Feasibility study explores potential for catastrophic failure of Upper Lake levees
- Elizabeth Larson
- Posted On
UPPER LAKE, Calif. — The condition and stability of Upper Lake’s levee system, highlighted in a feasibility study released by the county of Lake last year, has raised concerns about danger to the Upper Lake community and has led to the scheduling of a special workshop this week.
The Western Region Town Hall, or WRTH, is hosting the special workshop at 5:30 p.m. Wednesday, Sept. 28, at the Habematolel community center, 9460 Main St. in Upper Lake.
The area of concern includes the Middle Creek and Clover Creek diversion levees, the diversion structure and Old Clover Creek closure structure in Flood Zone 8, which the county calls the “Middle Creek Flood Control Project” and is operated by the Lake County Watershed Protection District.
The town hall’s members have raised the alarm about the levee conditions, the potential for breaches leading to significant flooding and damage to the historic town, as illustrated in the feasibility study released in December.
That led them in March to begin formally asking the Board of Supervisors by letter and in person to take action by agendizing a discussion with Water Resources Director Scott De Leon about remedies to keep citizens and property in Upper Lake safe from flooding, as well as to discuss funding, a timeline for improvements, and the program for maintenance to keep the Middle Creek flood bypass channel in continuous repair and a program for maintaining Clover Creek.
Such a discussion so far hasn’t happened, even though District 3 Supervisors EJ Crandell has been at the WRTH meetings, has heard the concerns firsthand, and in his capacity as chair of the Board of Supervisors is part of weekly agenda review.
However, WRTH members hope the board will finally take it up after the community weighs in at the Sept. 28 special workshop.
The Middle Creek Flood Control Project’s maintenance and condition has long been an issue. In 2007, the U.S. Army Corps of Engineers included Upper Lake’s levees on a nationwide list of those the agency said hadn’t been maintained at acceptable levels.
Over the years the county has faulted the Army Corps’ findings and said it has continued to maintain the levees.
However, this year, the levees appear to be unmown, overgrown with vegetation — particularly with trees blocking the channel — with large amounts of gravel buildup throughout the channels and under bridges.
Today, the Army Corps’ National Levee Database shows Upper Lake’s levees as being low risk due to the low population and the fact that the levees have withstood without previous breaches.
However, that database finds the potential for lower water depths in case of a breach when compared with the county’s feasibility study, reports that the levees haven’t had seepage issues — they have — and also doesn’t reflect “the overtopping without breach risk associated with the presence or operation of the levee system.”
Feasibility study looks at current levee conditions
In December, the Lake County Water Resources Department and staff from one of its consultants, Folsom-based Peterson Brustad Inc., presented a video of the Upper Lake levee feasibility study, formally known as the Middle Creek Flood Control Feasibility Study.
That video can be seen at the top of the page.
That study was years in the making. On Dec. 18, 2015, the Board of Supervisors approved a request to the California Department of Water Resources’ Small Communities Flood Risk Reduction Program, which assists local governments by providing technical and financial support to improve flood control facilities of the State Plan of Flood Control for small communities of under 10,000 residents, according to county documents.
In 2016, a consultant selection board chose Peterson Brustad Inc. to help the county apply to the program for the feasibility study and in July 2018 the county received a $499,971 grant.
On Jan. 8, 2019, the Board of Supervisors approved an agreement with Peterson Brustad in the amount of $499,971 to conduct the study.
Incidentally, Jan. 8, 2019, was the day Crandell was first sworn into office as District 3 supervisor, making approval of the study one of his first acts as supervisor.
The point of the study is to create alternatives that can improve the levee system and protect the Upper Lake community.
The study looked at ways to provide protection from a flood event that has a 1% probability of occurring in any given year. Such a flood is more commonly referred to as a 100-year flood event.
That study’s area was the boundary of Upper Lake, on the east side of Middle Creek and south of the Clover Creek diversion.
Particular focus is on the Middle Creek and Clover Creek diversion levees, the diversion structure and Old Clover Creek closure structure.
Those levees, the study is careful to note, are called State Plan of Flood Control, or SPFC, levees, which do not provide protection in a 100-year flood.
The Middle Creek and Clover Creek levees are meant to protect a population in the study area of 1,100 people, with critical infrastructure including a fire station; elementary, middle and high schools; Highway 20, which is the main evacuation route; and the Upper Lake Ranger District for the U.S. Forest Service, which services 260,000 acres of the Mendocino National Forest.
The study broke Upper Lake’s levees into two sections, the Clover Creek diversion and Clover Creek and the Middle Creek and Old Clover Creek downstream of Highway 20. It only considered left bank levees, which give direct protection to the study area.
It looked at known sedimentation issues, with 10 areas identified throughout the levee system. Among those areas are Alley Creek, the confluence of Middle Creek and the Clover Creek diversion, the Elk Mountain Road Bridge, the confluence of Middle Creek and Old Clover Creek, and the Middletown Creek and Scotts Creek confluence.
Erosion is another concern the study took into consideration along the levee banks.
Today, residents point to large amounts of gravel buildup that have resulted in reduced clearance in channels and under bridges. The levees also are heavily overgrown with trees and other vegetation and, in some areas, large cutbanks can be seen in the levee structures.
With all of the information from the study, the consultants modeled a 100-year flood event with the existing levees in place. That modeling found that overtopping of the levees would result in historic Robinson Lake filling up.
The resulting flooding could see up to 3 feet of water in much of the downtown, areas of 5 to 10 feet south of town and, in some areas, closer to 15 feet of floodwater.
Specific problems found with the levees include the Clover Creek left bank, which is not tied into high ground. That levee deficiency — which could lead to 1 to 3 feet of water in town — is one of the issues the study wants to address in its options, or alternatives.
Another deficiency in the system and the diversion structure is the lack of adequate “freeboard,” defined by the Federal Emergency Management Agency, or FEMA, as the vertical distance from the 1-percent-annual chance flood elevation up to the top of the levee.
Freeboard is important because FEMA requires 3 feet of freeboard and 4 feet of freeboard at bridge crossings in order to accredit the levee system.
A FEMA report titled “Freeboard Deficient Procedure Modeling and Mapping Non-Accredited Levees” explains, “Flood risk is dynamic and constantly changing. While levee systems are designed to manage a certain amount of floodwater, they can be overtopped or fail during flood events that exceed the level for which the levee was designed. Freeboard helps reduce the likelihood of overtopping (floodwater runs over the levee) during the design event, and it controls overtopping locations, helping to reduce the risk of catastrophic flooding.”
The modeling looked at several scenarios based on what would happen if there were breaches at specific sites along the levee system.
A breach at Middle Creek River Mile 3.4 would result in heavier flooding south of town and flow into Robinson Lake.
However, a breach at Middle Creek River Mile 3.9 could see floodwater depths of 3 to 10 feet, with additional issues from the flanking at the end of the Clover Creek levee. In that situation, water would come both from the north and the south, and then from the west to overwhelm the town.
An animation of that scenario starts at the 23:40 minute mark in the video above.
The next scenarios, a 100-year levee breach at Middle Creek River Mile 4.4 which is more upstream, similarly would be disastrous for the town, as would one at Clover Diversion River Mile 0.2, with both showing depths of up to 10 feet of water. A breach at Clover Diversion River Mile 0.9 would be slightly less severe, with depths in town of 3 to 5 feet.
A 100-year composite floodplain of the modeled breaches, which begins at the 30-minute mark in the video above, showed the fire station under about 7 feet of water, the Forest Service office would be under 1 to 3 feet, much of Main Street and even a portion of Highway 20 at the entrance of town and between the levees would be at risk of being under 10 feet of water, making it completely impassable.
Upper Lake’s schools, because they are built up more, are expected to be out of harm’s way, the study explained.
Testing shows levees fail to meet key structural criteria
The California Department of Water Resources did a nonurban levee evaluation of Upper Lake’s levees, examining them for erosion; landslide stability; through seepage and underseepage, which are drivers of breaches; freeboard; geometry; and hazard levels. It didn’t include geotechnical studies but was dependent on historical information.
As part of this study, the engineering company Kleinfelder conducted 20 geotechnical explorations along Middle Creek and the Clover Creek Diversion left bank levees, analyzing underseepage, through seepage and stability, and testing soil makeup.
The testing found that the Clover Creek Diversion did not meet the criteria for freeboard, through seepage or stability, but did meet the criteria for underseepage and rapid drawdown.
For the Middle Creek levee, the geotechnical exploration showed that it didn’t meet freeboard or through seepage criteria, but did for underseepage, stability and rapid drawdown.
With little geotechnical study having been done before, this new information is informing agencies and officials, helping them better understand with specificity the levee system’s issues, and be able to characterize if they have through seepage or underseepage, based on the FEMA standards for levee systems, the consultants said.
Kleinfelder came up with remediation options, including a seepage berm, which requires expanding the levee footprint on the landside to stop the seepage problems.
Another option is a shallow cutoff wall. The consultants said that’s the preferred remediation method, as it does not require any additional levee footprint so no additional land needs to be acquired. A shallow clay cutoff wall is put through the center of the levee, which is used to stop the seepage going through the levee and causing problems on the landside.
Those remediations will be taken into consideration as part of the alternatives.
The objectives of the project include 100-year protection, which could be achieved incrementally over time; helping local maintenance agencies respond; flood system and agricultural sustainability; willing sellers; managing residual risk; leveraging local assessments; and incorporating multiple benefits where feasible.
Constraints are the ability to cost share, deficiencies in the existing SPFC levees, regulations are difficult for a small community, adjacent channels, flooding depths that are “pretty significant” in Upper Lake, maintenance funding, ecosystem enhancements may be more difficult without willing landowners and making the system robust and resilient in anticipating of future climate change predictions.
The study includes a range of alternatives, with three in particular mentioned: fix in place the existing levees, sediment management and nonstructural improvements.
Some structural features could be included in multiple alternatives, and the selected alternative could be a combination of features from each alternative as the alternatives are refined in the evaluation process.
Still to be determined is how it will all be funded, which will be dependent on factors, including the alternative selected. A goal is to build a multi-benefit project that could include ecosystem restoration and recreation, which could be eligible for funding opportunities through state bond funding, some of them covering 100% of the costs.
That funding will be critical, because study presenters pointed out that levee projects are not cheap, and as such they may need to phase in aspects of the preferred alternative, as funding may not be available up front.
The alternatives
The study video presented three main alternatives.
Alternative No. 1 is fixing in place the existing levees that provide direct protection to the community, improving them to a 100-year level of protection, making freeboard and geotechnical fixes such as the shallow cutoff wall.
That alternative calls for raising and strengthening 2.7 miles of levees on Middle Creek and the Clover Creek Diversion, modifying or replacing the diversion structure, and extending the Clover Creek levee’s left bank by about 1,000 feet to high ground to prevent flanking. It also would address the Old Clover Creek closure structure’s deficiencies.
Alternative No. 2 calls for sediment management and vegetation clearing in the channel to convey 100-year flow, in addition to all of the aspects in Alternative No. 1.
Alternative No. 3 revolves around raising structures to be above the 100-year floodplain level, considering school modifications for an evacuation center, and other emergency action and evacuation enhancements, including improved mapping of the floodplain and levee breach timing, and evacuation routes and coordination.
All of the alternatives consider multi-benefit opportunities, such as helping the Clear Lake hitch, a large minnow native to Clear Lake that have been vital both to the ecosystem and to Pomo tribal culture, and which historically used Clover Creek for migration and spawning.
They’re listed as a threatened species under the California Endangered Species Act, and the study concluded the hitch should be taken into consideration for all alternatives in the Middle Creek watershed — promoting the ecosystem, reducing stranding and lowering channel velocities for migration.
Another multi-benefit opportunity cited is water quality. Such issues are well known in Clear Lake, and the study concluded that the alternatives should intend to increase water quality of runoff into Clear Lake.
Addressing issues with mercury, another challenge for Clear Lake, also is expected to offer another potential venue for project funding.
Regarding the next steps, the alternatives evaluation was to take place earlier this year. That step included layouts, geotechnical, hydraulics — flood flow, environmental and cultural, costs, benefits and economics, and community support.
At that time, an alternatives comparison was expected to be completed midyear, with the preferred alternative to be selected and information about that alternative to be emailed to stakeholders. That would be followed by submission of reports to the California Department of Water Resources by the end of the year with the pursuit of funding to begin.
However, Lake County Public Works and Water Resources Director Scott De Leon said last week that an alternative hasn’t yet been selected.
“The consultant is working through the process of assessing the choices using a matrix that evaluates various components of each option and ultimately assigns a score to each alternative,” De Leon said. “The top three alternatives will then be considered with a similar process until a preferred alternative is selected.”
Last week, following two days of hearings, the Board of Supervisors approved the final 2022-23 recommended budget. That final budget document does not include any funding for the levee alternatives.
In response to a question from Lake County News during the first day of the hearings about the levee improvements not being in the budget, De Leon said that work will be budgeted in the 2023-24 fiscal year.
Email Elizabeth Larson at This email address is being protected from spambots. You need JavaScript enabled to view it.. Follow her on Twitter, @ERLarson, or Lake County News, @LakeCoNews.