CC 10-17-2023 Item No. 5. Stevens Creek Boulevard Calabazas Creek Storm Drain Repair Emergency Work_Supplemental Report
CITY COUNCIL STAFF REPORT
SUPPLEMENTAL 1
Meeting: October 17, 2023
Agenda Item #5
Subject
Stevens Creek Boulevard/Calabazas Creek Storm Drain Repair Emergency Work
status report and authorization to continue.
Recommended Action
Receive a status update on the Emergency Work at Stevens Creek
Boulevard/Calabazas Creek Storm Drain Capital Improvement Programs (CIP)
project and authorize the continuation of the emergency action, by four‐fifths vote.
Background:
Staff’s responses to questions received from councilmember are shown in
italics.
(1) Please provide the extent of 54” Corrugated pipe replacement – how many
linear feet and wat will it be replaced with? Please send the plan set to
Council. (Moore)
The existing 54” corrugated metal pipe (CMP) is approximately 45 linear feet, and
extends westerly from the Calabazas Creek culvert, that passes under Stevens
Creek Blvd, to the first storm drain manhole on Stevens Creek Blvd (see
Attachment A for the plan set used to construct the pipe). The existing pipe will be
replaced with a 54” reinforced concrete pipe (RCP).
(2) How will the existing crushed pipe be replaced in simple terms? (Moore)
The existing failed CMP pipe will be replaced with a 54” RCP pipe, and the
excavated volume will be backfilled with controlled density fill to ensure all voids
are filled and to expedite construction activities.
(3) What was the in‐house cost estimate? (Moore)
The original estimate to replace this section of pipe was approximately $420,000.
(4) Why do we prefer to avoid emergency replacements? (Moore)
The City prefers to avoid emergency repairs as these types of projects typically
come at a premium cost. This is largely due to a need to address an emergency
repair quickly and to have a contractor commit resources with 100% focus on the
work. In the case of this project, the contractor will also be working some overtime
hours to reduce the construction duration an attempt to minimize impacts to the
public and make the repair before a substantial rain event occurs.
(5) Is further replacement imminent? Is more of this pipe expected to fail? If
not, why not? (Moore)
The remaining upstream portions of the storm drain main along Stevens Creek
Blvd consist of RCP pipe, which are in good condition. The City does not
anticipate any further failures of the main at this time.
(6) Why did it fail? Age? Rust? Any speculation? (Moore)
CMP pipe tends to oxidize and corrode over time, eventually leading to material
deterioration. Because of this, CMP pipe has an expected useful life of 40‐50 years.
The CMP pipe being replaced with this project is about 43 years old and has
undergone extreme deterioration along the bottom of the pipe (the pipe invert).
This deterioration resulted in the bottom being completely rusted away. For this
reason, the City initiated the FY23‐24 capital improvement project to replace the
pipe, and began initial investigations for the project in late September 2023. With
these investigations, it was apparent that the pipe had experience significant
degradation due to the winter storms in early 2023 and needed to be replaced
before the rains this winter.
(7) What hydrologic area fed this pipe (there should be a hydrology study
which determined the initial 54” CMP size)? Has that area changed?
(Moore)
The storm drain main serves the Stevens Creek Blvd corridor from Calabazas
Creek westward to approximately Highway 85 and includes large portions of land
both north and south of the boulevard. Storm drain mains are typically sized based
on hydraulic calculations for the drainage basin being served. For the drainage
basin in question, there is no indication that the size of the area served by the main
has significantly changed since it was first established.
(8) Was this pipe designed for a 10‐year storm or a 100‐year storm. (What has
Cupertino been designing the storm drain system to?). The last City
Council meeting indicated our storm drains are designed to a 10‐year
storm, I want to verify this. (Moore)
For at least the last 30‐years, the City has designed its storm drain mains to
convey a 10‐year storm. While, no hydraulic calculations could be found for this
storm drain main, when it was constructed, it appears to have been hydraulically
sized to convey a 10‐year storm. The City has not historically designed its storm
drain facilities to convey a 100‐year storm.
(9) Please provide the photographs showing the damage to the pipe which
may have led this emergency status. Why is it imminent danger? (Moore)
See Attachment B for before and after photographs of the damaged pipe. The repair
has been deemed a risk to public health and safety due to the potential for further
erosion of the subgrade below Stevens Creek Blvd and the possibility of a
significant sink hole within the street.
(10) When was the 54” CMP installed? As we are aware, these fail over time.
(Moore)
The existing CMP pipe was installed around 1979‐1980.
Attachments Provided with Original Staff Report:
None
Attachments Provided with Supplemental 1:
A. Plans for Calabazas Creek at Stevens Creek Blvd
B. Photos of Storm Drain Damage