Elizabeth Frieberg Meteorology 785 April 5, 2004 CCM Exam Question 2

Question 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-hour period. Further, she would like to design a low irrigation landscaping system keyed to the average seasonal rainfall. She has hired you to:

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 even at her site.

Part A: Determine the average seasonal rainfall at her site.

The average seasonal rainfall at the site would be inches. Due to the absence of rain data at this specific location, an interpolation of data from other sites was necessary. The manner in which this was approached was to locate the rainfall data for different locations in the surrounding area. I was able to interpolate the mean seasonal rainfall for the site with the aid of isohyet maps, provided by the Department of the Interior Water Resource Division, which display contours (lines of constant seasonal rainfall amounts).

Part B: Determine the range of seasonal rainfall amounts she can expect at the property most (67%) of the time.

The range that can be expected at this site (67% of the time) is inches to inches. The way that this was derived was by using a graph, Cumulative probability curves of annual precipitation, provided by Rantz. By examining the curve, and finding the range between 5 and 95 %, I was able to discern that the ratio of annual precipitation to Mean Annual Precipitation was at 5% = 1.51 and at 95% was 0.5. Using these values and multiplying by the mean seasonal rainfall, I was able to obtain the range the values for the client.

Part C: Determine the 100-yr 1 hour event at her site.

1.86 inches – Taken from Kent Field Data.

Part D: Determine the 100-yr 24 hour even at her site.

In terms of rainfall, return period is used in order to estimate how much time can pass between rainfall events of a given magnitude. For this example, the client wished to know about the 100 yr 1 hour and 24 hour event (or return period) at the site. What this means is that the return period for a 1 hour or 24 hour accumulation of rainfall is 100 years. There is a statistical probability that the total accumulated during that time frame happens once every 100 years.

11.07 inches – Taken from Kent Field Data.

The way in which I obtained this data was to use the data for Kent Field. With the knowledge of the topography of the area, Kent field was similar in area, and made the most logical sense to use.

Summary of Findings

Average seasonal rainfall at her site inches Range of seasonal rainfall amounts she can expect at the property most (67%) of the time inches to inches 100-yr 1 hour event at her site (Return Period) inches 100-yr 24-hour even at her site (Return Period) inches