EVALUATION OF NEW MODELS FOR SIMULATING EMBANKMENT DAM BREACH Tony L. Wahl Bureau of Reclamation – Denver, CO

What is CEATI International? Since 1891, the Canadian Electrical Association (CEA) has been the forum for electrical business in Canada In 1974, CEA initiated its R&D Program to serve the research needs of Canadian electric utilities In 1998, CEA’s R&D Program opened its doors to international participation In 2001, CEA Technologies Inc. (CEATI) was separated from the Canadian Electrical Association CEATI International is now the “Centre for Energy Advancement through Technological Innovation” ASDSO 2009

Interest Groups 14 Interest Groups in the areas of electrical energy… Generation Transmission Distribution Utilization Dam Safety Interest Group About 40 dam owners Jointly sponsors research & development projects Participants from Canada, the United States, Europe, Australia, and New Zealand ASDSO 2009

Dam Safety Interest Group (DSIG) Areas of Interest: Risk assessment for dam safety The use of geophysical methods in the diagnostics and monitoring of embankment dams Erosion and piping in dams Reliability of discharge facilities Ice loadings Probability (frequency) of extreme floods Emergency preparedness Testing of embedded dam anchors ASDSO 2009

Dam-Break Modeling: Recent History Lethal Dam Failures in 1970s Canyon Lake Kelly Barnes Laurel Run Buffalo Creek Teton 1977 DAMBRK model developed Could route peak breach outflows to determine inundation depths, flood consequences Could determine peak breach outflow, given a description of how a breach would develop ASDSO 2009

Modeling Breach Development Concrete dam failure modes (sliding, overturning, structural) are usually instantaneous and complete Embankment dam failures usually involve erosion, which takes time and depends on many factors Regression equations to relate breach parameters to dam and reservoir characteristics Many developed in 1980s and refined in 1990s Adequate for cases in which the area of interest was in the “far-field” Too crude for the “near-field” ASDSO 2009

Physically-Based Breach Modeling Dr. Danny Fread recognized need for modeling erosion processes to obtain better results in near field May 18, 1980 eruption of Mt. St. Helens created landslide dam on Toutle River Dr. Fread developed NWS-BREACH model to analyze possible breach of this dam NWS-BREACH released to public in 1988 ASDSO 2009

Modeling Developments in 1990s Flood routing capabilities much improved 2D modeling Integration with GIS to improve consequence analysis Little change in breach modeling during this time ASDSO 2009

CEATI Dam Erosion and Breach Project Since 2001 the DSIG has had an interest in improving the tools used to model embankment dam erosion and breaching Key Questions Will a dam breach? What is the outflow hydrograph? What is the warning time? Available methods mostly unchanged since late 1980s Regression models for predicting peak outflow Regression models for predicting breach parameters Breach erosion models, such as NWS-BREACH ASDSO 2009

Shortcomings of Available Methods Regression models for peak outflow No aid in determining whether breach occurs Little detail about hydrograph shape or warning time Regression models for predicting breach parameters Uncertainties are large, especially for time parameters Breach initiation time Breach formation time Breach erosion models (e.g. NWS-BREACH) Used sediment transport equations, not true erosion models Poor modeling of erosion of cohesive materials ASDSO 2009

Large-Scale Physical Tests Since 2000, many organizations have been performing small-scale and some large-scale embankment breach tests European IMPACT Project (22 lab tests and sponsorship of Norwegian field tests) Norwegian tests (23 lab tests, 5 field tests of 6-m-high dams) Agricultural Research Service (7 overtopping tests and 4 piping tests of 2-m-high dams) New breach erosion models under development Physically-based simulation of erosion processes Better modeling of the erosion of cohesive soils ASDSO 2009

Project Objectives Dam breach erosion project was initiated in 2004, with a focus on erosion and breach processes and prediction of breach outflow hydrographs at the dam We want to develop physically-based models for overtopping erosion and internal erosion leading to dam breach and facilitate the integration of those technologies into existing flood routing tools like HEC-RAS, MIKE11, Telemac, InfoWorks, etc. ASDSO 2009

Participants Electricité de France Case studies…erodimeter and piping erosion research Hydro Québec / Ecolé Polytechnique Montréal Numerical modeling of dam breach, development of Firebird breach model Bureau of Reclamation Laboratory testing…investigate erodimeters Agricultural Research Service Large-scale laboratory testing and development of SIMBA/WinDAM models HR Wallingford Large-scale testing (IMPACT project), developers of HR-BREACH model US Army Corps of Engineers Integration of breach modeling technology into HEC-RAS suite Elforsk AB Model evaluation Other interested parties and sponsors BC Hydro, Churchill Falls, Elforsk AB, EoN Vasserkraft, Great Lakes Power, Manitoba Hydro, New York Power Authority, Ontario Power Generation, Seattle City Light, Scottish & Southern Energy, National Weather Service ASDSO 2009

Phase 1: Information Gathering Project Overview Phase 1: Information Gathering Reviewed and assembled case-study and large-scale laboratory test data Reviewed and identified numerical models under development Phase 2: Model Development and Implementation Phase 3: Model Enhancement ASDSO 2009

Tasks in Phase 2 Evaluation of three numerical breach models SIMBA (ARS) HR-BREACH (HR Wallingford) FIREBIRD BREACH (Montreal Polytechnic) Evaluation of methods for quantifying erodibility of cohesive embankment materials leading to… Integration of breach modeling technologies into HEC-RAS dynamic routing model Potential efforts to facilitate integration with commercial flood routing models ASDSO 2009

The Models: Common Characteristics Models are all physically-based Models utilize quantitative input parameters describing erodibility of cohesive materials Models are intended to perform well without specific calibration to a particular case Models are not computationally intensive ASDSO 2009

The Models SIMBA – Simplified Breach Analysis (USDA-ARS) Simulates breach by overtopping of homogeneous earth embankments with negligible protection on the downstream face Four stage failure process surface erosion leading to development of a headcut on the downstream face of the embankment headcut advance through the crest to initiate the breach breach formation as the headcut advances into the reservoir breach expansion during reservoir drawdown Erosion formulas are fixed and most calibration factors have been determined from lab testing. Complete model is not calibrated to any specific data set. ASDSO 2009

The Models HR BREACH (HR Wallingford) Overtopping or piping-induced breach of cohesive, non cohesive and simple composite (i.e. zoned) structures. Simulated processes: Initial erosion of embankment surface protection (grass or rock cover) Headcut erosion through embankment Potential failure of breach side slopes by shear or bending Potential for sliding or overturning of core section Limited selection of erosion formulas Not calibrated to any specific data set ASDSO 2009

The Models FIREBIRD BREACH (Montreal Polytechnic) Overtopping-induced breach of homogeneous earthfill or rockfill dams One dimensional unsteady flow, St. Venant equations coupled with sediment continuity Able to handle transcritical flows Side slopes are evaluated for ability to resist sliding along a simple inclined face Choice of erosion formulas Can be more computationally intensive ASDSO 2009

Model Evaluation Evaluate model performance against large-scale laboratory tests and case-study data 2 ARS outdoor laboratory tests 2.3-m high homogeneous dams, overtopping 1 breach, 1 non-breach 3 overtopping breach tests performed in Norway during the IMPACT project (5- to 6-m high dams) homogeneous clay homogeneous gravel zoned embankment 2 real dam failures Oros (Brazil) Banqiao (China) ASDSO 2009

ARS Tests Two overtopped embankments, 2.3 m high SM Silty Sand, complete breach in 51 minutes CL Lean Clay, headcut damage, but no breach after 20 hours 2.5 orders of magnitude difference in erodibility of materials Constant inflow, small reservoir Hanson, G.J., Cook, K.R., Hunt, S. 2005. Physical modeling of overtopping erosion and breach formation of cohesive embankments. Transactions of the ASAE, 48(5):1783-1794. ASDSO 2009

Norwegian Tests - Part of IMPACT Three overtopped embankments, 5 to 6 m high Homogeneous clay, placed very wet Homogeneous gravel, surface frozen Zoned rockfill with moraine core Inflow regulated at upstream reservoir Clay dam: Peak inflow arrived shortly after initial breach… reservoir level went back up… peak outflow driven by peak inflow Flow regulation not attempted for gravel dam test Inflow was too little, too late for zoned test ASDSO 2009

Oros Dam (Brazil, 1960) 35-m high dam, failed by overtopping during construction Core material probably a Sandy Lean Clay, with PI=10 Well-compacted, except maybe last lifts ASDSO 2009

ASDSO 2009

Oros Dam - Summary Thick, erosion-resistant embankment, large reservoir Slow erosion 12 hrs to initiate breach 6.5 to 12 hrs to form breach and drain reservoir ASDSO 2009

Banqiao Dam (China, 1975) Hand-built dam with homogeneous earth shells and clay core wall of “arenaceous shale” Assumed to be poorly compacted and highly erodible 1 hr breach initiation 2 to 2.5 hrs to fully form breach ASDSO 2009

Evaluation Criteria Evaluate performance using Objective criteria initial inputs (best available information and judgment) optimized inputs Objective criteria Time to initiate breach (erode through crest) Time to form breach (reach full width) Final breach width Breach widening rate Peak outflow Subjective criteria Do models exhibit appropriate sensitivity? Ease of determining input data and selecting parameters Ease of operation ASDSO 2009

Current Status Team met at last year’s USSD meeting in Portland Members have been working this summer to perform the evaluation runs Group will meet again later this week to compare results and try to reach consensus on: Which models and model components are working well? What technologies are presently ready to be integrated into state-of-the-art models? Where is more work needed? SIMBA and HR-BREACH models are being integrated into USDA WinDAM and Wallingford Software InfoWorks products ASDSO 2009

Challenges TIME: Too many models, cases, scenarios Each case study presents unique evaluation challenges Real failures have questions about dam materials and erodibility, and about observed breach and outflow characteristics Lab tests have “real-world” logistical complications and limitations related to reservoir size Failure to accurately model breach initiation phase can require judgment to evaluate how well the model reproduced later stages of the breach process Evaluation process has already been extremely valuable ASDSO 2009

CEATI Information: Chris Hayes Director, Business Development 1155 Metcalfe St., Suite 1120 Montreal, QC H3B 2V6 (514) 866-5377 | www.ceatech.ca | info@ceatech.ca