A Long-Term San Francisco Bay Inflow Record Derived From Tides: Defining the Great Flood of 1862

Details A Long-Term San Francisco Bay Inflow Record Derived From Tides: Defining the Great Flood of 1862 A Long-Term San Francisco Bay Inflow Record Derived From Tides: Defining the Great Flood of 1862

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmgc13b1228j&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #GC13B-1228

Mathematics

Logic

1616 Climate Variability (1635, 3305, 3309, 4215, 4513), 1637 Regional Climate Change, 1812 Drought, 1821 Floods, 4560 Surface Waves And Tides (1222)

Scientific paper

Global climate studies and water resource management both benefit from definition of hydrologic change on time scales of decades to millennia. In the past, annual average flows have been estimated via tree-ring studies, while extreme flow events have been identified through high-water marks, an approach not applicable to tidal rivers. We introduce here a new approach to flow hindcasts, one applicable to the lower reaches of major river systems. It provides ~15-day flow estimates for the duration of tidal records, distinguishing high and low extremes (albeit with some averaging) and the seasonal cycle. The method uses the non-stationary response of estuarine tides to river flow. Given river flow and astronomical forcing, a forward model can hindcast estuarine tides. Conversely, given tides and astronomical forcing, an inverse model can hindcast flow. While river flow influences many tidal statistics, the detrended M2 admittance amplitude (with oceanic effects removed) was used here. An admittance is the ratio of an observed tidal constituent to the corresponding astronomical forcing. Constituent amplitudes and phases were defined through overlapping, windowed 761-hr harmonic analyses. San Francisco Bay has the longest tidal record of any Pacific station, 1854-date. An estimated inflow is available from 1930 forward. The 1930-1986 inflow record was used to develop an inverse flow model, which was verified using 1987-2003 inflows. Flows were then hindcast to 1859; earlier tidal data were found defective. The hindcasts show a period of high summer flows and low annual variability from 1859-1876. There are high annual average flows in 1859-1869, 1880-1881, 1889-1897 and 1903-1911 (most years), and 1914-1916. Flows were unusually low in 1877, 1887-1888, 1898-1899, 1912-1913 and 1929-1934. Major floods before 1930 occurred in 1862, 1868, 1881, 1890, 1904, 1907 and 1909. The winter 1862 flow (estimated two-week average ~14,000 m3/s) is almost twice the magnitude of the next two largest events in 1904 and 1997 (<8,000 m3/s two-week average flow). While the 1862 flood has long been thought to be the largest since 1850, this is likely the first estimate of flows into San Francisco Bay for this event.

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