HomeMy WebLinkAboutDrainage Reports - 01/04/2021November 4, 2020
Stormwater Engineering & Development
Review Division
City of Fort Collins Utilities
700 Wood Street
Fort Collins, CO 80521
Reference: Storm Drainage Memorandum
Heatheridge Lakes Condominiums, Building N Renovations Minor Amendment
Fort Collins, Colorado
To Whom It May Concern:
On behalf of Heatheridge Lakes Condominium Association, we provide this storm drainage memorandum
intended to summarize the drainage improvements to Building N and demonstrate the proposed
improvements will more effectively and safely convey the minor and major storm events. This memo will also
describe the location of the existing City floodplain/floodway and show the improvements will not encroach
into these limits.
This project proposes renovations to Building N and includes landscape, grading and drainage improvements.
The proposed drainage improvements intend to better convey stormwater around Building N (and to West
Hourglass Lake) while mitigating the routine flooding of the building frontage and potential inundation of the
building s finish floor elevation (FFE) which has been an issue historically. Sub-surface piping and inlets are
also proposed on the northeast side of the building to help alleviate the routing flooding at the southwest
corner of adjacent Building M. These drainage improvements are shown in the Grading Plan in the Minor
Amendment plan set titled Heatheridge Lakes Building N Landscape Renovations .
The following summarizes the criteria and design details of the proposed drainage improvement for this
project.
Basins and Hydrologic Criteria:
A basin map is included in Appendix A with contributory sub-basins defined and associated flowrates for the
major (100-year) and minor (2-year) design storm events calculated using the Rational Method. These basins
and flowrates are used for design of the drainage elements below. Modeling was performed using Hydraflow
Storm Sewer Extension for AutoDesk Civil 3D, version 2019.2.
Inlets and Piping:
Inlets and piping are proposed as described above and shown in the referenced grading plan. The sub-basin
shown in green results in flow rates of 0.16 cfs and 0.57 cfs for the 2-year and 100-year design events,
respectively. The resulting hydraulic grade lines show that the system will easily handle the minor event and
will even contain most of the major event below grade. Flows are intended to discharge into West Hourglass
Lake. Two grated inlets on MH3 and MH4 have the dual capacity to capture up to 1 cfs given the potential
head over the recessed rims and allows for up to 40% clogging. See Appendix B for hydraulic calculations.
City of Fort Collins Approved Plans
Approved by:
Date:
Dan Mogen
01/04/2021
Storm Drainage Memorandum
Heatheridge Lakes Condominiums, Build N Minor Amendment
Fort Collins, CO
November 4, 2020
Page 2
Building Frontage:
The blue sub-basin is proposed to consist of gravel with inlet and perforated pipe to convey precipitation
flows to the existing 8 pipe on the southwest side of the building that ultimately discharges to West
Hourglass Lake. The potential flooding of this area was analyzed in the event that the inlet and piping ceases
to function. The frontage sidewalk is proposed to be constructed such that the first point to flow from this
area will be through the northeast and southwest breezeways to either side of the building. The 100-year
event was calculated to be 0.06 cfs and modeled through the breezeway cross-section. The water surface
elevation was determined to be below the highest end of the sidewalk and below doorways/FFEs. See
Appendix C for this cross-section.
Northeast Conveyance:
The smallest cross-section was taken on the northeast side of the building to verify that upgradient flows
would not inundate FFEs during the major storm event. The 100-year event was calculated to be 1.89 cfs
based on the magenta colored sub-basin and modeled to show the water surface elevation would be over 1.5
lower than Building N s FFE. See Appendix C for this cross-section.
Floodplain/Floodway
The City designated floodway/floodplain flows though West Hourglass Lake. The shape files and AutoCAD
lines were provided to Sanderson Stewart from the City and input into the grading plan. The elevation was
conservatively taken from the cross-section upstream of the proposed outfall area. A conversion was used of
3.18 to bring from NAVD88 to NGVD29 with a final floodplain/floodway elevation of 5028.76. This line is
proposed to be staked to ensure that construction stays outside and above these limits. See Appendix D for
floodplain and conversion information.
Based on the analysis described above, we believe the intent to mitigate future flooding of Building N s
frontage/FFE, the alleviation of the routine flooding at Building M and provide a better conveyance on the
northeast side of the building will be achieved with these improvements. The City designated
floodplain/floodway is also proposed to be undisturbed.
Please feel free to call me at 970-893-2863 or email me at csonnier@sandersonstewart.com. if you have any
questions regarding this letter. Thank you.
Sincerely,
Charles Sonnier, PE
Senior Engineer/Project Manager
Heatheridge Lakes,Building N Renovations
Fort Collins,Colorado
Basin Map &Rational Calculations
Heatheridge Lakes,Building N Renovations
Fort Collins,Colorado
Inlets/Piping &Hydraulics
Capacity (cfs)
Capacity (cfs)
Heatheridge Lakes,Building N Renovations
Fort Collins,Colorado
Building Frontage and Northeast Conveyances
Heatheridge Lakes,Building N Renovations
Fort Collins,Colorado
Floodplain/Floodway
Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus
DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community²
0 50 10025 Feet
Heather Ridge
Legend
City_BFE
City_XS
City_Floodway
CITY OF FORT COLLINS MEMORANDUM
DATE: NOVEMBER 2013
TO: INTERESTED PARTIES NEEDING VERTICAL CONTROL IN THE CITY OF
FORT COLLINS
FROM: JOHN VON NIEDA PLS, CITY SURVEYOR
RE: UPDATED CITY OF FORT COLLINS VERTICAL CONTROL
The City of Fort Collins Survey Department is pleased to make available to you the expanded
listing of the City of Fort Collins Vertical Control Network.
Generally the vertical control monuments are a 2" aluminum cap set in a solid concrete
structure. The monuments are stamped “City of Fort Collins Benchmark” along with a distinctive
identifying number.
The elevations are given in feet to the nearest one hundredth of a foot (0.01'). The data was
obtained mostly using a Leica NA2002 Digital Level and most recently with a Leica DNA10
Digital Level and Digital Level Rod. The field data was gathered using Leica software. The data
was then transferred into Star*Lev Least Squares Level Net Adjustment software for
adjustment.
PLEASE READ
As a matter of common survey practice, please remember to always check between at
least two vertical control monuments. If you discover an unresolved error in the
published datum, or if a vertical control monument has been destroyed or is going to be
disturbed, please call the City of Fort Collins Survey Department at (970) 221-6605.
As of January 1, 2014, the City of Fort Collins will move to the NAVD 88 datum, requiring
any new projects to be surveyed and designed in this datum. The impetus to make this change
stems from FEMA’s conversion to the NAVD 88 datum, from the increasing convergence of
Surveying and GIS, and from the desire for consistency in Vertical Control with neighboring
municipalities. We recognize that this may cause inconvenience in the short term, and for this
reason we will continue to allow the NGVD 29 unadjusted datum (old “City Datum” – see note
below) for projects where surveying has begun prior to January 1, 2014 for a period of one year.
We have provided both NAVD 88 and old “City Datum” elevations for all benchmarks listed
herein
The City of Fort Collins is not responsible for errors in the “City of Fort Collins Vertical
Control Network” and makes no warranty, expressed or implied, in the accuracy thereof, and
shall not be responsible for damages or injuries suffered by a party as a result of any such
errors, whether such damages or injuries are direct, indirect, incidental or consequential.
Note:
The old “City Datum” had always been based upon “original” NGVD 1929 datum. In 1984 NGS re-leveled and
adjusted the vertical control along the Front Range including Fort Collins. In Fort Collins the adjustment amounted to
the new datum being 0.34' higher. Because Fort Collins had vertical control and flood mapping in place prior to 1984,
we had continued to use the “original” 1929 datum for our basis of vertical control. Therefore, the City’s elevations in
NGVD 29 are 0.34' lower than the published 1929 values on the remaining NGS benchmarks in the City.
Page 2
92-16 NORTH SIDE OF DOUGLAS RD. AT KENYON DR., ON THE
TOP OF AN IRRIGATION STRUCTURE.
NAVD88=5112.58
NGVD29 =5109.42
17-92 300 FEET SOUTH OF THE SOUTHWEST CORNER OF
EAST DRAKE RD. AND TIMBERLINE RD, IN THE TOP OF
AN IRRIGATION STRUCTURE.
NAVD88=4938.70
NGVD29 =4935.51
18-92 NORTH SIDE OF EAST DRAKE RD., EAST OF SILVER
CREEK DR.,ON THE TOP OF A STORM MANHOLE.
NAVD88=4970.15
NGVD29 =4966.96
19-92 SOUTHWEST CORNER OF EAST DRAKE RD. AND LEMAY
AVE , ON TOP OF A SANITARY SEWER PUMP STATION.
NAVD88=4978.40
NGVD29 =4975.21
20-92 NORTHEAST CORNER OF EAST DRAKE RD. AND
STOVER ST., ON A SIGNAL POLE BASE.
NAVD88=4999.12
NGVD29 =4995.94
21-92 NORTHWEST CORNER OF WEST DRAKE RD. AND
RESEARCH BLVD., ON A POWER TOWER BASE.
NAVD88=5042.84
NGVD29 =5039.67
23-92 NORTHEAST CORNER OF WEST DRAKE RD. AND
DUNBAR AVE, ON A POWER TOWER BASE.
NAVD88=5055.98
NGVD29 =5052.80
25-92 NORTH SIDE OF WEST DRAKE RD., ONE- HALF MILE
WEST OF TAFT HILL RD., ON A POWER TOWER BASE.
NAVD88=5124.86
NGVD29 =5121.68
26-92 NORTHEAST CORNER OF WEST DRAKE RD. AND
OVERLAND TRAIL, ON A POWER TOWER BASE.
NAVD88=5148.63
NGVD29 =5145.45
28-92 SOUTHWEST CORNER OF WEST PROSPECT RD. AND
CENTRE AVE. , ON A WATER VALVE PIT.
NAVD88=5010.65
NGVD29 =5007.48
29-92 APPROXIMATE 300 FEET SOUTH OF WEST PROSPECT
RD. AND SHIELDS ST., ON THE NORTH END OF THE
WEST BRIDGE PARAPET WALL.
NAVD88=5025.67
NGVD29 =5022.50
30-92 1600 BLOCK OF WEST PROSPECT RD, IN THE SOUTH -
WEST RETAINING WALL OF THE BRIDGE OVER THE
MERCER CANAL.
NAVD88=5058.22
NGVD29 =5055.04
32-92 NORTHEAST CORNER OF LAKE ST. AND OVERLAND
TRAIL, IN THE EAST HEADWALL OF THE BRIDGE OVER
THE PLEASANT VALLEY AND LAKE CANAL.
NAVD88=5119.42
NGVD29 =5116.24
33-92 SOUTHEAST CORNER OF ROSS DR. AND OVERLAND
TRAIL, IN THE SOUTH END OF A RETAINING WALL.
NAVD88=5143.33
NGVD29 =5140.15
34-92 WEST SIDE OF TAFT HILL RD. NORTH OF STUART ST ,
ON A STORM WATER DIVERSION STRUCTURE.
NAVD88=5088.78
NGVD29 =5085.60
NAVD88=5058.22
NGVD29 =5055.04