HomeMy WebLinkAboutStormwater Site Plan Review - PLN General - 10/24/2000 i
MASON COUNTY
PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER
Shelton,Washington 98584
DATE: Oct. 24`h, 2000
INTER-DEPARTMENTAL COMMUNICATIONS
TO: Jason Manassee, DCD—Planner
FROM: Alan A. Tahja, P/W - Co. Hydr. Engr.
SUBJ: Stormwater Site Plan Review NAME: _ Ouellette—Lilliwaup Post Office
A Stormwater Site Plan(SSP) prepared for the proposed Lilliwaup Post Office has been received
and reviewed by Public Works.
The plan conforms to standards of good engineering practices and principles for this time and
place, and appears to meet present standards for stormwater quality and quantity control.
I've been in contact with the project engineer, Lorna Taylor, and requested that they provide a
letter of concurrence or authorization to site their proposed bio-swale in the State Highway's
right-of-way. Authorization from the State to construct the swale in the right-of-way complicates
who is to maintain the Swale, but I expect Ouellette and WSDOT to be able to work something
out.
Akin to the right-of-way issue is completion of the Operation and Maintenance Covenant, its
recording, and delivery of a copy of the recorded document to Public Works for monitoring and
administration.
The right-of-way issue and O&M Covenant are the only details I see that still need addressing,
otherwise the SSP can be considered complete.
Please feel free to contact me at County extension 461 if you have any questions regarding this
department's review or these comments. I will keep you informed of any materials I receive
pertaining to the two remaining issues.
Si cerely,
Al n A. Tahja
File: H:\WP\STRMWTR\REVIEWS\Lilliwp-PO.doc E-6t6& I D
Taylor
Engineering RECEIVED
---- -- ]Consultants OCT 0 4 2000
URS OFIEWER
SEATTLE
PRELIMINARY
STORMWATER SITE PLAN REPORT
Post Office
in
Lilliwaup, Washington
M. Ay
of NNA Sy 0
0
� z
? 29351
�p <v
JSTV-
S�aNAL E�
EXPIRES 1141 of
Prepared by: Lorna M. Taylor, P.E.
Date: September 8, 2000
Revised: October 2, 2000
P.O. Box 1787 Issaquah,Washington 98027-0073 telephone(425)391-1415 fax(425)391-1551
Stormwater Site Plan Report
Lilliwaup P.O.
TABLE OF CONTENTS
Description Page Number
I. Project Overview..............................................:...... 1
II. Plot Plan............................... .............................. ... 1
III. Preliminary Conditions Summary................................ 1
IV. Off-Site Analysis...... ...... ......................................... 1
V. Analysis and Design................................................ 1
VI. Special Reports and Studies...................................... 2
VII Basin and Community Planning Areas........................ 2
Vill. Other Permits......................................................... 2
IX. Erosion and Sediment Control Plan............................ 2
X. Bond Quantities Worksheet, Retention/Detention
Facility Summary Sheet and Sketch, and Declaration of
Covenant.............................................................. 3
XI. Maintenance and Operations Manual.......................... 3
Appendices
Appendix A: Site Plan
Appendix B: Calculations
Appendix C: Soils Investigation Report
Appendix D: Declaration of Covenants Associated with Privately Maintained
Storm Drainage Facilities
TEC
Stormwater Site Plan Report
Lilliwaup P.O.
I. PROJECT OVERVIEW
This Stormwater Site Plan Report is submitted to Mason County in accordance with the
County-adopted Stormwater Management Manual for the Puget Sound Basin (Ecology
Manual). The project site is located southwest of Lilliwaup
Bay, west of SR 101 and Hood
Canal in Lilliwaup, Washington. See Figure 1 in Appendix B for a map of the vicinity_
The proposed project will construct a modular post office building and associated access,
parking, utilities, drainage features, and landscaping. The building sewer will be connected to
the existing septic system currently serving the existing post office building to the north. The
new development will cover approximately 0.57 acres spanning several leased lots, all fronting
on SR 101.
II. PLOT PLAN
The proposed site plan is shown in Appendix A.
III. PRELIMINARY CONDITIONS SUMMARY
Most of the 0.57 acre area to be developed is covered with small trees, brush, and grass. The
area is relatively flat and reportedly was a gravel borrow pit long ago. There is an existing post
office building just to the north of the proposed new building site. USGS mapping (see Figure
1 in Appendix B) shows that the site is just across SR 101 from Hood Canal at the base of a
steep hillside to the west. The soils underlying the site are very permeable with no signs of
high groundwater. The soils investigation report prepared by Bradley-Noble Geotechnical
Services for the project is included in Appendix C.
IV. OFF-SITE ANALYSIS
The site is adjacent to SR 101, which abuts Lilliwaup Bay on Hood Canal. There is no
downstream runoff path. Currently, runoff from the site infiltrates into the highly permeable
soil. There is a small ditch along the highway frontage, but it has no outlet. Any runoff from
the site that does occur would flow in the roadside ditch or across SR 101 to Hood Canal.
Uphill of the proposed development, is a steep slope, which extends approximately 250
vertical feet above the site.
V. ANALYSIS AND DESIGN
The runoff from the proposed project (including pervious and impervious areas) was analyzed
with the Santa Barbara Urban Hydrograph (SBUH) method, using the King County Surface
Water Management Design Manual computer program HYD.EXE and precipitation data from
the Ecology Manual. The developed system was modeled with a total of:
0.040 acres of roof surface
0.180 acres of asphalt paving
0.004 acres of concrete, and
0.346 acres of landscaping
compared with a bare dirt and brush pre-developed condition, which was modeled as fully
forested, to provide a conservative detention sizing.
The following table summarizes the pre-developed and post-developed flows generated by the
SBUH routine. The runoff was calculated for both the "pre-development" and "post-
I
TEC
Stormwater Site Plan Report
Lilliwaup P.O.
X. BOND QUANTITIES WORKSHEET, RETENTION/DETENTION FACILITY SUMMARY
SHEET AND SKETCH, AND DECLARATION OF COVENANT
Mason County's Declaration of Covenants Associated with Privately Maintained Storm
Drainage Facilities form is included in Appendix D. This form must be completed, notarized,
and filed with Mason County.
XI. MAINTENANCE AND OPERATIONS MANUAL
Retention/Detention Facilities
The detention/infiltration facility is a surface pond, located south of the new parking area. The
purpose of the pond is to infiltrate the runoff from the new impervious surfaces on the site into
the underlying soil. The pond has been sized to store some water when the infiltration
capacity of the soil is exceeded during extreme runoff events.
In order to function properly, the pond must be kept free of accumulated sediment, which will
reduce the infiltration rate into the underlying soil. The facility should be visually inspected for
sediment accumulation at least once each year and also after every major storm greater than
or equal to a 10-year return frequency. If the facility remains full of water for extended periods
of time, then the sediments should be removed from the pond.
Conveyance Systems
Pipes, trench drains, and swales/ditches transport runoff from one place to another, in this
case from catch basins and downspouts to the pond. To work properly, pipes must be kept
free of silt and other debris. If pipes become blocked, surface flooding will usually occur.
Catch Basins
Catch basins collect surface drainage and direct it into storm conveyance pipes. They help
prevent downstream p drainage problems by trapping sediment and other debris that would
otherwise flow downstream with the runoff. It is important to keep catch basins clean so
accumulated silt is not flushed into the infiltration pond during a significant storm. All catch
basins should be inspected at least once each year and after major storms.
3
TEC
APPENDIX A
SITE PLAN
�� ♦ `\ `\ GRAPHIC SCALE LEGEND-
.. ` ASPHALT PAVEMENT
• ' \` \ (01 rm I+ ® ETNwNC WALL ptl
\ \ \ EM
1 �090O CFO- ROOT CRAM //1 L
\ RIM GRAVEL
:TORN DRNNAZ LNE(50J 1)�•!c
HOED CANAL'
CONCRETE PAVEMENT PROJECT
LOCATION c on
\\�`• \ \ /' \ ! ,.� / ♦� �iT ` dra OR swAIF FLOW uN£
`t /. ♦/ . r GRASS FILTER a. C
/ ♦ ♦ SWAP AREA DRAM R C O
/ ! ` 'DVERfLDw
A N W !-
\ ` � / \c 'y'� � • , �` 'RWRAP RNIaiEO cArcN eA9N w ~
VICINITY MAP
!a. i .SECNSN'A' ` CLtANaur �T—(
1, /AI / • ';�. / +.GRAVEL \ N 1 517
GSPER90NNCN P£R ABBREVIATIONS:
,OETAL j/Cl ♦ _RIPRAP ARNCR AT PIPE
\ ! / ♦ \ % I/Cl PER DETAIL
OUT AO AREA:;RAM
APWA AMERICAN.'J&r WORKS ASSOCIATION
EA \ \' Ca CATCH aASM W
/ Cr�iClNR1 ♦ C.O. 1 DEVELOPMENT
�. / / / / OETAL 1/C ` DEV
\ Fr'. •i' DISP DISPERSION
It \ N / - - / •� •� •r / _♦. EAST 1
EIc
_ TO scow DRAIN !A
\ RCKKERf FAONC WALL IF /f /j IE \ ` L `IVERT E FLOOR
d
-�FA DETAIL
,y \ Cf }NDC FOOT
OA05%N ' ♦\ // v IS NORT�SCAii ¢<.
ly I / • . = ;
�;
/ 2
/
\ SGI;ON
SO • RAM:
CRY D
STD STArdARD �,�
\ / CANTILEVER CCNCRE TYP
rE ``� / / MSDOF wASfRJDiON STATE_{FAI7TMENF Y I�
RETANMG WALL PER DETAIL rOP T/' _� / , , 7AMSPS.'RTA:A1
i
y J i
DRAINAGE
r,PFNINC XTRIIDEu:GNCRE`_CL;R3
SET IN EPDXY
SITE PLAN _ INFILTRATION PCN S Off!
` ACING '9�'i•?j
to
: S _
. .. . . ...'. .S
•' 15l� - 15'ASSUMED
. .... ..... ......
I
\PROPCSEG rl 1 A55 SLOPE
9UILdND� SR SS f1rER - .
ASP+•ALr DFNVE 1 1
jTRIP _ I I F - r•etW x EMSTINc
i I.. SLOTTED CURB 1't l s I CcAPPRO%. Q
} 7 "RrEO HIGHT ,
I FLOW:`SPREADER }
. 20 . . . '; - OR ORAVEE ft= - .. - •T`PKAL: . . . .-1 1 AtAVEl 8A11A5T. - .. .. - . . .. . . .. _ . . _ .. _ ... . '
. 1 TRENCH/CI I•WN drM FOR]•OIXLER r-SLOr-ED_EXTRUDED Q
DETAiI T .0
.. I .'IYPERYEA&E CLAYjFwE - - - - i -CNC'<£TE :uRB
�!'ASP- . - SILT SOIL BARRIER
A< ,:,Lss a
. . . .
...CVER CRLS�'SLQFAONG.... .. - .. .. :. .. .. .... .... .. ... .. ... ... ... ... .. AAIi ., WASFILTER STRIP -
O
1 SCALE: 1' :. A •
. ... ... . . .. . . . ... . .. ... �J :.. . ... .. . . . . . . . . .. . . . .. . .... ..... .. . .. . .. . . l7�O 1. CRASS FILTER I
_ Q O • STRIP
.. EXISTING GRADE
: .. . . . . .. . .. 0 C1 O I
000
t 1 I 1 0 RE
a 00 0 g
141 MADE
• _ ... ... . - ... - .. . . . . .. - . . .. . . . . . . . . ,. .�FN . . . . - . . . . . . . .. .) O - 1--S' TREATED WOOD C
100"EAlt WAI R _ _ NOTCHED SPREADER BOARO I
•'•" - SIMILAR TO DE AIL /Cl
• POND BOTTW EFEV •20.5C - WASHED RIVER '
20 ... .... .. . .. .. . .. . . . . . . . . . W H
( 11/2 TO /+
� i
DISPERSION TRENCH
INFILTRATION POND //.���\' N rS 2
SCALE 1--5' T
sseo*,son 4.0 3/17/DD
APPENDIX B
CALCULATIONS
Lilliaup P.O. hyd.out
Preliminary Hydrology Anallsis
Date: 8/09/00
KING COUNTY DEPARTMENT OF PUBLIC WORKS
Surface Water Management Division
HYDROGRAPH PROGRAMS
Version 4.21B
1 - INFO ON THIS PROGRAM
2 - SBUHYD
3 - MODIFIED SBUHYD
4 - ROUTE
5 - ROUTE2
6 - ADDHYD
7 - BASEFLOW
8 - PLOTHYD
9 - DATA
10 - RDFAC
11 - RETURN TO DOS
ENTER OPTION:
SBUH/SCS METHOD FOR COMPUTING RUNOFF HYDROGRAPH
STORM OPTIONS:
1 - S.C.S. TYPE-1A
2 - 7-DAY DESIGN STORM
3 - STORM DATA FILE
SPECIFY STORM OPTION:
1
S.C.S. TYPE-IA RAINFALL DISTRIBUTION
ENTER: FREQ(YEAR), DURATION(HOUR), PRECIP(INCHES)
1 24.00 3.00
----------------------------------------------------------------------
******:************* S.C.S. TYPE-1A DISTRIBUTION ********************
't--AW6" 24-HOUR STORM **** 3.00" TOTAL PRECIP. *********
----------------------------------------------------------------------
ENTER: A(PERV), CN(PERV), A(IMPERV), CN(IMPERV), TC FOR BASIN NO. 1
.4 86.0 .1 98.0 15.0
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TC(MINUTES)
A CN A CN
6 .4 86.0 .1 98.0 15.0
PEAR-Q(CFS) T-PEAK(HRS) VOL(CU-FT)
.25 7.63 3953
.ENTER [d:) [path)filename(.esct) FOR STORAGE OF COMPUTED HYDROGRAPH:
6mu.hyd
SPECIFY: C - CONTINUE, N - NEWSTORM, P - PRINT, S - STOP
c
----------------------------------------------------------------------
ENTER: A(PERV), CN(PERV), A(IMPERV), CN(IMPERV), TC FOR BASIN NO. 2
.3 86.0 .2 96.0 6.9
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TC(MINUTES)
A CN A CN
•6 .3 06.0 .2 98.0 6.9
Page 1
ENTER: FREQ(YEAR), DURATION(HOUR), PRECIP(INCHES)hyd.out
10 24.00 6.50
----------------------------------------------------------------------
*`*****`***********` S.C.S. TYPE-IA DISTRIBUTION ********************
10-YEAR 24-HOUR STORM **** 6.50" TOTAL PRECIP. *********
----------------------------------------------------------------------
ENTER: A(PERV), CN(PERV), A(IMPERV), CN(IMPERV), TC FOR BASIN NO. 1
.4 86.0 .1 96.0 15.0
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TC(MINUTES)
A CN A CN
.6 .4 86.0 .1 98.0 15.0
PEAK-Q(CFS) T-PEAK(HRS) VOL(CU-FT)
.70 7.63 10739
ENTER [d:j [pathjfilename[.extj FOR STORAGE OF COMPUTED HYDROGRAPH:
10yu.hyd
SFEClEY: C - CONTINUE, N - NEWSTORM, P - PRINT, S - STOP
c
----------------------------------------------------------------------
ENTER: A(PERV), CN(PERV), A(IMPERV), CN(IMPERV), TC FOR BASIN NO. 2
.3 86.0 .2 90.0 6.9
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TC(MINUTES)
A CN A CN
� 6 .3 86.0 .2 98.0 6.9
PEAR-Q(CFS) T-PEAK(HRS) VOL(CU-FT)
.81 7.83 11227
ENTER [d:) [pathjfilename[.extj FOR STORAGE OF COMPUTED HYDROGRAPH:
l0yd.hyd
SPECIFY: C - CONTINUE, N - NEWSTORM, P - PRINT, S - STOP
n
STORM OPTIONS:
1 - S.C.S. TYPE-IA
2 - 7-DAY DESIGN STORM
3 - STORM DATA FILE
SPECIFY STORM OPTION:
1
S.C.S. TYPE-IA RAINFALL DISTRIBUTION
ENTER: BREQ(YEAR), DURATION(HOUR), PRECIP(INCHES)
100 24.00 9.00
----------------------------------------------------------------------
********`*****`***`* S.C.S. TYPE-IA DISTRIBUTION ********************
100-YEAR 24-HOUR STORM **** 9.00" TOTAL PRECIP. *********
----------------------------------------------------------------------
ENTER: A(PERV), CN(PERV), A(IMPERV), CN(IMPERV), TC FOR BASIN NO. 1
.4 86.0 .1 98.0 15.0
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TC(MINUTES)
_ A CN A CN
.6• .4 86.0 .1 98.0 15.0
PEAK-Q(CFS) T-PEAK(HRS) VOL(CU-FT)
1.03 7.83 15771
ENTER [d:j [pathjfilename[.extj FOR STORAGE OF COMPUTED HYDROGRAPH:
Page 3
•
........................
.. :::.::.
_ �. •.l.. � ... �{� �'t.... �;, ���,�:f'�.:fit
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.... .......: Tf �:.! __ - j-1 1'r.'_ :.�:�E .i !t:we..: iehp ... ••A1'..
...... .... ix-.,«r .. � F .•:'.:'. � �.t1► ••�,!+i«+«t+!::.".......' �� ,!y,cti ���
IS
f
w�r..z�as '"'..-"., •::�'.:_•: : fit:' ..::::::::�,O! �. ..:..:::.::::. � .. � _«..�.. . -
�f -
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bw
...... "
�....... !..�,
• J.
30
t
C y
v
.. . ...
:1.
i
............
x : ::: :: : :::
f Figure 1
�� r :.:
or fF r ::•.:
Site Location/USGS Map
Scale as Shown
D 7OPOQoa©d&O 1999 DOLUMe Yarmouth,ME 040% Semite Data:USGS
--i400R Scale:1:JOAM DetIJ 14-3 Datm WCS84
ENTER: FREQ(YEAR), DURATION(HOUR), PRECIP(ZNCHES)hyd.out
10 24.00 6.50
----------------------------------------------------------------------
****,r*********,r*,r*** S.C.S. TYPE-IA DISTRIBUTION ********************
***xx**** 10-YEAR 24-HOUR STORM **** 6.50" TOTAL PRECIP. *********
----------------------------------------------------------------------
ENTER: A(PERV), CN(PERV), A(IMPERV), CN(IMPERV), TC FOR BASIN NO. 1
.4 86.0 .1 98.0 15.0
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TC(MINUTES)
A CN A CN
.6 .4 86.0 .1 98.0 15.0
PEAR-Q(CFS) T-PEAK(HRS) VOL(CU-FT)
.70 7.83 10739
ENTER [d:] [path]filename[.ext] FOR STORAGE OF COMPUTED HYDROGRAPH:
10yu.hyd
SPF=y: C - CONTINUE, N - NEWSTORM, P - PRINT, S - STOP
c
----------------------------------------------------------------------
ENTER: A(PERV), CN(PERV), A(IMPERV), CN(IMPERV), TC FOR BASIN NO. 2
.3 86.0 .2 98.0 6.9
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TC(MINUTES)
A CN A CN
�`\\ .3 86.0 .2 98.0 6.9
PEAK-Q(CFS , T-PEAK(HRS) VOL(CU-FT)
.81 7.83 11227
ENTER [d:] [path]filename[.ext] FOR STORAGE OF COMPUTED HYDROGRAPH:
lOyd.hyd
SPECIFY: C - CONTINUE, N - NEWSTORM, P - PRINT, S - STOP
n
STORM OPTIONS:
1 - S.C.S. TYPE-lA
2 - 7-DAY DESIGN STORM
3 - STORM DATA FILE
SPECIFY STORM OPTION:
1
S.C.S. TYPE-lA RAINFALL DISTRIBUTION
ENTER: FREQ(YEAR), DURATION(HOUR), PRECIP(INCHES)
100 24.00 9.00
----------------------------------------------------------------------
*************x:***x* S.C.S. TYPE-IA DISTRIBUTION ********************
x**x***xx 100-YEAR 24-HOUR STORM **** 9.00" TOTAL PRECIP. *********
--------------------------------------------------------------------
ENTER: A(PERV), CN(PERV), A(IMPERV), CN(IMPERV), TC FOR BASIN NO. 1
.4 86.0 .1 98.0 15.0
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TC(MINUTES)
_ A CN A CN
•8. .4 86.0 .1 98.0 15.0
PEAR-Q(CFS) T-PEAK(HRS) VOL(CU-FT)
1.03 7.83 15771
ENTER [d:] [path]filenamef.ext] FOR STORAGE OF COMPUTED HYDROGRAPH:
Page 3
Lilliwaup P.O. rdfac.out
Preliminary Infiltration Pond Ana
l;sis
Date: 06/15/00
KING COUNTY DEPARTMENT OF PUBLIC WORKS
Surface Water Management Division
HYDROGRAPH PROGRAMS
Version 4.21E
1 - INFO ON THIS PROGRAM
2 - SBUHYD
3 - MODIFIED SBUHYD
4 - ROUTE
5 - ROUTE2
6 - ADDHYD
7 - BASEFLOW
8 - PLOTHYD
9 - DATA
10 - RDFAC
11 - RETURN TO DOS
ENTER OPTION:
R/D FACILITY DESIGN ROUTINE
SPECIFY TYPE OF R/D FACILITY:
1 - POND 4 - INFILTRATION POND
2 - TANK 5 - INFILTRATION TANK
3 - VAULT 6 - GRAVEL TRENCH/BED
ENTER: POND SIDE SLOPE (HORIZ. COMPONENT)
ENTER: EFFECTIVE STORAGE DEPTH(ft) BEFORE OVERFLOW
ENTER: VERT-PERM(min/in), PERM-SURFACE (0 = SIDES ONLY, 1 = SIDES AND BOTTOM)
ENTER (d:] [path]filename(.ext] OF PRIMARY DESIGN INFLOW HYDROGRAPH:
PRIMARY DESIGN INFLOW PEAR = 1.17 CFS
ENTER PRIMARY DESIGN RELEASE RATE(cfs):
ENTER NUMBER OF INFLOW HYDROGRAPHS TO BE TESTED FOR PERFORMANCE (5 MAXIMUM) :
ENTER (d:] [path]filename(.ext] OF HYDROGRAPH 1:
ENTER TARGET RELEASE RATE(cfs):
ENTER [d:] [path]filename(.ext] OF HYDROGRAPH 2:
ENTER TARGET RELEASE RATE(cfs):
ENTER: NUMBER OF ORIFICES, RISER-HEAD(ft), RISER-DIAMETER(in)
RISER OVERFLOW DEPTH FOR PRIMARY PEAK INFLOW = .19 FT -
SPECIFY ITERATION DISPLAY: Y - YES, N - NO
SPECIFY: R - REVIEW/REVISE INPUT, C - CONTINUE
SUMMARY OF INPUT ITEMS
1) TYPE OF FACILITY: INFILTRATION POND (2.00:1 SIDE SLOPES)
2) STORAGE DEPTH(ft): 2.00
3) VERTICAL PERMEABILITY(min/in): .50
PERMEABLE SURFACE: SIDES AND BOTTOM
Page 1
rdfac.out
1.20 .00 308.4 340.8
1.40 .00 379.6 372.0
1.60 .00 457.3 404.4
1.60 .00 541.5 438.1
2.00 .00 632.6 473.1
2.10 .46 680.8 491.0
2.20 1.31 730.8 509.3
2.30 2.40 782.6 527.9
2.40 3.69 836.3 546.8
2.50 5.16 892.0 566.1
AVERAGE VERTICAL PERMEABILITY: .5 MINUTES/INCH
SPECIFY: F - FILE, N - NEWJOB, P - PRINT IF/OF, R - REVISE, S - STOP
Stop - Program terminated.
Page 3
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6.3.5 NARROW AREA FILTER STRIPS
FIGURE 6.3S.A FILTER STRIP LENGTHS FOR NARROW RIGHT-OF-WAY
20.0
15.0
v
Flowpath
30 feet
10.0
a. 20 feet
sir
Vj
L
6�
-r
w 10 feet
5.0
0.0
0% 5% 10% 15% 20%
Filter Strip Slope
Note: minimum allowable filter strip length is 4 feet
6.3.5.2 DESIGN CRITERIA
Required and recommended design criteria for narrow area filter strips are the same as specified for basic
filter strips. Note that for roadway applications,gravel spreaders must meet the specification for shoulder
ballast given in Section 9-03.9(2)of the current Standard Specifications for Road,Bridge and Municipal
Construction, 1994compacted to 90 percent standard proctor.
1998 Surface Water Design Manual 9/l/98
6-65