HomeMy WebLinkAboutDrainage Reports - 05/10/1983STORM DRAINAGE STUDY
FOR
SILVERPLUME ESTATES
FORT COLLINS, COLORADO
APRIL, 1983
TMN
Engineering Consultants
■MINC
Engineering Consultants
Drake Terrace Suite 240
343 West Drake Road
Fort Collins, CO 80526
303/226-4955
April 28r 1983
Mr. Dan R. Jensen
d. Jensen Enterprises
P.O. Box 1007
Fort Collins, CO 80522
RE: Silverplume Estates
Dear Dan:
Presented herein is a storm drainage report for Silverplume
Estates. This report is in conformance with Federal Housing
Authority design criteria and the City of Fort Collins Storm
Drainage Design Criteriar dated January 1980.
Calculations pertaining to this report are enclosed at the back.
of this report.
Should there be any questions concerning procedures for analysis
or design, please contact us.
Very truly yours,
RBDr INC.
�gw-7�1
L14d G. McLauc
Robert M. Takedar P.E.
LGM/RMP:bh ✓ TA„;
Enclosures
TABLE OF CONTENTS
INTRODUCTION................................I
DRAINAGE BASIN DESCRIPPION..................1
DESIGNCRITERIA .............................2
DRAINAGEPLAN...............................2
CONCLUSIONS.................................4
ENCLOSURES: Calculations
Vicinity Nap with Drainage Plan
Detention Pond Design Drawing.
This report is prepared to provide the developer of Silverplume Estates
a proposed drainage plan and design which will provide for the proper control
of stormwater runoff. Silverplume Estates is a replat of Cedar Village Sixth
Filing and a portion of Cedar Village Fifth Filing. The project is located
generally south of Swallow Road and west of Dunbar Avenue in Fort Collins,
Colorado. A site vicinity map is enclosed for reference.
DRAINAGE BASIN DESCRIPTION
The general topography of the site slopes in a northerly direction at a
1-1.5% slope. Drainage from south of the site is intercepted by'a detention
pond facility developed for the Imperial Estates and Rossborough develop-
ments. Because of this facility and the drainage bypass ditch along the
south and west sides of Silverplume Estates, no offsite drainage enters the
site from the south or west.
On the east side of the project is the Silverplume P.U.D. and The Gables
P.U.D. Drainage plans for Silverplume P.U.D. indicate onsite detention for a
100-year.storm event to be released at a 2-year historical rate. Therefore,
contributing drainage to Silverplume Estates will be limited to the existing
2-year flow rate for the approximately 3-acre area of the P.U.D. which slopes
towards Silverplume Estates.
There are approximately 4.25 acres of The Gables which will be included
in the detention system of Silverplume Estates. This is for that portion of
The Gables which is a replat of Cedar Village 5th Filing. The remaining 3.45
acres of The Gables within the contributing basin, will have a detention
system similar to the Silverplume P.U.D., that is onsite detention for a 10D-
year storm event to be released at a 2-year historical rate.
1
Based on these existing conditions, the total contributing basin for
determining historical flows is 40.3 acres, of this area, detention for a
10D-year developed storm event will be required for 33.8 acres. The remain-
ing 6.5 acres is detained in the Silverplume and Gables P.U.D's as explained
above.
The amount of stormwater runoff for individual basins was determined by
the rational method. This procedure equates the design flow to the total
area, amount of rainfall on that area, and a multiplier that is determined by
the physical characteristics of the area under study.
The volume of stormwater detention storage was also determined by the .
rational method also called the mass diagram or accumulated runoff method.
The analysis is a comparison of cumulative runoff volume for the major storm
considered to the cumulative outflow volume permitted, given local regula-
tions or downstream considerations.
The bulk of the procedures, methods and numerical constants were ob-
tained from the City of Fort Collins Storm Drainage Design Criteria, dated
January 1980, and the Urban Storm DraManual, published by the
Denver Regional Council of Governments. Rainfall intensity duration curves
used in this analysis are those developed by the City of Fort Collins, dated
November, 1975.
Offsite flows are as explained in the drainage basin description section
of this report.
Runoff accumulating at the intersection of Dunbar Avenue and Swallow
Road from the areas on the east side of Dunbar are collected by the existing
2
curb inlets located within the intersection. As indicated in the storm
drainage study for Cedar Village Fifth Filing prepared by M&I, Inc., these
inlets were designed to handle the offsite flows from Silverplume P.U.D. and
The Gables P.U.D. The runoff from the west side of Dunbar which also accumu-
lates in the intersection, will also be collected at this point. The accumu-
lated flows are then piped directly to the detention pond located at the
northwest corner of Swallow Road and Claremont Drive.
The remainder of the site drains to the low point in Swallow Road
immediately south of the detention pond. Existing inlets at this point
collect flows which are then discharged into the detention pond.
As part of the replat for this project, it is proposed to vacate that
portion of Hanover Drive located between the original detention ponds for .
Cedar Village Fifth and Sixth Filings. In so doing, the detention system
becomes one facility for the entire development and is modified as indicated
in the construction drawings for the project. In general, the existing pond
remains as is and is enlarged to the west where Hanover Drive exists. In its
enlarged condition, the pond will have a storage capacity of 3.95 acre-feet.
Several existing concrete pans within the pond are designed for.nuisance
flows through the pond and will remain. The pond will have an outlet of 7.66
cfs which will discharge into the existing 21" RCP located in Hanover Drive
which is routed to the north and eventually flows into Spring Creek. A
spillway for the pond is located at Hanover Drive to allow for controlled
outfall should the pond overtop.
An.analysis was made for the major storm which assumed that the storm
sewer had been blocked and rendered ineffectual. The analysis shows that the
depth of ponding in the vicinity of Swallow Road and Dunbar is to height of
the road crown. Flows then overtop the pavement crown of Dunbar and flow
r]
down Swallow Road where they again pond over the inlets adjacent to the
detention pond to a height of the sidewalk. The flows then overtop the side-
walk and enter the pond. In both cases, ponding is minor and no damage would
occur to homes in the immediate area of the ponding.
This drainage plan for Silverplume Estates will satisfy all hydraulic
considerations as well as governmental regulations in providing the project
with both a safe and equitable method of stormwater management.
It is recommended that stormwater be routed as indicated herein and that
drainage facilities be constructed as designed for the project. In addition,
it is recommended that the Federal Housing Administration minimum require-
ments fo'r overlot grading be maintained.
4
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'VICINITY MAP
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