Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk Assessment5.3.1 Coastal ErosionNatureCoastal erosion can be described as the horizontal retreat of the shoreline. It is a part of a largerprocess of shoreline change that includes erosion and accretion, except along coastal cliffs.Coastal erosion is the movement of sediment from the shoreline into the ocean. Accretion is themovement of sediment onto a shoreline from the ocean. Many shorelines experience both erosionand accretion. If a balance of these two processes occurs, the shoreline is considered to be stable.Coastal cliffs generally erode in the form of a landslide into the ocean. Coastal cliffs cannotexperience accretion.Due to the potential cycles of erosion and accretion, coastal erosion is generally quantified overseveral years. Coastal erosion is measured as a rate, expressed either as a linear length of retreatcompared to time or as a volumetric loss compared to time.Coastal erosion on Guam can be caused by winds; ocean currents; storm surges; high surf;seismic activity; changes in the geometry of tidal inlets, river outlets, and bay entrances; manmadestructures and human activities, such as shore protection structures and dredging; and/orlocal scour around structures. La Nina and El Nino events also contribute, with El Nino causinglower sea levels but increased tropical cyclone activity, while La Nina causes less tropicalcyclone activity, but higher background sea levels. In addition, sea-level rise has an effect oncoastal erosion. Sea-level appears to have risen about 8 inches over the last century, with greaterrises over the last two decades.Human-built structures, such as properly engineered shore protection structures, can decrease therate of coastal erosion. Areas that are exposed to prevalent winds and open ocean waves oftenhave a higher potential to experience heavy coastal erosion than sheltered areas. The erosion ofcoastal cliffs can threaten the safety of land uses at the top of the cliffs. Coastal erosion can leadto sediment transport onto nearby reefs, which can lead to the decline of the health of these reefs.LocationThe entire coastline of Guam has the potential for coastal erosion hazards. The western coast ofGuam has experienced the most coastal erosion to date due to tropical cyclones and monsoonsurges that have produced high waves.Previous OccurrencesNo disaster has been declared on Guam due to coastal erosion. No comprehensive documentationis available regarding coastal erosion on Guam, and damage estimates due to coastal erosionhave never been specifically reported. As illustrated by the above discussion of the causes ofcoastal erosion, coastal erosion is almost always associated with another hazard. Many largetropical cyclones have made landfall on Guam or have come close to making landfall. Thesestorms all have resulted in storm surges, high surf, and high winds, all of which are key causes ofcoastal erosion. However, available historical records describe coastal erosion occurrences foronly a few storms. Therefore, it is probable that incidences that have caused coastal erosion havebeen severely underreported.Typhoon Andy in 1982, Typhoon Dale in 1996, Typhoon Halong in 2002, and Super TyphoonPongsona in 2002 were all documented to have caused coastal erosion. No specific details areavailable about the locations of coastal erosion for Typhoon Andy. Typhoon Dale contributed to5-6