Orestimba Creek Hydrology - Stanislaus County

Table 4FFA Results for Orestimba CreekFFA Statistics Final FFA Zero- RecordDuration Log Log Computed Adopted Cutoff Flow Low Outliers Years % Years Years ofMean Std. Dev. Skew Skew Flow (cfs) Years Cutoff Years Used Used RecordPeak 3.0782 0.5636 -0.59 -0.60 900.0 11 20 45 58.5 761-day 2.7177 0.5026 -0.43 -0.50 355.0 12 18 45 59.3 753-day 2.5011 0.5161 -0.46 -0.50 200.0 12 18 45 59.3 755-day 2.3638 0.5279 -0.51 -0.50 150.0 12 18 45 59.3 757-day 2.2651 0.5448 -0.50 -0.55 120.0 12 18 45 59.3 7510-day 2.1548 0.5552 -0.57 -0.60 90.3 12 18 45 59.3 7515-day 2.0283 0.5788 -0.69 -0.70 63.0 12 18 45 59.3 7520-day 1.9299 0.5905 -0.68 -0.70 49.0 12 18 45 59.3 7530-day 1.8071 0.6015 -0.59 -0.70 35.0 12 18 45 59.3 75Using the same cutoff years produces means and standard deviations that prevent the peak and floodflow volume-duration curves from crossing when plotted on a log-probability scale. Figure 3 plotsthe means against the standard deviations of the frequency curves for Orestimba Creek. The flowdata used are listed in Tables 3A and 3B. The data points and frequency curves are plotted onChart 1. Chart 2 shows just the peak flow frequency curve and its 90 percent confidence limits.The Orestimba Creek flow frequency curves and data from the West Stanislaus 2001 feasibilityreport are shown in Chart 4. Notice how the previous peak flow and 3-day frequency curves overestimatedthe less frequent floods.Orestimba Creek StatisticsMeans versus Standard Deviations0.620.60Log of Standerd Deviations0.580.560.540.520.500.481.8 2 2.2 2.4 2.6 2.8 3 3.2Log of Means1-day-30-day Peak Linear (1-day-30-day)Figure 3. Orestimba Creek Statistics, Means versus Standard Deviations14