Sunday, January 24, 2010



El Niño
2009-
2010













Every few years an El Niño appears with significant consequences to Southern California. El Niño brings heavy rains that can snarl traffic, cause significant mud slides, flooding, etc., as we have seen this last week. The ocean effects are not as immediately noticable, but are nevertheless significant.

This last week we had a major set of storms pass through Southern California. At the USC campus near downtown LA the total rainfall this week was about 4.5 inches. Up in the mountains there was as much as 12 inches in some spots. That’s almost a third of our annual average rainfall (~14 inches/year in LA) – more in one week than we received in the entirety of July 2006 to June 2007 when the total rainfall was ~4 inches.

So what happens to all of this water? If we could collect it all it would help significantly in alleviating our water shortage. As the city developed and began to pave over a lot of the land that had previously absorbed the rainfall, it began to have significant flooding problems due to the non-absorbed water pouring down creeks, streams, and streets. In response, in the 1930s the Army Corps of Engineers began the development of a stormwater drainage system to efficiently drain the excess water from the land to the ocean. As a result, although flooding does occur in some areas, it is not the problem that it would be without this storm drain system. But soon after the rains begin, the stormwater swollen streams start flowing into the ocean. On the west side of LA this occurs primarily through Ballona Creek that drains the west side of the LA into Santa Monica Bay. The Los Angeles, San Gabriel and Santa Ana Rivers drain regions that extend from the San Fernando Valley on the west to San Bernardino and Riverside on the east and these accumulated flows discharge into San Pedro Bay between Los Angeles/Long Beach Harbor and Huntington Beach.

After the clouds cleared we were able to see the ocean with an ocean color sensor on NASA’s Aqua satellite (source: http://www.sccoos.org/data/modis/modis_regions.php?r=3). The image above shows the remote sensing reflectance in the yellow-green part of the color spectrum. The colors indicate the concentration of particles in the water with blue being very low and red being very high. High concentrations of particles, indicated by the red colors, are a good indicator of where the stormwater plumes are located in the ocean. In the image you can see both Santa Monica Bay and San Pedro Bay south of LA/LB Harbor. This image shows that the storm water plumes extend from the mainland nearly to Catalina, stormwater from San Pedro Bay is flowing around Palos Verdes into Santa Monica Bay, and significant runoff was coming from the west side of Catalina Island near Cat Harbor. Of course, other smaller streams along the coast contribute to this runoff plume.

What are the effects of this runoff on the ocean?

Stormwater carries with it a lot of material from the land and drains. As we mentioned in the first blog, the stormwater contains a range of things that result from human activities and natural processes on land.
- Materials that fall on the roadways from our vehicles as the result of routine wear from tires, brakes, etc., and the leaking oil and other motor fluids from poorly maintained vehicles.
- Microbial contamination from agricultural animals, domestic pets, wild
animals, and occasionally human contamination
- Plant nutrients from fertilizers on agricultural land, golf courses, home
gardens, public gardens, etc.
- Other debris such as from the recent forest fires in the San Gabriel mountains
- Unspecified sources some of which we refer to as “midnight disposal, inc.”.

The consequences of these inputs are many:

Stormwater is often toxic to marine organisms very near where it enters the ocean. This toxicity is often due to metals such as zinc that is a component of automobile brake linings. As the stormwater dilutes into the ocean, this toxicity decreases. The microbial contamination, regardless of its origin, is often above the standards from the California Ocean Plan and causes public health agencies to close beaches for in-the-water activities for up to 3 days following a storm. The plant nutrients are the same nutrients that enable photosynthetic algae to grow in the ocean and we expect that after several days we will see blooms of phytoplankton in the stormwater plumes.

Sources:
NOAA NWS: http://www.cnrfc.noaa.gov/rainfall_data.php
http://www.lastormwater.org/Siteorg/general/lastrmdrn.htm
http://en.wikipedia.org/wiki/File:Santaanarivermap.png
http://www.sccoos.org/data/modis/modis_regions.php?r=3

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