HomeMy WebLinkAboutDrainage Reports - 07/15/1977.j
DRAINAGE REPORT
i
EVERGREEN PARK (3rd FILING)
FORT COLLINS, COLORADO
Prepared For
D. C. Miller Co.
1635 Blue Spruce Drive
Fort Collins, Colorado
15 July 1977
File No. 19-2-23
Prepared by
Northeast Engineering
817 E. Douglas Rd.
Fort Collins, Colorado
I
TABLE OF CONTENTS
Introduction --------------------------------------
Purpose-------------------------------------------
Design Criteria -----------------------------------
Procedure -----------------------------------------
Conclusions ---------------------------------------
Calculations --------------------------------------
Watershed Map
Rainfall Curves
1
1
1-2
3
3-4
5-7
VE
Lim
DRAINAGE REPORT
for
EVERGREEN PARK Ord Filing)
Introduction
Evergreen Park is located North of the downtown portion of
Fort Collins and East of College Avenue. The 3rd Filing
of Evergreen Park is located in the East i of Section 1,
Township 7 North, Range 69 West. The 3rd Filing drains in
a Southeasterly direction. The 3rd Filing contains
approximately 11 acres and is comprised of 19 multi -family
lots.
Purpose
This drainage report has been prepared to develop an over-
all drainage plan for the subdivision, to determine the
need for a storm sewer within the subdivision, to determine
and size necessary outfall facilities. In addition, the
100 year design storm runoff was determined to verify that
high water levels would not reach foundation elevations.
The report will serve as criteria for final design of the
necessary drainage facilities.
Desien Criteria
The Rational Method was used as the basic analysis tool to
determine rates of runoff for various design storms through-
out the subdivision. Volumes of runoff for the design
storms were also determined using the Rational Method to
calculate ordinates for the storm hydrograph. The Rational
Method is based on the formula Q = CIA Cf. The various
parameters are explained as follows
1. "Q" is the rate of storm water runoff in
cubic feet per second.
2. "I" is the rainfall intensity in inches per
hour based on a predetermined time of con-
centration. Intensities for this report
were taken from "Rainfall -Intensity -Duration"
curves published by Fort Collins and dated
November, 1975•
N
3. "C" is a runoff coefficient based on land
use and soil characteristics.
4. "A" is the area in acres contributing runoff
to the study point.
5. "Cf" is an adjustment coefficient which is
applied to the Rational Formula to take
into account antecedent moisture conditions
for major storms. Values of "Cf" are indi-
cated as followsc
Recurrence
Interval (years) 0f
to 10
1.0
25
1.1
50
1.2
100
1.25
The Rational Method is sufficiently accurate for non-complex
areas up to 200 acres in size.
Manning's Equation was used to determine flow capacities in
streets, swales, ditches and conduits. The modified
Manning's Equation was used to determine low flows within
the curb and gutter of streets. Thegeneral form of the
equation is Q = (0.56) (Z/n) (s) y)8/3. For high flows
above the top of the curb, the normal Manning's Equation was
used. This equation is written as Q = (A)/(1.486) (r) 2/3
(s)}/n. The "n" values used in both equations were 0.016
for paved street surfaces, 0.035 for grassed surfaces, and
0.013 for concrete surfaces.
The 2 year, 10 year, 50 year, and 100 year design storms
were used for the analysis. The 2 year storm was used to
determine the need for storm sewers within the subdivision.
The allowable inundation level used to determine the point
at which the storm sewer system should begin was the top of
curb. The 10 year storm was used to size facilities dis-
charging into the detention ponds. The 100 year storm was
used to determine the ability of the detention ponds to hold
the runoff contributed by the subdivision, and to determine
the probable maximum inundation levels and insure no flooding
of foundations.
Procedure
Ba::cd on the existing ground contours and on the street
layout indicated, street drainage patterns were set such
that the subdivision is divided into two watersheds with
two separate outfall points. Watershed No. 1 drains into
a detention pond at the intersection of Monterey Drive and
Conifer Street. Watershed No. 2 follows the existing
drainage course South along North Lemay Avenue. Minimum
grades on all streets were set at 0.4%. The ten year storm
peak rate of runoff was determined at each outfall point to
size the facilities discharging into the detention ponds.
The 100 year storm peak rate of runoff was determined for
each sub -basin. The two detention ponds were sized based
on the peak rate of runoff developed. The 100 year storm
peak rate of runoff was determined at each outfall point
to calculate the high water levels and insure that no
foundations would be at or below the 100 year high water
elevations.
Conclusions
Results of the analysis of each watershed will be discussed
as follows:
1. Watershed No. 1
Watershed No. 1 contains 8.36 acres, consists
of 17 multi -family lots, drains to the east
and outfalls into the detention pond proposed
for Evergreen Park 2nd Filing. The proposed
street system consists of four cul-de-sacs
draining between the end lots and down the
rear lot lines to the detention pond. The
100 year design storm runoff expected at the
outfall from each cul-de-sac is less than
5 c.f.s. and no curb overtopping will occur.
The swale between lots should be capable of
transporting; 5 c.f.s. without inundating
structures. One possible solution is shown
on page .2 of the calculation sheets attached
to this report. The outfall swale to the
detention pond must be capable of transporting
11.5 c.f.s. without inundating structures.
One possible solution for it is also shown
on page 3 of the calculations.
3
The detention pond in Tract 'C' has
approximately 0.5 additional acres to
accomodate the runoff from the 3rd Filing.
A depth of 2 feet will be sufficient to
store the additional runoff.
2. Watershed No. 2
Watershed No. 2 contains 1.65 acres, consists
of 2 multi -family lots, and outfalls to the
existing drainage south along the west side
of North Lemay Avenue. The proposed detention
pond blocks the existing flow patterns and
the developed rate of runoff will be less
than the existing rate of runoff, and there-
fore, no additional facilities are proposed
for this area.
4
/,lid :♦ / J
c� J'n v�rrS
i
6 /O — %3 S•O
lk
Fvx'
�S /
2 6 ro>>.,,.7cf
'Y• >14i'
141e11�-/ 411 rri/r+
J�yarir/C. K""n,ar
L
= A-WP
A ✓ '�� � /z
^y n
0.5
C2), v3o�
7 eTs OK Mom' o✓/��� Y/ ye%...,... +...1
Try L= C /oho e=
� r
UL = L > c tj
7—r i — B {t 5-love' O"
^� s
1
GOAlr6V sre££r •:..
WATERSHED
MAP