HomeMy WebLinkAboutDrainage Reports - 03/13/1992A•E DESIGN ASSOCIATES. P.C.
223 LINDEN STREET
FORT COLLINS. COLORADO 80524 ����✓�� 3 r�Iq z
(303) 493.8883 �
PROPOSED ADDITION AND RENOVATION
TO THE
CHRIST UNITED METHODIST CHURCH
301 EAST DRAKE ROAD
FORT COLLINS, COLORADO Final Approved Report .
Oaten
xx STORM DRAINAGE REPORT NARRATIVE
THE PROPOSED PROJECT
This project is located at 301 East Drake Road, Fort Collins,
Colorado in the Thunderbird Estates 1st Filing Subdivision. The
existing development comprises an entire block within the
subdivision upon which the existing sanctuary, class rooms,
fellowship hall, and narthex sit. In addition to this are
landscaped areas, sidewalks, four curb cuts with drive entries, and
the related asphaltic concrete parking lots.
The existing site, as currently developed, contains approximately
2.89 acres of existing development including approximately 10,600
GSF of structure and approximately 33,266 GSF of hard surfaced
sidewalks, driveways, and parking lots. The remainder of the site,
approximately 1.9 acres consists of landscaped turf area.
As proposed the project would convert approximately 6410 GSF from
turf area to a combined fellowship hall and classroom addition
including an expanded narthex. This addition and expansion as
designed, creates approximately 7630 GSF of horizontal projected.
roof area and an additional 800 GSF of hard surfaced sidewalks and
patio. With the exception of this addition, the vast majority of
the site remains "as is" without revision. Specifically, no new
parking areas or driveways or curb cut/street improvements are
required. For a graphic explanation of the project, please refer
to Drawings 1 and 1D.
THE EXISTING SITE DRAINAGE
As currently developed, the site has no storm sewer system
therefore all of the current runoff is distributed to the four
sides of the property then drained by an overland flow system
consisting of the existing street curb and gutter systems of East
Drake Road, Mathews Street, Thunderbird Drive, and Tulane Drive.
The site is situated within the Foothills Drainage Basin and due
to the original construction date of 1967, does not include either
on site storm water detention or retention. Also, since there is
no storm sewer system, roof drainage consists of exterior
downspouts.
A topographical survey of the site combined with a roof drainage
review, indicates that, due to the existing perimeter street
drainage and site grading, the site drainage can be subdivided into
six sub -basins; with the existing runoff shared by all. four
adjacent streets. These adjacent streets then generally direct the,
runoff easterly off site down East Drake and Thunderbird Drive.
For a graphic explanation of this, please refer to Drawing 1D,
which indicates drainage sub -basin limits and flow, downspout
locations, and the general flow of the adjacent street curb and
gutter system.
THE PROPOSED STORM DRAINAGE SYSTEM IMPROVEMENTS
Based on numerous discussions between A-E Design Associates, P.C.,
the City of Fort Collins Storm Water Utility Department, and the
Owner, the proposed storm water improvements related to the
increased runoff created by this project, is proposed as a typical
stormwater detention pond located within the existing turfed mall
area of sub -basin "A" at the south/central edge of the existing
development. This solution directs the release to Tulane rather
than Drake Road, capitalizes on the existing topography and
drainage of the site, eliminates the need for depressed or curbed
parking lots, and conveniently confines the storm drainage
improvements to within the same sub -basin as the impact; that being
sub -basin "A". For a graphic explanation of this solution, see
Drawing 1D.
To deliver the increased roof runoff and sidewalk areas to the
detention pond, a series of downspouts on the narthex expansion and
a minor drainage swale along the east and west eave lines of the
fellowship hall addition, will direct the runoff to the new
detention pond. Release to Tulane will be via a typical sidewalk
gutter to the existing flowline of Tulane. Emergency overflow
will be automatically provided due to direct overflow of the pond
to existing paved areas which also drain directly (via existing
curb cuts) to Tulane. Therefore, no specific emergency spillway
will be required.
THE STORM DRAINAGE ANALYSIS
Using the City of Fort Collins "Storm Drainage Design Criteria and
Construction Standards", consultations with Mr. Glen Schleuter of
the Stormwater Utility Department, selected excerpts from the
Denver Council of Governments "Drainage Criteria Manual", and the
topographical survey of Sub -Basin "A", a storm drainage analysis
was completed based on the rational method for small drainage
basins.
Using the above data and information, the historical runoff release
rate for, a two year, storm is 0.2 CFS; the developed release rate
as proposed, for a 100 year storm is 1.9 CFS; and the maximum
detention storage capacity occurring at T=60 minutes is
approximately 1855 CF.
Using an accepted equation for the release from a flooded inlet
orifice, combined with practical. considerations of restricted flow
and minimum orifice size, a 4" diameter release culvert
(transitioned to the 2-3/4" sidewalk gutter depth) was determined
to be appropriate.
The equation used is O=Cd A 2g(H-D/2)
where: O = flow in CFS
Cd = coefficient for a smooth orifice of 0.61
H = the inlet dept, in feet, at the high water level
g = 32.17 ft/sec/sec
D = the height of the orifice in feet
A = the area of the orifice in Sq. Feet
Also using this equation, the two on site sidewalk culverts
required at the two drainage swales were set at a minimum of 8"
diameter resulting in an approximate 0 of 1.5 CFS; well in excess
of the required minimum capacity of 1,e CFS divided by 2 = 0.9. CFS
for unrestricted flow to the detention pond.
A tabular summary of these calculations is on the following page
of this report and is also on Drawing 1D. Also included on 1D is
the sidewalk culvert schedule and the topographic information
related to existing spot elevations and new revised grading and
detention pond finished grade elevations. The provided maximum
storm detention capacity of 2300 CF was calculated using the plan
area of the proposed pond and several cross sectional areas based
on average water depths.
SUMMARY AND RECOMMENDATIONS
Based on this storm drainage analysis, review .of the proposed
addition, existing site development, adjacent storm drainage
patterns, and City of Fort Collins Stormwater Utility Department's
comments and direction, it is recommended that the additional storm
water runoff created by this project be managed as shown on Drawing
1D.
If constructed as designed, sub -basin "A" will be in compliance
with current City standards, the impact of this project on the
surrounding system will minimized, and the other four on -site
stormwater drainage sub -basins will see no measurable impact from
the proposed project.
Respectfull
fo A-E Des
,
Richard S.
Colorado 'Nc
submitted
n.'Associates, P.C.
Beardmore, PE
i:,),,14 2 7 1
As.
DRAINAGE ANALYSIS WITHIN THE SPRING CREEK
EXISTING SUB. BASIN "A" ANALYSIS
AREAT = 17,600 GSF = 0.40 ACRE
AREA ANALYSIS FOR REVISED SURFACES
TYPE C EXISTING AREA PROPOSED AREA
GRASS AND/OR LANDSCAPED 0.15 17,600 GSF 9170 GSF ✓
ROOF 0.95 0 GSF 7630 GSF J
HARD SURFACED 0.95 0 GSF 800 GSF
THEREFORE:
• HISTORICAL C = .1 5 (ALL GRASSMNDS APED)
• DEVELOPED C = 9170 (0.15) = 1376
+ 7630 (0.95) = 7249 l
+ 800 (0.95) = 760
9385 176
.53
C = 0
RELEASE RATE COMPARISON
• HISTORICAL - USING 3.3 INCHES/HOUR SINCE IT IS CONSERVATIVE ASSUMING
2 YEAR Tc = 5 MINUTES AND WILL GIVE LARGEST 2 YEAR STORM
0 HISTORICAL = 1.0 (0.15) (3.3 INCHES/HOUR) (0.40)
2 YEAR _ 0.2 CFS: THEREFORE ALLOWABLE NEW RELEASE RATE FOR
DEVELOPED SUB -BASIN 'A'
• DEVELOPED - USING 6. 8 INCHES/HOUR SINCE Tc = 1.87 (1.1 -10.53 x 1.251 V 200)
100 YEAR ✓ 1.01/3
TC = 11.55 MINUTES
0 DEVELOPED = 1.25 10.53) (6.8" /HR) (0.40) = 1.8 CFS v
100 YEAR
MASS DIAGRAM y USING C X A X I X IME X F= FLOW IN CF
ALL WABLE LEA FLOW RELEASE
TIME HISTO AL 2 EAR
FLOW VE PED 1 YEAR x1.2
FLOW
ETENTION /
5 0.06 3.3 X 300 X 1
59.4 0.22 X 8' X 300 X 1.25
660✓
600.6,/
10 0.06 3.3 600 X 1
118.810.22 X 7')( 600 X 1.25
1155 -"
1036. 2✓ �
15 0.06 3.3 900 X 1
178.2� 0.22 X 6 X 900 X 1.25
14135�
81�'
20 0.06 3.3 1200 X 1
237.6 0.22 X 5.2`�1200 X 1.25
1717
1479.4
30 0.06 3.3 1800 X 1
356.4�/0.22 X 4.2''kk,3600 X 1.25
2079"
1722.6
✓
60 0.06 3.3 3600 X 1
712.8✓ 0.22 X 2.6 %X 5400 X 1.25
2575
1862.2
90 0.06 3.3 X 5400 X 1
1069.2 0.22 X 1.FZ400 X 1.25
2821--
175.1.8-✓
120 0.0E 3. X 7200 X 1
1425.E ✓ 0.22 X 1.4'X 7200 X 1.25
2773✓
1347.4
• FROM THE ABOVE T80 GIVES LARGEST VOLUME DIFFERENCE -
THEREFORE:
/
VOLUME REOUIRED @ T„ =
1862.2 I�✓/
,.,,; , ~�
VOLUME PROVIDED AS DESIGNED
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