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TABLE 1
CONCLUSIONS
The drai,nage system for the .First Christian Church PUD has been
designed. and analyzed in accordance with current criteria.. This
drainage system is designed to tie to existing drainage facilities
in Eastborough and Drake/Lemay intersection. Following is a
summary of this report:
1. No upstream flow passes through the First Christian
Church site.
2. Runoff -to -rainfall coefficient based on final developed
conditions, not zoning.
3. Detention reduces major storm to 2-year historic release.
4. Southeast detention pond is a two -stage pond with back-
water on parking areas at return periods greater than
2-years.
-5-
METHODS AND ASSUMPTIONS
In general, the Preliminary -Draft Storm Drainage Design Criteria for
the City of Fort Collins, dated April 1979, was used as a - guideline
in all hydrologic and hydraulic calculations. A more specific des-
cription of the methodology follows.
Hydrologic Analysis
The Rational Method and Overland Time of Concentration formula were
used to determine peak runoff. Basins were defined on the basis of
proposed grading. The Runoff -to -Rainfall and Storm Frequency Increase
Coefficients (C and Cf) were applied based on actual pervious and
impervious areas. Table 1 in the appendix lists these coefficients
and the peak flows for the 2-year undeveloped and 2-year and 100-year
fully developed and Phase 1 developed conditions.
Detention
Because of fully developed conditions immediately downstream, detention
sufficient to reduce a 100-year developed event to a peak discharge
equal to the 2-year undeveloped storm is provided. Required detention
volume is determined by the Accumulated Runoff Method. Detention
ponds are shown on Sheet C2 with stage storage curves and maximum
water surface shown on Sheet C4.
Note that above elevation 64.5 storm water will spread over proposed
Phase 2 parking. Detention volume below this breakout elevation is
equal to volume required by a 2-year fully developed storm.
Culverts
Hydraulic Charts for Selection of Highway Culverts was used to deter-
mine culvert sizes.
Oriface
A discharge coefficient of 0.65 was assumed for orifice plate
calculations.
-4-
-3-
SITE CONDITIONS
General
The First Christian Church is located southwest of the intersection
of Drake Road and Lemay Avenue. The site, with,a net acreage of 24
acres, has been used as irrigated farmland.- Although it has laid
fallow this past season, no improvements have yet been placed on
the land.
Contributing Runoff
The general lay of the land is from the northwest to the southeast.
However, Lake Parkwood just offsite at the northwest corner is an
anomaly to this pattern, leaving the proposed site as a local high
point. Lemay Avenue and Drake Road both discharge toward Lake
Parkwood and are cut in lower than the existing church site. No
offstream runoff contributes to drainage on this site.
Basin and Proposed Detention
The site is located in the Foothills Drainage Basin and is approximately
3/4 of a mile from the drainageway. The majority of discharge from the
site is routed to existing culverts in Oxborough Lane and contributes to
Lake Sherwood. After routing through the lake, storm water finally
discharges into the drainageway.
Included in the appendix are copies of the agreements between Lake
Sherwood Venture and Vance concerning drainage easements, structural
improvements, and flood damage liability between the church site and
Lake Sherwood.
The entire area between the drainage basin swale and the site has been
platted at urban residential density. Drainage structures have all been
installed between the site and Lake Sherwood. For these reasons, deten-
tion volume to reduce a 100-year developed storm to a 2-year undeveloped
release rate is chosen as a design criterium.
-2-
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CORNER CONSU[TIN(i COMPANY po.box 646 foRT(dks,(dnRx1 ;k)522 p4)Nc:(303)482.U)50�
MANAC,EME VThW W XRWCT
FIRST CHRISTIAN CHURCH P.U.D.
DRAINAGE REPORT
ABSTRACT
This report is intended as a supplement to aid in the review of the
drainage plan_ shown in The First Christian Church PUD Utility Plans.
The drainage design is shown on Sheets C2 and C4 of these plans,
with both sheets included in reduced format with this report.
This report addresses the specific site conditions that impact the
drainage system. It also presents the assumptions, methodology and
final results of all drainage calculations. Although the PUD will
be phased construction, the hydrologic study assumed ultimate developed
conditions.
CORNER CONSU[TINC, COMPANY PO- bot646 f(vtUil4�C(AmAcbW,22 *)-,E: (303)482.6650�
MANAGEMENT/EW WEERWC
FIRST CHRISTIAN CHURCH P.U.D.
DRAINAGE REPORT
Prepared for
FIRST CHRISTIAN CHURCH
Fort Collins, Colorado ~
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August 25, 1980