Coastal Structures Table of Contents 1 INTRODUCTION 1.1 Category and Topics 1.2 Coastal Structures Focused Study Group 1.3 Current FEMA Guidance for Coastal Structures 2 CRITICAL TOPICS 3 AVAILABLE TOPICS 3.1 Topic 25: Add Guidelines and Specifications Text that States Study Contractors are not Required to Evaluate all Coastal Structures Using the Criteria in FEMA (1990) and Walton, et al. (1989); Add a Recommended Procedure for Mapping Flood Hazards at Transects with Coastal Structures 3.1.1 Description of Topic and Suggested Improvement 3.1.2 Availability 3.2 Topic 21: Clarify Guidance Regarding Treatment of Backfill/Topography when a Structure is Determined to Fail Under Base Flood Conditions, and is Removed from the Transect 3.2.1 Description of Topic and Suggested Improvement 3.2.2 Availability 3.3 Topic 23: Add Guidelines and Specifications Text that Buried Structures are to be Evaluated 3.3.1 Description of Topic and Suggested Improvement 3.3.2 Availability 3.4 Topic 27: Review Guidelines and Specifications and NFIP Regulations Regarding Treatment of Coastal Levees and Structures; Identify Conflicts; Review and Update TR-89-15 Structure Evaluation Criteria; Consider Requiring all Coastal Structures (existing and new) to Meet the Same Evaluation Criteria 3.4.1 Description of Topic and Suggested Improvement 3.4.2 Availability 3.5 Topic 24: Review Walton, et al. (1989) and Other Literature for Damage to Coastal Structures During Tsunamis 3.5.1 Description of Topic and Suggested Improvement 3.5.2 Availability 4 IMPORTANT TOPICS 5 ADDITIONAL OBSERVATIONS – HELPFUL TOPICS 5.1 Topic 22: Investigate Configurations of Failed Coastal Structures 5.1.1 Description of Topic and Suggested Improvement 5.1.2 Availability 5.2 Topic 26: Review Data on the Effects of Coastal Structures on Flood Hazards on Adjacent Properties; Review Flooding/Wave Effects Behind Structures 5.2.1 Description of Topic and Suggested Improvement 5.2.2 Availability 6 SUMMARY 7 REFERENCES Tables 1 Summary of Findings and Recommendations Figures 1 Review of concerns related to coastal armoring. 2 Excess Erosion Due to Seawalls 3 Sample Mapping of Flood Hazards at Failed Coastal Structure–through physical failure or insufficient return walls–and at Intact Coastal Structure Appendices A-1 FEMA 1990 Memorandum, Criteria For Evaluating Coastal Flood Protection Structures A-2 FEMA MT-2, Form 5, Coastal Structures Analysis Form A-3 FEMA Regulations for Coastal Levees, CFR Part 44 Section 65.10 Acronyms BFEs Base Flood Elevations CEM Coastal Engineering Manual CFR Code of Federal Regulations FEMA Federal Emergency Management Agency FIRM Flood Insurance Rate Map FIS Flood Insurance Study NGDC National Geophysical Data Center PWA Philip Williams & Associates SFHA Special Flood Hazard Area SPM Shore Protection Manual USACE U.S. Army Corps of Engineers 1 INTRODUCTION Existing FEMA guidance for treatment of coastal structures refers to seawalls, bulkheads, revetments and coastal levee-type structures, i.e., those that are intended to retain fill and offer protection against flooding and waves, and that are constructed along or parallel to the shoreline. Groins, jetties and detached breakwaters are not mentioned specifically, but should also be considered for flood hazard mapping purposes. A coastal structure can modify flood levels, wave effects and topography, both landward of, seaward of and adjacent to the structure, and must be considered during the mapping of coastal flood hazards. Two scenarios are commonly encountered: 1. Existing coastal structures are analyzed during a Flood Insurance Study, and their effects (if any) must be reflected by the resulting FIRM. This process is described in Appendix D to the G&S (FEMA, 2003). 2. Existing, new or proposed coastal structures often serve as the basis for revisions to FIRMs, and their stability and effects must be evaluated. The map revision instructions and form MT-2 (FEMA 2002) address this scenario. 1.1 CATEGORY AND TOPICS Seven coastal structures topics were identified at Workshop 1 and are identified below. There were no “Critical” topics identified. Five topics were designated “Available” and two were identified as “Helpful.” Each of these will be considered in this paper. 1.2 COASTAL STRUCTURES FOCUSED STUDY GROUP The Coastal Structures Study Group is made up of Bob Batallio, Ida Brøker, Kevin Coulton, Jeff Gangai, Darryl Hatheway, Jeremy Lowe, Ron Noble, and Chris Jones, who served at Team Leader. 1.3 CURRENT FEMA GUIDANCE FOR COASTAL STRUCTURES FEMA’s existing guidance for coastal structures is limited to the Atlantic, Gulf of Mexico and Great Lakes Coasts, as summarized in the G&S for Flood Hazard Mapping Partners (FEMA, 2003). Sections D.2.2.8 and D.2.3 address the Atlantic and Gulf of Mexico; nearly identical sections D.3.2.7 and D.3.3 address the Great Lakes. No coastal structure guidance specific to sheltered shorelines or the Pacific Coast exists in Appendix D, although it is reasonable to expect that existing guidance for other coasts will apply. Coastal Structures Topics and Priorities Atlantic / Gulf Coast Pacific Coast Non-Open Coast 25 Flood Protection Structures Review G&S language – (SC not required to evaluate all structures using 89-15); add new procedure for flood hazard modeling in the presence of coastal structures Failed Structures Clarify guidance that when a structure is determined to fail under base flood conditions, the structure is removed, fill/topo remains and is subject to erosion, wave analyses Buried Structures Add G&S language that buried structures are to be evaluated Coastal Levees v. Structures Review G&S and regs regarding treatment of coastal levees and structures; identify conflicts; clarify G&S that evaluations of all "structures" to be per 89-15 Structures - Tsunamis Review 89-15 and other literature for tsunami failure information/guidance Failed Structure Configuration Investigate configuration of failed structures Adjacent Properties Review data on (and add to G&S) effects of structures on flood hazards on adjacent properties, flooding/waves behind structures via adjacent properties Excerpts and major elements of the existing coastal structure guidance are summarized below: * “The crucial first consideration in evaluating a coastal structure is whether it was properly designed and has been maintained to provide protection during the 1-percent- annual-chance flood. If it can be expected to survive the 1-percent-annual-chance flood, the structure should figure in all ensuing analyses of wave effects (erosion, runup, and wave height). Otherwise, it should be considered destroyed before the 1-percent-annual- chance flood and removed from subsequent transect representations.” (Section D.3.3, paragraph 1). * Specific criteria for evaluating coastal structures are contained in a memorandum (FEMA 1990), reproduced in Appendix CS-1. The criteria are based in large part on a study performed by the USACE for FEMA (Walton, et al., 1989; also referred to as “TR-89- 15”), and cover such topics as: * Design parameters (water levels and wave heights; breaking wave forces), * Freeboard (above 1% stillwater level, and relative to the runup elevation), * Toe protection, * Backfill protection, * Structural and geotechnical stability (sliding, overturning, settlement, soil slip, ice and impact forces, etc.), * Materials (strength and durability, including stone size, filter characteristics, expected lifetime, etc.), * Adverse impacts, * Maintenance plan, and * Engineering certification. * Similar criteria are contained in the Coastal Structures Form (MT-2, Form 5, reproduced in Appendix CS-2) used to evaluate coastal structures as the basis for FIRM revisions. * In performing analyses for a Flood Insurance Study (FIS) FEMA (2003) directs the mapping contractor (partner) to obtain documentation for each coastal structure possibly providing protection from 1-percent-annual-chance flood. That documentation is to include the following: * Type and basic layout of structure; * Dominant site particulars, (e.g., local water depth, structure crest elevation, ice climate); * Construction materials and present integrity; * Historical record for structure, including construction date, maintenance plan, responsible party, repairs after storm episodes; and * Clear indications of effectiveness or ineffectiveness. Unfortunately, few FIS projects have sufficient funds to support a detailed evaluation of coastal structures, and the G&S call for development of “much of this information through office activity, including a careful review of aerial photographs. In some cases of major coastal structures, site inspection would be advisable to confirm preliminary judgments.” (Section D.2.2.8, last paragraph). * Cost considerations aside, the G&S also recognize that information about existing coastal structures may not be available or sufficient to complete a detailed evaluation. In such cases, the mapping contractor (partner) “shall make an engineering judgment about its likely stability based on a visual inspection of physical conditions and any historical evidence of storm damage and maintenance.” (Section D.2.3, second paragraph). 2 CRITICAL TOPICS There were no “Critical” topics identified in Workshop 1. 3 AVAILABLE TOPICS 3.1 TOPIC 25: ADD GUIDELINES AND SPECIFICATIONS TEXT THAT STATES STUDY CONTRACTORS ARE NOT REQUIRED TO EVALUATE ALL COASTAL STRUCTURES USING THE CRITERIA IN FEMA (1990) AND WALTON, ET AL. (1989); ADD A RECOMMENDED PROCEDURE FOR MAPPING FLOOD HAZARDS AT TRANSECTS WITH COASTAL STRUCTURES 3.1.1 Description of Topic and Suggested Improvement Sections D.2.3 and D. 3.3 of the existing guidance make reference to the FEMA criteria for the evaluation of coastal structures (FEMA, 1990; Walton, et al., 1989), and imply these criteria should be applied by study contractors, unless available information is not sufficient to perform detailed evaluations. The G&S should be revised to state clearly that detailed evaluations of all structures are not required of study contractors. Instead, the following structure evaluation procedure is recommended for inclusion in the G&S: 1. The Study Contractor should determine whether available information clearly indicates a coastal structure will fail or survive a base flood event, then perform the subsequent erosion and wave analyses on the indicated (intact or failed structure) profile. In the case of revetment type structures that tend to fail progressively, study contractors should be allowed the discretion to allow for partial – rather than complete – failure (see Topics 21a and 22). It should be clearly communicated to communities and property owners that Study Contractor structure performance determinations are for mapping purposes only, are not intended to substitute for detailed structural evaluations, and should not serve as a basis for Study Contractor liability in the event of structure failure. 2. If available information does not clearly point to survival or failure of a coastal structure, the Study Contractor may either: a) conduct a detailed evaluation using TR-89-15 procedures, or b) perform the erosion and wave analyses for both the intact and failed structure cases, and map the flood hazards associated with the more hazardous case. If option 2.b) is selected, the Study Contractor should clearly document the results of both cases (structure intact, structure failed) and specify which case is used for mapping purposes. Also, see section 5.1.1, Topic 22. Implications of not Performing Detailed Coastal Structure Evaluations During the FIS Flood study contracts typically do not have sufficient budget to carry out detailed evaluations of coastal structures, and study contractors commonly assume the structures will fail as a default condition (since they have not performed detailed evaluations). There are two important implications of this assumption: * Failed coastal structures may or may not yield the highest BFEs and greatest flood hazards. See Topic 22 for additional discussion. * Property owners frequently request (and receive) revisions to FIRMs after retaining engineers who perform detailed evaluations and certify that coastal structures will withstand the 1% flood event. As a result, the revised FIRMs may display highly irregular flood hazard zone boundaries and BFE lines, and may be constantly changing as additional detailed evaluations are performed. See Topic 27 for additional discussion. 3.1.2 Availability Information to address Topic 25 is available and easily incorporated into existing guidance. 3.2 TOPIC 21: CLARIFY GUIDANCE REGARDING TREATMENT OF BACKFILL/TOPOGRAPHY WHEN A STRUCTURE IS DETERMINED TO FAIL UNDER BASE FLOOD CONDITIONS, AND IS REMOVED FROM THE TRANSECT 3.2.1 Description of Topic and Suggested Improvement Existing guidance calls for the removal of a coastal structure (from analysis transects) when it has been determined that the structure will not withstand the 1% event (see Section D.2.3, first paragraph; Section D.3.3, first paragraph). However, no details are provided as to how such a removal should be accomplished for those types of structures contemplated by the G&S (seawalls, bulkheads, revetments, levees), and no details are provided regarding other types of coastal structures whose failure during a base flood event could affect coastal flood hazards (e.g., groins, jetties, detached breakwaters). Dealing with the former issue will be straightforward, but dealing with the latter will not. Guidance on how to predict the failure of groins and jetties – which usually fail by loss of profile (through settlement or displacement) and/or by becoming detached at their landward ends – is not readily available. Likewise, guidance on how to predict the failure of detached breakwaters (usually through loss of profile) is not readily available. The recommended approach can be divided into two components: * Topic 21a. For seawalls, bulkheads, revetments and coastal levees: remove the failed structure (or estimate a partial collapse of revetment structures, where appropriate) and alter the remaining soil to achieve its likely slope immediately after structure failure (note that this is not necessarily the same as the long-term stable slope in the case of bluffs and cliffs). This slope will then be subjected to an event-based erosion analysis and wave height and runup analyses. * Topic 21b. For groins, jetties and detached breakwaters: evaluate the overall condition and performance of the structures over time; determine whether the structures (or similar structures nearby) have been damaged or detached during prior major storms; document the structural damage and any resulting shoreline recession attributable to the structural damage; use this information to predict the likely shoreline configuration (in plan view) if the structures fail during the base flood. The altered shoreline will then be subjected to an event-based erosion analysis and wave height and runup analyses. Note that in the case of groins and jetties, it is unlikely that their failure will require “removal” from analysis transects (removal of a detached breakwater from a transect is more likely to occur). The effects of the structures on the shoreline configuration, however, will be removed. 3.2.2 Availability Information to address Topics 21a is available and easily incorporated into existing guidance. Existing guidance can be modified to mention Topic 21b, but detailed guidance is not readily available. Developing detailed guidance could require site-specific studies using analytical or numerical methods. Therefore, it is recommended that guidance be expanded to discuss removal of seawalls, bulkheads, revetments, coastal levees and that guidance allow for partial failure of revetments, where appropriate. Mention in guidance removal of the effects of groins, jetties, detached breakwaters on the shoreline. Develop specific guidance on how to remove the effects of groins, jetties, and detached breakwaters on the shoreline. 3.3 TOPIC 23: ADD GUIDELINES AND SPECIFICATIONS TEXT THAT BURIED STRUCTURES ARE TO BE EVALUATED 3.3.1 Description of Topic and Suggested Improvement Existing guidance is vague regarding those coastal structures that should be evaluated for their durability during the 1% flood event. The guidance is clear that exposed structures must be evaluated, but does not mention coastal structures that are known to exist, but are buried. The recommended approach is simple: Modify the G&S text to state that study contractors should: 1. Inquire as to whether buried coastal structures exist within their study area, 2. Mention the apparent presence or absence of buried coastal structures in the study documentation, 3. Apply evaluation techniques to buried coastal structures that are similar to those applied to exposed coastal structures. 4. Add examples to the G&S. 3.3.2 Availability Information to address Topic 23 is available and easily incorporated into existing guidance. 3.4 TOPIC 27: REVIEW GUIDELINES AND SPECIFICATIONS AND NFIP REGULATIONS REGARDING TREATMENT OF COASTAL LEVEES AND STRUCTURES; IDENTIFY CONFLICTS; REVIEW AND UPDATE TR-89-15 STRUCTURE EVALUATION CRITERIA; CONSIDER REQUIRING ALL COASTAL STRUCTURES (EXISTING AND NEW) TO MEET THE SAME EVALUATION CRITERIA 3.4.1 Description of Topic and Suggested Improvement There are potential inconsistencies in the treatment of coastal levees and other coastal flood protection structures, and in the evaluation of coastal structures. The issues are as follows: * Topic 27a – incomplete explanation of the differences between coastal levees and other coastal structures, and how the designation affects their treatment in flood hazard mapping; * Topic 27b – the evaluation criteria in Walton et al. (1989) should be reviewed in light of the methods contained in the Coastal Engineering Manual (USACE, 2002); and * Topic 27c – existing coastal flood protection (non-levee) structures can be incorporated into a coastal flood study based on engineering judgment, without meeting the same engineering and certification requirements for new or proposed structures; consider requiring all structures to meet the same criteria; maintenance plan criteria for private structures are problematic. Topic 27a: Coastal Levees vs. Other Coastal Structures There are two general classes of coastal structures that can provide some degree of protection against coastal flooding: coastal levees and other coastal structures. Coastal levees are structures that are designed to provide low-lying coastal areas with total protection during the 1% flood. In other words, the coastal levee must be substantial enough to prevent any flooding or wave overtopping landward of the levee crest. NFIP regulations (44CFR part 65.10; reproduced in Appendix CS-3) spell out the requirements a coastal levee must meet to be credited as providing complete protection from flooding, including a freeboard requirement specific to coastal levees – the crest elevation of the levee must be elevated at least two feet above the 1% stillwater elevation, and above the elevation of the 1% wave height or the maximum wave runup elevation (whichever is greater). Other coastal structures (seawalls, bulkheads, revetments) can be recognized on flood hazard maps as long as they remain intact during the 1% flood, even if they are overtopped. They can provide limited protection against flooding and waves, yet still be considered for flood hazard mapping purposes. These types of structures are often used by property owners to reduce flood hazards and to revise flood hazard zones on the FIRM (i.e., to change V zones to A zones or X zones). The G&S do not explain the differences between coastal levees and other coastal structures, do not discuss the different design and certification requirements, and do not discuss how the designation affects their treatment for flood hazard mapping purposes. On a related matter, one source of much discussion has been the maintenance plan criteria in FEMA (1990) and 44CFR65.10. The maintenance plan requirements in the regulations only apply to coastal levees, but in FEMA (1990), the same criteria apply to all coastal structures. This has been problematic since the criteria only allow certification of levees/structures where a maintenance plan has been adopted by and maintenance activities are the responsibility of a federal, state or community agency. Private structures will not be able to meet this requirement. As a practical matter, however, government agencies can require private owners to maintain their coastal structures. This effectively satisfies the intent of the maintenance plan requirement. Topic 27b: Update to Coastal Structure Evaluation Criteria FEMA coastal structure evaluation criteria (adopted in 1990) are based on a USACE report (Walton, et al., 1989). The report also forms the basis for the evaluation criteria contained in the G&S, in 44CFR 65.10, and in the flood map revision form for coastal structures (MT-2, Form 5). The criteria should be reviewed in light of more recent guidance and methods contained in the USACE’s Coastal Engineering Manual (CEM). Topic 27c: Consistency of Coastal Structure Design/Certification in Flood Studies and Map Revisions, Including Maintenance Plan Criteria Existing non-levee coastal flood protection structures can be incorporated into a coastal flood insurance study or restudy, without meeting all the specific requirements that new structures are expected to meet to justify a map revision. The study contractor documentation specified in Section D.2.2.8 can serve as the basis for the evaluation of existing coastal structures. The documentation includes: * Type and basic layout of structure; * Dominant site particulars, (e.g., local water depth, structure crest elevation, ice climate); * Construction materials and present integrity; * Historical record for structure, including construction date, maintenance plan, responsible party, repairs after storm episodes; and * Clear indications of effectiveness/ineffectiveness. Given the fact that the G&S allow the Study Contractor to develop much of this documentation through an office review of available data, engineering judgment using the above factors can determine whether an existing coastal flood protection structure is incorporated into the coastal hazard assessment, and whether it influences BFEs and flood hazard zones. In contrast, a new coastal flood protection structure is required to be certified with all supporting calculations and technical documentation specified in FEMA (1990) and Walton et al. (1989), including the maintenance plan requirement. It would appear – for consistency purposes – that a similar level of engineering and certification should be required of both existing and new/proposed structures. It is recommended that consistent engineering and certification requirements be used for existing and new/proposed structures, with an exception for the maintenance plan criteria for private structures (which are not adopted by government agencies; such agencies will not be responsible for maintenance). Maintenance for private structures should be the responsibility of private owners and enforced through deed restrictions instituted at the time of the FIS or map revision. Note that these recommendations will not only require a revision to the existing guidance in the G&S, they will require a significant increase in the level of effort (and cost) required for flood insurance studies, and will require a revision to FEMA’s (1990) adopted criteria for privately owned coastal structures. Making such changes is more than a technical issue, and will require FEMA policy change. 3.4.2 Availability Information to address Topic 27a is available and incorporated into existing guidance; however, inconsistencies will have to be resolved by FEMA. Information on Topic 27b is available in the CEM and changes to evaluation criteria can be proposed based on this information. Information related to Topic 27c is available; however, changes to require consistent engineering and certification requirements will necessitate FEMA policy changes and could have significant time and cost consequences. 3.5 TOPIC 24: REVIEW WALTON, ET AL. (1989) AND OTHER LITERATURE FOR DAMAGE TO COASTAL STRUCTURES DURING TSUNAMIS 3.5.1 Description of Topic and Suggested Improvement Sections D.2.3 and D. 3.3 of the existing guidance do not reference evaluation criteria that may be appropriate for coastal structures in tsunami-prone areas. While the existing guidance may be pertinent for non-bore type tsunamis, it will probably not be adequate for bore-type tsunamis. A review of the literature should be undertaken to document tsunami damage to coastal structures. Camfield (1980) summarizes the state-of-the-art as of two decades ago, and should be included in the review. More recent reports and information sources should also be reviewed. For example: * National Tsunami Hazard Mitigation Program: http://www.pmel.noaa.gov/tsunami- hazard/index.htm; * Tsunami data at the National Geophysical Data Center (NGDC): http://www.ngdc.noaa.gov/seg/hazard/tsu.html; * NOAA Tsunami Research Program: http://www.pmel.noaa.gov/tsunami/; * International Journal of the Tsunami Society, Science of Tsunami Hazards (available at http://epubs.lanl.gov/tsunami/ (see the article by J.F. Landers, L.S. Whiteside and P.A. Lockridge, Two decades of Global Tsunamis – 1982-2002, in Vol. 21, No. 1, 2003); * The Tsunami Research Group at the University of Southern California is dedicated to the investigation of tsunamis and some information may be found from their works: http://www.usc.edu/dept/tsunamis/; * Mitigation of local tsunami effects project: http://engr.smu.edu/waves/index.html; * Professor Philip L-F Liu at Cornell University is devoted to studies of the causes and effects of tsunami, and some information may be found in his publications: http://www.cee.cornell.edu/index.cfm; and * The O.H. Hinsdale Wave Research Laboratory at Oregon State University is designated by the National Science Foundation as a site for tsunami research. This tsunami model basin is presently the largest one in the world for analyzing the impacts of tsunami waves: http://wave.oregonstate.edu/. The G&S should be revised to incorporate revised coastal structure evaluation criteria for areas subject to bore-type tsunamis. 3.5.2 Availability Information to address Topic 24 is available. This effort should be coordinated with the Tsunami Study Group. 4 IMPORTANT TOPICS There were no “Important” topics identified in Workshop 1. 5 ADDITIONAL OBSERVATIONS - HELPFUL TOPICS 5.1 TOPIC 22: INVESTIGATE CONFIGURATIONS OF FAILED COASTAL STRUCTURES 5.1.1 Description of Topic and Suggested Improvement The discussion in Section 3.1 summarizes the current G&S treatment of failed coastal structures, namely, they are to be removed from the analysis transects. However, in the case of seawalls, revetments and similar structures, outright removal may not result in the highest BFEs and flood conditions. Moreover, in the case of revetments, partial failure rather than complete failure (and removal) may be a more appropriate scenario for analysis due to the creation of higher runup condition or greater depths of ponding. A proposed procedure for handling this situation was developed during the Whatcom County, WA, FIS (PWA, 2002). A modified PWA procedure is recommended for incorporation into the G&S as follows: * In the absence of structure certification, conduct coastal flood analysis for intact and failed conditions, and use the worst case for flood mapping; note that maintaining the results of both analyses may be useful in the event that map revisions are requested in the future based on intact structures; * Apply simple geometric approaches to estimate the failed condition for vertical or near- vertical rigid structures: * Estimate toe scour based on the Shore Protection Manual (SPM) or similar approximations (scour to the water depth at the structure toe, based on the largest unbroken wave anticipated at the toe); * Extend the toe erosion offshore a distance related to the incident wave length; * Presume the rigid structure breaks apart, into a rough, porous failed slope at 1.5:1. The slope is selected with the understanding that runup typically reaches a maximum at about this slope, which is also consistent with the potential angle of repose of rough angular material; and * Note that assuming a failed slope of 1.5:1 may lead to undermining of buildings situated very close to the coastal structure. This scenario should be investigated during Phase 2 to determine the appropriate mapping course of action. * In the case of revetments, consider whether complete or partial failure is more likely during the base flood, and model the selected failed condition. If the failure condition is uncertain, modeling of total and partial revetment failure can be carried out. In the case of the Sandy Point FIS, application of the above procedure indicated the failed structure condition typically did not yield the highest runup elevation, but could result in greater overtopping rates than the intact structure condition. Parts V-3 (Basco, 2003) and VI-5 (Burcharth and Hughes, 2003) of the CEM (and other documents – see Section 5.2.1) should be reviewed for possible guidance regarding the configurations of failed structures. However, it is proposed that the PWA method be considered an interim method (for seawalls, bulkheads and revetment type structures) and evaluated for future refinement. Methods for handling failed groins, jetties and breakwaters have not been proposed here, but may be considered for future enhancements of the G&S – see Topic 21b. 5.1.2 Availability Information to address Topic 22 is available. This effort should be coordinated with the Runup/Overtopping Study Group. 5.2 TOPIC 26: REVIEW DATA ON THE EFFECTS OF COASTAL STRUCTURES ON FLOOD HAZARDS ON ADJACENT PROPERTIES; REVIEW FLOODING/WAVE EFFECTS BEHIND STRUCTURES 5.2.1 Description of Topic and Suggested Improvement One of the coastal structure evaluation considerations included in FEMA (1990), FEMA (2002) and FEMA (2003) is adverse impacts. Unfortunately, the level of guidance contained in those documents is inadequate: * FEMA’s (1990) memorandum regarding the evaluation of coastal structures states: “All requests for flood map revisions based upon new or enlarged coastal flood control structures shall include an analysis of potential adverse impacts of the structure on flooding and erosion within, and adjacent, to the protected area.”; * FEMA’s (2002) flood map revision coastal structures form asks flood map revision requestors, “... will the structure impact flooding and erosion for areas adjacent to the structure? If yes, attach an explanation.”; and * FEMA’s (2003) G&S, section D.2.3, states, “... a structure might decrease flood hazards in one area while increasing flood and erosion effects at adjacent sites.” Impact of Coastal Structures (Seawalls, Revetments) on Adjacent Property Impacts can be divided into erosion impacts and hydraulic impacts. Erosion impacts will include the short- or long-term effects of a coastal structure on the topography of adjacent property. Hydraulic impacts will include such things as wave reflection, concentration of flow, etc. Fortunately, the literature contains numerous papers and studies related to erosion impacts: * Dean (1987) assessed commonly expressed concerns about seawall impacts. The assessment is summarized in Figure 1. * Fulton-Bennett and Griggs (1986) document case histories of 32 shore protection structures at sites between San Francisco and Carmel, CA. The report concluded that few of the structures survived the long-term test of time without some damage to the structure or the upland areas. Maintenance costs of the structures were much higher than originally anticipated. * Griggs, et al. (1994) summarized the results of field monitoring at sites in Monterey Bay, CA. They concluded after seven years of detailed monitoring that there was “an absence of measurable or significant differences” between the seawall backed beach and the natural beach. * Kraus and Pilkey (1988), and Kraus and McDougal (1996) present detailed literature reviews concerning the effects of seawalls on beaches. Both papers were published in the Journal of Coastal Research, the first being in a special issue devoted to the topic (Kraus and Pilkey, 1988). * McDougal et al. (1987) conducted laboratory and field investigations in Oregon to assess the impacts of shore protection structures on adjacent unprotected properties. The studies found the “excess erosion” on adjacent properties was consistent with the findings of Chiu (1977): the depth of excess erosion was found to be equal to approximately 10% of the seawall length (see Figure 2). Taken as a whole, these studies indicate the erosion effects of shore protection structures on nearby properties will vary, depending on the local coastal processes and morphology, sediment budget, and structure location/characteristics. However, the effects can be divided into three general categories: The effects of impoundment (sediment landward of the structure being prevented from eroding and nourishing the beach) and passive erosion (continuation of ongoing shoreline recession, resulting in a narrower beach in front of a structure) are relatively uncontroversial and can be quantified for a site. Figure 1. Review of concerns related to coastal armoring (Dean, 1987, as compiled by USACE, 2003). Figure 2. Excess erosion caused by seawalls (McDougal, et al., 1987). The effects of active erosion (postulated erosion and scour due to the presence of the structure) remain the subject of dispute and are more difficult to quantify. The previously mentioned work of Dean (1987), McDougal et al. (1987) and Kraus (1988, 1996) should serve as guidance for evaluating active erosion effects. Thus, this Focused Study concludes that the approximate or expected erosion effects of coastal structures can be determined for flood hazard mapping purposes. Guidance can be developed for study contractors to use in their evaluations. Looking forward, the more difficult issue will be how to incorporate this knowledge into FEMA policy regarding treatment of coastal structures: * If adverse effects of existing coastal structures are documented or of new/proposed structures are predicted, should mitigation be required? If so, in what form? * Should unmitigated effects be considered in flood hazard mapping (and is this getting into the future conditions area)? Should mitigation efforts be credited in flood hazard mapping (this is similar to the issue surrounding credit for beach nourishment)? * Should map revisions be permitted based on structures that are predicted or known to cause adverse effects on adjacent properties? This topic will undoubtedly be the subject of additional debate, and the work described in Table 1 is intended to provide limited technical guidance until the policy issues are resolved. The wealth of literature devoted to erosion effects of coastal structures does not exist for hydraulic effects. However, the hydraulic effects of many coastal structures can be approximated using the methods of hydraulics, fluid mechanics and wave mechanics, coupled with documents such as the Coastal Engineering Manual. There may be some instances where the hydraulic effects of large structures can be better addressed via numerical modeling, but this is expected to be the exception rather than the rule (at least for the near future). For the present, it is recommended that a general discussion of hydraulic effects be included in the G&S. Flooding and Erosion Behind Coastal Structures (Seawalls, Revetments, etc.) A second issue of importance to FEMA is whether the dimensions of a coastal structure are sufficient to prevent flooding and erosion from occurring landward of the structure during the 1% flood event. This issue will be important for both, flood insurance studies and the evaluation of flood map revisions based on coastal structures. Flooding behind a structure can be caused by overtopping of the shore-parallel section of the structure, or due to overtopping of the shore-perpendicular (return wall) section of the structure. Erosion behind a structure can be caused by undermining at the structure toe, overtopping, or other structural failures. The erosion can be initiated at or across the shore-parallel or shore- perpendicular sections. The G&S can be expanded to address these hazards, by stating that the TR-89-15-type analyses shall consider both the shore parallel and shore-perpendicular sections of coastal structures. For the mapping of flood hazard zones landward of structures determined to withstand the 1% flood event, the following procedure is recommended. Case 1, isolated structure with return walls: * Evaluate the shore-parallel and shore-perpendicular portions of the structure; * if the returns are too short or will not withstand the 1% event, remove the entire structure from the transect prior to further flood analyses (unless the structure is very long compared to the parcel frontage being evaluated), and * if the return walls are adequate, determine the mean overtopping rate across the shore-parallel section of the structure. * Map the resulting BFEs and flood hazard zone boundaries behind and parallel to both the shore-parallel section and any shore-perpendicular sections. This procedure assumes overtopping can occur over any section of the structure. See Figure 3; and * Calculate the maximum overtopping and determine if any ponding or drainage problems will exist behind the structure; adjust the mapped flood hazard zones and heights/elevations to reflect the ponding or drainage problems. Case 2, series of structures: This case will be encountered by Study Contractors, and will likely occur when a one property owner requests a map revision based on a portion of a single structure or one of a series of structures; * Consider each distinct structure separately – determine whether the land behind the structure is separated from adjacent lands by return walls; * if yes, evaluate as in case 1 above, unless the adjacent shore-parallel sections are long and will withstand the 1% flood event (in which case the return wall analysis and mapping are not required); and * if no, evaluate the adjacent shore-parallel sections for their stability during the 1% event. * if adjacent sections will not withstand the 1% event, the subject coastal structure may be damaged or destroyed as the adjacent structures fail (and may need to be removed prior to any flood analyses); and * if adjacent shore-parallel sections will withstand the 1% event, and if they are sufficiently long to preclude flanking behind the subject structure, continue as described below. * If the analysis goes forward, determine the mean overtopping rate across the shore- parallel section of the structure; and * Map the resulting BFEs and flood hazard zone boundaries behind and parallel to both the shore-parallel section and any shore-perpendicular sections. This procedure assumes overtopping can occur over any section of the structure. Check for ponding and drainage problems. Adjust the zones and BFEs along the boundaries with adjacent parcels, as dictated by the stability of adjacent coastal structures. Coastline Figure 3. Sample mapping of flood hazards at failed coastal structure – through physical failure or insufficient return walls – and at intact coastal structure (actual flood hazard zones and BFEs will vary with site/structure conditions). Note that the above procedures do not establish a minimum coastal structure length required to gain flood hazard mapping credit (either during an FIS or a map revision). However, as a first approximation, a structure length less than twice the mapped overtopping zone width behind the structure (see G&S Table D-7) would probably not provide significant flood hazard reduction for the area landward of the structure. For a more rigorous analysis, the minimum length required will depend upon: * whether the structure is intended to remove an area from the SFHA or merely to reduce the flood severity/BFE/zone, * the height of the structure and its associated base flood overtopping rate, * whether the structure is isolated or part of a longer structure, and * whether the subject parcel is isolated by return walls that can withstand the base flood event. Minimum structure lengths might be developed through analyses of selected structures and flood conditions, but this should be considered for future enhancements to the G&S. Recommendations and availability are summarized in Table 1. 5.2.2 Availability Information to address Topics 26a, 26b, and 26d is readily available. Information to address Topic 26e can be gathered and used, but may require greater effort. Addressing Topic 26c requires as much policy development as technical work. Therefore, Workshop 2 deleted Topics 26c and 26e from further consideration during the present project. 6 SUMMARY Table 1. Summary of Findings and Recommendations for Coastal Structures Mention in guidance: detailed TR- 89-15 evaluation/certification of coastal structures are not required during FIS, but discuss implications (see Topic 22) Expand guidance to discuss removal of seawalls, bulkheads, revetments, coastal levees; allow for partial failure of revetments, where appropriate. Mention in guidance, removal of the effects of groins, jetties, detached breakwaters on the shoreline. Develop specific guidance on how to remove the effects of groins, jetties, detached breakwaters on the shoreline. Mention in guidance: buried structures may exist, should be located and should be considered in analyses. 22 Revise Appendix D to differentiate coastal levee requirement from those for other costal flood protection structures; identify conflicts. Review CEM for new or additional guidance on evaluation of coastal structures; Consider requiring all structures (existing and new) to meet the same evaluation criteria. Review literature and revise guidance for coastal structure evaluation criteria in tsunami- prone areas. Review literature for treatment of failed structures; Revise coastal structure evaluation guidance to reflect PWA Interim method and literature review. Review literature and develop guidance for evaluating the erosion effects of coastal structures on adjacent properties. Review literature and develop guidance for evaluating the hydraulic effects of coastal structures on adjacent properties. Develop guidance for evaluating flooding and erosion from adjacent properties. Key: Coastal Area AC = Atlantic Coast; GC = Gulf Coast; PC = Pacific Coast; SW = Sheltered Waters Priority Class C = critical; A = available; I = important; H = helpful (Recommend priority italicized if focused study recommended a change in priority class) Availability/Adequacy “Critical” Items: MIN = needed revisions are relatively minor; MAJ = needed revisions are major “Available” Items: Y = availability confirmed; N = data or methods are not readily available “Important” Items: PRO = procedures or methods must be developed; DAT = new data are required; PRODAT = both new procedures and data are required 7 REFERENCES Basco, D.R. 2003 (July). Shore Protection Projects. In: J. Pope (ed.), Coastal Engineering Manual, Part V, Coastal Project Planning and Design, Chapter V-3 , Engineer Manual 1110-2-1100, U.S. Army Corps of Engineers, Washington, D.C. Burcharth, H.F., and S.A. Hughes. 2003 (April). Fundamentals of Design. In: S.A. 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