Modernization of High Hazard Dams in Austin, Texas. TFMA Spring Conference May 23, 2007

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1 Modernization of High Hazard Dams in Austin, Texas TFMA Spring Conference May 23, 2007

2 Modernization of High Hazard Dams in Austin, Texas City of Austin Glen Taffinder, P.E. Stormwater Pond Dam Safety Team Leader Freese and Nichols, Inc. John King, P.E. Jay Scanlon, P.E. Kim Patak, P.E. Principal Associate Design Engineer

3 Texas Administrative Code, Title 30, Chapter 299 Dams and Reservoirs Dam - Any barrier, including one for flood detention, designed to impound liquid volumes and which has a height of dam greater than six feet. (Definition continues with exceptions that do not apply to stormwater management ponds)

4 Definition of a Dam The junction of the downstream face of a dam with the ground surface. Lowest Point Cross-Section View

5 Definition of a Dam or the invert of the outlet pipe. Lowest Point Cross-Section View

6 Definition of a Dam Profile View..when measured from the downstream toe-ofdam or invert to the spillway crest.. Either > 6 feet

7 Definition of a Dam... when measured from the downstream toe-ofdam to the top of dam. Top of Dam Cross-Section View > 6 feet

8 High Hazard Dam

9 High Hazard Dam Note that the top of the dam is higher than the top of the house as evidenced by the electric power lines.

10 High Hazard Dam

11 2004 Pond Dam Safety Program Pond Dam Safety Group formed 1 FTE Requested TCEQ Rules Clarification Storm water facilities must meet State Rules No TCEQ review unless permanent storage of at least 200 ac-ft a builder/owner of a detention structure that meets the definition of a dam must design the structure to meet the minimum hydrologic criteria under the Chapter 299, Subchapter B provisions,

12 2004 Pond Dam Safety Drainage Criteria Manual Changes (2003) Require dam safety certification on plans % PMF based on size and hazard classification in 30 TAC, Chapter 299 Added PMP values to DCM (24 hour Rainfall Distribution) Program Dam Safety Certification Statement: I [name of professional engineer] Texas license number [number] certify that the design of the dam in this set of drawings fully complies with the safety regulations as codified in Title 30 of the Texas Administrative Code, Chapter 299, Dams and Reservoirs. This design is based on a size classification of [small, intermediate, large], a hazard classification of [low, significant, high] which results in a hydrologic design requirement of the dam safely passing % of the PMF.The dam will safely pass % of the PMF based on the hydrologic, structural and geotechnical analysis.

13 2004 Pond Dam Safety Program Drainage Criteria Manual Changes (2006) Simplified the Certification Statement Required all dams to safely pass 100% PMF Dam Safety Certification Statement: I [name of professional engineer] Texas license number [number] certify that the design of the dam in this set of plans can safely pass 100-percent of the Probable Maximum Flood based on the hydrologic, hydraulic, structural and geotechnical analysis using standard accepted engineering practices. (DCM 8.3.4)

14 COA Dam Safety Program What does the City want to achieve? Infrastructure Preservation Public Protection Design Consistency

15 COA Dam Safety Program Mitigation versus Conveyance Mitigate for the 2, 10, 25, and 100-year storm Convey the Probable Maximum Storm

16 COA Dam Safety Program Summary City wants to protect Citizens City wants to preserve Infrastructure

17 Modernization of High Hazard Dams in Austin, Texas Overview Inventory and Preliminary Assessment Phase I Analysis and CIP Phase II Design and Construction Questions and Answers

18 2003 Inventory Project Overview Phase I Analysis and Modernization Capital Improvement Plan development 2007 Construction documents and permitting for the first 3 dams to be upgraded 2008 Begin construction

19 2003 Inventory and Preliminary Assessment City owns and maintains >500 storm water ponds

20 2003 Inventory and Preliminary Assessment Database identified 200 possible dams (>6 ) Project Goals Structure Classification (dam?) Identification of dam-related risks Priority ranking of dam modifications Identification of pond dam safety program implementation requirements

21 2003 Inventory and Preliminary Assessment Approximately one-half of structures ruled out through survey and site visits Preliminary hazard classification assigned Aerial photography Field visits Preliminary Priority Rankings developed Recommendations for Pond Dam Safety Group

22 Goals Phase I Study Detailed analyses Dams from 2003 Inventory (66 structures) Detailed Hydrologic and Hydraulic Models Breach Analysis and Breach Inundation Mapping Alternatives analyses Preliminary Alternatives Analyses Project Cost Estimates Permitting Issues (environmental, Infrastructure, etc.) Land acquisition needs Develop a Capital Improvement Program Revised Priority Ranking Recommendations

23 Detailed Analyses Data Collection Hydrology and Hydraulics Stage-Storage-Discharge curves PMP depths and temporal distributions (COA and TCEQ) Ultimate land use conditions within the contributing drainage areas PMF runoff parameters Antecedent moisture condition (AMC) III, Composite SCS runoff curve number (CN), Time of concentration (TC), and Routings based on lag time; Development of the PMF inflow and stage hydrographs

24 Initial Findings Nine additional pond structures were eliminated (survey) 28 ponds do not receive full PMF runoff All of the dams, except for one, were classified as small, potentially high hazard dams

25 Breach Analyses Full Breach Analysis HEC-RAS unsteady models 4 dams Simplified Breach Analysis TCEQ Guidelines 53 dams

26 Simplified Breach Analysis Small and intermediate size dams only Derived from full breach models Qb = 3.1 * B * H^3/2, where Qb = peak total discharge from the breach, in cfs. B = bottom width of breach, assumed to be 3 * H for embankments or ½ the width of a spillway or concrete structure, in feet. H = maximum height of the dam, in feet. Qt = Qb + Qs, where Qs = peak discharge capacity from the spillway(s) with the reservoir at the top of the dam, in cfs

27 Simplified Breach Analysis Length (Lu) = * Ks*SQRT(2*C * H) Lu = Inundation length (miles) C = capacity at top of dam (ac-ft) H = Maximum Height of dam Ks= Qb / Qs [0.5 < Ks < 2.0] Interpolate flows from Qt at the dam to Qs at Lu. Determine Stage by normal flow or steady state backwater Increase n values by 25%

28 Full Breach Method

29 Simplified Breach Method

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31 Phase I Model Results 8 dams are not overtopped by the PMF 19 dams have less than six (6) inches of overtopping Average depth of overtopping is about 18 inches Residential Structures at risk: 2,871

32 Alternatives Assessment Determine depth of PMF overtopping Assess existing overtopping protection capabilities Determine height to which the dam s crest must be raised to prevent overtopping and spillway requirements Determine of overtopping velocities at the downstream toe Preliminary assessment of environmental mitigation, permitting requirements, infrastructure relocation, and land rights acquisition associated with the modernization alternatives Preliminary opinions of probable costs

33 Recommended Alternatives No action required Raise crest of dam Overtopping protection Diversion

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35 Modernization of High Hazard Dams in Austin, Texas John King, P.E. Principal, Austin Jay Scanlon, P.E. Associate Kim Patak, P.E. Water Resource Design Glen Taffinder, P.E. COA Pond Dam Safety Team (512) (512) (512) (512)