Exhibit 8 - Preliminary Drainage Report.pdfHILL LAKE BALLINGER PLAT
CITY OF EDMONDS, WA
Prepared for:
Laurie Hill
24223 76'h Avenue W_
Edmonds, WA 98026
Phone (206)-762-5239
Prepared by:
FLUKY-WYRICK do ASSOCIATES, INC.
3409 McDougall Avenue, Suite 102
Everett, WA 98201
425-259-0964
Prepared:
Rdy 15, 2008
IW.
-M�Mf
lUL 1 7 2005
PL
EXHIBIT 8
P-08.46 & 'SM -08-47
SECTION I -- PROJECT OVERVIEW
TABLE OF CONTENTS
SECTION1- PROJECT OVERVIEW...............................................................................»............................................ 3
EXISTING SITE CONDITIONS ........................... ------------------------ ..................... ......................................... ..............
......3
PROPOSEDSrM CONDITIONS---------------•-•-•--•--.......................------......-•--•-•--•-------...--------•--•-----...................-••...------....3
SECTION 2 - CONDITIONS AND REQUIREMENTS SUMMARY ........ ».................................................................... 6
SECTION3 - OFFSITE ANALYSIS................................................................................................................................. 7
SECTION 4 - FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN........................12
SITEHYDROLOGY (PARTS A&B)......................................... ..------------------..................................................•-----........12
PART A - EMSTING SITE HYDROLOGY --------- -•...... ..--............ ...................................................... ._...................... 12
PART B - DEVELOPED SITE HYDROLOGY ...... ....... .................----------------------------------------------------------------------------- 12
WATERQUALITY......................••-----------------------------------------------........—..........---................................------------...1-- 15
TABLE OF FIGURES
FIGURE1— VICINITY MAP............................................................................................................................................ 5
FIGURE2 — DOWNSTREAM BASIN MAP.............»...................................................................................................... 8
FIGURE 3 BASIN MAP (Source — Department of Ecology)..........................................................................................9
FIGURE4 — EXISTING CONDITIONS..........................................................................................................................10
FIGURE5 — SOILS MAP.................................................................................................................................................11
FIGURE 6 -- DEVELOPED CONDITIONS.....................................................................................................................16
Preliminary Drainage Report Page 2
Hill Lake Ballinger Plat July 2008
SECTION 1 - PROJECT OVERVIEW
SECTION 1- PROJECT OVERVIEW
EXISTING SITE CONDITIONS
The project is comprised of 5 existing lots, with three existing single family detached residences and
several outbuildings. An existing common driveway serves all three homes. This road takes access from
76th Avenue West, 378 feet north of its intersection with 244th Street SW. Two zoning designations are
found on the property, the westerly three lots are zoned RS -8 and the easterly portion is RSW 12.
An eight lot subdivision is proposed to be served by a public road accessing 76t` Avenue W. 76' Avenue
along the property frontage is currently improved with curb, gutter and sidewalk.
At the eastern boundary of the project is Lake Ballinger. The Ordinary High Water Mark of the lake was
flagged by Wetland Resources Incorporated. The firm also flagged the western boundary of the wetland
area adjacent to the lake. This wetland is a Category IV wetland, with a Department of Ecology rating of
25. A 35 foot buffer is required from the wetland.
The topography slopes down to the east to the lake. The on-site soils are Alderwood Urban Land
Complex.
PROPOSED SITE CONDITIONS
The site will be developed as an eight lot subdivision for the future construction of single family detached
residences. Potable water will be extended into the site from facilities located in 76 h Avenue W. Sanitary
sewer exists onsite east of the proposed lots. Stormwater detention will be provided by a stormwater
detention vault designed in accordance with Edmonds Municipal Code requirements and guidelines found
in the 1992 Department of Ecology Stormwater Management Manual for the Puget Sound Basin.
The project qualifies as redevelopment as defined by the Edmonds Municipal Code. EMC 18.03.060A
states that "any new development or redevelopment with more than 2,000 square feet of new impervious
surfacing shall be required to provide storm water quality controls." Permanent water quality measures
for stormwater runoff from pollution -generating impervious surfaces will be incorporated into the site
design with the provision of a Stormfilter filtration systems provided by Contech Stormwater Solutions
Inc.
EMC 18.30.060(l )(a) states that "all development sites less than one acre that discharge directly or
indirectly to a stream shall be required to limit the peak rate of runoff to the predeveloped condition two-
year, 24-hour design storm, while maintaining the predevelopment condition peak runoff rate for the 10 -
year, 24-hour and 100 -year, 24-hour storms. The proposed stormwater vaults have been designed to
meet this standard. Although the project does not propose to discharge to a creek, City staff has indicated
through the preapplication process that flow control is required. EMC 18.30.060(1) defines the
predevelopment site conditions: the predevelopment condition for all cases other than open water bodies
Preliminary Drainage Report Page 3
Hill Lake Ballinger Plat July 2008
SECTION 7 - PROJECT OVERVIEW
shall be considered meadows or young second growth forest. Second growth forest was utilized for this
site. A summary ofthe.Mnimum Requirements can be found in Section 2 of this report.
Preliminary Drainage Report Page 4
Hill Lake Ballinger Plat July 2008
SECTION 1 - PROJECT OVERVIEW
nGURE 1- V11clwy MAP
11INS
w E o:.
mivlLwon,
nim 0
IAKE
BALLU ER
SITE
VICINITY MAP
SCALE: 1"100O.'
Preliminary Drainage Repot# Page 5
Hill Labe Ballinger Plat July 2008
SECTION 2 - CONDITIONS & REQUIREMENTS
SECTION 2 - CONDITIONS AND REQUIREMENTS SUMMARY
I. STORMWATER DETENTION: An underground detention vault will mitigate for increased in
storinwater runoff. These facilities will limit the developed flow rates to 50°/o of the predeveloped
2 -year flow rate and will match the 10- and 100 -year predeveloped flow rates.
II_ PRESERVATION OF NATURAL DRAINAGE SYSTEMS: The proposed point of discharge is
to Lake Ballinger. The site currently drains to this system.
III. SOURCE CONTROL OF POLLUTION: Permanent water quality measures have been
incorporated into the site design. Residents of the proposed plat will park in garages. Garages are
an approved method of source control for residential developments_
IV. RUNOFF TREATMENT BMPS: Permanent water quality measures for stormwater runoff from
pollution -generating impervious surfaces will be incorporated into the site design with the
provision of a Stormfilter filtration system provided by Contech Stormwater Solutions Inc_
V. ' WETLANDS: There is a category IV wetland located adjacent to Lake Ballinger, east of the
proposed area of development. This wetland will be protected by a 35' wide buffer as required by
EMC.
vi. WATER QUALITY SENSITIVE AREAS: Staff did not indicate at the preapplication meeting
for the project that the site lies within a water quality sensitive area. Stormfilters provide a high
level of pollutant removal and can be designed to target specific pollutants such as phosphorous
and nitrogen.
VII. OFFSITE ANALYSIS AND MITIGATION: An offsite analysis is included in Section 3 of the
report.
VIII. BASIN PLANNING: The applicant is not proposing to develop or utilize a specific basin plan.
DC OPERATION AND MAINTENANCE: An Operations and Maintenance Manual will be prepared
at the time of construction plan review.
x FINANCIAL LIABILITY: Maintenance or performance bonds will be submitted at the time of
construction plan review and/or permit issuance.
Preliminary Drainage Report Page 6
Hill bake Ballinger Plat duly 2008
SECTION 2 - CONDITIONS & REQUIREMENTS
SECTION 3 - OFFSITE ANALYSIS
UPSTREAM ANALYSES
Upstream runoff is intercepted by 76th Avenue West. The improved road has an existing conveyance
system that conveys stormwater to the north and beyond the site.
DOWNSTREAM ANALYSES
Stormwater runoff from the site flows to the east to Lake Ballinger. Lake Ballinger has a surface area of
approximately 100 acre and an average depth of 15 feet.The Lake Ballinger outlets over a gravity dam
located on Mcaleer Creek. Mcaleer Creek drains to the south approximately 3-1/2 miles whereupon it
discharges to Lake Washington. Given the peak flow attenuation that will be provided by the Lake no
downstream impacts should occur given the proposed development and mitigation measures.
Preliminary Drainage Report Page 7
Hill Lake Batfinger Plat July 2008
FIGURE 2 — DOWNSTREAM BASIN MAP
Kl pp
AN
Preliminary Drainage Report Page 8
Hill Lake Ballinger Plat July 2008
SECTION 2 - CONDITIONS & REQUIREMENTS
FIGURE 3 —BASIN MAP (Source — Department of Ecology)
Lake Ballinger
i' Watershed
L -i5
City of Lyunwouii - 22%
City of Edmonds - 24%
.:, City of Mountlake
City of Shoreline .21%
Unincorporated
L�J
Snohomish Grin -
-
Commercial Properties
natcrshed 8oundary
Preliminary Drainage Report Page 9
Hill Lake Ballinger Plat July 2008
FIGURE 5 — SOIIS MAP
5 -- Alderwood Urban Land Complex, 2-8 percent slopes
5 — Alderwood Urban Land Complex, 8-15 percent slopes
Source: MRCS — National Resources Conservation Service
Preliminary Drainage Report
Hili Lake Ballinger Plat
Page 11
July 2008
APPENDIX - PRELIMINARY DESIGN CALCULATIONS
SECTION 4 - FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN
SITE HYDROLOGY (PARTS A & B)
PART A - EXISTING SITE HYDROLOGY
The existing site is developed. The predeveloped laud cover was modeled using 2nd growth forested and hydrologic
soil group C.
PART B - DEVELOPED SITE HYDROLOGY
The new proposed drainage system will collect surface runoff in a series of catch basins and carry water via
underground pipes to the onsite detention and water quality facilities. Runoff from the public road will be treated by
a Stormfilter catch basin. Runoff from roofdraints firym the proposed homes on lots 7 and 8 will be dispersed at the
edge of the wetland maintaining hydrology to this area_ The area contained in lots 7 and 8 will not be included in the
detention modeling.
Precipitation Depths
6 mo = 1.01"
2 yr = 1.50"
10 yr = 2.40"
100 yr = 3.50"
Predeveloped Event Summary:
Area
BasinID
Peak Q
Peak T
1.45
(cfs)
(hrs)
Predeveloped
0.03
12.00
Predeveloped
0.11
9.00
Predeveloped
0.25
8.67
Drainage Area: Predeveloped
Hyd Method: SBUH Hyd
Peak Factor: 484.00
Storm Dur: 24.00 airs
Area
Pervious 1.4500 ac
Tonal 1.4500 ac
Supporting Data:
CN Data
2nd Growth Forested
TC Data
Flow type: Description:
Sheet None Entered
Shallow None Entered
Peak Vol
Area
(ac -ft)
ac
0.0380
1.45
0.1053
1.45
0.2064
1.45
Loss Method:-
SCS
ethod:SCS Abs:
Inty
CN TC
81.00 133 hrs
81.00 1.4500 ac
Method Raintype
/Loss
SBUINSCS TYPE 1A
SBUHISCS TYPEIA
SBUHISCS TYPEIA
SCS CN Number
0.20
10.00 min
Event
2 yr
10 yr
100 yr
Length: Slope: Coeff: Travel Time
300.00 ft 2.00% 0.4000 75.53 min
120.00 ft 1.00% 5.0000 4.00 min
Preliminary Drainage Report Page 12
Hill lake Ballinger Plat July 2008
APPENDIX - PRELIMINARY DESIGN CALCULATIONS
Developed Event Summary:
BasinlD Peak Q
Peak T
Peak Vol Area
Method Raintype Event
------ (cfs)
(hrs)
(ac -ft) ac
/Loss .
Developed 0.19
8.17
0.0934 1.45
SBUH/SCS TYPEIA 2 yr
Developed 0.42
8.17
0.1889 1.45
SBUMSCS TYPEIA 10 yr
Developed 0.72
8.00
0.3133 1.45
SBUH/SCS TYPEIA 100 yr
Drainage Area: Developed
Hyd Method: SBUH Hyd
Loss Method: SCS CN Number
Peak Factor: 484-00
SCS Abs: 0.20
Storni Dur: 24.00 hrs
Inty: 10.00 min
Area
CN
TC
Pervious 1.4500 ac
91.54
0.37 hrs
Total 1.4500 ac
Supporting Data:
CN Data:
Imp ROW
98.00
0.2900 ac
Imp LOTS
98.00
0.3800 ac
Lawns Landscaping
86.00
0.7800 ac
TC Data:
Flow type: Description:
Length: Slope:
Coeff: Travel Time
Sheet None Entered
150.00 ft 2.00%
0.1500 19.79 min
Sheet None Entered
150.00 ft 2.00%
0.0110 2.45 min
Channel None Entered
25.00 ft 0.50%
42.0000 0.14 min
Control Structure ID: D-001- Multiple Orifice Structure
Descrip:
Multiple Orifice
Start El
Max El Increment
100.0000 ft
105.0000 ft 0.10
Orif Coeff:
0.62
Bottom El:
98.00 ft
Lowest Diam:
.1.4209 in
out to 2nd:
3.8700 ft
Diana 1.7578 in
Node ID: Vault
Desc: Manhole structure
Start El: 100.0000 ft
Max El: 105.0000 ft
Contrib Basin:
Contriib Hyd:
Length
32.0000 ft
Width Void Ratio
20.0000 ft 100.00
Prelira€iz►.ary Drainage Report Page 13
Hill Lake Ballinger Plat July 2008
APPENDIX - PRELIMINARY DESIGN CALCULATIONS
RLPCOWUTE [RLP] SUMMARY
10 yr Match Q: 0.1080 cfs Peak Out Q: 0.1077 cfs - Peak Stg 103.86 ft - Active Vol: 2471.30 cf
100 yr Match Q: 02484 cfs Peak Out Q: 0.2492 cfs - Pear Stg: 105.82 ft - Active Vol: 3723.14 cf
A 30% safety factor will be applied to the design volume.
3723 cf * 1.3 = 4,840 cf is required.
Prchntinary Drainage Report Page 14
Hill Lake Ballinger Plat July 2008
APPENDIX - PRELIMINARY DESIGN CALCULATIONS
WATER QUALITY
A Stormfilter catch basin from Contech Stormwater Solutions will provide water quality for pollution generating
impervious surfaces_ Runoff from rooftops will bypass the water quality devices and drain directly to the
conveyance system or detention vault_
Px 1fininary Drainage Report Page 15
Hill Lake Ballinger Plat July 2008
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