Sustainable Cities and Society 35 (2017) 350–364 Contents lists available at ScienceDirect Sustainable Cities and Society journal homepage: www.elsevier.com/locate/scs A framework for holistic designs of power line systems based on lessons learned from Super Typhoon Haiyan MARK ⁎ Kaoshan Daia,b, Shen-En Chena,c, , Mingyan Luoa, Gregory Loflin Jr.c a b c State Key Laboratory of Disaster Reduction in Civil Engineering and College of Civil Engineering, Tongji University, Shanghai 200092, China State Key Laboratory for Geomechanics & Deep Underground Engineering, China University of Mining and Technology, Xuzhou 221116, China Department of Civil and Environmental Engineering, University of North Carolina at Charlotte, Charlotte, NC 28223, United States A R T I C L E I N F O A B S T R A C T Keywords: Climate change Holistic design Power delivery systems Super Typhoon Haiyan Resilience Climate Change results in possible less frequent but more intensified storm events that include so called “super typhoons”. To ensure the resilience of the energy power delivery systems, this paper offers lessons learned from forensic investigations of the damaged power delivery systems from the Philippines after the 2013 Super Typhoon Haiyan. The objective is to investigate new strategies to design the power delivery systems, especially in the coastal regions, to defend against the intensified storm events brought on by climate change. A critical review of current power line system design approach, especially load design based on ASCE 7–10, is conducted. The review along with extreme loading calculations revealed limitations in current design philosophies. A design framework using a holistic approach that includes a strong backbone line system is suggested. The strong backbone line system is to be designed for extreme loading scenario with additional considerations including debris loading, wind-storm surge interactions and local geospatial condition with multiple wind directions. Preliminary design of a strong backbone system for McClellanville, South Carolina, power supply scenario is presented as a case study and is shown to be economically viable. 1. Introduction In December of 2015, the United Nations (UN) has come to another agreement to address the climate change issue – The Paris Agreement (COP-21) is the latest of several global efforts to limit greenhouse gas (GHG) emissions with the desire to deter the paths of extreme climate changes (UNFCCC, 2015), which have been identified to include warmer winters, draught, floods, erratic weather patterns, etc. Physical evidence of climate changes has already been identified at different parts of the world. Extreme climate change-associated events have occurred in recent years which resulted in significant impacts on local economics and a drastic increase in global disaster expenditures (IPCC, 2007). Many of these climate events are directly challenging the resilience of the regional populations. A great interest to the engineering community is what role super typhoons (wind forces in the Saffir-Simpson category of 4 or 5) play in the extreme weather projection of climate change (Krishna, 2009; McDonald, 2011; Nigam and Guan, 2011; Walsh, McInnes, & McBride, 2012). Research suggests that in terms of frequency, the increase in surface wind temperature would actually result in fewer tropical storms with likely increased intensities and wind speeds (Lin, Pun, & Lien, ⁎ Corresponding author at: University of North Carolina, Charlotte, United States. E-mail address: schen12@uncc.edu (S.-E. Chen). http://dx.doi.org/10.1016/j.scs.2017.08.006 Received 31 October 2016; Received in revised form 3 July 2017; Accepted 6 August 2017 Available online 12 August 2017 2210-6707/ © 2017 Elsevier Ltd. All rights reserved. 2014). For conditions that can create such events, several theories have been proposed. For example, Takagi and Esteban (2016) used statistics of the region to demonstrate a strong correlation between super typhoon frequencies and intensities and the regional sea surface temperature. The focus of this paper is on the power and energy delivery systems (power grid), which are essential to our daily lives, and can result in significant losses in local economics and even human deaths when subjected to extreme storm impacts. Vulnerability of the power systems in association with climate change is a critical area that desperately needs attention (Liu and Pang, 2012; Vickery and Lavelle, 2012). Recent extreme storm events including the hurricanes Katrina and Sandy in the US have already demonstrated the fierce impacts of these storms (Ewing, Liang, & Cui, 2014). Due to the complexity of the power delivery systems, which includes power generation, power transmission, power distribution and substations, etc., it is difficult to devise an effective method to enhance the performance of the power system for the impending climate changes. Super Typhoon Haiyan, formed on November 3, 2013 and dissipated on November 11, 2013, was one of the deadliest tropical cyclones along the West Pacific coastal regions. An estimated 6300 lives Sustainable Cities and Society 35 (2017) 350–364 K. Dai et al. Fig. 1. Haiyan Path and Townships encountered disasters. The effects of Haiyan have been most felt on the east coast of Leyte Island and southern tip of East Samar Island. Fig. 2 shows the track of Super Typhoon Haiyan with its mean velocity and Table 1 with the timed event of Super Typhoon Haiyan (reproduced from Kim, 2015). It is important to point out in Fig. 2 that the storm arrived at its highest speed and lowest pressure at landfall – the critical conditions for maximized hurricane effect. Most hurricane events are already at their down swing with reduced wind speeds upon landfalls. The flood flowed through Tacloban city reaching 7.0 m/s (Takagi et al., 2017). After Haiyan, the Island of Leyte experienced months of electricity and communication losses due to the loss of transmission and distribution structures and systems. Continental Americas experience similar seasonal cyclone impacts on both the Pacific and Atlantic sides – even though west Atlantics experienced far less category five storms than the western Pacific. in the Philippines were lost due to Haiyan (Del Rosario, 2014). Fig. 1 shows the townships that have experienced significant impacts from the storm. Current storm measurement techniques indicate (with ongoing debate) that Typhoon Haiyan may be the strongest storm ever recorded at landfall. Unofficially, records indicate wind speeds of 87.0 m/s, for one-minute sustained wind, which, if verified, are the strongest recorded wind speeds associated with a typhoon in history. The storm began as a low pressure system initially formed on November 3, 2013 near Micronesia. The low pressure system then intensified rapidly to a super storm on November, 6, 2013. At 23:00 UTC of November 7, the storm arrived at Tolosa of the Leyte Island and caused wide spread disasters, which included 722 deaths in the city of Tanauan, which is the second most severely damaged area by Haiyan in the Leyte Province (Yi et al., 2015). The significant storm brought precipitation reaching 615 mm between November 3 and 12 (Nguyen et al., 2014). Fig. 2. Track of Super Typhoon Haiyan Indicated on Google EarthTM (numbers indicate peak wind speed in mph). 351 Sustainable Cities and Society 35 (2017) 350–364 K. Dai et al. Table 1 Computation of Power Pole Design Requirements for Extreme Wind Loading Scenario (Wind Speed at 89.4 m/s, Risk Category IV, Exposure Category D and Location off the Florida Coast). Height above ground level (m) 0−4.6 6.1 7.6 9.1 12.2 15.2 18.3 21.3 24.4 27.4 30.5 36.6 42.7 kz 1.03 1.08 1.12 1.16 1.22 1.27 1.31 1.34 1.38 1.40 1.43 1.48 1.52 qz (kPa) 51.9 54.4 56.5 58.5 61.5 64.0 66.0 67.6 69.6 70.6 72.1 74.6 76.6 h (m) 4.57 1.52 1.52 1.52 3.05 3.05 3.05 3.05 3.05 3.05 3.05 6.10 6.10 Af (m2) 1.39 0.46 0.46 0.46 0.93 0.93 0.93 0.93 0.93 0.93 0.93 1.86 1.86 F (kN) 74 26 27 28 58 61 63 64 66 67 68 141 145 Not Including Wind Load on Conductors Including Wind Load on Conductors Pole Height (m) Base Shear (kN) Overturning Moment (kN m) Pole Height (m) Base Shear (kN) Overturning Moment (kN m) 4.6 6.1 7.6 9.1 12.2 15.2 18.3 21.3 24.4 27.4 30.5 36.6 42.7 74 100 126 154 212 273 336 400 466 532 601 742 887 169 306 490 722 1344 2176 3225 4494 6001 7734 9712 14453 20208 4.6 6.1 7.6 9.1 12.2 15.2 18.3 21.3 24.4 27.4 30.5 36.6 42.7 275 310 345 380 450 521 591 661 735 806 880 1031 1184 1088 1591 2155 2792 4247 5953 7901 10073 12568 15229 18218 25018 32866 Fig. 3. Haiyan Path and Townships encountered disasters. Liu and Pang (2012) simulated climate change influence on hurricane wind hazard scenarios using stochastic hurricane simulation and HURDAT model and concluded that there may be an average increase in wind speed of 14–19 m/s at the Northeastern US regions. Fig. 5 shows the increasing trend in the continuing average sea surface temperature (SST) in the Atlantic and the trend of continuing average of hurricanes in the US indicating an increasing trend since early 1900s. The intensifying hurricanes are defined based on averaging of core pressure and wind speed. Drawing from the lessons learned from Super Typhoon Haiyan in the Philippines, we present in this paper a discussion about the responses of the power systems against climate change. The documentation of the damaged structures from this storm event should increase our understanding of the damaging forces of the storm and it should help enhance the resilient design of coastal structures. Many of the observations and contributions from the forensic investigation groups can help enhance the post-disaster performance of the infrastructure and the built-environment. Nonetheless, recent storm records also indicate an increase in higher wind speed storms. Notably, Hurricane Patricia on October 2015 reached a sustained one minute wind speed of 96.0 m/s over western Atlantic Ocean. However, the storm had downgraded before reaching land and made landfall at 66.7 m/s. Fig. 3 shows the paths of Haiyan and Patricia along with several regional cyclonic events for the past 60 years (1945–2006). Climatology recognizes the cyclic warming and cooling cycles of the ocean. Currently, the Atlantics region is in the warming cycle and using hurricane landfall data, Vickery and Lavelle (2012) has found that the projection of warming Atlantic oscillations would result in higher hurricane activities and is consistent with historical data (110 year long term data) indicating that there is a high probability of Category 3 and above hurricanes for the near future. Fig. 4 shows how simulated warm and cold Atlantic oscillation activities compared with historical data (long term). The data are presented in ratio of warm or cold period versus long term data. It is shown that in most cases the warming period would result in significant increase in very intense hurricanes such as categories 4 and 5 storms for the entire coastal USA. Except Florida and northeastern regions, the category 3 hurricanes would increase for most coastal USA. On the other hand, for the cold period, there is generally a reduction of intense hurricanes with the exceptions of Florida and northeastern regions. 2. Effects of Super Typhoon Haiyan on electric utilities Because of the lack of storm data, the projected intensity of Haiyan 352 Sustainable Cities and Society 35 (2017) 350–364 K. Dai et al. Fig. 4. US Hurricane Event Projection during AMO. Fig. 5. Continuing Average Annual Sea Surface Temperature and Hurricane Events. 353 Sustainable Cities and Society 35 (2017) 350–364 K. Dai et al. Eastern Samar Island (Bricker et al., 2014; Kennedy et al., 2016). Based on post-disaster field observations, Yi et al. (2015) reconstructed a storm surge map for Tanauan, which does not match the storm surge hazard map provided by the local municipal office to the local residents. Kim (2015) used global climate modeling tools and simulated significant wave heights during Haiyan at 30 m in outer West Pacific. Finally, Nakamura et al. (2015) compared numerical modeling and actual field observations of storm surge and suggested that the storm surge height had reached 5 m at Tacloban on November 8. Other storm surge related studies involved the investigation of the coral reefs both as an indicator of the damaging effect of the storm waves (Engel, May, Brill, & Brueckner, 2015) and as a modifier of wave energy (Shimozono et al., 2015). Shimozono et al. (2015) suggested that the combined wave height from infragravity waves and sea swell can reach as high as 16–20 m (near Matarinao Bay). Based on our own field observations, the storm surge with wave setup effects can be much higher than 5 m at locations. This is especially true for the Port of Tacloban where coaster ships that did not evacuate from the shallow strait (with a spacing less than 2 km between Leyte and East Samar Islands) during the typhoon were forced to shore on the coastline of Balangay 70 in Tacloban. Some of the ships docked at shore sites are at least 2 m above average sea level – indicating that the wave height must be much higher to create the buoyancy that moved the ships. The Tomi Elegance (IMO: 8351508), for example with self-weight of 249 tons has a 600 dwt (deadweight tonnage) capacity, would have a draft of at least 5 m. Hence, the inundation height at the Port of Tacloban should be higher than 5 m. As part of the Cancabato Bay of the San Juanico Strait, Tacloban is uniquely located within a siting where the northeast traveling wind driven waves from the Pacific East can streamline into the Port of Tacloban and increase the water volume within the strait without encountering much obstruction. The San Juanico Strait is about 38 km long and limits the flow of the waves until reaching Babatñgon (Janbatas Channel), which opens into the Samar Sea. San Pedro Bay also has a particularly shallow sea floor with an average depth of about 20 m and a maximum depth of 36.6 m (Campos, 2003). As shown in Fig. 6, the direct waves and the reflected waves from East Samar were based on satellite observations was weaker than the actual ground observed intensity. Hence, the observations of structural damages collected on the ground are important data that can help establish the effects of Haiyan on built structures and supplement other disaster observations (Shimozono et al., 2015; Tajima et al., 2014). Disasters associated with Haiyan included heavy rain, storm surge, flooding, strong wind forcing, heavy debris accumulations and landslides, etc. As a result, many structures have experienced multiple damage scenarios (Chen et al., 2015). Other than wind loading, there is also evidence of high storm surges associated with inundations as well as high wave actions. Extreme water surface elevations during a storm surge can be amplified locally by regional bathymetries (Kennedy et al., 2011; Mori et al., 2014). However, extremely high resolution topography, bathymetry and atmospheric data are needed to accurately hind cast the actual surge heights. The near shore wave phenomenon associated with a tropical storm wind is complicated by time-varying incident forcing and wave-induced motions. As a result, the storm surge is a complex compilation of infragravity motion (short wave forcing), static sea swells, and long waves driven by winds (Contardo and Symonds, 2013; Holman and Bowen, 1979). Despite several satellites monitoring data, there are no actual measurements of the storm surge heights during Haiyan (Nguyen et al., 2014). The storm surges were estimated at 5 m in most climate models; however, considering local conditions, the wave height can be as high as 20 m at some locations, especially off Eastern Samar Island (Bricker et al., 2014; Kennedy et al., 2016). To determine the actual storm surge level during Haiyan, several studies have been conducted: Takayabu et al. (2015) used high resolution regional wave model to estimate the worst climate change effect in Tacloban and resulted in 5.15 m of surge; Lee and Kim (2015) used a comprehensive hydrodynamic model that included meteorological conditions and wave induced dissipations stress from wave breaks, whitecapping and wave breaks from variable ocean floor depths, to simulate storm surges; using historical data, Lapidez et al. (2015) determined vulnerable areas in Philippines using simulated Typhoon Haiyan-level storm surges. The storm surges were estimated at 5 m in most climate models; however, considering local conditions, the wave height can be as high as 20 m at some locations, especially off Fig. 6. Theorizing Landshoring of Cargo Ships at Tacloban as a Result of Wave Actions during Haiyan. 354 Sustainable Cities and Society 35 (2017) 350–364 K. Dai et al. Fig. 7. Communication Tower Collapse at Dulag. (Ormoc-Babatngon) owned by the National Grid Corporation of Philippines and step down to distribution lines. In Tabloban, most distribution line pole structures are either wood or concrete poles. Significant number of distribution/transmission pole structures has been damaged by wind and wave impacts, especially close to the coastal areas. Some mountain top transmission towers also sustained structural damages, but no complete destructions. Because coconut trees are the predominant regional hard wood tree types and many of the trees were brought down by the storm, there was a huge amount of debris, which also impacted the distribution lines. The combined effects from the storm on the power delivery systems resulted in an overhaul of the entire system at the disaster area. Forensic investigation of the failed power structures in Leyte and East Samar indicates that the failure profiles of pole structures can include tilting (leaning), buckling, uprooting of foundation, etc. Fig. 10 shows examples of failed distribution lines pole structures. Also shown in Fig. 10, the conductor cables can play critical roles to the loading and stability performance of the poles. There were buckled pole structures such as shown in Fig. 10c, which is a pole structure close to the Tacloban airport. However, due to the massive debris accumulation, it is not certain if the pole breakage was due to debris impacts or actually by flood water. Post disasters images from several internet sources showed many power pole structure failures (regardless of material type). forced together at the Port of Tacloban and resulted in strong waves that brought ships on land. Such convergence of waves unfortunately cannot appear in climate models which typically have a resolution of 1 km or higher. To study the effects of Haiyan on the power delivery systems, observations were made on both distribution and transmission structures at various locations on both Leyte and East Samar. Current reported study on Haiyan is based on data collected during a trip made in 2014 and only considered failed power and communication structures with forensic investigation of failed structure damages based on wind and flood water effects (Chen et al., 2016). A notable observation is the water damaged structures can reach as far as 1000 m from the coast. This is because many of the storm surges ran up the riverine system that connects to the Leyte Gulf. More detailed studies on storm damage effects on electric power systems for other storm events have been studied such as by Kwasinski, Weaver, Krein, and Chapman (2006). First of all, the strong wind forces are evident from the destruction of several communication structures near Tacloban and in East Samar: Fig. 7 shows a communication truss tower near the coast of Dulag that was damaged by the strong hurricane. The location is about 200 m from the coast and did not experienced storm impact. Protected by surrounding housing, the structure failure is all due to the strong wind. Also shown in figure is the coastal destruction near the tower. Fig. 8 shows examples of damaged communication towers including poles, truss poles and truss structures from both Leyte and East Samar. It is especially interesting regarding the structure shown in Fig. 8b, which experienced both severe wind load and water damages. Close ups shown in Fig. 9 revealed that the structure experienced separation of the base plate from the foundation indicating the strong insufficient base plate thickness to resist against structure pull out. There is also evidence that shows that the structure was immersed in inundating water for some time. The regional power supply is through a 138 kV transmission line 3. Conventional wind and flooding design considerations Most US power transmission and distribution structure designs are based on ASCE 7 (2013) wind and flood load provisions. The basic wind and flood requirements for structural design of transmission systems are derived from statistical and historical analyses of severe weather events. The current design method for wind uses a 50-year return period on “extreme radial ice thickness combined with 3-s wind velocities” shown by a base wind speed map for different design values to 355 Sustainable Cities and Society 35 (2017) 350–364 K. Dai et al. Fig. 8. Examples of Collapsed Communication (a) Pole, (b) (c) Truss Poles and (d) Truss Structure. represent the loading situation in different locations. The statistical analysis performed helps to determine how severe the weather loading should be based on how it has played out in the past as well as factoring in the potential increases due to anomalous weather events. The basis of design suggests a reliability-based design (RBD), which helps simplify the process of turning severe weather conditions into loading scenarios Fig. 9. Communication Truss Pole Structure Collapse at Tacloban near Picas- San Jose DZR Airport Road (Structure Experienced both Wind and Water Damages). 356 Sustainable Cities and Society 35 (2017) 350–364 K. Dai et al. Fig. 10. Distribution Line Pole Structure Failures in Tacloban. loadings. Tornados, microbursts, and downbursts are the qualified highintensity winds (HIWs). Tornado wind speed is based on published 30year return period map (Tescon, Fujita, & Abbey, 1979) where an F 5 tornado can reach wind speed as high as 142 m/s. While tornado wind is not the focus in current study, but possible induced tornado wind within a tropical storm has been recognized by researchers (Baker and Parker, 2009; Novlan and Gray, 1974). The design for flooding and storm surge varies from that of wind design. When designing for flood loads one must ensure that “structures shall be designed to resist flotation, collapse, and permanent lateral displacement due to action of flood loads” (ASCE 7). ASCE 7 (2013) also mentions that the effects of erosion and scour must be factored in with any flood loading of a structure. Flood hazard areas should be designed based on the design flood. Hydrostatic and hydrodynamic loads will be applied depending on the level of water in the area of the structure. Wave loads may be analyzed in three ways for the structure: the analytical method outlined in the code, an advanced modeling method, or based off of experimentation. The method described in the code first has one classify the wave zone as either a ‘V-zone’ or an ‘A zone’ having 0.9 m high or higher waves or less than 0.9 m waves, respectively. It is obvious that this wave height definition is not sufficient to address the storm surge scenario of Super Typhoon Haiyan. For a transmissions system structure the main wave load to focus on is the load of breaking waves on vertical pilings and columns. To calculate the main wave load, one needs to determine the specifics of the structure and its specific location and distinguish whether it is a salt water or fresh water wave that will be acting on the structure. One more thing to know is the wave height that will be acting on the structure, which can be found in conjunction with the local still water depth. All of these variables will give the values needed to find the force of the wave and be able to design for any flood or storm surge event. Fig. 11 shows summaries of the design approach for wind and flood loading using ASCE 7. It should be noted that the load designs only that can be used for design and analysis purposes. ASCE-7 is a generalized minimum design loading guide meaning it is suitable for any type of structure. Along with using the wind speed maps to find the base wind speed for the area of a structure, the design procedure requires one to take into account many other factors for the wind design requirements for a power transmission structure. The first step in the analysis is to assign a risk category to the structure. This is done based on how devastating the failure of this structure would be and is described in ASCE 7. One must also factor in the wind directionality, which is the effect of the shape of a structure on how it is loaded by the wind forces. How the surrounding area in which the structure will be exposed to wind force is also considered – This is done by selecting an exposure category for the location. These categories are described in ASCE 7 and are used to determine which maps and equations are used in the wind load design process. Another effect that needs to be factored in is the effect of the topography surrounding the area of the structure. ASCE 7 lays out different scenarios that the site topography may affect the loads to the structure and the specific sizes of said topographical effects. The localized gust factors that need to be factored in to account for the dynamic fluctuations of the equivalent static wind force also considered (for example, taken as 0.85 for rigid buildings). Another trait of the structure that needs to be used in the design is the enclosure classification of the structure. For transmission systems most will be classified as open and some will require protection for glazed openings depending on their geographical location. All of this variability plays into the design for wind of these structures and from all of these variations one can follow through ASCE 7 to select the appropriate coefficients, factors, and equations to correctly find the respective wind loads at the respective heights of a transmission system structure. Wind design for power transmission structures is further described in ASCE 74 (2010), where wind effects are further distinguished into extreme wind, high-intensity winds and ice and wind combined 357 Sustainable Cities and Society 35 (2017) 350–364 K. Dai et al. Fig. 11. ASCE 7 Load Design Approach for Wind and Flood. under current climate change scenario. considers the effects on a single structure and does not account for the interactions from the surrounding structures. The RBD approach allows ASCE 7 to account for the likelihood of a wind or water (flood) dominated design using factored load combinations. However, RBD is based on historical events and uncertainty of an extreme environment is not easily projected in its development. The ASCE 7–10 (2013) code has a maximum sustained wind speed of less than 90 m/s with return periods of 300 years, which may already be unrealistic for some coastal regions 4. Design considering climate change effects To address the resiliency of the power transmission systems under the effects of climate changes, most attentions are placed on the rapid recovery mechanisms including ensuring survivability by mechanisms to enhance the supply of power to the communities (Cooke, 2013). 358 Sustainable Cities and Society 35 (2017) 350–364 K. Dai et al. Table 2 Computation of Power Pole Design Requirements for Extreme Storm Surge Loading Scenario (Surge Height at 20 m, Risk Category IV, Salt Water). Height (m) Aeff (m2) Force (kN) D at top of water (cm) D at bottom (cm) M1 (kN m) M2 (kN m) Moment (kN m) 9.1 12.2 15.2 18.3 21.3 24.4 27.4 30.5 36.6 3.8 5.4 7.2 9.2 10.8 11.9 13.0 14.1 16.3 3616 5137 6817 8654 10212 11249 12286 13324 15398 33.6 33.6 33.6 33.6 36.0 41.5 47.0 52.5 63.4 50.1 55.5 61.0 66.5 72.0 77.5 83.0 88.5 99.4 13279 23608 36887 53117 68087 78463 88839 99215 119967 2168 5139 10037 17344 22699 22699 22699 22699 22699 15447 28747 46924 70461 90786 101162 111538 121914 142666 Prevention of disastrous outcomes from super typhoon impacts would require significant improvement of power delivery systems that includes hardening against potentially extremely high wind speed and storm surges. Using high resolution cloud-resolving global climate modeling, Tsuboki et al. (2015) suggests that the significant future super typhoons could attain wind speeds of 85–90 m/s. Most studies focused on the estimation of impacts of increased wind loads or flood loads including Davidson, Liu, Sarpong, Sparks, and Rosowsky (2003), Eidinger and Kempner (2012), Shafieezadeh, Onyewuchi, Begovic, and DesRoches (2013), and Pang, Chen, Liu, and Holmes (2013), which relies mostly on projection of historical storm data. A study on the effects of significant hurricanes of 2004, Cannon (2005) reported that most wood distribution and transmission poles would not have survived the wind forces driven by Category 4 and 5 hurricanes. The damaged pole systems reported are consistent with what we have observed in the Philippines, but the amount of pole structures damaged is more extensive. To enhance the resiliency of power delivery systems under climate change effects, EPRI (Electric Power Research Institute) described three critical elements of resilience: prevention, recovery, and survivability. EPRI (2013) further suggested the hardening of the power distribution system to counter climate effects. Citing increasing Category 4 and 5 hurricane events from 1970 to 2004, Peters, DiGioia, Hendrickson, and Apt (2007) recognized the challenges in upgrading and refurbishing existing power lines and suggested a survivability design strategy by developing a strong (hardened) backbone line system with improved structural reliability to enhance the resilience of the power infrastructure. The strong backbone line system approach allows focused investments from the energy industry to enhance the survivability of the power grid under extreme loading. The implementation of such design would require considerations at the system level – the grid manager needs to define the extent of the backbone line within the power grid. At the structure level, one would need to have the different loading quantification for different reliability levels. There is a general agreed difference in criticality between power distribution and transmission: Since existing power transmission systems lack redundancy due to limited alternate paths for electricity flow when compared to distribution systems, they are more critical to the electric infrastructure reliability. Pang et al. (2013) simulated a 230 kV transmission line system under simulated high wind hazard. The modeling technique included realistic line and support structure system modeling and was able to project critical failure locations within the line system. The modeling approach would help realize the system level vulnerability and resiliency and determine the critical geospatial information to establish the backbone system. However, the modeling technique does not include windborne or waterborne debris hydrostatic and dynamic loadings. Table 3 Pole Moment Capacity and Extreme Loading Moments Comparison (Including 10 m Storm Surge Scenario). Height (m) M (kN m) from Wind Loads M (kN m) from 10 m Storm Surge M (kN m) from 20 m Storm Surge Cracking Moment Capacity (kN m) Ultimate Moment Capacity (kN m) 9.1 12.2 15.2 18.3 21.3 24.4 27.4 30.5 36.6 3042 4763 6889 9448 12464 16077 20170 24953 36629 7724 14607 25037 39765 52487 57675 62863 68051 78427 15447 28747 46924 70461 90786 101162 111538 121914 142666 159 205 268 325 370 419 470 526 651 378 498 643 785 919 1056 1192 1329 1600 accumulations that can cause damages to both distribution and transmission line structures. Hence, considering the uncertainties in climate changes and the effects from extreme cyclone events such as Super Typhoon Haiyan, a holistic approach is needed to determine how conventional wind and flood loading scenarios can be modified to establish a framework for structure system design to deter the damaging effects from such storm events. To help understand the effect of extreme loading scenario for transmission line systems, an analysis using a wood pole structure is conducted. Table 1 shows the calculation of possible extreme loadings on a single pole using worst scenario of a 89.4 m/s wind speed and for a risk category IV and exposure category D. Both with and without powerline wind loads are considered in the calculations for up to 36.5 m high poles, which show that the extreme loadings may exceed the design capacities of most commonly used wood pole equivalent materials (wood, concrete and steel) available. Table 2 reports results for calculation of extreme storm surge height (20 m) effects on poles. Again extreme risk category (IV) is assumed. The load scenarios are considered separately; thus, are not representative of RBD design. Also wind/waterborne debris loading is not considered. Table 3 compares typical wood pole equivalent capacities where the extreme loading moment capacities would disqualify many pole classes and demand higher and stronger structures. To address extreme storm event, we suggest a design framework adopting the backbone system suggested by Peters et al. (2007). The backbone system approach recommends a hardened and critical primary power transmission system (strong line) coupled with a flexible design of secondary transmission and distribution power lines (flexible line) that, though not as structurally strong as the backbone system, would allow for rapid site access and effective line repair. The primary power transmission system represents a relatively permanent power delivery unit that would be strong enough to resist any speculated weather effects due to climate change scenarios and is suggested to design against a wind effect set at 89.5 m/s–111.8 m/s wind force and 20 m storm surge. With the hardened backbone system, there will be no extensive power outage and would allow for rapid power recovery for 5. Framework of a holistic design consideration From Super Typhoon Haiyan, we summarize that the category five storm can result in significant storm surge, strong winds and debris 359 Sustainable Cities and Society 35 (2017) 350–364 K. Dai et al. Fig. 12. Tree Down Analysis in Tacloban: Estimated over 44 Million Coconut Trees were Destroyed due to Haiyan with over 300 Million Remaining (Durst 2015). deforesting effect can result in significant loading on the structures. For the non-backbone transmission and distribution systems, multihazard loading conditions for coastal power systems should consider as load combination using the RBD approach. Hence, a detailed probabilistic analysis of different loading scenarios should be conducted to establish the RBD factored loadings (Fenton and Sutherland, 2011; Goodman, Vanderbilt, & Criswell, 1983; Goodwin, Mozer, & Digioia (1983); Peyrot and Dagher 1984). Current design wind velocity considerations identify the terrestrial conditions and the cluttering (buildings) of a site and divide into different exposure categories. However, such consideration works for a uniform wind over a landscape. Using Google Earth™ roof damage images, the static wind force projection can be determined which shows significant disturbance near the most populated areas in Tacloban. Fig. 13 shows the wind direction spatial display and the path of Haiyan. Finally, Fig. 14 summarizes the holistic power delivery system design and includes a strong backbone power transmission system (strong line) couples with flexible power transmission and distribution system (flexible lines). The backbone system should be designed based on extreme loading where storm effects should include revised wind and water (storm surge and flooding) loadings based on extreme wind speed and storm surge height. The flexible line should be designed based on factored loads that include wind and water loadings and wind and waterborne debris impacts. A detailed evaluation of design guidelines for power transmission and distribution systems is ongoing to generate more specific data for formulating the holistic design framework. More storm damaged structural data are also being collected to provide critical information that can help us understand the effects of super typhoons. nearby distribution lines. The extent of the design for the backbone system is an issue of economy and necessity. Using the holistic approach, the design of strong line structures should consider expanding the cleared right-of-ways from surrounding tress or structure and that the structures be placed on high grounds to minimize flood and debris loadings. Since power transmission structures are further away from urban areas, hence, have less distinct damage modes than distribution systems, i.e. less windborne or waterborne debris attack and less friction from surrounding ground cover, but more direct wind impacts. For windborne debris/projectile/missile impacts on structures, Vickery, Lin, and Twisdale (2003) and Lin and Vanmarcke (2008) have studied the extent of windborne debris/projectile impacts from hurricanes. However, both studies have not considered heavily wooded regions. Thus, those studies are still inadequate in providing a comprehensive view of damaging effects of an extraordinary hurricane. Hatzikyriakou et al. (2016) has pointed out the significance of waterborne debris impact on residential structures. Lessons learned from Super Typhoon Haiyan indicate that extreme wind speed, unprecedented storm surge heights, which run much further inland than typical typhoon events in the area and extensive debris accumulations are some of the critical factors that exceed current design requirements. There needs to be a quantification of the effects of solid wastes, debris and projectiles from trees down – this is especially critical for heavily wooded areas. One characterization of Super Typhoon Haiyan is the extensive amount of trees down that at some locations may exceed 90% of the original standing trees. Fig. 12 shows the extent of fallen coconut trees in the Haiyan-affected areas. Super Typhoon Haiyan revealed that the accumulation of solid wastes such as debris due to significant 360 Sustainable Cities and Society 35 (2017) 350–364 K. Dai et al. Fig. 13. Final Damaging Wind Directions Based on Roof Damages from GoogleEarth™ (Tacloban City Characterization Including Cluttering of Multi-Story (Typically One to Three Stories)) Structures with Occasional Higher Structures with No High Rise Structures. Fig. 14. Holistic Approach for Resilient Power Delivery System that Utilizes the Strong Backbone Transmission System Concept. 361 Sustainable Cities and Society 35 (2017) 350–364 K. Dai et al. Fig. 15. Strong Backbone Transmission Line Proposed for McClellanville, SC, Considering Loading at 89.4 m/s Wind Speed, Risk Category IV, Exposure Category D with no Storm Surge nor Debris Loading. 6. Economic and feasibility analysis of power supply system for McClellanville, SC Comparing to Table 2, the 20 m storm surge effect only increases the applied moment load by 25% to the 89.5 m/s wind force. For additional reliability, US National Academy Council recommended the purchase of the 365 kV:138 kV/200 MVA (RecX) transformer technology for rapid recovery during natural disasters (Cooke 2013). With a price tag of $7.5 million, such recommendation would be a challenge for the people of McClellanville. To determine the cost scenario and feasibility of the holistic design approach, the power transmission and distribution scenario at McClellanville, South Carolina, is considered. McClellanville is a small coastal town along the Atlantic west near Charleston, SC. The community has very small electrical distribution loads, as of 2016, it does not have any major transmission infrastructure. In 2005, a power supply alternative study was conducted by local power company and indicated that the construction of a power transmission line is the cheapest cost scenario to supply the township with sustained electricity. The study included all possible scenario plays including the construction of a local power generation, upgrade of existing distribution lines or substations. In all cases, the projected 30 years loss exceeds US $100 million (CEPC, 2005). Since the study, McClellanville was hit several times by major hurricanes in the area – notably and more recently, Matthews in October 2016. Hurricane Matthews resulted in more than a million dollar loss associated with the power grid failure. If possible future extreme climate events were to occur during next 30 years, the projected loss may push beyond US $200 million. A strong backbone, 115 kV transmission system for a 26 km stretch connecting from Belle Isle substation to McClellanville is suggested. Fig. 15 shows the proposed 26 km backbone transmission line extending from Belle Isle substation to McClellanville distribution point. The average distance of the line to the coast is about 8 km and eliminated possible loading due to debris and water inundation (historical flooding due to riverine and watershed rising did extend beyond Georgetown and Berkeley counties). Using the holistic backbone line design approach (Fig. 14), the strong backbone transmission line is sited away from likelihood of both storm surge and debris loadings and involved the construction of lattice self-supporting towers with additional guy lines that can withstand the extreme wind load moments outlined in Table 1. The total cost of the construction of the line is about $26 million (Western Electricity Coordinating Council recommendation for capital cost for per mile lattice transmission) and the projected 30 year total cost with loss is about $133 million. This is substantially lower than the 30 years cost ($163 million) of only upgrading the local distribution system without considering the effect of extreme climate effects. 7. Conclusions Extreme climate effects can result in severe damages to critical infrastructures. But the design against extreme events has not been addressed in current design codes. This paper summarizes the overall observations of power delivery systems investigated during recent Hurricane Haiyan in the Philippines. Haiyan is the world’s most powerful typhoon to date and has been deemed as a likely scenario of climate change induced tropical storms of the very near future. At the time of this study, two additional category 5 hurricanes have already occurred: Patricia (maximum one minute sustained wind speed of 96 m/s) on October 20, 2015 and Matthew (maximum one minute sustained wind speed of 72 m/s) on September 28, 2016. An understanding of the impacts of Haiyan can help in the improvement of the design of power systems that can help ascertain the survivability of coastal populations. Current power delivery system is designed on the basis for physical strength. Based on the Haiyan observations, a framework for designing a resilient power delivery system is suggested. The framework recommends a strong backbone power transmission system (strong line) coupled with flexible power transmission and distribution system (flexible lines) and holistic considerations of different damaging mechanisms. The strong backbone system should be designed based on extreme loading where storm effects should include revised wind and water (storm surge and flooding) loadings based on extreme wind speed and storm surge height. The flexible line should be designed based on factored loads that include wind and water loadings and wind and waterborne debris impacts. A case study for the power supply scenario in McClellanville, SC, is presented to demonstrate the feasibility of using the strong backbone system which consists of a self-supporting 115 kV transmission line supported on guyed trusses. 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This case study demonstrated that it is possible to establish a sustainable power transmission system that can help deter the effects of extreme weather disasters. While scientists continue to investigate the causes of extreme weather events such as supertyphoon Haiyan, designs against extreme conditions should be included for coastal populations to prevent escalating financial losses and losses of human lives. Lessons learned from Haiyan and similar storm events should continue to help improve the performance of the power delivery systems. Acknowledgements The authors would like to acknowledge the support from ASCE (American Society of Civil Engineers) on the Haiyan project. The authors would like to acknowledge National Science Foundation (NSF) Program Director Dr. Kishor Mehta for supporting this project under RAPID Response award 1433262. The authors are grateful to the local support from the Philippines India including PICE President Ernesto S. 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