Dam Breach Modeling and the Assessment of the Self Rescue Zone

An interesting and informative paper by Mr. Bruno Neves of Brazil on HEC-RAS dam breach modeling and determining the self rescue zone, as well as the timing for alerting residents of the self-rescue zone.  The concept is tested using HEC-RAS with hypothetical 2D breach scenarios of the Santa Branca Dam in the Paraíba do Sul River in São Paolo State, Brazil.  Included within the paper is a comprehensive review of some of the most widely used parametric breach equations used today.

ASSESsMENTOF self rescue zones REGARDING BREACH OPENING METHODS

 Bruno NEVES

ABSTRACT

Saving lives during dam break events is the main topic of this article, considering that a precise definition of the self-rescue zone (area in which authorities supposedly are not able to provide support to the population at risk in case of dam break, within the first minutes of the event) is crucial for planning mobilization of population in case of flood inundation. This paper presents a comparison of hydrographs, highlighting their dispersion through the self-saving zone, and sheds some light on issues of the hydraulic model.   

1.    INTRODUCTION

Despite the low probability of a dam break situation, around 0.0001 per year, this sort of hazardous event may cause significant damage and considerable loss of life (Medeiros, 2008).

In Brazil, federal law 12.334/2010 has set the National Policy of Dam Safety which defines obligations to the stakeholders of dams. Through National Agencies of Natural Recourses, details of this regulation are being defined.

ANA (National Water National Agency) and ANEEL (Electric Energy National Agency) agree in their regulations on the definition of the Self Rescue Zone, as follows: The downstream region of a dam where authorities are not able to provide assistance prior to the arrival of the flood wave in case of dam break alert, which is assumed to be 10 kilometers or the distance the front end of the rupture wave can travel in 30 minutes.

Colorado Department of Natural Resources (2010) regulates that the simulation of dam break events shall take place to characterize and identify locations which may be potentially threatened.

The bibliography shows diversity in modeling methods of breach opening which may lead to equally diverse results regarding potential damage in the self-rescue zone.

This paper cites details that may be considered when modeling a dam break event and sheds some light on results acquired from different breach opening equations. The US Army Corps of Engineers software HEC-RAS 5.0 (Hydrologic Engineering Centers River Analysis System) was used and applied to Santa Branca Dam, an earthen dam located in São Paulo state in Brazil.

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Probabilistic Methods for Dam Breach Modeling

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2010. All rights reserved.
I would appreciate any feedback from you all on this topic. It's something I've been thinking about for a while now. My biggest concern with the current practice of dam breach modeling is the overwhelming uncertainty associated with dam breach parameters. Not only the ultimate breach shape and development time, but things like the initiation mechanism of the breach, the discharge coefficients (both weir and orifice), and the progression rate. The deterministic approach we use leaves a bit to be desired in my opinion. Sensitivity analyses have shown that the breach outflow hydrograph can easily vary by 100% or more, based on the set of parameters used. I've been considering ways to generate a breach outflow hydrograph based on probabilistic methods. The idea being instead of providing our "best conservative guess" for the breach hydrograph, we can produce a 95% (or whatever percent) conditional non-exceedance hydrograph based on both overall peak discharge and also timing. Meaning, this is the dam breach hydrograph that will not be exceeded in peak value 95% of the time, given a dam failure for a given failure mechanism (overtopping or piping). This is done by assigning probablity distribution functions to each breach parameter, then run a Monte Carlo simulation using random assignments (within the minimum and maximum bounds and following the prescribed distribution function) for each breach parameter. Then we can plug the resulting 95% hydrograph (or the associated set of breach parameters to create that hydrograph) into our HEC-RAS unsteady flow model and resume our deterministic approach. At least we have taken the deterministic selection of breach parameters out of the analysis. I suppose at some time, the entire model could be approached with probabilistic methods, but first things first. In fact, HEC is currently working on implementing Monte Carlo simulation capabilities into HEC-RAS for a future release.

I wonder if any state Dam Safety office is ready for this type of analysis for preparing inundation maps for emergency action plans. I think it makes more sense.