Rainfall Report for 3 Corte Cayuga, Greenbrae, CA

By Agustin Diaz

A homeowner in the town of Greenbrae (3 Corte Cayuga), in Marin County, California, is building a drainage system for her home. She

has asked to determine: • Determine the average seasonal rainfall

at her site.• Determine the range of seasonal rainfall

amounts she can expect at the property most (67%) of the time.

• Determine the 100-yr 1 hour event at her site.

• Determine the 100-yr 24 hour event at her site.

Reporting Stations

1. San Rafael situated approximately 2 miles north of the home. 2. Corte Madera situated approximately 1.5 miles south of the home.

• Two stations near the home were used to determine the seasonal rainfall

Seasonal Rainfall and Variation

• Since areal variability does not vary much with latitude, a reasonable estimate for seasonal rainfall at the home would be 37 inches, and the homeowner can expect a range between 21.40 and 52.60 inches 67% of the time.

• 30-year average from 1967-19961. San Rafael- 37.18 inches2. Corte Madera - 37.34 inches

• Standard Deviation 1. Both stations had a standard devation of about 15.6 inches.

100-year 1-hour Rainfall event• San Rafael station was 1.76 inches. - No information was found for Corte Madera, but was found to be approximately 1.77 inches by multiplying San Rafael’s rainfall event by the ratio of 1.004303. - The ratio was obtained by dividing San Rafael’s seasonal rainfall from Corte Madera’s seasonal rainfall. • A good approximation for the 100-year 1-hour rainfall event at the home would be 1.765 inches.

100-year 24-hour Rainfall event• San Rafael station was 9.50 inches. - No information was found for Corte Madera, but was found to be approximately 9.54 inches by multiplying San Rafael’s rainfall event by the ratio of 1.004303. - The ratio was obtained by dividing San Rafael’s seasonal rainfall from Corte Madera’s seasonal rainfall. • A good approximation for the 24-year 1-hour rainfall event at the home would be 9.52 inches.

Information About Return Period• Return period for rainfall is used to estimate how often

certain extreme rainfall events will occur.• The 100-year 1-hour event of 1.70 inches means that on

average that amount of rainfall will occur every 100 years, so for a period 500 years the event will occur five times on average.

• The 100-year event does not mean that event will occur exactly every 100 years, and there is always a chance that the event can occur in consecutive years.

• Obtained through statistical analysis of rainfall data from the reporting stations with at least 30 years worth of data.