HomeMy WebLinkAboutItem 6c. Authorize On-Call Engineering Professional Services for a Hydrologic and Hydraulic Probable Maximum Flood StudyCity of San Luis Obispo, Title, Subtitle
Whale Rock Commission Report
Whale Rock Commission Meeting – May 28, 2026
Agenda Item 6c – Authorize On-call Engineering Professional Services for a Hydrologic and Hydraulic Probable
Maximum Flood Study
FROM: Shawna Scott, Assistant Director of Utilities
PREPARED BY: Noah Evans, Whale Rock Reservoir Supervisor
SUBJECT: AUTHORIZE ON-CALL ENGINEERING PROFESSIONAL
SERVICES FOR A HYDROLOGIC AND HYDRAULIC PROBABLE
MAXIMUM FLOOD STUDY
RECOMMENDATION
Authorize selection of a consultant from the City of San Luis Obispo’s Qualified Civil
Engineering On-Call Consultant List for engineering professional services consisting
of the preparation of a hydrologic and hydraulic probable maximum flood study, and
issuance of a purchase order, and any subsequent change orders, in an amount not
to exceed the total authorized budget of $500,000.
DISCUSSION
Background
During September 2025, staff received a letter
from Whale Rock’s Regulatory Authority, the
Division of Safety of Dams (DSOD) (Attachment
A). In this letter, the DSOD directs the Whale
Rock Commission to perform a hydrologic and
hydraulic study to verify that the Whale Rock
Dam and spillway can withstand a Probable
Maximum Flood (PMF) event without
overtopping.
The PMF is the theoretical upper limit of flood
runoff, representing the most severe, yet
feasible, combination of meteorological and
hydrological conditions in a specific drainage
basin. It is derived from the Probable Maximum
Precipitation (PMP) to ensure, with high
certainty, that infrastructure like dams and
spillways can withstand the largest potential,
though extremely rare, flood event.
The DSOD has directed the Whale Rock Commission to conduct this study to verify
that the original PMF used to design the Whale Rock Dam and spillway remains
applicable, representative of current weather patterns, and will not result in
overtopping during a PMF event.
WHALE ROCK DAM SPILLING DURING 2023
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City of San Luis Obispo, Title, Subtitle
Whale Rock Commission Report
Whale Rock Commission Meeting – May 28, 2026
Agenda Item 6c – Authorize On-call Engineering Professional Services for a Hydrologic and Hydraulic Probable
Maximum Flood Study
ENVIRONMENTAL REVIEW
The California Environmental Quality Act (CEQA) does not apply to the
recommended action in this report because the action does not constitute a “Project”
under CEQA Guidelines Section 15378 . The action is limited to a hydrologic and
hydraulic study and does not include any physical effects on the environment.
FISCAL IMPACT
Hydrologic and Hydraulic Study to Verify the Dam Can Pass the
Probable Maximum Flood
Funding
Category
Contribution
Source
Amount
Budgeted
Amount Spent
Capital Asset
Expense
2026-27 CIP
Contributions
$500,000 $0
ALTERNATIVES
The Commission may direct staff not to use the City of San Luis Obispo’s Qualified
Civil Engineering On-Call Consultant List and to release a Request for Proposals
(RFP). Firms on the Qualified Civil Engineering on -Call Consultant List have been
selected through a competitive qualifications-based bidding process, and have
previously performed studies and designed projects for Whale Rock, demonstrating
their expertise and knowledge of the facility. Advertisement of an RFP requires
additional staff resources and would delay the DSOD-mandated study by
approximately two months.
Attachments:
A - September 5, 2025, letter from the Division of Safety of Dams
Page 22 of 103
Docusign Envelope lD: E6966E92-1 D48-4478-8O37-AOB402B0FDEA
STATE OF CALIFORNIA - CALIFORNIA NATURAL RESOURCES AGENCY GAVIN NEWSOM, Governor
DEPARTMENT OF WATER RESOURCES
DIVISION OF SAFETY OF DAMS
2720 GAIEWAY OAKS DRIVE, SUITE 3OO
SACRAMENTO, CA 95833-3500
September 5,2025
Mr. Aaron Floyd, Deputy Director
Water Division
Whale Rock Commission
879 Morro Street
San Luis Obispo, California 93401
Whale Rock Dam, No. 1-40
San Luis Obispo Countv
Dear Mr. Floyd:
This is in reply to two letters dated June 14, 2019, and December 12,2024,with
multiple enclosures, from Whale Rock Commission (WRC) to the Division of Safety of
Dams (DSOD) in response to DSOD's letter dated May 24,2017, requesting a
comprehensive condition assessment of the spillway at Whale Rock Dam. The
enclosures reviewed for this response include the SpillwayAssessmenf report (Spillway
Report), by HDR Engineering, lnc. (HDR), dated May 20, 2019, and a final video survey
of the repaired spillway underdrain pipes by Nor-Cal Pipeline Services, dated October 5,
2023.
As discussed in an August 7,2025, call with Whale Rock Supervisor Mr. Noah Evans,
DSOD has reviewed the enclosures and has completed an independent comprehensive
review of the dam. From these reviews, DSOD has identified potential dam safety
concerns related to the dam's seismic stability and hydrologic capacity that will require
WRC to perfornr a comprehensive assessment of the dam system. At a minimum, this
comprehensive assessment must include the following:
1. A static and seismic stability evaluation of the dam using modern methods and
standards for dams.
2. A new high-quality subsurface exploration prior to the required seismic stability
evaluation since the past geotechnical investigations have not thoroughly
characterized the embankment and foundation alluvium.
DWR 9004 (New 8/25)
Page 23 of 103
Docusign Envelope lD: E6966E92-1 D484478-8O37-A6B402B0FDEA
Mr. Floyd
September 5,2025
Page 2
3. A hydrologic and hydraulic study to verify that the dam can pass the Probable
Maximum Flood (PMF) without overtopping. This needs to include visually
inspecting the condition of the stilling basin and evaluating the feasibility of
inspecting the spillway chute drains that were not previously inspected.
Following the hydraulic analysis, WRC must assess whether the current
condition of the spillway drainage system, given the observed presence of grout
intrusion, reduces the spillway's functionality to safely pass the PMF.
WRC must submit a plan and schedule for performing this comprehensive assessment
to DSOD by December 31 ,2025. This plan and schedule needs to incorporate a plan
for filing an alteration application for the required geotechnical exploration along with the
work plan and associated filing fees.
ln addition, HDR recommended several maintenance-level repairs in the Spillway
Report be performed, which include sealing of cracks and joints, repairing of spalls, and
grinding down wall offsets at joints. As documented during DSOD's recent periodic
maintenance inspection, these repairs have been satisfactorily completed with the
exception of grinding down adverse offsets at spillway walljoints. As required in the
recent inspection report, WRC submitted an August 19,2025,letter with a plan and
schedule to complete this work, which is currently under DSOD review.
HDR also identified an area of seepage in the spillway at Station 2+77. WRC must
inspect this area at least monthly, and any observations of seepage or turbidity must be
incorporated into the dam's annual instrumentation report. WRC needs to continue
performing regular spillway inspections and maintenance, including monitoring previous
repairs and sealing developing cracks to ensure that the spillway can safely pass flows,
and notify DSOD of any change to the spillway, including the projection of spill events.
lf you have any questions or need additional information, you may contact Dam Safety
Engineer Richard Keizer at (916) 612-4802 or Project Engineer lan Maki at
(916) 820-77e5.
Sincerely,
Sh."utu- h^rs for
Erik J. Malvick, Ph.D., P.E., G.E
Division Manager
Division of Safety of Dams
DWR 9004 (New 8/25)
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