The URL can be used to link to this page

Home My WebLink About08.12.25 Follow-up - Five Mile Basin Documents Presenting at August 12 BOS MeetingFrom:Pack, Joshua To:BOS Cc:Hightower, Scott; Scott DeLeon; Nuzum, Danielle Subject:Follow-up - Five Mile Basin Documents Presenting at August 12 BOS Meeting Date:Tuesday, August 12, 2025 1:24:11 PM Attachments:PW.Five-Mile Basin Update August 2025.2.PowerPoint.pdf Revised Emergency Rationale Letter with attachment.pdf Thank you for the opportunity to provide an update on the Five-Mile Basin and for the Board’s direction in addressing the short-term concerns. At today’s meeting, the Board requested some of the documentation previously shared with the Board and the public. I’ve included a copy of the emergency rationale letter outlining the urgent need for action, as well as the PowerPoint presentation I presented earlier today. These are public documents, so please feel free to share them with stakeholders as appropriate. I will continue to provide updates to the Board on our next steps in the coming months. As always, I appreciate and value your continued support of this project and our department. – Josh Joshua Pack Director of Public Works 7 County Center Drive Oroville, CA 95965 530.538.7681 Phone 530-538-7171 Fax jpack@buttecounty.net Five Mile Basin Flood ConcernsShort-Term Treatment Alternatives Joshua Pack Butte County Director of Public Works Board of Supervisors August 12, 2025 Five-Mile Basin Background Since June 2025 Department directed to explore short-term alternatives Coordination with regulatory agencies and stakeholders Engaged with State and Federal Representatives Chico City Council –scheduled to consider adoption of resolution supporting County action on August 19th Regulatory agency status –permits submitted Regional Water Quality Control Board (RWQCB) California Department of Fish and Wildlife (CDFW) United States Army Corps of Engineers (USACE) Federal earmark funding was unsuccessful Park Fire -Upcoming Winter Concerns Second winter post-fire may present increased risks: Roots fully decayed with minimal regrowth Minimal vegetation to help stabilize slopes Increased potential for debris flows, mudslides, and flooding Increased risk to Chico and surrounding communities Short-Term Urgency Action Alternatives Alternative 1 –Routine maintenance actions Alternative 2 –Sediment removal Alternative 3 –Targeted treatments to support stormwater conveyance Alternative 1 –Routine Maintenance Focus on treatment actions currently under our Routine Maintenance Agreement (RMA) with CDFW Provides limited benefit Replicates 2025 winter storm season response -mobilize staff and resources to mitigate issues Largely dependent on storm size and severity Avoid actions that would violate existing regulatory agency permits or authority Alternative 2 –Sediment Removal Restore Basin to original condition as constructed in the 1960s Removal of nearly 70,000 cubic yards of material Address scour issues near Big Chico Structure Removal of vegetation and storm debris Creates additional space in the Basin Alternative 2 –Concerns During a storm, extra capacity is filled by floodwaters Work may impact the ability to discharge floodwaters efficiently out of the Basin May not have tangible impacts on flood levels Increased risk of sediment deposit with unpredictable impacts on floodwaters Significant costs (>$4 million) Doesn’t address root issues or resolve short-term concerns May lead to considerable regulatory violations Alternative 3 –Targeted Treatments Targeted treatments to improve stormwater conveyance and address short-term flood risk Channel excavation to redirect flow depths & velocities away from Big Chico Diversion Structure Make repairs to scour damage at Big Chico Diversion Structure Reduce impacts from vegetation Redistribute stormwater flows Support sustainable operations Reduce the risk of additional sediment loads Findings –Sycamore Weir and Lindo Channel Sediment does not appear to affect flows negatively Woody vegetation in recently deposited sands risks encroaching on flows, potentially raising flood elevations These Structures do not need significant amounts of urgent treatment Findings –Big Chico Creek and Structure Big Chico Creek approach to structure has narrowed from 180 feet to 50 feet Sediment load has created a natural levee, directing stormwater flows directly at the structure Structure designed to handle a fraction of stormwater flows during peak events Fast-moving water adds pressure, which may increase flood water elevations higher Floodwaters eroding the adjacent levee, potentially affecting structural integrity of levee and structure Targeted Treatment Areas of Focus 1 234 Big Chico Creek and Structure Levee of sediment (hatched area) diverts stormwater flows directly into Big Chico Creek Stormwater rises quickly due to Big Chico Structure’s capacity Floodwaters are diverted (blue arrow), creating significant scour This activity threatens the levee and structure Vegetation Concerns Small zone of dense, mid-channel vegetation Emerging saplings and other vegetation at Sycamore Weir Has potential to increase flood surface elevations Vegetation has grown more dense this spring and summer Findings –Big Chico Creek Rediverting and reestablishing stormwater flows to Lindo and Sycamore can provide much-needed relief to Big Chico Creek Structure Lindo and Sycamore were designed to handle most of the stormwater flow Alternative 3 –Targeted Treatments Alternative 3 –Targeted Treatments Delivered through the Job Order Contracting Program 20-day project duration Install a construction entrance to the basin Cleaning and grubbing Channel slot excavation and reformation Off -haul additional sediment and gravel Project completion before October 15 Estimated cost of $500k Budget amendment approved by the Board earlier today Alternative Comparisons Criteria Alternative 1 Routine Maintenance Alternative 2 Sediment Removal Alternative 3 Targeted Treatments Benefit Minimal Brief, temporary stormwater storage Up to 3 feet reduction in peak surface elevations Permanence No No Yes -Self-sustaining Regulatory Risk Minimal Considerable to severe Modest to High Costs Minimal Likely >$4 million $500,000 Fully Funded Yes –CSA 24 No –supplemental action needed Yes –CSA 24 Board Action Provide direction to staff 1144 65th Street, Suite B Oakland, CA 94608 (510) 250-9189 August 8, 2025 Joshua Pack, Director Butte County Department of Public Works 7 County Center Drive Oroville, CA 95965 RE: 5-Mile Basin Emergency Action Rationale Mr. Pack: Following receipt of the attached email dated August 7th from Matthew Roberts, a representative of the Corps of Engineers, wherein Mr. Roberts states there is no indication of an immediate threat to life, property, or essential infrastructure in order to support an Emergency Activity Project under the Regional General Permit #8 (Emergency Activities), please accept this letter outlining NCE’s rationale for urgent actions to be taken during the dry season to address significant flood risks at the 5 Mile Basin facility above the City of Chico California. By way of this correspondence, we respectfully request that you initiate official action by your Board of Supervisors and the City Council for the City of Chico to concur with the findings and recommendations of this letter. Background NCE was engaged by Butte County Public Works to develop alternatives to reduce flood risk related to 5 Mile Basin. This facility has received limited maintenance over the last 20 years, and the recent (2024) Park Fire and associated peak flow events in November 2024 and February 2025 have resulted in community concerns regarding flood risk at the facility. Potential concerns were also raised by State Agencies as described in the Watershed Emergency Response Team (WERT) report for the Park Fire, which states “Flood control infrastructure on Big Chico Creek at the 5-Mile Diversion System (BC-04) may be impacted by increased postfire streamflow, sedimentation, and debris accumulation…[and] aggradation along the diversion system has altered the hydraulics such that an elevated head develops on the Big Chico Creek gates that causes flows above design limits to pass.” The WERT report identiﬁes future risks from the 2024 Park Fire that intensively burned a signiﬁcant portion of the Big Chico Creek upper watershed. The result (already observed over the 2024/5 winter) was larger ﬂoods than expected (up to 70% larger relative to similar pre-ﬁre rainfall volumes), and substantially more sediment production. Extensive post-ﬁre sand bar and ﬂoodplain deposits can be observed between the 5 Mile facility and the mouth of the watershed (roughly 2 miles upstream). Some of these deposits are 5-6 feet deep. P a g e | 2 The engineering and geomorphology of the facility’s existing conditions are complex. The original facility was designed and constructed in the 1960s by the US Army Corps of Engineers (USACE). The USACE “stilling basin” design objective for the facility is to slow and distribute flows into three primary downstream channels (Sycamore, Lindo, and Big Chico). However, over the last 2 decades, a significant accumulation of bedload sediments (primarily gravels and cobbles in a sandy matrix) and emergent riparian vegetation (willows, sycamores, alder, cottonwoods, and others) have developed along the right bank of Big Chico Creek immediately downstream of the pedestrian bridge, and opposite the hydraulically smooth flood wall along the left bank. Additionally, substantial erosion of several key landforms associated with the original 1960-era design have widened the channel from a design width of 140 ft to as much as 240 ft (the natural channel width is closer 50—70 ft), and have resulted in a loss of part of a levee extension that partially protected the Big Chico structure from direct head-on flow orientations. The existing configuration funnel flows directly at the Big Chico Creek Diversion and affects flows entering the 5 Mile Basin in a manner that significantly reduces channel stability within the facility. The result is a current channel expression that directs up to 12-foot-deep flows with estimated velocities of 8-15 ft/s directly into the Big Chico headwall structure (per the City’s HECRAS model). Furthermore, a roughly 8500 ft2 (0.2 acre) patch of dense vegetation has encroached into the active floodway at a critical location within the facility. This “wall” of vegetation increases the water surface elevation during large events by roughly 1.0 to 2.4 feet and has established itself as an effective grade control on the bed of the channel, elevating the bed by 4 feet relative to the downstream reach. A graphic illustration of this feature is shown in Figure 1 below. Figure 1) Signiﬁcant sediment accumulation and riparian vegetation along the west bank of Big Chico Creek is highlighted above. P a g e | 3 More critical (and urgent) is the potential containment risk for the Q100 design flow. Using a 1D, steady state hydraulic model (HECRAS), Northstar concluded that the Q100 storm would be contained by the facility with 1.7 feet of freeboard in the vicinity of Big Chico diversion. The City of Chico’s HECRAS model (developed by Wood Rodgers) indicates Q100 water surface elevation equal to, or just slightly below (within a few tenths of a foot) the existing levee / headwall elevation. However, as steady-state 1D models, neither study appears to factor the runup effect associated from velocity head, and neither study factored the effect of vegetation that currently exists within the basin. The City of Chico model shows velocity of 8- 15 ft/s within the vicinity of Big Chico diversion structure, which would theoretically super-elevate the flood surface against the headwall by an additional 1.0-3.5 ft. If these preliminary calculations are correct, the Q100 storm event is likely to overtop the Big Chico headwall and/or surrounding levee. Given the information described above, NCE focused our review on immediate ﬂood risks related to the current condition of 5-Mile Basin. A summary of current impacts to the physical structure and ﬂows into the facility include: 1. Channel alignment now forces 100% of peak ﬂows directly into the headwall structure for the Big Chico Diversion, which is only capable of passing 10-15% of these ﬂows 2. Big Chico Creek approaching the Big Chico diversion structure has been narrowed from a design width of 140 ft to 50 ft under existing conditions. 3. The reduced opening into the 5-Mile Basin has resulted in high-velocity ﬂows which has caused localized scour adjacent to the base of the levee directly above the Big Chico Creek Diversion and excavated approximately 0.9 acres of inset ﬂoodplain that was part of the original 1960-era design. 4. Dense riparian vegetation has encroached within a 100 ft longitudinal section of the active channel, increasing water surface elevations by an estimated 1.0 to 2.4 feet. 5. The super-elevation of ﬂood water surface, together with the increase in ﬂood water elevations due to vegetation suggest the Big Chico Creek Diversion Structure and/or the adjacent levees are incapable of containing 100-year storm event ﬂows. • According to both post-event ﬁeld evidence and ﬁrst-person reports, the February 4, 2025 event came within a few tenths of a foot of overtopping the levee (and may have even “splashed” over in a few locations). That event (estimated at 10,000 cfs) was only 5/8ths of the USACE design storm of 16,000 cfs. 6. The 2024 Park Fire eﬀects are likely to a) Continue to deliver large volumes of sediments (sands, silts, and clays) b) Increase the magnitude, frequency, and possibly duration of peak ﬂows c) Increase the bedload transport rate of gravels entering the facility P a g e | 4 These findings suggest a strong need to address the current condition with an emergency project prior the upcoming winter season, which may deliver additional peak flows that will stress the facility. The risk to the downstream community and infrastructure of not taking immediate emergency action include: • Increased risk of flooding of residential properties and businesses • Impacts on roadway, water, and sewer infrastructure • Increased threat to public safety • Potential for significant monetary impacts to local property owners, businesses, and government agencies Proposed Actions NCE has identiﬁed 5 urgent, short-term treatments that should be addressed prior to this coming winter that will assist in reducing short term ﬂood risk. The core of NCE’s approach is allowing the river to do much of the work (as intended by the original 1960’s design) of re-distributing sediment in ways that support long-term, sustainable function of the facility. We note that these actions are the ﬁrst step in what the County envisions as a long-term management strategy to update and restore function of the 5-Mile facility. Given the urgency of these issues, and the limited time to address them before the winter season arises, NCE is seeking regulatory relief from the typical permitting process to expedite construction of these emergency actions. The objective is to reduce ﬂow depths and velocity (to the extent practicable) away from the Big Chico diversion headwall, making repairs to damage within the Basin from the 2024/5 winter, and reduce the negative eﬀects of local vegetation. NCE expects these actions to reduce the ﬂood surface elevation in the event of a large peak ﬂow event during this upcoming winter season. These actions are premised on our current understanding of the system and will be subject to change pending ongoing hydraulic modeling eﬀorts. Emergency treatments include: 1) Restoring / re-activating a historic back-channel to divert ﬂows away from Big Chico diversion. This action involves light “pilot-channel” excavation (working estimate of 1500 cubic yards) and construction of temporary hydraulic structures (rock vanes and/or graded riﬄe) to guide natural ﬂows into this relict channel feature. 2) Filling two 4.5-foot-deep scour holes immediately adjacent to the levee and Big Chico diversion structure. These scour holes risk undercutting the levee, and possibly the structural integrity of the diversion headwall and/or levee. Long-term solutions will likely require restoring or improving the levee geometry in this vicinity. 3) Removing emerging trees / shrubs that have naturally recruited in recently deposited (Park Fire) sands immediately upstream of Sycamore Weir. These emerging trees and shrubs risk negatively P a g e | 5 aﬀecting the performance of the weir, potentially increasing water surface elevations in the facility pool during peak events. 4) Carefully and selectively thin / prune about 1/3 acre of existing stands of dense mid-channel vegetation (shrubs and trees). This vegetation directly impinges ﬂows and acts as a de facto grade control, resulting in about 2 feet of additional gravel deposits from last winter. These trees also likely increase the water surface elevation during extreme events. Note, we anticipate leveraging much of this vegetation for future restoration of the adjacent inset ﬂoodplain, so treatments will be conservative. 5) Selective experiments to support future sediment transport and restoration design / management studies. This may include local pot-hole sampling, tiny sediment traps (trenches / pits), tracer studies, scour piles, or similar. These may involve some instrumentation infrastructure (e.g., datalogger enclosures, conduit, etc). Details are currently under development. NCE’s proposed emergency treatments are designed to minimize impacts to habitat and recreational values while addressing these urgent issues. The information gained from this work will also inform continued long-term management planning. For additional information, please feel free to contact Scott DeLeon, our Principal Project Manager, Sincerely, NCE Scott DeLeon Principal Project Manager Michael Liquori Principal Water Resources Practice Lead Attachment From:Roberts, Matthew James CIV USARMY CESPK (USA) To:Blodow, Carson@Waterboards; Jepson, Lillian M CIV (USA) Cc:Pack, Joshua; Scott DeLeon; Emily Pullman Subject:RE: Butte County 5-Mile Basin Emergency Maintenance Project - Application for 401 Certification Date:Thursday, August 7, 2025 1:04:18 PM Attachments:image003.png image004.png Carson, Good afternoon. Lilly Jepson will serve as the U.S. Army Corps of Engineers Project Manager for this action; however, she is currently on leave this week and will be attending mandatory training next week. We understand from your recent communication with the applicant that you are reviewing the nature of the proposed impacts and whether they should be characterized as temporary or permanent. Based on our preliminary review and the information provided to date, it does not appear that the proposed activity qualifies for authorization under Regional General Permit 8 (Emergency Activities), as there is no indication of an immediate threat to life, property, or essential infrastructure. We wanted to ensure that this understanding is clearly communicated at the outset of the coordination process. The Corps remains committed to working collaboratively with the Water Board and the applicant to evaluate the proposed activity through the standard permitting procedures, consistent with applicable regulations under Section 404 of the Clean Water Act. Please don’t hesitate to reach out if you would like to schedule a time to discuss further. Respectfully, Matthew Roberts Chief, CA North Section Regulatory Division/Sacramento District US Army Corps of Engineers (Primary) 530-723-3355 (Secondary) 530 223-9538 (fax) 530-223-9539 *We want your feedback! Take the survey: https://regulatory.ops.usace.army.mil/ords/f?p=136:4 * Our customer service hours are 9am to 3pm Monday through Friday. * Visit our website at www.spk.usace.army.mil/Missions/Regulatory.aspx for more info and to sign up for public notices.