HomeMy WebLinkAboutDrainage Reports - 05/09/1995 (2)April 27, 1995
FILE: 95027
City of Fort Collins
Stormwater Drainage Utility Department
235 Mathews Street
Fort Collins, CO 80525
RE: Revised Drainage Report and Calculations for the
Expansion of the Shepherd of the Hills Lutheran Church
ATTENTION: Glen Schleuter
Dear Glen:
The Shepherd of the Hills Lutheran Church is proposing the expansion of their existing Church facility
located at 1200 South Taft Hill Road, Fort Collins, Colorado. This expansion includes two additions to the
Church on the west side of the existing facility measuring approximately 4,500 square feet and one
expansion on the east side of the Church measuring 450 square feet.
A drainage report for the Church property was submitted to the City of Fort Collins in 1987 by John Clark.
This report addressed the change in impervious area as a result of the paving of their gravel parking lot.
This report addressed the drainage on the east side of the building, which we have revised to show the result
of the addition. The parking lot was graded to provide on -site detention for drainage on the east side of the
building. We have not deviated from the existing grading on the east side of the building but have
increased the percentage of impervious area to the drainage calculations. Please see our addendum to the
report done in 1987 for our revised calculations.
The additions to the Church on the west side will be constructed within the minimum 25-foot setback from
the Taft Hill Road right-of-way. The construction of the additions will not result in the regrading of the
grassed area, as the lawn is mature with trees and shrubs which will not require relocation or regrading.
Therefore, our historic storm runoff routes have not changed as a result of the additions on the west side of
the Church.
Runoff from the west side of the Church historically drains to the north, eventually emptying into
Clearview Channel in Avery Park. After review of the construction plans for the additions (as previously
stated), the developed runoff patterns will not deviate from the historic patterns.
The Engineering Co.
2310 East Prospect
Ft. Collins, CO 80525
Phone (303) 484-7477
FAX (303) 484-7488
City of Fort Collins
Stormwater Drainage Utility Department
April 27, 1995
Page Two
After discussions with the City of Fort Collins Stormwater Utility Department, we felt the developed runoff
from the site would not require on -site detention due to the proximity of the site to Clearview Channel. We
felt the runoff peak from the Church site (west side) would pass prior to the peak runoff in Clearview
Channel, as modeled by Lidstone & Anderson, Inc. (See enclosed report by Lidstone & Anderson, Inc.)
We contacted Lidstone & Anderson, Inc. and obtained information regarding the time to peak of Clearview
Channel in the proximity of the Church site. We have enclosed these calculations from Lidstone &
Anderson, Inc. in the Technical Appendix of this report.
After determining the peak runoff times from the Church site (west side), we found the time of
concentration for the peak runoff condition from the Church to be less than the peak runoff time in
Clearview Channel. Therefore, we do not propose any additional on -site detention as a result of the
addition to the Church. The runoff from the Church site will not impact the peak storm runoff in Clearview
Channel. Please see our Technical Appendix for our calculations.
In review of the City's Erosion Control Standards concerning wind erosion, we have the following
comments. In accordance with Section 8.1.1.1, Exclusions, the additions to the Church will encompass
approximately 5,000 square feet of area disturbed by construction. This area may vary depending on the
amount of disturbance beyond the actual footing limits. As soon as the area is disturbed, forms will be set
to place concrete for footings and eventually the floor slab. As we interpret your exclusions, because the
site is less than one acre, because 35% or more of the area will be covered with gravel bedding for the
floor slabs, and because the existing building will provide a wind barrier, the additions to the Church would
be exempt from the Wind Erosion Control.
In review of the City's Erosion Control Standards concerning rainfall erosion, we have the following
comments. Construction is scheduled for April through October. We will need to be appreciative of the
tendency of rainfall to carry sediment prior to the resodding of disturbed areas. We intend to control
sediment loss during construction by structural methods and to control sediment loss after construction by
resodding.
As the historic drainage pattern for this drainage basin is defined by the existing topography and will not be
disturbed during construction, we intend to place straw bale barriers as shown on the enclosed exhibit.
Straw bale barriers will be placed at the start of construction on the west side of the existing building.
Barriers shall be placed as shown on the enclosed exhibit where the historic drainage pattern flowing to the
north concentrates itself into a defined Swale. This location was selected to provide a location where runoff
would concentrate and allow the sediment to drop prior to flowing onto the existing asphalt parking lot on
its way to Clearview Channel. Maintenance of the barrier will include daily inspection by the contractor
(especially after rainstorms) to insure the straw bales have not dislodged or become separated. If either
condition occurs, the contractor will replace the disturbed bales. The straw bale barriers will be removed
once landscaping provisions (resodding) are in place following construction.
City of Fort Collins
Stormwater Drainage Utility Department
April 27, 1995
Page Three
Please review this material on behalf of the Church. They have been delayed in obtaining their building
permit until this report has been approved by your department. They are anxiously awaiting your approval
to begin construction.
If you have any questions regarding this material, please do not hesitate to call.
Sincerely
//yours
,
JRick R. Pickard, P.E.
The Engineering Co.
Technical Appendix
Determine the Time of Concentration in the Developed Condition
To determine the time of concentration, we will use the Rational Method. To determine the time of
concentration for our proposed runoff flow route, we can use either the following equation or the "Overland
Time of Flow Curves" enclosed as Figure 3-2.
Tc = 1.87 ( 1.1 - CCf) Dos / So.33
where: Tc - time of concentration, minutes
S = slope of basin, %
C = rational method runoff coefficient
D = basin length, feet
Cf= frequency adjustment factor
As the use of Figure 3-2 is limited to 500 feet, we must determine the length of flow route for our
application.
Length of flow route, = 450 ft.
Slope of flow route = (69.92 - 64.25) / 450 = 1.3 %
Composite runoff factor (C) _ ((75 ft. x 0.95 )+ (375 ft. x 0.20)) / 450 = 0.33
Cf= 1.25, from table below
Rational Method Frequency Adjustment Factors
Storm Return Period
Frequency Factor
(years)
Cf
2 to 10
1.0
I I to 25
1.1
26 to 50
1.2
51 to 100
1.25
Note: The product of C times Cf shall not exceed 1.00
Therefore: The time of concentration for the developed flow path is as follows:
Tc = 0.87 x 0.1 - 1.25 x 0.33) x 4500.50) / 1.30.33 = 25.0 minutes
n
In review of the 100-year hydrology at Clearview Channel at the South Taft Hill crossing (as prepared by
Lidstone & Anderson, Inc.), the peak storms pass at conveyance elements 59 and 61 with times higher than
the time of concentration for the Church site. Therefore, as the peak runoff from the Church site will pass
prior to the peak runoff in Clearview Channel, it is feasible to assume no on -site detention is required due
to the proximity of the Church site to Clearview Channel.
Review of Detention Volume Requirements for the East Addition to the Church
The construction of approximately 450 square feet of additional building will require a check of the
detention volume requirements as described by John Clark in his drainage report submitted to the City in
1987. Those calculations are represented by a redlined copy of John Clark's original report and
calculations. Please see enclosed sheets. Our calculations indicate adequate storage volume in the parking
lot detention pond after construction of the addition on the east.
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NO. 95-027
The Engineering Company
FORT COLLINS, COLORADO
SHEPHERD OF THE FILLS
CHURCH ADDMON DRAINAGE EXHOT
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DYR I EL DCIVOGNIG) RRP DAT'R WAGON THIS The Engineering Company SHEPHERD OF THE HL.LS EXHI�i
BCAid Y.SP ePmm PRa. NO.95-027 FORT COLLINS, COLORADO CHURCH ADDMON , � 1
DRANAGE