CCM Exam Question 2 Marin County Homeowner Rainfall Data Report

A homeowner in the town of Greenbrae (3 Corte Cayuga), in Marin County, California, is building a drainage system for her home that will be designed to handle the 1 hour 100-yr rainfall at the home site. In addition, the roof design is extremely sensitive to rainfall sustained over a 24 h period. Further, she would like to design a low irrigation landscaping system keyed to the average seasonal rainfall. She has hired you to: (a) Determine the average seasonal rainfall at her site; (b) Determine the range of seasonal rainfall amounts she can expect at the property most (67%) of the time; (c) Determine the 100-yr 1 hour event at her site; (d) Determine the 100-yr 24 hour event at her site. Make sure that explanatory material is provided to the homeowner for each of your findings. Criteria:

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(a) Average seasonal rainfall: Using data from Water Resources Division of the U.S. Department of the Interior (S.E. Rantz), the average seasonal rainfall for this sight was interpolated at about 35 inches.

(b) Determine the range of seasonal rainfall the customer can expect at the property most (67%) of the time: Since Greenbrae has no rainfall records, data from the Kentfield location is used.

Kentfield Average Annual Rainfall: Kentfield Standard Deviation: Kentfield Coefficient of Variation on this data: / =.35 Greenbrae Average Annual Rainfall: 36 Greenbrae Standard Deviation: 36 *.35= + or Using the Coefficient of Variation, rainfall data for Greenbrae can be calculated: Therefore, there is a 67% chance that rainfall at the Greenbrae location will range from 23.4 to 48.6 inches per year.

(c) Determine the 100-year 1 hour event What is a “100-Year Event”?: The 100-year event can statistically occur once every 100 years. This means there is a 1% chance any given year will have the rainfall equivalent to the 100-year event. It’s important to note that in any given year the 100-Year event is always a 1% chance, even if it just occurred the previous year. It is simply a statistical measurement based upon previous data.

Once again the data for Kentfield will be used to calculate the 100-year hourly rainfall event for Greenbrae: Greenbrae Annual Rainfall: 36 Inches Kentfield Annual Rainfall: inches Ratio: 36 / =.77 Therefore, the 100-year 1-hour event at Greenbrae would be: Kentfield 100-year 1-hour event * Ratio of *.77 = 1.43 inches A ratio can be used to calculate Greenbrae’s data which eliminates the effect of topography:

(d) Determine the 100-year 24-hour event Kentfield 100-year 24-hour event * Ratio of.77: *.77 = 9.21 Inches