The San Gabriel river is situated in California, and runs through Los Angeles. It has the most extensive water control system in the world, in a basin 3,000 km^2.
The Ovens lake is 400km from LA, Sacremento river is 600km away and Parker Dam is 400km away. Between them 300 million m^3 of water is transported to LA daily.
Problems regarding the river- why is flood control needed?
LA is the fastest growing urban area in the USofA. In 1890, LA had a population of 11,000, which was largely made of native American tribes, but by 1990 it had a population of 11 million. In 2015 the population was 3.884 million people.
The headwaters where the San Gabriel starts are in mountains up to 3,000 m high. The valleys are unstable and steep-sided. They are fairly straight with gullying along hillsides; overall they are v-shaped with a steep gradient.
Where the valleys meet with the lowlands below, there are large alluvial fans of depositional material, and there was, previously, large amounts of river braiding.
- Large population growth
- Quickly saturated soil in mountains during rain
- Little vegetation present in the mountains- semi arid environment
- Concrete surfaces in town
In 1938 there was a flood of the San Gabriel due to a lack of river controls. This was during the economic depression that affected much of the world, so there simply wasn’t funding to help prevent flooding. There was also less money available to build so that structures could withstand flooding.
The Los Angeles Flood Control Authority was set up in 1915, after flooding in 1914. The first flood control works started in 1917. There are five main components (listed in order from the headwaters):
- Check dams on upper tributaries. These stop debris from the uppermost points in the stream.
- Debris dams at exits from the mountains. These collect dirt and sediment carried from higher up. This reduces damage caused by flooding and reduces the chances of there being blocked channels.
- Control dams. These control the flow of water downstream. Water is trapped behind and released at a steady rate as water levels lower. The first control dams along the river are 25 km from the source and control water flowing from 500 km^2
- Spreading grounds. Water is absorbed into the soil here- a sort of holding pen is built where water is trapped. These occupy vast areas with only a shallow layer of water, so water readily either evaporates or percolates into the soil. One examples of these is Rio Hondo.
- Concrete-lined channels. These control the direction of travel of the water. A deep channel with a large hydraulic radius is provided, so water travels efficiently down it. As it is very deep, even if water gets past the other measures, such as if there is a storm and a lot of water falls away from the mountains, then there is still a very low chance of flooding. There are over 640 km of concrete lined channel.
Only 2% of the rainwater within the San Gabriel basin ever reaches the sea. The rest percolates into permeable rocks, evaporates, or is used by people.
The dams have to be emptied after rain to remove the debris. The debris is either dumped elsewhere or used as aggregate for engineering.
The Whittier Narrows are an example of a site where a single dam has blocked two tributaries.
One of the large dams is the Santa Fe dam, which is 7 km long and contains an area of water of 445 hectares.
Issues with the management
- Less sediment reaching the coast.
- Beaches are not being built up
- Removes natural attraction of the beaches
- Removes natural shoreline protection from the beaches, putting beach-side properties at risk.
(Image Sources: https://en.wikipedia.org/wiki/San_Gabriel_River_(California) https://dpw.lacounty.gov/wmd/watershed/sg/ http://pubs.usgs.gov/wri/wrir034279/wrir034279.pdf https://www.kcet.org/shows/departures/the-other-river-that-defined-la-the-san-gabriel-river-in-the-20th-century )