Presentation 18: Inspection, Maintenance, and Monitoring of Service and Emergency Spillways FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways MWH Inspection, Maintenance and Monitoring of Service and Emergency Spillways Daniel L. Johnson MWH Americas Current Condition * Change in Mentality * Attention paid to safety of dams * Understanding of design events * Owners’ awareness FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Levels of Experience * Rarity of large flood events * Denver snowstorm of 1913 * Big Thompson Flood of 1976 * South Platte Flood of 1965 * Events do occur and spillways are leading cause of failures FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Personnel Issues * Inspection Knowledge Needed * Failure modes * Service spillways see more use than emergency spillways * Emergency spillway may have never been used FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Inspection Issues * Capability to meet design criteria * Conditions and components for successful operation - Located on abutment - Located on dam * Condition assessment * Changes with time FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Inspection Issues * Observation of operation * Annual flood * 5, 10 and 25 year flood * Normal flows give indication of ability for successful operations * Normal flows may be most critical for maintenance FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Maintenance * Maintenance is typically not frequent * Emergency spillway may be forgotten * Repairs are necessary to maintain in as-designed condition * Concrete - Movement, foundation erosion, toe and head erosion * Earth - Slope protection, erosion of channel, abutments, toe, head * Deleterious materials FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Special Issue * Over-the-dam spillways need additional attention * Frequent use as they are cost effective using RCC * Induces new failure mode * Increases frequency of emergency spillway usage Monitoring * Monitoring is needed - to estimate performance - to set a maintenance/rehabilitation plan * Measurements of - movement, cracking, deterioration, aging issues * Documentation of - surveys, photos, checklists and notes FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Data Usage * Review of monitoring data - by personnel experienced and qualified * When first gathered to understand current condition - as comparison to historic records for evaluating changes * Reporting of results to owner and safety agencies Conclusion * Spillways constructed of engineered materials age * Criteria may not be up to date * Modern designs may have less robust components * Inspection, maintenance and monitoring may be last hope FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Presentation 19: Unlined Spillway Erosion Risk Assessment FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Unlined Spillway Erosion Risk Assessment Johannes Wibowo Evelyn Villanueva Don Yule Darrel Temple Photo of Tuttle Creek, KS Unlined Spillway Erosion Risk Assessment Problem Statements: * Spillway erosion analysis encounters variable nature of geometry, geologic material and unpredictable flood events. * Dam Safety Portfolio Analysis needs a tool to determine the probability of spillway damage. Photo of Painted Rock, AZ FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Unlined Spillway Erosion Risk Assessment Research Objectives: * Develop a tool to assess the probability of damage on unlined spillway erosion * Develop a tool to prioritize unlined spillway/channel remediation projects Photo of Saylorville, IA Unlined Spillway Erosion Risk Assessment Photo of DMAD spillway shortly after failure (1982) FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Unlined Spillway Erosion Risk Assessment Canyon Dam Spillway, Texas Date: July 6, 2002 Flow: 66,000 cfs, 250 yrs flood Duration: 12 days Spillway Width: 1260 ft Material: Limestone Unlined Spillway Erosion Risk Assessment Photo of Canyon Dam FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Unlined Spillway Erosion Risk Assessment Risk Assessment Process of Answering Three Questions 1. What can go wrong? 2. What is the likelihood it will go wrong? 3. What are the consequences if it does go wrong? Unlined Spillway Erosion Risk Assessment 1. What can go wrong? * Local Scouring * Spillway Breach * Headcut Erosion * Dam Breach FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Unlined Spillway Erosion Risk Assessment 2. What is the likelihood it will go wrong? * Uncertainty of Flood Event * Uncertainty of Material Parameters * Uncertainty of Performance of the Unlined Spillway Unlined Spillway Erosion Risk Assessment 3. What are the consequences if it does go wrong? * Spillway Partial Damage - Lightly Damaged - Severely Damaged * Spillway Breach - Population at Risk - Loss of Economic Value FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Unlined Spillway Erosion Risk Assessment Spillway Erosion Models * USDA (Temple et al., 1994 * Modified USDA (KCD, 1995; ERDC, 2002) * Annandale (1995) * REMR (WES, 1998) Unlined Spillway Erosion Risk Assessment Phase of Erosions Rock Top Soil Original Surface Vegetal Detachment Head-cut Development Head-cut Advancement FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Unlined Spillway Erosion Risk Assessment Event Tree Sedimentary Erosion Occurred Head-cut Developed Head-cut Advanced Spillway Breach Dam Breach Volcanic Spillway Flow Intact Local Scour Big Pot Hole Spillway Breach Local Damages Partial Damages Dam Breach Partial Damages Unlined Spillway Erosion Risk Assessment Development of Head-cut Load: Hydrograph Governing Equations: Te = y(d = Ad)S Ds/dt = kd[te –tc] FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Unlined Spillway Erosion Risk Assessment Parameters Te = effective stress Y = unit weight of water D = normal depth of flow S = surface slope De/dt = erosion rate Kd = detachment rate Tc = threshold stress Unlined Spillway Erosion Risk Assessment Head-cut Advance Load: Hydrograph Governing Equations: Dx/dt = {C(A A0) (A A0) > 0 0 (A –A0) < 0 Dx/dt = Rate of headcut advance C = empirical parameter A = Hydraulic attack A0 = Threshold level FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Unlined Spillway Erosion Risk Assessment Erosion Model – Threshold Line Maximum qH Erodibility Index Kh Eroded Threshold Line Not Eroded Unlined Spillway Erosion Risk Assessment Erodibility Index Kh Kh = Ms x Kb x Kd x J6 Ms = Material Strength Number Kb = Block Size Number Kd = Joint Shear Strength Number Js = Joint Orientation Number FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Unlined Spillway Erosion Risk Assessment Maximum Hydraulic Attack E = w x q x h E= Maximum Hydraulic Attack W = Unit weight of water Q = Unit discharge H = Energy line drop Unlined Spillway Erosion Risk Assessment Logistic Regression * Regression for Binary Outcomes - Occurrence (Erosion) - Non-Occurrence (No Erosion) * User of Logistic Regression Method - Medical - Business * Probabilistic Liquefaction Analysis FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Unlined Spillway Erosion Risk Assessment Logistic Regression * Odds ration p/1-p * Logit transformation Ln[p/1-p] = b0 + blx p = 1/1 + cxp [-(b0 + blx)] p = probability of occurrence b0-b1 = regression parameters x = independent variable Unlined Spillway Erosion Risk Assessment Multiple Logistic Regression P=1/1 = exp [-(b0 +b1x1 + b2x2 + .. + bnxn)] p = probability of occurrence b0 bv .., ba = regression parameters xl, x2, .., xq = independent variables FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Unlined Spillway Erosion Risk Assessment Multiple Logistic Regression for Spillway Erosion P = 1/1 + exp [-(b0 + b1Ka + b2qH)] Kh = Erosion Index, Material Resistance qH = Maximum aH, Hydraulic Attack Unlined Spillway Erosion Risk Assessment Result of Multiple Logistic Regression p = 1/1 + exp [-(1.71 – 3.9Kh + 3.364 qH)])] p = probability of erosion Kh = Erosion Index, Material Resistance aH = Maximum aH, Hydraulic Attack FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Unlined Spillway Erosion Risk Assessment Logistic Regression for ERDC Threshold Unlined Spillway Erosion Risk Assessment Logistic Regression for Annandale Threshold FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Unlined Spillway Erosion Risk Assessment Independent Variables * Hydrograph - Peak unit discharges (cfs/ft) - Flood durations (hrs) * Spillway Geometry - Lengths (ft) - Slopes (degrees) * Material Index - Erosion Indexes Unlined Spillway Erosion Risk Assessment Ordinal Logistic Regression Sj = F (Material, Peak Discharge, Duration, Average_Slope, and Length) Data: Case Histories (USDA and COE) Damage Levels: No Damage 0 – 0.05% Lightly Damage 0.06 - 15% Moderately Damage 16 – 40% Severely Damage 41 – 75% Breach 76 – 100% FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Unlined Spillway Erosion Risk Assessment Ordinal Logistic Regression Sj = 1.515 Log_Kh + 8.635 Log_q – 1.581 Log_Dura + 0.807 Slope_av + 3.975 Log_Length Probability Formulation: No Damage = 1/ (1 + exp (Sj-k1)) Lightly Damage = 1/ (1 + exp (Sj-k2)) - 1/ (1 + exp (Sj-k1)) Moderately Damage = 1/ (1 + exp (Sj-k3)) - 1/ (1 + exp (Sj-k2)) Severely Damage = 1/ (1 + exp (Sj-k4)) - 1/ (1 + exp (Sj-k3)) Breach = 1 - 1/ (1 + exp (Sj-k4)) K1, k2, k3, and k4 = boundary parameters from regression Unlined Spillway Erosion Risk Assessment Input Unit Disch. (cfs/ft) 112.1 41.8 104.4 163.5 Duration (hours) 120 570 210 3 Erosion Index1 Kh 1/ 5340 28 103 0.01 Ave. Slope (deg) 1.4 1.32 14.04 1 7.2 Length (ft) 2200 520 230 1340 155 Probability Output No Damage 0.001 0.990 0.000 0.029 0.000 Lightly 0.019 0.009 0.002 0.275 0.000 Moderate 0.305 0.001 0.047 0.609 0.000 Severe 0.629 0.000 0.639 0.085 0.003 Breach 00.046 0.000 0.312 0.002 0.997 FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Unlined Spillway Erosion Risk Assessment Photo of the Bluestone, WV dam Photo of the Dalles, OR dam The Dalles, OR Q = 2,290,000 cfs Bluestone WV Q = 430,000 cfs Erosion Index (Kh) 1960 2734 Stream Power (Kw/mw) 125.4 22.3 Probability of Erosion 0.012 0.000 Unlined Spillway Erosion Risk Assessment Simulation Using USDA Model * Monte Carlo * Latin Hyper-Cube No Damage 0 – 0.05% Lightly Damage 0.06 – 15% Moderate Damage 16 – 40% Severely Damage 41 – 75% Breach 76% - 100% Unlined Spillway Erosion Risk Assessment Prioritizing Process Ranking the outcome: Risk = P occurrence * P failure * Consequences FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways Unlined Spillway Erosion Risk Assessment Future Research * Erosion Index needs to be refined * Geophysical Exploration will be useful for volcanic areas * Effect of spillway channel geometry (curving, narrowing) * Three dimensional erosion (side erosion) FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways