Glendale (PDF) - Hazard Mitigation Web Portal - State of California

Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 8 – Floodsimproved channels. During periods of urban flooding, streets can become swift movingrivers and basements can fill with water. Storm drains may also back up withvegetation and debris causing additional, localized flooding.• Debris Flows: Another flood related hazard that can affect certain parts of thesouthern California region is debris flows. Debris flows most often occur in mountaincanyons and at the foothills of the large mountains that serve as backdrop to the area.However, any hilly or mountainous area with intense rainfall and the proper geologicconditions may experience one of these very sudden and devastating events.Debris flows, sometimes referred to as mudslides, mudflows, or debris avalanches, arecommon types of fast-moving landslides that generally occur during periods of intenserainfall or rapid snow melt. They usually start on steep hillsides as shallow landslidesthat liquefy and accelerate to speeds that are typically about 10 miles per hour, but canexceed 35 miles per hour. The consistency of debris flows ranges from watery mud tothick, rocky mud that can carry large items such as boulders, trees, and cars. Debrisflows from many different sources can combine in channels, and their destructive powermay be greatly increased. They continue flowing down hills and through channels,growing in volume with the addition of water, sand, mud, boulders, trees, and othermaterials. When the flows reach flatter ground, the debris spreads over a broad area,sometimes accumulating in thick deposits that can wreak havoc in developed areas.Dam Failure FloodingSeismically induced inundation refers to flooding that results when water retention structures(such as dams) fail due to an earthquake. Failure of these structures can also result from othercauses, such as overtopping, foundation problems, or construction errors. Statutes governingdam safety are defined in Division 3 of the California State Water Code (California Departmentof Water Resources, 1986). These statutes empower the California Division of Dam Safety tomonitor the structural safety of dams that are greater than 25 feet in dam height or have morethan 50 acre-feet in storage capacity.Dams under State jurisdiction are required to have inundation maps that show the potentialflood limits in the remote, yet disastrous possibility, that a dam is catastrophically breached.Inundation maps are prepared by dam owners to help with contingency planning; theseinundation maps in no way reflect the structural integrity or safety of the dam in question.Because dam failure can have severe consequences, FEMA requires that all dam owners developEmergency Action Plans (EAP) for warning, evacuation, and post-flood actions. Although theremay be coordination with county officials in the development of the EAP, the responsibility fordeveloping potential flood inundation maps and facilitation of emergency response is theresponsibility of the dam owner. Dam owners are also required to prepare and submitemergency response plans to the State Office of Emergency Services, the lead State agency forthe State dam inundation-mapping program. Cities and counties are required by State law tohave in place emergency procedures for the evacuation and control of populated areas within thelimits of dam inundation. In addition, recent legislation requires real estate disclosure upon saleor transfer of properties in the inundation area (AB 1195 Chapter 65, June 9, 1998; NaturalHazard Disclosure Statement).There have been a total of 45 dam failures in California since the 19 th century. The mostsignificant dam failures in southern California are listed in Table 8-3, and the two mostsignificant dam failures, St. Francis Dam in 1928 and the Baldwin Hills Dam in 1963, aredescribed further below.Table 8-3: Dam Failures in Southern California2006 PAGE 8- 13