HomeMy WebLinkAboutDrainage Reports - 10/05/1987.,
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THE MALLARDS �u5� JS
SUPPLEMENTAL DRAINAGE REPORT /
V PAO,1�
to 'Master storm drainage resort
for the Landings PUD
October 5, 1987
The S T EUVA9 f WHTTiER Co.
P.O. BOX 5387
LOVELAND, CO 80537
the STEWART WHITTIER co.
Engineering & Surveying
PO BOX 5387 Loveland, Colo. 80537 tele.303-663-2065
Oct. 5, 1987
Intermill Land Surveying, Inc.
1301 N Cleveland
Loveland, CO 80537
Gentlemen;
Enclosed herewith is a supplemental drainage report for the
•Mallards• at the Landings PUD.
If there are any questions, please contact this office.
Sincerely,
Stewart Whittier, PE
SW/ms
TABLE OF CONTENTS,
Letter of Transmittal i
Table of contents
Scope
1
Location 1 -
Method 1
Calculations 2
Summary of calc. 2 yr storm Table 1
Summary of calc. 10yr storm Table 1
Summary of calc. 100 yr. ` Table 1
Preliminary drainage &
Utility plan Plate 1
SCOPE
The foregoing drainagereport is a supplement to the "Master storm
drainage report for the Landings PUD' prepared by Cornell Consult-
ing Co. January 1979. This supplement conforms to the'City of
Ft. Collins storm drainage criteria and construction standards',
I'iay 1984.
LOCATION
The study area is located in the south half of Section 36, T7N,
H69W and more particularly located in the southwest corner of
drainage area 10 of the above mentioned master drainage report.
The land is presently vacant. There is a drainage ditch used to
carry runoff from the Boardwalk Apartment site to the detention
pond along the eastern boundary of this project. It is proposed
to abandon this ditch and carry the runoff in a metal pipe.
METHOD
The rational method has been used to determine the runoff and
using a 2 year and 100 year storm frequency. The land is pres-
ently zoned UP. The plan is to eliminate the existing ditch
and carry the water underground to the existing detention pond
to the east of the property. Wherever possible the lots will
be graded to drain to the streets. The street will be used to
carry the runoff to drop inlets which will carry the water to the
existing detention pond to the east.
The original runoff was calculated for map area 10 to be 73 cfs
for 100 year storm and 33 cfs for 2 year storm. Area 10 has a
total of 45 acres. The proposed site is approximately 8.2 acres.
A reasonable porportional portion of the total designed runoff
IL r
would be /45=0.18 or 0.18x/�13.3 and 0.18x33=5.9 respectively.
�0 3
For the present development the runoff is 35.4 and 10.7 respec
tively. Part of the reason for the difference in the amount of
runoff now and that of 1979 is that there has been a cerable
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charge in the city standard since that time. L Wka, 5�04� ay� (-A
CALCULATIONS
In order to calculate proposed runoff, the rational formula,
Q=CfCIAO
where:
Q= flow in cfs
I= rainfall intensity, in/hr
A= acres
C= runoff coefficient
Cf=frequency adjustment factor (Cf for 2 yr storm=l, 100yr=
1.25)
Overland time flow curve and rainfall intensity curve is
based on Ft. Collins design criteria. A composite "C'
factor of 0.45 was used for lots and 0.95 for paved streets.
(2)
The results of the 'above calculation for both .the 2 year and 100
year storm are tabulated in Table 1. �54&
? Cep
The pan across the cul-de-sac is calculated form(M—odified1Manning 's
formula and nomograph found in fig 4-1 Ft. Collins design
// criteria.
A.i pan 8 ft wide and 4" deep is satisfactory.
8' _
z=8/0.33=24.2
n=0.016
z/n=24/0.016=1515
s = 0 . 0 0 7 5
Q=4cfs, >3.3, sec OK
The street encroachment for RLP zoning for local streets for
initial storm is no curb topping similarly for the major storm
the encroachment is that the water over the crown shall not exceed
6'. Using the modified Manning formula and nomograph found on
fig. 4-1, the street cross section as proposed on sheet 2 of the
plan is satisfactory. �y ,
INITIAL STORM (2year) y-�-7
Q=0.56(z/n)S2y3 where; z/n=18/.016=1125 n-0.0y&
S=0.0075
y=0.5
Q=7cfs is greater than 3.3, section OK
MAJOR STORM (100year)
By inspection, section OK since the greatest flow, llcfs will
not exceed 6' over the crown. '
Since the amount of runoff released by the pond at the Boardwalk
apartment is not certain, it is assumed that the l' dia. pipe is
adequate. Therefore, use 21" pipe from inlet to inlet 'A', 24' dia.
(3)
from inlet "A" to inlet "C% from inlet "C" to outlet at pond use
27". From Steel Drainage handbook, fig 4-18, HW/D is less than or
equal to 1.0 (10year storm),
Q21'-14cfs
Al
Q24"=20cfs CO l l �N5
Q27"-30cfs r�
therefore pipe selections are adequate. Standard oll
end sec-
tions are to be placed at each end of the proposed pipe.
The drop inlets are to be 2'x3'x36" deep.
The runoff from area 4 and 6 is to drain directly to the detention
pond and the runoff from area 5 will drain to Whaler's Way and
enter the existing drainage facility. The runoff from area 3
is to drain to the rear of the lots to a 10 foot drainage ease-
ment and as shown on sheet 2 of the plan.
(4)
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