Dam Hazard Consequences Assessment

1. Dam Hazard Consequences Assessment ASFPM May 2011 James Demby – FEMA Sam Crampton - RAMPP Mary Shaw - RAMPP

2. Brief History of the Project • Project Goal: To develop a process that could be recommended to communities to execute an economic, social and environmental consequences of dam failure • This was presented at 2010 ASFPM • Created initial draft guidance document (summer-fall 2010) • Relied heavily on existing procedures recommended for examining the vulnerability to hazards • Conducted initial pilot study (Winter 2010 - Spring 2011) • For NRCS Flood Control Dam Y-15, Gwinnett County, GA • Currently revising guidance document • Utilizing a more qualitative approach and lessons learned from pilot study • Currently preparing for a second pilot study • Lake Barcroft Dam, Fairfax County, VA • Will use to further refine procedures in draft document

3. Initial Draft Guidance Document • Prepared Initial Draft Guidance Document • Used a How-To guide approach with worksheets... • Heavily based on existing FEMA guidance about evaluating the consequences or potential losses due to hazards • Problem • The other documents develop an understanding of probability leading to an analysis of risk • Risk is a function of probability and loss • With dam failure • Probability of collapse may be very low • Probability of non-failure incident / malfunction may be higher • Generally, probability of a particular type of dam failure is unknown • Assessment of consequences, not risk since we are no looking at the probability of the dam failing

4. Dam Hazard Consequences Assessment NRCS Dam Y-15 Pilot Study #1 • NRCS Dam Y-15 (Georgia) • 41’ high earthen flood control dam • Rehabilitated in 2007-2008 to create RCC Spillway for 6-hour PMF • Experienced a ~1/4 PMF in September 2009 • High detail GIS data available from County (LiDAR, Building footprints, tax data etc.) • Recreated September 2009 Event for Original Spillway Configuration (Assuming Dam would have Breached) • Assumed breach at peak elevation • Routed breach downstream until incremental depth < 1.5 feet • Applied, tested and refined the methodologies of draft guidance document

5. Identifying Inundated Areas • Used HEC-RAS Model and Identified the: • Inundation extent • Arrival time • Inundation depths

6. Identifying Assets and Population at Risk (PAR) • Residential Structures • Planning and development department provided occupancy per structure • 2.8 per structure average (SFH) • Elementary School • Online research/county coordination to determine student and staff numbers Temporal Population Adjustment • Non Residential Structures • Assumed 1.4 persons per parking Space

7. HAZUS Economic Loss Assessment User Defined Facilities Method (UDF) • More time consuming, more detailed approach • User defined assets, values and building types • $1.9 million of building related loss • General Building Stock Method (GBS) • Less time consuming, simplified, less detailed approach • HAZUS uses census data and assumes average distribution • $97 million of building related losses ~50× difference between UDF and GBS Method • Gwinnett County largely developed post-FIRM • Does good floodplain management explain difference?

8. Assessing Social Consequences • Loss of Life • Brown and Graham Method was applied • When warning time is: • <15 minutes: • Probable Loss of Life = 0.5(PAR) • 15 to 90 minutes: • Probable Loss of Life = PAR0.6 • >90 minutes: • Probable Loss of Life = 0.0002(PAR) • Population at Risk (PAR) • 0 – 15 Arrival Zone = 168 • 15 – 90 min Arrival Zone = 251 • > 90 min Arrival Zone = 55 • Warning time defined as the pre-breach warning, plus the arrival time • Loss of life highly sensitive on the warning time • Performed a sensitivity analysis to determine value of advanced warning systems

9. Assessing Social Consequences • Social Consequences Assessment • Economic Resource • Infrastructure, jobs, tourism • Environmental Resources • Pollution, hazards, wildlife • Public Services • Courts, transit, water, sewer, power • Public Health and Safety • Care facilities, emergency services, disease, sanitation • Recreation and Leisure • Historical/cultural sites, lakes, trails • Social Cohesion • Activities, religious facilities

10. Assessing Environmental Consequences • Pollutants • Commercial buildings and garages within inundation zone, potential source of chemical pollutants • No major pollutant threats (gas stations, chemical plants, wastewater plants etc) • Y-15 sediment storage capacity of 334-acre feet, 91-acre feet estimated as of 2009. • Extensive sediment deposit potential downstream, possible heavy metals • Higher turbidity • High potential for stream bank erosion exposing sewer lines

11. Next Draft (currently under development) • Places greater emphasis on type of Dam Failure Scenario • Sudden Collapse • Worst case • Planned Collapse • Time to evacuate the inundation area • Incidents • Inability to function as flood control • Uncontrolled releases (human/mechanical errors) • Focus of recommended process is for community with limited resources, little capability with HAZUS • Will assess and refine more simplified approaches to a consequence assessment • Assumptions are Required by Local Community • How long it would take to repair / rebuild • Relationship between number of lives lost and number of injuries

12. Recommended Process Outline of tentatively recommended process: • Gather data on dam • High hazard / Large impoundment... • Identify Assets & Characteristics of Assets • Include structures, infrastructure, vacant structures, open space, agricultural land • Mention importance of CIKR – allow local definition • Measures gathered for each asset in inundation area • Occupancy • Replacement Value, content value • Jobs • Sales / Usage / Productivity • Existence of alternative space / cost of alternative space • 1st Floor Elevation, type of construction • Dependencies, interdependencies, function

13. Recommended Process Continued • Identify Potential Impact • Collapsed, damaged, leaking tanks, business temporarily closed, permanently closed... • Loss of life, injury • Specify Consequences or Implications of Impacts • Economic consequence • Cost of evacuate, rescue, repair, debris removal, • Social consequence • Loss of social organizations, anxiety... • Environmental consequences • Pollution

14. Recommended Process Continued • Assess the consequences • Economic costs may be • Covered by insurance or grants • Paid out-of-pocket by homeowners, business owners, tax payers • Social consequences may be • Obstacles that can be overcome • Positive • Negative and enduring • Environmental consequences may be • Obstacles that can be overcome • Positive • Negative and enduring • Pollution • Final step • Consider path forward • Further study to determine probability • Plan to mitigation in very long-term through zoning...

15. Dam Hazard Consequences Assessment • Lessons Learned from Initial Draft / Pilot Study • Quantitative approach only misses too many potential consequences • Anxiety • Loss of confidence in government • Disruption of social fabric • HAZUS’s Limitations must be carefully evaluated • Caution must be applied when using GBS method • Model is expensive to implement / requires advanced user capabilities • Path forward may be unsatisfactory • Further study rather than project / mitigation

16. Dam Hazard Consequences Assessment