HomeMy WebLinkAboutSTAMPED - 2023-11-02 Brotherton Field Conditions Resolutions Memo Page 1 of 6
4815 Center Street
Tacoma, WA 98409-2319
(253) 474-9449 / sittshill.com
Brent K. Leslie, P.E., S.E.
Kathy A. Hargrave, P.E.
Larry G. Lindell, P.E., S.E.
Michael A. McEvilly, P.L.S.
Andrew J. Boileau, P.E., S.E.
David C. Boileau, A.I.A.
Memorandum
To: Nathan Janders, P.E., Development Engineering Manager
From: Don Davis, P.E.; Damian Matzen, E.I.T.
Date: November 2, 2023
CC: Scott Lindley, J.M. Riley Company; Jim Castino, Castino Architecture
Subject: Brotherton Stormwater-Utility Modification
During construction of the stormwater system for the new Brotherton Cadillac car dealership, several
conflicts with existing utilities have been encountered in the field by the contractor. This memo
itemizes the conflicts, summarizes our proposed plan revisions, and provides backup stormwater
calculation information as necessary.
Conflict #1: Domestic water and irrigation water services were found to be in conflict with the
proposed storm pipe between CB11 and CB-E.
Our initial proposal for adjustment of the stormwater system in the vicinity of the domestic and
irrigation water crossings was deemed infeasible due to invert elevation offsets in CB11 which resulted
in an unacceptable level of water water detained upstream of CB11. However, the pipe segment
between CB11 and CB-E had already constructed based on our initial findings that the vertical step at
CB11 would not negatively impact the hydraulic capacity of the system, and the contractor’s need to
restore the surface in front of the carwash for the owner’s business.
Therefore, the attached plan set includes proposed replacement of the 12” pipe upstream of CB11 to
remedy the vertical discrepancy in CB11. The replaced pipe between the existing upstream catch basin
and CB11 will be installed at a 0.44% slope (minimum slope required to achieve 3 feet per second (fps)
full flow pipe velocity, see Appendix A), and result in an invert elevation approximately one-inch higher
than the incoming 12” pipe in that structure. Since the crowns in that structure will not match, we have
prepared a backwater analysis using Autodesk Storm and Sanitary Analysis (SSA) software to
demonstrate that the upstream stormwater system will not be negatively impacted by the one-inch
step, see Appendix B.
It is expected that this pipe replacement will occur at a later date, when the contractor’s job trailer can
be relocated elsewhere on the site. At that time, the contractor shall also excavate the 12” pipe
crossing at the domestic and irrigation water services, and insert ethofoam pads between the utilities.
Conflict #2: The existing 8” public water main which runs alongside the westerly property line of
the Cadillac site was found to be in conflict with the proposed stormwater pipe segment between
CB-D and CB-E.
The proposed storm cannot be installed below the existing water due to elevation constraints in the
downstream pipe network that we are tying into. We understand that the City of Renton has special
design guidelines for non-standard utility separation, per Section 3-11.3 of the February 2021 Water
Utility Design and Construction Standards. Therefore, we are proposing to route the storm over the
existing water, with a minimum of 6” separation and with CL52 ductile iron pipe. In order to meet the
0.44% minimum slope for 3fps velocity, the entire pipe run from CB-E through CB-G upstream will need
to be adjusted.
DEVELOPMENT ENGINEERING
NJanders 11/09/2023
Page 2 of 6
Another challenge stemmed from the aforementioned pipe slope adjustment. The downstream invert
elevation of the pipe between the stormwater pump station and CB-B, an overflow pipe in the event
the pumps are non-functional, had to be raised to an elevation such that the pipe between these two
structures would be flat. We considered raising the upstream end of the pipe, but it is in conflict with
the lid of the pump station structure.
Therefore, we selected an alternate location for the overflow pipe from the system detained behind
potentially inactive pumps. We are now proposing to install a pipe between CB10 to CB-D. If the
pumps are non-functional, stormwater will back up the entire system behind the pump station until it
reaches the elevation of this new overflow pipe.
We have used SSA to model the stormwater system under a 100-year, 24-hour rainfall event, with the
pumps disabled. The attached plan and profile excerpts from SSA, Appendices C and D, respectively,
have been prepared to demonstrate that the Cadillac collection and conveyance system will
satisfactorily drain via the new overflow pipe location. Pipes will surcharge, but stormwater structures
will not overtop in the event the pumps are non-functional.
Conflict #3: The existing 8” public water main in the SW 12th St ROW was found to be in conflict
with the proposed sanitary sewer outfall from the Cadillac site.
Similar to the above scenario, a gravity connection to the sanitary sewer main in 12th underneath the
existing water main is not feasible. We are seeking approval of routing of the sanitary sewer service
over the water main per the provisions of Section 3-11.3 of the aforementioned design guidelines
document. This segment of the sanitary sewer service will be constructed with ductile iron pipe with
just over 1’ of clearance to the existing water main.
By adjusting the sanitary sewer service to pass over the water main, we observed another conflict
between the sanitary sewer service and storm pipe connecting CB09 and CB10. The revised plans
indicate that we will raise the storm pipe and replace with 8” DIP, to provide clearance for the sanitary
sewer pipe.
Page 3 of 6
APPENDIX A
Stormwater pipes less than 0.50% must meet a minimum full flow pipe velocity of 3 feet per second.
The following Manning’s Equation calculations shown below illustrate the minimum slope required in
a full flow pipe to maintain 3 feet per second.
12 inch diameter pipe under full-flow conditions must have a slope of 0.44% or greater
8 inch diameter pipes under full-flow conditions must have a slope of 0.75% or greater
Pipe Diameter D 12.0 (inches)
Manning's "n" n 0.013
Slope S 0.44 (%)Formulas:
Flow Depth d 12.00 (inches)
Flow Depth Ratio d/D 1.00 (%) d/D = (d/D)
Flow Point Angle q 360 (deg)q = 2*COS-1((1-(2*d/D))
Flow Area A 0.785
(ft2)A = 1/8*(RADIANS(qr)-SIN(qr))*(D/12)2
Wetted Perimeter WP 3.142 (ft) WP = p*(D/12)*qr/(2*p)
Hydraulic Radius R 0.25 (ft) R = A/WP
Flow Q 2.370 (cfs)Q = (1.49/n)*A*R2/3*S0.5
Velocity V 3.017 (fps)V = Q/A
Pipe Diameter D 8.0 (inches)
Manning's "n" n 0.013
Slope S 0.75 (%)Formulas:
Flow Depth d 8.00 (inches)
Flow Depth Ratio d/D 1.00 (%) d/D = (d/D)
Flow Point Angle q 360 (deg)q = 2*COS-1((1-(2*d/D))
Flow Area A 0.349 (ft2)A = 1/8*(RADIANS(qr)-SIN(qr))*(D/12)2
Wetted Perimeter WP 2.094 (ft) WP = p*(D/12)*qr/(2*p)
Hydraulic Radius R 0.167 (ft) R = A/WP
Flow Q 1.047 (cfs)Q = (1.49/n)*A*R2/3*S0.5
Velocity V 3 (fps)V = Q/A
Page 4 of 6 APPENDIX B – BACKWATER ANALYSIS AT EXISTING CB UPSTREAM OF CB11
Page 5 of 6 APPENDIX C – BACKWATER ANALYSIS FOR CADILLAC ON-SITE STORMWATER NETWORK, ASSUMING PUMPS ARE NON-FUNCTIONAL
Page 6 of 6 APPENDIX D – BACKWATER ANALYSIS FOR CADILLAC ON-SITE STORMWATER NETWORK, ASSUMING PUMPS ARE NON-FUNCTIONAL