accounting for sediment and geomorphic processes in the
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Accounting for Sediment and Geomorphic Processes in the DWR Recommended Option for Yolo Bypass Expansion
Presented by:Jeremy Thomas, CH2M
Developed with:Mitchell Swanson, CH2MMichael Mierzwa, P.E. DWR
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• CVFPP 2017 Update context• Features of State Recommended Option for Yolo
Bypass• Setting – key features and geomorphology past
and present• Issues related to sedimentation risks• Issues related to erosion• Natural analog – Elk Slough• Current guidance
Overview of Presentation
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• Central Valley people, property and assets at risk• Current flood risk management path unsustainable • Lack of funding for capital works and for ongoing
operations and maintenance of existing infrastructure
• In 2008, the Legislature enacted the Central Valley Flood Protection Act, which authorized and required development of the Central Valley Flood Protection Plan (CVFPP) to address these issues
A Stressed System, the Need for Action
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• CVFPP is a dynamic, programmatic plan, updated in five year cycles – CVFPP first adopted in 2012, first “Update” in 2017
• 2017 Update has same goals 2012 CVFPP• The planning horizon is the 30 years • Refines and updates the State Systemwide Investment
Approach (SSIA)• Adds specificity about recommended near and longer-
term investment and financing approach• Provides broad guidance about more resilient risk
management• Coordinated and aligned with other major flood
management efforts
2017 Update to the CVFPP
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• Technical analyses informing a reasonable, balanced and cost-effective approach
• Emphasis on sustainable, integrated flood management
• Diverse array of actions to improve flood protection
• CVFPP Public Draft December 2016
Technical Work to Support CVFPP Goals
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Yolo Bypass Multi-Benefit Projects
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BWFS Recommended Option for Yolo Bypass
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Expanded Fremont Weir
Notch for Fish Passage
Connecting Channel
Levee reconfiguration and floodplain habitat in Elkhorn Basin
Expanded Tule Canal Toe Drain corridor
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BWFS Recommended Option for Yolo Bypass
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Tule Canal Toe Drain riparian and wetland corridor expansion 35 miles through Yolo Bypass to North Delta
Potentially significant habitat corridor from Delta to Upper Sacramento and Feather Rivers
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Restoration Concepts for Tule Canal and Toe Drain
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Yolo Bypass: Existing and Notch Conditions Fremont Weir Tule Canal
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Sediment deposition zone -millions of cubic yards managed over past 30 years (Singer et al, 2008).
Proposed notch passes up to 6,000 cfs Dec-Mar with spill beginning at 18,000 cfs +/-(existing spill at 48,000 cfs +/-)
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• All Sacramento River BWFS Yolo Bypass Options assume a notch in Fremont Weir for floodplain inundation and fish rearing operations per RPAs
• Sediment, channel stability and vegetation will have critical influence on flood, ecological, and agricultural O&M requirements, practices, and costs in the Yolo Bypass
• Linkages to North Delta, EcoRestore, and Deepwater Ship Channel in southern portion of Bypass (nutrient export, DSC maintenance, sea-level rise, etc.)
• Study goals:− Provide guidance for geomorphic processes supporting ecosystem
features (i.e. channel form, width, and adjustment) while minimizing flood O&M and maintaining compatibility with agriculture
− Identify potential countermeasures for channel stability and sedimentation
Issue: Geomorphology and Sediment Transport in the Yolo Bypass
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• Sedimentation:− Fate of sediment entrainment from Sacramento River through
notch− Low stage in Yolo Bypass – location of depositional area(s)
with narrow Tule Canal and surrounding Ag fields− Sedimentation during Weir Spill events for connecting
channel located in existing historic depositional zone for 4,000 ft +/- ds of Weir
• Erosion:− Erosion risk connecting channel and Tule Canal Toe Drain
with flows up to 6,000 cfs − Long term meandering risk - developing meander belt wider
than designed corridor
Potential Risk Factors
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• Would Fremont Weir notch create a significant sedimentation risk?
• Preliminary estimate of notch flow diversion (based upon water intake estimated sediment diversion rates from Sac River)
• Conclusion: Yes, sediment delivery could range between 1,000 to 20,000 tons / year and could present a new O&M challenge
• Further analysis is recommended under range of sediment supply and flow ranges and potential locations of sedimentation
Fremont Weir Notch Sediment Load
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What’s known?• The area downstream of
Fremont Weir has been subject to sedimentation requiring periodic removal and/or redistribution
• Natural setting = natural levees and crevasse splay / distributary channels
Fremont Weir Sediment Load with Weir Spill
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• Sedimentation impairment and historic sediment removal
• Reduced weir overflow during flood events
• Sediment removal in recent years:
− 1986-1991: 3.5 million cubic yards
− 2007: 0.9 million in 2007• Conclusion: high
sedimentation risk for notch, Connecting Channel and Tule Canal
Fremont Weir Sedimentation History
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Erosion Risk Factors:• Erosion evident in existing sedimentation zone with
predominately sand and silt sizes with potentially erosive flows
• Hydraulic change with notch flow (6,000 cfs) and periods of low stage in Bypass (> 30 days) means periods of increased erosive force
• Existence of rock protection along Tule Canal Toe Drain indicates erosion under present conditions; notch flows would increase erosive force
• Conclusion: erosion risk is high for Connecting Channel and Tule Canal Toe Drain with a notch in Fremont Weir
Yolo Bypass Erosion Risk in the Connecting Channel and Tule Canal Toe Drain
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Possible Geomorphic Analog - Elk Slough
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Elk Slough Analog Characteristics
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• Key issue is potential for meander belt development over time – is current corridor width adequate for unlined channel?
• If straight channels are retained, then long term protection needs should be estimated – rock lining and long term O&M (including sediment management)
• A wide, meandering, dynamic habitat corridor would create superior habitat compared to a straight channel with no active geomorphic processes
• Need for better understanding of geomorphic processes and long-term landscape evolution as a result of proposed modifications in the Bypass
Channel Design and Elk Slough Analog
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• Although erosion and sedimentation risks are identified, it is not yet known the potential magnitude of the risks
• Natural geomorphic setting of Fremont Weir and area immediately downstream indicates sedimentation will be a long-term, ongoing issue; potential for changes in volume and location of sedimentation zones with a notch are unknown
• Use of Elk Slough analog indicates that the Tule Canal Toe Drain corridor is undersized in the current Recommended Option; or will require extensive bank protection and sediment management
• Use of Elk Slough analog indicates potential for establishing a self-maintaining, meandering channel through the Bypass that could provide a continuous habitat corridor from the North Delta to the Upper Sacramento River
• Further planning and design in the Yolo Bypass will require deeper analysis of sediment transport, erosion, and deposition
• Need to account for climate change and SLR factors
Conclusions
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