HomeMy WebLinkAboutHARMONY & I-25 - ODP - ODP140001 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTOverall Development Plan (ODP)
Drainage Report
for
Harmony and I-25
submitted to:
City of Fort Collins,
Colorado
April 30, 2014
Harmony and I-25 ODP Drainage Report
TABLE OF CONTENTS
Page
1.0 Introduction
1.1 Scope and Purpose ..................................................................................................... 1
1.2 Project Location and Description ................................................................................. 1
1.3 Previous Studies ......................................................................................................... 3
1.4 Limitations of Future Projections ................................................................................. 3
2.0 Historic/Existing Conditions ......................................................................................... 3
3.0 Developed Conditions Plan
3.1 Design Criteria ............................................................................................................. 7
3.2 Low Impact Development ............................................................................................ 8
3.3 Floodplain Management .............................................................................................. 8
4.0 Conclusion ....................................................................................................................... 10
5.0 References ....................................................................................................................... 10
Figures
Figure 1. Vicinity Map ................................................................................................................... 2
Figure 2. Gravel Pit Information .................................................................................................... 4
Figure 3. Outfall to the Poudre River ............................................................................................ 6
Appendices
Appendix : ODP Calculations and Supporting Information
Harmony and I-25 ODP Drainage Report
Page 1
1.0 Introduction
1.1 Scope and Purpose
This report has been developed in accordance with the City of Fort Collins Stormwater Criteria
Manual requirements and complies with City Overall Development Plan (ODP) submittal
requirements. This report presents the results of an ODP level drainage analysis for the first
phase of the ODP at the Harmony and I-25 project.
1.2 Project Location and Description
The Harmony and I-25 development is a 266-acre site located at the southwest quadrant of
Harmony Road and Interstate 25 in the west half of Section 3, Township 6 North, Range 68
West. The site is generally bounded by the following roadways: Strauss Cabin Road on the
west, Harmony Road on the north, I-25 on the east and Kechter Road on the south.
Surrounding properties include: the Arapaho Bend Natural Area and the Harmony Road
Transportation Transfer Center on the north side of Harmony Road, Island Lake Marina and the
Eagle View Natural Area on the south side of Ketcher Road, the Chandler property west of
Strauss Cabin Road, the Mountain Life Church and the Budurus properties located along
Strauss Cabin Road further to the south. East of the property is the interstate and the Town of
Timnath.
The ODP boundary for this overall development includes the Harmony Gardens parcels but
does not include the existing Shell gas station parcel at the northeast corner of the site. This
development site was annexed into the City of Fort Collins in 2009 under what was called the
“Riverwalk Annexation”.
The Fossil Creek Reservoir Inlet Ditch (FCRID) and the Boxelder Ditch are located along the
western side of the site with the FCRID being further to the west. The FCRID basically creates
the western property boundary south of Rock Creek Drive. The Boxelder Ditch bisects the
Chandler property, crosses Strauss Cabin Road and enters this site approximately 1700’ south
of Harmony Road.
This development is located within the Cache la Poudre watershed and City of Fort Collins
Poudre River drainage basin. This project site sits within the FEMA regulated floodway and
floodplain, per FEMA FIRM Map 08069C1013F, dated December 19, 2006. The floodplain and
floodways lines are shown in Figure 1 on the next page.
This Overall Development Plan (ODP) drainage report analysis is limited to the northwest
portion of the property that is outside of the half-foot floodway line (west of the half-foot floodway
line) and north of the half section line, shown as the “Limits of Development” on the ODP plan.
The reason for this is because new land development is prohibited within the floodway and this
ODP is not proposing the make any adjustments or changes to the location of the floodway at
this time. In addition, there are no land uses within the ODP boundary shown below the half
section line because of the proposed zoning south of the half section line. That zoning is
proposed to be RUL “Rural Lands”. This development project is proposing a mix of primary and
Harmony and I-25 ODP Drainage Report
Page 2
secondary uses allowed in the Harmony Corridor zoning district only. Potential uses are
convenience shopping, restaurants, and offices. Since only the north half is currently zoned for
development, only that portion was analyzed with this ODP drainage report.
Figure 1. Vicinity Map
Harmony and I-25 ODP Drainage Report
Page 3
1.3 Previous Studies
• North I-25 EIS, August 2011
• Poudre River Drainage Basin Master Plan
1.4 Limitations of Future Projections
The analysis contained in this study is based upon the Limits of Development for the Overall
Development Plan. It is important to note that details regarding type, configuration, and location
of stormwater improvements will be provided in subsequent steps of the entitlement for this
project.
2.0 Historic/Existing Conditions
Historically and prior to the 1980’s, this area was grasslands and utilized for farming. Since the
early 1980’s, this property had been mined for gravel. Starting in 1981, the Stute Pit (located
within the Harmony Gardens parcel) was mined for sand and gravel for the construction
industry. Subsequently in the later 1980’s and into the 1990’s, the Weitzel Pits and then the
Wellington Downs Pit were also mined for sand and gravel. With the exception of the Harmony
Gardens parcel and the constructed pad site at the Harmony Road and Strauss Cabin
intersection, a great majority of this property was mined for gravel. See Figure 2 for gravel pit
locations and information.
Today, the Stute Pit is a lined and state certified reservoir. This reservoir provides irrigation
water for the Harmony Gardens tree nursery. The Wellington Downs Pit is also a lined and state
certified reservoir.
The Weitzel Pits are simply inactive gravel mine pits that have become ponds due to the
watertable elevation in the area. The west pit (denoted as #1), the largest of the Weitzel Pits,
has top of bank elevations that vary from 4838 to 4849 (NAVD 1988), and a varying bottom of
pit elevation, with the lowest approximate elevation at 4823. The water surface in this pit is at an
approximate 4838 elevation, resulting in a maximum water depth of 15’. The other two Weitzel
Pits, denoted as #2 and #3 are configured similarly and also have a similar water depth.
Harmony and I-25 ODP Drainage Report
Page 4
Figure 2. Gravel Pit Information
Harmony and I-25 ODP Drainage Report
Page 5
The constructed pad site at the Harmony Road and Strauss Cabin Road intersection is
approximately 13 acres in size. This area was filled and a portion of this area is included in a
LOMR-F. Note that a portion of the filled area is located within the 0.5’ floodway and will be
required to be removed as a part of this project. The fill is to be removed to pre-fill elevations
and certified.
According to the Natural Resources Conservation Service (NRCS) the project site is
characterized by Type B and C soils. Existing slopes within the northernmost 13 acres are
basically flat, and the bank slopes into the existing gravel pits/ponds vary from 3:1 to 4:1. The
NRCS soils data for this site can be found in the Appendix.
Because of the past mining activities on the site, the historic drainage patterns have been modified
somewhat because the old gravel pits now act as large retention basins for stormwater. Weitzel Pit
#2 has a top of bank elevation at 4936 in the southeastern finger and this is where this pond
would overtop and spill south into an adjacent drainage way in a significant storm event. The
water surface elevation in this pond has been observed to be somewhat below the top of bank
elevation. The drainage way that originates at the south finger of #2 follows the east property
boundary between the interstate and the Wellington Downs pit. The drainage way outlets into
two culverts under Kechter Road and this is assumed to be the historic stormwater release
location for this site. Please refer to Figure 3 for the outfall route to the Poudre River.
Harmony and I-25 ODP Drainage Report
Page 6
Figure 3. Outfall to the Poudre River
Historic stormwater runoff rates from this site have been calculated based upon the site as it
was prior to mining activities.
Harmony and I-25 ODP Drainage Report
Page 7
3.0 Developed Conditions Plan
3.1 Design Criteria
The overall drainage plan presented in this report has been developed in accordance with the
City of Fort Collins Stormwater Criteria Manual and the City of Fort Collins submittal
requirements for an ODP. The drainage plan presented is based upon the allowable developed
stormwater release rates prescribed for the Poudre River drainage basin, that being the 2-year
historic rate; and an integrated stormwater management program that will incorporate “Low
Impact Development” (LID) principals into the design.
The purpose of this developed condition analysis is basically to establish the maximum runoff
rates from the “Limits of Development” within the ODP and to determine the 100-year detention
volume required. The existing Weitzel Pits, now groundwater fed ponds, will be utilized as
detention and water quality basins for stormwater runoff from this site. Specifically, the ponds
shown as Ponds #1 and #2 in Figure 2, or as the area shown as the detention basin on the
Grading and Drainage Plan, is the planned area for detention.
Developed runoff analysis for the ODP is based on runoff coefficients for zoning classifications.
For this project, the zoning classification is H-C (Harmony Corridor), and the minor storm runoff
coefficient is 0.95, and 1.0 for the major storm.
The allowable release from the ODP site is 9.8 cfs (2-yr historic release rate). The 100-year
release from the undeveloped areas adjacent to the ODP site (east of the Limits of
Development) is 57.2 cfs. This amount will be treated as a pass-through flow. The resulting
amount of stormwater passing from the detention basin will be approximately 67 cfs (9.8 +
57.2). Stormwater release from the detention basin will need to be provided through a controlled
release structure, likely to be a spillway that would need to be built below the historic ground
surface so as not to fill in or affect the floodway.
In addition, there is an offsite basin to the west of Strauss Cabin Road that currently drains via
the Boxelder Ditch. A conveyance will be provided to this property to convey at least the 2-yr
historic flows (allowed release after development) thru the ODP site. (100-year conveyance of
this flow will be provided if development on that parcel does not occur first.) This flow will be
considered a pass through flow and will be added to the allowed release from the detention
pond.
The required amount of stormwater detention for the full development of the land use parcels in
the “Limits of Development” area is 16.2 ac-ft plus an additional 20% (City criteria), resulting in
19.4 ac-ft. The area of the Weitzel Pond #1 that is outside of the “Limits of Development” and
Weitzel Pond #2 will constitute the detention basin for this project. The approximate water
surface area for these two ponds combined is approximately 18 acres, resulting in
approximately 1.1’ of stormwater storage depth above the groundwater elevation. The detention
water surface elevation is still below the Base Flood Elevations as published on the current
FEMA maps.
Harmony and I-25 ODP Drainage Report
Page 8
• Groundwater elevation ~ 4836 (NAVD 1988)
• 100-yr detention water surface elevation ~ 4837
• Base Flood Elevation (BFE) in this area ~ 4842
A WQCV calculation has been provided, however, based on conversations with City stormwater
staff, the existing gravel ponds can be used as water quality features for the stormwater
releases from this site. The required WQCV for this site is 2.25 ac-ft. It is assumed that the large
Weitzel ponds will more than account for these requirements.
Stream stability analysis, erosion and sediment control and open water and wetlands mitigation
are beyond the scope of this report and will be addressed separately or in a subsequent PDP
drainage plan.
3.2 Low Impact Development
LID practices are planned to be employed at this site. LID approach to stormwater management
basically aims to mimic the pre-developed stormwater hydrology. Some of the techniques used
to accomplish this are:
• Conserve Existing Amenities by identifying areas of the site that have value and that
should be protected
• Minimize Impacts by preserving natural drainage patterns where possible, reducing
paved areas, increasing stormwater drainage flow path lengths.
• Minimize Directly Connected Impervious Areas (MDCIA) by utilizing green belts and
vegetated areas where possible to slow down storm water travel time and provide
increased filtration and infiltration
Per City criteria, LID techniques are required to be used to treat stormwater runoff for 50% of
any newly added impervious areas and 25% of any newly added pavement areas must be
treated using permeable pavement technology. Grass buffers, rain gardens and permeable
pavements are a few of the LID techniques that may be utilized at this site.
3.2 Floodplain Management
The ODP project site sits within the FEMA regulated floodway and floodplain, per FEMA FIRM
Map 08069C1013F, dated December 19, 2006. More specifically, flood zone designations within
this project site are as follows:
• Flood Zone AE, FLOODWAY AREA: Flood Zone AE is determined to be a Special Flood
Hazard Area subject to inundation by the 1% annual chance flood. The 1% annual
chance flood has a 1% chance of being equaled or exceeded in any given year. Zone
AE is where base flood elevations have been determined. Floodway areas in zone AE
show the location of the channel of the stream plus any adjacent floodplain areas that
must be kept free of encroachment so that the 1% annual chance flood can be carried
without substantial increases in flood heights.
Harmony and I-25 ODP Drainage Report
Page 9
• Flood Zone AE, 100-YEAR FLOODPLAIN: Flood Zone AE is determined to be a Special
Flood Hazard Area subject to inundation by the 1% annual chance flood. The 1% annual
chance flood has a 1% chance of being equaled or exceeded in any given year. Zone
AE is where base flood elevations have been determined.
This site is situated outside of any City of Fort Collins defined floodways and floodplains.
The “Limits of Development” as shown in the ODP maps are outside of the half-foot floodway.
However, we recognize that a majority of this area still resides within the 100-year floodplain.
Land uses proposed for the “Limits of Development” do not include any critical facilities,
essential service critical facilities, hazardous materials facilities, government services,
residential, or mixed-use. All non-residential structures planned to be built within the “Limits of
Development” will be elevated and flood-proofed 2 feet above the 100-year flood elevations
(shown as Base Flood elevations on the FIRM maps.)
With the eventual development of all areas within the ODP, floodplain modeling is anticipated as
an integral part of the design process for the planning and layout of this site. The design team
for the site understands that a part of standard course for this project, a Conditional Letter of
Map Revision will need to be submitted and approved by the City and FEMA to show how the
flood may eventually be re-routed and channelized through the site so as to allow for additional
building areas to be created within the overall ODP.
Harmony and I-25 ODP Drainage Report
Page 10
5.0 Conclusion
The City of Fort Collins and Urban Drainage Stormwater Criteria Manuals have been used to
establish the criteria for a developed condition runoff plan within the “Limits of Development” for this
ODP. This overall drainage report has provided the general concept for providing stormwater
routing, collection and detention.
This ODP shows that the “Limits of Development” are outside of the half-foot floodway and that
detention and WQCV requirements can be met within the existing Weitzel Ponds and that the
detention water surface is below the 100-year flood elevations.
6.0 References
1. City of Fort Collins Stormwater Criteria Manual
2. Fort Collins Amendments to the Urban Drainage and Flood Control District Criteria
Manual, January 2012
3. Urban Drainage and Flood Control District Criteria Manual, Volume 3, August 2011
4. Federal Emergency Management Agency, 2006, Flood Insurance Rate Map, Larimer
County, Colorado, Map Number 08069C0994F
5. Federal Emergency Management Agency, 2006, Flood Insurance Rate Map, Larimer
County, Colorado, Map Number 08069C1013F
6. Natural Resources Conservation Services, 2013, Web Soil Survey: Soil Survey Area
Larimer County Area, Version 8, Dec 23, 2013
7. Colorado Division of Reclamation, Mining & Safety website
APPENDIX
ODP Calculations
and
Supporting Information
Harmony I-25
Basin Overland Flow Average Channelized Channel Channel Channelized
Length, D Overland Slope Tov Tov Tov Flow Length Slope Velocity Time (Tt)
(ft) (%) 2-year 10-year 100-year (ft) (%) (ft/s) (min) 2-year 10-year 100-year
Existing Conditions
North 1/2 0.20 0.20 0.25 500 0.5 47 47 45 2170 0.5 0.5 72.3 119 119 117
Proposed Conditions
ODP (HC) 0.95 0.95 1.00 500 1.0 6 6 4 350 1.0 2.0 2.9 9 9 7
Basin Time of Concentration
Frequency Adj. Runoff Coefficients (C*Cf)
C2 C
10 C100
Overland Travel Time Time of Concentration
Tc = Tov + Tt (min)
Harmony I-25
2 - year 10 - year 100 - year 2 - year 10 - year 100 - year 2 - Year 10 - Year 100 - Year 2 - Year 10 - Year 100 - Year
Existing Conditions
North 1/2 140.0 0.20 0.20 0.25 119 119 117 0.82 1.40 2.86 23.0 39.2 100.1
Proposed Conditions
ODP (HC) 60.0 0.95 0.95 1.00 9 9 7 2.30 3.93 8.80 131.1 224.0 528.0
Time of Concentration, Tc (min)
Basin Peak Discharge
Rainfall Intensity (in/hr) Peak Discharge (cfs)
Basin Basin Area Basin
(ac)
Frequency Adj. Runoff Coefficients
SUMMARY OF DETENTION PONDS ANALYSIS AND DESIGN
ODP
100 YR. STORM
Q = DEVELOPED SITE RUNOFF
C*Cf = 100-YR RUNOFF COEF. = 1.00 Max C*Cf = 1.0
I = RAINFALL INTENSITY
A = ON-SITE AREA = 60.0 ac
Q=CCfIA = 60.0 I cfs Max C*Cf = 1.0
RELEASE RATE, 2 yr historic = 9.84 cfs
MASS BALANCE METHOD
100-YR DEVELOPED VOLUME 100-YR REQUIRED
RAINFALL SITE INTO MAX. RELEASE VOLUME DETENTION
Tc INTENSITY RUNOFF POND RATE RELEASE VOLUME
(min) (in/hr) (cfs) (cf) (cfs) (cf) (ac.-ft.)
10 7.72 463.20 277,920 9.84 5,904 6.2
15 6.52 391.20 352,080 9.84 8,856 7.9
20 5.60 336.00 403,200 9.84 11,808 9.0
25 4.98 298.80 448,200 9.84 14,760 10.0
30 4.52 271.20 488,160 9.84 17,712 10.8
35 4.08 244.80 514,080 9.84 20,664 11.3
40 3.74 224.40 538,560 9.84 23,616 11.8
45 3.46 207.60 560,520 9.84 26,568 12.3
50 3.23 193.80 581,400 9.84 29,520 12.7
55 3.03 181.80 599,940 9.84 32,472 13.0
60 2.86 171.60 617,760 9.84 35,424 13.4
70 2.60 156.00 655,200 9.84 41,328 14.1
80 2.40 144.00 691,200 9.84 47,232 14.8
90 2.20 132.00 712,800 9.84 53,136 15.1
100 2.05 123.00 738,000 9.84 59,040 15.6
110 1.90 114.00 752,400 9.84 64,944 15.8
120 1.80 108.00 777,600 9.84 70,848 16.2
WATER QUALITY CAPTURE VOLUME (WQCV) Extended Duration (Dry)
% Impervious = 95 %
Required Storage From FIG 5-1 0.45 "/Ac.
WQCV = (Req. Storage/12)(A)= 2.25 Ac-ft
TST Inc.
Consulting
Engineers
4/29/2014
Fort Collins Rational Method.xls
Hydrologic Soil Group—Larimer County Area, Colorado
(ODP Land Uses Area Only)
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/27/2014
Page 1 of 4
4485000 4485100 4485200 4485300 4485400 4485500 4485600 4485700 4485800 4485900
4485000 4485100 4485200 4485300 4485400 4485500 4485600 4485700 4485800 4485900
499700 499800 499900 500000 500100 500200 500300 500400
499700 499800 499900 500000 500100 500200 500300 500400
40° 31' 25'' N
105° 0' 13'' W
40° 31' 25'' N
104° 59' 41'' W
40° 30' 53'' N
105° 0' 13'' W
40° 30' 53'' N
104° 59' 41'' W
N
Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84
0 200 400 800 1200
Feet
0 50 100 200 300
Meters
Map Scale: 1:4,800 if printed on A portrait (8.5" x 11") sheet.
MAP LEGEND MAP INFORMATION
Area of Interest (AOI)
Area of Interest (AOI)
Soils
Soil Rating Polygons
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Soil Rating Lines
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Soil Rating Points
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Water Features
Streams and Canals
Transportation
Rails
Interstate Highways
US Routes
Major Roads
Local Roads
Background
Aerial Photography
The soil surveys that comprise your AOI were mapped at 1:24,000.
Warning: Soil Map may not be valid at this scale.
Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil line
placement. The maps do not show the small areas of contrasting
soils that could have been shown at a more detailed scale.
Please rely on the bar scale on each map sheet for map
measurements.
Source of Map: Natural Resources Conservation Service
Web Soil Survey URL: http://websoilsurvey.nrcs.usda.gov
Coordinate System: Web Mercator (EPSG:3857)
Maps from the Web Soil Survey are based on the Web Mercator
projection, which preserves direction and shape but distorts
distance and area. A projection that preserves area, such as the
Albers equal-area conic projection, should be used if more accurate
calculations of distance or area are required.
This product is generated from the USDA-NRCS certified data as of
the version date(s) listed below.
Soil Survey Area: Larimer County Area, Colorado
Hydrologic Soil Group
Hydrologic Soil Group— Summary by Map Unit — Larimer County Area, Colorado (CO644)
Map unit symbol Map unit name Rating Acres in AOI Percent of AOI
22 Caruso clay loam, 0 to 1
percent slope
C 41.6 58.0%
60 Larim gravelly sandy
loam, 5 to 40 percent
slopes
B 1.4 1.9%
64 Loveland clay loam, 0 to
1 percent slopes
C 13.8 19.2%
81 Paoli fine sandy loam, 0
to 1 percent slopes
B 0.5 0.8%
101 Stoneham loam, 1 to 3
percent slopes
B 1.4 1.9%
103 Stoneham loam, 5 to 9
percent slopes
B 3.8 5.3%
105 Table Mountain loam, 0
to 1 percent slopes
B 9.3 13.0%
Totals for Area of Interest 71.8 100.0%
Hydrologic Soil Group—Larimer County Area, Colorado ODP Land Uses Area Only
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/27/2014
Page 3 of 4
Description
Hydrologic soil groups are based on estimates of runoff potential. Soils are
assigned to one of four groups according to the rate of water infiltration when the
soils are not protected by vegetation, are thoroughly wet, and receive precipitation
from long-duration storms.
The soils in the United States are assigned to four groups (A, B, C, and D) and
three dual classes (A/D, B/D, and C/D). The groups are defined as follows:
Group A. Soils having a high infiltration rate (low runoff potential) when thoroughly
wet. These consist mainly of deep, well drained to excessively drained sands or
gravelly sands. These soils have a high rate of water transmission.
Group B. Soils having a moderate infiltration rate when thoroughly wet. These
consist chiefly of moderately deep or deep, moderately well drained or well drained
soils that have moderately fine texture to moderately coarse texture. These soils
have a moderate rate of water transmission.
Group C. Soils having a slow infiltration rate when thoroughly wet. These consist
chiefly of soils having a layer that impedes the downward movement of water or
soils of moderately fine texture or fine texture. These soils have a slow rate of water
transmission.
Group D. Soils having a very slow infiltration rate (high runoff potential) when
thoroughly wet. These consist chiefly of clays that have a high shrink-swell
potential, soils that have a high water table, soils that have a claypan or clay layer
at or near the surface, and soils that are shallow over nearly impervious material.
These soils have a very slow rate of water transmission.
If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is
for drained areas and the second is for undrained areas. Only the soils that in their
natural condition are in group D are assigned to dual classes.
Rating Options
Aggregation Method: Dominant Condition
Component Percent Cutoff: None Specified
Tie-break Rule: Higher
Hydrologic Soil Group—Larimer County Area, Colorado ODP Land Uses Area Only
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/27/2014
Page 4 of 4
760 Whalers Way Bldg C, Suite 200 Fort Collins, CO 80525 ideas@tstinc.com
970.226.0557 main 303.595.9103 metro 970.226.0204 fax www.tstinc.com
Survey Area Data: Version 8, Dec 23, 2013
Soil map units are labeled (as space allows) for map scales 1:50,000
or larger.
Date(s) aerial images were photographed: Apr 22, 2011—Nov 18,
2011
The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
imagery displayed on these maps. As a result, some minor shifting
of map unit boundaries may be evident.
Hydrologic Soil Group—Larimer County Area, Colorado
(ODP Land Uses Area Only)
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/27/2014
Page 2 of 4