Dam Breach Inundation Analysis Using HEC-RAS And GIS TWO CASE STUDIES IN BRITISH COLUMBIA, CANADA Canadian Dam Association 2014 Conference - Banff Alberta
Presenters 2 Dwayne Meredith, P.Ag. KWL Vernon Ahmad Asnaashari, P. Eng. In BC
Introduction 3 Presentation Outline Introduction Dam Break Modelling Purposes Case Studies Challenges and Lessons Learned Recommendations
Introduction 4 CDA 2007: The analyses leading to consequence assessment and classification of the dam typically include the following steps: 1. Characterization of hypothetical dam breach, 2. Flood wave routing, 3. Inundation mapping, and 4. Evaluation of the impacts
Introduction 5 Objective Summarize challenges and lesson learned during the modelling and simulations of two case studies.
Introduction 6 Why is this important? 1. Understanding potential consequences from an unanticipated release of water; 2. Derives dam classification; 3. Affects design requirements; 4. Assigns regulated activities; 5. Necessitates level of planning (ie OMS and EPP)
Introduction 7 Historic (and Recent!) Events Testalinden Mt. Polley Photo Credit: CBC.ca Photo Credit: CBC.ca
Two Case Studies 8 Pinaus Lake Dam dam dimensions: 1.7 m high x 23.1 m wide small dam replacement concrete gravity to sheet pile weir impoundment : 2.65 million m 3 Cold Spring Creek Dam dam dimensions: 4.6 m high x 22.0 m long concrete gravity structure impoundment: 1,600 m 3
Two Case Studies 9 Pinaus Lake Dam
Two Case Studies 10 Cold Spring Creek Dam
11 Dam Break Modelling
12 Tools Guideline Engineering Judgment Dam Safety
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Dam Failure to Inundation Area 14 Photo Credit: FEMA
Dam Breach Model Output 15 1. Estimate Breach Outflow under extreme events 2. Route the Breach Outflow in the DS Creek 1. Flow at DS locations 2. Water Levels 3. Flood arrival time 4. Velocity 5. and more (if needed)
Challenges 16 Data Availability and Resolution Coarse topographic data Shallow water Incremental water level Site specific PMF value and hydrograph BC PMF Estimator Creager Graph Model Stability
Challenges 17 Model instability Model Crashing!
I like the blue bar 18
Challenges 19 Factors Affecting Case Studies Models Stability Channel slope Low flow conditions (before breach) e.g. Sunny day cases Big changes in flow (after breach) Cross section spacing Computational time step Manning s n values Initial flow/water level conditions Breach parameters (B, t)
Cold Spring Creek Dam Model 20 dam dimensions: 4.6 m high x 22.0 m long impoundment: 1,600 m 3
Coarse Topographic Data (TRIM) 21 Cutting Cross Sections (Scale 1:20,000) Horizontal line in cross sections In floodplain Channel data from field measurement Pilot channel
Cold Spring Creek Dam Model 22 Incremental Water Level Q=1 m 3 /s Cold Spring Creek Breach Outflow (Sunny-Day failure)
Cold Spring Creek Dam Model 23 Incremental Water Level Cold Spring Creek Breach Outflow (Flood condition failure)
Cold Spring Creek Dam Model 24 PMF (NO Breach) El. 941.67 m PMF (Breach) El. 941.75 m Cold Spring Creek Breach Outflow (Flood condition failure)
Lesson Learned 25 Cross Sections Spacing Steep slopes => closer cross sections High velocity area => closer cross sections Cross sections spacing: Not too close, Not too far (Balanced!) Maximum spacing: Dr. Fread's Equation, Samuals Equation Trial and error Balance with time step
Lesson Learned 26 Junction Bed Elevation At creek junctions (confluences), tributary and river bed elevation should be the same or very close. Otherwise, model will crash.
Lesson Learned 27 Manning n Adjustment Increase Manning n DS of Dam Account for associated sediment, debris and turbulence for a dam breach This is a conservative assumption Increase Manning n on Steep Slopes Auburn Cofferdam
Lesson Learned 28
Effects of Large Water Bodies Downstream 29 70 Q p =65 m 3 /s 60 50 Q p =50 m 3 /s Little Pinaus Lake Inlet Little Pinaus Lake Outlet Flow (m 3 /s) 40 30 20 10 0 12:00 PM 03:00 PM 06:00 PM 09:00 PM 12:00 AM 03:00 AM 06:00 AM 09:00 AM 12:00 PM 03:00 PM 06:00 PM 09:00 PM Time Step (Hours)
Lesson Learned 30 Qp=65 m 3 /s Qp=50 m 3 /s
Lesson Learned 31 Select "Mixed Flow Regime" to help model stability
Thank you! 32 Questions? Dwayne Meredith, P.Ag. DMeredith@kwl.ca (250) 503-5817 Ahmad Asnaashari, P.Eng. ahmadasna@gmail.com (604) 345-6016