1991-05-15 Drainage Impact on County Ditch No. 9WESTWIND ADDITION
ALBERTVILLE, MINNESOTA
Drainage Impact on
County Ditch No. 9
MEYER-ROHLIN, INC.
FJ~K3INEF.RS-WVD SURVEYORS m! font 25 K a~rera +1~nn.353r3 Phone 612.682-na r
May 15, 1991 E-9001. O
Exhibit 1
MEYER-ROHLIN, INC.
ENGINEERS AND LAND SURVEYORS
1111 HIGHWAY 25 NORTH
BUFFALO, MINNESOTA 55313
I hereby certify that this plan, specification,
or report was prepared by me or under my direct
supervision, and that I am a duly Registered
Profess~ybnal Engineer under the laws of the
e State of Minnesota.
. Meyer
g. No. 6218
MEYER-ROHLIN, INC.
ENGINEERS-LAND SURVEYORS 1111 Hwy. 25 N., Buffalo, Minn.55313 Phone 612- 682-1781
May 15, 1991
Wright County Board of Commissioners
Wright County Courthouse
10 NW 2nd Street
Buffalo, MN 55313
Re: Westwind Development
Albertville, Minnesota
Honorable Chairperson and Commission Members:
The Westwind Addition, located in Section 35, Township 121,
Range 24 in the city of Albertville, is a development of an
existing farmstead into an area of urban low-density housing. A
portion of the original farmstead with its buildings, croplands and
pastures generally drained to the southwest into the upper reaches
of County Ditch No. 9. Peak drainage flows to County Ditch No. 9
under these agricultural conditions have been calculated to be 37
cfs for the two-year storm event and 179 cfs for the 100-year storm
event. These waters drained overland directly into County Ditch
No. 9.
The drainage program for the Westwind Addition is to continue
the drainage by storm sewer in the same southwest direction, but
contain all storm drainage into a detention pond adjacent to County
Ditch No. 9. The flows will be stilled within the pond, dropping
all sediment, and then outletted into County Ditch No. 9 at a
controlled rate equivalent to the pre-development rate for a
two-year storm event (37 cfs). The detention pond will have a
capacity of approximately 9.8 acre-feet of storage, or 60% greater
than the 100-year storm event from the development.
We find no negative impact upon County Ditch No. 9 or County
Ditch No. 21 from this controlled flow. Realistically, for all
flows, an improvement will result in eliminating the. high
erosion-creating runoff rates which carry much siltation into the
county ditch system.
Thore P. Meyer, Professional Engineer Robert Rohlin, Licensed Land Surveyor
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Worksheet 2: Runoff curve number and runoff
Project (/(~~~sT-wi TJ ~ EC-c~Gl(~t "~" gy ~1 Da[e fir- ~. ~,/
Location ~C~-~~, ~,,, Checked Date
Circle one: Present Developed ~, 2 q ~ G ~2 nM S
1. Runoff curve number (CN)
Soil name
and Cover description 1/ Area Product
h
d CN - of
y
rologic (cover type, treatment, and cv CN x area
group hydrologic condition; N i' ~I acres
percent impervious; ~ `" `~' ^mi~
(appendix A) unconnected/connected impervious
area ratio) ~
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1/ Use only one CN source per line. Totals ljiq ~ ,~~/~
CN (weighted) = total product ~~/S /~l_!' _!
total area _ ,/ ~13, Use CN
X90
2. Runoff
Frequency .............................. yr
Rainfall, P (24-hour) .................. in
Runoff, Q ........... .......... in
(Use P and CN with table•2-1, fig. 2-1,
or eqs. 2-3 and 2-4.)
Storm ~J1 Storm A2 Storm ~l/3
~QD
2, 7 58~
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D-2 (210-VI-TR-55, Second Ed., June 1986)
~'Vorksheet 3: Time of concentration (Tc) or travel time (Tt)
__ Project _~ i 1 ~ ~y•D ~L ~J 1=-i.~ M1~uT BY 't"~ ~ Date s g~
••, Location ~~~1-~/ ~ l_Ll Checked Date
Circle one: Present Developed 2 Y2 '~ f~CJ V!Z_'g-~z U.tS
Circle one: T Tt through subarea
NOTES: Space for as many as two segments per flow type can be used for each
worksheet.
Include a map, schematic, or description of flow segments.
Sheet flow (Applicable to Tc only) Segment ID
1. Surface description (table 3-1) ............
2. Manning's roughness coeff., n (table 3-1) ..
3. Flow length, L (total- L < 300 ft) .......... ft
4. Two-yr 24-hr rainfall, P2 .................. in
S. Land slope, s .............................. ft/ft
6. TC 0.0o75(no)0'8 Compute Tt ...... hr
P2 s
Shallow concentrated flow Segment ID
7. Surf ace•description (paved or unpaved) .....
:8. Flow length, L ..... ......................... ft
9. Watercourse slope, s ....................... ft/ft
• 10. Average velocity, V•(figure 3-1) ........... ft/s
11. Tt 3600 V Compute Tt ...... hr
i~
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Channel flow ~ Segment ID C '~ ,y--~'~'1~
z~
12. Cross sectional flow area, a ............... ft2 ~2.0
13. Wetted perimeter, pw ,,,,,,,,,,,,,,,,,,,,,,, ft ~~~
14, Hydraulic radius, r pg Compute r ....... ft 6 d ~
w ,
15. Channel slope, s ........................... ft/ft C,I.~D~
16. Manning's roughness coeff., n .............. ,~ .,
1.49 r2/3 sl/2
I7. V Compute V ....... ft/s 2. (D 3
n
18. Flow length, L ............................. ft Z Z S Q
19. Tt = 3600 V Compute Tt ,,,,,, hr ~, 2 + = qnZ/
20. Watershed or subarea Tc or Tt (add Tt in steps 6, 11, and 19) ......, hr ~•`>~
~ (210-VI-TR•55, Second Ed., June 1986)
D•3
Worksheet 4: Graphical Peak Discharge method
Project ~~ 1JI IVY IJEVI~t r~u,~. rr, By -Ty7r~ Date
Location l..t~E2T V t ALL Chec-kj-e~d-~~ Date
Circle one: Present Developed 2 Y `~ I a0 t~ ~2 '4TC~2 M ri
1. Data:
Drainage area . ~.q~&~lU.. Am = 1078 mil (acres/640)
Runoff curve number .... CN ~/~ (From worksheet 2)
Time of concentration .. Tc = -zJ`.'~~-~- hr (From worksheet 3)
Rainfall. distribution type = - ~ (1, IA, II, III)
Pond and swamp areas spread
throughout watershed ...... _ ~ percent of Am ( acres or mil covered)
2. Frequency ...............................
3. Rainfall, P (2~i-hour) ...................
yr
in
Storm 1 Storm 2 Storm 3
Z ~~
2. ~ s~ gs"
4. Initial abstraction, Ia ...~~~~,~~~~~~~~~ in ~- 6 ~ ~ '~O 3
(Use CN with table 4-1.)
5. Compute Ia/P ............................ t,l.Z~aD~ ~ ~20Z
6. Unit peak discharge, q ......,.~...~.~.~ csm/in ~ ~
(Use Tc and Ia/P with exhibit 4 _~)
7. Runoff, Q ...............................
(From worksheet 2).
8. Pond and swamp adjustment factor, Fp ...~
(Use percent pond and swamp area
with table 4-2. Factor is 1.0 for
zero percent pond and swamp area.)
9. Peak discharge, q ......~~~~~~~~~,~~~,~~
P
(Where qp = quAmQFp)
in ~. ~j 3.0~
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cfs ~3~> ~ / ~9
D-4 (210-VI•TR•55, Second Ed., June 1986)
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