EVALUATION OF NEW MODELS FOR SIMULATING EMBANKMENT DAM BREACH

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

2. 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

4. 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

5. 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 extremefloods • Emergency preparedness • Testing of embedded dam anchors ASDSO 2009

6. 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

7. 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

8. Physically-Based Breach Modeling • Dr. Danny Fread recognizedneed for modeling erosionprocesses to obtain betterresults in near field • May 18, 1980 eruption ofMt. St. Helens createdlandslide 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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 magnitudedifference in erodibility ofmaterials • Constant inflow, smallreservoir 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

23. 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 arrivedshortly after initial breach…reservoir level went back up…peak outflow driven by peakinflow • Flow regulation not attempted forgravel dam test • Inflow was too little, too latefor zoned test ASDSO 2009

24. 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

25. ASDSO 2009

26. 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

27. 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

28. Evaluation Criteria • Evaluate performance using • 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

29. 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

30. 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

32. CEATI Information:Chris HayesDirector, Business Development1155 Metcalfe St., Suite 1120Montreal, QC H3B 2V6 (514) 866-5377 | www.ceatech.ca | info@ceatech.ca