HomeMy WebLinkAbout21008 Storm Revision Memo_Approved
To: Nathan Janders, PE
From: Core Design Inc
Date: August 20, 2025
Re: Towns on 12th - NE 12th Street Storm Revision
This memo serves as justification for the design of the August 20, 2025 storm revision to the Towns on
12th Plans. This revision is necessary in order to minimize the potential ponding at the east end of the
proposed road widening along the project frontage of NE 12th St.
The proposed storm revision, as detailed in the revised civil plans dated August 20, 2025, includes
installation of one catch basin along the new curb line of NE 12th St, approximately 7.5 feet from the end
of the improvement, as well as a catch basin on the shared property line between Lots 1 and 10, beyond
the east end of ROW 1, with 8-inch diameter ductile iron piping connecting these catch basins to existing
catch basin 30.
Route Justification
The connection of this proposed conveyance to the on-site system has been determined to be the best
option over the alternatives of connecting to the existing storm on the south side of NE 12th Street or to
the east at the intersection of NE 12th Street and Union Ave NE.
Connection south across NE 12th Street is not feasible due to the shallow existing storm, existing water
within the ROW which would be in direct conflict with a storm crossing, and the newly placed overlay in
that area. Connection east at the intersection of NE 12th Street and Union Ave NE has been dismissed as
an option due to the impact to neighboring driveways and excessive distance, 280 feet, when compared to
the proposed option which only requires 139 feet of pipe. As such, the proposed solution is the most
efficient option with the least impact to offsite and public property.
Vault Capacity Justification
The final approved TIR included a treatment trade in order to justify the release of this area. This area
swap is summarized in Table 1 below, as pulled from Section 4.1.3 of the final approved TIR for the
project.
Table 1 Approved TIR Treatment Trade Areas
NE 12th Street Area Swap
Released (sf) Collected (sf) Difference (sf) Difference (ac)
Impervious (Road) 3,265 4,011 746 0.02
Pervious 1,366 3,854 2,488 0.05
Total Area 4,631 7,865 3,234 0.07
DEVELOPMENT ENGINEERING
yqi 09/18/2025
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In the proposed condition, The majority of the area previously categorized as “released” will be collected.
The design change proposed no change to the upstream collected road area, therefore all area in excess of
the original total released area will be treated as flow through for the vault. See Table 2 below for a
summary of the new proposed condition.
Table 2 Revised Treatment Trade Areas
NE 12th Street Area Swap
Target Surfaces Non-Target Surfaces
Released (sf) Newly
Collected (sf) Subtotal (sf) Newly
Collected* (sf)
Upstream
Collected (sf) Subtotal (sf)
Impervious 156 3,109 3,265 2,928 4,011 6,939
Pervious 73 1,293 1,366 0 3,854 3,854
Total Area 229 4,402 4,631 2,928 7,865 10,793
*Existing pavement between edge of existing pavement and centerline not subject to treatment. “Newly
Collected” area is represented by green hatched area in the attached exhibit, totaling 7,330 square feet.
All Non-Target Surfaces, less the Released Target Surfaces, as presented in Table 2 above, will be
considered as flow through for the vault no subject to water quality or flow control. Since the design of
the detention vault had already accounted for the treatment of the newly collected target areas, and the
newly collected non-target areas are considered flow through, the existing vault design will function as
designed and constructed to meet the flow control requirements.
Size Justification
In order to avoid the newly installed power within the right-of-way sidewalk, an 8-inch ductile iron pipe
has been proposed rather than the 12-inch diameter minimum typically required within city ROW. This
reduced diameter is proposed in order to maximize the available clearance between the recently installed
power and the proposed pipe. The maximum capacity of the proposed 8-inch pipe at the minimum
proposed slope, 0.55%, as calculated using Manning’s equation, has been compared to the 100-year peak
flow of the Newly Collected Areas, tabulated in Table 2, as determined utilizing WWHM to ensure
adequate conveyance capacity is provided.
Per Manning’s equation, the capacity of an 8-inch ductile iron pipe (n-value of 0.012) flowing full at
0.55% is 0.973 cubic feet per second. The areas for use in WWHM to determine the 100-year flow to the
proposed catch basin inlet (CB 30B) in the ROW have been simplified into Table 3 below from those
presented in Table 2.
Table 3 CB 30B Tributary Areas
Target Areas
(sf)
Non-Target
Areas (sf) Total (sf) Total (ac)
Impervious 3,109 2,928 6,037 0.138
Pervious 1,293 0 1,293 0.030
Total Area 4,402 2,928 7,330 0.168
Per WWHM, the runoff rate for the 100-year storm from this tributary area is 0.1414 cubic feet per
second.
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Figure 1 CB 30B Tributary Area Flow Rates
Based on these results, the 100-year peak flow to the proposed inlet is 0.1414 cfs, less than the capacity of
the proposed 8-inch diameter pipe of 0.973 cfs. Therefore, the proposed 8-inch pipe is adequately sized to
convey runoff from the newly collected area.
Sincerely,
CORE DESIGN, INC.
Holli Heavrin, PE
DESIGNE N G I N E E R I N G P L A N N I N G S U R V E Y I N G12100 NE 195th St, Suite 300Bothell, Washington 98011425.885.7877 Fax 425.885.796321008