storm drainage report - marin county
TRANSCRIPT
MARIN GENERAL HOSPITAL
WEST WING ADDITION
GREENBRAE, CALIFORNIA
STORM DRAINAGE REPORT
PREPARED BY:
KPFF CONSULTING ENGINEERS
45 FREMONT STREET, 28TH FLOOR
SAN FRANCISCO, CA 94105
FOR:
MARIN HEALTHCARE DISTRICT
C/O VERTRAN ASSOCIATES, LLC
242 BUTTERFIELD ROAD
SAN ANSELMO, CA 94960
DATE: MARCH 29, 2019
KPFF Consulting Engineers Marin General Hospital
March 29, 2019 Storm Drainage Report
Page 1 of 3
45 Fremont St, 28th Floor San Francisco, CA 94105 415.989.1004 kpff.com
INTRODUCTION
KPFF Consulting Engineers has compiled this report to evaluate the proposed stormwater
design for the planned Marin General Hospital West Wing Addition project. The scope of this
report is the compilation of the analysis examining drainage patterns, stormwater runoff,
and the storm drain system for the purpose of meeting the requirements of the authorities
having jurisdiction for permitting of the project.
The Marin General Hospital West Wing Addition project consists of several improvements to
the existing campus located at 250 Bon Air Road in Greenbrae, California. The proposed
redevelopment consists of the remodel of the West Wing building (ground and partial first
floor) on the Marin General Hospital campus along with a building addition at the north side,
site improvements of approximately 80,000 square feet and a new median curb cut and left
turn lane on Bon Air Road.
EXISTING CONDITIONS
The existing project site consists of the existing west hospital wing, surface parking lots, and
undeveloped landscape areas throughout the site. The Marin General Hospital site is located
northwest of highway US101, bound by the Larkspur City limits on the south, Bon Air Road
and Creekside Park on the north and west, and Marin County Property on the east.
The project site generally slopes from east to the west with elevations ranging from
approximately 10-15’ above mean sea level. The catch basins drain directly into the Marin
General Hospital storm drain system. The existing surface lot drains to the existing on-site
storm drainage system and conveys the stormwater runoff through the site into the Marin
County public drainage system in Bon Air Road which eventually drains directly to Corte
Madera Creek through a 72-inch diameter outfall located just southwest of the West Wing
main Hospital entrance.
PROPOSED IMPROVEMENTS
The proposed Marin General Hospital West Wing Addition project consists of a number of
improvements, including the following:
• Addition to existing West Wing building totaling approximately 3,900 square feet.
• Addition of site improvements of approximately 80,000 square feet including new
round-about and drop off area.
• Addition of new median curb cut and ADA path across Bon Air road along with left
turn lane on Bon Air Road.
Runoff from the proposed West Wing addition will be mostly captured by the pervious
asphalt pavement and bioretention. One bioretention area will capture runoff from the new
West Wing addition structure and treat low volume storms before releasing into the Marin
KPFF Consulting Engineers Marin General Hospital
March 29, 2019 Storm Drainage Report
Page 2 of 3
General Hospital drainage system via perforated pipe underdrains. Runoff volumes that
exceed the scope of treatment requirements will enter overflow drains.
In order to comply with Table 1-1 of the BASMAA Post-Construction Manual which requires
the area of impervious surfaces created or replaced is less than 50% the original pre-2015
condition, pervious pavement will be utilized in the vehicular travelways as shown in the
exhibits provided in Appendix B.
Impervious area exhibits for each phase is shown on sheets C14.1-C14.2. Plan sheet C14.1
shows that the pre-developed existing impervious area of 389,613 sq. ft., of which 50% is
194,807 sq. ft. Plan sheet C14.2 shows that after the final phase of the Marin General
Hospital project, the total impervious area added/replaced in the entire site is 188,267 sq. ft.
The total impervious added/replaced is roughly 6,000 sq. ft. less than 50% of the original
pre-2015 condition.
STORMWATER ANALYSIS SUMMARY
Stormwater analysis for the Marin General Hospital West Wing Addition was performed in
two phases. First, in order to meet Marin County Municipal Code Section 24.04.520,
impervious and pervious areas were calculated for each phase of the project to determine
the hydrological effect on the site. This analysis is intended to confirm that the post-
developed site has less impervious area than the pre-developed condition and therefore will
have no additional impacts to the Marin County public drainage system. Second, a hydraulic
design is prepared for the proposed Marin General Hospital drainage system upgrades. This
design is intended to confirm that proposed pipes are sized adequately to convey the design
storm. In both cases, the 100-year design storm is used.
For the West Wing Addition project site, the pre-developed existing has a total impervious
area of 32,756 sq. ft. The total impervious area post construction of the site is 12,117 sq. ft.
Therefore post-development impervious area is less than the pre-developed existing
impervious. Using the rational method, the decrease in total impervious area will lower the
overall curve number of the site and therefore decrease the peak runoff volume of the site
during a storm event.
HYDRAULIC ANALYSIS SUMMARY
Rational methodology was employed for the hydraulic modelling. The proposed drainage
pipe network was designed using AutoCAD Civil 3D 2018 to conform to the proposed site
plan. Inlet structures, manholes, and conveyance pipes were all modelled into a single 3D
model of the project. Once this information was correctly placed, the network was exported
to Hydraflow Storm Sewers, an extension of AutoCAD Civil 3D. With catchment area
characteristics applied to each inlet and an applied 100-year storm event intensity, the pipe
network was analyzed to confirm the pipes are capable of conveying the peak flow. A
KPFF Consulting Engineers Marin General Hospital
March 29, 2019 Storm Drainage Report
Page 3 of 3
hydraulic grade line (HGL) at the Bon Air Road point of connection was set at 7.0’ amsl,
approximately equal to the 100-year flood water surface elevation (WSEL).
Hydraulic Model
Rainfall intensity logged in the NOAA Rainfall Atlas was used to create a 10-year and 100-
year IDF curve for the project site. This IDF curve was applied to the catchments and overall
model for the West Wing Addition project. Ideally, all storm water would be conveyed in the
pipes and no surcharge would be applied at the inlet structures and manholes. Several
structures did have a hydraulic grade line above the crown of the inlet and outlet pipes. In
each of these structures, however, the HGL did not encroach to within 2.0-feet of the grate
elevation during the 100 year storm. The surcharged inlets and associated elevations are
listed in the table below:
Structure Name HGL Elevation (ft) Rim Elevation (ft) Freeboard (ft)
SDMH #1 7.06 11.76 4.70
SDCB #3 7.09 12.71 5.62
JB #2 7.54 13.17 5.63
SDCB #2 8.03 11.27 3.24
SDCB #1 8.14 11.38 3.24
DI #1 7.65 14.33 6.68
The 10-year design storm was also analyzed to confirm that no surcharge occurred in any
storm drain structure.
See Appendix B for more information.
CONCLUSION
The proposed improvements associated with the Marin General West Wing Addition project
will not have an increased impact on the Bon Air Road public storm drain system, and the
private storm drainage system associated with the Hospital Replacement Building is
designed to meet the requirements of Marin County Municipal Code Section 24.04.520.
APPENDIX A
VICINITY MAP & PLAN SET
V
A
N
V
A
N
N
O
P
A
R
K
I
N
G
HOSPITAL
REPLACEMENT BUILDING
DMA #1
DMA #3
DMA #7
DMA #8
DMA #6
(E) CENTRAL
WING
(E) WEST WING
DMA #4
BON AIR ROAD
DMA #5
DMA #2
1"=20'
STORMWATER CONTROL PLAN
WEST WING ADDITION
C7.1
N
O
T
F
O
R
C
O
N
S
T
R
U
C
T
I
O
N
Revisions:
Key Plan/
North Arrow
Issue
Date:
Scale:
Sheet
Title:
Sheet
Number:
5510 Lincoln Blvd. Suite 250
Los Angeles, California 90094
Phone (310) 829-2249
Fax (310) 829-1736
Revision # Revision Description Revision Date
Perkins Eastman
Client
Project
PE
Job Number:
OSHPD
Number:
35587.00
MGH: WW REMODEL/ADDITION
STRUCTURAL ENGINEER
MECHANICAL / PLUMBING / ELECTRICAL ENGINEER
CIVIL ENGINEER
LANDSCAPE ARCHITECT
INTERIOR DESIGNER
KPFF Consulting Engineers
45 Fremont Street, 28th Floor
San Francisco, CA 94105
Mazzetti
220 Montgomery Street, Suite 650
San Francisco, CA 94104
KPFF Consulting Engineers
45 Fremont Street, 28th Floor
San Francisco, CA 94105
SWA
2200 Bridgeway Boulevard
Sausalito, CA 94966
Forrest Perkins
100 Montgomery Street, Suite 2300
San Francisco, CA 94104
Consultants:
Marin Healthcare District
AIM
J. H
JAS
ON
MAY 31, 2019REN. DATE
NO. C-27994
AINROFILACFOET
ATS
T
D ARCHI TEC
ESNECIL
250 Bon Air Road
Greenbrae, CA 94904
McCarthy Building Companies
1265 Battery Street, 3rd Floor
San Francisco, CA 94111
General
Contractor:
Construction
Manager:Vertran Associates
250 Bon Air Road
Greenbrae, CA 94904
OSHPD
Approval:
TIME STAMP:
01/31/2019 - Design Review
Application Backcheck
C
MYRO
N
STATE OF CALIFORNIA
REENI
GNELANOISSEFORPDERETSIGER
L
NABEI
No. 84348
LIVIC
N
CHA
O
Exp. 09/30/2019
1COUNTY BACKCHECK 01/31/2019
1
1
2COUNTY BACKCHECK 2 03/29/2019
2
2
APPENDIX B
HYDROLOGIC MODELLING DATA & RESULTS
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