HomeMy WebLinkAboutHEARTHFIRE (HOFFMAN) PUD - PRELIMINARY (REVISED) - 31-95A - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE REPORTHoffman P.U.D. Preliminary Drainage Report
I F. REFERENCES
1. City of Fort Collins, Colorado - Storm Drainage Design Criteria and Construction
Standards, 1984 EDITION.
2. Urban Drainage and Flood Control District - Urban Storm Drainage Criteria
Manual, 1990 Revisions.'
I3. Dry Creek Drainageway Planning, prepared by Gingery Associates, Inc.
January, 1980.
I4. Larimer County Storm -water Management Manual, prepared by Resource
IConsultants, Inc., May, 1979
5. Master Drainage Plan and Study, Richards Lake Property, by Downing Leach,
October, 1986
6. Wellington Quadrangle, Colorado, U.S. G.S. 7.5 Minute Series (Topographic),
1965, Photorevised 1980.
7. Report of a Preliminary Geotechnical Investigation for Richard's Lake P.U.D.
Larimer County, Colorado prepared by. Empire Laboratories, Inc.
November, 1986.
8. Reviewer Comments, City of Fort Collins, Colorado - Stormwater Utility
June, 1995
Merrick & Company Page 16
I AINFRA\15011782\PRELIMI \H FMNDRNG. N15
Hoffman P.U.D. Preliminary Drainage Report
1. Summary of Drainage Concept
The existing site currently drains to the southwest to on -site existing low/wetland
areas or to Richards Lake. The proposed development drainage plan will
continue that pattern, with downstream impacts consistent with the historic flow
rates and volumes and meet the requirements of City of Fort Collins criteria.
A Drainage Plan has been prepared and is included in the back of this report.
Included in the Appendices are calculations to support the swale, detention
pond, street and inlet capacities, and stormdrain pipe designs.
3. Summary of Erosion Control Concept
A majority of the site drains to the on -site wetland/detention pond. Sediment
control barriers will protect the wetlands from siltation. Disturbance of the
wetlands will be minimal. Rainfall induced erosion into Richard's Lake will be
prevented by minimal surface grading along the shore of the lake. Silt fences will
protect the on -site existing wetland and will help prevent wind erosion. Overlot
grading will be followed by temporary seeding, utility installation and paving.
Gravel filters and hay bales will protect street inlets and swales respectively.
The proposed development erosion plan will provide protection in excess of the
required effectiveness and with downstream impacts consistent with historic
patterns. The erosion control plan meets the requirements of City of Fort Collins
criteria.
Respectfully submitted,
MERRICK & COMPANY
Glen A. Tulk, PE
Project Manager
Merrick & Company Page 15
IAIN FRA%150117821PRELIM I U1 FMN DRNG. N 15
Hoffman P.U.D. Preliminary Drainage Report
Richard's Lake. Silt fences will be placed to protect the on -site existing wetland
and to prevent wind erosion. Overlot grading will be followed by temporary
seeding, utility installation curb and gutter placement, and street, paving. Gravel
filters and hay bales will protect street inlets and swales respectively. The
proposed development erosion plan will provide protection in excess of the
required effectiveness. Downstream impacts of runoff will be consistent with
historic patterns.
2. Specific Details
The Hoffman P.U.D. lies within Moderate to Low Rainfall and Wind
Erodibility Zones as shown by City of Fort Collins' maps. Soils on the site
include the Fort Collins Loam, Kim loam, and the Nunn Clay as detailed in
the 1986 Geotechnical Investigation for the Richard's Lake P.U.D. by
Empire Laboratories, Inc. Erosion is a potential problem during
construction and after until exposed soils are revegetated. Erosion control
performance standards were calculated according to- criteria presented in
the City of Fort Collins Erosion Control Reference Manual for Construction
Sites. Temporary seeding with mulch will be used for areas that are
overlot graded. Permanent seeding with mulch will be used for areas that
are final graded. Permanent seeding with mulch will follow installation of
stormsewers and any swales. Gravel filters will be placed at all curb inlets
immediately after installation. Straw bales will be placed in all erosion -
prone swales. Outfall velocities will be at velocities low enough to prevent
erosion, rip -rap or other protection will be provided to prevent scour and
bank erosion.
Wind erosion will be minimized by phased grading and construction, and
placement of wind barriers and silt fences when cleared areas exceed the
maximum 200 foot criteria as presented in the City of Fort Collins Erosion
Control Reference Manual for Construction Sites.
F. CONCLUSIONS
1. Compliance with City Requirements
The designs and calculations included in this report comply with the 1984 City of
Fort Collins Storm Drainage Design Criteria. Hydrologic and hydraulic
calculations were based on accepted practices for the Rational Method as
contained in the City of Fort Collins Storm Drainage Design Criteria and in the
Urban Drainage and Flood Control District Criteria Manual. Calculations of the
runoff rates and storm system for this project are included in Appendices 1 and 2
of this report.
Merrick & Company Page 14
IAINFRAl150117021PRELIM 1 \HFM NDRNG. N 15
Hoffman P.U.D. Preliminary Drainage Report
and Storage Company and compatible with the historic flow path. (Refer to
Appendix 2 for outlet design).
The detention release structure will have a water quality release outlet which will
release the 2 year runoff over a 12 hour period as specified in the Urban
Drainage Manual, Volume 3. The 2 year storm generates 179709 cubic feet of
runoff in 180 minutes; the 12 hour water quality protection release rate is
therefore 4.15 cfs. The outlet design of the 100 year release structure will
deduct the water quality release so that the total 100 year release in maintained
at 16.6 cfs.
Wetland Management
Off -site wetland/pond B to the north will experience minimal disruption from the
development of the proposed Hoffman P.U.D. The wetland will continue to retain
runoff generated in historic basin OH2, the existing topography surrounding
Pond B provides 23.4 acre feet of storage capacity above the average water
level of 5108 feet.. The proposed Hoffman P.U.D. will not reduce that off -site
capacity. The proposed emergency outlet improvements for Pond B will allow the
wetlands' water level to fluctuate as it has historically. The emergency outlet
structure is designed to release to the southern, on -site Pond A only in the event
Pond B water levels exceed 5110 feet.
On -site wetland/pond A will be minimally disturbed by the proposed Hoffman
P.U.D. development. Grading will slightly modify the northwest quarter of the
wetland, and a detention outlet structure with emergency spillway will be
constructed outside of the existing wetland on the southeast side of the
topographic depression. Water quality and vegetation in the wetland will be
protected by a proposed sediment trapping gravel berms located downstream of
each stormwater pipe inlet to the wetland. These berms will slow inflowing storm
runoff allowing sediments and adsorbed pollutants to settle out in sediment
basins upstream of the wetland. Periodic maintenance of these sediment traps
by the homeowners association will insure continued water quality in the
wetland. Detention releases from the detention pond will utilize a water quality
outlet structure that protects the Richard's Lake receiving waters from sediments
and pollutants. (See detention release structure design detail sheet).
E. EROSION CONTROL
1. General Conceit
A majority of the site drains to the on -site wetland/detention pond. Rainfall
erosion will be prevented by minimal disturbance of the lots along the shore of
Merrick & Company Page 13
LAIN FRA\15011782\RRE LIM t \H FMNDRNG. N 15
II
Hoffman P.U.D. Preliminary Drainage Report
1
1
1
1
1
1
1
0
1
1
1
1
—,
1
L
I
Street and Inlet Capacities
Appendix 2 contains spreadsheets showing the allowable 2 and 100 year street
flows for a variety of street slopes. Detailed inlet sizing will be presented with the
final drainage report, preliminary sizings are included in the Appendix for key
design points. All inlets will be sized to maintain street capacities within City
criteria for the 2 and 100 year storms.
LZI • u,- - �- -� •i
The Hoffman P.U.D. site historically drained to either a privately owned lake or to
an on -site wetland. The City of Fort Collins requires detention of the Major (100
year] Storm Provision to accept off -site flows discharging through the property
is also necessary. Using the Rational Method as outlined in UD&FCD's
Drainage Criteria Manual Volume 2 "Storage", a required detention volume of
cubic feet was calculated for the Hoffman P.U.D. assuming a release rate from
the detention/wetland of 16.6 cfs, the historical 2 year flow from Historical.
Basin H2. This assumes that the undetained releases to Richard's Lake from
developed Basin P1 are approved. The net increase flowing to Richard's Lake
is:
Historical
2 year 16.6 cfs
100 year 97.0 cfs
Developed
16.6+22.3=38.9 cfs
97.0+85.3=182.3 cfs
The proposed grading and existing topography of the Hoffman P.U.D. limit the
areas that can effectively be drained through the proposed detention pond. The
proposed grading directs developed flows to detention/wetland pond A from 81.4
acres of the site. However, the detention volume is calculated for the entire
105.5 acres of the Hoffman P.U.D. with release at the required 1.6.6 cfs historic
2 year runoff rate for the original 60.6 acres (Basins H2 and OS1) draining to the
central wetland area on the site. The proposed 81.4 acres includes 1.6 acres of
Historic basin H3, and 4.4 off -site acres. Due to the steep slopes along the
northeast shore of Richard's Lake, Basin P1 will discharge to the Lake
undetained. Easement to discharge these undetained flows and the detention
pond releases will be obtained from the privately owned Richard's Lake. Off -site
flows from off -site basin OH2 and any discharges from wetland/Pond B are not
included in the detention sizing of Pond A; it is assumed that any future
development of Basin OH2 will be detained on that site. Proposed Pond A is
designed with 1 foot of freeboard and an emergency release structure which
would only discharge if the water levels exceeded 5086.4 feet. The outfall will be
designed to discharge at non -erosive flow velocities acceptable to Water Supply
Merrick & Company
Page 12
I:\INFRA\75011782\PRELIM1 \HFMNDRNG.N15
1
Hoffman P.U.D. Preliminary Drainage Report
I
wetland/detention pond (Pond A). Basin P2 is anticipated to create a peak minor
storm flow of 15.7 cfs, and a major storm flow of 56.4 cfs. (Refer to Hydrologic
calculations, Appendix 1). Flows in Basin P2 will be conveyed by street, curb,
and gutter to proposed inlets, and then by storm sewer to the on -site wetland A.
Basin P
Basin P3 consists of 44.22 acres located in the center of the site. The basin will
be developed as 28 multi -family townhomes on 4.8 acres, the rear yards of 21
single family residences surrounding the wetland, and includes approximately
27.8 acres containing the wetland open space. The basin is proposed to have a
2 year runoff coefficient of 0.25, and a 100 year coefficient of 0.45. The flows
generated in the developed portions of basin P3 will be directed by proposed
curb and gutter to the wetland. The calculated 2 year flow generated by Basin
P3 is 22.9 cfs; the 100 year storm generates 137.3 cfs.
Basin P4
Basin P4 is the 13.73 acres bordering the north and eastern sides of the wetland
(Pond A). This basin is proposed to be fully developed with single family
residences and landscaped open space. The basin is calculated to have a 2 year
runoff coefficient of 0.45. Flows from basin P4 will surface flow via proposed
curb and gutter to proposed inlets and then by storm sewer to the
wetland/detention pond (Pond A). Basin P4 will generate 9.3 cfs. in the minor
storm, and 38.7 cfs in the major event.
Basin P5
Basin P5 is the 2.84 acres across the drainage divide at the northwest corner of
the site. The present site plan shows minor development of this small parcel.
Drainage from Basin P5 presently sheet flows to the northwest.
The 100 year peak on -site developed design flows to Richard's Lake (undetained
from 25.40 acres) from Basin P1 is 85.3 cfs, the 2 year storm generates 22.3 cfs.
The on -site developed flows to the wetland/detention pond area 43.7 cfs for the
minor storm, and 187.8 cfs for the major storm event. The wetland collects
drainage from 78.34 acres with the design presented here. Detailed summary of
the minor and major storm rates are provided in Appendix 1. Also included are
the relevant calculations for the off -site flows.
Merrick & Company Page 11
hINFRA\15011782TRELIM1 WMNDRNG.N15
Hoffman P.U.D. Preliminary Drainage Report
feature. Disturbance of this off -site wetland will be minimal since improvements
will be at elevations above the established wetland.
2. Specific Details
The site has been divided into 5 primary on -site basins and 2 off -site basins.
(See included Preliminary Drainage Plan). Primary basins are designated with a
P, the off -site basins are labeled with OS. For the purpose of calculating gutter
and street flows and inlet capacities, developed sub -basins were delineated and
designated with the primary basin name followed by a letter (A, B, C, etc as
P1 A, etc.).
MEMIall
Basin P1 consists of the southwestern part of the site adjacent to Richard's Lake.
Basin P1 contains 25.40 acres and is proposed for development as single family
residences, landscaped lots, and streets. The site plan shows slopes to the
southwest with sheetflow off the rear yards of the lots bordering Richard's Lake.
Basin P1 includes a small basin (2 acres) of rear yards along the west property
line which will contain a proposed swale to convey undetained flows to the south
directly into Richard's Lake. The remainder of the basin discharges to Richard's
Lake by sheet flow off year yards, or by storm sewer pipe. All of the flows
generated in basin P1 will enter Richard's Lake undetained. The proposed 2 year
runoff coefficient for Basin P1 is 0.45. The basin is anticipated to create a
developed peak flow for the minor storm of 22.3 cfs and 85.3 cfs for the major
storm.
Developed street flow in Basin P1 will be directed by grading to proposed curb
and gutter and then flow southwest to a proposed curb inlet which discharges to
Richard's Lake. Preliminary inlet and pipe sizing is presented here, with the final
design to be presented with the final drainage plan. The inlet will be sized to
maintain gutter flow within the allowable street capacity for the 2 and 100 year
storms.
Basin P2
Basin P2 consists of the northwestern quarter of the site. Basin P2 contains
17.41 acres and is proposed to be fully developed with residences and
landscaped open space. With the highest density proposed residential
development, the basin is proposed to have a 2 year runoff coefficient of 0.45,
and a 100 year coefficient of 0.60. Flows from basin P2 will surface flow in
proposed curb and gutter to proposed inlets and storm sewers and swales to the
Merrick & Company Page 10
IAINFRA\15011782\PRELIM 1 \HFMN DRNG. N 15
Hoffman P.U.D. Preliminary Drainage Report
volume requirement was determined using a storage inflow and
release hydrograph. The detention volume provided was
determined by planimeter of the proposed detention pond area in
Tract A. The available volume was calculated as that above the
elevation of the existing wetland average water level.
Spreadsheets for both calculations are included in the Appendix.
d. The Initial (Minor) storm was calculated using the 2-year recurrence
interval for Residential Land Use as required by the City of Fort
Collins' Design Criteria. The Major storm was calculated using the
100-year recurrence interval.
3. Hydraulic Criteria
a. The storm sewer pipe hydraulic capacities used for the system
design are as outlined in UD&FCD's Drainage Criteria Manual
Volume 2. Swale capacities and design depths were calculated
using Manning's equation and follow procedures referred to in
UD&FCD's Drainage Criteria Manual. Inlet capacities will be
calculated following the City of Fort Collins Storm Drainage Design
Criteria and follow procedures referred to in UD&FCD's
Drainage Criteria Manual.
D. DRAINAGE PLAN
1. General Conceit
This drainage design is based on the 2-year recurrence interval storm for the
initial storm, and the 100-year recurrence interval storm for the major storm.
Proposed site improvements will direct the off -site flows through the site by
means of curb and gutter, stormsewers, and graded swales. The developed on -
site flows will be directed away from the houses and into the streets or swales
where they will flow to the proposed wetland/detention pond located in the center
of the site or to Richard's Lake. Disturbance and grading around Richard's Lake
and its existing wetlands will be avoided. Disturbance and grading of the on -site
wetlands (Pond A) will be the minimum necessary to provide the needed
detention outlet structure and emergency spillway proposed for the Hoffman
P.U.D. detention pond. Detention releases will meet the restrictive criteria of the
owners of Richard's Lake, and will include a water quality outlet.
Provision for pass through of any discharges from off -site wetland B in the
unlikely event it overflows will be made. Improvement to the existing off -site
Pond B/wetland outlet will be made in cooperation with the owners of this off -site
■ Merrick & Company Page 9
IAINFRA\75011782\PRELIM? V iFMNDRNG.N15
Hoffman P.U.D. Preliminary Drainage Report
evaporation and percolation to the groundwater table and no flows enter the
Hoffman P.U.D.
C. DESIGN CRITERIA
1. References and Regulations
a. "Richard's Lake Subdivision, First Filing Preliminary Drainage
Plan", by Downing -Leach 10/86, was reviewed and utilized to
assess off -site contributions to the proposed Hoffman P.U.D. site.
No previous drainage studies for the project site have been
prepared per available information. No drainage study for the
adjacent property to the north, was available at the time this
preliminary report was prepared.
b. This report and the analysis and design of the storm drainage
facilities for the Hoffman P.U.D. site are based upon regulations
outlined in the City of Fort Collins "Storm Drainage Design Criteria
and Construction Standards Manual" May, 1984 and Urban
Drainage and Flood Control District's "Urban Storm Drainage
Criteria Manual", 1989.
C. The Dry Creek Master Plan by Gingery Associates, Inc., 1/80 was
reviewed for this preliminary report.
2. Hydrologic Criteria'
a. The design rainfalls were determined using the "Rainfall - Intensity
- Frequency for City of Fort Collins, Colorado" (Graph included in
Appendix 1) from the City's Storm Drainage Design Criteria
Manual.
b. The Rational Method was used for calculation of storm runoff
associated with this development. Overland flow time of
concentrations were calculated based on the "Overland Time Of
Flow Curves" (included in Appendix 1) from UD&FCD's Drainage
Criteria Manual. Travel times for channelized flow were calculated
using the "Estimate Of Average Flow Velocity" (included in
Appendix A) also from the UD&FCD's Criteria Manual.
C. The City of Fort Collins Criteria specifies detention of the 100 year
(major) storm and release at the 2 year historic rate. The detention
Merrick & Company Page 8
IAIN FRA\150117821P RELIM 1 \H FMNDRNG. N15
Hoffman P.U.D. Preliminary Drainage Report
drainage divide between the area draining northwestward toward wetland pond
A and a gentle eastward sloping pasture area off -site towards the Master
Planned Richard's Lake P.U.D. property to the southeast. Off -site flows enter
Historic Basin H4 along the east property line as sheet flow from the
Serramonte Highland Subdivision. A portion of these off -site flows are directed
to the southeast across the Richard's Lake P.U.D. property, and a portion are
directed by existing slopes to the wetland pond A. Basin H4 generates a 2 year
storm flow of 2 cfs and a 100 year storm flow of 11.9 cfs and passes through
respectively 0.2 cfs and 0.5 cfs of off -site flows.
Historic basin OH2
Off -site historic basin OH2 is defined by the drainage divide caused by Douglas
road on the north and County Road 13 on the west and includes the residences,
farmland and oil field area to the north of the proposed development. Basin OH2
is somewhat developed with approximately 10 homes, barns and oil field sheds,
tanks, and graded dirt roads. Basin OH2 covers 50.2 acres. Runoff generated
in basin OH2 drains internally by sheet flow to an existing wetland/pond (Pond
B). Under normal conditions, the wetland/pond drains by percolation to the
water table and evapo-transpiration and does not discharge any flows off -site.
During extremely high water events, a poorly graded channel on the southwest
side of wetland/pond B would allow it to discharge to the Hoffman P.U.D. on -site
wetland, Pond A. These high water releases could only occur when water levels
in Pond B exceed 5110 feet elevation. The existing available storage volume in
the Basin OH2 wetland/retention pond is 23.4 acre-feet at the 5110 foot contour,
which is 7 times greater than the 100 year historic runoff of 3.2 acre feet.
Historic basin OS1
' Off -site historic basin OS1 is defined by a drainage divide in the landscaped
yards of the Serramonte Highlands subdivision. Basin OS1 is developed with
approximately 3 homes with landscaped yards. Runoff generated in basin OS1
drains by sheetflow to the southwest. This off -site runoff enters the Hoffman
P.U.D. site along the northeast property line and eventually flows to the existing
on-site/wetland/pond (Pond A).
3. Outfalls Downstream From Properly
The southern 40% of the existing site drains by overland flow and swale to the
south into Richard's Lake. The northern 60% of the existing site drains to the
existing on -site wetland -pond area which will only releases flows to Richard's
Lake during extremely high water events. Off -site flows in basin OH2 presently
flow to wetland/pond B. Under normal conditions, this wetland drains by
' Merrick & Company Page 7
I:IINFRA%150117821PRELIMI W FMNDRNG.N15
Hoffman P.U.D. Preliminary Drainage Report
western boundary of Historic basin H1. County Road 13 is superelevated at a
point about 1200 feet south of Douglas Road. This superelevation causes a
drainage divide at the west turn in the road, and prevents the crossing of flows
generated west and north of the road. An existing roadside swale collects flow
from the east 1/2 of County Road 13. This swale discharges into a pasture
located on the west side of the site. The County Road 13 swale collects the
gravel road runoff from an area of approximately 0.33 acres. Basin H1 is defined
on the south west by the water level of Richard's Lake. The northeast flank of
the basin boundary is created by an existing topographic divide which separates
Richard's Lake from the on -site low, wetlands/pond area to the north (Pond A).
Total flows generated by Historic Basin H1 are 10.2cfs and 60 cfs for the 2 year
and 100 year storms respectively.
Historic basin H2
Historic basin H2 comprises the majority of the site and includes the central,
closed basin area which drains internally to the existing wetland/pond (Pond A).
Basin H2 covers 59.4 acres. This sub -basin presently sheet flows to the
wetland. Under normal conditions, the wetland/pond drains by percolation to the
water table and evapo-transpiration. During high water events, a graded
channel on the south side of the wetland allows it to discharge to the southwest
to Richard's Lake. These high water releases occur when water levels in Pond
A exceed 5089.5 feet elevation. In like manner, Pond B located off -site to the
north, overflows through a graded channel into Pond A during high water events
when water levels exceed 5110 feet in the off -site pond (Pond B).
Historic Basin H2 generates 16.6 cfs in the 2 year event; and 97 cfs during the
major storm event.
Historic basin H3
Historic basin H3 is a small (2.9 acre) area along the southeast property line of
the site where existing natural slopes direct flows towards the Master Planned
Richards Lake P.U.D. property to the southeast. These flows are directed to the
southwest on that property and eventually reach Richard's Lake. The basin
generates a 100 year storm flow of 3.9 cfs.
Historic basin H4
Historic basin H4 is a small (5.6 acre) on -site area along the east property line of
the site where the topographic high caused by the oil service road defines a
Merrick & Company Page 6
IAIN FRA115011782XPRELIM 1 \HFMNDRNG. N 15
Hoffman P.U.D. Preliminary Drainage Report
The proposed project will be impacted by two off -site sub -basins.
a. Off -site basin OS1, located immediately to the east of the central part of
the proposed Hoffman P.U.D. site is presently developed as single family
houses and landscaping at a density of 0.5 units per acre. OS1 is 1.6
acres of the Serramonte Highlands Subdivision. OS1 is calculated to
have a developed 2 year runoff coefficient of 0.25, and a 100 year runoff
coefficient of 0.45. The existing 100 year peak flow to the Hoffman
P.U.D. site from Basin OS1 is calculated to be 4.6 cfs. This runoff flows to
the southwest across an existing dirt track and enters the site along the
east. An existing poorly defined swale conveys these flows to the
southwest to the wetlands (Pond A).
b. Off -site basin OS2, 1.4 acres, is presently developed as farm yard and oil
service yard with some small sheds, and small outbuildings. The 100
year peak flow from OS2 is calculated to be 4.7 cfs. The flows rom this
off -site basin currently enter the north side of the proposed Hoffman
P.U.D. site as sheet flow and drain to the existing on -site wetland pond
(Pond A).
C. Off -site Historical OH2 contains 50.2 acres of farmland, residences, farm
buildings, and oil field buildings located to the northeast of the proposed
Hoffman P.U.D. site. Under normal conditions this basin does not
discharge runoff to the south onto the proposed site. Basin OH2 has an
existing wetland/pond with an average water level of 5108 feet elevation.
At 5108 feet, the north, off -site wetland covers 6.7 acres. The highest
closing contour for the 15.67 acre closed area containing this
wetland/pond is 5111 feet.
d. There are no major drainage ways on the proposed development site.
2. Drainage Patterns Through Property
The site can be divided into four historic sub -basins and 2 off -site historic basins.
On -site Historic basins are designated with an H, the off -site historic basins are
labeled with OH. (See Figure 1, Appendix 1- Historic Basin Map).
Historic Basin H1
Historic Basin H1 contains 40.6 acres of pasture sloping to the south and
southwest. The basin presently drains to Richard's Lake. The basin is defined
on the north by a natural high point located about 250 feet south of the
intersection of County Road 13 and Douglas Road. County Road 13 defines the
Merrick & Company Page 5
I:\INFRA\15011782\PRELIMI\HFMNDRNG. NI
Hoffman P.U.D. Preliminary Drainage Report
B. HISTORIC DRAINAGE
1. Overall Drainage Basin Description
The site is located in the major historic Dry Creek Drainage Basin.
The portion of the major drainage basin in which this project is located is
presently partially developed. Major basin drainage is generally from northeast to,
southwest and then southeast with major defined man-made channels fed by
sheet runoff and local irrigation and roadside ditches. The slopes vary across
the site from 12.6% to as low as 0.5%, with an average of approximately 4
percent. Elevations range from 5131 on the north to 5062.5 feet which is the
average water level in Richard's Lake. Present zoning for the site calls for
residential development. Jurisdictions in the drainage basin include
unincorporated Larimer County, and the City of Fort Collins. The site lies entirely
within the City of Fort Collins.
The existing tributary sub -basin in which the site lies is defined by County Road
11 to the east, Richard's Lake on the south, County Road 13 on the west, and
Douglas Road (County Road 54) on the north. Runoff from landscaped rear
yards of the Serramonte Highlands Subdivision to the east enter the site as
sheetflow along a portion of the eastern border of the property. Douglas Road
on the north forms a man-made drainage divide preventing southward flows from
the farmland north of the road. On the west side of the property, the eastern half
of County Road 13 is superelevated where it turns west; this superelevation
prevents flows from crossing from the north and west. A ditch along the east
side of County Road 13 drains into the site at the location of the west bend in the
road. No flows enter the site from the south.
The site slopes to the south west at an average of 4%. An oil field service road
located at the southeast corner of the site acts as a drainage divide diverting
runoff from a small area off -site to the southeast and directing the majority of the
flows generated in the southeast corner of the site to the on -site wetland (Refer
to Historic basin map, Basin H2). The existing drainage pattern of the majority of
the site is surface flow draining to the existing on -site wetland area. The historic
on -site 100 year storm flow to this wetland is calculated to be 97 cfs (2.4 cfs/ac).
Approximately 40 acres on the south side of the site drain southwestward into
Richard's Lake (Refer to Historic Drainage Basin Map, Basin 111). The historic
Ion -site 100 year storm flow to Richard's Lake is calculated to be 68.4 cfs (1.7
cfs/ac) See Appendix 1 - Historic Runoff calculation).
I Merrick & Company Page 4
1AIN FRA\15011782\PRELIM 1 \HFMNDRN G. N 15
I
Preliminary Drainage Report
F
C � W
OR
C �
17 •i/�•/ •c�CA27E� ••.
.............
••-••••• .U�....
Richard
Lake
HOFFMAN P.U.D. Site
cr
MA Da
A/
............. rndenmeie L NMEfPr.
T`1.
Lake-
n ~ C DER
ry RTLEAFFEPL
c Z
r$ ............
W _
Scale: 111=1/2 Mile
STA WY
LT
Vicinity Map
Figure 1
Merrick & Company Page 3
Hoffman P.U.D. Preliminary Drainage Report
Phase 2 - 98 units of medium -density (7.84 D.U. per Acre) single family
residential use (please refer to Parcel A on the Site plan).
Phase 3 - low -density (29 units at 3.05 D.U. per acre) single residential
use (Parcel C).
Phase 4 - 25 single family residential units at low -density (1.26 D.U. per
Acre) adjacent to the Serremonte Highlands subdivision (Parcel D).
Phase 5 - medium -density (28 units, 5.71 D.U. per acre) patio homes
located at the southeast end of the wetlands (Parcel E).
Phase 6 - planned as medium -density 34 patio home units, 4.15 D.U. per
acre, (please refer to Parcel B on the Site plan).
The on -site wetlands will be set aside as a water quality open space. The permanent
wetland presently covers approximately 12.6 acres within a proposed 23.5 acre tract
(Tract A). The entire project gross density is proposed at 2 units per acre, 3.15 units
per net acre. The developed site will have a 2 year composite runoff coefficient of 0.39.
Merrick & Company Page 2
I:\INFRA\15011782\PRELIM I \HFMNDRNG. N I5
Hoffman P.U.D. Preliminary Drainage Report
A. INTRODUCTION
This report analyzes the drainage impacts of the proposed Hoffman P.U.D.
development. The design considerations for this project are to provide a
' drainage system capable of handling runoff generated on the proposed site as
well as convey the existing runoff from off -site tributary areas. In order to
accomplish this, the off -site and on -site drainage areas are studied as they are
impacted by the proposed project. This report was prepared following
guidelines and criteria presented in the "Fort Collins Storm Drainage Design
' Criteria and Construction Standards", 1984 and the "Urban Storm Drainage
Criteria Manual" (USDCM), 1992.
' 1. Location
The project is located in the West half of Section 30, Township 8 North, Range
68 West, of the Sixth Principal Meridian, City of Fort Collins, Larimer County,
' State of Colorado. (see Figure 1, page 2 - Vicinity Map) The project site is
bordered on the north by Douglas Road and single family homes located on
lands in unincorporated Larimer County and on the southwest by Richards Lake.
County Road 13 borders the site on the northwest. Developed single family
homesites of the Serramonte Highlands subdivision, unincorporated Larimer
' county border the northeast property line, the undeveloped Richard's Lake
Master Plan area lies adjacent the site to the southeast.
t2. Property Description
' The project site contains 105.5 gross acres of which approximately 31.141 acres
are to be drainage tracts and open space and approximately 74.415 acres will be
developed as streets, and as single and multi -family residences. The site has
' previously been farmed and used as grazing pasture. It contains 2 producing oil
wells of the Ft. Collins Field which produces from the Cretaceous Muddy
sandstone. The existing vegetation consists of mainly native grasses and
weeds. The soils on -site are silty to sandy loam type. The north part of the site
has moderate wind and rain erodability, the portion of the site adjacent to the
lake has low erodibility for wind and rainfall. The site slopes to the southwest at
an average of 4% with slopes ranging for 0.5% to 12% on the site. The south
' half of the site presently drains to Richard's Lake, the northern portion of the
Hoffman P.U.D. site drains to an existing on -site closed topographic low spot
t which retains water and supports a wetland. The proposed development will
occur in 6 phases:
Phase 1 - low -density (30 units, 2.28 D.U. per Acre) single family
residential (please refer to Parcel F on the Site plan).
Merrick & Company Page 1
111N FRA\15011782\PRELIM 1 \H FMN DRNG. N 15
Hoffman P.U.D.
Preliminary Drainage Report
TABLE OF CONTENTS
PAGE
INTRODUCTION..........................................................................................................................
1
GENERAL LOCATION AND DESCRIPTION.............................................................................. 1
Location........................................................................................................................................
1
PropertyDescription.....................................................................................................................
1
VicinityMap............................................................................................................................:1,
3
B. HISTORIC DRAINAGE....................................................................................................
4
Overall Basin Descriptions................................................................................................
4
Drainage Patterns Through the Property ..........................................................................
Outfalls Downstream from the Property
5
7
............................................................................
C. DESIGN CRITERIA...........................................................................................................
8
References and Regulations......................................................................................:......
8
HydrologicCriteria.............................................................................................................
8
HydraulicCriteria...............................................................................................................
9
D. DRAINAGE PLAN.............................................................................................................
9
GeneralConcept...............................................................................................................
9
SpecificDetails................................................................................................................
10
E. EROSION CONTROL PLAN..........................................................................................
13
GeneralConcept.............................................................................................................
13
SpecificDetail......:...........................................................................................................
13
F. CONCLUSIONS..............................................................................................................14
Compliance with Standards .................................
:.......................................................... 14
Summary of Drainage Concept.......................................................................................
14
Summary of Erosion Control Concept............................................................................
14
G. REFERENCES................................................................................................................16
H. APPENDICES
Appendix 1- Hydrologic Calculations.............................................................................. 17
Appendix 2 - Hydraulic Calculations............................................................................... 18
Appendix 3 - Erosion Control Calculations..................................................................... 19
Merrick & Company Page 2
1AINFRA11 5011782%PRELIM1IHFMNORNG. N15
Hoffman P.U.D.
Preliminary Drainage Report
Prepared for:
Hoffman P.U.D.
Richard's Lake Development
1412 Richard's Lake Road
Fort Collins, Colorado 80524
Contact: Mr. Bill Yunker or Mr. Craig Hash
(303) 484-1921
Prepared by:
MERRICK & COMPANY
2450 S. Peoria Street
Aurora, Colorado 80014
Contact: Mr. Glen A. Tulk, PE
Telephone: (303)-751-0741
Merrick & Company Job Number: 15011782
Merrick & Company Page i
LAIN FRAY15011782\PRELIM? \H FMN DRNG. N 15
HOFFMAN P.U.D.
PRELIMINARY DRAINAGE
AND EROSION CONTROL REPORT
August 3, 1995
Revised November 14, 1995
Presented to
The City of Fort Collins
Prepared by
Merrick & Company
❑00
EIOEI MERRICK
cooEngineers 6 Architects