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Home My WebLink AboutMulti-Hazard Mitigation Plan: 12. High Surf CIVIL DEFENSE AGENCY COUNTY OF HAWAII 920 ULULANI STREET HILO,HAWAII 96720 r e�-KT'Nc f 12. High Surf Chapter 12:Hazard Analysis—High Surf CHAPTER 12 - HIGH SURF 12.1 Description of Hazard High waves that inundate coastal areas are generated by strong winds or seismic events. The seismic-type wave is called a tsunami and covered in Chapter 9, on page 9-1. Damaging wind-generated waves occur from distant storms in the Northern and Southern Hemisphere, tropical storms, hurricanes, and localized Kona storms (see Figure 12-1).North-facing shores receive annual North Pacific swells in winter ranging from 10 to 20 feet. Usually damage- causing events from north swells are over 20 feet. Larger northeast trade waves are typically 2 to 4 feet; however, well-developed trade swells produce high waves of 6 to 8 feet that have caused damage. South-facing shores are exposed to Kona storms and southern swells, which have caused damage at heights of only 4 to 6 feet. Tropical storms and hurricanes affect any and all shorelines especially during summer and fall with damaging high waves of 10 to 30 feet. Hawaii b as WE!to 1651 Apr A,5.6 H a Hpn-.. aemxn Damaging high Upola Point waves* and 1 JanI,I 1990 Feb B-9 Rou Kohala 1e96 Fee s-+2x39 n high waves due ) WaiplO 993 9.91110 HFamen � 1596 Fee fbl]8-12nNNE _ s.H.owmaN,in Hm _t r +996 N.s 6 24 fi H awes,avrsaq. to hurricanes 1986 JenbJ+96-, nNCwall Kawalhae'WaTM Islantlwlde high wages _ ! oec 15- +919 29 High wvrc 1011—2129 an Hns Nov zl2t Hph— .1 10]6 Feb 9-1""euA 1015 H.vts iihw 1918.1.111-1a'o"_ 1010 J-11-16 Hlghsess Ioee Ma.1-11 Hier— zer. Keahole _ Hilo Onekahakaha 1902 11 N hH - 993�1s1eHF.me..W WastCwst/Kona Polnf _ = Beach 1954 Nw2B 11 % 1969 Jan 23 Illpn Fe sunveta6hwAn....... 19x9 F b1s elvn sun.r:99 c.eu Kaa- 1a68 e 20 Hl or au" Konona ,969.1", Hlgncunalm.l.uear 0 20mi 1x11 Jane".h wn Magic ' 1012 0.5596-]0 Son Sands - 5919 Ja.6-11lgh wneM9[eas1 P a Ka Iln ,915- I— ni9n sun.l Me9lc sn..aa Basco BCaCh {� 0 20km A.9H-16 Kp SYn 1996 Feb 6 Hsh Burt 11"ee.it Oplhikao 1066 Fab 22 Hqe iwt Nenr—I Beach Lakes and reservoirs 19e1 Ja.9-19 rW., Kefapana Kaimu 0-20001eet ,9925e.1+HMU.rty131.1 Jun zs-�" 25s r(t! South Puna Coast 2000-4000 feet rtam.n9m.nauer Sx+Bll',10-120 - 1912 1.16 Hlgn.udul 40006000 feel ,\:. 1962 au9 Is H"waves ec.ml 6000-8000 feel `~,� Kau :M N«e,110"-01 'Il—u.ream 8000-10.000 feel _ 1� 1919 Aug 11-M Su1251, Isar Jd 25 I" ane Kapet 10.000-12,0001ae1 1620 Ju11929nwH 1666 Jul ZZ-b 10.19n ISO 1 Coast � � liar,Jul l Il 6 wd low A°a'+-6 over 12,000 feel lees Hlen sun I9a5 Ju12510-1 sn sun +gas o6c�a1 1965 Jul d 1065 S..S0.Hqn cu0 H82 Sop Is H Q Urban areas 19re M., n,,,nyn 3ml 196e Ju 22-z3 eqn wd /,�, 1966 Aup 46; r —Hlghwaya 1969 Jrn e-11 n M.dazlertr:...e Streams 1M JW 10-20 Hph wri 1992 S.F 1 LIW —Canals ' S Int +9w] .9n.12-1sn H Hurricane TS Tropical storm 10 K Height 01 wri(teal) z-1s n 'Does not include high waves due 19 tsunamis c' Figure 12-1. Storm Wave Directions and Events for Hawaii Island Large wind-generated waves can also cause storm surges (or overwash). A storm surge is a rise in the water level caused by wind forces driving water against the coast (wind set-up) or 12-1 Hawaii County Multi-Hazard Mitigation Plan Chapter 12:Hazard Analysis—High Surf by wave forces (wave set-up). If the surge occurs at high tide, water height is even greater. The water rise enables the storm waves to reach further inland with the associated scouring and erosion caused by the wave forces. In studying the aftermath of Hurricane Iniki, the Corps of Engineers determined that wave set-up was the stronger component than wind set- up,unlike the mainland coastal areas.66 12.2 Significant Historic Events Hurricane Estelle illustrates an example of Hawaii County's susceptibility to wind and wave setup, as well as storm events. On July 22, 1986, the eye of Estelle passed over 100 miles south of Hawaii County. In addition to the high spring tide, high waves generated from Estelle, crashed on the shores of the Big Island. From available reports regarding Hurricane Estelle, major damage in Hawaii occurred at the Vacationland area. The high waves washed away 5 beachfront homes and severely damaged dozens of others. According to records at the Hawaii County Planning Department, 18 houses suffered minor damage that totaled $42,500. In addition, 12 houses had major damage that totaled $194,000 and 7 houses were completely destroyed with an estimated property damage of $160,883. 12.3 Probability of Occurrence High surf events occur quite frequently on all coasts of Hawaii County; however events which actually cause damage to property are far less common. The FIRM coastal zone flood classifications give probabilities of coastal flooding in zones that may be impacted by episodes of high surf as shown in Table 12-1. Although the coastal flood zones were not developed exclusively to address the impacts high surf (development of these zones is discussed in Chapters 9 and 10) they do provide a conservative delineation of areas that may be at risk. Table 12-1. FIRM Coastal flood zone classifications ZONE DESCRIPTION Coastal areas with a 1%or greater chance of flooding and an additional hazard associated with storm F V waves.These areas have a 26%chance of flooding over the life of a 30-year mortgage.No base flood elevations are shown within these zones. Coastal areas with a 1%or greater chance of flooding and an additional hazard associated with storm VE,VI -30 waves.These areas have a 26%chance of flooding over the life of a 30-year mortgage.Base flood elevations derived from detailed analyses are shown at selected intervals within these zones. 66 Fletcher,C,B.Richmond,E.Grossman,A.Gibbs,Atlas of Natural Hazards in the Hawaiian Coastal Zone, Prepared in cooperation with University of Hawaii,State of Hawaii Office of Planning,and National Oceanic and Atmospheric Administration,U.S.Geological Service Geologic Investigations Series I-2761, 2002.,p. 17. 12-2 Hawaii County Multi-Hazard Mitigation Plan Chapter 12:Hazard Analysis—High Surf 12.4 Risk Assessment The economic risk associated with the impacts of high surf is encompassed within the loss prediction and risk assessment of the flooding Hazard. 12.5 Mitigation Strategies Mitigation strategies to address the impacts of high surf are encompassed within the mitigation strategies of hurricane(storm surge), flooding, and tsunami hazards, since all of these will produce similar and likely larger impacts then high surf alone. 12-3 Hawaii County Multi-Hazard Mitigation Plan