Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk Assessmentactive faults have been identified, and these faults do show evidence of movement that is likelyQuaternary or even late Quaternary.Liquefaction and Lateral Spreads: Specific data, including depth to water table, boring logs,blow count information, and detailed age data for the geological units, none of which areavailable for Guam, are generally used to determine the probability for liquefaction and lateralspreading to occur. However, as described above, based on recorded occurrences from 1975-2011, an earthquake is likely to be felt on Guam every 4-5 years. Liquefaction and lateralspreading on Guam may occur as a result of these earthquake events.5.3.5 FloodingNatureFlooding is one of the most common natural hazards; it occurs whenever rainfall accumulates inan area faster than it can drain off or can be absorbed by the soil. This accumulation causes anoverflow from a water body onto an adjacent floodplain. However, all floods are not alike, anddifferent areas are susceptible to different types of flooding. Guam is vulnerable to coastalflooding, riverine flooding and stormwater runoff, and flash flooding. Flooding on Guam is oftenassociated with tropical cyclones; this connection is described below, but for further informationspecific to tropical cyclones, see Section 5.3.15 (Tropical Cyclone). Severe flooding can alsooccur without a tropical cyclone.Coastal Flooding: Coastal flooding in the western North Pacific Ocean is primarily due toinundation from ocean water that is associated with storm surges and wind-driven waves. Astorm surge occurs when the seawater surface that is near to or under the eye of an approachingtropical cyclone is elevated in the shape of a mound. This event occurs because of the extremelow pressure that exists under and near to the eye of a tropical cyclone. The sea level can be asmuch as 5 feet higher than normal during a storm surge. The strong winds associated with atropical cyclone produce wind-driven waves that ride on top of the storm surge. With theaddition of an astronomical high tide, incursion of seawater onto a normally dry land area (i.e.,coastal flooding and inundation) can occur.When a tropical cyclone passes directly over a small island, the whole shoreline of the island canbe exposed to coastal flooding. Bays, river outlets, and reefs that occur on or close to a coast canexacerbate coastal flooding. The small size and restricted entrances of some bays can act to focusand contain the high water associated with storm surges and wind-driven waves. Strong windsoften drive this high water toward the shore; the result can be a coastal flooding event. Raisedsea levels can prevent rivers and streams from draining into the ocean and can cause the water inthe rivers and streams to back up. This backup of water can result in flooding near the outlet ofthese waterways.Generally, reefs have a damping effect on storm surges and wind-driven waves. The shallowerthe water over a reef and the wider the reef, the more damping the effects of the reef on a stormsurge and wind-driven waves. Conversely, the deeper the water over the reef and the narrowerthe reef, the more coastal inundation and coastal flooding that can occur. Coastal flooding andinundation are greatest when wind-driven waves riding on top of a storm surge approach a reef ina perpendicular direction. The force of the incoming storm surge can reduce the normal drainageof water across a reef and cause water to build up on the inside of a reef. This water buildup can5-22