HomeMy WebLinkAboutSDP 2016-0001; CARLSBAD OAKS NORTH LOT 24; ADDENDUM SWQMP FOR CARLSBAD OAKS NORTH LOT 24; 2020-12-14
SWQMP Addendum for Carlsbad Oaks North, Lot 24
TO: City of Carlsbad DATE: 12-14-2020
FROM: Michael Baker International
SUBJECT:
PROJECT ID:
RELATED PROJ. ID:
DRAWING NO:
PERMIT NO:
SWQMP Addendum
SDP 2016-0001
CD 2019-0024
DWG 502-3A
GR2017-0015
Introduction
This technical memorandum serves as an addendum to the approved SWQMP prepared by Excel
Engineering for the Carlsbad Oaks North, Lot 24 project. The original SWQMP was approved by
the City of Carlsbad in 2017 (DWG 502-3A). The project represents, construction change number
2 for drawing. The change includes converting recently completed parking stalls and landscape
areas into a building extension. This addendum includes a description of the proposed changes,
as compared to the approved SWQMP, as well as supporting calculations.
Approved SWQMP
The approved SWQMP is for the entire site and utilizes the City’s pollutant control worksheets
and a SWMM analysis to show compliance with water quality and hydromodification
requirements. BMP-E has two tributary drainage areas (DMA 5 & 11). The focus of this addendum
is expansion of BMP-E to accommodate a slight increase in impervious area within DMA 5. No
other DMA or BMP will be updated in this construction change.
Proposed Revision
This construction change will increase the building footprint that is within DMA 5 by an additional
6,400 square feet. The expanded footprint will encroach onto parking/landscaped areas that
have recently been built and will increase the overall imperviousness of DMA 5 by 1,400 square
feet. Drainage patterns will not be altered as a result of the proposed improvements.
BMP-E will be modified to provide additional storage to meet water quality and HMP
requirements for the additional 1,400 square feet of impervious area associated with this
construction change. BMP-E will be expanded by 115 square feet and the recently constructed
riser will be retrofitted with a 1” orifice 0.2’ above the basin’s surface area. The SDHM analysis
maintains the low flow orifice diameter and the expanded basin area have an underdrain that
connect to the existing perforated PVC underdrain.
Michael Baker We Make a Difference
INTERNATIONAL
Technical Memo: SWQMP Addendum for Carlsbad Oaks North, Lot 24
Page 2
Worksheets B-2.1 (for Design Capture Volume) and B-5.1 (Simple Sizing Method for Biofiltration)
have been used to re-size BMP-E based on the newly added area from DMA 5. By modifying the
layout, increasing the surface area, BMP-E will adequately treat the 85th percentile storm water
volume generated from the proposed improvements (See attached worksheets).
The San Diego Hydrology Model (SDHM) has been used to analyze the hydromodification storage
provided by BMP-E. The SDHM input and output is included herein.
Results & Conclusions
The proposed improvements described in this addendum will occur within DMA-5. DMA’s 1-4
and DMA’s 6-14 will remain unchanged. BMP-E has been increased by 115 square feet and the
riser has been modified to now include a 1” orifice located 0.2’ from the basin bottom. This new
basin size adequately treats and mitigates the water from the added 1,400 square feet. The new
biofiltration footprint area for BMP-E has increased from 4,199 square-feet to 4,314 square-feet.
The DMA Exhibit from the approved SWQMP has been included herein with revisions added to
clarify the newly proposed modifications. An excerpt from the construction change is also
provided showing the newly proposed BMP.
Technical Memo: SWQMP Addendum for Carlsbad Oaks North, Lot 24
Page 3
Certification
I hereby declare that I am the engineer of work for this project, that I have exercised responsible
charge over the design of the project as defined in Section 6703 of the business and professions
code, and that the design is consistent with current standards.
I understand that the check of project drawings and specifications by the City of San Diego is
confined to a review only and does not relieve me, as Engineer of Work, of my responsibilities
for the project design.
December 14, 2020
Brian Oliver RCE 45045 Date