Flood Recurrence Frequency of 12-30-05 Storm

By Thembi Borras

On December 30th of 2005 rivers and streams rose and spilled over their banks and already saturated soil moved. Ken Montgomery, proprietor of the Anderson Valley Nursery, recorded 7.58" of rain fell between 9am on the 30th and 9am on the 31st and 27.46" fell during the month of December. According to Ken, December 2005 was the wettest December in at least 70 years, and the second wettest month in at least 70 years. Ken reports that even more stunning than the record setting monthly total is that the 27.46" fell in the last two weeks, in fact it was one of the driest Decembers until December 17th.

On the 31st, it was dry enough to get out and see what the high water left behind or had taken away. Silt and water were left behind in homes and structures that were flooded by the Russian and Navarro Rivers. Soil, trees, logs and lumber were taken by the River and deposited downstream. The North Fork Navarro River tried to take a nearby cabin; it lifted it off its post and pier foundation and carried it downstream as far as the electrical ground wire would allow.

A woman I met in the aftermath of her Oak Manor home having been flooded said the home had not flooded in the 30 years that she lived in the subdivision. Therefore, based on anecdotal evidence the flood flow had not been as high as on 12-30-05 in 30 years at the Oak Manor site.

Determining how frequently the flood flow associated with the 12-30-05 storm might be expected to happen is not simple. It is a function of the historical data available in a specific geographic area. The United States Geological Survey (USGS) gage on the mainstem Navarro, located 5.3 miles upstream from the mouth, has been in operation for the last 54 years providing discharge data. On the morning of 12-31-05, this gage recorded the peak discharge of 55,700 cubic feet per second, using this data in the Gumbel method the flood recurrence frequency was estimated to be 25 years. The Gumbel method is considered a quick and dirty way to estimate the flood recurrence frequency using peak annual discharges. The prediction becomes better the more years data is collected. The flood recurrence frequency of the 12-30-05 flood in watersheds in the Fort Bragg area was thought to be lower (e.g. 12 years) and in watersheds in Sonoma County thought to be much higher (e.g. 50 years).

A portion of this production was gleaned from communication with Dennis Slota, Hydrologist at the Mendocino County Water Agency.