Was all that Los Angeles River flood control concrete necessary?

Thursday, 18 December 2014
William C Patzert1, Shelley Shaul Regalado2, Steve LaDochy3, Pedro C Ramirez4 and Josh K Willis1, (1)NASA Jet Propulsion Laboratory, Pasadena, CA, United States, (2)Citrus College,, Earth Science, Azusa, CA, United States, (3)California State University Los Angeles, Los Angeles, CA, United States, (4)Cal State Los Angeles, Los Angeles, CA, United States
In 1938, heavy rains over the Los Angeles Basin resulted in widespread and costly flooding of the Los Angeles River floodplain. In response to the resultant damage, 51 miles of the River was concreted from the San Fernando Valley to the Pacific Ocean. Today proposals to modify the river to capture more water and to restore it to a more natural state have been approved. Through comparison of rainfall data, we test whether channelization can adequately handle the extreme flooding events occurring since 1938. Between February 27th to March 3rd 1938, two major storms resulted in 14.1 inches of rain in Pasadena, CA leading to the flooding of the Los Angeles River, 115 fatalities, the destruction of 5,601 buildings, and to $627 million (2011 dollars) in damages. Downtown Los Angeles averages 15 inches of precipitation a year, while the San Gabriel Mountains, where most of the Los Angeles River watershed rainfall is collected, typically receive more than 40 inches of rain annually. Eight record storms, each with rainfall totals over 11 inches, since the 1938 flood could have created devastating deluges were it not for channelization. Presently, at full stage the channelized Los Angeles River can accommodate a discharge of 129,000 cfs. During the 1938 flood event the discharge peaked at 68,000 cfs above Arroyo Seco and 79,000 cfs below Firestone Blvd. A similar storm event today would have led to increased discharge due to urbanization. Since 1938, the greatest discharge recorded at the same stations was 52,200 and 74,400 cfs during the February 16th 1980 storm. Although damage was substantial during this storm, river channelization prevented fatalities and much damage. To date, the channelization of the Los Angeles River has been successful in flood control. However, our research shows that southern California precipitation is becoming more intense which may result in increased flooding. Any future modifications to the river must be prepared to handle the extreme flooding events and water managers must also be mindful of these events.