HomeMy WebLinkAboutRIGDEN FARM, 5TH FILING, TRACT I, SETTLERS GREEN - PDP - 56-98J - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTA
0 WATER QUALITY
Water quality was addressed and constructed in the Rigden Farm Filing One drainage
study and development (see introductory letter).
CONCLUSION
The findings and recommendations of this report have been obtained according to the
documents listed in this report and accepted common engineering practices. There is no
other warranty either expressed or implied.
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0 DRAINAGE FACILITIES
J2 Cross -Pans
Drainage from above Mercy Way must cross Mercy Way in a cross -pan, which has a
sideslope of'h"/foot. The area discharging to this cross pan is 0.47 acres (sub -basin A).
The time of concentration was determined to be 11 minutes, giving a 10-year storm
rainfall intensity of 4.25"/hr, and a flow across the pan of 1.06 cfs. Using
"FLOWMASTER" software, the depth of pan is 0.13', resulting in a cross -pan width of
6' .
This drainage combines with the next sub -basin (B) of 0.465 acres. The time of
concentration for sub -basin B is 10 minutes; the time of travel for sub -basin A drainage
to reach the lower cross -pan is 55 seconds. Therefore, the maximum flow at the lower
cross -pan will occur in 10 minutes 55 seconds. This intensity is 4.257hr, and the 10-year
flow for sub -basin B is 1.04 cfs. The combined flows at the lower cross -pan is therefore,
2.60 cfs. Using FLOWMASTER; the depth of the pan is 0.18", resulting in a required
cross -pan width of 8'.
Pipe Diameter in Minipark
The first pipe is at a 3.33% slope, and subsequent pipes are at a higher slope; therefore,
` the diameter of the first pipe may be used for the pipes below it. Using FLOWMASTER,
is and a 2.60 cfs flow through a PE pipe, the diameter required is 8".
Sidewalk Drain
Drainage from the upper sub -basin of 1.06 cfs plus some drainage from Mercy Way
itself must cross the sidewalk. Using a conservative 1.5 cfs, FLOWMASTER calculates
the width of a 4" deep sidewalk drain to be 0.85" therefore a 12"-wide sidewalk drain
was selected.
EROSION CONTROL DURING CONSTRUCTION
The critical times for erosion and sedimentation is during construction of the
development itself. Prior to construction the developer will have the contractor (per
plans) place straw bales where storm drainage would collect. Wind and water erosion
will be contained by silt fences on the south and east sides of the development. A
temporary detention pond will be constructed prior to each stage of building construction
for sediment control.
The BMP's in the original drainage plan will be followed by this project (see attached
measures at the end of this report.
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TABLE 1
ORIGINAL AND PROPOSED C FACTOR (IMPERVIOUSNESS)
Basin Area in Original Original New New
Settlers Green C C'Cf C C"Cf
116 b
2.13
0.71
0.88
0.53
0.66
116c
2.44
0.65
0.81
0.53
0.66
117a
0.28
0.74
0.92
0.53
0.66
Total
4.85
Wt.Average
0.68
0.87
0.53
0.66
Due to a modification in grading and drainage with this project, 0.5 acre, which originally
drained toward Topeka Drive now drains to Custer Drive. Two-thirds of the 0.5 acres
comes from Basin 116b, and one-third comes from Basin 116c. Since there is a 2 1 %
flow reduction due to the reduced imperviousness, 1.27 cfs is diverted from Basin 116b
to 117a, and 2.04 cfs is diverted fro 116c to 117a. Table 2 summarizes the revised flows.
TABLE 2
REVISED FLOW CALCULATIONS WITH NEW GRADING PLAN
Original
New
Basin
Area
Original CCf
New CCf
0(100yr)
Q(100yr)
(cfs)
(cfs)
116a
2.13
0.88
0.66
20.7
16.4
116b
2.44
0.81
0.66
16.7
13.2
117a
0.28
0.92
0.66
23.9
25.3
Total
61.3
54.9
The net decrease in flow to Topeka Drive is 7.86 cfs. The net increase in flow to Custer
Drive is 1.41 cfs, or 6%, which is considered low, and can be handled by the existing
drainage facilities.
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same year, groundwater was found at elevation 4905 east of this site by Terracon during a
Geotechnical investigation for Rigden Farm Filing One. However, during utility
trenching in August and September, 2000, no groundwater was found as low as elevation
4895 just east of Settler's Green. During construction of Rigden Farm, the project
manager had no groundwater problems, and it was assumed that the groundwater
discovered in 1997 was a perched lens related to agricultural irrigation infiltration; and
that the extensive utility trenching had drained this perched groundwater.
PROPOSED CONSTRUCTION
The proposed construction will be traditional residential single-family and condominium -
style multifamily housing. Utilities are installed to the site.
lmperviousness will be reduced from that assumed in the original drainage study for
Rigden Farm Filing One. Therefore, no detention basins are anticipated. A catch basin
has been installed for drainage from Tract I, and will be used for approximately 4/5t' of
the drainage from Basin I I6c.
This project will use all existing development and utility service lines, and will add a
narrow street and alley in conformance with the new Fort Collins' "Narrow Street and
Alley Standard." A private service drive will lead to a six-plex planned at the Corner of
Rigden Parkway and Custer Drive. The direction of drainage complies with the original
drainage report except for 0.5 acre, which runs to Custer Drive instead of Topeka Drive,
as assumed in the original study.
PROPOSED DEVELOPMENT DRAINAGE CONDITIONS, 100-YEAR STORM,
The C factor in the Rational Formula, which is the degree of imperviousness, is reduced
by this project in relation to the original drainage study. Following is the calculation of the new C factor:
Total Drainage Area: 4.85 a.
Roof area = (24 homes* 1120 sf) + (26 garages*75 sf) + (2 triplexes* 2400 sf) + (6
townhouses*2400 sf) = 1.257 acres @ C=0.95
Concrete/Asphalt Paving = 1.068 acres @ C=0.95
Gravel parking space = 12*414 sf = 0.025 acres @ C=0.50
Park land = 0.175 acres @ C=0.10
Lawns = 2.325 acres @ C=0.15
Proposed Avg C =(.95*2.325)+(.50*.025)+(.10*0.175)+(.15*2.325) / 4.85 = 0.53
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INTRODUCTION
This drainage report provides hydrology analysis using City of Fort Collins rainfall data
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and hydraulic investigation to decide the method to drain the storm water from the
property to storm drainage facilities. The project is located in Section 29, Township 7
North, Range 68 West of the 6t' P.M., City of Fort Collins, Larimer County, Colorado.
The site lies east of Rigden Parkway between Des Moines Drive and Custer Drive. The
drainage report is prepared and compiled from the following references:
1- Volume 3, Urban Storm Drainage Criteria Manual: Urban Drainage and Flood
Control District; Denver, Colorado
2- City of Fort Collins Storm Drainage DesignCriteria and Construction Standards
3- Topographic map prepared by Frederick Land Surveying, Loveland, Colorado.
4- Soils and Geotechnical Report by Earth Engineering Consultants, Fort Collins.
5- Drainage Study for Rigden Farm Filing One P.D.P, May 1999, by JR
Engineering, Fort Collins
This drainage report will present and discuss how this project relates to the approved
Rigden Farm Filing 1 drainage study. That development was a combination of single-
family residential lots and large tracts to be sold to developers for complete development.
Tract I, which is the majority of this project, was assumed by the original drainage study
to be multi -family; however, only three multifamily buildings are proposed now, with
most of the new lots to be single-family with studio -homes constructed above separated
garages.
SITE LOCATION AND CONDITIONS
The site is located at the northeast corner of Rigden Parkway and Custer Drive. The
surrounding streets are totally developed, and utilities have been run to the eight single-
family lots already platted.
The project site is 4.85 acres. See the attached General Location and Existing Drainage
map on the previous page.
The soils are lean clay and sand, and alluvial sand with gravel where significant cutting
of the grades were done. Drainage is primarily toward the east -north-east, except at the
southern 1/5°i of the site, which drains east -south-east. See the original drainage report
for detention, treatment, and ultimate discharge details.
GROUNDWATER
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In July 1997, extensive groundwater and groundwater pollutant studies were done by
Stewart Environmental Consultants for the CSU Research Foundation. Groundwater was
found at elevation 4907 in the area of this project on that date. On September 30 of the
LOCATION PLAN
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TABLE OF CONTENTS
Title Page
LETTER OF TRANSMITTAL
2
TABLE OF CONTENTS
4
LOCATION MAP
5
INTRODUCTION
6
SITE LOCATION AND CONDITIONS
6
GROUNDWATER
6
DRAINAGE BASIN MAP
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PROPOSED CONSTRUCTION
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PROPOSED DEVELOPMENT DRAINAGE CONDITIONS
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TABLE 1: ORIGINAL AND PROPOSED IMPERVIOUSNESS
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TABLE 2: REVISED FLOW CALCULATIONS
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DRAINAGE FACILITIES
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EROSION CONTROL DURING CONSTRUCTION
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WATER QUALITY
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CONCLUSION
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ATTACHMENTS: FLOWMASTER WORKSHEETS FOR FACILITIES
EROSION CONTROL PLAN FROM ORIGINAL
DRAINAGE REPORT FOR RIGDEN FARM
FILING ONE
A minor change in the flow direction will be necessary with this project, and will be
discussed here. Backup calculations are enclosed, to include a reiteration of the erosion
control measures to be followed by the developer of this project.
If you have any questions or need further information please feel free to call me at 970-
484-0533.
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30067
October 3, 2000
Glen D. Schleuter, Storm Drainage Division
City of Fort Collins Utilities Department
700 Wood Street
Fort Collins, Colorado, 80521
RE: DRAINAGE, EROSION CONTROL & WATER QUALITY REPORT FOR
SETTLER'S GREEN PROJECT DEVELOPMENT PLAN
Dear sir:
Enclosed is the report for the subject development. The hydrology data and hydraulics
analysis presented here meet the City of Fort Collins Storm Drainage Design Criteria and
Construction Standards, dated April 29, 1997.
The original drainage report for Rigden Farm Filing 1 was performed by A. Reed, fR
Engineering, LTD., on May 30, 1999, and this site is covered by that report. The Master
Plan study for Rigden Farm is the "Foothills Basin (Basin G) Master Drainage Plan" by
Resource Consultants, Inc., 1981, and lies within Reach 2 of the Foothills Basin. Other
dainage reports were used by Mr. Reed, as stated in his report.
The major portion of the site, Tract I, was originally assumed to be 100% multifamily
development in the fR Engineering report. The site was composed of sub -basins I I6b
and 116c and a small portion of sub -basin 117a (see site map attached). The original site
was calculated to have an overall imperviousness of 68%. The new development, which
is primarily single family with a park area, has an overall imperviousness of 53%.
Therefore, the new site development would have a calculated runoff of 15.1% less than
that calculated in the original drainage study. Therefore, no detention and no additional
water quality facilities are required by this development. Also, water quality was
addressed in the original report, and is handled by facilities already constructed.
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