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PUBLIC WORKS - Director Dan Newton
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Requested Action(s)
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a) Proclaim that an emergency condition exists concerning the Simpson Road Bridge due to the apparent progression of the Thomes Creek’s primary flow path toward the northerly stream channel increasing the potential for scour and erosion at, or near, the bridge’s north abutment
b) Find that immediate action is necessary to reduce the potential threat to the Simpson Road Bridge, a critical transportation and emergency-access facility
c) Authorize the Director of Public Works to proceed with an interim, observation-based gravel-extraction and sediment-management project at Nicol Pit, subject to applicable permits, regulatory approvals, environmental requirements, and technical conditions
d) AGREEMENT - Approval and authorization for the Public Works Director to sign the Agreement with Tehama Asphalt to provide extraction services in accordance with the Simpson Road Bridge erosion threat mitigation and gravel removal operations effective upon date of signing and shall continue for a period of five (5) years (Subject to receipt of required insurance documentation)
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Financial Impact:
The gravel extraction and sediment management project will not result in a direct cost to the County. The agreement with the gravel extractor provides that the only compensation to extractor is to keep the material extracted with limited conditions.
Estimated staff time to oversee the agreement, coordinate with regulatory agencies, and monitor the project are not anticipated to result in annual budget increases.
Background Information:
The Simpson Road Bridge crosses Thomes Creek and provides important transportation and emergency access in Tehama County. Recent observations and comparative aerial imagery indicate that the main flow path, or thalweg, has progressed toward the northerly stream channel and may be concentrating flow closer to the bridge’s north abutment.
The apparent channel migration, combined with the potential for substantial winter storm flows and additional sediment movement, creates an urgent concern regarding the continued stability of the bridge.
The concept of limited gravel extraction is intended to evaluate whether carefully controlled sediment management can reduce flow concentration near the north abutment without compromising natural or structural bed controls. The proposal is not presented as a permanent solution or final engineered design.
The project would be implemented with Tehama Asphalt under a formal agreement addressing access, operational responsibilities, temporary haul-road construction, removal of temporary fill, restoration, and monitoring. The initial action would be incorporated into a multi-year monitoring and adaptive-management program. Future work may be modified or discontinued by regulatory entities based on observed channel and bridge conditions.
1. Simpson Road Bridge
The Simpson Road Bridge is a critical County transportation facility. It provides access for residents, emergency services, agricultural operations, and other users. Damage or loss of the bridge could result in significant public safety, transportation, economic, and emergency-response impacts.
The bridge’s north abutment is located adjacent to an active portion of the Thomes Creek channel. Changes in the location and concentration of flow may expose the abutment to increased velocity, turbulence, bed degradation, bank erosion, or localized scour.
2. Nicol Pit and Proposed Gravel Management
The proposed action involves limited extraction of accumulated gravel from the Nicol Pit area of Thomes Creek. The project concept contemplates the use of two push-pull scrapers to remove material from the channel and transport it to an upland location outside the ordinary high water mark (OHWM).
The proposed extraction is intended to evaluate whether limited sediment management can:
§ Reduce excessive flow concentration toward the north abutment.
§ Evaluate whether limited extraction affects local channel conveyance and flow concentration.
§ Evaluate whether accumulated sediment or debris is contributing to localized flow concentration or obstruction.
§ Influence the location of concentrated flow without destabilizing the channel.
§ The project is not intended to establish a permanent channel alignment, guarantee protection of the bridge, or replace a final engineered bridge-protection or channel-management project.
3. Emergency Condition and Risk Context
The County has identified an apparent and developing risk to the Simpson Road Bridge based on the following conditions:
§ The main flow channel is progressing toward the northerly stream channel.
§ Flow may be concentrating closer to the north abutment.
§ The northerly line of the streambed appears to have migrated or eroded approximately 19 feet over the past two years based on a preliminary review of Google Earth imagery.
§ Continued channel migration could increase exposure of the north abutment to erosive forces.
§ Winter storms could produce higher flows, increased velocity, and additional sediment transport.
§ The bridge is a critical transportation and emergency-access facility; and
§ delay could reduce the County’s ability to implement a controlled, monitored response before the next major storm event.
Given the potential consequences of continued channel migration or scour, staff considers the situation sufficiently serious to warrant an emergency oriented, interim risk reduction response while required technical and regulatory work proceeds.
1. Project Purpose and Objectives
The purpose of the proposed project is to reduce apparent near-term risk to the Simpson Road Bridge while gathering information needed to evaluate longer-term conditions and solutions.
The project objectives are to:
§ Evaluate whether limited gravel extraction can reduce flow concentration near the north abutment;
§ Improve local conveyance without causing undesirable increases in velocity or scour;
§ Avoid removal or disturbance of important bed-control features;
§ Maintain or improve access for project monitoring and emergency response;
§ Establish baseline conditions before work begins;
§ Monitor channel and bridge response during and after the project;
§ Identify conditions requiring modification, suspension, or termination of work; and
§ Provide information for future engineered or permanent bridge-protection solutions.
§ The project is an interim, observation-based risk-reduction concept requiring technical validation, rather than as a completed design.
2. Proposed Method of Operation
§ Removal of accumulated gravel from the designated work area.
§ Transport of excavated material to an upland stockpile or placement area outside the OHWM.
§ Construction of a temporary haul road or access feature using only the minimum material necessary for equipment ingress and egress.
3. Alternatives Considered
§ Alternative 1: No Action
o Under the no-action alternative, the County would not conduct gravel extraction or other interim sediment-management activities.
o This alternative would avoid immediate construction impacts but would leave the apparent channel migration and potential north-abutment scour risk unaddressed. Continued storm flows could result in additional erosion, increased exposure of the abutment, or loss of transportation access.
§ Alternative 2: Limited Interim Gravel Extraction
o This is the proposed alternative. It would involve limited, monitored gravel extraction and temporary access improvements, followed by restoration and multi-year monitoring.
o Potential benefits include:
§ Immediate reduction of accumulated sediment in the targeted area.
§ Possible reduction in flow concentration near the north abutment.
§ Improved understanding of channel response.
§ Creation of information useful for future design decisions.
Potential risks include:
§ Rapid sediment refilling.
§ Increased local scour.
§ Bank erosion.
§ Loss of bed-control features.
§ Unanticipated upstream or downstream channel changes.
§ Environmental impacts.
§ Temporary construction impacts.
§ These risks would be addressed through conservative limits.
§ Professional review, monitoring, stop-work criteria, and adaptive management.
§ Alternative 3: Permanent Engineered Bridge or Channel Protection
o A permanent solution could include engineered bank protection, abutment protection, grade-control measures, channel improvements, bridge modification, or replacement.
o This alternative may provide a more durable response but would require additional engineering, funding, design, environmental review, permitting, and construction time. It is not necessarily feasible as the immediate response to the current concern but should remain under consideration.
§ Alternative 4: Monitoring Without Initial Extraction
o The County could conduct additional surveys and monitoring without implementing the proposed extraction.
o This option would improve the County’s understanding of current conditions but would not provide an immediate physical response to the apparent channel migration and potential scour risk.