Amendment to stormwater for additional imperviuos area.pdfDON N'EL ENGINEERING, F C.
Consufffng Civil Engineers
660 -21 V1 Street SW; #300
Mountlake Terrace, WA, 98043
April 29, 2016
Jenn�fer Lambert
Public Work Depai-tweet
121 5"' Ave N Edimonds, WA 98020
RE. Neurmm) ...190 Stinset Ave N(9BI-D20150224)
Dear Jennifer,
I have 6wm 4iformed that an additional 200 SF of new irnpervlous hasbeen added tc) this
project at construction beyond what was originally proposed iri the drainage report, I have been
asked to verify that thestorm water irffllo�afion system is able to handle the ckmnge,
Project coverage (See Neurrian Onsite Drainage Repor-t dated jrt IN 24,2015) was
chmiged as follows:
Proposed Per-vimv,, 1,585 SF - 200 SF
Proposed limpervious 4,548 SF'
Rooftop 1,,956 SF
Porches - 148 SF
C,oncrete 'Walk = 566 SF + 200 SF
New, Pavement
Total New Irnpervious d,5t8 ST"
ImperviOUS iS Still ICSS th,911 5,000 SF, so the proect is still as Small Prcr; e
�ct,
J
The lmffltrafiorr was desived as fbflows�
Beginning ofDrainage Report Citation
"" Desigp constralrrts:
Total site contributing irnpervious PROPOSET) -- 4,348 SF
Measured Infiltralion Rate =- 2 jrAir
hifiltraffim rate: 0,5 in/hr afterapplicafion of'Safety Factor = 4
Seected BMP� Gravelless trerich
An,alysis,
Per Cil Y r (yf I 'Amonds handout 9E'72 B (1,11) Stornw
ialer BM-Psimplified sizin.g�,):Table
5, Sizing factor for Gravel tes,s Chamber with 0.5 un/hr2.70/'o SA
(impervious) * 2.711,1o' 11.091'1 (4,348 SFarra 3/1 00) = 117.E LF Figure A-4,
typical gravelless chamber products komn to meet gravelless cliamber criteria
hiclude Storm',Fecfi SC -3 d 0: Sionn Tech SC -31 Ct Chamber:
( I
Iiiamber desiLpi width: 34 inches (2.83 fect)
Chamber design ierq;th 5A inches, (7,12 feeo
Surface area per charnber; 36 51 SF
744-0916 (CIELL) 425 359'-2717 (FAX) 425 744-09-46
LF (required) = 117, 1 LFrequircd/';,t2/un it -, 16.48 or t7 units offEK'-310
Distribution:
Front Yard Area 4 cliambers 28,,48 LF
1(ear Yard Area 4 -chambers - 28AS LF
Under Driveway Area 9-ch,,imkiers 64,(,')8 LF
Sub-Toml = 121 .04 LF'],, 1 17A LF
(.3ravelless Charnber Requirements,
a. Charnbcw ter glh snail be deterrnined usingthe simpfified sizh,ig inethod,
Done.
b, "The infiltration rate used t(,a determine the required length, shall be the design, cir "Iong-
term," rate and must be cMculated using correction factors in the Ecology Manual,
Lon, terser Rate pi-e.)viekd 1�y the soils rqwwt as, 2 inane with SF:= 4; J)esigrl Rate = M HVIMR
�? eepfinear
ell Void (storage) volmne provided b.y the chamber aibic, ft er shall be at least 2,6
foot and infijtj°atjVe SLfftUe Under the chumber footprint shall be at least 2.8 squar6 FL J per
linear fbot. Chamhe)selcwted is the Storm Tech ,5(7-310 Che,'imber, which meets fim crileria,
at. hivert elevation of the oveTflOw conveyance shall be set above the storage resei-voir,
Done,
Coriclusiom
Edinands handout #E72 B suggests asing Storm Tech SC -310 as a gravelless chamber mrith a length
eqUivalent of 117 S,F M11 safisfy, the drainage requirement For this project, CaICUIPIting the designed
surface area produced by S(.-3 I O�, it ww; determined that 17 chanibers would create adequate detcrition
ptus surfi,,c1("e area tcinfiltrate the impervious area ftom the sitc-""
End of'Otation
Additic"i aff 200 LF of impeiMous is an increme d--rf'4,6%, 1 177 LF of chaMbers Was required by, the
City Design Table, & 21,04 LF prcMded, are extra 3.2%, In addition, Orie lsoH provided an iTYMITMi011 NMC
(It, 7"INA-M. aocording to the Geotechnical Report.safely factor of 4 wm used in the CICSigT! (2 is
normally consrdcrod sufficient) increasing the capacity above the design requirement.
There Ore, there is su icient exce,ss cal.wacitV Imilt into the to accominodate a 5r'
inct-cose on thisprqjee-'t.
If you have any queslions, 111case caflams, , at 425-744.0916,