Orestimba Creek Hydrology - Stanislaus County

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Orestimba Creek near NewmanAccurate analysis of flood flow requires a reliable, representative time sample of randomhomogeneous events for the study location. Orestimba Creek’s arid climate and heavilyunconsolidated sediment produce non-homogeneous basin runoff characteristics. Orestimba is a lowflow threshold stream. Significantly higher losses occur on the lower flows than on the high flows.Some of this is due to the arid climate and the inability of the soils to hold moisture for an extendedperiod. This is apparent in the drainage area’s vegetative cover of mostly grasses, which quickly dieoff as spring and summer approach. Storms must be closely spaced to have significant runoff. Onlyin 1 out of 10 years, for example, is there enough runoff to sustain the growth of young Sycamoretrees in Orestimba Creek’s lower streambed below the foothills. Several other factors, factors thatcan be observed by plotting the flows on a log probability scale, lead to the conclusion that the basinrainfall-runoff response is non-homogeneous. These include the following: (1) There is a constantslope for the largest 26 out of 76 years of data ((the largest third of the flows (33% probability andless)). But this slope changes for the next third of the data and again for the last third of the data,which contain an average of 12 years of zero flows. (The slope of the data indicates its standarddeviation. A reasonably constant slope indicates homogeneity.) (2) A second factor is that 12 of 75years of recorded runoff for the basin show no flows at all. This occurred at the same time that anadjacent basin, Puerto Creek, with the same climate and similar upper basin geology had runoff foreach of Orestimba Creek’s zero flow years. The main cause is the dramatic difference in the rainfallrunoffresponse depending on antecedent rainfall and its magnitude. (3) A third factor is seen in thecomparison of the flows and statistics of the upper quartile of Orestimba Creek flows with a similarwatershed’s flows (i.e., Del Puerto). This permits the comparison of flood frequency estimates andstatistics with those that might be expected in a homogeneous region.Orestimba Creek emerges from the foothills about five miles upstream of the Orestimba Creek nearNewman gage. The shallow stream flows through moderately deep valley fill alluvium composed ofcoarse-grained unconsolidated sediments (Reference c). These unconsolidated sediments inducelarge channel losses at the beginning of the flood season, causing much of the flow for smallerevents to be adsorbed and flow under the surface of the streambed itself. As a result, many lowrainfall years show zero flow at the gage. This flow regime quickly stores early or light runoff untilthe storage capacity is satisfied. Then—and only then—is there surface runoff. As a result, 12 of the76 years of record show no flow at the gage for the drier years. The flow regime changes as channelstorage increases: the wetter the antecedent conditions and the larger the storm, the greater thepercentage of storm runoff.The Orestimba Creek near Newman analysis used the log-Pearson Type III distribution with theconditional probability adjustment option of Appendix 5 in Bulletin 17 B. This adjustment isrecommended to obtain the frequency curve for stations where the record of annual peaks istruncated by the omission of peaks below a gage base and years with zero flow and/or low outliercriteria. Bulletin 17B recommends not eliminating more than 25% of the flow. This study eliminates40% of the flows. The upper 60 percent of the data was used (highest 45 years of record) todetermine points needed to establish the statistics. Using 60% of the flows minimizes the nonhomogeneityof the flows and provides enough data to estimate the 0.5, 0.1, and 0.01 exceedenceprobabilities needed to compute conditional probability adjustment. The regional characteristics andother stream characteristics indicate that the removal of these data brings the frequency curvemoments into closer alignment with Del Puerto Creek and other streams on the west side of thevalley that have not been mentioned.12
Chart 2, the peak and volume flow frequency curve for Orestimba Creek near Newman, shows theannual flow data on a log-normal probability scale. Chart 3 plots Orestimba Creek near NewmanPeak Flow Frequency curve and the peak flow 5% and 95% confidence bands. The upper half of thedata better represents less frequent floods than the whole data set, because at this point in the datathe flow regime is more stable and homogeneous (it also resembles the slope of the Del Puerto Creekdata (see Chart 1)). The slope of this curve is represents to the standard deviation of the data.Del Puerto Creek basin is north of Orestimba Creek. It emerges from the foothills just upstream ofthe Del Puerto Creek stream gage, has a similar climate, elevation range, and normal annual basinprecipitation, but is a little over half its size (72.6 square miles versus 134 square miles). UnlikeOrestimba, this streambed has accumulated very little alluvium sediment above the gage and thecreek doesn’t have any zero-flow years of record.The upper half of the Orestimba Creek flows correlated well with the overlapping water years of DelPuerto Creek. Based on results from REGFREQ (The Corps’ regional frequency program), peakshad a correlation coefficient of 89 percent and all the duration data, 1-day through 30-day, had acorrelation coefficient greater than 90 percent.To isolate higher flows for FFA analysis, the low flow cutoff option was used. The FFA steps usedto determine the final statistics are documented in Tables 3 and 4. Orestimba Creek’s final statisticsare listed in Table 4. Means and standard deviations of the curves were checked for consistency byplotting them against each other (see Figure 3). The values plot very close to the best-fit regressioncurve. Since those points generally follow a straight line, the curves will not cross. Skews weresmoothed to produce reasonable volume mass curves for flood wave development (Reference d).Table 3 shows the FAA output of themultiple curve statistics only consideringnon zero-flow years. It provides the peak,maximum 1-, 3-, 5-, 7-, 10-, 15- 20-, and 30-day flow volumes for Orestimba Creek. FFAincludes some low outliers in some of thesedurations, but does not influence thesestatistics.Table 4 shows the final statistics of the FFAcomputer run and the final adopted skews. Italso shows the run’s setup parameters forcutoff flows, zero flows, total number ofzero and low cutoff flows, total number ofyears, and percentage of years used.Table 3FFA Statistical Output before Screening Input forOrestimba CreekZero-Duration Mean Std. Dev. Computed Years of FlowLog Log Skew Record YearsPEAK 3.0800 0.7192 -1.33 76 111-DAY 2.7289 0.6332 -1.18 75 123-DAY 2.5205 0.6352 -1.20 75 127-DAY 2.2901 0.6641 -1.22 75 125-DAY 2.3833 0.6547 -1.31 75 1210-DAY 2.1791 0.6758 -1.26 75 1215-DAY 2.0556 0.6948 -1.29 75 1220-DAY 1.9592 0.7067 -1.29 75 1230-DAY 1.8353 0.7227 -1.23 75 1213
Page 1: Orestimba Creek HydrologyA Reevalua
Page 4 and 5: 8. Del Puerto Creek Flood Volume Ma
Page 6 and 7: Figure 1. General Map4
Page 8 and 9: Photo 2. Orestimba Creek, looking s
Page 10 and 11: The current analysis included an ad
Page 12 and 13: TABLE 2ADel Puerto Creek near Patte
Page 16 and 17: Table 4FFA Results for Orestimba Cr
Page 18 and 19: TABLE 5AOrestimba Creek near Newman
Page 20 and 21: TABLE 5BOrestimba Creek near Newman
Page 22 and 23: TABLE 7Salado Creek at DamsiteFlow
Page 24 and 25: Figure 4. Translation of average na
Page 26 and 27: Figure 5. Subarea Map. Report hydro
Page 28 and 29: Orestimba Creek Flood Volume Durati
Page 30 and 31: Table 9 shows the ratios of Orestim
Page 32 and 33: Orestimba Creek Near Newman, CA(at
Page 34 and 35: Orestimba Creek Subarea 4ORSUB4Main
Page 36 and 37: 6. REFERENCESa. U.S. Army Corps of
Page 38 and 39: 36Chart 1
Page 40 and 41: 38Chart 3
Page 42 and 43: 40 Chart 5
Page 44 and 45: Table A1Orestimba Creek Flood Hydro
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PeriodDay-hrTable A3Crow Creek Loca
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Table A5Orestimba Creek Local Flowf
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PeriodDay-hrTable A7Orestimba Creek
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Orestimba Creek Hydrology - Stanislaus County

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