To: Honorable Mayor and Members of City Council
From: Phil Brun, Utilities Director
Prepared By: Addison LeBlanc, Assistant Engineer
TITLE:
Title
Conn Creek Spillway Alternatives Assessment
LABEL
RECOMMENDED ACTION:
Recommendation
Authorize the Utilities Director to execute a contract with GEI Consultants, Inc., in an amount not to exceed $290,480 for the Conn Creek Spillway Alternatives Assessment required by the California Division of Safety of Dams (DSOD), and determine this action is exempt from CEQA.
Body
DISCUSSION:
Conn Creek Dam was constructed in 1948 and had significant upgrades to the spillway in 1968. Since then, the City and DSOD have continued annual inspections and maintenance with no major upgrades. The City has been submitting engineering analyses and studies to DSOD, as part of the annual maintenance inspections which include recommendations that aid in reviewing the aging infrastructure. The hydrology analysis was reviewed in 1984 and the seismic analysis in 1992.
GEI performed the Spillway Condition Assessments required by the DSOD under the Spillway Re-evaluation program created in response to the 2017 Oroville Spillway incident. The assessments provided necessary interim repair recommendations in 2017, which were performed in 2019, and further investigations in 2020 for repairs planned for July 2022.
The Spillway Re-evaluation program includes updates to the hydrologic and hydraulic analysis of the spillway as part of the long-term planning. Recent updates to the hydrology reports that are used to analyze the hydraulic capacity of the Spillway were included in an amended report and submitted to DSOD for review of the probable max flood (PMF). DSOD’s review confirmed the report’s findings that the spillway capacity needs to be increased to meet current design standards. DSOD requested the analysis be further refined with feasibility and alternatives analysis to meet modern design standards and address the deficiency.
The planned repairs in 2022 include exploration of materials below the concrete surface of the spillway and are considerations for design alternatives based on the condition of the subsurface structure. These are unique design and performance criteria for alternatives and are part of GEI’s current work.
The alternatives assessment is an important budgeting tool for the upcoming Master Plan update and allows the long-term planning for maintaining the City’s local water supply. GEI has reviewed the historical rehabilitation, upgrades, hydrologic and hydraulic analysis, and current conditions of the spillway for DSOD through the spillway condition assessment. Based on this, their intimate knowledge is integral in the feasibility of upgrading the spillway to ensure its long-term use while meeting current design standards of modern spillway structures as mandated by DSOD. The most cost-effective and timely result for the project is to continue services using the specialized engineering expertise that has gone into this extensive review and assessment.
The outcome of the alternative’s analysis will be three preferred design options presented to DSOD for approval. This will allow the City to move forward with full design and seek federal grant funding.
Staff recommends that the City Council authorize execution of the Agreement for Hennessey Spillway Alternatives Assessment, pursuant to Napa Municipal Code Section 2.91.050, Exceptions for Specialty Items. A written determination from the City Manager supporting this recommendation is included as Attachment 1.
FINANCIAL IMPACTS:
The Agreement provides a not to exceed expenditure of $290,480. Compensation will be on a time and materials basis in accordance with the rates outlined in the negotiated agreement and the written determination in Attachment 1. Sufficient funding for the Project is composed of existing appropriations for the Hennessy Spillway Flow Expansion (WS23UT01) and Hennessy Spillway Spot Repairs (WS20UT01).
CEQA:
The Utilities Director has determined that the Recommended Action described in this Agenda Report is not in-and-of-itself a “project” (pursuant to CEQA Guidelines Section 15378) since it does not result in a physical change in the environment.
However, the Recommended Action is a part of a larger “project” that will be subject to environmental review in accordance with CEQA at the “earliest feasible time” prior to “approval” consistent with CEQA Guidelines Sections 15004 and 15352. The larger “project” is Conn Creek Spillway Modifications, and staff plans to bring back a CEQA analysis to Council prior to approval to construct the Project.
DOCUMENTS ATTACHED:
ATCH 1 - Written Determination for Specialty Items - Conn Creek Spillway Alternatives Assessment
NOTIFICATION:
None.