HomeMy WebLinkAboutSHENANDOAH PUD - PRELIMINARY - 47-95 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTShenandoah P.U.D.
Preliminary Drainage Report Page 8
APPENDIX
PAGE
Exhibit1.......................................................................... 1
Exhibit2.......................................................................... 2
Exhibit3.......................................................................... 3
Exhibit4.......................................................................... 4
Ridgewood Hills P.U.D. Basin B-2 Calculations .......................... 5- 7
"HY8" Computer Output for Existing 24" RCP ........................... 8-10
"HY8" Computer Output for Existing 48" RCP .......................... 11-13
Eagle Point "Watershed Module" Methodology .......................... 14- 15
Design of North Louden Irrigation Ditch ................................... 16- 17
Design of Culverts for North Louden Irrigation Ditch ................... 18- 32
Shenandoah P.U.D.
Preliminary Drainage Report Page 7
REFERENCES
1. Fossil Creek Drainage Basin Master Drainageway Planning
Study, City of Fort Collins, Larimer County, Colorado, Simons,
Li & Associates, Inc., August, 1982.
2. Overall/Preliminary First Filing Drainage and Erosion Control
Study for Ridgewood Hills P.U.D., Fort Collins, Colorado,
RBD, Inc. Engineering Consultants, May 2, 1994.
3. Soil Survey of Larimer County Area, Colorado, U.S.
Department of Agriculture, Soil Conservation Service and
Forest Service, Issued December, 1980.
4. Storm Drainage Design Criteria and Construction Standards,
City of Fort Collins, Colorado, May, 1984.
5. Drainage Criteria Mauna], Urban Drainage and Flood Control
District, Wright -McLaughlin Engineers, Denver, Colorado,
March, 1969.
6. Final Drainage and Erosion Control Study for Ridgewood Hills
P.U.D. First Filine , Fort Collins, Colorado, RBD, Inc.
Engineering Consultants, November 23, 1994.
Shenandoah P.U.D.
Preliminary Drainage Report Page 6
V. CONCLUSIONS
5.1 Compliance with Standards
All drainage analyses have been performed according to the City of Fort Collins
Storm Drainage Design Criteria Manual (SDDCM) and the Urban Drainage and Flood
Control District's Drainage Criteria Manual.
5.2 Summary of Draina eg Concept
The overall drainage concept for the Shenandoah site has taken into consideration all
studies known to have an influence on the proposed site. The hydraulic and hydrologic
(~ analyses conducted for this study have been performed in a manner consistent with standard
practice and principles of civil engineering.
The operations of the North Louden Irrigation Company will be uninhabited due to
the proposed development of the site. The user south of Benson Lake who is fed by a lateral
n extending from the Shenandoah site through the existing 48" RCP under College Avenue will
not experience an interruption in service.
It is understood that a signed relocation agreement between the developer and the
�. ditch company will need to be obtained prior to approval of the Final Utility Plans for
Shenandoah P.U.D..
Shenandoah P.U.D.
Preliminary Drainage Report Page 5
will be released at a rate based on either the capacity of existing downstream conveyance
elements or by approved criteria adopted by adjacent developments.
Developed stormwater has been routed and released from on -site facilities to mimic
the historic drainage pattern of the site. This includes routing roughly the same developed
area as historic to the Non -Contributing Area so as to avoid transferring runoff across basins.
Detention Pond D-1 will be released to the existing twenty-one (21) inch RCP storm
sewer, coming from Pond #39 of Ridgewood Hills P.U.D. First Filing, at a rate of 0.19
cfs/acre or 0.19cfs/acre x 7.004 acres= 1.33 cfs. This release rate is based on a portion of
the 2-yr historic flow at the existing 24" RCP as calculated by RBD, Inc.(See Ref.6) .
Preliminary analysis of the existing 21" storm sewer indicates it has the capacity to accept
this additional release and the future anticipated release from both Phase I and Phase II of
Ridgewood Hills of 15.6 cfs (To be documented at final submittal).
Detention Pond D-2 is located in a natural depression adjacent to College Avenue
which has an existing 24" CMP extending from the Shenandoah site north, to the future
Fossil Creek site. This pond will release developed stormwater from the site through a
proposed pond outlet pipe, following the historic flow pattern, to the existing twenty-four
(24) inch RCP. The release rate from this pond will be 0.19cfs/acre.
Detention Pond D3 is located in the center of the Shenandoah site and will detain
developed stormwater runoff from the site to be routed to the south and through two (2)
existing twenty-four (24) inch RCP's extending from the site east under College Avenue.
Easements will be required downstream for this developed discharge and at the time of this
report, letters of intent from effected property owners were being obtained by the developer.
The south end of the site is proposed for future uses at this time. Developed
stormwater from this area will be conveyed south to two(2) existing 36"x 58" horizontal
elliptical pipes extending under College Avenue to the east and daylighting north of County
Road 32. The Fossil Creek Drainage Basin Master Drainageway Plan will be used in the
design of future drainage appurtenances. Downstream easements will also be required for
conveyance of future developed stormwater. At the time of this report, these easements were
being coordinated with effected property owners and governing municipalities.
4.5 General Drainage Concept, con't
The off -site flow of 16.5 cfs from Basin B-2 estimated by RBD., Inc. for the future
Ridgewood Hills P.U.D. will be routed through the Shenandoah site to the existing 48" RCP
under College Avenue and then to Benson Lake. The actual area of Basin B-2 which
historically went to Benson Lake compared to what is proposed for future developed
conditions needs to be evaluated by the engineer for Ridgewood Hills P.U.D. at the time of
development to see is this historic contributing area is exceeded.
Shenandoah P.U.D.
lPreliminary Drainage Report Page 4
IJ 4.3 Hydrologic Criteria
The Rational Method has been used to estimate preliminary peak stormwater runoff
from the development site. The initial 2-year and major 100-year design storms have been
used in the evaluation of the proposed drainage system. Rainfall intensity data for the
Rational Method will be taken from Figure 3-1 of SDDCM. In addition, rainfall
distributions specific to the site will be taken from the NOAA Atlas 2 for the Western United
States.
4.4 Hydraulic Criteria
The computer program "Watershed Module" by Eagle Point, shall be used to route
� storms through the proposed on -site detention facilities using the Modified Puls Method (See
II pages14- 15, Appendix). This modeling incorporates:
• NOAA Rainfall Distributions specific to the site using the Western Atlas,
• Combined inflow hydrographs,
• Reservoir stage/storage/discharge curves,
• Reservoir outlet structures.
The computer program "Storm Sewers Module" by Eagle Point will be incorporated
in the design of on -site storm sewers. This module is used in conjunction with the
"Watershed Module" to provide an accurate design of storm sewers systems.
These modules have been used because of their accuracy and ability to accept
computer generated reservoir data taken directly from the proposed utility plans through
AutoCAD.
The computer model "HY8" developed by the U.S. Department of Transportation,
Federal Highway Administration, has been used to analyze the capacity of existing and
proposed culverts.
4.5 General Drainage Concept
This preliminary drainage report addresses developed stormwater as a result of the
proposed improvements as shown on the Preliminary Utility Plans for Shenandoah P.U.D..
Areas of the Shenandoah site designated for future development will be addressed only with
regard to their overall intended drainage scheme.
Developed stormwater will be released to downstream properties as per ensuing
agreements coordinated with the City of Fort Collins Stormwater Utility Department as they
relate to this development.
Developed stormwater will be collected, conveyed and detained on -site in three (3)
detention facilities. Depending upon the location (drainage basin) of the pond, stormwater
Shenandoah P.U.D.
Preliminary Drainage Report Page 3
acres. This area includes future phases of the Ridgewood Hills P.U.D. site, the future Fossil
Creek Nursery site and the Shenandoah site.
The report "Final Drainage and Erosion Control Study for Ridgewood Hills P.U.D.
First Filing", by RBD, Inc., dated November 23, 1994, page 4, identifies this Non -
Contributing Area as being 125 acres (using U.S.G.S. contour mapping at 10' intervals)
which is fairly consistent with the findings of Northern Engineering Services which used
more accurate field information. Also using actual field survey information and the
computer program "HY8", a rating curve for the existing 24" RCP was developed (See
Pages 8-10, Appendix) to determine its capacity.
3.2 Historic Drainage Patterns
The site drains generally from west to east, at slopes ranging from 3.5% to 15%, to
four (4) low points adjacent to South College Avenue. At two (2) locations (the existing 24"
and 48" RCP's), stormwater from the non-contributing area is conveyed under College
Avenue to Benson Lake. Culverts at the remaining two (2) low -points convey stormwater
under College Avenue to the east across an existing drainageway. Off -site runoff enters the
site along the entire west propertyline from land which is currently vacant but which is
proposed to be future phases of Ridgewood Hills P.U.D..
IV. DEVELOPED DRAINAGE BASIN
4.1 Developed Conditions
The proposed development of the site includes residential and commercial properties
with local and collector streets, open spaces, utilities and on -site detention facilities. The
existing North Louden Irrigation Ditch will be relocated the length of the site with the design
and construction to be approved by and coordinated with the ditch company.
Future stormwater release from Ridgewood Hills P.U.D. Basin B-2 (100-yr = 16.5
cfs, See pages 5- 7, Appendix) will be routed through the Shenandoah site to the existing 48"
RCP and on to Benson Lake.
4.2 Design Criteria and References
Drainage criteria outlined .in both the City of Fort Collins Storm Drainage Design
Criteria Manual.(SDDCM) and Stone Drainage Criteria Manual by the Urban Drainage and
Flood Control District have been used for this Final Drainage Study. The
"Overall/Preliminary First Filing Drainage and Erosion Control Study for Ridgewood Hills
P.U.D. prepared by RBD, Inc. Engineering has also been referenced.
Al Shenandoah P.U.D.
�1 Preliminary Drainage Report Page 2
11. SITE LOCATION AND DESCRIPTION
2.1 Site Location
The project is located in the East Half of Section 14, Township 6 North,
Range 69 West of the 6th Principal Meridian in Fort Collins, Colorado. The project is
bounded by South College Avenue (U.S. Route 287) to the east, County Road 32 to the
south, and W. Trilby Road on the north. (See Vicinity Map).
2.2 Site Description
The Shenandoah site covers approximately 95.8 acres, the majority of which is
pasture land with a single residence and outbuildings. The Soil Survey of Larimer County
Area. Colorado (Reference 3) was used to identify the soils on -site. A total of seven (7) soil
classifications were found at the site ranging from Longmont Clay, with low permeability
and slow runoff, in the low-lying areas to Fort Collins Loam with moderate permeability and
rapid runoff (See Exhibit 1, Pg. 1 Appendix).
2.3 Irrigation Ditches and Reservoirs
The North Louden Irrigation Ditch currently meanders down the center of the site
flowing from south to north at slopes ranging from 0.0562% to 0.1163%. Robert Benson
Lake is located east of the site across College Avenue and currently receives historic runoff
from the proposed Shenandoah site.
III. HISTORIC DRAINAGE BASIN
3.1 Major Basin Description
The site is located in the Fossil Creek Drainage Basin (See Exhibit 2, Pg. 2
Appendix). According to the "Fossil Creek Drainage Basin Master Drainageway Plan" on -
site detention is not required. Release rates from the site will vary depending upon location
and shall be predicated upon criteria adopted by adjacent developments (Ridgewood Hills
P.U.D.) and the capacity of downstream conveyance elements.
A portion of the Shenandoah site is located in a Non -Contributing Area which drains
to Robert Benson Lake (See Exhibit 3, Pg. 3 Appendix). The south half of the site is part of
Subcatchment 55 included in the SWMM modeling for the Fossil Creek Drainageway
Planning Study.
Within this Non -Contributing Area is a portion (49.262 acres) which is tributary to an
existing 24" RCP extending under College Avenue from the future Fossil Creek Nursery site
to Robert Benson Lake. It has been determined, based on surveyed topographic mapping,
that the total area west of College Avenue which is tributary to the 24" RCP is 115.812
Preliminary Drainage Report
for
Shenandoah P.U.D.
I. INTRODUCTION
1.1 Objective
To provide a preliminary drainage scheme for the proposed Shenandoah
development based on design criteria and master drainageway planning adopted by the City
of Fort Collins, Colorado. Specific objectives as part of this study are:
1. To adopt the hydrology and hydraulics contained in the Fossil Creek
Drainage Basin Master Drainageway Planning Study
(Reference 1),
2. To consider all off -site stormwater runoff from adjacent properties and
IF
the design intent of future adjacent developments (Reference 2),
3. To consider utilizing any existing adjacent stormwater appurtenances,
4. To consider any possible adverse effects downstream of the
development due to developed stormwater.
1.2 Mapping and Surveying
A field survey of the site was conducted by Landstar Surveying, Inc.
Loveland, Colorado in August, 1995. Based on this field survey information, topographic
mapping was computer generated by Northern Engineering Services, Inc. with a contour
interval of one (1) foot.
1.3 Site Reconnaissance
A site visit was conducted on Friday October 13, 1995 by the project
engineer. Based on the topographic mapping, existing drainage basins and land use were
confirmed. The location and dimensions of existing culverts and storm inlets were verified
as well as their condition and flow direction.
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Northern Engineering Services, Inc.
November 20, 1995
City of Fort Collins
Stormwater Utility
235 Mathews
Fort Collins, Colorado 80522
RE: Shenandoah
Fort Collins, Colorado
Project Number: 9535.00
Dear Staff:
Northern Engineering is pleased to submit this Preliminary Drainage Report for
Shenandoah for your review.
This report was prepared in compliance with technical criteria set forth in the
City of Fort Collins Storm Drainage Design Criteria and Construction Standards
manual.
If you should have any questions or comments as you review this report, please
feel free to contact me at your convenience.
Sincerely,
NORTHERN ENGINEERING
0326
Mary B. Wohnrade, P.E. � '
420 S. Howes, Suite 202 Fort Collins, Colorado 80521 • (970) 221-4158 • Fax (970) 221-4159
Preliminary Drainage Report
for
Shenandoah P.U.D.
City of Fort Collins, Colorado
November 20, 1995
Prepared For:
Colorado Land Source, LTD.
9085 East Mineral Circle, Suite 260
Englewood, Colorado 80112
Prepared By:
Northern Engineering Services, Inc.
420 South Howes, Suite 202
Fort Collins, Colorado 80521
(970)221-4158
Fax (970)221-4159
Project Number: 9535.00