21210 72ND AVE W.PDF11111111111111
14976
21210 72ND AVE W
STREET HLE
RECEIVED
DEC.. 2 8 1979
Dir. Of Public Works
Reid, Middleton & Associates, Inc.
ENGINEERS* SURVEYORS e PLANNERS
324 Main Street, Edmonds. Washington 98020 0 (206)775-3434
Revised December 27, 1979
November 6, 1979
File No. 50981
STORM WATER RETENTION/DETENTION CALCULATIONS
FOR
LA PIERRE CONDOMINIUM COMPREHENSIVE PLAN
Existing Site Condition'
Area = 12-35 acres
The existing area is an abandoned gravel pit which now has light plant
growth an.d substantial areas covered with concrete.
Topography - 3 percent westerly plus 2 percent easterly from 500 feet
.west of the east line. Due to this topography, only the east 5 acres flow
easterly contributing to flows offsite. The allowable discharge rate will
be limited to that acreage.
To provide an outlet to the west basin of the proposal, an outlet will
-be constructed at the southeast corner of the property to discharge into the
existing 30 inch storm sewer.
Therefore, using Yrjanainen-Warren method for simple retention/detenti6n
basin design, the following calculations are to determine the required volume
of this development:
Tc 500/(0.4)(60) + 10 = 30.8 minutes
1 0.82 in/hr @ 10 year storm frequency (Seattle curve)
Prarate CE (Existing)
4.3 acres sand & gravel @ C - 0.25
0.70 acres concrete @ C 0 90
V_2 Q to
4
CE = .30(0.25)+0.20(0.90) �v
0.34 C
5.00
OV--
,oraLe %-F % ULurej
Open planter area = 5.21 acres @ 0.25
Building & impervious area = 2.48 acres @ 0.90
Asphalt road = 4.66 acres @ 0.85
(5.21)(0.25)+(2..48)(0.90)+(4.66)(0.85)
12-35 Mi
Q existing CIA = 0.34)(0.82)(5.00) = 1.39 c.f.s.
1_Q allowable 1.39 c.f.s.'
No off -Site watershed drains to the depression.
Determine the volume of retention using a-10 year storm.
QE - 1.39 - = 0. 185
Qo = TC X CF = 12.35(o.TiT
Assume orifice
F11625
0 1 F = 72.6 min.
T = -25 + 0 T
.1 5
C> e—
S-T
'AL
Revised December 27, 1979
November 6, 1979
File No. 50981
Page 2
STORM WATER RETENTION/DETENTION CALCULATIONS FOR LA PIERRE CONDO. COMP. PLAN (cont.)
VS 2820(72.6) _ 4o (o. 185) (72. 6) = 1560
727 + 25
VT 1560 (12-35)(0.61) = 11.752 c.f.
Use 11,755 C-f-
Due to topography and future site conditions, a staged system of
retention.basins in series will be used to detain the required volume. The
"Level Storag . e Routing" method will be used to determine flows and storage
requirements for this series of basins. An allowable flow is determined
for the various stages so as to insure filling of each separate detention
basin. Due to contributory effects of upstream basins to downstream basins,
a larger volume will be required at the lowest basin to prevent the system
for overflowing during the design storm.
The calculations are lengthy and, therefore, are submitted in hand-
written form herewith.
Size Pipes Due to Flow
QIF = 1.59 c.f.s. Q2F = 3.13 c.f.s.
Q3F = 1.75 c.f.s. Q4F = 4.28 c.f.s.
Q5F = 1.04 c.f.s. Q6F = 0.96
Area I
Q/Sve = 1.59/(.005)y' = 22.5
Areas I & 2
4.72/(.0136)' = 40.5z 46
Areas 1, 2 & 4
9/(.005)'h = 127.3 e 1 29
Areas 1-4
10-75/(0.005) 152 173
Areas 1-5, IZ
11-79/(0.005) ' = 167 173
12" he]. alum. o.k.
12" he]. alum. o.k.
21" hel. alum. o.k.
24" he]. alum. o.k.
24" he]. alum. o.k.
Sizing the Detention System
Required volume of storage - 11,755 c.f.
The following is a tabulation of the unit volumes associated with
yarious catch basins and pipe used in this system. 30% voids is assumed
in washed gravel beds.
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Revised December 27, 1979
November 6, 1979
File No. 50981
Page 3
STORM WATER RETENTION/DETENTION CALCULATIONS FOR LA PIERRE CONDO. COMP. PLAN (cont.)
1211 pipe 0.785 c-f./l.f.
8 pipe 1.767 c.f./l.f.
24" pipe 3.142 c.f./l.f.
301 1 pipe 4.909 c.f./l.f.
CBMH Type 2 12-57 c.f./v.f.
CB Type 16 5.79 c.f./v.f.
CB Type 15 3.98 c.f./v.f.
24 pipe in 3' x 41 qravel bed 5.8o c.f./,I.f.
using 30% voids
30" pipe in 4.5' x 4.5' gravel bed 9.51 c.f./l.f.
using 30% voids
The following is a tabulation'of the design volumes backed up behind
the various catch basins with 'orifices:
Yolume behind CB No. 21
210 l.f.' - 30"
CB No. 21, Type
1030.9
2, 2.83 VJ.
35.6 Vvo
CB No. 22.,' Type 2, 1.78 v.f.
Subtotal
22.4
1088.9 C.f. by —
Volume behind CB No. 16
105 I.f. - 18" CO 16 to CB 23
185.5 c.f. V,
27 I.f. - 24 CB 16 to CB 17
84.8
186 I.f. - 24" 3,' x 4' gravel bed CB 17 to CB
18 1078.8
85 I.f. - 30'' CB 18 to CB 19
417.3
25 I.f. - 12" CB 19 to CB 20
19.6
67 I.f. - 12" full above CB 20�
52.6
49 I.f. - 12" partially ful I 4/2-x 0.78
19.3 0 V-
CB NO. 16, Type 2, 3.99 v.f.
50.2
CB No. 17, Type 2, 3.85 V.f.
48.4
CB No. 18, Type 2, 2.92 v.f.
36.7
CB No. 19, Type 2, 2.50 V.f.
31.4
CB No. 20, Type 15, 1.44 v.f.
5.7
.CB No. 23, Type 2, 1.50 v.f.
18.8
Subtotal
2049.1 0 r-
Volume behind CB No. 9
60 I.f. - 30'' CB 9 to CB 10
294.5
105 I.f. - 30" CB 10 to CB 11
515.4 cliANG Or-, GUW
CB No. 9, Type 2i 5.61 v.f..
70.5
CB No. 10, Type 2, 2.65 v.f.
33.2
CB No. 11, Type 2, 1.91 v.f.
24.o
Subtotal
937.6
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Revised December 27, 1979
November 6, 1979
File No. 50981
Page 4
STORM WATER RETENTION/DETENTION CALCULATIONS FOR LA PIERRE CONDO. COMP. PLAN (cont.)
Sizing the Detention System (cont.)
Volume behind CB No. 7
100 I.f.
2 4"
in
3 x
41 gravel bed.CB 7 to CB 12
580. 0
120 I.f.
24"
in
3 x
4' gravel bed.CB 12 to CB 1*3
696. 0
30 I-f-
2411
CB
12 to
CB 13
94.2L--
75 I.f.
24''
CB
13 to
CB 14
235.6w--
140 I.f,
24"
CB
14 to
CB 15
439.9-"
150 I-f-
30"
CB
7 to
C8 8
736.4
220 I.f. -
24,,
CB
8 to
C9 9
691.2
CB No. 7,
Type
2,
6.2 v.f.
77.9
CB No. 12,
Type
2,
3.98
v.f.
50.0
CB No. 13,ype
2,
3.23
v.f.
40.6
CB No. 14,
Type
2'
2.88
v.f.
36.2
CB No. 15,
Type
2:
2.18
v.f.
27.4
CB No. 8,
Type
2,
3.73
v-f-
46.9
8o i.f. 12."
CB
15
to CB
16
62.8
Subtotal
3815.1 0 Y--
Volume behind CB No. 2
505 I-f- - 24" in4.5' x 4.5' gravel bed
95 I-f- - 18" CB 4 to CB 5
CB No. 2, Type 2, 4.50 v-f,
CB No. 3, Type 2, 3.25 v-f,
CB No. 4, Type 2, 2.75 v.f.
CB No. 5, Type 16, 1.78 v.f.
CB No. 6, Type 2, 2.50 v.f.
Volume behind CB No. 26
130 1 - f - - 30''
CB No. 26, Type 2 , 3. 15 v. f
CB No. 27, Type 2, 2 .50 V. f
Subtotal
Subtotal
Total Volume
ik
1�0
48.03-041a) q,q\
167.9
56.6
4o.8
34.6
10.
31.
5144.6 c.f. OV-
638..l
39.6
31.4
709. 1 c . f
13,744. 4 c. f .
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