Revised Storm Report~r~;'~`
February 22, _2007
Proponent: Mr. Jack Long
Contact: Jack Long
Pacific Frontier
The One Eleven. Building
Prepared by: Jeff McInnis, P.E.
Petra Engineering, LLC
1306 North I Street
Tacoma, WA 98403
Ph: (253) 752-7617
( STREET E "I~AG0141A; L~"A f 98403 ~ PHC7NE {231752--1#il l 1 ~r'1k {2~3}7b1-8582 i WV4nrV.PETR~ENG.ty ET
TABLE OF CONTENTS
STORMWATER MANAGEMENT AND EROSION/SEDIMENTATION CONTROL REPORT
Section 1-Proposed Project Description .................................................................................................................. 4
Section 2 -Existing Hydrology .................................................................................................................................. 4
Section 3 -Developed Hydrology ............................................................................................................................... 4
Section 4 -Hydrologic Analysis ................................................................................................................................. 5
Section 5 -Conveyance System Analysis &Design .................................................................................................. 5
Section 6 -Special Reports & Studies ....................................................................................................................... 5
Section 7 -Other Permits ........................................................................................................................................... 5
Section 8 -ESC Analysis &Design ............................................................................................................................ 5
Section 9 -Bond Quantities, Facility Summaries, & Declaration of Covenant ..................................................... 6
Section 10 -Conclusions .............................................................................................................................................. 6
Appendix ...................................................................................................................................................................... 7
2
I hereby state that this Drainage and Erosion/Sediment Control report for Crystal Springs has been prepared by me
or under my supervision and meets the minimum standards of care and expertise which is usual and customary in
this community for professional engineers. I understand that The City of Yelm does not and will not assume
liability for the sufficiency, suitability, or performance of drainage facilities designed by me.
' ~~. ~"
~ :~~
4C i Wt's, ~.'
~ ~ ,: ,~
,.x` I ~'~
~f"
"r: .y, k~
d
~~® X77 ~-'C? N~~~, ,~ ~ ~
S'T'E= \~
au ti
EXPlR~S_ p5-24- ~
Stormwater Management and Erosion/Sedimentation Control Report
Section 1 -Proposed Project Description
This report accompanies the Planned Residential Development (PRD) submittal for the development of
the 29 unit mulit-family project named Crystal Springs on Crystal Springs Road in Yelm, WA. The site is
located east of Crystal Springs Road in the northern extremity of the City of Yelm, directly across from the
intersection between Crystal Springs and Coates Street in Section 19 of Township 17N and Range 2E.
The project involves the creation of 29 residential units on 6 lots, approximately 17,000 Sq. Ft. of
associated private roads, and the associated recreation tracts and open space areas to satisfy the PRD
requirements of the City of Yelm.
Section 2 -Existing Hvdrolo
The appendix contains apre-developed hydrology map showing the site as it exists today. As can be
seen from that map, the site remains undeveloped and covered in grasses and a small number of trees.
The overall site proposed for development is 180, 790 Square Feet, or 4.15 Acres. Although the overall
parcel is significantly larger than 4.15 acres, the western portion of the site is being segregated through a
Boundary Line Adjustment process and sold to a third party.
The site is sloped to the east, fairly steeply at the western portion and fairly shallow farther to the east.
The site is entirely contributory to the small. creek bed known as Yelm Creek. This creek bed has been
dry during the duration of this project, but historically has flowed through the rainy season. There is an
associated floodplain to elevation 330 which is noted on the plans submitted with this report.
Yelm Creek is considered a Critical Area. A Critical Areas Report has been compiled and submitted by
Talasaea Consultants under separate cover. This Critical Areas Report will address the reduction of the
CAO buffer to accommodate the proposed design.
Section 3 -Developed Hvdrolo
In the developed condition, we are proposing the installation of a 22' wide internal road system consisting
of approximately 14,622 Sq. Ft. of new impervious. In addition, the building pads will comprise
approximately 31,776 square feet of impervious area, with the driveways encompassing approximately
10,247 square feet of impervious area. When tallied, the impervious area created by the project will tofal
56,645 square feet, or 1.30 acres.
.The proposed storm drainage system, designed to comply with the 1992 DOE Stormwater Management
Manual for the Puget Sound Basin, will include a detention pond at the eastern portion of the project site.
This storm pond has been sized to receive runoff from the site. The developed Z, 10, and 100-year
storms have been estimated using the SBUH methodology using the Stormshed® software. The pond
was then sized to reduce flows to the pre-developed flows of 50% of then-year, and the 10 and 100-year
flowrates. Prior to entering the storm pond, the flow will be routed through a Vortechs Model 2000 for
stormwater treatment. Once through the water quality device and attenuated by the 6,700. cubic foot
storm pond, the stormwater will be released to the Yelm Creek system through a spreader trench system
located just outside of the 100-year flood plain elevation.
Section 4 -Hydrologic Analysis
A hydrologic analysis of the developed conditions has been completed using StormShed software and the
SBUH methodology using a Type 1A hyetograph. On-site soil has been classified as Hydrologic Soil
Group C as a result of the on-site soil exploration compelted by Gary Flowers, LEG. Although the SCS
mapping (included in appendix) shows the site as containing both Spanaway and McKenna soils, the on-
site exploration did not turn up any infiltratable soils on the site.
The results of the detention analysis, showing the expected runoff values from the impervious areas
contributing to the WQ and detention systems, are tabulated below:
Event Pre-Develop Flow (cfs) Developed Flow (cfs) Attenuated Flow (cfs)
2-yr/24-hr .1960 (50% - .0980) .6074 .0990
10-yr/24-hr .4183 .9366 .4179
100-yr/24-hr .6667 1.2448 .6660
The detention pond sizing was performed utilizing the rates presented above and is presented in the
appendix. The appendix contains the hydrology calculations as well as pre and post-development maps,
Section 5 -Conveyance System Analysis & Design
The on-site conveyance systems (pipes, and roof drain tightline) have been designed to convey the 25-
year peak flows in accordance with the 1992 DOE manual. The simple mannings pipe conveyance
analysis is provided in the appendix. As no conveyance issues are anticipated due to the steepness of
the site, a backwater analysis has not been performed.
Section 6 -Special Reports & Studies
A geotechnical report was prepared by Gary A. Flowers, PLLC for this project and has been submitted
under separate cover. This report was prepared only for use by the design engineer in designing
pavement and for exploration of the feasibility of on-site infiltration.
Also a Hydrogeologic Assessment was prepared by GeoEngineers addressing potential; high
groundwater flood hazard areas on or around the site. This report was submitted under separate cover.
Section 7 -Other Permits
Building Permits will be obtained to construct the buildings.
Section 8 -ESC Analysis & Design
Erosion and sediment control will be provided through the use of silt fence, Catch Basin Protection, and a
stabilized construction entrance. Since the site slopes to the southwest, silt fence will be concentrated on
the southwest portion of the site. With the soil types .present on the site, a sediment pond is neither
practical nor preferred, as runoff will not reach it prior to infiltration. Instead, treatment at the point of
.runoff is preferred and will be employed oh this project.
The ESC features employed on this project include:
• Stabilized construction entrances
• Catch basin inlet protection
• Silt fence
5
Track-walking and hydro-seeding as necessary.
It is important to note that the responsibility for erosion control rests with the contractor, as only he can
respond to changing site conditions. The measures included on the ESC plan should be considered a
starting point, only.
Section 9 -Bond Quantities, Facility Summaries, & Declaration of Covenant
These Items will be supplied at the time of complete CD submittal, as will the declaration of covenant.
Section 10 -Conclusions
The storm drainage system, as designed, will control stormwater runoff quantity and quality and will work
well with the proposed residential PRD and the surrounding areas. The detention system will operate in
accordance with the requirements of the City of Yelm and the 1992 DOE manual. This system will be
designed in accordance with the standards of care and quality typical in this area for projects of this type.
6
Appendix
Exhibits
1, Soils Map and description
2. Pre-Developed Hydrology Map
3. Developed Hydrology Map
4. StormshedT"" Calculations, including pond sizing
5. Conveyance Analysis
7
~~~
/ /
~ G~
(,;
1/
i
I
1
I
!Y
/ ~/"s>y
~~~ ~ ,i
0
O
"' o
I
o ~
~
~
o_
,II
r+~ U ^~
O
~ ~
~aW
Z~~
°a ~ Q ;``
000
~ ~ w
°
~ ~t ..
m ~,.
°~,~
~~
,' ~;~
,
~ ~
,
~~ z
~v
fii"'
<
~
~ ~ ~~,
~~
~. ~
v 4 ,
~
~
,,
^^
VJ ~ ~~
`
S ~,~.. II ~ ~r
I
f \~_~5~ ~ ~i• ~~,
y l
` ~f ~ I ~ A, 1 {
i
/
Y
r ~ y f -~.
`fi
r'
,~ /;
~
'
~ ~~ ~
:i:S,, i ~ ,,~~ '.
., ~r '~~
. .. *. _
Appended on: 07:13:54 Tuesday, September 12, 2006
Post 100-year Event Summary
v nt , Peak Q (cfs) ;Peak T (hrs) ~Hyd Vol (acft) Area (ac) :Method Raintype
6-month 0.3386 7.83 0.1078 1.3000 SBUH TYPElA
2 year
0.5991
7.83
:~ 0.1922 ~
1.3000 ~.
j SBUH TYPElA'
10 year 0.9235 7.83 0.2999 ~ 1.3000 ~ SBUH TYPElA'
__
_~
25 year
:_ ~ _ ~ _
1.0844
_
~~~ _._ ~ __ .
7.83
_::~
~ e _ . _ _ m_ ~
0.3539
~
~W:
~ ____ _
1.3000
~~ _ _~____ ~ _.. e~
SBUH TYPEIA
W
__
100 year __.
_
1.2448 ..__....
~
7.83 .__
.
0.4079 _.__
__.
1.3000 :. _...:a __.._._~.....
,SBUH TYPElA
Record Id: Post 100-year
Method
SBUH
____ TYPElA
!Hyd Intv 10.00 min ;Peaking Factor 484.00
;Abstraction Coeff 0.20
;Pervious Area (AMC 2) 0.00 ac ~DCIA 1.30 ac
Pervious CN _
~ 0.00 ;DC CN ~ 98.00
.;Pervious TC ~ 0.00 min ADC TC 0.56 min
Directly Connected CN Calc
_ _ _ __
~~~~~~~~~~~~~~~~~~~ ~~Description ~ ~ ! SubArea Sub cn
_ __
.~..._.~.~. ________~~ ~ ._~e ~-__....a.~ ~ _.._
Impervious surfaces (pavements, roofs, etc) 1.30 ac 98.00
__._ ~ _,..__„_DC Composited CN (AMC 2) __,________ _„~~__ ~~~~~ ~ ~ 98.00
Licensed to: Petra Engineering, LLC
Appended on: 07:14:05 Tuesday, September 12, 2006
Post 10-year Event Summary
_
Event __ __
:Peak Q (cfs)'~Peak T (hrs)Hyd Vol (acft) ,;Area (ac) .;Method Raintype
~
___.
6-month. : 0.3431 7.83 0.1078 ~~ 1.3000 ~ SBUH ~TYPEIA
2 year 0.6074 7.83 ~ 0.1922 ` 1.3000 'SBUH ;TYPElA
_ ~
10 year
0.9366 ~ ____
7.83 _ erg _~
~ 0.2999 _ ___
1.3000
~~ SBUH _._~ ~ ~ _.
TYPElA
25 year __
1.0998 _ ____
7.83 ,- _ .~ _w..._.__ .
0.3539 ... ..._
1,3000 _ __
SBUH _.~~ __m
TYPElA
~._
100 year w_____ __...~ _
1.2626 ...._. _____.
7.83 ;___.. __
0.4079
_
1.3000
_r....._
SBUH
. _...._
TYPEIA
Record Id: Post 10-year
w ~_
?Design Method ~ ~ _ ~ _ ~_ ___ _____.~.
, SBUH ___ ~. __ _.._... __..__
'Rainfall type . __
TYPElA~
_~.__ ~
Hyd Intv m.____~ww_ ~_.___..~.,.,
10.00 min ~~_~M .._.__~~~:;: .__.;.
jPeaking Factor ..._.~.._.. ____..~~:
484.00
___..___._._.___.....~....__ .._.. __..___.
____
Abstraction Coeff _~ m
. __
~~~ 0.20 ~~~ ~.
E ~~.._..._ .......................~.~..,,..,..
_:~ ___.._.._._ _
,Pervious Area (AMC 2)
_~ ...~
~ 0.00 ac
.. _ _.____T _ ~ __._. . ~__~ ~
; C A
...:
1.30 ac
__ ~W_____
;Pervious CN
~. _.._~~~_ ~__ ~ ~_
; 0.00 _
_; C N~ _ ___..~.-_~ ._ T.~T_~_____
98.00
;Pervious TC _:.__ _--__.:~~ . ~ ___
~ 0.00 min _ w _ .___.__ __
DC TC __. ~:~.._..~~w
0.86 min
Directly Connected CN Calc
` Description SubArea Sub cn
~mm_
Impervious surfaces (pavements, roofs, etc ~ 1.3 w~
0 ac _ ,_ne~ _._.~
98.00
_. ~._ _ ~.._... ..._
'; _.~...._. ~__._._ .__.__._~ _ ~ _ .~._.__w _ __.......___..._ ......_._..._ _-__.___
DC Composited CN (AMC 2) ~ ~ _._.._._._ ....
1 98.00
Licensed to: Petra Engineering, LLC
Appended on: 07:14:15 Tuesday, September 12, 2006
Post 2-year Event Summary
Event Peak Q (cfs) Peak T (hrs) Hyd Vol (acft)
___ ;Area (ac) ;Method ~Raintype
~6 month : 0.3431 7.83 0.1078 ~ 1.3000 ,3 SBUH TYPElA
2 year 0.6074 7.83 0.1922 1.3000 ;SBUH TYPElA'.
_
10 year
0.9366 ~
7.83
0.2999
1.3000
~ SBUH q.~__
TYPEIA'
i~ 25 vear
: 1.0998 a_~
7.83 . _ ~; _ ~
0.3539 __~~ ~~~
1.3000 . A~_~
_SBUH ~ ..
=TYPElA''
100 year, 1.2626 7.83 0.4079 1.3000_...; SBUH TYPElA
Record Id: Post 2-year
_~~ _.~ ~ ~
jDesign Method _~
SBUH ~ _.._.~~ __~-W _._~~
iRainfall type ___~~
TYPElA
~Hyd Into 10.00 min Peaking Factor 484.00
'~ _~. ~ ~~ ';Abstraction Coeff 0.20
i~Pervious Area (AMC 2) 0.00 ac DCIA _ 1.30 ac
..?Pervious CN 0.00 DC CN ~ 98.00
Perv
ious TC
0.00 mm
ADC TC
.0.86 min
Directly Connected CN Calc
.... .._ ...... ~._ _. ~ _. ~____ __...... ~ ~ ~ ~ _...me _...~
( Description SubArea ~ Sub cn
~ ~ ~ ~~ ~ __~ ~ rt....m ._.~ _.~ a~~ ~ __~
Impervious surfaces (pavements, roofs, etc) 1.30 ac 98.00
DC Composited CN (AMC 2) 98.00 'i''
Licensed to: Petra Engineering, LLC
Appended on: 07:14:29 Tuesday, September 12, 2006
Post 6-month Event Summary
Event Peak Q (cfs) ,P eak T (hrs) Hyd Vol (acft) :Area (ac) Method Raintype
6-month ; 0.3386 '' 7.83 0.1078 1.3000 :SBUH ~TYPElA
2 year
~ 0 5991
:~ ~ 7.83
:e 0.1922
~
~
~ 1.3000 ;SBUH ~TYPElA
__
10 year ~ _.
_ --
0.9235 ~ ____ __.
7.83 ~ .
_.__,
._
~ 0.2999 .._ ~
~ 1.3000 _ ~ _~~ ~__.
,SBUH TYPElA
-m.~
25 year _~ ens _~
1.0844 ; ~.
7.83 ~ _._~. ..~._
0.3539 ~_.~
1.3000 . ~~_~ ___ Mm.
3 SBUH ETYPElA'
100 year 1.2448 _ 7.83 0.4079 ! 1.3000 'SBUH ~TYPEIA
Record Id: Post 6-month
;Design Method ~ SBUH :Rainfall type TYPElA
~Hyd Inty
10.00 min
€Peaking Factor ___
~ 484.00
~~,..._.
.._._..:mm_.~.__..~~~._ __...__._.._W ~----_~______.~...._.._.~.._
?Abstraction Coeff ~ ~ _ ~~____
0.20
3Pervious Area (AMC 2) 0.00 ac 'DCIA 1.30 ac
;Pervious CN ~
.
_ 0.00 DC CN 98.00
.
;_
__
;Pervious TC r
0.00 min
DC TC
0.56 min
` Directly Connected CN Calc ':
z,
e.m~ ~ .,.~ _. e~ ~ ~~ __-_m__
Description SubArea ,. Sub cn ;;
~.,
~: ~~.
Impervious surfaces (pavements, roofs, etc) 1.30 ac 98.00
DC Composited CN (AMC 2) 98.00
Licensed to; Petra Engineering, LLC
Appended on: 07:14:38 Tuesday, September 12, 2006
Pre-100-Year Event Summary
Event :Peak Q (cfs) ;Peak T (hrs) s~Hyd Vol (acft) ;Area (ac) Method ;Raintype
ii=6-month
_
:.m: 0.0526 ;
;:::: 8.17
~ 0.0374
_ _ ~ 1.3000 „SBUH .TYPElA'
_.v~.
2 year _.
.._.
0.1960 .__
8.17 ? _
0.0981 _ ~~~ ___.T_~
1.3000 4 .._ ~ ~ ___.
;SBUH ~TYPElA'
10 year
_
. ~ 0.4183
-_ 8.00
~ 0.1884 1.3000 ,SBUH :TYPEIA
.
.
25 year _ e ~ ___
, 0.5409 ~
:::: ~
~
8.00
~ P .~ ~
0.2364 .~ ~
1.3000 . ~ ~~. ~:~:.
SBUH TYPElA'
_._..___ ~.
100 year ~......_ _._.._,_ __.
~ 0.6667 _.._ ~ ~_.._.__
8.00 ~ ~ ~v.__~~:
0.2856 .... W.:~_r .:::
1.3000 ...__. ~ ~ _~ ~ ::~.._
;SBUH ;TYPElA
Record Id: Pre-100-Year
~Desi n Metes _._ __~ _ ~~
g hod _~ ~ __~.~_ _ .._w
~ SBUH
...
Rainfall type
TYPElA
Hyd Intv
~ ~ 10.00 min ;Peaking Factor 484.00
~ Abstraction Coeff ! 0.20
;Pervious Area (AMC 2)
__ _..
_ 1.30 ac DCIA 0.00 ac
_
_
.._ ___~ _
Pervious CN ___-___
87.00 ~ ~ ~~ .
DC CN v_.__~~
0.00
___ ~. _______
:Pervious TC
r-._.
19.32 min
_~ _.. ~ ~ ~ _ _.... r
ADC TC
~
__
0.00 min
__
~ Pervious CN Calc
~~ _~. _~~_ ~~_~___:~ _~~.. ~~ :: .::~ _ ~ W:_.~. __-~ ___-~
Description ~ SubArea Sub cn '`
Herbaceous 1.30 ac ~ 87.00 '~`
Pervious Composited CN (AMC 2) 87.00 `?
Pervious TC Calc
_~. ~~ n_~ ~.~
Type ~ Description E Lengt~~ ~~ ~~ ~~~~~ ~ ~~
h ~ Slope ~ Coeff MLSC ~ TT
__.__~ ._._ ~mm _._~_.. ~__. _~.__.~ _W.:~ __.~W ._ ____..: ,__ _::.~..__._~~.~.._~
Sheet ;Dense grasses :0.24 X50.00 ft $22.00% ''0.2400 X2.00 m ;3.97 min
~_._._~ _
_w .....~~ .....__~ ...... ~__ ~ . ~._.__.....__
Sheet Dense grasses :0.24 ;100.00 ft (3.00% ~0 2400 (2.00 m 15.35 min
Pervious TC 19.32 mm
Licensed to: Petra Engineering, LLC ~~ ~~~~~
Appended on: 07:14:47 Tuesday, September 12, 2006
Pre-10-Year Event Summary
~ Event ; Peak Q (cfs) ;Peak T (hrs)Hyd Vol (acft) ;Area (ac) MethodRaintype
__
',6-month; 0.0526 E 8.17 0 374 1.3000 ~ SBUH TYPElA
2 year 0.1960
_ 8.17 0.0981 1.3000 SBUH TYPElA
_
10 year
~ _
~_
0.4183
~
~- _ - ~ ;
8.00 ~
~ ~:._. __._:~:.
0.1884
:
: .. _ .. _._.~::
1.3000
__ :: -.::::.:
SBUH ~~:_e~ - :...::...._...
TYPEIA''
-
___
25 year _
_~___
0.5409
e...
8.0
0 .
__-_-_ _..
0.2364
_~
1.3000 _ _
SBUH _.
~
TYPEIA
F100 year; 0 6667 8.00 0.2856 1.3000 SBUH _TYPElA
Record Id: Pre-10-Year
Desi ~n Method ~~
g ~~ SBUH Rainfall e _
typ ,
___..~
~ TYP
E l A
Hyd Into 10.00 min Peaking Factor ~ 484.00
~ ~ ,A raction Coeff ~ 0.20
;Pervious Area (AMC 2) 1.30 ac DCIA 1 0.00 ac
;Pervious CN 87.00 DC CN ~~ ~ 0.00
. ~ _~.. ~ n._.._._ _._
Pervious TC __ ____.~
19.32 min _.._ ~.___ _...~
DC TC ___. ~_ ___v _~~
0.00 min
~
::
~W
W Pervious CN Calc
:-
~
~::
.
.
_
_
` Description
~~
::W
~ ~
_ ~ _
_
_
~~ m:m ~ ~- e~ _
~ SubArea
a
. ~d - ~ ~e ~__
Sub cn
~
,
__..m
_____._
( Herbaceous __.
.A. _
m _. ~.._ ~
~ 1.30 ac _.em _~_~
87.00
Pervious Composited CN (AMC 2) 87.00
__
~ _.~:T.~__ _ _ ~ ~ - ~_ ~ _
Pervious TC Calc _____
..~ m~ ---._~ ~~m ~
Type Description _~.~ ~~ ~ _ m_~
i Length Slope ;Coeff ~s
~ Misc ~ ~~ e~,~
~~ TT
_._..~
_ __._~ ~ ._ ._...~.~ ._.. ~
Sheet Dense grasses :0.24
~
,
~ ~.._....~ _ ~.
50.00 ft X
_.._.~u ~_.~:.n _. ~
22.00% 0.2400 '=.2.00 m
~w ..____ ... ~._.._
;
3.97 min
=Sheet Dense asses :0.24
~ gr
=100 00 ft X3.00% :0.2400 2.00 in ,
..~...
~~15.35 min
~ Pervious TC =19.32 min ''
Licensed to: Petra Engineering, LLC
Appended on: 07:14:56 Tuesday, September 12, 2006
Pre-2-year Event Summary
!= Event ;Peak Q (cfs) £ ak T (hrs) Hyd Vol (acft) ;Area (ac) ;Method Raintype
_.
~6-month
0.0526
_ 8.17
~
0.0374
~ 1.3000
SBUH _TYPElA
2 _.
year
W~.~
0.1960
8.17
0.0981
:.3000
~ SBUH TYPElA
10 year ___ ~
; 0.4183 ~ w_e _
~ 8.00 ~
_ _.Pe~ ~
0.1884
_ _. ____
1.3000 ~ _____a_ m
~ SBUHTYPElA
5 year
- __ _~-
0.5409
~_ m~
.:
~ 8.00
_~ _. ~
0.2364
I
:~ _.ne
1.3000 __
~ _~ _~
SBUH TYPElA'
100 year 0.6667 8.00 0.2856 1.3000 'SBUH 'TYPElA
Record Id: Pre-2-year
Design Method ~ SBUH Rainfall type ~ TYPElA
#Hy I tvm~ ___.,_~~__,_. _. 10.00 min `Peaking Factor 484.00
;Abstraction Coeff 0.20
;Pervious Area (AMC 2)
t x 1.30 ac
, ~DCIA 3 0.00 ac
_
Pervious CN _
_ _
87.00 __ _ _ _ ___
=DC CN
0.00
Pervious TC _ .__..:m ~ _ _
19.32 min _ _w ~: m~ _ _ _
DC TC ~ _____:- ___.. ~ ~_
0.00 min
_.
_.
~, ~~
Pervious CN Calc ~
~
Description
~ 1 ~~ SubArea Sub cn
~_~ _ ~~~~~~
Herbaceous ~_~~ ' ~ ~~ 1 3.0 ac ~~~ ~~~~ 87.00 ~~_~~'
~ __~~
._.__.__ _ ~ _~ ___ ~:.._.. ~..w~__.__._~~ ___. ~ _ : _.~:~: __.~.. ~~mm .~
Pervious Composited CN (AMC 2) ~
~ £ _ _. _~ _...~:_._._. ~ ~,
87.00
} W
Pervious TC Calc _ _.m_
Type Description ~ Length ~ Slope Coeff ~ Misc TT '''
=Sheet ,Dense grasses :0.24 ~
?50.00 ft ,22.00% :;0.2400 2.00 m ;3.97 min
}Sheet ;Dense grasses :0.24 €100.00 ft X3.00% .;0.2400 ;2.00 m ;15.35 min
Pervious TC 19.32 min
Licensed to: Petra Engineering, LLC
Appended on: 07:15:07 Tuesday, September 12, 2006
Record Id: MH Release
Descnp. ;Prototype Structure :Increment ;0.10 ft
Start El.~~ 100.0000~ft Max El.~ ;105.0000 ft`
Onf Coeff :0.62 ;Lowest Orif El. (_
_~ _ _ _ 98.00
::Lowest Diam' 1.5820 in ;Dist to next (2.2300 ft ,
ED2 ~ ~ 5.0684 in :;Dist to next ' 0.3100 ft~.~
_...~~_. _ ...._W:_~__.... ~._..~:~w _____~:~ _ ~ ~_ _.., _ ....~
D3 ;0.5859 in ,Dist to next '..0.0000 ft
~I02.3000 ft -.:0.2874 cfs :1114.9000 ft 1.3037 cfs'
._._ ._ ..._...~. .._,._~ ....._. _m:.: _..,...:_ _.._.
::3.02.4000 ft 0.3926 cfs -105.0000 ft :l .3267 cfs
=102.5000 ft0.4696 cfs :145.1000 ft 1.3493~cfs!
_. _._...~~~ rv, ~ __ ~~
~1~5.0000 ft_1.3267 cfs
Licensed to: Petra Engineering, LLC
Appended on: 07:15:15 Tuesday, September 12, 2006
Record Id: N-001
'=Descrip: Prototype Record.:Increment :0.10 ft
'`Start El. _ ;100.0000 ft mm Max El. '105.0000 ft
;Storage Node Pond Discharge Node MH Release
Record Id: Pond
~Descrip: :Prototype Record ;Increment X0.10 ft
;Start El ;100.0000 ft ;Max El.103.0000 ft'
__ __ __
~.__
;Length
;17.3300 ft Width :50.0000 ft
,,Length ssl ;3.OOh:ly jLength ss2 ~ 3.000Oh:ly
ssl 3.OOh:lyWidth ss2 3.000Oh:1
Record Id: MH Release
,(Descrip:
__ ;Prototype Structure
_ Increment ;0.10 ft
Start EI _
_
__._......_
100.0000 ft
S __
~_ _
_ ..
_. .
Max E1.
..~ _.m._..~
~' 105.0000 ft
i
~m
Onf Coeff _ _
_._..
~0 62
...__ ~ ___ ~ _ ._....
:~.~~ 'Lo
west Onf El '98.00
!Lowest Diam
;1.5820 in
_ ;Dist to next ~.~.._.~W ~ ._..:
2.2300 ft
_
~D2 ~~ ~_ .__
15.0684 in ~._~._--____~~.
;Dist to next ...x_.~..~_.....
X0.3100 ft
~.~..._
_.~ ..~~
=D3
...._._..~__m.m_
0.5859 in
_ ~_.~. _ ~__._.~
Dist to next
,
X
~ ..____... _..._.
0.0000 ft
.~..~
Licensed to: Petra Engineering, LLC
PIERCE COUNTY STORMWATER MANAGEMENT and SITE DEVELOPMENT MANUAL
6.5.9 Correction Factor to Pond Volume
When designing runoff quantity control facilities, the Engineer shall apply a correction factor to the
design volume when using a unit hydrograph method to model 24-hour storm events. The
correction factor is shown in Figure 6.1 and shall be used to increase the proposed pond volume
by the applicable percentage, i.e., 10,000 c.f. + 30% = 13,333 c.f.. The pond depth may not be
increased and the orifi may not be changed. This factor is based on site impervious cover. If the
facility is to serve for water quality also, the pond depth cannot be increased beyond the criteria
established in Chapter 7. NOTE: This factor does not apply to infiltration facilities or facilities
sized with a continuous simulation model such as HSPF however, the design engineer is
encouraged to incorporate an appropriate factor of safety based upon the level of detail included in
the model.
FIGURE 6.1
Volume Correction Factor to be Applied to Runoff Quantity
Control BMPs Based on Site Impervious Cover
~` .~
6-10