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

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CHAPTER 3 Upper Basin In-channel Flood Storage and Restoration Opportunities Basic Conclusions: A very substantial volume of flood storage exists in the upper watershed. Focus to date has been on the Soap Lake subbasin of the upper Pájaro and lower Llagas and Uvas tributaries. This area is part of the Lake San Benito basin and is very flat with poorly integrated drainage. Most of the basin is underlain by hydric soils and is in agriculture. The RMC reports have tentatively outlined 30,000 ac-ft of flood storage over 7900 acres at an average depth of over 3 feet. That is the area that is subject to flooding in the 100-year flood, and approximately corresponds to a portion of the FEMA flood delineation map (see Map C for a portion of that map). Our team has identified a larger upper Pájaro River area subject to inundation to an average depth of 1.5 feet that gives about the same de-facto storage volume (see Map C). Our team has identified about 3000 acres of the RMC 7900 acres that could be excavated to enhance flood storage for an additional 7700 ac-ft of storage. The excavated material could be used for nearby protective berms and fill to allow some non-agricultural land uses in the areas subject to very shallow infrequent inundation of 1 foot or less. The net result is about 7000 ac-ft of added storage above the passive 30,000 ac-ft that already exists. On the San Benito River and its tributary Tres Pinos Creek, about the same 30,000 ac-ft of de-facto passive flood storage exists today, but it is located along in-channel and channel margin areas along the braided channel itself. This total 60,000 ac-ft of storage capacity modifies the runoff characteristics of the Pájaro River today and all flood control designs assume that such storage is functional and in place. Diking of sewage lagoons and active in-channel mining operations subtract from that storage and increase downstream peak flows. Today’s San Benito River is diked and modified so that storm flow volumes and peaks derived from that tributary should be increasing. We find that those channel changes have occurred progressively after the late 1940’s and 1950’s. We estimate that opportunities exist to enhance flood storage on the lower San Benito River below Tres Pinos for an added 14,700 ac-ft without encroaching on areas outside of the current (1996) FEMA-defined 100-year active flood zone. An example of an area suitable for restoration of natural overbank flood storage is shown in Fig. 10. Thus the total potential augmentation of flood storage above Chittenden is on the order of 22,400 ac-ft. This added volume of in-channel and near-channel storage has a direct effect on flood peaks in the lower Pájaro River Valley, lowering the flood peaks by about 10,000 cfs and the stage below Murphy’s Crossing by about 4 feet for the 100-year design event (see PWA Lower Pájaro report, 2003). We believe that there are incentives for costs of upstream flood storage augmentation to be borne by local landowners. We believe that a river parkway plan can be combined with such augmentation to protect, stabilize, and enhance biotic and cultural resources upstream in a win-win situation so that costs of downstream flood protection are reduced while biologic and water quality values are increased throughout the watershed system. DRAFT 7/22/03 32 Pajaro Watershed Flood Management
We also find that on the order of 6000 ac-ft of flood storage on the San Benito River south of Hollister was lost before 1955, and that this cannot be recovered today because housing and other structures are now located on that portion of the floodplain. MAP C: Storage areas considered in this report. Inset: 1996 FEMA 100 year flood area Theory: A balance between a river and its floodplain is necessary for the system to function without continual artificial (human) input or damage from floods and/or bank erosion. Natural watershed systems are drained by waterways that store sediment and water both in the channel and on its floodplain. The floodplain is constructed by the river itself as a self-regulating feature for storage of floodwater that exceeds the volumes that can be carried in the natural stable channel. Sediment that cannot be carried by the system during short flood periods is stored as bars and other deposits in the channel and on the floodplain (Curry, 1981) awaiting the next flood flows. DRAFT 7/22/03 33 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 33: California State University Robert
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 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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