HomeMy WebLinkAbout217 W. TRILBY RD. - PDP - PDP120006 - REPORTS - DRAINAGE REPORT (3)PRELIMINARY DRAINAGE REPORT
For: 217 West Trilby Road
Fort Collins, CO 80526
Prepared For:
AFFORDABLE ROOFING, LLC
217 West Trilby Road
Fort Collins, CO 80526
March 20, 2013
Project No. 39685.00
Prepared By:
JR Engineering, LLC
2900 South College Avenue, Suite 3D
Fort Collins, CO 80525
Page i
TABLE OF CONTENTS
Table of Contents ............................................................................................................................ i
Engineer’s Certification Block......................................................................................................... ii
General Description and Location ................................................................................................. 1
Site Description and Location ..................................................................................................... 1
Site Soils...................................................................................................................................... 1
Storm Drainage Criteria ................................................................................................................. 2
Hydrologic Criteria...................................................................................................................... 2
Hydraulic Criteria........................................................................................................................ 2
Drainage Basins, Historic and Proposed Runoff............................................................................. 3
Historic Basins............................................................................................................................. 3
Developed Basins ........................................................................................................................ 4
Stormwater Pollution Prevention................................................................................................... 5
Temporary Erosion Control......................................................................................................... 5
Permanent Erosion Control ......................................................................................................... 5
Summary & Conclusions................................................................................................................. 5
References....................................................................................................................................... 6
Appendices...................................................................................................................................... 7
Site & Soil Descriptions and Characteristics........................................................................... A-1
Proposed Rational & Detention Pond Calculations................................................................. A-2
Sleeve w/ Drainage Plans……………………………………………………………………Sleeve
Page ii
ENGINEER’S CERTIFICATION BLOCK
I hereby certify that this Preliminary Drainage Report for 217 W Trilby, Fort Collins, CO 80526
was prepared by me (or under my direct supervision) for JR Engineering, LLC and the owners
thereof and meets or exceeds the criteria of the City of Fort Collins Stormwater Design
Standards.
________________________________________
Timothy J. Halopoff, PE
Registered Professional Engineer
State of Colorado No. 37953
Page 1
GENERAL DESCRIPTION AND LOCATION
SITE DESCRIPTION AND LOCATION
217 W. Trilby is located in the NORTHEAST QUARTER OF SECTION 14, TOWNSHIP 6
NORTH, RANGE 69 WEST OF THE 6TH P.M., CITY OF FORT COLLINS, COUNTY OF
LARIMER, STATE OF COLORADO. More specifically, 217 W. Trilby is a 2.485 ac. piece of
property that is currently used as a residence and a large open yard, but is zoned LMN and will
support office uses on the east & west side of the property once subdivided.
The future use of the site (2.485 ac.) entails subdividing the existing lot into 4 lots and
dedicating additional Trilby Right-of-Way (0.083 ac.). The existing residence will remain on the
new Lot 1, consisting of 0.344 ac. There will also be 3 lots created for the purpose of
constructing 3 office buildings complimented with garage spaces. Lot 2 (0.323 ac.), will be
located on the east side of the North Louden Ditch and will support a new office development
with a footprint of 4,500 s.f. The remainder of the property on the East side of the ditch will
become Lot 3 (0.520 ac.) and will support an office containing another 4,500 s.f. footprint. On
the west side of the North Louden Ditch/Drainage Swale will be Lot 4. Lot 4 consists of 1.215
ac. and will support an office structure with a garage in the back; having a foundation footprint
of 6,000 s.f.
The site generally slopes from the southwest to the northeast with slopes ranging between
2.8% and 11.0%. The property is generally covered with sparse native grasses. The North
Louden Ditch traverses the property from the southeast corner to the northwest corner,
flowing northwest and ultimately flowing into a corrugated metal pipe and crossing W. Trilby.
The westerly portion of the property (proposed Lot 4) receives a small area of off-site flow
(2.450 ac.) from the southwest and together with the on-site Lot 4 flows, accumulate a total
area of 2.937 ac., which currently already drains to two points on the ditch. On the east side of
the ditch, which is where the residence is located, 0.740 ac. drains east to two existing sumps
on the east property line. Ultimately, the historic flow from the residence and the north
portion of Lot 2, flow to the northeastern corner of the property and into the West Trilby
Road right-of-way. The Southeast portion of the property, southern Lot 2 and Lot 3 drain to
the pond on the east side neighbor. The small portions of land on the extreme north of both
sides of the ditch drain to the south roadside borrow ditch on Trilby.
SITE SOILS
The 217 West Trilby site consist of loams including: Fort Collins Loam, Thedalund Loam and
Wiley Silt Loam. Fort Collins Loam (Classification #36), having 3-5% slopes, generally has a
combined surface layer thickness of approx. 20 inches. Runoff is moderate, and the hazards of
wind and water erosion are moderate. Thedalund Loam (Classification #108), having 3-9%
slopes, occasionally has a surface layer of clay loam and small areas of Kim, Cushman, and
Midway soils. Runoff is medium, and the hazard of erosion is moderate. Wiley Silt Loam
(Classification #119), having 3-5% slopes, occasionally has a subsurface layer of silt loam or
loam. Runoff is medium, and the hazard of erosion is medium.
All of the loams on this site are classified as being in the Type B hydrologic group. According
to USDA/Natural Resource Conservation Group, the earth on-site has a moderate infiltration
rate when thoroughly wet. These soils consist chiefly of moderately deep or deep, moderately
well drained or well drained soils that have a moderately fine texture to moderately coarse
texture. These soils also have a moderate rate of soils transmission.
Supporting figures can be found in the Appendix, page A-1.
Page 2
STORM DRAINAGE CRITERIA
This preliminary report was prepared to meet or exceed the City of Fort Collins storm water
criteria. The City of Fort Collin’s Storm Drainage Design Criteria and Construction Manual
(with all current 2011 Revisions)(FCSDDCCM) and the Urban Drainage Flood Control
District’s (UDFCD) Drainage Criteria Manual (USDCM) Volumes 1, 2 and 3 were referenced as
guidelines for this design.
HYDROLOGIC CRITERIA
The rational method was performed to calculate the peak runoff rates for each basin.
Weighted percent imperviousness and weighted runoff coefficients were calculated for each
basin using USDCM Tables RO-3 and RO-5 based on Natural Resources Conservation Service
(NRCS) Type B hydrologic soil classification and surface characteristics of each basin. The time
of concentration was calculated using USDCM Equation RO-3 and the intensity was calculated
using the corresponding storm rainfall depth and USDCM Equation RA-3. To account for the
City of Fort Collins’ IDF Curve, Coefficient 3 of the UDFCD’s intensity formula was adjusted to
0.7867. The City of Fort Collins area has 2-yr 1-hr rainfall depth of 0.82 inches and a 100-yr 1-
hr rainfall depth of 2.86 inches. These depths do account for the 1997 adjusted rainfall depths.
HYDRAULIC CRITERIA
The hydraulic calculations provided for in this report are limited to Detention Pond
calculations. This report allows us to prove that the system of proposed detention ponds is
able to reduce the 100-year developed condition flows to the 2-year historical release rates;
thereby satisfying the City’s requirements. Once this Preliminary Report is accepted and
approved, hydraulic calculations for storm drain pipe, water quality structures and rip-rap will
be provided within the Final Drainage Report for 217 West Trilby.
The ultimate 217 West Trilby storm drainage system will be designed to convey the minor and
major storm events through the property with the proposed inlets, storm sewer pipes, and
swales for the flows being calculated by this preliminary report. Per the requirements provided
by the City of Fort Collins Storm Drainage Design Criteria and Construction Manual, all inlets
and storm pipes will be designed to convey the 100-year storm flows. During the major storm
event, the developed condition 100-yr storm, the accumulated water depth will be held to a
maximum depth with a minimum of 1’ freeboard to the top of all building foundations.
Additionally, the detention ponds will be designed to detain all onsite flows and maintain 1’ of
freeboard. Since this report shows that there is no change to any of the offsite flows, they will
pass the 100-year offsite flows over the weirs.
All swale and pipe outlets will be protected with turf mat or riprap; whichever the City
requires. Storm sewer pipe outlets will be protected using the requirements set by the
USDCM for the protection of downstream conveyance channels and culverts.
Page 3
DRAINAGE BASINS, HISTORIC AND PROPOSED RUNOFF
HISTORIC BASINS
217 West Trilby is an infill development piece of property. As such, the existing and proposed
drainage basins will need to follow the generally accepted principle of releasing the 100-yr.
developed condition storm flow at the same location and magnitude as the 2-yr. historic
condition flow rate. There are offsite areas that flow across the proposed development site.
Referring to the “Existing Drainage Plan” (Appx. - Sht. 1 of 2, included in the sleeve at back),
the following describes the existing condition drainage basins.
Existing Offsite Drainage Basins (OF):
Sub-basin OF1 consist of 1.52 acres that flows northeasterly, across private property
and enters the onsite development property at the northwest corner; where the flow is
conveyed by Basin ON1 to the North Louden Ditch irrigation channel.
Sub-basin OF2 consist of 0.93 acres that flows easterly, across private property and
enters the onsite development property at the southwest corner; where the flow is
conveyed by Basin ON2 to the North Louden Ditch irrigation channel.
Sub-basin OF3 consist of 2.20 acres that flows easterly, across private property and
the storm drainage travels on the south curb and gutter/southerly roadside ditch of
Trilby Road. Along this path, the drainage from Sub-basin OF3 stays north of the
subject development property and for this reason, is excluded from the
hydraulic/detention pond calculations.
Sub-basin OF4 consists of 0.56 acres that flows northeasterly, across private property
and the drainage travels along the south side of the subject property. Along this path,
the drainage from Sub-basin OF4 stays north of the subject development property and
for this reason, is excluded from the hydraulic/detention pond calculations.
Existing Onsite Drainage Basins (ON):
Sub-basin ON1 consists of 0.43 acres that flows easterly, across the northwest ¼ of
the development property, conveying flow from Sub-Basin OF1 and its own flow,
making the total area flowing to the North Louden Ditch at the bottom of ON1 1.95 ac.
Sub-basin ON2 consists of 0.92 acres that flows northeasterly, across the southwest
¼ of the development property, conveying flow from Sub-Basin OF2 and its own flow,
making the total area flowing to the North Louden Ditch at the bottom of ON2 1.85 ac.
Sub-basin ON3 consists of 0.55 acres that flows easterly, across the southeast ¼ of
the development property. This Sub-basin drains to a sump area located midway of the
easterly property line and flows directly to the neighboring property, where it makes its
way back to Trilby’s south borrow ditch.
Sub-basin ON4 consists of 0.23 acres that flows easterly, across the northeast ¼ of
the development property. This Sub-basin drains to a sump area located at the top of
the easterly property line and flows directly to the neighboring property, where it
makes its way to the south borrow ditch at Trilby.
Page 4
DEVELOPED BASINS
The proposed sub-basins have been designed to mimic the historic basins, both on and off-site.
We have achieved proposed sub-basins that are close to the same acreages on-site and have
intentionally designed our sub-basins such that they are exactly the same in the off-site
condition. This has enabled us to simplify the hydrology calculations, thereby simplifying the
detention pond calculations as well. Furthermore, because we have been able to keep the off-
site sub-basins the same for the existing and proposed conditions, we were able to avoid re-
calculating and reiterating the information provided for off-site in the prior section. Referring
to the “Proposed Drainage Plan” (Appx. - Sht. 2 of 2, included in the sleeve at back), the
following describes the proposed condition on-site drainage basins.
Proposed Onsite Drainage Basins (ON):
Sub-basin ON1 consists of 0.40 acres and flows from the pick-up point where Off-site
Sub-basin OF1 enters the site in the northwest. From there, we routed the flow
through a series of two detention ponds connected by a small storm pipe. The two
ponds act as one detention pond. Following the water quality/detention structure, the
flows are released to flow into the relocated North Louden Ditch.
Sub-basin ON2 consists of 0.54 acres and flows from the pick-up point where Off-site
Sub-basin OF2 enters the site in the southwest. From there, we routed the flow down
the back of the parking lot retaining wall in a trickle pan. That pan conveys flows to the
detention pond in the rear of the Lot 3 building. This detention pond utilizes retaining
walls and provides adequate detention.
Sub-basin ON3 consists of 0.70 acres and flows from the high points on the bermed
areas east of the relocated North Louden Ditch and will flow directly into the new
native/low water tolerant grassed detention pond located in the rear of the proposed
building on future Lot 3. From there, we routed the flow down a trickle pan and
through the water quality/detention structure. Following the water quality/detention
structure, the flows are released to flow over a rip-rap spreading pad and directed
toward the same swale as the existing condition.
Sub-basin ON4 consists of 0.33 acres and flows from the high points on the bermed
areas east of the relocated North Louden Ditch and into inlet drains toward the north
east corner of the property. Once the flows are conveyed to the northeast corner,
they will be released into a detention pond that may need to utilize retaining walls to
achieve the appropriate detention volume. We have shown that the small volume
necessary for detention is kept below 1’ of ponding in any part of the parking area.
In conclusion, the detention pond elevations created by the waters calculated in the
Appendices will be discussed in the following section entitled “Summary and
Conclusions”.
Page 5
STORMWATER POLLUTION PREVENTION
TEMPORARY EROSION CONTROL
A temporary erosion control plan is to be implemented for the site during construction.
Temporary erosion control measures include, but are not limited to, slope and swale
protection provided by the use of erosion control wattles/sediment control logs, silt fence
placed around areas of disturbance, construction vehicle tracking pads at entrances, designated
concrete truck washout basins, designated vehicle fueling areas, inlet protection, etc. All
temporary erosion control measures are to be removed after they are deemed unnecessary.
PERMANENT EROSION CONTROL
Permanent erosion control measures include, but are not limited to, the constructed
detention/water quality pond, riprap pads placed for culvert outlet protection, seeding and
mulch placed to enable and established vegetative growth, etc. Long-term maintenance of these
erosion control measures shall be the responsibility of the owner of the property.
A general erosion control plan will be included with the Final Development Plan, Utility Plan set
and in the appendix of the Final Development Plan Drainage Report. Ultimately before
construction of this project, a more detailed erosion control plan will need to be performed
within the Storm Water Management Plan that must be submitted to the State of Colorado and
the City of Fort Collins.
SUMMARY & CONCLUSIONS
To summarize the flow rates and detention pond characteristics, we have shown calculations as
per the City’s requirements for Preliminary Drainage reports in section “A-2, Proposed
Rational and Detention Pond Calculations”. For the required detention pond release rates, we
calculated the 2-year historic on-site flows for contributing basins and reduced that flow by the
area of development contributing to the pond. The remaining areas are left undisturbed. We
then used the modified Detention Pond Calculator “FAA12” (Fater, Judish & Li; 1998) to
calculate the required pond size, to which the volume calculated for Water Quality, was then
added. The weir size was calculated using the 100-year storm flows from both the historic off-
site and developed on-site, which assumes that the outlet structure is completely clogged.
Pond 1 was calculated to need 0.043 acre-feet of volume and 0.044 acre-feet is provided. Pond
2 was calculated to need 0.135 acre-feet and 0.138 acre-feet is provided. Pond 3 was calculated
to need 0.057 acre-feet and 0.068 acre-feet is provided. Pond 4 was calculated to need 0.083
acre-feet and 0.085 acre-feet is provided. All of the weir sizes account for the 100-year flows
that enter the respective detention ponds.
In conclusion, all of the City of Fort Collins hydrology calculations were performed using the
required methods and all of the 217 Trilby Project drainage amenities, meet or exceed the
necessary drainage feature requirements. Furthermore, this preliminary drainage report and
the complimenting Overall Utility Plans show appropriate grading, permanent landscaping,
riprap, and on-site construction facilities (e.g. storm pipes, inlets, etc.) that will be calculated
hydraulically and discussed in the Final Drainage Report and on it’s accompanying Maps.
Page 6
REFERENCES
1. Urban Storm Drainage Criteria Manual (Volumes 1, 2, and 3), Urban Drainage and Flood
Control District, June 2001.
2. Storm Drainage Criteria Manual and Construction Standards, City of Fort Collins Storm
Water Utility, City of Fort Collins, Colorado, Updated January, 1997.
3. “Hydrologic Group Rating for Larimer County Area, Colorado”, USDA-Natural Resources
Conservation Service, National Cooperative Soil Survey. Web Soil Survey URL:
http://websoilsurvey.nrcs.usda.gov. [July 21, 2010]
Page 7
APPENDICES
Site & Soil Descriptions and Characteristics (A-1)
Proposed Rational & Detention Pond Calculations (A-2)
Existing & Proposed Drainage Plans (Sleeve)
Page A-1
SITE & SOIL DESCRIPTIONS AND CHARACTERISTICS
Page A- 2
PROPOSED RATIONAL & DETENTION POND CALCULATIONS