Pájaro River Watershed Flood Protection Plan - The Pajaro River ...
Pájaro River Watershed Flood Protection Plan - The Pajaro River ... Pájaro River Watershed Flood Protection Plan - The Pajaro River ...
The gages at Chittenden (11159000) and at Hollister (11158500) have similar 100-year peak floods, and have the highest maximum discharge of the gages analyzed. The gage at Chittenden (11159000) has the largest drainage area, but does not have the largest 100-year maximum discharge estimate, in part because its data set is longer and thus the confidence is better (lower interval). The results for the 100-year flow decadal analysis for each gaging station are listed in Figure 2. 140000 Decadal analysis 100 year flood discharge cfs 120000 100000 80000 60000 40000 Chittenden Gage at 156 Hollister Pacheco C. Uvas C. 20000 0 1940 1950 1960 1970 1980 1990 Figure 2: 100 year flood estimates by decade All gages show a small increase in the estimate of the 100-year flood discharge from the time period of 1960 to 1990. A decrease is also seen in all gages from the 1950 estimate to the 1960 estimate. Discussion The above analysis was conducted using the standard reference as required of the Corps of Engineers. It requires a series of adjustments for extreme value rare events to account for their statistical rarity and for the non-symmetrical distribution of precipitation and runoff. An oversimplified way of looking at such data sets is that it is either raining or it is not raining. If it is not raining, the amount of rain is 0.0 and cannot get any less. But if it is raining it can almost always rain harder and get wetter. “Dry” is a fixed value but “wet” is not. The adjustments are made using a table to fit the data to a certain log-transform that Mr. Pearson called Type III and that fits a great many precipitation-related data sets. The Corps’ has chosen to design for a 40,100 cfs peak at Murphy’s Crossing, below Chittenden. That chosen value is subject to many caveats. The actual calculated DRAFT 7/22/03 25 Pajaro Watershed Flood Management
value for the maximum possible 100-year flood at Chittenden is closer to 43,500 cfs using the Corps’ methodology. But there is flood storage at Murphy’s Crossing, in the Aromas area (Aromitas Lake of Jenkins) and in the Soda Lake area just below the Chittenden gauge. USGS actually gauges the Pájaro during high flows at the bridge at Aromas, not at Chittenden several miles upstream. Earlier chosen design floods were higher, but the current value is not unreasonable. Because the lower river is formed by spillover from the upper watershed, drainage area does not increase in a linear fashion downstream. This is a unique watershed. As we shall show, the channel capacity at Soap Lake and in the San Benito River increases in a very nonlinear fashion for flows above about 22,000 cfs as gauged at Chittenden. Thus the log plot of flows versus return period above that discharge tends to “flatten” (see the X’s or actual values in Fig 6 above versus the calculated Log-Pearson III curve). That is, high flood flows tend to be smaller than would be predicted based on the full period of record because of the shape of the channels in the upper watershed and their faultdammed characteristics. The Corps’ design value is thus conservative in that it is above reasonably probable values. The flow record was disaggregated into separate decades and each was assessed individually to look for trends. In practice, one should not use a single gauging station to predict a flood magnitude beyond two-times the length of the actual record. That is, to estimate a 100-year flood, one needs at least 33 years of peak flow record. Thus, the predictions based on 10-year periods do not reflect actual 100-year flow predictions, but do give potential clues regarding changes in flood frequency through time. From this analysis we see that the 1955 Christmas storm at Chittenden in an otherwise non-remarkable decade would have forced prediction of a much larger 100- year event, but that the more frequent large events in later decades change that predicted value. The Christmas, 1995, flow at Chittenden was estimated at 24,000 cfs and was only exceeded there by the February 1998 event at 25,100 cfs. The March 1995 event was estimated at 21,500 cfs. Thus, the 40,100 cfs figure being used by the Corps’ for a design value is 160% of the maximum historic peak in 62 years of instrumental record. The February 1938 storms caused the levees to break in the lower Pájaro (Monterey Herald, 2-12-38) and flooded the Watsonville area with a reported 3 feet of water. A newspaper photo of the lower Pájaro Valley below the town of Pájaro (Fig 7) at about the location of Highway 1 today looks very much like the 1995 conditions. DRAFT 7/22/03 26 Pajaro Watershed Flood Management
- Page 1 and 2: California State University Robert
- Page 3 and 4: Table of Contents Executive Summary
- Page 5 and 6: Pájaro River Watershed Flood Manag
- Page 7 and 8: that today bears the name Pájaro.
- Page 9 and 10: Map B. 1908 Parcel Map of a portion
- Page 11 and 12: Map A 1875 based on 1854 land surve
- Page 13 and 14: Upper Watershed, San Benito and San
- Page 15 and 16: Himalaya and Alaska are there possi
- Page 17 and 18: DRAFT 7/22/03 17 Pájaro Watershed
- Page 19 and 20: The current project attempts to rec
- Page 21 and 22: California State University Robert
- Page 23 and 24: where: g adj is the adjusted skew g
- Page 25: 70000 Hollister upper confidence in
- Page 29 and 30: Regional Analysis The trend in perc
- Page 31 and 32: Interestingly, the maximum consecut
- Page 33 and 34: California State University Robert
- Page 35 and 36: We also find that on the order of 6
- Page 37 and 38: Figure11 Cartoon showing how incisi
- Page 39 and 40: Although Hospital Road shows some a
- Page 41 and 42: clearly attenuate (reduce) flood pe
- Page 43 and 44: average 8-foot depth of water that
- Page 45 and 46: culverts and fill. Other parts of t
- Page 47 and 48: Our modeling of enhanced flood stor
- Page 49 and 50: APPENDICES & Figures APPENDIX 1: No
- Page 51 and 52: APPENDIX 5 Example of Historical Ch
- Page 53 and 54: APPENDIX 2: 1995 and 1998 Storm Com
- Page 55 and 56: http://12.45.109.6/pls/portal30/get
- Page 57 and 58: 50 Corralitos Creek Hydrograph & Co
- Page 59 and 60: Figure 16 - San Benito River channe
- Page 61 and 62: Fig 15 Stream Barb Structures DRAFT
- Page 63 and 64: Corps’ Data presented Alternative
- Page 65 and 66: California State University Robert
- Page 67 and 68: . illustrations, on an interim repo
- Page 69 and 70: APPENDIX 3 Streambank Property Owne
- Page 71: Historical Change in San Benito Riv
<strong>The</strong> gages at Chittenden (11159000) and at Hollister (11158500) have<br />
similar 100-year peak floods, and have the highest maximum discharge of the gages<br />
analyzed. <strong>The</strong> gage at Chittenden (11159000) has the largest drainage area, but<br />
does not have the largest 100-year maximum discharge estimate, in part because its<br />
data set is longer and thus the confidence is better (lower interval).<br />
<strong>The</strong> results for the 100-year flow decadal analysis for each gaging station<br />
are listed in Figure 2.<br />
140000<br />
Decadal analysis<br />
100 year flood discharge cfs<br />
120000<br />
100000<br />
80000<br />
60000<br />
40000<br />
Chittenden<br />
Gage at 156<br />
Hollister<br />
Pacheco C.<br />
Uvas C.<br />
20000<br />
0<br />
1940 1950 1960 1970 1980 1990<br />
Figure 2: 100 year flood estimates by decade<br />
All gages show a small increase in the estimate of the 100-year flood<br />
discharge from the time period of 1960 to 1990. A decrease is also seen in all gages<br />
from the 1950 estimate to the 1960 estimate.<br />
Discussion<br />
<strong>The</strong> above analysis was conducted using the standard reference as required of the<br />
Corps of Engineers. It requires a series of adjustments for extreme value rare events<br />
to account for their statistical rarity and for the non-symmetrical distribution of<br />
precipitation and runoff. An oversimplified way of looking at such data sets is that it is<br />
either raining or it is not raining. If it is not raining, the amount of rain is 0.0 and<br />
cannot get any less. But if it is raining it can almost always rain harder and get wetter.<br />
“Dry” is a fixed value but “wet” is not. <strong>The</strong> adjustments are made using a table to fit<br />
the data to a certain log-transform that Mr. Pearson called Type III and that fits a great<br />
many precipitation-related data sets.<br />
<strong>The</strong> Corps’ has chosen to design for a 40,100 cfs peak at Murphy’s Crossing, below<br />
Chittenden. That chosen value is subject to many caveats. <strong>The</strong> actual calculated<br />
DRAFT 7/22/03<br />
25<br />
<strong>Pajaro</strong> <strong>Watershed</strong> <strong>Flood</strong> Management
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