height across the breach varies greatly, adjust as needed to ensure the levee height represents the total breach.

Some conservatism is introduced with the shear stress assumption; therefore it is recommended that a best estimate k_d be chosen. If specified soil information is unknown, the modeler may estimate a k_d value from a series of preselected values. These values were determined by RMC Technical Leads with an evaluation of data from the TAMU database developed by Briaud Et. Al. These selections are based on lumped measured erodibility of USCS soil types.

Examples and comparison of breaching results will be provided. This will include a comparison to a historic breach, as well as programmatic breaching efforts in support of upcoming risk assessments.