Stormwater Summary 2-21-06
Schoell Madson
Planning Engineering Surveying
15050 23rd Avenue North
Plymouth, MN 55447
T 763-746-1600
F 763-746-1699
www.schoellmadson.com
February 21,2006
Paul Oehme
City of Chanhassen
7700 Market Boulevard
PO Box 147
Chanhassen, MN 55317
CITY OF CHANHASSEN
RECEIVED
MAR 0 6 2006
CHANHASSEN PLANNING OEPT
RE: Stormwater Summary
Christensen Subdivision
Dear Paul,
This report is in response to the 1-3-06 city comments regarding site hydrology and pipe-sizing
calculations for the Christensen Subdivision project.
Existing Conditions
In general, the existing site flows from north to south, with runoff north of the existing driveway
collecting in a low point which is drained under the driveway by an 18" CMP culvert. Using the
available topography and interpolation, the drainage area for the existing culvert is approximately
48,300 square feet, with 1,890 square feet of impervious, or 3.91 %. The culvert is 41 feet long,
and has a slope of 1.17%. With this information, the amount of runoff was calculated to be:
Q=c*j*a = .42 * 5.6 * 1.11 = 2.61 cfs
The existing 18" culvert can handle 12.30 cfs at 1.17%, so there is currently plenty of capacity.
Proposed Conditions
The proposed development will not alter the drainage area of the culvert, but will increase the
amount of impervious surface within it. This will be accomplished with swales on either side of
the proposed house and a second 18" CMP culvert beneath the proposed driveway. Under the
proposed conditions, 4,188 square feet of impervious will be added to the existing drainage area,
bumping the total up to 6,078 square feet, or 12.6%.
Q=c*i*a - .47 * 5.6 * 1.11 = 2.92 cfs
The new impervious area only generates an additional 0.31 cfs of runoff, and so the existing
culvert will still be more than able to handle it. The proposed culvert will only take water from the
northeast of the house & driveway, and it will be sloped at 0.50%. At this slope, the 18" pipe can
handle 8.04 cfs, and given the fact that the total runoff is only 2.92 cfs, the proposed 18" CMP will
also be more than adequate for the amount of water flowing through it.
Site Hydrology
Using HydroCAD software, the entire site was modeled to determine runoff from the 2, 10, & 100-
year, 24-hour rainfall events. The results of the analysis are listed below, and the complete
calculations are attached to this report.
Maximum Rate of Runoff (cfs)
24-Hour Existing Proposed
Rainfall Event
2-Year 0.72 0.95
10-Year 1.96 2.30
100-Year 4.06 4.51
If you have any questions regarding this report, please feel free to contact me at 763-746-1608 or
iaredj(@.schoellmadson.com.
Sincerely,
~~
Jared Jones, E.I.T.
Schoell & Madson, Inc.
Existing Runoff
Proposed Runoff
Bg
Drainage Diagram for Christensen Subdivision
Prepared by Schoell & Madson, Inc. 2/20/2006
HydroCAOO 7.00 sIn 001876 @ 1986-2003 Applied Microcomputer Systems
Christensen Subdivision
Prepared by Schoel! & Madson, Inc.
HydroCAD@ 7.00 sin 001876 @ 1986-2003 Applied Microcomputer Systems
Type /I 24-hr 2- Year Rainfal/=2.80"
Page 2
2/20/2006
Subcatchment 1 S: Proposed Runoff
Runoff =
0.057 af, Depth= 0.65"
0.95 cfs @ 12.03 hrs, Volume=
Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-24.00 hrs, dt= 0.05 hrs
Type II 24-hr 2-Year Rainfall=2.80"
Area (sf)
8,929
37,029
45,958
CN Description
98 Paved parking & roofs
65 Woods/grass comb., Fair, HSG B
71 Weighted Average
Tc Length
(min) (feet)
10.0
Slope Velocity Capacity Description
(ft/ft) (ft/sec) (cfs)
Direct Entry,
Subcatchment 1 S: Proposed Runoff
Hydrograph
~
~
~
o
u:
Type II 24.hr 2':'Year
Rainfall=2.80"
Runoff Area=45,958 sf
Runoff Volume=0.057 af
Runoff Depth=0.65"
Tc=10.0 min
CN=71
0- .
o
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time (hours)
1- Runoff~
Christensen Subdivision
Prepared by Schoel! & Madson, Inc.
HydroCAO@ 7.00 sin 001876 @ 1986-2003 Applied Microcomputer Systems
Type /I 24-hr 2- Year Rainfal/=2.80"
Page 3
2/20/2006
Subcatchment 2S: Existing Runoff
Runoff
0.72 cfs @ 12.04 hrs, Volume=
0.046 af, Depth= 0.52"
=
Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-24.00 hrs, dt= 0.05 hrs
Type" 24-hr 2-Year Rainfall=2.80"
Area (sf)
4,854
41,104
45,958
CN Description
98 Paved parking & roofs
65 Woods/grass comb., Fair, HSG B
68 Weighted Average
T c Length
(min) (feet)
10.0
Slope Velocity Capacity Description
(Nfl) (fUsee) (cfs)
Direct Entry,
Subcatchment 2S: Existing Runoff
Hydrograph
0.8.
0.75
0.7
0.65
Typen24-hf2~Y~ar
0.6 Rainfall=2.80".
0.55 . RUnoff Area::::45,958sf
0.5
Runoff Volume=O.046af
. ; , .
Runoff Depth=O.52"
. Tc::1().()min
0.3
CN=68
~ 0.45-
~
;t 0.4
o
u:: 0.35
0.25
0.2
0.15.
0.1
0.05
o
o 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time (hours)
1- Runoff~
Christensen Subdivision
Prepared by Schoel! & Madson, Inc.
HydroCAD@ 7.00 sIn 001876 @ 1986-2003 Applied Microcomputer Systems
Type 1/24-hr 10-Year Rainfal/=4.10"
Page 4
2/20/2006
Subcatchment 1 S: Proposed Runoff
Runoff =
2.30 cfs @ 12.02 hrs, Volume=
0.128 af, Depth= 1.46"
Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-24.00 hrs, dt= 0.05 hrs
Type II 24-hr 10- Year Rainfall=4.10"
Area (sf)
8,929
37,029
45,958
CN Description
98 Paved parking & roofs
65 Woods/grass comb., Fair, HSG B
71 Weighted Average
Tc Length
(min) (feet)
10.0
Slope Velocity Capacity Description
(fUft) (ft/sec) (cfs)
Direct Entry,
Subcatchment 1 S: Proposed Runoff
Hydrograph
~
~
~
o
ii:
Type I 24.hr10-Year
2 .. Rai nfall::::4. 1 0" ..
Runoff Area=45,958 sf
Runoff Volume=O.128 af
Runoff Depth=1.46"
Tc=10.0min
CN=71
0- .
o
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time (hours)
1- Runoff~
Christensen Subdivision
Prepared by Schoel! & Madson, Inc.
HydroCAD@ 7.00 sin 001876 @ 1986-2003 Applied Microcomputer Systems
Type /I 24-hr 10- Year Rainfal/=4. 10"
Page 5
2/20/2006
Subcatchment 2S: Existing Runoff
Runoff
1.96 cfs @ 12.03 hrs, Volume=
0.111 af, Depth= 1.26"
=
Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-24.00 hrs, dt= 0.05 hrs
Type II 24-hr 10- Year Rainfall=4.10"
Area (sf)
4,854
41,104
45,958
CN Description
98 Paved parking & roofs
65 Woodslgrass comb., Fair, HSG B
68 Weighted Average
T c Length
(min) (feet)
10.0
Slope Velocity Capacity Description
(ftlfl) (ftlsec) (ds)
Direct Entry,
Subcatchment 2S: Existing Runoff
2
.l!!
..!:!.
Type II 24-hr 10-Year
Rainfall=4.10"
Runoff Area=45,958 sf
Runoff Volume=0.111 af
Runoff Depth=1.26"
Tc=10.0 min
CN=68
~
~ 1
ll..
o
o 1 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time (hours)
1- Runoff I
Christensen Subdivision
Prepared by Schoel! & Madson, Inc.
HydroCAO@ 7.00 sIn 001876 @ 1986-2003 Applied Microcomputer Systems
Type 1/24-hr tOO-Year Rainfal/=5.90"
Page 6
2/20/2006
Subcatchment 1 S: Proposed Runoff
Runoff =
0.247 af, Depth= 2.81"
4.51 cfs @ 12.02 hrs, Volume=
Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-24.00 hrs, dt= 0.05 hrs
Type II 24-hr 100-Year Rainfall=5.90"
Area (sf)
8,929
37,029
45,958
CN Description
98 Paved parking & roofs
65 Woods/~rass comb., Fair, HSG B
71 Weighted Average
Tc Length
(min) (feet)
10.0
Slope Velocity Capacity Description
(ft/ft) (ft/sec) (cfs)
Direct Entry,
Subcatchment 1 S: Proposed Runoff
Hydrograph
5
4
Type II 24-hr 100-Year
Rainfall=5.90" ·
Runoff Area=45,958 sf
RUnoff VoIUme=O~247. af
. .
Runoff Depth=2.81"
Tc=10.0 min
CN=71
3
J!!
~
==
o
u:
2
0- . .
o 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time (hours)
1- Runoffl
Christensen Subdivision
Prepared by Schoell & Madson, Inc.
HydroCAD@ 7.00 sIn 001876 @ 1986-2003 Applied Microcomputer Systems
Type /I 24-hr 100- Year Rainfal/=5.90"
Page 7
2/20/2006
Subcatchment 2S: Existing Runoff
Runoff =
4.06 cfs @ 12.02 hrs, Volume=
0.223 af, Depth= 2.54"
Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-24.00 hrs, dt= 0.05 hrs
Type II 24-hr 100-Year Rainfall=5.90"
Area (sf)
4,854
41,104
45,958
CN Description
98 Paved parking & roofs
65 Woods/~rass comb., Fair, HSG B
68 Weighted Average
Tc Length
(min) (feet)
10.0
Slope Velocity Capacity Description
(ft/ft) (fUsee) (cfs)
Direct Entry,
Subcatchment 2S: Existing Runoff
Hydrograph
4
3
Type II 24-hr 100-Year
Rainfall=5.90"
RunoffArea=45,958 sf
Runoff Volume=O.223 af
Runoff Depth=2.54"
Tc=10.0 min
CN=68
~
~
~
o
ii: 2
o
o 1 2
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time (hours)
1- Runoff~