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TECHNICAL INFORMATION REPORT
FUR
WEDGEWOOD LANF.
DIVISION 4
REhTON, V`ASHINGTON
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Prepared by: Scutt R. Borgeson, P.E. � �
Project Manager: Edgar T. Jones, P.E. ,�� �'��:z o;• �'
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Core Project No.: 01045 ! F,;'""' '`�
Date: December,2Q05 �'����AL
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ENGINEERING PLANNING SURVEYING
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WEDGEWOOD LANE - DNISION 4 !�
TABLE OF CONTENTS �',
I. Pro'ect Overview il
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II. Conditions & Requirements Summary
III. Offsite Analysis
A. Upstream Tributary Area I'
B. Downstream Analysis I
C. Bypass Area
IV. Flow Control and Water Quality Facility Analysis and Design
A. Hydrologic Analysis '
V. Conveyance System Analysis and Design �
VI. ESC Analysis and Design
VII. APPENDIX
Developed Hydrology Map '�,
Backwater Calculations Spreadsheet I�
Core Design,Inc. WEDGEWOOD LANE—DIVISION 4 Page 1 '
I. PROJECT OVERVIEW
The Preliminary Plat of Wedgewood Lane — Division 4 is located in the city of Renton,
East of Hoquiam Avenue (142°d Ave. SE) and south of N.E. 10`t' Street. Please see the
Vicinity Map on the next page. This project was previously known as Cambridge Lane.
This project site currently has one single-family residence and an outbuilding. A
Category III Wetland with a seasonal stream cutting through it runs along the southern
portion of the property. The existing ground is covered with tall grass and forestland.
The property's eastern and southern boundaries border the proposed plat of Wedgewood
Lane Divisions 1 and 3 and to the west is an undeveloped parcel. On the north side of
N.E. 10�' Street, across from this project is the future Wedgewood Lane Division 5
project, which currently has just one single-family residence. Please see the Existing
Conditions Exhibit at the end of this section for a visual depiction of the existing site.
The same project owner that developed the first three divisions of Wedgewood Lane is
developing this project. During the design of the first three divisions, the detention and
water quality facilities were sized to accommodate the anticipated developed runoff of
this Division 4 project. The Technical Information Report for Wedgewood Lane —
Divisions 1, 2 and 3 is included in the Appendix of this report. This report is intended to
' supplement that report and will simply provide the additional information necessary to
verify that the stormwater facilities currently under construction in Division 3 are
adequate for the Division 4 project. '
Division 4 of Wedgewood Lane is required to satisfy the requirements of the 1998 King
County Surface Water Design Manual (KCSWDIV�. The stormwater pond in Division 3,
that will also serve this project was designed to satisfy those requirements. The pond will ',
hereafter be referred to as Pond A. This project will also incorporate recommendarions
from the 2005 KCSWDM, including utilizing low impact development features in order
to incorporate the latest methodolo�y and to 'r�eip sustain tne neaith ur on-site and oif-s��e
wetlands. Basic dispersion of roof runoff is the primary low impact development BMP
that will be used. The roof and footing drains of 5 lots from Division 4, totaling 0.77
acres, will bypass the detention pond and will be dispersed to the on-site wetland through
multiple dispersion trenches, in accordance with the "Basic Dispersion" provisions of the
2005 KCSWDM, specifically Section 5.2.2 and Appendix C. The dispersion trenches
help to mimic the existing conditions and maintain the health of the wetland area by
distributing stormwater throughout the wetland complex. The lots that will be dispersed ',
are shown in detail on the Developed Conditions E�ibit. '',
Pond A will also provide water quality enhancement with a wet pool. The outflow of the I
pond will be discharged via a slotted drain flow spreader to the adjacent Type II Wetland. ',
Please see the Developed Conditions E�ibit at the end of this section for a depiction of �
the site plan and the proposed stormwater facility.
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Core Design, Inc. WEDGEWOOD LAlvE—DIVISION 4 Page 2
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VICINITY MAP
Not to Scale
Core Design, Inc. WEDGEWOOD LAlVE—DIVISION 4 Page 3
II. CONDITIONS & RE UIREMENTS SUMMARY III
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Please refer to the Technical Information Report for Wedgewood Lane — Divisions 1, 2 and 3, II,
included in the Appendix of this report. j
Core Design, Inc. WEDGEWOOD LAiVE—DIVIS[ON�3 Page�3
III. OFFSITE ANALYSIS
A. UPSTREAM TRIBUTARY AREA
The Wedgewood Lane - Division 4 project doesn't have any significant upstream tributary area.
N.E. 10�' Street intercepts stormwater runoff from property to the north and it is then conveyed to
the east in a roadside ditch. Runoff from the property to the west flows generally to the south
and therefore doesn't reach the project site. To the east is the Wedgewood Lane—Division 3 site
and it is lower in elevation than the project site.
B. DOWNSTREAM ANALYSYS
Drainage from the Wedgewood Lane — Division 4 property naturally sheet flows across the site
to an on-site Category III wetland and seasonal stream to the south. During the wetter months,
the seasonal stream conveys the stormwater through the Wedgewood Lane — Division III site to
an offsite Category II wetland where it merges with a larger wetland complex and forms Honey
Dew Creek. This drainage continues offsite under S.E. 116`�' Street through a 24" CMP culvert.
From here the drainage continues north in a well-defined channel for more than 1/4 mile. In the
vicinity of the 1/4 mile downstream point, Honey Creek has a bank-to-bank width of
approximately 5' —10' with an intermittent free stone channel approximately 2' — 5' wide. The
stream was dry with no evidence of recent surface water flows, when walked in August, 2002.
Substantial debris was observed within the stream. A neighbor on the east side of the creek said
the creek contains consistent flows through the winter until around May. Some moist soil was
noted in adjacent areas. From the 1/4 mile downstream point, Honey Creek drains northerly
throu�h a heavily w�oded. brush� area for approximately another '/4 mile, where it turns westerly
ror approximatety u�U ieet and tlows under Hoquiam Avenue NE via a z4�� concrete cutven. i he
creek continues flowing through a heavily farested, brushy area. for approximately 1,200 feet,
where it enters a 48" concrete storm drainage system in Duvall Avenue NE. At this intake, the
creek was approximately 10' — 15' wide with no evidence of recent surface flows. From Duvall
Avenue NE, the creek continues flowing westerly via an underground system within a
commercial development near the intersection of Duvall Avenue NE and NE Sunset Boulevard.
The creek continues northwesterly through established channels and eventually enters May Creek
approximately 2 '/2 miles from the site, which discharges to Lake Washington north of
Kennydale. No evidence of erosion or flooding was found during this downstream analysis. See
the attached Downstream Tributary Area map for a graphical depiction of this route.
Core Design,Inc. `VEDGEWOOD Lr�'VE—DIVTSION 4 Page�
C. BYPASS AREA
As previously stated, the roof and footing drains of 5 lots from Division 4, totaling 0.77 acres
will bypass the respective detention facilities and will be dispersed to the wetlands through
multiple dispersion trenches. The 2005 KCSWDM encourages the implementation of best
management practices (BMP) for flow control and provides credits for their use in Section 5.2.2 '
of the manual. The "Basic Dispersion" BMP will be utilized, which will help to maintain the
health of the wetland to the east. The impervious area of each of the dispersed lots will be
modeled as 50% impervious and 50% grass in the sizing of the detention ponds for their
respective basins. The dispersion trenches have been designed and sited to satisfy the criteria of
Section C.2.4.4 of the KCSWDM. This section of the manual dictates that 10 LF of trench is
required for every 700 SF of impervious area that is tributary to it, up to a m�imum of 50 feet.
It also specifies that a vegetated flowpath length of 25 feet is required and that the "outer edge of
the vegetated flowpath segment for the dispersion trench must not overlap with other flowpath
segments". These criteria and the others specified in the manual have all been satisfied. The
impervious areas used to determine each dispersion trench length are summarized in the table
below.
Lots to be Lot area Max.Impervious Impervious Area Total
dispersed (Acres) Area(SF;incl. for Pond Sizing Dispersion
(Basin A lots house,driveway (S� Treoch
are listed first and atio Len th Feet
Lot 3 0.15 2,800 1,400 40
Lot 4 0.14 2,800 1,400 40
Lot 5 0.14 2,800 1,400 40
Lot 6 0.16 2,800 1,400 40
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Core Design,Inc. WEDGEWOOD LANE—DIVISION 4 Page 6
IV. FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN
A. HYDROLOGIC ANALYSIS
The drainage analysis for the sizing of Pond A was performed as documented in the Technical
Information Report for Wedgewood Lane—Divisions 1, 2 and 3, included in the Appendix of this
report. Wedgewood Lane—Division 4 was included in the developed tributary area for the pond.
The breakdown of impervious and pervious areas for Division 4 is repeated below in order to
show adjustments that were made based on refinements to the project specifications. The total
tributary area contributed to the pond from this project remained the same at 1.70 acres,however,
the impervious surface was reduced by 0.21 acres and the till grass component was increased by
the same amount. Due to these changes, the pond will have excess capacity. Therefore, no
further modeling was deemed necessary.
Of the ten lots, the maximum impervious area allowed far nine of them was determined to be
2,800 SF. This value is intended to account for the houses, driveways, patios and walkways that
will be constructed on each of them. Please refer to the Divisions 1-3 report in the Appendix for
the actual calculations used to size Pond A
BASIN A AREA BREAI�OWN
Wedgewood Lane—Division 4
Basin A-Offsite Basin A-Offsite
Pre-Developed Developed
Area (Acres) Area (Acres
_ �..�,s- .�.,�, _.�.,_��.....�,.,.��..
'I'i11-Forest �.�i -
Till—Pasture 0.08 -
Till—Grass - 0.53
Impervious- 0.11 -
Existin structures
Impervious-Lots - 0.32
5 2,800s
Lots to be Dispersed - 0.16
(50% Impervious &
50% Pervious)
5 2,800s
Impervious-Private - 0.10
Access Drivewa
Impervious- - 0.59
Fronta e
Total 1.70 1.70
Core Design,Inc. WEDGEWOOD LANE—DIVISION 4 Page 7
V. CONVEYANCE SYSTEM ANALYSIS AND DESIGN
The flow for the conveyance and backwater pipe sizing was attained using the rational formula as
follows:
Qioo=C* IR *A
Where,
Qioo=Peak flow(cfs)for a 100-year storm event
C=0.90 Impervious, C=0.25 lawns
IR�eak rainfall intensity(inches/hour) for a 100-year storm event
A=drainage subbasin area(acres)
The value of IR is attained through the following formula:
IR=PR*aR*�Tc��-bF�
Where,
IR�eak rainfall intensity(inches/hour) for a 100-year storm event
pR=total nreci�itatinn fr�m,a l 0�-vear. 24-hoiu storm event=3.95in.
aR=coefficient from Table 3.2.1.B (KCSWDIVn=2.61
bR=coefficient from Table 3.2.1.B (KCSWD1Vn=0.63
Tc=time of concentration(minutes) =10 (assumed) ;,
Therefore, ;
IR=(3.95)*(2.61)*(10)�-0.63� 'I
=2.41 in/hour I
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For Division 4, the sub-basin area(A) that is tributary to the storm system is 1.38 Ac. However, �ii
the conveyance system that will be constructed in N.E. 10�' Street will also serve the future
Wedgewood Lane - Division 5 project, which is on the north side of the road. Division 5 covers
2.24 acres and will have 13 single-family lots. Most of the lots aze 50' wide, so 2,800 S.F. of
impervious area per lot was used to account for the houses, driveways, patios and walkways that
will be constructed on each of them. For the three lots that are 55' wide, 3,050 S.F. of
Core Design,Inc. WEDGEWOOD LANE—DIVISION 4 Page 8
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impervious was assumed. Division � will also be responsible for adding sidewalk on the north
side of N.E. 10�' Street, which will cover appro�mately 3,900 S.F. The total impervious area
anticipated for Division 5 that will be tributary to the N.E. lOth Street conveyance system is
approximately 0.95 Ac. Therefore, approximately 1.29 acres will be covered by pervious
surfaces. The composite runoff coefficient"C"was calculated as follows:
Cco�osrrE=[((1.01+0.95)*0.90) +((0.53+1.29)*0.25)] /(1.38 +2.24) = 0.61
The stormwater flow for the 3.62 acres that is tributary to this conveyance system is therefare:
Q ioo=(0.61)*(2.41)*(3.62)
Qioo=5.32 cfs
A portion of the frontage improvements that will be constructed for Division 5 along Hoquiam
Avenue will be tributary to a separate conveyance system that discharges to a roadside ditch to
the south of the N.E. l Oth Street. This separate system will also receive stormwater from existing
and future improvements on Hoquiam Avenue extending approximately 750 feet north of N.E.
10�' Street. This tributary area measures appro�mately 45,000 S.F. and at full build out, will
have anproximatelv 38,000 S.F. �f imnervi�us surfaces with the remainder bein� nervious. The
composite runoff coefficient"C"was calculated as follows:
, CCOMPOSITE-L�O.g7*0.90)+ (0.16*0.25)] / 1.03 = 0.80
The stormwater flow for the 3.94 acres that is tributary to this conveyance system is therefore:
� Q�oo=(0.80)*(2.41)*(1.03)
Qioo= 1.99 cfs
As you will see on the Backwater Calculations Spreadsheet in the Appendix, the conveyance
system has at least 1 foot of freeboard during the 100-year storm event, so it complies with the
requirements of the KCSWDM and the City of Renton.
Core Design,Inc. WEDGEWOOD LANE—DIVISION 4 Page 9
VI. ESC ANALYSIS AND DESIGN
Pond A, located in Wedgewood Lane—Division 3, will also serve as the sediment retention pond
for Division 4. For more information on this facility, please refer to the Technical Information
Report for Wedgewood Lane—Divisions 1, 2 and 3, included in the Appendix of this report.
Core Design,Inc. WEDGEWOOD LANE—DIVISION 4 Page 10
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VII. APPENDIX '
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Core Design,Inc. WEDGEWOOD LANE—DIVISION 4
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ENTRAVCE E\TR4\CE EXIT OLTLET I�LET APPROAC'fi BE\D JU�CTION
PIPE PIPE �IAV\I\G'S 0[TLET I\LET PIPE FLO��' �'ELOCITY TAfL��`ATER FRICTIO\ HGL HEAD HEAD COni'ROL CO\7ROl, VELOCITI yEAD HE.4D HEADVI`ATER DEPTH
FROM TO FLOW LENGTH DIA. n EI.EVATION ELEVATIO\ AREA VELOCITY HEAD ELEVATIOiV LOSS ELEVATIOr LOSS LOSS ELEVATIOn' ELE1'ATION HEAD LOSS LOSS ELEYATION BEL01V RIA1
CB CB (CFS) (FEET) (i\) VALI:E (FEET) (FEET) (SQ FT) (FT/SEC) (FEET) (FEET) (FEET) (FEET) (FEET) (FEE7� (FEET) (FEET) (FEET) (FEET) (FEE� (FEET) RI�1 EL (FEET)
BASIN A
B 31A CB 2 5.3 205 l2 0.012 456?9 -��?.-? 0.79 6.77 0.?1 4».i4 3.86 4�9.20 0.�6 0.?1 460.?? 460.21 0.0' 0.00 0.07 460.2? 466.4i 6.2
B 2 CB 4 1.6 210 12 0.01? -��'.?2 -1(i��ii 0.79 2.10 0.07 �+60.�'i p,38 466.50 0.03 0.07 466.60 �16650 O.UO 0.00 0.01 466.61 469.14 2.53
B 2 CB= 0.4 ]16 12 0.012 -�57J2 44�.62 0.'9 0.�6 0.00 460.27 0.01 460.28 0.00 0.00 460.29 A�36.62 0.00 O.Oi 0.00 46030 461.30 LO �
DITCH CB� 2.0 5? 1? 0.012 468.4? 468.77 0.79 2.�3 Q10 �369.42 0.1-1 469.77 0.0� 0.10 46292 469.77 0.0S 0.00 0.01 469.85 471.80 1.95
CB 5 CB�A 1.7 40 12 OAl2 465.77 469.17 0.79 2.21 O.OS 469.85 0.08 470.17 0.04 0.08 470Z8 470.17 0.00 0.0? OA1 470.31 4?1.80 1.4
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12/19/2005 CORE DESIGN,INC. PAGE 1
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