Flood & Dam Inundation Hazards

According to flood maps prepared by the Federal Emergency Management Agency, no part of Monterey Park lies within a one hundred year flood zone.[2] Regional drainage improvements adequately protect the city from flooding associated with major storm events. Also, the city's distance from the Pacific Ocean minimizes exposure to tidal wave (tsunami) hazards resulting from off shore earthquakes.

The only flood hazards of concern involve Garvey Reservoir and the Laguna Basin (see Figure SCS-4 (PDF)). Garvey Reservoir, owned by the Metropolitan Water District of Southern California (MWD), stores municipal water supplies for MWD customers. The Laguna Basin is a flood control facility integral to the regional system maintained by the Los Angeles County Department of Public Works. A major seismic event has the potential result in dam failure or seiche conditions at these facilities. A seiche can occur as a result of ground vibrations initiating water wave motion. If wave amplitude is high enough, the water may slosh over the shore or barrier containing the water body and flow onto surrounding properties.

Inundation Zones

Figure SCS-4 (PDF) identifies the flood inundation areas for Garvey Reservoir and the Laguna Basin that would result from dam catastrophic failure. As illustrated for the Laguna Basin, the inundation area is limited to the interchange of Interstate 710 (Long Beach Freeway) and Interstate 10 (San Bernardino Freeway). Thus, private property within the city is not threatened by this hazard.

Dam Flood & Failures

Garvey Reservoir lies impounded behind a north dam and a south dam. MWD completed a substantial overhaul of the facility in 1999 to address seepage and ensure overall reservoir integrity. The state Department of Conservation, Division of Dam Safety conducts periodic dam inspections to verify the dams' ability to withstand seismic stresses. In the unlikely event of a conjectured catastrophic failure at Garvey Reservoir, properties to the north and south could be flooded. The estimated average flood depth is five feet. As Figure SCS-4 (PDF) shows, failure of the north dam would create two flood zones:

• The first affecting the steep, undeveloped valley immediately east of the reservoir
• The second flowing north, impacting properties roughly between Alhambra and New Avenues to Garvey Avenue

If the south dam failed, flood waters of average depth six to seven feet would cascade down the slope bank and into the residential neighborhoods below. At the Pomona Freeway, the water would spread laterally along the north side of the freeway before flowing through freeway undercrossings.

Footnote

The 100-year flood is defined as that major flood event that has a 1% or greater chance of occurrence in any one year. Flood hazard planning practices addresses such storms, as well as, for example, 50-year and 500-year events.