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 ...
1967 flight lines and the full digital 1998 federal digital orthophoto quadrangle series. We learned that there were three classes of change in the San Benito River channel that all affect downstream flood peak heights. There were two different kinds of land use changes that affect runoff timing and volume to the upper Pájaro channel derived from San Benito and Santa Clara counties. The changes we document can be summarized in 5 classes as follows: 1. Those where direct channel incision prevents or reduces overbank flood storage onto a floodplain along the river. Rather than model the degree of incision necessary to affect flood storage on floodplains, we simply noted abandoned floodplains recognized by soils and vegetation. This kind of change greatly accelerates passage of floodwaters downstream, except where the channel incision intercepts the groundwater surface and vegetation thus chokes the channel to slow water velocity. 2. Those where channel widening with or without a deeper central channel (thalweg) effectively increase the capacity of a channel and thus reduce the height of a flood and access of those waters to their floodplain. This kind of change accelerates flood runoff because the water remains in the channel and flows at a higher velocity than would overbank floodplain flow. 3. Those associated with a change from a multi-thread or braided channel to a single more efficient channel, often accompanied by reduced in-channel vegetation. This kind of change accompanies incision and is favored where a central channel is deliberately graded or confined to protect banks from erosion or to prevent lateral migration of the channel, as for example where sewage lagoons or highways are being protected. This kind of channelization change greatly accelerates flow and reduces flood storage. 4. Those associated with a straightening and cleaning of seasonal or flood-period temporary drainage channels on the floodplain. This was observed today only in the Soap Lake area but these same constructed drainage channels also are seen in 1917 mapped on the now-abandoned floodplain south of Hollister. This class of changes reduces the time that overbank floodwater remains out of the channel, thus having a modest impact on downstream flood height. 5. Those associated with dams and flood control structures and bank protection measures that harden banks, reduce bank and bed roughness, and reduce infiltration capacity and land surface runoff detention during intense rainfall events. Public works projects such as bridges, spillways, and highway berms tend to reduce bank and bed friction and thus accelerate runoff. The farther upstream or farther from the channel that these works are found, the less the degree of direct impact on peak flood heights. No matter how intense the rainfall or how long its duration. Uvas, Chesbro, and Hernandez reservoirs DRAFT 7/22/03 38 Pajaro Watershed Flood Management
clearly attenuate (reduce) flood peaks for events when they are not full and spilling. The RMC report concludes that: “The three large reservoirs in the watershed – Hernandez, Uvas and Chesbro – have been very effective in reducing the peak discharges of the more frequent events and, in the case of Hernandez Reservoir, have been effective in reducing peak discharges across the frequency spectrum.” (RMC Hydro Technical Memorandum, 2000). That report concluded that, in 1937 before the three major water supply reservoirs were constructed, the 100-year discharge at Chittenden would have been about 12 percent larger than today. We disagree. That modeled value is based on observed historical attenuation of flood peaks below those reservoirs. We investigated the watersheds above two of those reservoirs and did not find evidence of hillslope overland flow in the oak woodlands that represent the conditions that existed in the reservoir basins before they were constructed. We thus disagree that the 100-year peak intensity rainfall and runoff event would be detained or attenuated by a full and spilling reservoir system. The opposite should be the case because a full reservoir with super-elevation at the spillways will not absorb or detain any more rainfall and thus peak discharges at the extreme event are increased unless these water supply reservoirs are first drawn down. Flotsam around the shorelines of Uvas and Chesbro show that they both have filled to above the elevations of the spillway inverts. Dikes along both Llagas and Uvas creeks in Santa Clara County and significant channelization and straightening of the primary channels had led to loss of fish passage and high velocity channel erosion in some places. Much of this is now being repaired and channel roughness elements are being put in place to try to rebalance these tributaries. Our impressions were that the channels themselves are now as rough or rougher than were their natural antecedents, particularly where filled with Arundo and other plants, and tortuously threaded through urban areas. Thus, acceleration of runoff is minimal (Fig 13 photos are examples of Llagas conditions) DRAFT 7/22/03 39 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 and 26: 70000 Hollister upper confidence in
- Page 27 and 28: value for the maximum possible 100-
- 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: Although Hospital Road shows some a
- 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
clearly attenuate (reduce) flood peaks for events when they are not<br />
full and spilling. <strong>The</strong> RMC report concludes that:<br />
“<strong>The</strong> three large reservoirs in the watershed – Hernandez, Uvas and<br />
Chesbro – have been very effective in reducing the peak discharges of<br />
the more frequent events and, in the case of Hernandez Reservoir, have<br />
been effective in reducing peak discharges across the frequency<br />
spectrum.” (RMC Hydro Technical Memorandum, 2000).<br />
That report concluded that, in 1937 before the three major water<br />
supply reservoirs were constructed, the 100-year discharge at<br />
Chittenden would have been about 12 percent larger than today.<br />
We disagree. That modeled value is based on observed historical<br />
attenuation of flood peaks below those reservoirs. We investigated<br />
the watersheds above two of those reservoirs and did not find<br />
evidence of hillslope overland flow in the oak woodlands that<br />
represent the conditions that existed in the reservoir basins before<br />
they were constructed. We thus disagree that the 100-year peak<br />
intensity rainfall and runoff event would be detained or attenuated by<br />
a full and spilling reservoir system. <strong>The</strong> opposite should be the case<br />
because a full reservoir with super-elevation at the spillways will not<br />
absorb or detain any more rainfall and thus peak discharges at the<br />
extreme event are increased unless these water supply reservoirs are<br />
first drawn down. Flotsam around the shorelines of Uvas and<br />
Chesbro show that they both have filled to above the elevations of the<br />
spillway inverts.<br />
Dikes along both Llagas and Uvas creeks in Santa Clara County and<br />
significant channelization and straightening of the primary channels<br />
had led to loss of fish passage and high velocity channel erosion in<br />
some places. Much of this is now being repaired and channel<br />
roughness elements are being put in place to try to rebalance these<br />
tributaries. Our impressions were that the channels themselves are<br />
now as rough or rougher than were their natural antecedents,<br />
particularly where filled with Arundo and other plants, and tortuously<br />
threaded through urban areas. Thus, acceleration of runoff is minimal<br />
(Fig 13 photos are examples of Llagas conditions)<br />
DRAFT 7/22/03<br />
39<br />
<strong>Pajaro</strong> <strong>Watershed</strong> <strong>Flood</strong> Management
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