Loss-Consistent Categorization of Hurricane Wind and Storm Surge Damage for Residential Structures

Transcription

1 Loss-Consistent Categorization of Hurricane Wind and Storm Surge for Residential Structures Carol J. Friedland, P.E., Ph.D. 1, Marc L. Levitan, Ph.D. 2 1 Assistant Professor, Department of Construction Management and Industrial Engineering, Louisiana State University, Baton Rouge, LA, USA, [email protected] 2 Associate Professor, Department of Civil and Environmental Engineering, Louisiana State ABSTRACT University, Baton Rouge, LA, USA, [email protected] This paper presents the application of a new scale to describe the level of to residential s in wind, flood and combined wind and flood events. While the development of the Wind and Flood (WF) Scale is the topic of a separate document in preparation, this paper provides a brief background of the need for a scale that can describe the combined caused by wind and flood hazards and shows application of the new scale for assessment of residential after Hurricanes Katrina and Ike. The basis of the WF Scale is the HAZUS Wind Scale, which is expanded and modified to address levels and types of associated with hurricane storm surge and other inundation flooding. INTRODUCTION single scale exists that adequately describes all aspects of hurricane, and the scales that do exist are not compatible for direct comparison of states. Several scales categorize wind from hurricanes, but none capture the full range of behavior in detail. Scales such as the Saffir-Simpson scale and Enhanced Fujita (EF) scale relate wind speed with degree of for particular types. The Saffir-Simpson scale provides a general range of, but does not provide sufficient detail to be applied to specific s. The Enhanced Fujita scale, developed for tornadoes, has been utilized by some to describe hurricane wind ; however, the degree of (DOD) indicated for specific wind speeds may not be appropriate for s constructed to higher coastal standards. FEMA s HAZUS-MH Wind Model [1] contains a scale that describes typical levels of caused by hurricane winds (and ensuing from wind/rain intrusion); however, caused by storm surge is not considered. HAZUS and other sources have developed loss curves that describe increasing as a function of wind speed for various types of construction, but continuous scales are generally not useful for categorizing observed. While the wind engineering community generally uses discrete scales to describe in terms of physical parameters, methods used to categorize flood typically consist of single variable-dependent continuous functions that relate the depth of floodwaters to defined in terms of economic loss, expressed either in terms of currency [2] or as a percentage of the value[3]. Some research has focused on identifying collapse potential of residential s as a function of flood depth and velocity (e.g.,[4]), but such studies generally provide only an empirical function, above which structural failure occurs and below which only inundation is present. A few discrete scales do exist that describe flood consequences in terms of physical [5]; however, they do not adequately segregate the higher levels of.

2 Further, as evidenced by Hurricanes Katrina, Rita and Ike, the source of (wind or flood) is often in question. A new scale is needed that will allow assessment of for the entire structure, rather than having to classify wind or flood as the source of the, as the cause may not always be readily apparent. An initial attempt at such as scale was developed immediately after Hurricane Katrina to aid in field data collection [6]. However, subsequent field investigations and analysis of the hurricane data have revealed significant shortcomings to that first combined scale, and have highlighted the need for a more comprehensive and loss-consistent categorization. The WF Scale considers the combined from hurricane wind and flood on a loss consistent basis. Regardless of the mechanism, the physical is described in a manner that accounts for the presence or absence of structural, as well as treating repairable non-structural on an economically consistent basis. This scale has been constructed using scales from both the HAZUS-MH Wind and Flood [7] models, as well as guidance from the US Army Corps of Engineers, the European Macroseismic Scale (EMS-98)[8], and others. WIND AND FLOOD DAMAGE SCALE states associated with hurricane storm surge are often much more significant than that is described by either wind or flood categorization schemes. The HAZUS-MH model describes hurricane wind on a five category scale: no or very minor, minor, moderate, severe, and destruction. One of the indicators for the highest wind state, destruction, is defined as loss of more than 2 of roof sheathing. While this level of may result in significant loss from water intrusion or may lead to caused by internal pressurization, this is clearly a very different state than partial collapse or complete collapse of a structure, commonly experienced along storm surge-affected coastlines. The two collapse classifications are typically used in the assessment of earthquake to s and have been employed by researchers assessing to s after tsunami events [9]. The WF Scale categories were developed through enhancement and modification of the standard FEMA HAZUS-MH Wind Model classifications to include both collapse and partial collapse categories (Figure 1), as well as to provide detailed guidance in the assessment of combined. or Very Minor Minor Moderate Severe Range of Considered by FEMA s HAZUS-MH Wind Model Very Severe Partial Collapse Collapse Range of Typically Associated with Hurricane Storm Surge Figure 1: categories associated with hurricane wind and storm surge for residential s The WF Scale is presented in Table 1, where the shaded WF indicators represent the signatures that require corresponding classification, while non-shaded cells are general values for each state. Figures 2-5 demonstrate the application of the scale using examples from assessments completed after 2005 Hurricane Katrina and 2008 Hurricane Ike. While the housing types encountered in coastal Mississippi and Galveston, Texas, are markedly different, the scale is applied to consistently describe observed. As in Table 1, the shaded WF indicators in Figures 2-5 represent the signatures that require

3 corresponding classification, while non-shaded cells provide general values for each state. The following details explain the assessment for each structure: WF-0 Katrina and Ike visible, no floodwater impacted the s WF-1 Katrina Minor cladding to side of house, no flooding within the WF-1 Ike Breakaway wall with no to WF-2 Katrina structure (porch) failure, with to main WF-2 Ike Cladding, does not expose the structure interior, wrap not intact WF-3 Katrina Wash through of cladding, structural system still in place, minor roof deck failure (<2) WF-3 Ike Approximately 2 roof deck failure WF-4 Katrina Structural (repairable) to porch and portions of the WF-4 Ike Roof deck exceeding 2 (roof deck is also removed from backside of the ) WF-5 Katrina Unrepairable structural to more than 2 of the WF-5 Ike Elevated structure is racked WF-6 Katrina and Ike Total structural failure of the WF DAMAGE SCALE APPLICATION FOR HURRICANES KATRINA AND IKE Building attributes and were classified for nearly 2000 residential structures following Hurricanes Katrina and Ike based on review of high definition video and still images collected during field reconnaissance missions in coastal Mississippi and Galveston, Texas. Figures 6 and 7 provide summary attribute and information for both datasets, including (a) number of stories, (b) foundation type, (c) approximate base floor elevation, (d) calculated depth of flooding (calculated as surge elevation minus approximate base floor elevation, (e) WF state from the video review (not including flood depth), and (f) WF state including flood depth. Attribute details that were not able to be assessed for destroyed s are indicated, and base floor elevations for unassessed s are included in the < 1 ft category in Figure 6. The s were assumed to be of wood construction, as all s with exposed structure appeared to be wood framed.

4 Table 1: Wind and Flood Scale [10] State Qualitative Wind Description Qualitative Surge/Flood Description Roof Cover Window/ Door Roof Deck Structure Wall Structure Roof Structure Structural "Stillwater" Flood WF-0 or Very Minor Little or no visible from the outside. broken windows, or failed roof deck. Minimal loss of roof cover, with no or very limited water penetration. or Very Minor floodwater impacts the. ne WF-1 WF-2 WF-3 WF-4 WF-5 WF-6 Minor Maximum of one broken window, door or garage door. Moderate roof cover loss that can be covered to prevent additional water entering the. Marks or dents on walls requiring painting or patching for repair. Moderate Major roof cover, moderate window breakage. Minor roof sheathing failure. Some resulting to interior of from water. Severe Major window or roof sheathing loss. Major roof cover loss. Extensive to interior from water. Very Severe Complete roof failure and/or, failure of wall frame. Loss of more than 2 of roof sheathing. Partial Collapse House shifted off foundation, overall structure racking, unrepairable structural (structure still partly intact). Collapse Total structural failure (no intact structure). Minor Breakaway walls or appurtenant structures (staircases carports, etc.) d or removed without physical to remaining structure. floodwater impacts the. Moderate Some wall cladding from floodborne debris or high velocity floodwater. Breakaway walls or appurtenant structures (staircases carports, etc.) d or removed with physical to remaining structure. Severe Removal of cladding from "wash through" of surge without wall structural. Very Severe of wall frame, repairable structural to any portion of the or cases of unrepairable structural, not to exceed 2 of the plan area. Partial Collapse House shifted off foundation, overall structure racking, unrepairable structural to > 2 of the plan area. Structure is still partly intact. Collapse Total structural failure (no intact structure). > and 1 >1 and 50% Typically Typically Typically One window, door, or garage door failure > one and the larger of 20% & 3 > the larger of 20% & 3 and 50% 1 to 3 panels >3 and 2 >2 >2 Slab, pile scour with no apparent Cracked slab with visible deformation Racking of elevated structure, without to, with to Minor cladding with wrap intact Moderate cladding that does not expose structure interior, wrap not intact "Wash through" ne >2 Any repairable structural or 2 unrepairable Unrepairable structural (>2) Total structural failure > 0 ft and <2 ft (one story) or <5 ft (two+ stories) >2 ft (one story) or >5 ft (two+ stories)

5 WF Rating WF-0: or Very Minor WF-1: Minor Hurricane Katrina Example Hurricane Ike Example WF Indicators Roof Deck Structure Structural Roof Deck Structure Structural ne > and 1 One window, door, or garage door failure Slab, pile scour with no apparent, without to Minor cladding with wrap intact ne Figure 2: WF-0 to WF-1 Classification Examples for Hurricanes Katrina and Ike

6 WF Rating Hurricane Katrina Example Hurricane Ike Example WF Indicators >1 and 50% > one and the larger of 20% & 3 WF-2: Moderate WF-3: Severe Roof Deck Structure Structural 1 to 3 panels, with to Moderate cladding that does not expose structure interior, wrap not intact > 0 ft and <2 ft (one story) or <5 ft (two+ stories) > the larger of 20% & 3 and 50% Roof Deck >3 and 2 Structure Structural "Wash through" >2 ft (one story) or >5 ft (two+ stories) Figure 3: WF-2 to WF-3 Classification Examples for Hurricanes Katrina and Ike

7 WF Rating WF-4: Very Severe WF-5: Partial Collapse Hurricane Katrina Example Hurricane Ike Example WF Indicators te: the backside of the roof also experienced roof deck failure, causing > 2 roof deck loss Typically Roof Deck >2 Structure Structural Cracked slab with visible deformation Any repairable structural or 2 unrepairable Typically Roof Deck >2 Structure Structural Racking of elevated structure Unrepairable structural (>2) Figure 4: WF-4 to WF-5 Classification Examples for Hurricanes Katrina and Ike

8 WF Rating WF-6: Collapse Hurricane Katrina Example Hurricane Ike Example WF Indicators Typically Roof Deck >2 Structure Structural Total structural failure Figure 5: WF-6 Classification Examples for Hurricanes Katrina and Ike

9 Hurricane Katrina Number of Stories t Assessed 4 1 Story 43% Hurricane Katrina Raised Floor 21% Piles 6% t Assessed 4 (a) 3 Story 0% 2 Story 1 (b) Slab 31% Hurricane Katrina Approximate Base Floor Elevation 1-2 ft 23% 3-6 ft 8% 8-10 ft < 1 ft 64% Hurricane Katrina Calculated Flood Above Floor > 11 ft 27% 0 ft 6% < 2 ft 10% 2-6 ft 17% (c) (d) 7-10 ft 40% Hurricane Katrina WF State - Physical Only WF-6 4 WF-0 21% Hurricane Katrina WF State - Physical + Flood WF-0 WF-6 4 WF-1 WF-2 9% WF-1 14% WF-3 3 (e) WF-5 WF-4 WF-3 7% WF-2 6% (f) WF-5 WF-4 Figure 6: Hurricane Katrina inventory and characteristics Based on a review of Figure 6, the foundation type and structure elevation appear to have significantly contributed to the amount of sustained from Hurricane Katrina s storm surge. Figure 6(d) shows approximately 70% of s were flooded by more than 7 feet. Over half of the s in the database experienced some level of structural (WF-4 to WF-6), resulting in significant debris generated from d and destroyed s. Both the hazard environment and the coastal construction techniques employed in Galveston, Texas, limited the amount of experienced in the study area. Galveston did not experience the most severe wind, surge and wave forces; however many residences on Bolivar Peninsula were completely destroyed as a result of Hurricane Ike and areas that were most heavily impacted were inaccessible at the time of the field survey.

10 Differences in severity noted in Figure 6 and 7 (e) and (f) show the usefulness of the inclusion of flood hazards in overall assessments. Significant portions of the Hurricane Katrina dataset were subject to storm surge inundation, which is reflected in the increased levels of observed when the flood hazard is included. The Hurricane Ike dataset in Galveston, Texas, was exposed to less severe storm surge as a result of a lowered hazard environment and the predominance of pile foundations. As shown in Figure 7 (e) and (f), few changes in state were observed as a result of inclusion of surge inundation depth. The WF Scale does allow assessment (including collapse and partial collapse) without identification of the mechanism of the. Hurricane Ike Number of Stories t 3 Story Assessed Hurricane Ike t Assessed 2 Story 46% 1 Story 50% Piles 9 (a) (b) Hurricane Ike Approximate Base Floor Elevation t Assessed > 10 ft 11% 8-10 ft 8 < 8 ft Hurricane Ike Calculated Flood Above Floor 2-4 ft 1% > 4 ft < 2 ft 0% 0 ft 97% (c) (d) Hurricane Ike WF State - Physical Only Hurricane Ike WF State - Physical + Flood WF-2 10% WF-0 24% WF-2 11% WF-0 23% (e) WF-1 63% (f) WF-1 63% Figure 7: Hurricane Ike Building Inventory and Characteristics

11 CONCLUSIONS This paper has presented the application of the new Wind and Flood Scale. The primary need that the WF Scale addresses in hurricane assessments is the ability to describe to residential s, regardless of the mechanism. This ability allows for a more complete understanding of the overall level of caused by hurricanes. The WF Scale provides a method to describe total on a loss consistent basis, allowing description of the overall levels of. These descriptions of have multiple applications, including loss of life estimates, emergency management planning and response activities, and improved understanding of the overall performance of residential s exposed to hurricane conditions. REFERENCES [1] FEMA, Multi-hazard loss estimation methodology - hurricane model, HAZUS-MH MR2 Technical Manual, Federal Emergency Management Agency, Washington, D.C., 2006a. [2] E. C. Penning-Rowsell and J. B. Chatterton, The benefits of flood alleviation: a manual of assessment techniques, Aldershot: Gower (1977). [3] USACE, Generic depth- relationships for residential structures with basements, Economic Guidance Memorandum 04-01, U.S. Army Corps of Engineers, Washington, D.C., [4] CH2M Hill, Potential flood s, Willamette River System, Department of the Army Portland District, Corps of Engineers, Portland, OR, [5] I. Kelman, Physical flood vulnerability of residential properties in coastal, eastern England, PhD Thesis, University of Cambridge, [6] J. A. Womble, S. Ghosh, B. Adams, and C. Friedland, Advanced detection for Hurricane Katrina: integrating remote sensing and VIEWS TM field reconnaissance, Hurricane Katrina Special Report Series MCEER, Buffalo, NY, [7] FEMA, Multi-hazard loss estimation methodology flood model, HAZUS-MH MR2 Technical Manual, Federal Emergency Management Agency, Washington, D.C., 2006b. [8] G. Grünthal ed, European macroseismic scale 1998 (EMS-98) Cahiers du Centre Européen de Géodynamique et de Séismologie, Luxembourg, [9] H. Miura, A. C. Wijeyewickrema, and S. Inoue, Evaluation of tsunami in the eastern part of Sri Lanka due to the 2004 Sumatra earthquake using remote sensing technique, in: 8th U.S. National Conference on Earthquake Engineering (8NCEE), (San Francisco, CA), [10] C. J. Friedland, Residential from hurricane storm surge: proposed methodologies to describe, assess and model, PhD Thesis, Louisiana State University, 2009.