Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk Assessmentlead to increased coastal flooding. A channel in a reef or a location where a reef folds into acoastline can serve as funnel for large amounts of water to come onto the shore in the form oflarge waves. This phenomenon can also result in increased levels of coastal flooding.Storm surges can also result in coastal flooding in urban areas by causing storm drains to back upin low-lying areas. When a storm surge elevates the sea water to the same elevation as or abovethe elevation of storm drain outlets that drain into the ocean, the storm drains are not able todrain, and the storm runoff backs up into the storm drains and onto the nearby streets.Riverine Flooding and Stormwater Runoff: The most common type of flooding is riverineflooding, also known as overbank flooding, refers to fresh water sources. Riverine floodplainsrange from narrow, confined channels in the steep valleys of mountainous and hilly regions towide, flat areas in plains and coastal regions. The amount of water in the floodplain is a functionof the size and topography of the contributing watershed, the regional and local climate, and theland use characteristics of the floodplain. In steep valleys, flooding is usually rapid and deep, butof short duration, and in flat areas flooding is typically slow, relatively shallow, but can last forlong periods of time.On Guam, flooding in rivers and streams is typically caused by prolonged periods of rainfallfrom tropical cyclones or monsoon surges. These systems can saturate the ground and overloadthe rivers and reservoirs in numerous smaller basins that drain into larger rivers. Intense rates ofrainfall (e.g., 1 inch per hour) can lead to flash flooding. Flash flooding is most likely to occur invalleys in mountainous areas. The northern ecoregion of Guam has essentially no streamdrainages because the area consists primarily of permeable limestone, which results in rapidinfiltration of water even during heavy rainfall. Little or no surface water runoff occurs in thisarea. However, the southern ecoregion of Guam has a dense network of streams. This areaconsists primarily of volcanic rocks, which have limited infiltration capacities.Heavy rainfall and the associated flooding can cause large amounts of soil and debris to enterinto rivers. Debris can also enter rivers if it has been blown there by severe winds (see Section5.3.11 [Severe Wind]). Debris that becomes a part of floodwaters can cause damage to theculverts and bridges that floodwaters are either flooding through or around. This type of debriscan completely dam bridges, culverts, and other drainages and cause floodwaters to bypass thesestructures and enter into areas that typically do not receive floodwaters. The dammed drainagescan suddenly give way and cause flash-flood conditions farther downstream. In addition, largeamounts of soil and debris can become deposited on the reefs surrounding tropical islands. Thisdeposition of silt and debris is highly detrimental to the health of the coral polyps that create andmaintain the reefs.Although heavy rainfall events associated with tropical cyclones, particularly typhoons, andmonsoon surges are well documented and acknowledged as a major hazard, Guam alsoexperiences heavy rainfall events that are not associated with tropical cyclones or monsoons.These events result in significant stormwater runoff that may overwhelm local stormwatersystems and local river and stream systems, if any, and cause localized flooding. Although thistype of flooding is recognized as a hazard, the areas subject to this type of subhazard have notbeen well documented to date and are not identified on FIRMs because these events are typicallyrelatively small in size and extent.Flooding due to stormwater runoff or street flooding often occurs when storm drains cannotconvey the amount of water that could flow through them. This hazard can be due to high rates5-23