[aslkahuna]

The latest Weatherwise has an article about the 1861-62 "Nochian" flood in California which by all accounts was the worse flood to ever visit the state in any record since and even preceeding the arrival of Europeans in the area. It is characterized as a 30000 year event so extreme was the event. The synoptic pattern leading to this flood was NOT an El Niño pattern but instead featured a slowly sagging Arctic front and a MJO induced Pineapple Express. This pattern is very similar to the patterns for the 1955 and 1964 floods in CA and NoCA/OR respectively (the former being the worst flood of the 20th Century in CA) except that it was renowned for remarkable persistence and the fact that as the whole pattern shifted south snow was recorded in the portions of the Central Valley not underwater (the 1955 flood pattern ended with snow levels below 1000 feet). Based upon experience with the later flood in 1955, it is likely that peak discharge rate of the combined Sacramento and San Joaquin Rivers into SFO Bay very likely exceed the record rates ever recorded at St. Louis for the Mississippi River (the flow rates for the 1955 floods were about 1 million cfs). The 1861-62 floods caused 100's of millions of dollars of damage which inflation adjusted would be under 10 billion dollars but the population of CA in 1862 was 1% of the current population so a repeat could cause many 10's of billions worth of damage.

One result of the 1861-62 flood was the development of Flood Control structures of the Central Valley and most notable the Sacramento Riven and it's major Tributaries, the Feather, Yuba and American rivers. A similar system was set up for the San Joaquin. The system consists of the usual array of dams and levees but also includes weirs (overflow spillways) that send floodwaters into bypasses. The largest bypass is the Yolo which ranges from 10-20 miles wide and 15+ feet deep which has the I-80 causeway crossing it. When dry the land is used for crops and grazing livestock with the latter moved when the bypass floods. Of all the weirs on the Sacramento, only the Colusa Weir which empties into the Yolo bypass has gates. ArizWx has the best purple prose to describe what it's like to drive the causeway over the Yolo when it is full but imagine a 20 mile wide river. In 1955, the Yolo was full and this was despite a levee break near Yuba City at the confluence of the Feather and Yuba rivers. Given what happened in 1955, it is likely that combined inflow into SFO Bay from the Delta as well as the rivers of the South Bay Area and North Bay Area which were also in high flood resulted in not only massive turbidity increases but that the salinity of the Bay would have been drastically reduced.

Steve

[sprite]

I read the story in Weatherwise. If a similar flood was to occur nowadays, damage would be massive, wouldn't it? :o

[aslkahuna]

Probably on the order of 50-100 billion dollars worth of damage or more because one has to remember that the 1861-62 flood affected ALL of CA. Not only would the Sacramento and San Joaquin River systems be in high flood but the Russian and Klamath Rivers up north, The Coyote, Uvas, and Guadelupe Rivers in the South SFO Bay Area plus the major creeks like Los Gatos, Penetencia, Silver and Stevens and the San Lorenzo River and Soquel Creeks into Monterey Bay. In the lesser flood of 1955 (the worst of the 20th Century) All but far southern CA was in high flood and 78 people died with hundreds of millions in damage in a synoptic pattern nearly identical to 1861-62. At the peak of the 1955 flood, all highways in the Bay Area were underwater and San Jose was literally cut in two by flooding. We lived on high ground but we couldn't even get into downtown San Jose 5 miles away because the roads were flooded. The San Lorenzo River in Santa Cruz rose 40 feet in two hours in Santa Cruz and the Birdge in Soquel (50 feet high) had to be blown up to free a rock and log jam so that the town could be saved. Five years later huge stumps of tree washed down the creek still littered the beach at Capitola. In 1955, the population of Santa Clara County was maybe 160000 with San Jose at 110000. Now the County is home to Silicon Valley and San Jose is well over a million in population. Imagine the impact if something like 1955 or 1861-2 were to hit Silicon Valley. The impact would be second only to a repeat of the 1906 Earthquake.

Steve

[AUZUR]

San Jose proper is bigger than San Francisco isn't it? Population wise that is.

[ColdFront77]

Yes:

.......San Jose in 2007: 953,679
San Francisco in 2005: 739,426

[aslkahuna]

San Francisco can not really grow much in population since the City occupies virtually the entire County and can not extend beyond the County line. SJC proper is a bit under a mil but like most cities the metro area which would include Milpitas, Berryesa, Almaden, Santa Clara, Morgan Hill, and the County areas such as the East Foothills would push things over that mark. Then add the other cities in the County such as Campbell, Los Gatos, Cupertino, Saratoga, Mountain View and Sunnyvale and you add a whole bunch more.

Steve