3.3: California Floods

In southern California, most flooding is the result of heavy precipitation over periods of one or two days. The short streams and steep watersheds emptying onto lowlands that may be heavily populated, produce large volumes of water within short periods and damage is often severe. The problem is sometimes compounded by the denuding of large areas of the watershed by fire during the previous season. The Mediterranean climate what often experiences drought, results in sparse vegetation and thinner soils, so mudslides, flashfloods and mass wasting is more prevalent.

The west slopes of the coastal ranges in the central and northern parts of the state also experience flooding because of heavy precipitation over a period of only a few days. These streams are usually longer than those of southern California and require a longer time to build up a flood potential. The Eel and Klamath Rivers, as well as others in the northwestern part of California, are larger rivers than most found in the state, like the Los Angeles or Kern Rivers. The Klamath drains a basin of more than 12,000 square miles. In these streams, a flood buildup may extend over a period of a week or longer.

The streams of the Sierra Nevada and Cascades overflow either because of rainfall or snowmelt, or from a combination of these. With the construction of more dams and reservoirs on these streams, the frequency of damaging floods decreases. Most of the streams are still capable, however, of causing occasional major damage along their downstream reaches. The extreme southern portion of the San Joaquin Valley has no direct drainage to the sea. Excessive runoff from the southern Sierra Nevada into this area can result in the temporary enlargement of Buena Vista Lake and Tulare Lake, which is often helpful towards irrigation for agriculture.

The Los Angeles flood of 1938 is often used as a case study when reviewing extreme weather events. The Los Angeles flood of 1938 was one of the largest floods in the history of Los Angeles, Orange, and Riverside Counties in southern California. The flood was caused by two Pacific storms that swept across the Los Angeles Basin in February-March 1938 and generated almost one year's worth of precipitation in just a few days. Between 113–115 people were killed by the flooding. The Los Angeles, San Gabriel, and Santa Ana Rivers burst their banks, inundating much of the coastal plain, the San Fernando and San Gabriel Valleys, and the Inland Empire. Flood control structures spared parts of Los Angeles County from destruction, while Orange and Riverside Counties experienced more damage.

Figure 3.4: Aftermath of Flood Waters Draining in the Pacific Ocean, Near Malibu, California- 1938. Image by Los Angeles Department of Water and Power is in the public domain.

Winds

California lies within the zone of prevailing westerlies and on the east side of the semi-permanent high-pressure area of the northeast Pacific Ocean. The basic flow in the free air above the state, therefore, is from the west or northwest during most of the year. The several mountain chains within the state, however, are responsible for deflecting these winds and, except for the immediate coast, wind direction is likely to be more a product of local terrain than it is of prevailing circulation.

During the winter, storm tracks move further south. Wind direction and speed are modified by migratory pressure centers. With a strong high-pressure area over the Great Basin and an intense low-pressure area approaching the coast from the west, strong and sometimes damaging winds occur, usually from an easterly or southeasterly direction, especially along the coast and in the coastal mountains. As the storms move inland the winds veer to southerly and southwesterly directions, and high wind speeds may occur anywhere within the state with the greatest velocities at high elevations.

Under a slightly different configuration of these pressure systems, winds tend to flow out of the deserts of the Great Basin towards and into California’s Central Valley, the Southeastern Desert Basin, and the South Coast. Such wind situations are identified in southern California by the name Santa Ana Wind. The air is typically very dry and originated from high pressure systems over the desert regions of the southwest, bringing in northwestern winds to Southern California. The winds are strong and gusty, sometimes exceeding 100 MPH, particularly near the mouth of canyons oriented along the direction of airflow. It is a situation that occasionally leads to serious fire suppression problems and often results in the temporary closing of sections of main highways to campers, trucks, and light cars.

Figure 3.5: Illustration of the Santa Ana Winds. Map by the NOAA’s National Weather Service is in the public domain.

A similar circulation pattern creates the Diablo Winds of the Sacramento and San Joaquin Valleys. As a result of compressional heating of air flowing out of the Great Basin this situation results in pronounced heat waves in summer. In winter, the result is usually a rather mild temperature accompanied by a dry, persistent wind that many persons find unpleasant.

The typical northwest wind of summer is reinforced by the dynamics of the thermal low-pressure area located over the Central Valley and the Southeastern Desert area. In the San Francisco Bay Area, there is a marked diurnal pattern in the strength of the wind even though an onshore circulation tends to continue throughout the 24-hour period. This helps to carry locally produced air pollution products away from the Bay Area but creates problems for the regions immediately south and east of the source area.

In the Los Angeles area, however, the Basin is almost completely enclosed by mountains on the north and east. Coupled with this is a characteristic of the air along most of the coastal area of California. The vertical temperature structure (inversion) tends to prevent vertical mixing of the air through more than a shallow layer (1,000 to 2,000 feet deep). The geographical configuration and the southerly location of the Basin permit a regular daily reversal of wind direction—offshore at night and onshore during the day. With the concentrated population and industry, pollution products tend to accumulate and remain within this circulation pattern.

Another local characteristic of the northwest wind alongshore is the creation of a jet effect in the vicinity of some of the more prominent headlands. The most outstanding of these currents of air is found off and to the south of Point Arguello. Here a strong jet of air is projected southward past San Miguel and San Nicholas Islands, driving a huge eddy as much as 200 miles in diameter. The air swings eastward near San Diego then northward and westward along the coast to rejoin the southward flowing air at the west end of the Santa Barbara Channel. Similar but smaller eddies form in the vicinity of the Golden Gate, just south of Point Reyes, and south of Monterey Bay around Point Surr. Wind speeds in the immediate vicinity of these major headlands can be two or three times as great as the wind flow at nearby points.

During periods of moderate to strong westerly flow at upper levels over the central part of the state, particularly during the winter and spring, the well-known ‘Sierra Wave’ is created in the Eastern Sierre area. Although this phenomenon is particularly useful to sailplane enthusiasts, it can also be a hazard to the unwary pilot.