Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk Assessment5.3.10 Salt SprayNatureSea salt deposition can occur throughout Guam. This hazard is caused by tropical cyclones andresults from two processes. The first process involves ocean spray that is carried upward by thesurface winds of a tropical cyclone to mix with rain. The concentration of salt in rain on Guam isunknown but is detectable. The second process occurs when ocean spray from large wavescrashing along the coastal cliffs is blown inland by severe winds. Under this process, the strongerthe winds the further inland the sea salt is deposited.Sea salt deposition can devastate agriculture and other plants, can cause heavy corrosion, and canaffect electrical facilities. Some of the effects associated with salt spray (the devastation ofagriculture and plants and power outages from shorts in electrical facilities) can be observedalmost immediately, but corrosion occurs over a long period, has a cumulative effect on thesurface it is affecting, and is difficult to observe immediately after a tropical cyclone.LocationAll of Guam is susceptible to salt spray, but areas near the shore experience the greatest effect.Previous OccurrencesLittle documentation is available regarding historical sea salt deposition events. Becausecorrosion is difficult to detect immediately after an event and even more difficult to attribute to aspecific event, no documentation is available for hazard events that cause corrosion. Therefore,sea salt deposition has likely had a much larger effect than historical hazard events suggest. AsWERI (1999) has stated, “It is likely that none of the island escapes the deposition of salt duringeven a weak tropical storm, but areas near shore feel the greatest effect.” Three historical saltsprayhazard events are described below.• February 14, 1864: Father Francisco Resano documented that a tropical cyclone caused croploss due to sea salt deposition.• November 23, 1992: When the eye of Typhoon Gay crossed Guam, the northern part of theeye-wall sheared off, resulting in little rain falling on the northern part of Guam but verysevere winds. Sea salt deposition was heavy across northern Guam. Salt was deposited onpower lines, transformers, pumps, generators, vegetation, and most other exposed surfaces.The vegetation of northern Guam was devastated and took nearly 4 years to recover.• January 24, 2000: “Strong winds caused by a surge in the winter monsoon moved acrossGuam. Several power outages were reported in the Cabras Island area because of salt sprayon insulators.”Probability of Future EventsA hazard event from salt spray is related to the combination of high winds, generally from atropical cyclone, high seas, and low quantities of rainfall. No standard method exists todetermine the probability of such an event. As discussed above, historical records of these eventsare rare (approximately once every 40 to 50 years), but the event is likely underreported.5-37