In 2017 Yolo County received a grant from the California Department of Water Resources Small Community Flood Risk Reduction Program to complete a feasibility study with a primary goal of reducing flood risk to Knights Landing. The scope of the study is to identify preferred structural and non-structural elements, compare implementation costs and schedules and identify local funding requirements to assess options which will reduce the flood risk to the Knights Landing Levee Basin. The study team considered potential solutions to reduce flood risk while sustaining agriculture and the regional economy, providing safe access to the river, improving riverine habitat viability and addressing regional levee maintenance governance.
Background and Existing Conditions
Knights Landing Basin is located along the Sacramento River in the northern boundary of Yolo County, at the northwest end of the Yolo Bypass. It is bounded by the Knights Landing Levee System (KLLS) with approximately 15.2 miles of levees that provide protection from flows in the Sacramento River on the East; the Knights Landing Ridge Cut on the West; the Colusa Basin Drain in the North, and the Yolo Bypass in the South. The community of Knights Landing, a disadvantaged community with an estimated population of 1,000 (2010, US census survey) is located in the northwest portion of the basin at the confluence of the Knights Landing Ridge Cut, the Colusa Basin Drain, and the Sacramento River.
Historically, the community of Knights Landing flooded in 1878 because of Sacramento River levee failures. In 1983, the Knights Landing Ridge Cut flows were impaired by high stages in the Yolo Bypass, ultimately flooding numerous homes. In 2012, the Federal Emergency Management Agency (FEMA) performed a Flood Insurance Study for Yolo County and mapped the community of Knights Landing into the Zone A flood insurance rate zone corresponding to the 1-percent annual chance floodplain. This is because the levees protecting the Knights Landing Basin do not meet FEMA criteria for accreditation and are assumed to fail.
Key stakeholders were identified in the early stages of the study that included residents, landowners and levee maintenance agencies such as County Service Area 6 (CSA-6) and the Knights Landing Ridge Drainage District (KLRDD) to provide input throughout the study process.
The study team leveraged numerous state and federal studies of the Knights Landing Levee Basin and input from the locals to identify flood risk problems. Several locations with underseepage, through seepage, erosion, stability and freeboard problems were identified along the Knights Landing Levee Sysytem (KLLS). Along the right bank of the Sacramento River, through seepage was identified near levee mile 1.0, 2.5, 3.0 and 4.0 and underseepage was identified at levee mile 1.0 and 4.0 along with freeboard deficiency at levee mile 0.5. Stability and erosion problems were identified between levee mile 5.0 and 6.0 along the left bank of the Knights Landing Ridge Cut. KLRDD also noted erosion along the Colusa drain. In addition, erosion and stability problems were also identified along Yolo Bypass. However, problems along Yolo Bypass are not addressed as part of this study as California Department of Water Resources (DWR) is currently addressing them as part of larger Sacramento River Basin wide flood reduction efforts. Applicable levee improvements were examined and evaluated to remediate these levee concerns.
Flood waters from a potential breach on the Sacramento River levee will flow from North to South towards Yolo Bypass. However, due to the existing topography of the area, the Basin acts like a bathtub with water backing up from Yolo Bypass in the South to the community of Knights Landing in the North. To protect the community from these floodwaters, a cross levee was examined. Several locations were identified for this cross levee based on available historical levee performance, recommendations of previous studies, and stakeholder input.
Formulation of Structural Alternatives
The study goal is to achieve a 100-year level of flood protection for Knights Landing through potential improvements to the surrounding levee system and possible addition of a new cross levee on the south side of the community.
The study established the following objectives:
- Improve Flood Risk Management/Provide for Flood Risk Reduction
- Reduce the risks of flooding to life, property, and critical infrastructure
- Improve flood system resiliency and facilitate adaptation to future climate variability
- If feasible, attain a 100-year level of flood protection for the community of Knights Landing in accordance with FEMA’s guidelines pursuant to Code of Federal Regulations (CFR) Section 65.10.
- Promote Ecosystem Functions/Enhance Habitat Restoration
- Improve the dynamic hydrologic and geomorphic processes in the Knights Landing region to enhance ecosystem functions
- Increase and improve the quantity, diversity, quality, and connectivity of riverine aquatic and floodplain habitats
- Contribute to the recovery and sustainability of native species populations and overall biotic community diversity
- Reduce stressors related to the current operations and future improvements of the Knights Landing flood protection system that negatively affect at-risk species
To address these identified problems, thirteen structural alternatives were formulated using a combination of cross levee and applicable levee improvements. These thirteen alternatives were evaluated qualitatively using flood risk reduction, ecosystem benefits, agricultural sustainability and recreation criteria and narrowed down to six alternatives. These six structural alternatives were further evaluated in detail with potential levee remediations that were used to develop feasibility level cost estimates. These cost estimates along with financial ability to pay analysis were utilized to select a preferred alternative.
Findings and Recommendations
The preferred structural alternative includes:
Levee Improvements along the Right Bank of Sacramento River:
290 ft of combination berm and ditch fill
1,011 feet of drained stability berm
2,623 feet of cutoff wall on the right bank of Sacramento River
793 feet of cutoff wall at levee mile 2.5
878 feet of cutoff wall at levee mile 3.0, and
2,400 feet of cutoff wall and 3,157 feet of combination seepage-stability berm at levee mile 4.0.
Levee Improvements along the Left Bank of Knights Landing Ridge Cut
4,825 feet of drained stability berm and waterside rock slope protection for erosion repair
A new cross levee from levee mile 4.9 on the right bank of the Sacramento River and would run approximately 6,800 feet southerly toward levee mile 0.9 of the left bank of Knights Landing Ridge Cut. The total cost for these levee improvements and construction of the cross levee is estimated to be approximately $72 million. The preferred structural alternative can be implemented in several phases due to any potential financial constraints and sources of funding. Levee improvements surrounding the community along the right bank of Sacramento River and left bank of Knights Landing Ridge Cut could be implemented first to protect the community from potential levee failures adjoining the community. Levee improvements outside the community near levee mile 2.5, 3.0 and 4.0 on the right bank of Sacramento River would be implemented next, to address reducing the consequence of a potential levee failure downstream of the community.
Several non-structural measures were studied for their potential to reduce residual flood risk. The most implementable for this region are voluntary flood proofing, voluntary flood raising and changes to the National Flood Insurance Program (NFIP). The study did evaluate a relief cut along the Yolo Bypass levee, but this did not prove to be effective.
In addition to these structural and non-structural measures, ten habitat restoration concepts were also identified as part of this Study. The project team qualitatively evaluated each one based on a number of factors. These factors included their ability to provide ecological uplift, whether they include or support recreational activities, their cost to construct and operate, the estimated permitting complexity, their effects on agricultural sustainability, the overall feasibility of implementing the improvements, and their contribution to reducing flood risks. Finally, this evaluation explored a range of opportunities that could achieve the Central Valley Flood Protection Plan objectives for multiple benefit projects, consistent with the Conservation Strategy, including specifically integrating ecosystem restoration with flood risk management.
Three concepts with the highest potential to be implementable in connection with the flood risk reduction alternatives were identified. These are the Grays Bend Riparian Enhancement concept, the Portuguese Bend Enhancement concept, and the Knights Landing Ridge Cut enhancement concept. For each of these concepts, the excavated soil could be used as a source material for the construction of berms for improvements along the Sacramento River or for the cross levee. Also, habitat creation would potentially offset any riparian habitat impacts that may occur due to levee repairs along the Sacramento River.
The Grays Bend Riparian Enhancement concept improves the quality and quantity of shaded riverine aquatic habitat along the left bank of the oxbow channel by widening both the area of inundated riparian habitat and expanding the width of the riparian fringe forest. The target species for this enhancement concept are birds dependent on healthy riparian habitat including Western Yellow-billed Cuckoo, Least Bell’s vireo, and tricolored black bird, as well as other native species such as giant garter snake and western pond turtle.
The Portuguese Bend Enhancement concept improves floodplain connectivity. The low-lying areas are targeted for improved connectivity. Most of these low-lying areas have higher ground surrounding them that block water during lower to intermediate flows. Targeted excavation would connect these low-lying areas to the river so that they inundate earlier and more frequently. Increasing the area of inundation through targeted excavation would enhance the ability of this area to contribute to localized groundwater recharge.
The Knights Landing Ridge Cut Enhancement concept includes excavating the mid-channel island within the Ridge Cut to increase the channels capacity, to reduce cross channel erosion, and to provide material source to construct the cross levee. Although the riparian vegetation would somewhat reduce the additional conveyance capacity that would be achieved with channel excavation, it would provide the ancillary benefit of helping achieving the State’s objectives of restoring species habitat and ecosystem function. Specific species that could benefit include Giant Garter Snake, valley elderberry longhorn beetle, tricolored blackbird, Swainson’s hawk, Western Yellow-billed Cuckoo, and Least Bell’s vireo.
Finally, the study evaluated recreational opportunities. The team reviewed the recommendations from the Yolo County’s Knights Landing Revitalization Study completed in 2016 and solicited stakeholder input. There were three recommendations, focused on recreational enhancements within the community identified in the study including a new public park on a portion of the Sci-Tech Academy Charter School site, a promenade overlook on the Sacramento River, and improving the Knights Landing boat launch area. Additionally, the study identifies recreational opportunities along the levee that extends along the northwestern edge of the community adjacent to the Colusa Basin Drain and parallel to Reed, utilizing the new cross levee to provide a loop trail that will allow residents to walk or run completely around the community, and to utilize the seepage berm between mile 4.5 and 5 for parking when locals are fishing and accessing the river instead of using the existing top of levee and waterside embankment. |