Hurricane Surge Hazard Analysis · Practice of Surge Hazard Analysis Organized according to five...

Hurricane Surge Hazard AnalysisThe State of the Practice and

Recent Applications for Southeast Louisiana

Southeast Louisiana Flood Protection Authority—East

Bob Jacobsen PE, LLCConsulting Coastal Hydrologist

April 2013

Report

Objectives

Objectives1. Describe State-of-the-Practice.

2. Evaluate 2005-09 analysis in context of SOP.

3. Recommend improvements to analysis.

Additionally:

4. Assess implications for HSDRRS performance.

5. Recommend revisions to HSDRRS design.

6. Recommend other improvements to surge risk management.

“Forward Looking”

Practice of Surge Hazard Analysis

Organized according to five Subjects/Tasks:

Report provides a detailed discussion of scientific and technical

literature, methodologies, assumption, and limitations.

Previously Reviewed at Committee Meetings.

A surge hazard analysis that supports a FIS and

HSDRRS accreditationis good for NFIP purposes—but may not be good enoughfor the SLFPA-E’s mission.

We’ll Skip the Background(Surge Hazards, Surge Risk Management, the 2005-09 Analysis)

Objective 1

Findings

State of the Practice(2)

1. Advances in Estimating Surge Hazard

Significant reductions in systemic & local bias over past several years driven by:

� Improving hurricane surge forecasts—e.g., 2012 Hurricanes Isaac and Sandy.

� Subsequent GoM/So. Atlantic coastal FISs.

� Academic research.

� NOAA, USACE, and other FEMA/DHSprograms.

Five important advances.

Texas

North Carolina

South Carolina

Georgia &NE Florida

Cent. Atl. FloridaSW

FloridaNW Florida & Alabama

Big Bend Florida

� Important storms since Katrina/Rita/2005.

� Gustav & Ike/2008 Isaac/2012.

� Research on storm frequencies and characteristics.

� Importance of Holland B,Integrated Kinetic Energy, intensification, decay to surge hazard.

� The role of the Loop Current and coastal water temperature in local hurricane return period, intensity, wind field extent, and dynamics.

� Surge threat of large, slow moving, low intensity hurricanes.

i) Hurricane Climatology

� Better representation of wind/surge/wave physics in models.

� Code developments—tight coupling with wave models.

� Mesh resolution/quality/node attributes—elevations & Manning’s n.

� Execution speed and ability to complete extensive model performance tests.� High performance parallel computing

(HPPC).

� Recent models much more reliable, robust, and locally realistic.

ii) Modeling

� HPPC speeds and capacity now allow for many more storms in the joint probability analysis (JPA)

� Facilitates a more detailed,more sophisticated, Joint Probability Method with Optimal Sampling (JPM-OS).

� Surge Response-OS approach is valid for exterior surge but cannot be applied to polder inundation.

iii) Surge Hazard JPA

Can improve greatly on IPET Polder Inundation JPA.

� HPPC efficiency allows for more rigorous treatment of surge, wave, erosion and collapse fragility, and other probabilities.

� Full consideration of residual risk, uncertainties, and design “conservatism”—true “risk-based” design.

� Beyond basic reach-by-reach overtopping hazard assessments.

iv) JPA for Polder Inundation

� Greater HPPC efficiency enables more comprehensive future scenarios.

� Varying regional response to RSLR, coastal erosion, levee settlement, etc.

v) Future Conditions Hazard Analysis

Coastal Louisiana in 2100Blum and Roberts 2009

2. Estimate Uncertainties Very Large

� Epistemic (normally distributed) uncertainty, σ large percent of median estimate.

� Very wide bell curve.

� Aleatory uncertainty also very large.

� Despite advances in accuracy.

� Pace of future uncertainty reduction likely to be very slow.

II. Objective 2

Findings

2005-09 Analysis(12)

Four Major Low Bias Factors: Under-

Estimate Surge Hazard

1. Climatology does not include large, slow-moving, low intensity hurricanes for 100-yr surge hazard.

� No Gustavs or Isaacs

2. Surge model contains superseded approaches and mesh contains outdated topographic, bathymetric, land cover, and HSDRRS data.

� Hurricane Katrina hindcast validation under-represented surge height by >1.5 ft along south Lake Pontchartrain.

3. JPA approach and small (152) storm set overly smooth interpolation and extrapolation of regional surge response to variations in hurricane characteristics.

� No Camille-like storms for evaluation of 500-yr surge.

4. 2057 analysis does not consider key coastal land losses; applies a uniform surge increase for most of east-bank.

Three Other Important Bias Factors

5. USACE’s specific approach to computing the CDF adds a small High Bias “Cushion.”� Modestly increased the 100- and 500-yr exterior surge

hazard estimates—at one location by 0.4 and 1.1 ft, respectively.

6. Approach to characterizing foreshore wave heights.� Rayleigh distribution of wave heights.� Breaker parameter (Hs/Depth) of 0.4� Combined effect could mitigate or exacerbate—don’t

know.

7. JPA for the 2011 conditions did not re-simulate all the storms in the set. Again, don’t know the effect.

Five Issues on Quantifying Uncertainty

8. No variation in hurricane dynamics—intensification/ growth/decay.

9. Combined wind/surge/wave model uncertainties indicated by other recent studies.

10. Uncertainties in JPA method and CDF integration.

11. Uncertainties in polder inundation hazard analysis have not yet been addressed:� Seepage

� Overtopping

� Breaching

12. Analysis for wave overtopping at 10% exceedance level does NOT use actual statistical uncertainties.� Substituted drastically lower σ for exterior surge—

some reaches by a factor >4.

� Did not vary the wave height with depth (in Monte Carlo).

� Rainfall� Pumping� Internal routing

Objective 1& 2 Bottom Line

The bias and uncertainty issues mean the 2005-09 surge hazard analysis is outdated for southeast Louisiana surge risk management purposes beyond the 2013 NFIP FIS and HSDRRS accreditation.

Objective 3

Recommendations

Improving SurgeHazard Analysis

(2)

1. Update Entire Current and Future Surge

Hazard Analyses to SOP—ASAP

� See list of specific recommendations in each of five tasks.

� Share with CPRA, USACE, and other federal, state, and local agencies.

� Assert leadership in regional surge hazard analysis.

� Institutionalize regularly revisions as part of the state’s Master Plan process.

Provide better median and confidence limits estimates for exterior surge, waves, overtopping, and polder inundation over a full hazard range.

Table ES.1. Recommendations for Updating the Southeast Louisiana Surge Hazard Analysis

2. Encourage Critical Research for

Surge Hazard Analysis

� List of key research recommendations.

� “Living” technical guidance documents to improve the consistency and quality of methodologies.

� Lessons learned from other regional surge studies.

� Best practices for various risk management applications.

Organize a cooperative partnership with CPRA, FEMA, USACE, and NOAA—target specific improvements in surge hazard analysis.

Objective 4

Implications

HSDRRS Levee Performance*(6)

*Addresses HSDRRS levee design elevation, which minimizes current 100-yr surge wave overtopping. Floodwalls were designed higher, to future100-yr overtopping condition (with RSLR), due to the excessive cost of

subsequent crown increase. Addresses design elevation—notconstruction, which may be different due to overbuild for settlement or

geometry requirements.

1. Higher Median Estimate,100-Yr SurgeBias Factors Nos. 1 + 2 will likely raise median 100-yr surge estimate >2 ft at some locations.However, re-analysis needed to assess combined influence of all bias factors throughout region.

� Design resulted in reach-specific 100-yr freeboard depending on the estimated wave conditions.

� Increases to the 100-yr surge not likely to exceed crowns, but could reduce freeboard below NFIP 2 ft min.

Current 100-yr Freeboard, ft

1. Higher Median Estimate,100-Yr Surge

Substantially increases median estimate of levee

overtopping.

Bias in foreshore waves would add

even more to overtopping.

x20x5

x0.75x0.17

2. Higher Median Estimate, 500-Yr Surge

Bias Factors even more likely to raise median estimate of 500-yr surge and overtopping throughout the region.

� Significant impact on reaches with low 500-yr freeboard.

� Major threat of free flow overtopping and interior-side erosion and breaching.

� Reach in St. Charles Parish north of Airline Highway has only 0.5 ft of 500-yr freeboard—could show a negative freeboard under a revised analysis.

0

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-0 .2 -0 .1 0 0 .1 0 .2 0 .3 0 .4 0 .5 0 .6 0 .7 0 .8 0 .9 1 1 .1 1 .2 1 .3 1 .4 1 .5 1 .6 1 .7 1 .8 1 .9 2

3. Higher Confidence Intervals

0.01

2005-09

Median Estimate

0.110% Exceedance Estimate

100-yr Overtopping Estimate, cfs/ft

Revised quantification of uncertainty will notably

increase exceedance levels for 100- and 500-yr surge, waves,

and overtopping.

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-0 .2 -0 .1 0 0 .1 0 .2 0 .3 0 .4 0 .5 0 .6 0 .7 0 .8 0 .9 1 1 .1 1 .2 1 .3 1 .4 1 .5 1 .6 1 .7 1 .8 1 .9 2

0.01 0.1

3. Higher Confidence Intervals

0.05

Potential Revised

Median Estimate

0.88

Potential Revised 10% Exceedance Estimate

100-yr Overtopping Estimate, cfs/ft

Revised quantification of uncertainty will notably

increase exceedance levels for 100- and 500-yr surge, waves,

and overtopping.

3. Higher Confidence Intervals

Current 500-yr/10% Overtopping Estimate, cfs/ft

Revisions for bias and uncertainty together can increase estimates of

current overtopping hazardsat 10% exceedance level by an Order of Magnitude.

4. Revised Overtopping Will Surpass

100-yr Design CriteriaRevised 100-yr median & 10% exceedance estimates of overtopping at many reaches will surpass erosion criteria—0.1 & 0.01 cfs/ft. Requires much higher crowns if current criteria not revised.

5. Higher Polder Inundation RiskHigher overtopping raises polder inundation hazard; but cannot be estimated without an improved JPA.

� IPET indicated 100-yr inundation volumes are far lower for surge than rain-fall only.

� Need polder inundation hazard analysis with final HSDRRS design.

� Add IHNC/GIWW and Seabrook Barriers and other upgrades.

� 500-yr polder inundation hazard and uncertaintywill be higher;need to evaluate..

IPET 2009

6. Higher 2057 RiskUpdate for bias will show significant spatially varying impacts to estimated future 100-/500-yr surges, waves, and overtopping.� Taking into account varying regional impact of RSLR,

coastal erosion, and land cover change. � For example, NO-East Land Bridge.

50-yr Land Loss Projection, CPRA 2012 Master Plan

Objective 4 Bottom Line

These six performance concerns indicate that HSDRRS

levees as designed have significant shortcomings for surge risk management

purposes beyond the 2013 FIS accreditation.

Objective 5

Recommendations

HSDRRS Design(4)

1. Revise Exceedance Level

� Revise the estimation of 100-yr surge (SWL), wave, and overtopping exceedance levels to reflect the total statistical (including non-normal) uncertainty.

� SLFPA-E should then work with CPRA and USACE on determining if the 10% exceedance level is appropriate.

Not every perimeter system needs to address the same level of uncertainty.

SLFPA-E risk management different from SLFPA-W, PLD, South Lafourche, Plaquemines, Grand Isle,

Vermilion Parish, etc.

2. Add Three Exceedance Level Criteria

i. Uniform, system-wide minimum freeboard for the 100-yr/exceedance surge (SWL).

� In addition to NFIP required minimum freeboard above the median 100-yr exterior surge still water.

� System should greatly reduce the risk of free flow overtopping at 100-yr hazard.

� Work with USACE and CPRA to define.

� For example, St. Charles Parish north of Airline Highway:

� 10.8 ft 100-yr surge;

� 14.0 ft current crown design (3.2 ft freeboard);

� >15 ft at exceedance level accounting for total uncertainty.

2. Add Three Exceedance Level Criteria

ii. Reach-specific 100-yr/exceedance overtopping rates

� Instead of a uniform 0.1 cfs/ft, could be higher or lower depending on reach erosion control measures.

� Base allowable 100-yr/exceedance overtopping rate on latest research for erosion control measures.

� But not to exceed an overtopping limit to minimize polder inundation impact.

� Revised estimates of local 100-yr/exceedance surge and waves will likely require upgrading wave breaking or crown elevation (to reduce overtopping at exceedance level), and/or interior-

side erosion control, for many reaches.

� Uniform design criterion for the median 100-yr overtopping (0.01 cfs/ft) could be eliminated if it provides no needed benefit.

2. Add Three Exceedance Level Criteria

iii. Reach-specific 500-yr (resiliency) exterior SWLs, waves, scour velocities, overtopping rates, and seepage rates—at appropriate exceedance levels.

� Based on reach conditions, latest performance research, and considering full range of failure mechanisms.

� SLFPA-E should work with CPRA and USACE to clarify HSDRRS authorization, and modify as needed, to allow selecting and prioritizing resiliency projects from among the alternatives to optimize reduction in polder residual risk.

� Resiliency projects should include the option of further raising reach crowns—e.g., for reaches with very low 500-yr freeboard.

CPRA 2011

3. Accelerate Resiliency

� Especially those not likely to be rendered obsolete in the near future.

� If USACE construction funds are not available SLFPA-E should obtain state/local funding.

� Levee reaches with low freeboard (e.g., St. Charles Parish) should receive high priority.

SLFPA-E should work with CPRA and USACE to expedite reasonable resiliency measures.

4. Ensure Systematic Approach

1st Priority: Minimize 100-yr wave overtopping in accordance with design criteria at exceedance level.

Surveys, Inspections, & Maintenance.

2nd Priority: 500-yr resiliency at exceedance level.

3rd Priority: “High Level.”

For each polder consider components as equally important “links in a chain.”

Cannot allow weaker, more fragile links—e.g., gates that are difficult to close.

CPRA 2012 Master Plan

Objective 6

Recommendations

Improving Surge Risk Management

(7)

Improve Surge Risk Management1. Complete studies to identify cost-effective internal

compartmentalization projects.

2. Maintain and improve critical natural “Multiple Lines of Defense” per CPRA 2012 Master Plan.

3. Investigate impacts of levees below HSDRRS.

4. Continue to look for cost-effective enhancements of NO East Land Bridge.

5. Support an “All Flood Hazards Analysis.”

6. Support drainage and transportation infrastructure upgrades consistent with “all flood hazards.”

7. Broaden NFIP participation.

Closing

QuestionsDiscussion