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Event-Based Erosion FOCUSED STUDY REPORTS EVENT-BASED EROSION Event-Based Erosion FEMA Coastal Flood Hazard Analysis and Mapping Guidelines Focused Study Report February 2005 Focused Study Leader Bob MacArthur, Ph.D., P.E. Team Members Kevin Coulton, P.E., CFM Bob Dean, Sc.D. Darryl Hatheway, CFM Maria Honeycutt, Ph.D. Jeff Johnson, P.E. Chris Jones, P.E. Paul Komar, Ph.D. Chia-Chi Lu, Ph.D., P.E. Ron Noble, P.E. Trey Ruthven, P.E. Dick Seymour, Ph.D., P.E. 1 FEMA COASTAL FLOOD HAZARD ANALYSIS AND MAPPING GUIDELINES FOCUSED STUDY REPORTS EVENT-BASED EROSION Table of Contents 1 Introduction........................................................................................................................... 1 1.1 The EBE Team and Approach ........................................................................................ 3 1.2 Present FEMA Guidance on Event-based erosion Related to all Priority Categories .... 3 1.2.1 Factors in Beach, Dune and Bluff Erosion ....................................................... 8 2 Critical Topics....................................................................................................................... 9 2.1 Critical Topic 30: Geometric Erosion Assessment for the Pacific ................................ 9 2.1.1 Description of Topic and Suggested Improvement .......................................... 9 2.1.2 Description of Procedures in the Existing Guidelines .................................... 10 2.1.3 Application of Existing Guidelines to Topic – History and/or Implications for NFIP................................................................................................................ 10 2.1.4 Alternatives for Improvement......................................................................... 12 2.1.5 Conclusions for Topic 30 (Geometric Erosion Assessment for the Pacific) .. 15 2.1.6 Recommendations for Topic 30 (Geometric Erosion Assessment for the Pacific) ...................................................................................................... 17 2.2 Critical Topic 33: (Shingle/cobble erosion assessment).............................................. 18 2.2.1 Description of Topic and Suggested Improvement ........................................ 18 2.2.2 Description of Procedures in the Existing Guidelines .................................... 19 2.2.3 Application of Existing Guidelines to Topic – History and/or Implications for NFIP................................................................................................................ 19 2.2.4 Alternatives for Improvement......................................................................... 22 2.2.5 Recommendations........................................................................................... 22 2.3 Critical Topic 35: Guidance for Erosion Assessments in Sheltered Areas.................. 23 2.3.1 Description of Topic and Suggested Improvement ........................................ 23 2.3.2 Description of Procedures in the Existing Guidelines .................................... 24 2.3.3 Application of Existing Guidelines to Topic .................................................. 24 2.3.4 Alternatives for Improvement......................................................................... 25 2.3.5 Recommendations........................................................................................... 26 2.4 critical Topic 39: Primary Fontal Dune ....................................................................... 26 3 Important Topics................................................................................................................ 26 3.1 Topic 34: Develop Improved Geometric Methods Which Consider Cobble/Shingle Effects........................................................................................................................... 26 3.1.1 Description of Topic 34 and Suggested Improvement ................................... 26 3.1.2 Description of Procedures in the Existing Guidelines .................................... 27 3.1.3 Alternatives for Improvement......................................................................... 27 3.1.4 Recommendations........................................................................................... 29 3.2 Topic 36: Guidance for Erosion Assessments in Sheltered Areas............................... 30 3.2.1 Description of Topic and Suggested Improvement ........................................ 30 3.2.2 Description of Procedures in the Existing Guidelines .................................... 30 3.2.3 Application of Existing Guidelines to Topic .................................................. 30 3.2.4 Alternatives for Improvement......................................................................... 30 3.2.5 Recommendations ................................................................................................. 31 3.3 Topic 37: Review Atlantic-Gulf Coast 540 SF Criterion ............................................ 32 3.3.1 Description of Topic and Suggested Improvement ........................................ 32 i FEMA COASTAL FLOOD HAZARD ANALYSIS AND MAPPING GUIDELINES FOCUSED STUDY REPORTS EVENT BASED EROSION 3.3.2 Review of 540 SF Criterion ............................................................................ 33 3.3.3 Recommendations........................................................................................... 34 3.4 Topic 38: Physics- or Process- Based Erosion Assessment......................................... 36 3.4.1 Description of the Topic and Suggested Improvement................................... 36 3.4.2 Description of Procedures in the Existing Guidelines .................................... 38 3.4.3 Application of Existing Guidelines to Topic .................................................. 38 3.4.4 Alternatives for Improvement......................................................................... 38 3.4.5 Recommendations........................................................................................... 45 4 Available Topics.................................................................................................................. 49 4.1 Topic 31: Add/revise G&S language regarding bluff erosion in Atlantic/Gulf areas.. 49 4.1.1 Description of Procedures in the Existing Guidelines .................................... 49 4.1.2 Application of Existing Guidelines to Topic .................................................. 49 4.1.3 Alternatives for Improvement......................................................................... 50 4.1.4 Recommendations........................................................................................... 51 4.2 Topic 32: Develop Improved Geometric Methods for Bluff Erosion in the Atlantic and Gulf Areas .................................................................................................................... 51 4.2.1 Description of Topic 32 and Suggested Improvement ................................... 51 4.2.2 Description of Procedures in the Existing Guidelines .................................... 51 4.2.3 Application of Existing Guidelines to Topic .................................................. 51 4.2.4 Recommendations and Approach................................................................... 52 4.3 Topic 41: Long-Term Erosion/Future Conditions ....................................................... 52 4.3.1 Description of Procedures in the Existing Guidelines .................................... 52 4.3.2 Recommendations and Approach ................................................................... 53 4.4 Topics 42 & 43: Add guidance regarding the treatment of nourished beaches in flood hazard mapping ............................................................................................................ 53 4.4.1 Description of Procedures in the Existing Guidelines .................................... 53 4.4.2 Recommendations and Approach ................................................................... 53 4.4.3 Topics 42 & 43: Availability ......................................................................... 54 5 Helpful Topics..................................................................................................................... 54 5.1 TOPIC 40: CALCULATE VERTICAL EROSION DEPTHS.................................... 54 5.1.1 Description of Topic and Suggested Improvement ........................................ 54 5.2 Availability ................................................................................................................... 55 6 SUMMARY ......................................................................................................................... 55 7 REFERENCES....................................................................................................................64 Tables 1 Summary of Findings and Recommendations for Event Based Erosion.............................. 56 2 Preliminary Time and Cost Estimate for Guideline Improvement Preparation.................... 61 ii FEMA COASTAL FLOOD HAZARD ANALYSIS AND MAPPING GUIDELINES FOCUSED STUDY REPORTS EVENT-BASED EROSION Figures 1 Definition sketch of frontal dune reservoir (from FEMA, 2003) ........................................... 5 2 Current FEMA treatment of dune retreat and dune removal .................................................. 6 3 Pacific Northwest erosion model (Komar et al., 2002) ........................................................ 14 4 Pacific Northwest
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