Pájaro River Watershed Flood Protection Plan - The Pajaro River ...

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parkway system that can be implemented with no or minimal outside funding. Graniterock has already demonstrated a willingness to propose such action and assisted our study through their generous sharing of their aerial survey data. If conservation easements or land trust arrangements can be implemented for parts of the upper Pájaro watershed in conjunction with these major landholders, this may facilitate faster completion of potential storage volumes. We can help to facilitate such planning and implementation. Suggested Enhancement Options for Upper Pájaro River: The Upper Pájaro River, along the Santa Clara-San Benito County line is fundamentally different than the San Benito River. Here a channel is incised up to 25 feet below the Lake San Benito lakebed, but because the riverbed has historically carried a reliable supply of influent groundwater, a dense finger of riparian forest characterizes most of the channel. This mature riparian forest of cottonwood, alder, maple, and willow has a dense woody instream fabric of logs and mid-channel growth, with a diverse pool structure. Although only 100 m wide in places, this riparian corridor provides high quality wildlife habitat and, apparently, allows anadromous fish passage into Llagas and Uvas creeks. Because the riparian forest is so dense and woody debris so prevalent, flood stages rise rapidly and go overbank onto the old San Benito lakebed. Local landowners report that flooding reaches the old lakebed level at a frequency of 10 years or less. Because of the high regional groundwater levels that seasonally saturate up to the lakebed silt cap, the soils of the area are classed as hydric and, unless cropped continually, revert to wetland conditions with emergent wetland plants. Farmers have constructed drainage channels across these lands to carry shallow groundwater and rainfall into the Pájaro. We were able to meet with local landowners and/or farm leaseholders. We learned that this Soap Lake area, just south of Gilroy, and situated along Highway 25 between Gilroy and Hollister, may be a target for extensive development. An ongoing effort sponsored through the Pájaro River Watershed Flood Prevention Authority and AMBAG seeks to establish flood or conservation easements for the Soap Lake basin. Our sources suggest that the opportunity costs for development are so great that contiguous easements may be very difficult to obtain. While the site looks like marginal agricultural land used for little more than growing hay or grazing with small areas of row crops, it is in fact being leased back to local farmers and kept in agriculture as an interim holding pattern while development options are considered. If some of these lands would be wetlands were it not for continual agricultural use, then federal regulations will make it necessary to maintain agricultural uses or raise the lands or protect them with dikes and levees to permit non-agricultural uses. Should this be the case, a need for local fill may provide an opportunity to encourage landowners to excavate the 3-foot deep lake-silt cap immediately adjacent to the river. This could increase the flood storage. DRAFT 7/22/03 44 Pajaro Watershed Flood Management
Our modeling of enhanced flood storage considers opportunities for local landowners to enhance wetland status in areas now in agriculture through excavation of 2 to 3 feet of native surface lake silt (see Map C). There is no assurance that the expensive regarding efforts would be cost-effective for owners wishing to develop parts of the margins of Soap Lake for housing or other non-agricultural uses. Preliminary discussions with representatives of landowners have not discouraged us from considering three-foot excavation in about 2.5 square miles of lower Soap Lake along the Pájaro River for onchannel flood storage augmentation, yielding 4800 ac-ft of new storage in addition to the existing Soap Lake flood volume. We have also modeled an added 2.25 square mile area extending westward to the existing railroad bed berm, adding an additional 2880 ac-ft of new storage, and raising the land elevation between Highway 101 and the rail line above flood levels. This kind of tradeoff must be approved by all regulatory agencies. In essence, a marginal non-functional wetland area now in agriculture is converted to functional restored planted natural wetland in exchange for allowing fill of the edges of the Soap Lake basin that are only wet during sustained 100-year flood events at the present time. Of the 30,000 ac-ft Soap Lake basin storage volume, some 700 ac-ft of natural storage would be traded for about 7000 ac-ft of enhanced functional wetland habitat storage. This is also a winwin situation if a developer or regional agency can be found to champion that large-scale set-aside, and if the regulatory agencies favor it. CONCLUSIONS Over 60,000 ac-ft of flood storage exists on or very near the channels of the upper Pájaro watershed. Soap Lake comprises an important part of this, but only a part of the storage that can be modified or lost with upstream development. Approximately 22,400 ac-ft of storage enhancement is readily possible. Most of this storage is no longer active and no longer accessible to the river because of stream channel incision, levees and berms, and diversions. Restoration of this volume can reduce downstream peak flood heights by on the order of 4 feet during a 100-year flood. The cost of this flood reduction is believed to be less than the cost of protective works downstream that achieve the same level of protection. DRAFT 7/22/03 45 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 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 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: culverts and fill. Other parts of t
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 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

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