HomeMy WebLinkAbout03240 - Technical Information Report �
0 BAIMA & HOLMBERG INC.
�
N
"' � It� � 3� ���
TECHNICAL INFORMATION REPORT
for
Annie's Place Short Plat
December 20, 2004
�vtSGD `E(�S10�
�KU P �
ti���'oF W
� Q � �
a � �_
�`' �
i ��.��,��;3�.� ��w
SSI�NAL ��
EXPIRES 11/30/06
Baima& Holmberg, Inc. Job No. 1004-025
Prepared For
Bill Hughes
15220 Cedar Grove Road -
Issaquah, WA 98027 AP� 2 � 2005 ',
�,.,,.
3z�o
100 FRONT STREET SOUTH • ISSAG�UAH • WASHINGTON • 98027-3817 • (425) 392-0250 • {425) 391-3055
TABLE OF CONTENTS
SECT'ION TITLE
1 PROJECT OVERV�VV
2 CONDTTIONS AND REQUIREMENTS SUMMARY
3 OFFSITE ANALYSIS
4 FIAW CON17tOLAND WATER Ql',�1.1"1��� 1�.�(�11:17��-
� ANALYSIS AND DESIGN
� 5 CONVEYANCE SYSTEM ANALYSI� .��ti U t�I�:ti 1 G�
6 SPECIAL REPORTS AND STUDIES
7 OTHER PERNIITS
8 ESC ANALYSIS AND DESIGN
9 BOND QUANTITIES,FACII.ITY SUMMARIES,AND
DECLARATION OF COVENANT
10 OPERATIONS AND MAINTENANCE MANUAL
SECTION 1
PROJECT OVERVIEW
Project Overview
This project involves the development of a 1.08-acre parcel into 3 single-family
lots. T'he site is located at 150 Lyons Avenue NE, about 150' south of NE 2�
Street. The site has an existing house/driveway/yard with a portion on the east
side covered with woods. The site generally slopes down to the south towards a
wetland area located on the south part of the property. Per the SCS soil maps, the
site is underlain with Alderwood sandy loam/till soil.
Upstream Tributary Area
A small area north of the site drains south through the site to the onsite wetland at
the south property line.
Level 1 Downstream Drainage Analys�{
The site drains to an existing wetland at the south �����,����t� !�n� t:,;_ I�i�>,�;
continue flowing south about 600' to a 12" storm system (B). Flows enter the
CB/pipe system flowing south then turning east at a catch basin{C). This flo�ti
continues east, then south through a 12" pipe, crossing the east end of the cul-de-
sac of SE 2�Court, about 800' downstream of the site(D). This pipe system
continues about 200' to the south side of an existing R/D pond (E), discharging
onto a rock pad. Flows continue south into King County then east then south
about 800' throu�h A 5'± wide stream(F)though Maplewood Park to near the
� north end of 148 Avenue SE. Here flows enter a 30" inlet pipe (G) and continue
to drain south through a 30" CB/pipe system in 148'� (H} about 700' to a King
County stormwater R/D pond(I)near the intersection of 148�Pl SE/SE 139th Pl,
well beyond '/a mile downstream of the site. Flows outfall fi-om the pond to
another Kin�County stormwater R/D pond(n near the intersection of 148'�Pl
SE/SE 142 Street, about 1200' beyond (about 3,700' downstream from the site).
That pond outfalls to a ravine leading down to the Cedar River, about 2 miles
downstream from the site.
Proposed Stormwater Control
Included calculations show that the site is exempt from both detention and water
quality requirements. The Lot 1 roof and a portion of onsite private,joint
driveway will drain to the adjacent, frontage Lyons Avenue drainage system to be
constructed and discharged into the adjacent ditch. Runoff from the Lot 2 and 3
houses and most of the onsite private,joint driveway will be dispersed into the
onsite wetland bui�er.
i i i
� i �
i i +
i �� ,�. i �
� I � TAX IAT NO.084710-0061 I
� I �
� I I ,
I � I II
( � � I
I � �� � Ik � �
l I ` I
s�' ^o o 'e aoo.a . ��a
+ , _ _� I
I I �iz
F �o.' tz• +r
I �. ' �.....��. ��� 8 ��.
J� ri ��� � � E . 36' i
I � � � ' �� � 8�' s�.�8 .
� �
I w ; I � Y�� 3
..' 8 5• 8,
� CI � � t2 � 6
8
J ,� �$.. � . 24• �-::.�; 2 6• '
'�a y� {�6' ^
I '� t' t 6' ��Y 8.
I ~ I � � � 6, 6. (2 6' I �
t 4' 414 � Q
I 2 � ' �� '''
; � •
I a � I� --- --- 36�
,�� �, , M,z ,
� �- T (✓ .��-�.
I � � � � �. r il'-f � .�A-11
25' B' /
I � ( w 4C1�1 --� •�. - BUFFER M g„ �Q>�
Oi ��u r.�"cE � J _.. . .� �yA—�. �r e..
I ` , i •� � / / �j? Z4 36„ � 4'
.�r �
� I �-3 � �
� a i CONC. N88'IY'43'�' 3pp��• .
I + „ � .�.,,.� �
O s
� � � � � ��
I ,�, i� � I �yA_� TAX LOT N0.084710-0081
I
I . �
I I �
I I (
I I : �
I II �
�� � �
I I s I
I � � I
N Site Plan
i�� = so�
. x----- ............... — —•—•—•—•_�_•
— �
" (F.M4NIFOLD RD.1�"� b�lr4�
�� �" �oy B2N5 N io � , ,
I u I.^.: 2ae N C � y� e�.n�wv,w7u� ( 1]5) ` 1P0�
I LI z 'L�' ( 56� �m r T w ' � -- �)
< � 'Z � - W ,n M 15 � ..�� f !Q� �
N O 4 — n � �
I �pf+ • '� /f O � � W � w • i nn
o '" � �lp,µ5 � 2
� � PS ¢ � �e I
� � `� 4 W V 6 I
7°� O � � O �I O ^ O_ O w ,�b r p� Z
...� � , ., , J � �J � P � J
�p � TRD� "., d �
• �
.,fi ��,y �M ^ R � � I �� 64 6J �6Y 01 N =aN � �� a � i
r�.� r�7 r� �� s eo �� Y J �
i R �i,'� �C'�J ZJ � �a � � � s • Q a 1
q p m �
s.E.:-:y:;•>I2aTM;•>:>;:R.•:::;:;•:::•::..,:a ' � p ' bC' �I
.0 M! J TAACT� I K, � � z
� N Mlf S I
i
� � 0 � Y� � I !. � 4 �+ �t'� a ar �L k� ttA n 1 i
� � � '
2 ,w .!� M n'« I
x aut � R I T t 6 3 k' 3RD > ! ST. X �u ,
� r •
� (1) � I 1� wr �." �b �y � ]I �t jp' I ',.
: N.E� �i r I,i � i
� I21 I , � u � � �7 � �� tt = �
0 �$ � � ?No � x y� as a 5 �
H° I oa q% � 73 �. pe ^ .
•.!7 300.7 I
� _ � i
(3) 0.67A1C.�C I � � � p µ C�. Ji }4 � !T I - I
!� '� I
o � xso � I g�50 A � e� rn.o �� „� n
C7' K • I
' I . 0.• � I O � riucr c N R �.no cr. �.a ri « ( i
• w �
� � ' W 4 � �� � ea �z n ri 8 �� '
I ,
S.P.48E030 � � I.38 A 0.�� N O I 6T 6! 6! i0 � � � tll� 2 4 5 6 7 '�
�o ' I p u � � CE AR R�V PA K D V. I s i�
. O' : �: � �� .� O �
s S�FSMT, a � $ S.E. 132ND ST. NV' O CENTIIAL AVE.) = I �
, w�is sx.rn 4i e;..... �oae, aoo.a�
« �
,� 25 �� QI� �' � � .� � �� ... y� I
A I _ "'�
n Bte pO „� O Q .
� ❑ � 9 ' I
M =
� - _'� AC K n LOA * > y�,Cj IT�� � I�,
� � ►-�= ��., O� �.. O O� .�` p (
«�> �* � �. �oa O�
s �SM�o,
� g 11_�I) a: V� .
»��. � (3) M W � .��
n
� , ZS '� . R �"4I nO SP 379i05 N e,�, .. Q� I.
i �
F• V�� �• g RE �4�TRAC "� I
W J � J :•-'
> - K n ' Q; ZI
a sei.a s�.0 W , t»i�st °)
- -------------- �
r � o
� i Q I
� R '� + + s
� � � �
� � 29 � # � 14 +' � ; �• �
a —� q , • � �3 ��i
. ^MM. • O � p
t o�,n � �� U � z
� i
� ;, � Mr pQ� . � � �.
(SlAI) � � + t M (S! 6! � b
� , r � 1 f 3 � 6 6 7 � 1 p 1'•
s � ` � �� � — �p� � � F'�
� ' � , ��.
A , ,
� � � � vn ■ . . a
p a 3'Z - �. � � 15 _ g S.E� 2ND CT. � r
- � • �` �'I
C �es __J An �I I
� V
^ i
• � b p y a T0.� �• El�w
I
-' � p-� R H w< <o �_ t0 9 k ❑ Z E N 17 I! --'��
�nce --9=-= _=====g' u 1
d' �ni.�� i5�i,ao
70 0 i �0.52 Ac. � I ;�
❑ Z � 0,,, "' : '\
,.H 2 '�°� I
8
•a�N 3 � �_� I
� W4 .~ xv .i
0 0.4)� 5 I
�=S � � Vicini Ma
U5 , - :I � �
: 70 si
�� _ M . m� _ °`Q, t I
King County Department of Development and.Environmental Services
TECHNiCAL INFORMATION REPORT (TIR) WORKSHEET
_ . _ . _
Rart 1 �PROJECT��WNER=AND; . - :Part 2'=:PROJECT=LOCATION AND_ , �. . _
�RROJEGT�ENGWEER �-; . - :DESCRIPTION . - -
Projec�wner � � S ` Proje t Name �
�-�- ��..- Ll��- �
Addris�Z � �p�2 ��� Location .
G� ,/ p Township 2✓
Phor�e �7�j0�j//¢�`�f -G�QZ � Range 5.
- ...��Section �
Project Engineer • �� _
���
Company �
Address/Phone �
P..art 3 IYP:E�F�E#�MIT � _ �: : _-. -: ; _. T .
�1PFL�CATION� _,z� , ,.;` - -�� _Part� �O�"HER RE'J{EWS AND P.�ftM1TS
Subdivison � - DFW HPA Shoreline Management
�( Short Subdivision COE 404 Rockery
�` Grading ' � DOE Dam Safety Structural Vaults
Commercial FEMA Floodplain Other
� _ Oth2r COE Wetlands -
Part;� ;SITE?COMNIUNITI(_AND�3RAINAGE BA5lN _ �, } - �""` � � -
Commun'ity �`.,.� D�
� I
. Drainage Basin /
� �
� _ _.. , ; :;_ . . .; _
Patt'6. SITEr,CNARACTEF�ISTCGS ' _ ' ` -
River Floodplain .
x Weilands
Stream 6 �
: Seeps/Springs
Critical Stream Reach
High Groundwater Table
Depressions/Swales
Groundwater Recharge
Lake �
Other
Steep Slopes
�
!, , ,. . _. . -
:-Part 7':SOlLS�- _
Soil Type Slopes Erosion Potential Erosive Velcoties
� �P -���2'`�?'� [�r.v
j � .
Additional Sheets Attached
:� . ., ., .. .
'Part B.,;DEVELOPMEN�UMlTATJONS= � : - -
� REFERENCE LIMITATIOWSITE CONSTRAINT
Ch. 4—Downstream Analvsis ����� l
Addfional Sheets Attached
- - - _, e
`Part 9 JESC REQUIREMENTS - _ ' a � _ _
= :� :.:_ _ .�: : - - �
� �- -_ { - -
_
__ _- ..,. :
_
� . �. . __... ...�. . _ . . .,.__ _._ . T .,.. . . .,:. _. _.. _:� _ - -_
r
MINIMUM ESC REQUIREMENTS MINIMUM ESC REQUIREMENTS
DURING CONSTRUCTION AFTER CONSTRUCTION
_ Sedimentation Facilities _ (�Stabilize Exposed Surface .
y Stabilized Construction Entrance - .. �Remove ancl Restore Temporary ESC Facilities
�Perimeter Runoff Control � �Clean and Remove All Silt and Debris '
(� learing and Graing Restrictions � nsure OperaGon of Permanent Facilities '
(/Cover Practices Flag Limits of SAO and open space
VConstruction Sequence preservation areas '
Other Other
_ Part j 0 :SURFACE 1NATER SYSTEM .y . -_ , . -- - . I
_. .
:. ,: , :. . .. ..
_;.:: : .
_ . _ _ _ _ . : �
Grass tined Tank Infiltration � Method of Analysis '
Channel Vault Depression �
Pipe System . Compensation/Mitigati
Energy Dissapator Flow Dispersa! on of Eliminated Site
Open Channel Wetland Waiver Storage
' Dry Pond •
Stream Regional
Wet Pond Detention
Brief Description of System Operation � � t'L f� ��
Facility Related Site Limitations
Reference Facility Limitation
Part�� ��'RUCTUE�ALANALY3[S 'Pait'12�,-EASEMEf�1TSlTRACTS
Cast in Place Vault Drainage Easement
Retaining 1�lall Access Easement
Rockery>4' High Native Growth Protection Easement :
Structural on Steep Slope Tract :
" Other - Other
; , - ;_ , : = - , �
Part�3 S(GNATUREOFPRQFESSIONALENGINEER = _
� _. � � _ _ - -
::.: ... ,.. :� . .�.. . .. .... . .:.:. � . ._ -
;� ,�
� .. f , ._.._ ._.
_ .... ,:_:_�. ,._ .�_ .
. : ..., ._ . :___
. ,_
, '�
t or a civil engineer under my supervision my supervision have visi2ed the site. Actual site
conditions as observed were incorporated into this worksheet and the attachments. To the best of �
my knowledge the information provided here is accurate.
Si ned/Dafe
JOB
BAIMA & HOLMBERG, INC. SHEETNO. OF
100 Front Steet South
ISSAQUAH, WASHINGTON 98027-3817 CALCULATEDBY DATE
(425) 392-0250 FAX (425) 391-3055
CHECKED BY DATE
SCALE
_. .... �.
_ ��/E_ � C���� ��ti�
, _
. �.� � _ �!� �� �- �-sb "� 't. �, � '� ._
_ ,
rt�cJN�i� i21�'� __ _ N� �cu/���5r.ja�:�� �
�'_(�►�ti,�.,�►� ��t� . _ :_� V�t�'
_. _ �
�
�
: _ �r���,�
__ _ _ _ . _ _ _ _ ._ _
__ _ - fi _ � I.,-c��r- � � __ 'ZZ�{��� t-_ � �� 5�
: _ _ - _
(f�Q� - ��nl t�:��
_
__
.
_ -r-- : �c,n�r- z 2���� � : �- ���_s�
J
; rzc r�e�/� _ .
���_ _ ,�� � _
__ __ . _ _ _ _ _
. _
�-- (._(,o '� � . 2`� S 51� _�t- _ 9.� � s � : _
� :
. _ _ ___ . _ ���� �� �.,...� _.
►�c v�
_ __
, _ _
_
— � 3 � � `��= -�' _�23 �, � � _� � 2�55� -t _ 32�12s�
__
� ��1�) ;_ _ �tyvT 1 � ((,DT _�_ � E�or�
/
__ . _
__ . __ _ .
�` f�,o �G, �� -- C� , � �,�C __ . _
� __
__ . _ _
_ _
'� ��; " _� •�,°� � � .� ���� ` C'� `1�5� G �-i�55_
�
__
_ __ _ __ _ __ _
:_ . _ . U���, �(��5 �r� ���v?�v� . _ _ .
: _ __ __ _
_ _ �tmt� � _
- .2�c�
� � _
�����,,���,�,��� �
SECTION 2
CONDITIONS AND REQUIRMENTS S Y
�
r--�
V1
M �
Z �
� � .
� �
�
�
0
Project Overview
This project involves the development of a 1.08-acre parcel into 3 single-family ,
lots. The site is located at 150 Lyons Avenue NE, about 150' south of NE 2°a
Street. The site has an existing house/driveway/yard with a portion on the east
side covered with woods. The site generally slopes down to the south towards a
wetland area located on the south part of the property. Per the SCS soil maps, the j
site is underlain with Alderwood sandy loam/till soil. �i
Upstream Tributary Area ,
A small area north of the site drains south through the site to the onsite wetland at '
the south property line. '
Level 1 Downstream Drainage Analysis
The site drains to an existing wetland at the south property line (A). Flows
continue flowing south about 600' to a 12" storm system (B). Flows enter the
CB/pipe system flowing south then turning east at a catch basin(C). This flow
continues east, then south through a 12" pipe, crossing the east end of the cul-de-
sac of SE 2nd COUrL, about 800' downstream of the site (D). This pipe system
continues about 200' to the south side of an existing R/D pond (E), discharging '�
onto a rock pad. Flows continue south into King County then east then south
about 800' throu�h A 5'± wide stream(F)though Maplewood Park to near the '
north end of 148 Avenue SE. Here flows enter a 30" inlet pipe(G) and continue
to drain south through a 30" CBlpipe system in i48�' (I� about 700' to a King
County stormwater R/D pond(I)near the intersection of 148�'Pl SE/SE 139�'Pl,
well beyond '/4 mile downstream of the site. Flows outfall from the pond to
another Kin� County stormwater R/D pond (n near the intersection of 148�'Pl
SE/SE 142 Street, about 1Z00' beyond (about 3,700' downstream from the site).
That pond outfalls to a ravine leading down to the Cedar River, about 2 miles
downstream from the site.
Proposed Stormwater Control
Included calculations show that the site is exempt from both detention and water
quality requirements. The onsite private,joint driveway will drain to the adjacent,
frontage Lyons Avenue drainage system to be constructed and discharged into the '
adjacent ditch. Runofffrom the houses will be dispersed into the onsite wetland
buffer.
�
�
�
�
�
wE��
� �► �`j` � ` � L�/`
� — /1 � �
� // � =� _ _ 4 �
x _ �� � � \ _
,��
����� � 11� � X,�.
\, �i .• � `\ _ � %� �` �� X,� �
` �.� i � �
�� �f, 1 � i `� , `� �, 11
� �X _ J � �� � �
�� i`,� � � -'� �� � ��, � '�
�� • S���,� ` \ ��,
� . �� � / /� x'ic.n I
� \ � � � �� \' � �
\ r� •• \ � � I �
,.i
�� \� ; �� �s� ,� �— �I� �\` \ ��
.�
u� `1 �.,�' — � �1 �� \ �� z,�,
- tt..0! � - _ . �
Xu� ` � � � f I �\ �\ t2S
� � �� N� �1 1�� X
:..� i�� ..�� �1 \ �-- �� �
Xas.a �' _ � � � �I 1 �� ��� � ♦��` /
�,� � ,� � G � �e i �i i �� \�\ � `� �� _� -
X � ��,�_� �ii i ii �f, � � �
,�, .. / ' �o� � �_ ,�'�� �\ ._—_--
" ��^i r `� �� }� ��� �^�� � i
� � \ �,—��� �\ c__` � x,um
,-----��\t3 5�-- -__-x --- - -'=>��__�=T=�=���=���
� _- --- � � , .
� ._�-- --- ��
\�� I --��i��/r ���—�`\J^✓ �` � I �"\ �\`--^—
♦
1 l� � ,. / ��\ I � � !� ��------,
I / � / 1 � � ,1 / � ��`�% _���---�
i � i � i ti
�� � ,_� � � i '�� �_ � �___.—
� � � � � � � . �____,__
.'� ���� ., �`--� � i� �` � ��
��--� �� � � \ i� ' ( —�.��, �
�1� �
� ���.��\ //i-- ��/ , j l � � �' xm. �
\r--�� ' � ' , \ r .
/ �
j�� i// � � j �� 1 X � X„wa
/—�`J `.r �1\ �`,/ / r/ �x»az �\
/�i/�� � �� / ! nau � ♦
�_�—�� � � /
r--� j � ,ts.t� �•-��..----�—�
� •
X,,..s. x„�.�. 1 l� x �' � %T '�
i �
j j ^ • '�� ���--''
i I . ,�u,.� � i '
�_, � � X„� .' �i ��L��' � ;
—� x .r � / �� ��' / i
�n x x �� � _� �`\� ���
m
nu
x .,� r"�
Xus�s �
x
Y � �
�" ... .... .._..-�5 — t 3 '
N DOWNSTREAM DRAINAGE MAP i� = Zoo�
�I . � t � —� � ; . � �i► ,� ��, ' ./ \� w � .� �;/vi .
1 ��� �� �� , " � �� � `��' �� �! x �{ ��� ����`�s:, �
� � . �� , - , � ,r ,. •�- �3,. _r ��
. ��� � •�� .
� ,� ._�,���� �+► �� R-, ,
� ,s. � �,� �,. ,► ,, ;t �
� " � '! � � r � ' � \ .\ ! ����,. �..:;;,,�� � ':
�� ��� " /��- ��� �� .� • ��� ��� ..�. .
� ���� � � r ���lC � � 4''� �� ,}
��� I� ��� � � �1 ✓ � �\�\�� ..If. L���tYT p�9��1(fji.. '�
��� l ' �� � r � I� � ����� '��Ir �.�� �+.� �^�
�� R* ;� � \�. `�___ �'=<,., .,��;�" �.����ri�i��
� ; � � �� � (�,� � ;� s� r �+ ,� �� . �%.F ;�_ i t..
„�+ii � �� � +..� 1� �#i � ,j .! _�.i 1�.\ \�`��� ,Fi�"., �i:�itj���' ' �
� r �.� 3�� �__ �� ,✓_-� ,�.rt�•;.,. _�, ►
/ � , � ,:-..
/ � ' � , � I � I��� �� ��� � � ���, ,i�=�!{i:fJ+�`� ��A ► �
♦ g�
� ��� / �'"� "'�� • ' � .���1f7�'��+ i � � �
' q . ��— � .t L R1Z •�ji ►,, �1�i,7' ,�-
� � � �",,.�;; ,+;��(u,s' S1 �� 1
� � , :� � `r.. �' ' i� , 1,� � �
� � �_..� - - ;� , ,,.�.f- , f�� f� � ,
.� � �' ,t� .�rtf� ��✓,��„►�y���
� � + i�� ��- � �,�/�i I � �t E�' ��•,` • � I
.•.� � � ��il� � � N../ �ii���� ' ,:�_ �' �' ��7�'� , /
, l
J _ .r_. r �-.__�, .� �,��.� �
��i�j� � ' --� ��-. ,�.�� ���-- —� ,-� � , �^ , �
� �� r ,� fr iI�
�� � �{i��J � � �� ' r���� `����' � � • �.' �j�1 � ��,���� �i �
� M�' 7\ i� � �':' � "� .7,��� r��, .. /�. .�•,�._R �/�,
� � �� ,�•; � �.�� , � �r1•;R:��;,`-''` •
. ' + ` r ��. �r�! r` '•�L/ tl'r���� ��I
} t �
+ . L �i�' JIL:. %�� �u+ � /=7 � .
r,- . ; •� ; �; � �; , ,� �;, , • �
.,,,
� . r• � yr � � �� � ' �
;
���+� ���, �� '' �� ,1��,\_-��, �����rc � I ��:� ;�:
� .
� � ��1��+/ ��� � � � �� � � .��`�,1����I�=,� �'�+.,��`���'h� ,,.�.,,...� .
�J�11� � � �, � ��I I�I iI /�.��
• � r � �� ` ,, 1 r� 4 —'C I-i/ ���.��y ��,7�i �.
" � � , � .,. � �1'i,l��/1 r ,.«�-.
- ��� � s
� _ _�. f .• � � � ;w ' � �� � ,�� : �a:� _-
�; . _ � .a S�'�";��._� � �}� r `t �--. .,
'' � �r � � t � ��
� �� � �i. 1/ e . -_ � . � � �,l;`���' �..'�' r
�� � �� _ 1 - _ n ��.,�.� `� �r�' '� .
I �, !�. I�� �� ♦ �."_ '�� � t'�, ``� � 1u rt:ii� `w �
_r:Jl..�.A��. � . ` r � r-'�.,
� '-• J. : �.�,��►• •, , �^�;+�,' ��i, ��.,•• • .L•.���� r�,��r��11�!� ..�,
� w� � 1f -�., -�i> •� �.--_-_ � . .^_,_��,,,•, ,,
�� w �_.��•i �f+'� �`�1-��ii+:,�� �; � ���iL���:�� II�� �
� - � �;,%�' �' � . , �� I A �`�,
�.�� i I) �I � 1�i��►+♦,��� .rt�i�' 1 �' 1 L:o'�/�.��; l�
� �L���1�d��.� ��• �II���:;� I � �r�������� If ��, r � ��
��'� - �!�► �.� -- _ •� i� � .,,.-.,��L��—.��r�:;�.,....r„�, r� ,
� � j- . �.�� i �� �, ► ' y -+-�'� �i� ;����:r ,`:;,:-..:.
J . a, + w� .y.. i . ��.—:. i� al, )111.�� /'�1�`!;•...✓�–: r
I �� •. � ..I� '+����t,r 1� ��'� , � ��� .l,Y�r r'1�����r�3 �i1%/�I�i�„I`�' �y+r• •
► /�i_�:r�� ,i � •% ,.,.• �,I-! � rtrJj�„�,�,►.,���-'" � .,
` � � r . , ,�� },.�.:- •.. �,r�r� -��� - _ *+'l('=- :,�� . �,s.
' � �.��
�,'�,. � � _ �� =-•�, ;� �. �.�1-�. .�������z�:�:.r,_���.��„�a,r�,�� „ * �� �
�. ,� �� a�. ..��(�n _ .,,,,�1. __ �I - _ --�...�.►11 . -� ,,..,.�-
I w:.�i 7 = —_�- �, �� ;.. •�, J.� �• �► ;'` .—�,—..�► >j � �'�
� �. �_J � �� �._,r;-.� • - � ��. ,�-I r�.. ,.�L�,.!„_ �,, �.�
��-- ,! �� �'��;�. �,r-j�`�,�����.,��%y�'��" � ��`� �`� " �I
� ��( . uu��=---;-�, r?`•� �
���/1 . .
��s' '.� 1 � �w�Ji,ri��i� ��� r�1!��1 �'1 � p'�:�:�,w �
... - l �� I ��-� �- , L��,�:_-���,;�, � ��,.
�1 �� � �1•
� ./ .r��i`•�r•- r�r,
, ����"`1,\ j \ ; ' � I���"' �r�w• r..� �����-`��nri'��]� Z�`�a � S �
,.Nr
�� � � �1� � P �'�`/lh� 1�`�..�� �ri �� ��������`,�� i'��� � �
� �) ` \�r��1P� � i�r��, , �'` �■ t;+�s� � I ��
� � � � ��� �� �s ^� ry r� ,� , .'1' `�.,. �'�! �j,.� 1�.iRR% �
I ` i y � �I,�\��� �/��7 ��I�:.Y'�� �i ��;x
��.����� � � � i �,,• �h�- _ ��i S�:
l` ��` � _ ,� r. �-... r�� ,� � .��r►,, :� �, —�� ��>�
_��._,.._���_�� - ---...��.� � ���1".*�� , � .� ;.�/�� �� 1 - �- � � :�- ■ .n\ ,- ,�,�
�•—.:•�� — -------• ---.._�_-.__ /L�r��� � . . �:
��r�.��� �._:.-�..� I ��� f ���I �
,iZ�� r --- -�.--,—�--�-. __ � ,:, --�►_—���� i��.�.��1.;�����
,.=- LI[
�•l����1�� � �i i� ; t�� �V� W ' ���v��'t f,� r�ni t� ����i�j.�il(r����1M'�,+���1�!�1�'���..�
/ i�'� �i �d�,:.�i.�♦. rid��� •':==• `q f�
SECTION 4
FLOW CONTROL AND WATER QUALITY FACILITY
ANALYSIS AND DESIGN
Annie's Place Stormwater Design
We are proposing to provide dispersion for stormwater control for the roofs and driveways for this
3-lot short plat. The following information is used for stormwater design:
✓
• The site has 0.75-acres of area in Lots 1, 2 and 3 that is outside of the wetland and buffer.
• Road widening, curb/gutter and sidewalk will be constructed on 0.07-acres of frontage
(within the R/W)for a total of 0.82-acres being developed.
• The existing conditions have impervious surfaces of 0.07-acres of roofs, driveway,
sidewalk and gravel shoulder, about 0.25-acres of woods, and about 0.50-acres of grass.
The KCRTS 100-year peak is 0.156-cfs for the existing condition.
• The site is very flat, draining south to the onsite wetland and buffer(the wetland extends to
the south, perhaps several hundred feet). ,/
• The developed conditions impervious a.rea is assumed to be 55% of the lot areas outside of
the wetland and buffer.
• Lots 2 and 3 (0.45-acres outside of wetland and buffer) can disperse flows from both the
driveway and the roofs at 50' or greater from the south property line, through at least 50'
of natural buffer vegetation. Lot 1 (0.30-acres outside of wetland and buffer)would
disperse the roof closer than 50' from the south property line, also through natural buffer
vegetation(the driveway would disperse through greater than 50' of a combination of lawn
and natural buffer vegetation.
.
• The 1998 KCSWDM allows the dispersed impervious surfaces of an urban single family
subdivision to be treated as"non-effective impervious area" for stormwater calculation by
modeling as grass if the dispersed flowpath is greater than 50' {the wetland and buffer
qualify as this protected dispersed flowpath). See sections 3.2_2.1 pages 3-27& 3-28 and
5.1.2 pages 5-9.
• Using the above criteria, the developed conditions impervious surface for hydraulic
calculation is the frontage improvements plus 55% of the Lot 1 outside of wetland and
buffer, i.e.: 0.07+ 0.55*0.30 = 0.24-acres impervious area.
The impervious area of Lots 2 and 3 will be treated as"non-effective impervious area", i.e.
grass. The total pervious area=0.82—0.24 = 0.58-acres,pervious�ass.
• The KCRTS 100-year peak is 0.236-cfs for the developed condition.
• The KCRTS increase in 100-year peak flows is 0.080-cfs, under the 0.1-cfs threshold for
detention being required.
•
• Therefore, detention is not required for tlus project all onsite
impervious surfaces will be dispersed or tightlined to the public storm '
system in Lyoas Avenue. Collection trenches will be constructed along
the driveway and tightlined to a dispersion trench along the wetland ',
buffer. The roofdrains from Lot 2 and 3 will be connected into the 'i
dispersion trench. The dispersion trench will keep the wetland hydrology
intact. Lot 1 runoff will be tightlined to the public storm system in Lyons
Avenue as much as possible.
.� t oo�{ --m z s
BAIMA & HOLMBERG} INC. �,E�,,,o oF
100 Front Steet South
ISSAQUAH, WASHINGTON 98027-3817 CALCUTATEDBY � DATE_l�C��
(425) 392-0250 FAX (425) 391-3055
CNECKED BY DATE
SCAI.E
_
: _. _. i
__ .- .-.: ----- _ __ I��,� C,�_L�c-.-�- ----- --�.- - -.- -... . __ ... .
__ . _._.�.._._.._. __ -- --.- --.. -- - --.. - --.-- --- --- . --.--- .__ . _ . --.---..
. .
__ _�____� _ _ . _ ` _ ._ ._ . _ _. . _. �_.._ _ ._ __ . _ . __._
: . �� � CoNp� �a� .�-.- --- - - - -- - - - -- - -
�:.
__. . _ . : _
_
� : �
: : : : .
.
:
. .
__..—:--_.-_.._.._.�-:-. ; �u�`t���4 ��.r�1���. 5��. :_.. -f-_._�'�1�� _�R ..
�4-✓�+¢�i
... .__...__..._ _�__ :_._._._. _ _. _ _ _._ _ _ _ .._ -- - _ ---...._ _._ __ __ _ ___ ___ __---- --.... _....
: : ° � ;
:_ _ ..
,.
: : � Q• - -. ._ .- -- -.
`_ �. _ - --
�.. __. �. . .. ..
_.__....._.......__. ____._._e__._ _ _ _.. :�4�_...�- . _�___�._.: ._._ _... _. _ �Z�._c.�- -- -- - - - - - - -- - - -
: : �- : �-� ��,� - - -- -
: �
-- � ___:. ___ _.._:__�_.-=--.--� ---- _. ----- - _ - - - ----- - _ _ - -. ----- --- -_ - - - -- -- --------- -- ----.- ---- ----- - --
. .:
: _: :. �_..
. r � : }.
- --_.______ :_ _�__ - ..--.:---Y--------- --- -- - --- ---- - -- -- - -- ---- - -.- -- - -- --- --- ------- - - -- - - -- --- _--
: : . : : : :
;
;.
: : :
. _ .
: . : : ;
� _ ._ .
__.
: - . . . : .
: �. :� : E� . __ : _���.__ _._:_ �•.�.�w--.-. _�-�-:_�'�r� _�_.__ t __.
___�_---�-.---�,:.---..-- -...---_ --_ __
..�
; , : . : : �: ,��'�
��� S�� �.
, ,� , � :
: � : : :
r�
�:
�
�
: : : ; : : _ . _. _ _
- : - :_ _ - - _ __ .._ _
...__._.._ _..�_ _:__ _� ._..
__._._.r.._._.____ . --Y-.---�-._.... ---... - - - - ------ ---- ----- - - - --- --- - - - --- ---- -- - -
: : :
: : :
� - - -
: _ � _ -- - -- -- - - . -
� _ :.
: .
. .
: : :
.; .
_ __ ____ __ _ _ _ _ . ____ __ ____ _ _ ___ ___ __ _ __ - - - __ _
---. - . --.-.---- - - -- ---- - - -- - - --- - - --- - - - ---- - - -
: .Y : � � -
: :
_ _ . : :.
� �
; .
_
: .
: .
� : —
. :
�.. ��
. � :
...:.
��:
. .. 2
-_-. _�__-�__.__.:.2_.Z�.__...�_5�.:.�-� ._�`S�-�._ _._._..._....--:--...� - - -s
-_._._:__._�' - - -- ----- -- -- -- - -- -._ _..
� : : .
. . . = :
. :
; : ;
��
. . . . ; :
�
__.__�_--..Y...._._:...... _�-- ---:----.�_._.__-___.._ .__:__ _ ---- - --- . - --- --._ ..- - -- ..- -----�-- ----- - - ---�-- �-- ---- -._ _..
_:_ __ �. :._ ___. :._ ...
: . . . . . _.: _.:.- -- ---- - - - -
; : : :
� � ; : �
- �._ _:. `
. : : ; , ,
: ; ; ` :
; �
, :
. : °
. : .; �.
..D.�`�4� � �
___�_.._..�._._._.---.------.----=--- - -- --�---�-.- — — ---- --.- ---. -.-- -- .
�. ;
; :
,. :
. . �-----r--.--�.---.�----- . . . -�--� --;-.--- ---
;
: � : :
..-.--..---....---:--_LL____:___....:._. --;___ __ . --- -- - --- -- -- -- -- - -- - -- ---- --- -- --- ---- -- __ _ .._ ...- ------ -. ---
. : : � _ : .
. . . _: _: . : �. _
: ; : : : -
: : i ' : �-c�o oD S __ _ _ . _ - ---
�
;t
" � �5,�
--------._.___:._. ___A. . _.._._-.-- -:--__.__ _:. ---=- _��.:__ . __ _ - --- --- ---- - - - - -- - - --- - -
: : : �
. : :
.
� : : :
� _ .
_.
. , . : ;
:
�. �
. . : � , .
. , : . . . .
: � ; : ; : : _ _ -- - _ _ _ . _
.;.
_.._�.....----.._..__._ z __..� ..__._._..._...__...._.____---____.._�_._.__._.._. ----- --.-- --- -_. _.._ _.. _ _ _ - -- -- - ---
, ; :
. . . . F . .a. .:.-.. ..5... ..., �F�
i i ' /,
. ! E
�! ' i i ' i y i i
. . , . , . ' : '. �. ....-i.. ' ._.�
. . ,
: • . .
, . . .
. . �_��.�•�_�._.,�- .p����.-.----.�___ _._c�. �Q�,G: - - �5 -
-.-.-._.---;---._...._.....Y-.�i_-�-- - --- - --- - ,-�� �.. ---� -- ---
: : ; '
4.
' '..----`...._ : t. � . ... ...... __. .._ .... .... .. ..._ ._.. ._-- -.._- —'- —.._ .._..._
..__._:__._._... ...,-. .._.___.._......_—.f--._.._--'-- �� - — - - - --- - - -� — - ---- --- — ---- ---- - --
i i i � € € ; [ � -
... . .�
: ; : : : : : :
�� ��^y,��
� -: /�
...l�
: : : � : : : :
; :
,
�_—�._.___s_._.__.._._._.�_._._...._._......._.._._._.�._...._._;_.__.....�._._...._:.._.._.__�_._...._ -
_....._._.__.. . .�--�-- -_ _...- ---- -....-- -...--—
+—�-�-�;--�-�: : . : : . : -�-€---.�._..__.�.. �-- -�----� - -- -�- --- - -
: : ; : : : : :
: : : : : ; ;
: � : �
. . . . : ; ;_..
:.
..:.
i E i i '
:..
: : � : : • : : : : : : : : : • : �
. . . . [ E '; : .
. i ' i
.r-...
_�
..""'__�_"._�.__.._a..._....."e'_' __' ..._ __" '__ '_.. _ ..... _.._" "_"' ""' '....._ "___' _ ..___ _... __' _'__ .._' "___' '_' '_""_'_ ' .._.. '_ '_"_'_'. _...
i i � ? Yi
. �
_.. ._......
. E :
. . _..
......... ......
_..
. . . . i
2
ti
l
. . ' ; ;
< :
-_---._._....._._....�...._....�.----:-._..._.:U. _t� .:._._..._: .. _ _ :__. _.. _ .�.;�_..._._..° _...._:._..._____�.i�.�' - -- ------�-�- --...
; , : : - -- -
� : : : 5
r . 4. ���. yF.
: . . :
: :.
. . : ' ��
:
. � :
: : � :
.._... .... ""_Y"__'_�_'_'""�_'__ _ "''._ _ ...... ._ _ .... __. .. _... "' _ " _ ' _"_ ' ''" __"_ ''.. __.. ..
? { ; :
«— =
. i e i ' /
� _. .._o-._ : ...
e i i i E i 'w�,
....... '.-"'_'...}...__.._:...._......_._i ..._._i._.__.i._.__._r.._ . .... �]�./�7 ... _ . . ." '_' _ _ '....' "'_" "__ ' '. _..._ ...._ .. _....
€ i i i i i
. 6 : .
_ ...�'�{./
. . . i i i
= �t �(O G��.:
. . . . . . . : : , ;�
_ -�----�---:-�--�=--�--------- -�- ---- ---- --- ----- -�- --- -._.. _.... _ _ ----° -�-- ------ --�-- ----- ---- --- �------- --. _
: � : : : = }.
1
j : i i i
i
: : � :
1
....._""_'___i._.....Y..'__.._�_"...__..__._.-.(—._.._._:--..'__ __'_""" '"" ""'_'_"' __" _ _____"_ '_' "'" '_"_ ""' """_'_'_ _...._ '_'__ '__"""" _""'...._ ._.. .....
: : i i i i E. � �.
i i � i i i i . . .�. . �
�
..i.... ._�.. .�
_i.
. , —�•
; i i e �
i
_ : i j [ i i i
: . : : : � : : � i i � ._.. ._.. ... ._.. ..._ .... "_.. _..
._.; .' .i.._ ..�.... .1.. ..�... .'. ._:.. ...e
. . _ '_.....' ' ` . . . ._..
._....__....._.._._..«___'...r.._._.....;.____...�.""'"""__'_.'_'_'_'_"__'«'_"'_".._._.._.._...a " ' � '_" "—'__ ' _ _ ' _ _' ' '_' _ "'
r - •-�-�--�`-----'-�--�-�•—�--.�__._._._._._._.;. --
i ; ; : i i : € i : `
. . . : � € S € d : :
. � �� � �
j
7 �
_ v : '
'. ..i.. . i.. ..
$ i i i i f.
i e
-.:. .....e.. ...y....
. . . . ' i
i �
._._.."'"__—'_'..i""__""...5........_...i._"._.�_.__._i_"'_ ' '_"_ ___ "_' "___ _""' _"_ "___ "" __'____ __' __ '____'_"._._ —. _....____" _"______'._ ......
i i j ?
, .._i..
: � : : i i '
�.� .� �..
. . i i `- ! ..... _._ ..... ...... ......
....:... ; ___
................_i._._......+.............' ..... .._. .. . .... ...... ._... ...... ...._ ...... .
�
' r '
..........___....._............_...e._.__.._.._...._...3.............•._.._._.......__...._'"..._......_...._ • � ' _ .._ _._._ .._ . _ '
; . . � . . .. . ..'_ .......... _._..........-'s__' '
. . . . . . . . . . _.�.... ._'.
Pi100ULf IO1-115i�SMh1�L1 I►rieAl
�oe
BAIMA & HOLMBERG, INC. SHEEfNO. OF
100 Front Steet South
ISSAQUAH, WASHINGTON 98027-3817 CALCULATEOBY DATE
(425) 392-0250 FAX (425) 391-3055
CHECKED BY DATE
SCALE
,. _..;.._.__,_ �V�'C.� �D Cb�7�� 77t91��
�
_
� . ,
�jV�i�•• �'T''��' .= /¢ � V� 3 �/`f'G T ���,t7� '�' 4r V �/¢�
�'vT"Sr D� �'� �,vv T � tdT Z�3 ��
� � C � C �
w�rt,'�D � 13v��Q , : _
; �. +� • �Z r4G, _ .
_---__.
a� - �, �s.� o.� t � . o-��� = o , Z �,�� :
� —
_ , �5 0lo t:r�T c1 �iQ,miv7�� _ _
� �
/ � -- � r. ._. ...
�
_ __ _ �
_ �4 - � � � Z, o ,Z�•�c = �, sg �-� ����
_ ,
_ _ ;_�-�� t�+ �,�v rov s 14-i�-�tr4 m n�. _ �.oZ' Z r� 3
:
�
_ _
. !�t � C. � � 2S�� � 5p' � r.,
�. ��L TL'4',�vp ` T3����r�-
� �
�. ,;4 ��Z 5�G. 3, 2.z.:I �Sw IJ� Z�i�
:_
Lnr Z�3 ����'"' f2covs s�t'���,s
;�,�r �r 2 �r� N��- --�G7"r vt� ���,�lo�s
� � ,�'"� �'��� I�I'Lf�R `' a N D fl 12-� 1l�-�r4TEp �s
'��`� �� L w�l �2� .����c/z� tt�
: ��ti � ,
_ .
Ct9 N � � Z viVS ������i�-f/G G ��GY��$
u�ti4 ��CzTs ���� ��-1w�vT
_ ._. .
��, �.o� � c.�s � ��n = �• Z3 (�.��s
� o
����z��► o � � b, Z3� - 0,�5� = o,ogo�,�
� o ���a
, ,P � � L
- - --1- - - - - - - - 1-- - - - - - 1 - - - -
LYONS AVE N.E. (146th AVE SE)
EDCE Of ASPHAL
_'.,'_'"'_� - --�----`_w"'S00'18'4 7a.._.._�.._._ .__
� —�� '��"�`��+ J N �O N/�T�/ — — — �
� ;��- ��; - :��9 � �,2°"'T� �k • � �C��4ugL
��� � r��5���t��''� ��o�c.�K.---
..?s
�;
�D O
C
N
m
` � d�6 s� t�� �i���� s�����G
I � r�
�
u � ,�- p, �� '� � �-
. , �
� BN� ��
�,' �D 5�N5•
0Zo C N /�� �
i � �v! Ll _
tl A � rs .; o
N �� A �
\' �
f ' a m
b�
� � � � -�
� � , - �
N .� � � N
i
/ p �.� ,'�
� � �
v\ � � �t�x��j` F O' .
A ,�� ` - .��s- `L � j� O
.� a � � '
d � � � N o Zs�G
�, - ��
� �D S
�> ��
� ro �, =
i ^� `
,� ! �
�� ,� �
` > � �
_ ,..
�� � S00'18'44"W ���+�� Existing Conditions
N
� N
A
� 1" = 40'
,: -�
---- __ _. _ _ _----� ---_.
20' ACCESS
f���/I!�� /� EASEMENT
' ��w seano�oe•e aoo.az'
.._.�_____.___.�___.._,._.._.._ _����_�/_.�_..___ ,;. , -_________
N � 'I �
M �
� � � �
; ,��� ,���,
m �.�.� �°��-��� �,�� � + '�
q� m
g t w 4�� _ _. A �,.�w,.�..ws.wwx�.�r. � ��
V\ g
N � � _�� � V� � � /� �
�O # g �,(J��` � � �
f � �,�-�•,,,, / �
J � �' '" --.� �
r �,r� � � /� ` a
= g � `p"� / 25� 36..
�' t«�. ,»�•••� BUFFER � 16 �-�2
< yp y� � � ^�����r► �' 6 �A-=0 .
� ml �' ��--� ~�r- - � � "� ��_��� ` �..�A-11
� I 8,�
/ � .
! � / �A-6 6„ �A-d
� `� �,y sEws�nvE nR�
I �*�_.._..—i-A-6-'' ' �. �_j'. EASEMENT
�� SENSIIIVE AREA �
V��� �� EASEIAENT 36" 24
� � WE7LAND 24�� WETIAND � 24..
1 � •� � is�
� A 3
N88'19'42'W 3pp,g2•
�� J�IS ��f i �i � � ���� �
� �
filter strip
9�� 5'min .
overflow �6"min ✓,, drive`n►a
�— ✓
�� ��,%i
a cc /� �'
���2��-3�4�� Q p o p�°o o �
� washed rock ���o ope o „ NOTE: Same length dimensions
� oe, o ep, 18 min and site limitations as typical system
�\j ` p4Q� �Q a �
V � °n�o�° ° o
1
�� 2'min �
� � `� )� � y/�
� v vv
�� X-SECTION ��S ���ON �
AI TC �
Existing Conditions KCRTS Peaks
Flow Frequency Analysis
Time Series File:1004u.tsf
Project Location:Sea-Tac
---Annual Peak Flow Rates--- -----Flow Frequency Analysis-------
Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob
(CFS) (CFS) Period
0.071 4 2/09/O1 2:00 0.156 1 100.00 0.990
0.042 7 1/05/02 16:00 0.087 2 25.00 0.960
0.087 2 2/27/03 7:00 0.075 3 10.00 0.900
0.027 8 8/26/04 2:00 0.071 4 5.00 0.800
0.043 6 1/OS/OS 8:00 0.068 5 3.00 0.667
0.075 3 1/18/06 16:00 0.043 6 2.00 0.500
0.068 5 11/24/06 3:00 0.042 7 1.30 0.231
0.156 1 1/09/08 6:00 0.027 8 1.10 0.091
Computed Peaks 0.133 50.00 0.980
Developed Conditions KCRTS Peaks
Flow Frequency Analysis
Time Series File:1004d.tsf
Project Location:Sea-Tac
---Annual Peak Flow Rates--- -----Flow Frequency Analysis-------
Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob
(CFS) (CFS) Period
0.110 4 2/09/O1 2:00 0.236 -----1 100.00 0.990
0.078 7 1/05/02 16:00 0.135 2 25.00 0.960
0.135 2 2/27/03 7:00 0.115 3 10.00 0.900
0.071 6 8/26/04 2:00 0.110 4 5.00 0.600
0.088 6 10/28/04 16:00 0.108 5 3.00 0.667
0.115 3 1/18/06 16:00 0.088 6 2.00 0.500
0.108 5 11/24/06 3:00 0.078 7 1.30 0.231
0.236 1 1/09/08 6:00 0.071 8 1.10 0.091
Computed Peaks 0.202 50.00 0.980
,I �
,
i
I
� I
5.1.2 DOWNSPOLTT DISPERSION SYS'I'EMS
5.1.2 DOWNSPOUT DISPERSION SYSTEMS
Downspout dispersion systems are splash blocks or gravel-filled trenches which serve to spread roof
iunoff over vegetated pervious areas. Dispersion attenuates peak flows by slowing entry of the runoff into
the conveyance system,allows for some infiltration,and provides some water quality benefits.
0 BASIC REQUIREMENTS
1. Downspout dispersion is required on all subdivision single family lots which meet one of the
following criteria:
• Lots greater than or equal to 22,000 square feet where downspout infiltration is not being
provided according to the requirements in Section 5.1.1 (p.5-5).
• Lots smaller than 22,000 square feet where soils are not suitable for downspout infiltration as
determined in Section 5.1.1,and where the design criteria below can be met.
2. Projects utilizing roof downspout controls as flow control BMPs per Section 5.2(p.5-13)are required
to provide downspout dispersion if downspout infiltration is not feasible or applicable as deternuned '
in Section 5.1.1,and if the design criteria below can be met.
❑ ESIGN CRITERIA I
1. Downspout trenches designed as shown in Figure 5.1.2.A,(p.5-10)shall be used for all downspout
dispersion applications except where splash blocks are allowed below.
2. Splash blocks may be used for downspouts discharging to a vegetated flowpath at least 50 feet in
length as measured from the downspou:.o the downstream property line,structure,SAO steep slope,
stream,wetland,or other impervious surface. Sensitive area buffers may count toward flowpath
lengths.
3. If the vegetated flowpath(measured as defined above)is less than 25 feet on a subdivision single
family lot,a perforated stub-out connection per Section 5.13 (p. 5-11)may be used in lieu of
downspout dispersion. A perforated stub-out may also be used where implementation of downspout
dispersion might cause erosion or flooding problems,either on site or on adjacent lots. This
provision might be appropriate,for example,for lots constructed on steep hills where downspout
discharge could be cumuladve and might pose a potential hazard for lower lying lots, or where
dispersed flows could create problems for adjacent offsite lots. This provision does not apply to
situations where lots are flat and onsite downspout dispersal would result in saturated yards. Note:
For all other types of projecu, the use of a perforated stub-out in lieu of downspout dispersion shall
be as specified in Section 5.2(p. 5-13)for implementation of}low control BMPs.
4. For sites with septic systems,the dischazge point of all dispersion systems must be downgradient of
the drainfield. This requirement may be waived if site topography clearly prohibits flows from
intersecting the drainfield.
5. The approved plans for single family subdivision projects must show dispersion trench details and
which lots they apply. If a facility sizing credit is being claimed,the approximate location of the
house and associated flowpaths must be shown for each lot proposed for credit. Altematively,details
of typical lots may be provided to depict approximate locations of flowpaths and septic systems(if
applicable).
6. If the project is a subdivision,the following note conditioning single family residential building
permits on compliance with the approved dispersion systems shall be recorded with the plat or short
plat:
"Single family residences constructed on lots created by this subdivision must provide downspout
dispersion systems according to the details shown on the approved plans."
1998 Surface Water Design Manual 9/1/98
5-9
SECTION 5.1 ROOF DOWNSPOUT CONTROLS
FIGURE 5.1.2.A TYPICAL DOWNSPOUT DISPERSION TRENCH
o a ° ° v o 0 0 °� level outlet
op oao ao 00 0
Q Q oD 9a d p d
Q° � o � o� _�
Q o o q0 a Qv
V 9 � Qo° oapo4 -.►
ap �oo � apoo 0
° °� o � o p ° _�
p D � p D p O o v � p
n d�0 p Q 0 O �p d�� � �
6 min o �oo a o°a�o a� oo n
—4 perf pipe
o Q°dp� '�voaoo�
n o V4od odc��
18 min oo°ao o�oaoO9�„�� 11I2"-3I4"washedrock
v �
a O Q Q�o QQ Q�p Qu' Q
00 p O p0 D �O p
p �OQ O � Q� p � 0�00
Q � �0��0 �Q OQ �Oo 0
Q O � O O 0 � O � O D
h—24" min�
TRENCH X-SECTION
NTS
slope —�
small catch basin or yard drain
/ 2' x 10'
level trenches
<_700 sq. ft.
Type I CB
>700 sq.ft. /
standard
dispersion trench
w/notched grade board
length 10' per 700 sq. ft.
of roof area,
see Figure 4.2.2.0
PLAN VIEW OF ROOF
NTS
9/1/98 1998 Surface Water Design Manual
5-10
3.2.2 KCRTS/RUNOFF FILES MET'HOD—GENERATING TIME SERIES
• All onsite,predevelopment foresdshrub cover and all offsite foresdshrub cover is defined as"forest,"
irrespective of age. Post-development onsite land use is defined as forested only if forested areas aze
,- in a sensitive area buffer or aze otherwise protected and will have a minimum 80%canopy cover ,
within 5 years. In urban areas,unprotected onsite forest cover should be treated as either
pasture or grass in the post-development analysis. In rural areas,unprotected forest cover should �
be assumed 50%grass,50%pasture.
• The HSPF grass parameters were developed by the USGS study of regional hydrology and have ,
generally been interpreted as providing the hydrologic response for"urban"grasslands(lawns,etc.),
which have relatively low infiltration rates and are drained effectively. The HSPF"pasture"
parameters were developed by King County DNR to provide a hydrologic response intermediate to the
USGS forest and grass parameters,as might be typified by ungrazed or lightly grazed pasture with
good grass cover. Because it is impossible to adequately control grassland management after
development,all post-development grassland should be modeled as"grass"(with the ezception of
unprotected forest in rural development as noted above). All predevelopment grassland should be
modeled as"pasture"except for redevelopment of azeas with predevelopment land use densities of 4
DU/GA or greater(which are modeled as grass). I
❑ CALCULATION OF IMPERVIOUS AREA I
Total Impervious Coverage i
Table 3.2.2.D (p.3-28)lists percent impervious coverage for use in KCRTS analysis of eadsting
residential areas. The tabulated figures are useful in offsite analysis that includes lazge developed
residential areas,making a detailed survey of impervious coverage impractical.
Impervious coverage for proposed residentia�and commercial development must be estimated for each
specific proposal. Impervious coverage of streets,sidewalks,hard surface trails,etc.,shall be taken from-
� layouts of the proposal. House/driveway or building coverage shall be as follows:
� For urban residendal development,the assumed impervious coverage shall not be less than 4,000 .
square feet per lot or the maximum impervious coverage permitted by code(K.C.C.21A.12.030),
whichever is less.
• For rural residential development,the assumed impervious coverage shall not be less than 8,000
square feet per lot or tne maximum impervious coverage pernutted by code,whichever is less.
• For commercial or mulri-family development,impervious coverage shall be estimated from layouts of
the proposal.
Effective Impervious Area
The net hydrologic response of an impervious azea depends on whether that area is effectively connected
(usually by pipes or a channel)to a storm drainage s�stem. The impervious area that the u�er in��tc o
K�T�is the"Effective Impervious Area"(EIA),the total impervious area multiplied by the effective
imperv�ous fraction(see Table 3.2.2.E,p.3-28).
�Non-effective impervious area(i.e.,total impervious area tess EIA)is assumed to have the same
hydrologic response as the immediately surrounding pervious area. For example,for existing
residential areas with rooftops draining to splash pads on lawns or landscaping,the non-effective portion
of the roof areas would be treated as pasture for predevelopment conditions(if DU/GA<4.0)and ass
for vost-develQ=ment c itions. Note: Credits for in,f ltration/dispersion of downspouts on individual
1�in proposed single fa»uly residential subdivisions are applied separately on a site-specific basis(see
Note 3, Table 3.2.2.E).
The effective impervious fraction can be selected from Table 3.2.2.E or determined from detailed site
surveys. With the exception of figures for compacted gravel and dirt roads and parking lots,the figures in
Table 3.2.2.E are average figures cited by the USGS (Dinicola, 1990).
1998 Surface Water Design Manual 911/98
3-27
SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS i��I'HODS
TABLE 3.2.2.D PERCENT I?�4PERVIOUS COVERAGE FOR EXISTING RESIDENTI�L AREAS
Dwelling Units/Gross %Impervioust'� Dwelling Units/Gross %Impervious
Acre Acre
1.0 DU/GA 15�2� 4.5 DU/GA 46
1.5 DU/GA 20 5.0 DU/GA 48
2.0 DU/GA 25 5.5 DU/GA 50
2.5 DU/GA 30 6.0 DU/GA 52
3.0 DU/GA 34 6.5 DU/GA 54
3.5 DU/GA 38 7.0 DU/GA 56
4.0 DU/GA 42 7.5 DU/GA 58
For PUDs,condominiums,apartments,commercial businesses,and •
industrial areas,percent impervious coverage must be computed.
Notes:
�'� Indudes streets and sidewalks.
�1� These figures should be adjusted by the effective impervious fraction given in Table 3.2.2.E, 'rf I
applicable. Values from Table 3.2.2.E may be interpolated as necessary. '
��-� 4 �# � -; TABLE 3.2.2.E EFFECTIVE I�IPERVIOUS FRACTION��� r 4
� �� -� a
��� ��... � . ��... .
Land Use Predevelopment Post-Development
Commercial, Industrial, or Roads with Collection System 0.95 1.00
MuIU-Family or High Density Single Family��(>4 DU/GA) 0.80 1.00�3�
Medium Density Single Family�2� (4 DU/GA) 0.6fi 1.00�3�
Low Density Single Family�2�(1 DU/GA) 0.5fl 1.00�3�
Rural�2�(<1 DU/GA) 0.40 1.00�3�
GraveVDirt Roads and Parldng Lots, Roads without 0.50 0.50
Collection System
Notes:
�'� The effective impervious fraction is the fracdon of actual total impervious area connected to the
drainage system. These figures should be used in the absence of detailed surveys or physical
inspection (e.g.,via pipe,channel,or short sheet flowpath).
�2� Figures for residential areas indude roadways.
�'� Where downspout infiltration is used, roofs are not counted as impenrious area when sizing the flow
cont�ol facility. Roofs are considered arass where downspouts are dispersed in rural residential
development, or w e�ispersed flow aths exceed 50 feet in urban residential development(see `
Section 5.1.2 for limitations on roo ownspout ispersion .
9/1/98 1998 Surface Water Design Manual
3-28
SECTION 5 �
CONVEYANCE SYSTEM ANALYSIS AND DESIGN
' SECTION 6
',
�
SPECIAL REPORTS AND STUDIES �'�
�
H
� �
0
H a
w �
�
0
- - - . _._- _. _: � � ___ _ __..
_ _ _ _. ,��x,.,a.,�:._; ',_;,
SECTION 8 I
ESC ANALYSIS AND DESIGN
SECTION 9
BOND QUANTITIES, FACILITY S ES, AND
� DECLARATION OF COVENANT
. °!
��,
� �
SECTION 10
OPERATIONS AND MAINTENANCE I��iANUAL