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. Estimated Peak Flow Values for Arroyo De La Guna near Pleasanton, California FEMA vs. Gage Analysis vs. Regression Equations Drainage Area = 405 square miles FEMA Published Gage Analysis Regression Equations Recurrence Flow Values Estimated Peak Flow Estimated Peak Flow Interval (years) (cfs) (cfs) (cfs) 10 7,000 8,200 9,500 20* or 25** N/A 11,500* 17,500** 50 13 ,500 15,700 26,800 100 17,000 18,700 36,600 Desil!n Discharl!e The results of the flood frequency analysis using 41 years of annual peaks from the gage represent a more reliable estimate of the peak flow values for Arroyo De La Guna. In Contrast, the regional regression equations are fairly outdated and represent an "averaging" affect for many streams in the central coast area of California. The peak flow results from the regression equations are considerably higher than the gage analysis results. In order to be consistent with common engineering practice. the gage analysis results will be adopted for the purposes of this study. . Although an official peak flow for February 2, 1998 has not been published by the USGS at the time of this study, it is estimated that the 1998 flood event on Arroyo De La Guna was on the order of a 1 O-year to 25-year event. The selected design discharge for this study is 11,500 cfs. This represents a flow having a recurrence interval of about 20 years. In other words, there is a 5% chance that that flow would be equaled or exceeded in any given year. However, it does not mean that the design discharge will happen every 20 years. For example, a stream may have three 20-year floods in a row, and then may go for a hundred years before having another one. It is impossible to say when the big floods will happen. Flood events larger than the 20-year event can and will happen at some point in the future. In terms of providing good protection to the homeowner, there is benefit vs. cost balance that must be taken into consideration when choosing a design discharge. Designing for a 100-year or greater flood event will provide more protection, but at a much greater expense. Designing for a lesser flood will provide good protection at a lesser expense. A 20-year design will provide reasonable protection while keeping the construction costs as low as possible. IV. STREAM BANK ST ABILlZA TION AL TERNA TIVES . Riprap Riprap has proven to be an effective means to deter erosion along channel banks, in channel beds, and adjacent to hydraulic structures. It is a popular choice among engineers for erosion 4