Santiago Creek Dam

Santiago Creek Dam is jointly owned by the Irvine Ranch Water District (IRWD) and Serrano Water District (SWD). The districts were jointly notified by the California Division of Safety of Dams (DSOD) that the Santiago Creek Dam is part of a statewide list of 93 dams that are required to conduct an extra spillway assessment in addition to the ongoing yearly inspection program. 

DSOD Spillway Re-evaluation Program

DSOD established the Spillway Re-evaluation Program in 2017 to assess dam appurtenant structures, including spillways, to confirm they meet current safety standards. This is an on-going screening process that includes the assessment of the spillway’s design and construction and geologic attributes while concurrently reviewing the dam owner’s maintenance and inspection program, the spillway’s historical performance, and any previous spillway repairs. Agencies statewide are working closely with DSOD to expedite the development of the required assessments and restore any known areas of disrepair.

Santiago Creek Dam Spillway Assessment

In June 2017, DSOD requested a comprehensive condition assessment of the nearly 90-year old Santiago Creek Dam spillway. The districts retained GEI Consultants (GEI) in August 2017 to perform the assessment.

The first phase of the assessment, which was completed in February 2018, included evaluation of available information on the spillway design and construction, review of the geologic setting, and surface observation of the concrete spillway. Based on the findings, GEI recommended that a second phase of the assessment be conducted to better define and understand the condition of the spillway.

The second phase of the assessment included non-destructive evaluation to determine the thickness of the concrete spillway, borescope exploration to confirm the presence or absence of voids beneath the spillway, concrete core sampling and strength testing, video survey of the spillway underdrain system, and additional geologic mapping in the area immediately downstream of the spillway.

GEI completed the second phase of the assessment in December 2019, which concluded that the concrete spillway structure remains useable, but is nearing the end of its useful life. The assessment further recommended that the planning phase for either the repair or the replacement begin now, and that the spillway replacement is completed within the next 10 years. Based on the findings of the spillway assessments, the districts have decided to immediately proceeding with planning efforts to replace the spillway.

Santiago Creek Reservoir Interim Lake Level Operations Plan

While those planning efforts are underway, and until the replacement spillway is constructed and placed into service, IRWD and SWD have proactively implemented the Santiago Creek Reservoir Interim Lake Level Operations Plan to minimize the use of the spillway.  This plan defines the following operating parameters for the lake:

Summer Season (April 2 – October 30):

The lake will operate without restrictions. Maximum water surface elevation may reach up to the maximum lake water surface elevation of 794-feet.

Winter Season (October 31 – April 1):

Starting October 31, the water level in the lake will be below elevation 762.5-feet. If at any time during the winter season the water level in the lake reaches or exceeds an elevation of 762.5-feet, IRWD and SWD will implement measures to reduce the water level to below elevation 762.5-feet.  Starting March 14 of every winter season, IRWD and SWD will monitor the water level in the lake and the weather forecast to determine if the water level could be increased to a maximum elevation of 772.5-feet for the remainder of the winter season.

Santiago Creek Reservoir Outlet Tower Analysis

As part of a continued effort to maintain and inspect dam facilities, the districts retained AECOM to perform a seismic stability analysis of the existing Irvine Lake outlet tower. The analysis, which was completed in July 2017, concluded that the outlet tower should be replaced with a new sloped intake structure.  In February 2019, AECOM completed the Santiago Creek Dam Outlet Tower Replacement Preliminary Design Report (PDR), which includes abandoning the existing outlet tower, utilizing the existing outlet pipeline under the embankment dam, and constructing a new inclined intake structure. In November 2019, DSOD approved the concepts summarized in the PDR, and the Districts are proceeding with the simultaneous development of both the outlet tower replacement and the spillway replacement projects under the expert technical guidance of two nationally-recognized dam engineering consultants, AECOM and GEI.

To learn more about IRWD’s dam safety inspection program please visit this page.