"Dam Breach Modeling with Unsteady HEC-RAS: Common Techniques and Assumptions Compared"

United States Society on Dams Annual Conference, New Orleans, LA
April 25, 2012
By Sunit Deo, PE, CFM and Scott Muchard, PE, CFM

Flooding from dam failures can devastate communities by causing extensive property and infrastructure damage, injuries, and loss of life. To prepare for this possibility, the Texas Commission on Environmental Quality (TCEQ) Dam Safety program recently implemented new rules requiring owners of high and significant hazard dams to submit an Emergency Action Plan (EAP), including dam breach analyses and inundation mapping. Breach analyses of small and intermediate size dams can be performed by a "simplified breach method" as defined by TCEQ. However, for large high hazard dams, TCEQ guidelines recommend the use of full unsteady, dynamic wave models to better represent downstream impacts and inundation limits of a dam breach flood wave. Unsteady flow modeling can provide more accurate estimates of peak flood elevations and their arrival times at specific downstream damage points. However, use of unsteady and dynamic wave models, such as the relatively recent unsteady flow dam breach component within HEC-RAS, can be time consuming, expensive, and have extensive data requirements. Careful planning of the breach analysis methodology and assumptions is required.

The recent TCEQ requirements have resulted in many new dam breach analyses performed for existing large high hazard dams across Texas. This presentation focuses on planning and preparation strategies, tips, and lessons learned while developing dam breach analyses by the "full breach analysis method" using HEC-RAS unsteady-state modeling, following TCEQ H&H Guidelines for Dams in Texas.  Hydraulic parameters such as downstream boundary conditions, receiving stream conditions, and tributary flows are particularly important for breach analyses.  Estimating and choosing dam breach parameters such as size, shape and time to form is another important aspect of the dam breach analysis process. Various methods and guidelines exist for estimating these parameters.

At the end of the day, developing a stable running model without warnings or errors is very tricky. This presentation provides tips that can be helpful in developing a stable unsteady-state HEC-RAS dam breach model.