HomeMy WebLinkAboutPermit File BLD-2021-0065 4305 Cherry Court (3) kLISSER
& ASSOCIATES
LAND SURVEYING / LAND-USE CONSULTATION
January 22,2021
Landed Gentry
Art Shaw
Via e-mail art@landedgentry.com
Re: Lot 5 Cherry Court Short Plat
Dear Art:
Per your request, please accept this letter as verification that we set control staking for the
construction of the house and office/shop buildings on proposed Lot 5 of the Cherry Court Short
Plat.
The staking was performed on January 4, 2021.
Let me know if you have any questions or if you need any additional documentation to support
the work we performed.
Since•:-�
s
G. Lisser, P.L.S.dice .
BGL/mm
LISSER & ASSOCIATES, PLLC
320 Milwaukee Street / P.O.Box 1109 / Mount Vernon,WA 98273 / (360)419-7442 / Fax: (360)419-0581 / Email:bruce@lisser.com
P:1Letters119-130 012221.doc
Ravnik & Associates, Inc.
CIVIL ENGINEERING & LAND-USE PLANNING
December 18,2020
City of Anacortes .j
P.O.Box 547 111
'1(0Anacortes,Washington 98221
DEC 2 2.2020
Attention: Mr.Steve Lange
Department City of Anacortes Engineering ep rtment kU
ATY Of ANAGOHTES
Reference: Cherry Court
Post Construction Stormwater and Plan Review Submittal
Dear Steve-
This Post Construction Drainage Report and analysis has been prepared to address the as constructed condition and
future lot development within the Cherry Court residential short plat as assure the project provides sufficient
stormwater BMP's as required by the 2014 DOE Manual and the City of Anacortes to mitigate the increased stormwater
runoff created by the development of this Short plat along with the future home site development on each lot.
Currently,the road,sidewalk,detention pond and utilities have been installed per the civil design and as-constructed
documents submitted with this Post Construction Drainage Report. The constructed detention pond size and
Stormwater control structure information noted within this report have been confirmed by post construction
information. A 2-foot wide drainrock-filled interceptor trench system has been installed along the southerly and
westerly side of the site to provide the project area relief from subsurface flows/perched ground water being
contributed to the site from offsite, up gradient properties. It should also be noted that recent surrounding Orchards
development to the west and south have installed deep utilities such as sewer systems which also aid in intercepting
groundwater which was historically perched within this project area.
The only significant change with this Post Construction Drainage Report is how each lots future development is proposed
to be mitigated onsite at the time of individual building construction. At the time the original report was prepared the
DOE Manual standard of 4,200 square feet(3,200 sf of house/1,000 sf driveway)for each of the 5 lots. Now that one lot
has had the house and site specifically designed, and other preliminary lot fits have been completed by the owner it has
become notable that the total previously assumed 4,200 sf of hard surfacing on each lot is not sufficient. Thus below is
a summary of how the project will mitigate the increase and specifics for each lots required onsite LID provisions needed
to meet the requirement of the Post Construction Drainage Report.
Sections of the previous report have been updated within the attached Post Construction Drainage Report information
as noted for reference.
The same overall site basin area of 3.11-acres has been used,as was previously used,for this project's submittal. This
new Post Construction analysis will use seven more specific sub-basin areas to analyze the site. This will include one for
each new lots and one to represent the right of way and pond areas as noted. 2.61-acres of Pasture and 0.5 acres of
Forest were used for the Pre Developed condition as was allowed with the submittal of the preliminary plat and fill and
grade permit plans prepared for this project.
Two small infiltration systems have been added to the onsite site design to collect clean roof runoff from two of new
house sites, Lot 1 and Lot 6 located along the north side of the site. These roof drain infiltration systems will only serve
1633 LINDAMOOD LANE I P.O. Box 361 • BURLINGTON, WA 98233
PH: (360) 707-2048 • FAX:(360) 707-2216
clean roof runoff and are required to be the footprint size denoted,with a minimum 0.67-foot(8-inch)depth of clean
drainrock. Within the infiltration system a 4-inch perforated pipe shall be installed and connected to discharge to a yard
drain with a simple riser overflow set at the top of the drainrock elevation. This overflow riser will allow rare, large
storm events,which exceed the proposed roof infiltration systems capacity to infiltrate,to overflow from these systems
into the plats storm system and be routed to the detention pond located in the northeast corner of the site. Only a very
small amount of infiltration 0.4 and 0.6 gal/sf/day(0.027 in/hr to 0.040 in/hr)has been accounted for within these roof
infiltration systems as noted within the attached soils testing provided by Linda Spicher in November,2020.
Furthermore information provided by Geotest test pits information was also reviewed within these. The areas within
the northerly side of Lots 1 and 6 where these systems are proposed were noted to have had historical perched water
depths at approximately 24-29-inches below the surface,which has likely been lowered due to the surrounding utility
development done for the Orchards development located along the south and west(up-gradient)sides of the site with
this project. in addition,the interceptor drain installed with the short plat infrastructure for this project along the south
and west sides have likely further reduced the subsurface flows. Restrictive soils containing clay and/or area deemed
"not suitable"are noted as being at approximately 41-inches and 52-inches below ground surface per the Spicher test
hole information.
Below is a summary of the areas accounted for within the attached report which were used to determine that based on
the asbuilt detention pond and control release structure,along with the noted various onsite LID measures notes the
system as proposed is sufficient to result in a"Pass"of the project based on the Post Development WWHM analysis.
Refer to the attached Post Development WWHM Report and views provided with the Post Construction Drainage Report
for more specific information.
Post Developed Conditions(3.11 acres)
Right of Way(ROW)Sub-Basin(0.52 ac)
0.19 ac Road
0.05 ac Sidewalk(Modeled as landscape as back-sloped to lawn allowance per DOE BMP T5.18)
0.07 ac Pond
0.21 ac Lawn(Modeled as Pasture per DOE BMP T5.13)
0.52 ac
Lot 1-Sub-Basin(0.39 ac)
0.10 ac House(Roof drains routed to infiltration system 105'x 16.5'x 0.67'deep drainrock filled, 0.4 gal/sf/day*)
0.02 ac Driveway(Sheet Flow Dispersed 50'downhill north,modeled as Disp,)
0.27 ac Lawn(Modeled as Pasture per DOE BMP T5.13)
0.39 ac
Lot 2-Sub-Basin(0.35 ac)
0.09 ac House(Dispersed over 25'via Dispersion Trench modeled as 50%lawn/50%Roof)
0.02 ac Driveway(Sheet Flow Dispersed downhill north 25'modeled as 50%lawn/50%Driveway)
0.24 ac Lawn(Modeled as Pasture per DOE BMP T5.13)
0.35 ac
Lot 3-Sub-Basin(0.38 ac)
0.10 ac House(No onsite BMP's proposed)
0.03 ac Driveway(Sheet Flow Dispersed 25'downhill north,modeled as 50%lawn/50%Driveway)
0.25 ac Lawn(Modeled as Pasture per DOE BMP T5.13)
0.38 ac
Lot 4-Sub-Basin(0.48 ac)
0.10 ac House(Dispersed over 50'via Dispersion Trench)
0.07 ac Driveway(Sheet Flow Dispersed 25'downhill east and north, modeled as 50%lawn/50%Driveway)
0.31 ac Lawn(Modeled as Pasture per DOE BMP T5.13)
0.48 ac
Lot 5-Sub-Basin(0.64 ac)
0.10 ac House(Dispersed over 50'via Dispersion Trench)
0.02 ac Garage/Shop(Dispersed over 25'via Dispersion Trench,modeled as 50%lawn/50%Driveway)
0.07 ac Driveway(No onsite BMP's proposed)
0.45 ac Lawn(Modeled as Pasture per DOE BMP T5.13)
0.64 ac
Lot 6-Sub-Basin(0.35 ac)
0.09 ac House(Roof drains routed to infiltration system 78'x 16.5'x 0.67'deep drainrock filled,0.6 gol/sf/day*)
0.03 cc Driveway (Sheet Flow Dispersed 50'downhill north or east depending on layout, modeled as Disp)
0.23 ac Lawn(Modeled as Pasture per DOE BMP T5.13)
0.35 ac
Totals: ROW+Lots 1-5:
0.52ac+0.39ac+0.35cc+0.38ac+0.48ac+0.64ac+0.35ac = 3.11 ac
*Infiltration rates per Linda Spicher Soils Report dated 11/24/20
In summary the 3.11 ac project area with the Post Developed Unmitigated condition(701)is estimated to generate the
following Flow Frequency per Post Construction WWHM at the projects Point of Compliance(POC):
2 year 0.1435 cfs
5 year 0.2094 cfs
10 year 0.2617 cfs
25 year 0.3383 cfs
50 year 0.4038 cfs
100 year 0.4770 cfs
With the incorporation of the detention pond,control release structure constructed,and onsite LID measures such as
Sheet Flow Dispersion for some of the driveway areas as noted,Roof Drain Dispersion for some of the lots as noted,and
the two proposed Roof Downspout infiltration systems as noted,the following Flow Frequency for the Post Construction
Post Developed Mitigated Condition(801)is as follows:
2 year 0.0280 cfs
5 year 0.0587 cfs
10 year 0.0914 cfs
25 year 0.1534 cfs
50 year 0.2197 cfs
100 year 0.3088 cfs
Based on this information Post Construction WWHM analysis has determined that the system with onsite LID BMP's and
the currently as-constructed stormwater facilities receives a"Pass". Refer to the attached Post Construction WWHM
Report and Views attached with this Post Construction report.
When more specific lot development site plans have been designed there are potential other ways to BMP's that can
also mitigate the increase of onsite stormwater. Instead of using the two roof drain infiltration systems for Lots 1 and 6
house development as noted above,the owner could alternatively use the prescriptive roof drain infiltration trench
system noted within the DOE Manual Section 3.1.1. Per this section it allows the use of 125 lineal feet of infiltration
trench for each 1,000 square feet of roof area,based on the onsite soils being a Sandy Loam. The Sandy Loam
classification is based on the soils triangle along with and test pit data provided by Geotest within the original
geotechnical report for this project. Other alternatives that could be used include pervious driveway sections,or
traditional driveway surfacing with a drainrock reservoir below if it is pretreated as required by DOE by use of an
oil/water separator vault or the like. As with any house development that has not been specifically designed yet there
will be various ways to mitigation stormwater impacts. If alternative provisions,such as the prescriptive roof drain
trench approach,or pervious driveways,etc.are to be used at house development other than what has specifically be
provided within the Post Construction report and associated WWHM model,a new drainage report and analysis should
be provided to the City for approval to assure it confirms to the current City and DOE drainage requirements.
In summary the Post Construction Drainage Report has been revised to allow more new hard surface on each lot,
however additional LID provision on each lot as specified provides the necessary mitigation to meet the requirements of
the continuous model software(WWHM)and should not cause a significant impact.
Please feel free to contact me to discuss further,can provide further information,or if I can answer any questions.
Thank you,
Heike Nelson,PE
12.11.20 Model Trench Infil Bed
House Site Basins 1-6: Total Impery Total Pery Total imp+Pery
LOT Lot area House Onsite UD Driveway Onsite UD Allowed Lawn Required trench&type Dralnrock Drain rock
4 (sf) (ac) (sf) (ac) notes for Roof: (sf) (ac) notes for DW: (ac) (sf) (Pasture)(sf) (ac) (sf) (ac) cf(18-in) cf(8-In)
1-Custom 16969 0.39 4465 0.10 House Roof Infil Sys(108x16Sx0.67)' 843 0.02 Dfsp 50' ;0.12 5308 • 11661 0.27 16969 0.39 OR Percrip 558 LF 2'wide Infil Trench / 1116 SF x 18" 1114.75 cf 1161 cf
2-Mod 2323 15404 0.35 4104 0.09 House Roof Dlsp 25' 998 0.02 Disp 25' 0.12 5102 10302 0.24 15404 0.35 59 LF Disp Trench
3-Mod 2411 16682 0.38 4255 0.10 None 1403 0.03 Disp 25' :0.13 5668. 11014 0.25 16682 0.38 61 LF Disp Trench
4-Mod 2411 20524 0.47 4265 0.10 House Roof Disp 50' 2925 0.07 Dlsp 25' :0.17 7190 13334 0.31 20524 0.47 61 LF Disp Trench
5-Custom 27799 0.64 4246 0.10 Rouse Roof Dlsp 50' 3000 0.07 None 0.12 5227 19593 0.45 27799 0.64 61 LF Disp Trench
960 0.02 Garage/Shop Roof Disp 25' Ind in above 14 LF Dfsp Trench
6-Mod 2323 15272 0.35 4104 0.09 House Roof Infil Sys(78x 16.5x0.67)** 1102 0.03 Dlsp 50' 0.12 5206 10066 0.23 15272 0.35'OR Percrlp 513 IF 2'wide Inn Trench / 1026 SF x 18" 1024.5 cf 862.29 cf
Totals 2.59 25449 0.59 10271 0.24 .0.77 33702 75970 1.74 Z59
House Lots 1-6 *Assuming 0.027 in/hr Driveway Lots 1-6 House+Driveway Lawn Total Lot 1-6 Imp+Pery
"Assuming 0.0401n/hr
ROW Basin(0..52 ac): _
POND 0.07 0.07 AC
SIDEWALK 0.05 0.05 AC
ROAD 0.19 0.19 AC
PASTURE 0.21 0.21 AC
0.52 0.52 AC
Total Project Sumary:
Impervious
Proposed Roof 0.587 ac
Proposed Driveway 0.236 ac
Proposed Road 0.190 ac
Pond 0.070 ac
Total 1.083 ac
Pervious
Sidewalk(backsloped) 0.05 ac (Modeled as fawn since backsloped)
Pasture 1.977 ac Total Project Basin
Total 2.027 ac 3.110 ac
Pasture Lawn Roof Driveway Disp Pond
ROW 0.21 0.05 0.19 VIM
Lot 1 0.27 0.1 0.02 Roof to Infiltration trench/DW dispersed over 50'
Lot 2 0.24 0.055 0.055 Roof dispersed over 251/DW dispersed over 25'
Lot 3 0.25 0.015 0.1 0.015 DW dispersed over 25'
Lot 4 0.31 0.035 0.035 0.1 Roof dispersed over 50'/DW dispersed over 25'
Lot 5 0.45 0.01 0.01 0.07 0.1 House Roof dispersed over 50'/Shop Roof dispersed over 25'
Lot 6 0.23 0.09 0.0 Roof to infiltration trench/OW dispersed over 50'
1.96 0.165 0.3 0.355 0.25 0.1 3.1100
• PROPOSED 16.5' WIDE j
INFILTRATION SYSTEM
(ROOF DRAINS/CLEAN WATER ONLY)I
TRENCH CAN BE BUILT IN FILL
WITH GUIDANCE FROM THE 8.25 -8,25'
PROJECT GEOTECH AND FILL
BELOW IS PLACED AS REQUIRED I CONSTRUCT TRENCH FOR
BY THE 2014 DOE MANUAL PERFORATED PIPE, SEE THIS
TRENCH BOTTOM TO BE 2" BEL01
PIPE INVERT. ENTIRE TRENCH TO
BE FILLED WTTH FREE DRAINING
GRANULAR MATERIAL SUCH AS
FILTER FABRIC COMPACTED FILL BUCKSHOT, PEA GRAVEL, DRAIN
TOP AND SIDES ROCK, OR CLEAN FREE—DRAINING
OF TRENCH I GRAVEL HAVING A MINIMUM 30%
VOIDS.
'—� EX. GROUND J
LOT 1 DOWNSPOUT INFILTRATION }J >}
SYSTEM IS ESTIMATED TO 12-14- 1
BE 108' LONG x 16.5' WIDE x 12"-14
8" DEPTH OF DRAINROCK (OR I 8" MIN
EQUAL FOOTPRINT) Ilr
LOT 6 DOWNSPOUT INFILTRATION
SYSTEM IS ESTIMATED TO 4" PERFORATED
BE 78' LONG x 16.5' WIDE x STORM DRAIN CONNECTED RESTRICTIVE SOIL
8" DEPTH OF DRAINROCK (OR TO YARD DRAIN WITH 4" RISER LAYER 41"-54" PER
EQUAL FOOTPRINT) OUTLET TO CHERRY COURT STORM SPICHER SOIL TEST PITS
SYSTEM. TOP OF RISER OUTLET
(df TO BE = TOP OF DRAINROCK
LOT 1 AND 6
DOWNSPOUT/ROOF DRAIN
INFILTRATION SYSTEM
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