HomeMy WebLinkAbout03274 - Technical Information Report - Downstream Analysis and Prelim Drainage Calcs .
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�� Core Design, Inc.
CORE 1471 1 N E.29fh Place Suite iF 101
Bellevue,Washington 98007
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LEVEL ONE DOWNSTREAM ANAL�SIS AN�
PRELIMIN%�IZY STORM DRAINAGE
CALCLTI,ATIONS
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WEDC EW�OD LANE
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WEDGEWOOD LANE
TABLE OF CONTENTS
l. Project Overview
2. Offsite Analysis
3. Flow Control and Water Quality Facility Analysis and Design
i
Core Design,Inc. WEDGEVVOOD LANE PRELIMIIVARI'PLAT Page i
1. PROJECT OVERVIEW:
The Preliminary Plat of Wedgewood Lane is located in the city of Renton, East of 142°d Ave. SE and
south of S.E. 116�' Street, which cunently is not established right-of-way. Please see the Vicinit��
Map on the next page. This project site includes multiple parcels that currently have approximately
7 single-family residences and associated garages and outbuildings. A Type II Wetland runs just
outside of the eastern property line along the majority of the property. T'he existing ground is
covered with tall grass and forestland. The property's eastern boundary borders the proposed Plats
of Rosemonte and East Renton and other undeveloped parcels, all of which are in King County.
This preliminary plat will be developed in three divisions. The adjacent Preliminary Plat of
Aspenwood will be developed separately by Harbor Homes,but the stormwater runoff from it will be
tributary to the Wedgewood Lane plat's stormwater system. We are designing the stormwater
detention and water quality facilities to serve all three divisions of the Wedgewood Lane project, as
well as the Aspenwood project that is currently being reviewed by City staff. We feel that
constructing one system to handle the stormwater from all of these projects will benefit the residents
in the long term with reduced per lot maintenance expenses and will improve the overall appearance
of these neighborhoods, as compared to the option of having smaller, individual facilities for each of
the projects. In order to further enhance these neighborhoods, our client has directed us to design the
facilities to have a more natural appearance than traditional ponds. Core Design Inc. has discussed
this proposed system with the City of Renton in meetings and with a letter dated December 20, 2004
and received conceptual approval of the concepts presented herein in a response letter dated January �
6, 2005.
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King County requires Level �- Flow Control and Water Quality for the site. Detention will be
provided by the detention pond located in Division 3. Water quality enhancement will be provided
by two separate facilities, a bioswale and dead storage in the pond. The current pond configuration
has been designed to provide enough dead storage to enhance the water quality of approximately
55% of the total azea of the three Wedgewood Lane divisions and Aspenwood. We are proposing
that a bioswale be utilized to provide treatment for the remaining 45% of the project. The bioswale
would provide the required treatment for the water quality storm event and then discharge the
stormwater into the conveyance system, upstream of the wet pond. The wet pond will provide
primary water quality enhancement for stormwater runoff from a little more than half of the site and
will also provide some secondary treatment for the runoff that will have already been treated by the
bioswale. Although the wet pond and bioswale proposed for this project will each be sized for the
flows from just a portion of the overall site, there will still inevitably be some enhanced protection
for the stormwater that goes through both facilities. All drainage discharging from the pond will be
discharged via a flow spreader to the onsite Type II Wetland.
A separate detention and water quality facility will be designed to serve six lots in the northeast
corner of the site, which are located in Division 3. The design for this system will be submitted
separately with plans for that division. Also, the roof and footing drains from 22 lots, totaling 3.07 _ /,
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acres, will be dispersed to the wetlands through multiple flow rea •s,�ssed in the letter to
t he Ci ty o f Renton da te d Decem ber 2 0, 2 0 0 4. T hese sprea ders wi ll distri bute t he stormwater across
a large portion of the wetland, in order to mimic the existing conditions.
Core Design, Inc. WEDGEWOOD LANE PRELIMINARY PLAT Page 1
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FIGURE 1: VICINITY MAP
DATE SEPTEMBER �004 1471 1 NE 29th Pl4ce Suite 101
Bellevue, Washrngton 98007
D RAWN FAK
425.885.7877 Fax 425.885.7963
APPROVED ETJ
DESIGN
PROJECT MANAGER ENGINEERING � PLANNING • SURVEYlNG
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2. OFFSITE ANALYSIS:
This offsite analysis was performed on August 29, 2002 and on March 25, 2003.
Upstream Tributary Area i
The only upstream tributary area that will affect the development is the east half of Hoquiam Ave.
NE for the majority of the site's frontage. This would include the area from the centerline crown of
the road to the project's property line. Stormwater runoff from this area will be collected in the
developed project site's system.
Downstream Analysis
Drainage from the Wedgewood Lane property naturally sheet flows across the site to an offsite Type
II wetland where it merges with other wetlands to the northeast from the project site to form 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/a mile. In the
vicinity of the 1/a 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 '/a mile downstream point, Honey Creek drains northerly through a heavily wooded, brushy �
area for approximately another 1/a mile, where it turns westerly for approximately 800 feet and flows
under Hoquiam Avenue NE via a 24" concrete culvert. The creek continues flowing through a
heavily forested, 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 Boulevazd. The creek continues northwesterly through
established channels and eventually enters May Creek approximately 2 1/z 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. �'�'EDGE�'�'OOD LANE PRELIlI7INARY PLAT Page 3
3. FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN:
A. Hydraulic Analysis
The drainage analysis for the pond was modeled using the King County Runoff Time Series
software. The site's soils are Alderwood (AgC & AgD), KCRTS group Till. Please see the
Appendix for the Soils Map and Rainfall Regions and Regional Scale Factors map. The site is
located in the Landsburg rainfall region with a location scale factor of 0.85.
EXISTING CONDITIONS
The undeveloped site mostly sheet flows into the off site Type II wetland located just east of the
eastern property line. The area to be improved covers approximately 23.6 acres not including lots
that will drain directly to the wetland and the drainage pond itself. The existing ground cover is a
combination of unpervious surfaces, till-pasture and till-forest. T'he zoning for the site is R-8 and R-
4. Per Table 3.2.2.E in the 1998 KCSWDM, the effective impervious fraction for 1 DU/GA is 0.50.
The impervious area for the existing site is 0.86 acres. The effective impervious area is 0.50*0.86
acres = 0.43 acres. Please see the Existing Conditions Exhibit for additional information. The
following information was used for generating the time series and flow frequencies shown further
below.
EXISTING CONDITIONS Total Area =23.6 acres
GROUND COVER AREA(acres)
Till-Pasture/Till Forest 5.13 / 18.04
Effective Im ervious* �.43
�` Using a fraction of 50% for both driveways and buildings per Table 3.2.2.E
Flow Frequency Analysis
Time Series File:01045ex.tsf
Project Location:Landsburg
---Annual Peak Flow Rates--- -----Flow Frequency Analysis-------
Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob
(CFS) (CFS) Period
1.92 2 2/09/O1 18 :00 2.68 1 100.00 0.990
0.402 7 1/05/02 16 :00 1.92 2 25.00 0.960
1.27 4 2/28/03 16:00 1.65 3 10.00 0 .900
0.388 8 8/26/04 1:00 1.27 4 5.00 0.800
1.18 5 1/05/05 10:00 1.18 5 3 .0a 0.667
0.945 6 1/18/06 21:00 0.945 6 2.Oa 0.500
1.65 3 11/24/06 5:00 0.402 7 1.30 0.231
2.68 1 1/09/08 7:00 0.388 8 1.10 0.091
Computed Peaks 2.43 50.00 0 .980
Core Design, Inc. WEDGEWOOD LANE PRELIMINARY PLAT Page 4
DEVELOPED CONDITIONS
The developed site will consist of 145 single-family residences with associated roadways and
utilities.
The impervious area was calculated using the criteria in the 1998 King County Surface Water Design
Manual page 3-27 and K.C.C.21A.12.030. The proposed development is urban residential. The site
has R-8 and R-4 zoning. The m�imum impervious surface per lot is 75%.
The input values and output data for the KCRTS analysis are below and on pages 6-10.
DEVELOPED CONDITIONS Total Area= 23.6 acres
GROUND COVER AREA(acres)
Till-Grass (Landsca in ) 12.0
Im rvious 11.6
Flow Frequency Analysis
Time Series File:01045dv.tsf
Project Location:Landsburg
---Annual Peak Flow Rates--- -----Flow Frequency Analysis-------
Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob
(CFS} (CFS) Period
4.73 4 2/09/O1 2:00 8.79 1 100.00 0.990
2.66 8 12/03/O1 17:00 5.72 2 25.00 0.960
4.38 6 9/10/03 15:00 5.26 3 10.00 0.900
5.26 3 8/26/04 1:00 4.73 4 5.00 0.800
4.30 7 10/28/04 18:00 4.45 5 3.00 0.667
4.45 5 10/22/05 17:00 4.38 6 2.00 0.500
5.72 2 11/21/06 9:00 4.30 7 1.30 0.231
8.79 1 1/09/08 7:00 2.66 8 1.10 0.091
Computed Peaks 7.76 50.00 0.980
As shown in the KCRTS output that follows, the required detention pond size was deternuned to be
248,796 cf. The preliminary grading and utiliry plans submitted concurrently with this report show a
pond design that will provide 253,800 cf of live storage for detention from the live-dead interface
elevation of 429.0 to the design water surface elevation of 443.0. The final design configuration,
including the details of the orifice control structure, will be provided with the final engineering plans
following preliminary plat approval.
Core Design, [nc. �'�'EDGE«'OOD LANE PRELIIVIINARI'PLAT Page 5
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Detention Routing Calculations
Retention/Detention Facility
Type of Facility: Detention Pond
Side Slope: 3.00 H:lV
Pond Bottom Length: 165.00 ft
Pond Bottom Width: 65.00 ft
Pond Bottom Area: 10725. sq. ft
Top Area at 1 ft. FB: 34749. sq. ft
0.798 acres
Effective Storage Depth: 12.00 ft
Stage 0 Elevation: 0.00 ft
Storage Volume: 248796. cu. ft
5.712 ac-ft
Riser Head: 14.00 ft
Riser Diameter: 18 .00 inches
Number of orifices: 2
Full Head Pipe
Orifice # Height Diameter Discharge Diameter
(ft) (in) (CFS) (in)
1 0.00 2.25 0.514
2 8.75 3.13 0.607 6 .0
Top Notch Weir: Rectangular
Length: 1.38 in
Weir Height: 11.80 ft
Outflow Rating Curve: None
Stage Elevation Storage Discharge Percolation Surf Area
(ft) (ft) (cu. ft) (ac-ft) (cfs) (cfs) (sq. ft)
0.00 O.OQ 0. 0.000 0.000 0.00 0.
0.02 0.02 0. 0.000 0.�21 0.00 0 .
0.05 0.05 0. 0.000 0.030 0.00 0.
0.07 0.07 0. 0.000 0.036 0.00 0.
0.09 0. 09 0. 0.000 0.042 0.00 0.
0.12 0. 12 0. 0.000 0.047 0.00 0.
0.14 0.14 0. 0.000 0 .051 0.00 0.
0.16 0.16 0. 0.000 0 .056 0.00 0.
0.19 0.19 0. 0.000 0.059 0.00 0.
0.46 0.46 0. 0.000 0.093 0.00 0.
0.74 0.74 0. 0.000 0.118 0.�0 0.
1.01 1.01 a. 0.000 0.138 0.00 0.
1.29 1.29 0. 0.000 0.156 0.00 0.
1.56 1.56 0. 0.000 0.172 0.00 0.
1.83 1.83 0. 0.000 0.186 0.00 0.
2.00 2.00 0. 0.400 0.194 0.00 10725.
2.27 2.27 2946. 0.068 0.207 0.00 11100.
2.55 2.55 6110. 0.140 0.219 0.00 11495.
2.82 2 .82 9265. 0 .213 0.231 0.00 11881 .
3.10 3 .10 12648. 0.290 0.242 0.00 12287.
3.37 3 .37 16019. 0.368 0.252 0.00 12683 .
3.65 3 .65 19629. 0.451 0.262 0 .00 13100.
3 .92 3.92 23221. 0.533 0.272 0.00 13507.
4.20 4.20 27062. 0.621 0.281 0.00 13935.
4.47 4.47 30881. 0.709 0.290 0.00 14353.
4.75 4.75 34961. 0.803 0.299 0.00 14792 .
5.02 5. 02 39013 . 0.896 0.308 0.00 15221.
5.29 5.29 43181. 0.991 0.316 0.00 15655.
Core Design, Inc. WEDGEWOOD LANE PRELIMINARY PLAT Page 6
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5.57 5.57 47628. 1.093 0.324 0.00 16110.
5.84 5.84 52038. 1.195 0.332 0.�0 16555.
6.12 6.12 56739. 1.303 0.340 0.00 17022.
6.39 6.39 61396. 1.409 0.347 0.00 17477.
6.67 6.67 66356. 1.523 0.355 0.00 17955.
6 .94 6.94 71267. 1.636 0.362 0.00 18421.
7.22 7.22 76493 . 1.756 0.369 0.00 18910.
7.49 7.49 81663 . 1.875 0.376 0.00 19386.
7.76 7.76 86962 . 1.996 0.383 0.00 19868 .
8 .04 8 .04 92595. 2.126 0.389 0.00 20374.
8.31 8 .31 98163 . 2.254 0.396 0.00 20866.
8.59 8 .59 104077. 2.389 0.402 0.00 21383 .
8.75 8.75 107522 . 2.468 0.406 0.00 21680.
8 .78 8 .78 108174 . 2.483 0.409 0.00 21736 .
8.82 8.82 109045. 2.503 0.418 0.00 21811.
8.85 8.85 109700. 2.518 0.431 0.00 21867.
8.88 8.88 110357. 2.533 �.448 0.00 21923 .
8 .91 8.91 111015. 2.549 0.470 0.00 21980.
8 .95 8 .95 111896 . 2.569 0.496 0.00 22055.
8 .98 8 .98 112558 . 2 .584 0.524 0.00 22111.
9.01 9.01 113223 . 2 .599 0.547 0.00 22168.
9.04 9.04 113888 . 2 .615 0.556 0.00 22224.
9.32 9.32 120186 . 2 .759 0.619 0.00 22756.
9.59 9.59 126399 . 2.902 0.668 0.00 23273 .
9.87 9.87 132992 . 3 .053 0.711 0.00 23815.
10.14 10.14 139493 . 3.202 0.750 0.00 24344.
10.42 10.42 146386 . 3 .361 0.785 0.00 24897.
10.69 10.69 153181 . 3.517 0.818 0.00 25436.
10 .96 10.96 160122 . 3 .676 0.849 0.00 25980.
11.24 11.24 167476 . 3 .845 0.878 0.00 26550.
11.51 11 .51 174719 . 4 .011 0.906 0.00 27105.
11.79 11.79 182390 . 4 .187 0.933 0.00 27686.
11.80 11.80 182667 . 4.193 0.934 0.00 27706.
12 .07 12 .07 190224 . 4 .367 0.993 0.00 28272.
12 .35 12 .35 198223 . 4.551 1.080 0.00 28864.
12 .62 12 .62 206094 . 4.731 1.180 0.00 29441.
12 .90 12 .90 214422 . 4.922 1.290 0.00 30044.
13 .17 13 .17 222613 . 5.110 1.420 0.00 30631.
13 .45 13 .45 231275. 5.309 1.560 0.00 31246.
13 .72 13 .72 239792 . 5.505 1.710 0.00 31844. ,
14.00 14 .40 248796 . 5.712 1.870 0.00 32469. ''
14.10 14 .10 252054 . 5.786 2 .340 0.00 32694. I
14.20 14 .20 255335. 5.862 3 .190 0.00 32919.
14.30 14 .30 258638 . 5.938 4 .290 0.00 33145.
14.40 14.40 261964 . 6.014 5.600 0.00 33372.
14.50 14.50 265313 . 6.091 7.070 0.00 33600.
14.60 14.60 268684 . 6.168 8 .510 0.00 33828.
14.70 14.70 272078 . 6.246 9.040 0.00 34057.
14.80 14.80 275495. 6.325 9.540 0.00 34287.
14.90 14.90 278936 . 6.403 10.010 0.00 34518.
15.00 15.00 282399. 6.483 10.450 0.00 34749.
15.10 15.10 285886. 6 .563 10.880 0.00 34981.
15.20 15.20 289395. 6 .644 11.280 0.00 35214.
15.30 15.30 292928. 6.725 11.670 0.00 35447.
15.40 15.40 296485. 6.806 12 .040 0.00 35681.
15.50 15.50 300065. 6.889 12 .400 0.00 35916.
15.60 15.60 303668. 6.971 12.750 0.00 36152 .
15.70 15.70 307295. 7.055 13 .090 0.00 36388.
15.80 15.80 310945. 7.138 13.420 0.00 36625.
15.90 15.90 314620. 7 .223 13.740 0.00 36863 .
Core Design, Inc. `i'EDGEVVOOD LANE PRELII�ZINARY PLAT Page 7
Hyd Inflow Outflow Peak Storage
Target Calc Stage Elev (Cu-Ft) (Ac-Ft)
1 8.79 ******* 6.68 14 .47 14.47 264434. 6.071
2 4.73 1.92 1.69 13.68 13 .68 238565. 5.477
3 5.72 ******* 1.65 13.61 13 .61 236225. 5.423
4 3.14 ******* 0.84 10.92 10.92 15903�. 3.651
5 2 .80 ******* 0.82 10.69 10 .69 153173 . 3.516
6 4.45 ******* 0.59 9.19 9 .19 117353 . 2.694
7 5.26 ******* 0.37 7.35 7 .35 79010. 1.814
8 2.37 ******* 0.34 6.03 6 .03 55169. 1.267
----------------------------------
Route Time Series through Facility
Inflow Time Series File:01045dv.tsf
Outflow Time Series File:rdout
Inflow/Outflow Analysis
Peak Inflow Discharge: 8.79 CFS at 7:00 on Jan 9 in Year 8
Peak Outflow Discharge: 6.68 CFS at 9:00 on Jan 9 in Year 8
Peak Reservoir Stage: 14.47 Ft
Peak Reservoir Elev: 14.47 Ft
Peak Reservoir Storage: 264434. Cu-Ft
. 6.071 AC-Ft
Flow Frequency Analysis
Time Series File:rdout.tsf
Project Location:Landsburg
---Annual Peak Flow Rates--- -----Flow Frequency Analysis-------
Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob
(CFS) (CFS) (ft) Period
1.69 2 2/09/O1 20:00 6.68 14.47 1 100.00 0.990
0.337 8 11/05/O1 3:00 1.69 13.68 2 25.00 0.960
0.844 4 3/O1/03 7:00 1.64 13 .60 3 10.00 0.900
0.372 7 8/26/04 7:00 0.844 10.92 4 5.00 0.800
0 .818 5 1/05/05 16:00 0.818 10.69 5 3.00 0.667
0.591 6 10/28/05 2:00 0.591 9.19 6 2.00 0.500
1.64 3 11/24/06 8:00 0.372 7.35 7 1.30 0.231
6 .68 1 1/09/08 9:00 0.337 6.02 8 1.10 0.091
Computed Peaks 5. 02 14 .36 50.00 0. 980
Care Design,Inc. R'EDGE�'�'OOD LANE PRELI�IINARY PLAT Page 8
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Duration Comparison Anaylsis ��
Base File: 01045ex.tsf
New File: rdout.tsf
Cutoff Units: Discharge in CFS
-----Fraction of Time----- ---------Check of Tolerance-------
Cutoff Base New °sChange Probability Base New %Change
0 .473 � 0.96E-02 0.90E-02 -6.5 I 0.96E-02 0.473 0.427 -9.8
0 .520 � 0.81E-02 0.85E-02 5.9 I 0.81E-02 0.520 0.555 6.7
0.567 � 0.69E-02 0.78E-02 12.9 I 0.69E-02 0.567 0.616 8.6
0.614 � a.59E-02 0.70E-02 17.6 I 0.59E-02 0.614 0.658 7.3
0 .660 � 0.51E-02 0.58E-02 14.0 I 0.51E-02 0.660 0.681 3 .1
0 .707 � 0.41E-02 0.46E-02 12.2 � 0.41E-02 0.707 0.734 3 .7
0.754 I 0.35E-02 0.38E-02 8.3 � 0.35E-02 0.754 0.766 1.6
0.801 � 0.30E-02 0.27E-02 -7.2 I 0.30E-02 0.801 0.789 -1.5
0 .848 � 0.24E-02 0.19E-02 -22.0 I 0.24E-02 0.848 0.814 -3 .9
0.894 I 0.21E-02 0.16E-02 -19.8 � 0.21E-02 0.894 0.837 -6.4
0.941 � 0.17E-02 0.14E-02 -20.0 � 0.17E-02 0.941 0.881 -6.4
0 .988 � 0.15E-02 0.12E-02 -20.2 I 0 .15E-02 0.988 0.915 -7.4
1.03 I 0.14E-02 0.11E-02 -19.3 � 0.14E-02 1.03 0.947 -8 .5
1.08 � 0.12E-02 O.10E-02 -15.1 � 0.12E-02 1.08 1.01 -7.0
1.13 � 0.99E-03 0.91E-03 -8.2 � 0.99E-03 1.13 1.10 -2.8
1.17 I 0.95E-03 0.78E-03 -17.2 � 0.95E-03 1.17 1.12 -4.4
1.22 � 0.83E-03 0.67E-03 -19.6 I 0.83E-03 1.22 1.15 -5.6
1.27 � 0.73E-03 0.54E-03 -26.7 � 0.73E-03 1.27 1.20 -5.3
1.32 � 0.65E-03 0.49E-03 -25.0 � 0.65E-03 1.32 1.23 -6.7
1.36 � 0.55E-03 0.41E-03 -26.5 � 0.55E-03 1.36 1.27 -7.1
1.41 I 0.44E-03 0.36E-03 -18.5 ( 0.44E-03 1.41 1.34 -4.8
1.46 � 0.38E-03 0.31E-03 -17.4 I 0.38E-03 1.46 1.40 -3.8
1.50 � 0.31E-03 0.24E-03 -21.1 � 0.31E-03 1.50 1.46 -3.1 ,
1.55 � 0.23E-03 0.20E-03 -14.3 � 0.23E-03 1.55 1.52 -2.1
1.60 � 0.18E-03 0.13E-03 -27.3 I 0.18E-03 1.60 1.57 -1.6 '
1.64 I 0.15E-03 0.65E-04 -55.6 I 0.15E-03 1.64 1.58 -3.6 I
1.69 � 0.11E-03 O.00E+0a -100.0 � 0.11E-03 1.69 1.62 -3 .8
1.74 � 0.11E-03 O.00E+00 -100.0 I 0.11E-03 1.74 1.62 -6 .4
1.78 � 0.82E-04 O.00E+00 -100.0 I 0.82E-04 1.78 1.64 -8.0
1.83 � 0.49E-04 O.00E+00 -100.0 I 0.49E-Q4 1.83 1.66 -9.4
1. 88 � 0.49E-04 O.00E+00 -100.0 I 0.49E-04 1.88 1.66 -11.7
Maximum positive excursion = 0.052 cfs ( 8.9�)
occurring at 0.579 cfs on the Base Data:01045ex.tsf
and at 0 .630 cfs on the New Data:rdout.tsf
Maximum negative excursion = 0.243 cfs (-12 .7$}
occurring at 1.91 cfs on the Base Data:01045ex.tsf
and at 1 .67 cfs on the New Data:rdout.tsf
Core Design, Inc. �'�'EDGE�i'OOD LANE PRELLI�INARY PLAT Page 9
__
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Core Desi�n, Inc. WEDGEWOOD LANE PRELI'4iINARY PLAT Page 10
Water Quality Volume Calculations
A. WATER QUALITY POND
King County requires that Basic Water Quality Treatment be provided for development of this site.
The dead storage portion within the wet pond will be used to provide water qualiry treatment. The
required dead storage was sized according to Section 6.4.1.1 of the 1998 KCSWDM. The following
variables were used in the calculation:
Volume Factor(fl = 3
Rainfall =0.039 feet or 0.47 inches
Area=developed basin
Where A, = area of impervious surface (s fl
Atg= area of till soil covered with grass (s fl
A�= area of till soil covered with forest(s�
Ao= area of outwash soil covered with grass or forest (s�
Vr= f'�`[0.9A; +0.25A�g+0.1 A,f+ 0.01 Aa] x (R/12)
The water quality volume (Vb) for the entire site has been calculated below. In this case the pond
will serve as the primary water control facility for 55°Io of the proposed development and provide the
other 45�/o with secondary treatment:
Vb= 3*[(0.9)(505,300) + (0.25)(522720)]0.047/12 = 68,790 CF (Entire Site) '
Portion going directly to pond (55% of total) II
(Vb) Entire Site *0.55 = 37,834.5 CF (55% portion going to pond) I
The proposed pond will contain the dead storage from the live-dead interface elevation of 429.0 feet
with a surface area at that elevation of 8,512 sf. The top of sediment storage in the pond will have a
surface area of 1,392 sf at an elevation of 421.0 feet. This layout will provide 39,600 cf of dead
storage.
0 ;�,.,f r,'(f-- /t�✓�Ji` ✓='�5'�'�' '( � �.
i �� /f� �1 , l� ^ af�. �
� ' � � l�•l �
B. BASIC BIOFILTRATION SWALE � '�'7 ;„ JE .- -;- ,:��' -�_"'-'�� �
��° ;r� �.
Basic Water Qualiry treatment will also be provided with a biofiltration swale designed per Section
6.3.1 of the 1998 KCSWDM. The biofiltration swale utilizes a design flow equal to 60% of the
developed 2-year peak flow rate, as deternuned using the KCRTS model with 15-minute time steps.
The 2-year flow rate using 15-minute time steps is 8.52 cfs and is listed in the KCRTS output
included at the end of this section. The water quality flow is therefore, 60%*8.52 cfs = 5.11 cfs. A
flow splitter will be utilized to distribute 45% of the water quality storm event to the biofiltration
swale, while the remaining 55°Io continues to the wet pond for treatment. T'he design flow for the
biofiltration swale was calculated as 45%*5.11 cfs =2.30 cfs. Included at the end of this section is a
Core Design,Inc. WEDGEWOOD LANE PRELIMINARY PLAT Page 11
Microsoft Excel spreadsheet that calculates the flow rate and velocity given a bottom width, flow
depth, side slopes, Manning's n, and longitudinal slope. The bottom width was calculated to be 14.0
feet with 3:1 side slopes and a longitudinal slope of 1.9%. Manning's n was assumed to be 0.2 for
shallow flow conditions and depth of flow was assumed to be 0.33 feet since the swale will be
infrequently mowed. Per the Excel spreadsheet shown on the next page, the calculated flow is 2.30
cfs, which is equal to the required water quality flow and the calculated velocity is 0.467 fps which is
less than the maximum allowable of 1 fps.
The required length of the swale was calculated with the intent of achieving a 9 minute hydraulic
residence time. L= 540*velocity=540 sec.*0.467 fps = 252 feet.
Conveyance capacity of the swale was checked using the same bottom width, side slopes,
longitudinal slope, and a Manning's n equal to 0.030 per the 1998 KCSWDM Table 4.4.1.B for an
earthen, winding and sluggish swale with grass and some weeds. The 100-year flow rate calculated
for the entire site was multiplied by 0.45 to account for the amount of flow that makes it to the
bioswale leaving 8.52 cfs "0.45 = 3.96 cfs.
The 100-year flow can be conveyed through the swale at a maximum depth of 0.15 feet, which is less
than the water quality flow depth of 0.33 feet. Please see the Excel spreadsheet that follows. The
velocity through the swale during the 100-year return period is 1.88 fps, which is less than the
m�imum 5 fps.
T'he bioswale has been designed with a 14-foot wide bottom with a concrete or wood divider down
its centerline to effectively create two 7-foot wide swales. The bioswale will have 3:1 side slopes, a
longitudinal slope of 1.90%, a length of 255 feet, and a total depth of 0.83 feet, which equates to a
flow depth of 0.33 feet plus 0.50 feet of freeboard.
Flow Frequency Analysis
Time Series File:0104515.tsf
Project Location:Landsburg
---Annual Peak Flow Rates--- -----Flow Frequency Analysis-------
Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob
(CFS) (CFS) Period
6 .08 7 2/08/O1 19:00 21.67 1 100.00 0.990
5 .98 8 5/06/02 7:15 14.19 2 25.00 0.960
12 .41 4 9/10/03 13:45 13 .89 3 10.00 0.900
14 .19 2 8/25/04 23:30 12.41 4 5.00 0.800
8.52 6 9/10/05 16:45 9.68 S 3 .00 0.667
13.89 3 10/22/05 16:15 8.52 6 2.00 0.500
9.68 5 11/21/06 8:00 6.08 7 1.30 0.231
21. 67 1 1/09/08 7:30 5.98 8 1.10 0. 091
Computed Peaks 19.18 50.00 0. 98�
Core Design,Inc. �i'EDGEVVOOD LANE PRELIMINARY PLAT Page 12
1
Initial Bioswale Sizing
BIOFILTRATION SWALE ANALYSIS (WQ) ,
WALE DATA:
OTTOM WIDTH(FT)= 14.00 JOB NAME: WEDGEWOOD LANE
EF1'SIDE SLOPE(L:1)= 3. JOB#: 01045
GHT SIDE SLOPE(R:1)= 3. OPERATOR: S.R.B.
OTTOM SLOPE(FT/F"T)= 0.019 DATE: 12/17N2004
�IANNING'S n= 0.2
FLO�'V FLOW WETTED HYDRAULIC SWALE FLOW
DEPTH AREA PERIMETER RADIDS CAPACITY VELOCITY
(FEET) (S FT) (FEET) (FEET) (CFS) (FPS)
0.00 0.0000 0.0000 0.0000 0.000 0.000
0.05 0.7075 14.3162 0.0494 0.098 0.138
0.10 1.4300 14.6325 0.0977 0.311 0.217
0.15 2.1675 14.9487 0.1450 0.613 0.283
0.20 2.9200 15.2649 0.1913 0.993 0.340
0.25 3.6875 15.5811 0.2367 1.445 0.392
0.30 4.4700 15.8974 0.2812 1.965 0.440
0.33 4.9467 16.0871 0.3075 2.308 0.467
0.35 5.2675 16.2136 0.3249 2.549 0.484
0.40 6.0800 16.5298 0.3678 3.197 0.526
0.45 6.9075 16.8460 0.4100 3.905 0.565
0.50 7.7500 17.1623 0.4516 4.672 0.603
0.55 8.6075 17.4785 0.4925 5.497 0.639
0.60 9.4800 17.7947 0.5327 6380 0.673
0.65 10.3675 18.1110 0.5724 7320 0.706
0.70 11.2700 18.4272 0.6116 8.316 0.738
0.75 12.1875 18.7434 0.6502 9.368 0.769
0.80 13.1200 19.0596 0.6884 10.476 0.798
0.85 14.0675 19.3759 0.7260 11.638 0.827
0.90 15.0300 19.6921 0.7633 12.856 0.855
0.95 16.0075 20.0083 0.8000 14.129 0.883
1.00 17.0000 20.3246 0.8364 15.456 0.909
Core Design, Inc. WEDGEWOOD LANE PRELIMINARY PLAT Page 13
Final Bioswale Sizing
BIOFILTRATION SWALE ANALYSIS (WQ)
WALE DATA:
OT'I'OM WIDTH(FT)= 14. JOB NAME: WEDGEWOOD LANE
EFT SIDE SLOPE(L:1)= 3.00 JOB#: 01045
RIGHT SIDE SLOPE(R:1)= 3.0� OPERATOR: S.R.B.
OTTO'_11 SLOPE(FT/FI')= 0.019 DATE: 12/17//2004
ANNING'S n= 0.03
FLOW FLOW �'�'ETTED HYDRAULIC SWALE FLOW
DEPTH AREA PERIMETER RADIUS CAPACITY VELOCITY
(F'EET) (S FT� (FEET) (FEET) (CFS) (FPS)
0.00 0.0000 0.0000 0.0000 0.000 0.000
O.aS 0.7075 14.3162 0.0494 0.651 0.919
0.10 1.4300 14.6325 0.0977 2.072 1.449
0.15 2.1675 14.9487 0.1450 4.084 1.884
0.20 2.9200 15.2649 0.1913 6.619 2.267
0.25 3.6875 15.5811 0.2367 9.633 2.612
030 4.4700 15.8974 0.2812 13.099 2.930 i
0.35 52675 16.2136 0.3249 16.997 3.227
0.40 6.0800 16.5298 03678 21311 3.505
0.45 6.9075 16.8460 0.4100 26.030 3.768
0.50 7.7500 17.1623 0.4516 31.145 4A19
0.55 8.6075 17.4785 0.4925 36.649 4.258
0.60 9.4800 17.7947 0.5327 42.536 4.487
0.65 10.3675 18.1110 0.5724 48.802 4.707
0.70 11.2700 18.4272 0.6116 55.442 4.919
0.75 12.1875 18.7434 0.6502 62.455 5.125
0.80 13.1200 19.0596 0.6884 69.837 5.323
0.85 14.0675 19.3759 0.7260 77.588 5.515
0.90 15.0300 19.6921 0.7633 85.706 5.702
0.95 16.0075 20.0083 0.8000 94.190 5.884
1.00 17.0000 20.3246 0.8364 103.041 6.061
Core Design,Inc. WEDGEWOOD LANE PRELIlI'IINARY PLAT Page 14
APPENDIX
Core Design, Inc. «'EDGE�'�'OOD LANE PRELIl�iINARY PLAT
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F I GURE SOSL �1�P
DATE -TANNA�R �0�5 14711 NE29th Plpce Suite 101
Bellevue, Washington 98007
D RAWN 5�g 425.885.7877 Fqx 425.885.7963
APPROVEC E7�T � DESIGN
PROJECT MANAGER I
� ENGINEERING • PIANNING • SURVFYfNG
SECITON 3.2 RUNOFF COMPUTATION AND ANALYSI3 MEIHODS
•
FIGURE 3.2.Z.A RAINFAi.L REGIONS AND REGIONAL SCALE FACTORS
ST 1.0/
ST 1.9 _ ST 1.0 LA 0.8 LA 0.9 �► i.0 LA 1.2
SHONONISN COUNTY
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9/1/98 1998 Surface Water Desigi Manual
3-22
SEC. 10, TWP. 23N, RGE 5E., W,M. .
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� �. �Y -I- d� A � 1560 140TF1 AVF. N,E., SU/IF f00
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o � N � p ,�, � �A� 1 w � 1�� D A T E J A N. 2 0 0 5 S H E E T OF
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� �. � DRAWN __ SOM - OZO4J'r
` �� ,�� ��r�' y ��' ��� i: ,T;���i '!
� � Planning/Building/PublicWorks Department
�' � Gregg Zimroerman P.E.,Administrator
Kathy Keolker-Wheeler, Mayor
January 6, 2005
Scott Borgeson, P.E.
Core Design, Inc.
14711 N.E. 29`�Place, Suite#101
Bellevue, WA 98007
Subject: Wedgewood Lane Preliminary Plat '
Storm Drainage Management Concept
Dear Mr. Borgeson:
After review of the concepts outlined in your letter dated December 20, 2004 the City of Renton
finds both approaches practical and acceptable with the addition of a few conditions for both the
submittal and the plat.
Surface Water facility serving multiple plats —The City of Renton agrees that reduced per lot
maintenance expense and improved overall appearance is a worthy goal:
Surface Water Treatment through "training" bio-swale and wet pond facilities — When
submitted with the preliminary plat, all calculations and the drainage report need to show
specifically and thoroughly all surface water is being treated meeting the intent of the KCSWD:�I ,
and no downstream creek or river is endangered. A plain language discussion/narrative is
required as a cover for the drainage report with the preliminary plat for presentation and hearing
purposes.
Natural Appearance - The City of Renton agrees that a natural shape is aesthetically more
desirable. A detailed landscaping and maintenance plan including a plant inventory will be
required with the preliminary plat submittal. The CC&R's for the plat will need strict tanguage
regarding landscaping and maintenance responsibilities.
��'etland recharge/dispersal trenches — Dispersal trenches aze not usually a preference as
maintenance and erosion control are continuing concerns, most especially when the new
homeowner starts landscaping. Given the soils and the proximity of the wetland, dispersal
trenches will be considered. The CC&R's for the plat will be required to address maintenance of
dispersal trenches and monitoring downstream erosiou. The language will have to clearly state
that the dispersal trenches are not to be removed, covered or damaged and give the homeowner's
association some monitoring responsibility if erosion occurs. The wetland-monitoring plan will
need to address this issue as well. You may expect conditions of this sort to be addressed in the
SEPA review of the preliminary plat and may wish to be prepared with a proposal on how to
address these concerns.
Roof Area as grassed surface —This is an interesting argument. When presenting the drainage
report be prepared with all square footages, calculations and references to suppoR the concept and
show how the intent of the BMP is met. Staff believes the argument is supportable.
�� —
1055 South Grady Way-Renton,Washington 98055 `L'� � .l�l r1 � s�
Ir�,,..:_.�.,,,.,,,,_._:__��o:..__.._.,.,,�__.�,;,,�. on�: ._�...,._.. .�1 H E:?U O P T H F C U R?'E
♦
Basically> the City of Renton agrees with the proposals for water treatment and dispersal in
concept, subject to verification of all calculations and approva! of plant materials, landscaping,
erosion control and wetland monitoring. Maintenance will also be carefully reviewed at
submittal, so providing as complete information as possible witl� the preliminary plat is
recommended.
If you have any questions, please feel free to contact me at 425-430-7299.
Sincerely,
�a. {{ ��-��,�,
a}�n Kittrick -
Development Engineering Supervisor
Public Works Inspections &Permits
Development Services Division
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.
December 2U, 20U�3
Core No. 01045
Mr. Neil Watts
City of Renton
1055 South Grady ��'ay
Renton. �VA 98055
Subject: �'�'edgewood Lane Preliminar,y Plat— LU:� 03-082
Storm Drainage 1�lanagement Concept
Dear Mr. VVatts:
We are in the process of preparing the preliminary plat submittal docunlents for the
Wedgewood Lane project that Northward Development is pursuing. As a part of this
effort, we are looking into designing the storm�vater detention and water quality facilities
to sen�e all three divisions of the Wedgewood Lane project, as well as Harbour Homes'
adjacent Aspenwood project that is currently being reviewed by City staff. We feel that
constructing one system to handle the stormwater from all of these projects will benefit
the residents in the long term �;�ith reduced per lot maintenance expenses and ���ill
improve the overall appearance of these neighborhoods, as compared to the option of
having smaller, individual facilities for each of the projects. In order to further enhance
these neighborhoods, our client has directed us to design the facilities to have a more
natural appearance than traditional ponds.
Through this design process, we have found that if ��-e give the detention and «ater
quality pond a more natural shape, while also receiving the additional stormwater from
the Aspenwood plat, it ��ill be impossible to provide the required detention and water
quality volumes without a decrease in the project density. We have designed the pond to
have gentle 3:1 side slopes around the majority of its circumference, but have steepened
the slopes to 2:1 where the pond will abut the rear yards of several lots. This was done to
maximize the attainable volume «�hile also adding visual variety to the pond's
appearance. Since the rear lot lines of these lots will be fenced, access to the side of the
pond with the 2:1 slopes will be restricted. The remainder of the pond will not be fenced,
so that it's natural shape and landscaping enhancements will be a visible site amenity.
As you discussed with Ed Jones last week, «�e have determined that the current poild
configuration can provide enough dead storage to enhance the ��-ater quality of
approximately 55% of the total area of the three Wedge��-ood Lane divisions and
Aspenwood. We are proposing that a bioswale be utilized to provide treatment for t11e
remaining 45% of the project. The bios���ale ��-ould provide the required treatment for the
water quality storm event and then discharge the storni��-ater into the conveyance system,
upstream of the«�et pond. The�vet pond «-ill pro�ide primary«�ater quality enhancement
for stormwater runoff from a little more than half of the site a�1d will also provide some
secondarv treatment for the ninoff that ���ill have alreadv been treated bv tlie hios«-ale.
�_� I ����I� �� 'ic�t�_.
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' 01/17/OS Neil Watts— City of Renton Page: 2
The King County Surface Water Design Manual, 1998 edition (KCSWDM), does not
appear to specifically address utilizing multiple types of water quality facilities to meet
the manual's `Basic VVater Quality' requirements. The manual does require that sites.
whose locations dictate more stringent «�ater quality controls, provide multiple �i-ater
quality facilities. By providing multiple facilities in a `treatment train', the risk of
pollutants leaving the site is further reduced. Although the wet pond and bioswale
proposed for this project will each be sized for the flows from just a portion of the overall
site, there will still inevitably be some enhanced protection for the stormwater that goes
through both facilities. We don't see any reason that this design �vill be any less effective
than a solitary system, but since this is not a method that is explicitly discussed in the
KCSWDM and since this approach is necessary in order for the proposed facilities to also
serve the Aspenwood project. we «�ould like to gain assurancc from the City that tllis
approach ��vill be acceptable.
Another topic for which we «ould like� to �e�t }�our appro�°al has to do «-ith ���etland
recharge. The project site is adjacent to a large wetland complex that lies to the east. The
site generally slopes down to the east, from Hoquiam Avenue towards the wetland. Since
development regulations require that stormwater that falls on the developed site be
collected and con��eyed in a tight line com�eyance system to a detention and���ater quality
facility, a significant portion of the area that is currently tributary to the wetland �vill no
longer reach the wetland until the pond's outfall, which is do���nstream from much of thc
wetland. In order to maintain the wetland's current hydrologic patterns, we propose to
direct the roof runoff from approximately 20% of the developed lots directly to the
wetland, utilizing level spreaders to disperse the stormwater at the edge of the 50'
wetland buffer in multiple locations. These downspout dispersion trenches will be
designed in accordance with the specifications shown in the King County Surface Water
Design Manual, 1998 edition (KCSWDM), including Figure C.2.F. This is the samc
approach that was approved last year by the City of Renton for Northward
Development's Brandall Village Short Plat project (LliA 03-082, Permit U031246).
Since applying this same design concept to the Wedgewood Lane project is necessary for
the proposed detention and «-ater quality pond to also serve the Aspenwood project, ���c
would like to gain assurance from the City that this approach will be acceptable.
In accordance with the KCSWDM, this wetland recharge design concept requires that the
permitting agency grant an exemption from Core Requirement #3 — Flow Contol. On
page 1-27 of the KCSVVDM, exemptions from Core Requirement #3 are identified. �,r�;� �
Specifically, item 2-B specifies that, "If the project is a single family residential project,��ss '�`�` �✓
flo�v control BMPs must be applied within the threshold discharge area as specified in ,r�.r • �'
Small Site Dminc�ge Requiremef�ts (detached Appendix C)." Chapter C.2 - Flo«� Control � ; �
BMPs includes the following language that dictates that dispersion be utilized:
"Dispersion or infiltration BMPs are required for small site proposals applying for
single family residential building permits or short plats, except in the� followin�
case: If the existing lot is smaller than 22,000 square feet, or if the short plat
creates lots less than 22,000 square feet, then infiltration is required if suitablc
soils are present. lf soils will not support infiltration, then other flow control
BMPs must be considered. If site constraints make dispersion and infiltration
unfeasible, then the site may be connected to the storn� drainage s}stem «ith
perfo�-ated tightlille connection (see Section C2.5. p.C-?3).��
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� O1%'17�'OS '.�eil �'�'atts Citv of Renton Page: �
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Due to the till soils present on this site, which are inhe�rently poar draining, infiltration
would not be feasible. Dispersion is an available alternative, however, since adequate
space exists to allo«� dispersion without adverse downstream impacts, due to the large
� wetland complex that will be preserved as open space. Although the lots that will utilize
dispersion will be 6,000-9,000 square feet in size, when the wetland area that they �;�ill
��' discharge to is also considered, the dispersion area is effectively larger than the required
� 22,000 square foot lot arca. So, the intent of the KCS�'�'D'�l requirement yuoted above is
� satisfied.
Section 5.2.2 — B!VIPs for Reducing Facility Size states, "Tlie Ilovv control Bi�1Ps
presented in this section may be used to reduce the size of required flow control facilities
implemented as described below." This refers the reader to the follo�ving criteria #2, "If
roof runoff is dispersed according to the requirements of Section 5.1.2 (p. 5-9) on singlc
family lots greater than or equal to 22,000 square feet, and the vegetated flowpath of the
roof runoff is 50 feet ar longer, the roof area may be modeled as grassed surfacc rather
than impervious surface when sizing the required flow control facility."
As discussed above, this project effectively exceeds the requirement for at least 22,000
square feet of lot area due to the wetland preserved native gro«�th protection area. Also, a
minimum 50 foot vegetated flo«- path ben��een the dispersion trenches and the delineated
wetland edge can be provided, due to the 50' wetland buffer that will already be
provided. Therefore, the criteria under Section 5.2.2 can b��at�ed and for this reason.
we propose that the roof area of approximat 30 lots�e modeled Till Grass in sizing
the proposed detention pond. The drivew nd r these lots ��vill still be
tributary to the detention pond and ���ill be modeled as imper��ious surfaces during the
facility design process.
'i l expect that you will fmd that the design concepts presented in this letter will adequatel��
protect the public and satisfy the requirements of the King County Surface Desi��
Manual. We would appreciate it if you would send us a �irritten reply to signify your
� approval of these two design elements, so that we can incorporate them into the plans we
are preparing for the Wedgewood Lane preliminary plat submittals. If you have any
questions or comments that you would like to discuss, you can call Ed Jones or myself at
425.885.7877 ar reach us via e-mail at ETJ��GoreDe�i��»I�lc.com or
SfZI3':r�CorcDc�ii��inc.con�. T}laT� vou. � �
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S i nc erelti�.
CORE DESIGN, I�C.
Scott Bor�cson. P.E.
Ci�-i1 Envine�er
cc: Dick Gilro�°, North���ard Developi��ent
Patrick Gilroy. �Iorth���ard De��elopment