20170125091435.pdfCcivil & structural
ENGINEERING engineering & planning
DRAINAGE MEMORANDUM #2
To: Jeanie McConnell, Engineering Program Manager, City of Edmonds
From: Greg Guillen, PE, SE, Principal
cc.
Date: November 10, 2016
Re: Edmonds Post Office Stormwater Site Plan Report
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The Edmonds Post Office project's report created by LPD Engineering, PLLC dated April 3, 2014 sized facilities for both
phases of the project. Phase I has already been mostly constructed.
Our only concern through our report review was that the detention system appeared to be sized for a site with 0.04
acres of pervious surfaces and 0.96 acres of impervious surfaces, resulting in a 6 foot diameter, 50 lineal foot detention
pipe. A more appropriate area count would be 0.02 acres of pervious surfaces and 0.98 acres impervious surfaces. LPD
used the correct areas for the conveyance and other calculations.
We had thought, in a previous submittal, that the pipe had actually been installed at 62 lineal feet (per construction
plans) to accommodate this difference in areas. Through the comment process, we found this to be incorrect and that
the 50 lineal feet was used instead.
The detention pipe flow control structure calls for a 1.875 inch diameter orifice along with a 12 inch wide, 9 inch tall
notch. In our experience, the orifice and notch combination works very well in meeting flow control duration
standards. On this project, however, we are only required to match peak flows prescribed by the City of Edmonds
Stormwater Supplement Manual. By adjusting the flow control structure so that the notch is only 6 inches tall instead
of 9, the pipe passes the flow control test.
If the notch on the Phase 1 drawings has not been cut yet, it should be cut at 6 inches instead of 9 inches. If it has been
cut, CG Engineering's recommendation is that a pipe coupling be installed in such a way as to block the notch's bottom
3 inches. Attached please find the WWHM calculations for what we believe to be the current as built condition, and for
the recommended fix to bring the pipe into compliance.
If you have any additional questions or concerns please do not hesitate to call.
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11/ 1U/ 10
DISCLAIMER
This recommendation is the professional opinion of CG Engineering PLLC based on the information provided. This memo was prepared subject to the
standard of care applicable to professional services at the time the services were provided.
250 4th Avenue South, Suite 200
Edmonds, WA 98020
ph.425.778.8500 1 f.425.778.5536
www.cgengineering.com
Western Washington Hydrology Model
PROJECT REPORT
Project Name: edmondsmixedph2 (PHASE 1 AS BUILT PIPE)
Site Address:
City
Report Date 11/8/2016
MGS Regoin Puget East
Data Start 1939/1.0/1.
Data End 2097/08/31
DOT Data Number: 02
WWHM3 Version:
Name : Basin 1
Bypass: No
GroundWater: No
Pervious Land Use Acres
C, Lawn, Flat .02
Impervious Land Use Acres
ROOF TOPS FLAT 0.98
Name 'rank I
Tank Name: Tank 1
Dimensions
Depth: 6ft.
Tank Type Circular
Diameter 6 ft.
Length : 50 ft.
Discharge Structure
Riser Height: 5.5 ft.
Riser Diameter: 12 in.
NotchType Rectangular
Notch Width 1.000 ft.
Notch Height: 0.750 ft.
Orifice 1 Diameter: 1.875 in. Elevation: 0.5 ft.
Element Flows To:
Outlet 1 Outlet 2
MITIGATED LAND USE
Flow Frequency Return Periods for Mitigated. POC #1
Return Period
Flow(cfs)
2 year
0.159543
5 year
0.212252
10 year
0.253447 Fail (over 0.25)
25 year
0.313153
50 year
0.363574
100 year
0.419444 Pass (under 0.45)
This programand accompanying documentation is provided as -is' without warranty of any kind.. The
entire risk regarding the performance and results of this program is assumed by the user, Clear Creek
Solutions and the Washington State Department of Ecology disclaims all warranties, either expressed
or implied, including but not limited to implied warranties of program. and accompanying documentation.
In no event shall Clear Creek Solutions and/or the Washington State Department of Ecology be liable
:for any damages whatsoever (including without limitation to damages for loss of business profits, loss
of business information, business interruption, and the like) arising out of the u.se of, or inability
to use this program. even if Clear Creek Solutions or the Washington State Department of Ecology has
been advised of the possibility of such damages,
Western Washington Hydrology Model
PROJECT REPORT
Project Name: edmondsmixedph2 (FIX WEIR "I."O BRING PIPE TO COMPLIANCE)
Site Address:
City
Report Date 11/8/2016
MGS Regoin Puget East
Data Start 1939/10/1
Data End 2097/08/31
DOT Data Number: 02
WWHM3 Version:
Name Basin 1
Bypass: No
Groundwater: No
Pervious Land Use Acres
C, Lawn, Flat .02
Impervious Land Use Acres
ROOF TOPS FLAT 0.98
Element Flows To:
Surface Interflow Groundwater
Tank 1, Tank 1,
Name Tank 1
Tank Name: Tank 1
Dimensions
Depth: 6ft.
Tank Type Circular
Diameter 6 ft.
Length : 50 ft.
Discharge Structure
Riser Height: 5.5 ft.
Riser Diameter: 12 in.
NotchType Rectangular
Notch Width 1.000 ft.
Notch Height: 0,500 ft. * * 1 111 REV]..SE E I T ][NC NOTCH TO 0.5 ft.
Orifice 1 Diameter: 1.875 in. Elevation: 0.5 ft.
Flow Frequency Return Periods for Mitigated.. POC #1
Return Period
Flow(cfs)
2 year
0.158812
5 year
0.209892
10 year
0.249605 Pass (under 0.25)
25 year
0.306903
50 year
0.355092
100 year
0.408309 Pass (under 0.4.5)
This program and accompanying documentation is provided 'as -is' without warranty of any kind. The
entire risk rogwAing the performance and results of this program is assumed by the user. Clear Creek
Solutions and the Washington State Department of Ecology disclaims all warranties, either expressed
or implied, including list not limited to implied warranties of program and accompanying documentation.
In. no event shall Clear. Creek Solutions and/or the Washington State Department of Ecology be liable
for any damages whatsoever. (including without limitation to damages for loss of business profits, loss
of business information, business interruption, and the like) arising out of the use of, or inability
to use this program. even if Clear Creek Solutions or the Washington State Department of Ecology has
been advised of the possibility of such damages.