HomeMy WebLinkAboutHARMONY 23 - PDP - PDP160031 - REPORTS - DRAINAGE REPORTProject Development Plan (PDP)
Preliminary Drainage Report
for
Harmony 23
submitted to:
City of Fort Collins,
Colorado
November 23, 2016
760 Whalers Way
Bldg C, Suite 200
Fort Collins, CO 80525
970.226.0557 main
303.595.9103 metro
970.226.0204 fax
ideas@tstinc.com
www.tstinc.com
November 23, 2016
Mr. Dan Mogen, E.I., C.F.M.
Development Review Engineer
City of Fort Collins Utilities
700 Wood Street
Fort Collins, CO 80521
Re: Project Development Plan (PDP) for H23
Project No. 1219.0002.00
Dear Mr. Mogen:
We are pleased to submit this Preliminary Drainage Report for H23. This report
analyzes the developed stormwater runoff from the project and provides
stormwater infrastructure and water quality management for runoff. This report has
been prepared based on the City of Fort Collins Stormwater Criteria Manual and
complies with the PDP submittal requirements for a final drainage report.
Respectfully,
TST, INC. CONSULTING ENGINEERS
Derek A. Patterson, P.E. Jon Sweet, P.E.
DAP/JFS/jrm
Harmony 23 Preliminary 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 ......................................................................................... 4
3.0 Developed Conditions Plan
3.1 Design Criteria for Hydrologic Analysis ....................................................................... 6
3.2 Drainage Plan Development ....................................................................................... 7
3.2.1 Street Capacity ................................................................................................... 8
3.2.2 Inlet Design ........................................................................................................ 9
3.2.3 Storm Sewer Design .......................................................................................... 9
3.2.4 Channel Design ................................................................................................ 10
3.2.5 Riprap Design ................................................................................................... 10
3.2.6 Detention Pond Design .................................................................................... 10
3.3 Low Impact Development .......................................................................................... 12
3.4 Floodplain Management ............................................................................................ 14
4.0 Conclusion ....................................................................................................................... 15
5.0 References ....................................................................................................................... 16
Figures
Figure 1. Vicinity Map ................................................................................................................... 2
Figure 2. Outfall to the Poudre River ............................................................................................ 5
Figure 3. Four-Step Process for Stormwater Quality Management ............................................ 10
Appendices
Appendix A: Rational Method Hydrologic Analysis
Appendix B: Street Capacity and Inlet Analysis
Appendix C: Storm Sewer Design
Appendix D: Channel Design
Appendix E: Riprap Design
Appendix F: Detention Pond Design
Appendix G: FIRM Maps and Original Floodplain Workmap
Appendix H: Excerpts from Reference Reports
Appendix I: Proposed Drainage Maps
Harmony 23 Preliminary Drainage Report
Page 1
1.0 Introduction
1.1 Scope and Purpose
This report presents the results of a preliminary drainage analysis for the Harmony 23 (H23)
project. This report has been developed in accordance with the City of Fort Collins Stormwater
Criteria Manual requirements and complies with Project Development Plan (PDP) submittal
requirements.
1.2 Project Location and Description
The H23 project is a multifamily development situated on approximately 25 acres. The project is
located in the northeast corner of Section 4, Township 6 North, Range 68 West, more
specifically at the southwest corner of the intersection of Harmony Road and Strauss Cabin
Road. The site is generally bounded by the following roadways: Strauss Cabin Road on the
east, Harmony Road on the north, and the Fossil Creek Reservoir Inlet Ditch (FCRID) along the
south and west. Surrounding properties include: The Arapaho Bend Natural Area on the north
side of Harmony Road, the Harmony & Strauss Cabin Convenience Shopping Center
Subdivision (development recently approved) to the east of Strauss Cabin Road, Brookfield
Subdivision to the southwest, and undeveloped land to the west.
The Boxelder Ditch cuts through the site starting north at Harmony Road and running south
through the middle of the site where it crosses Strauss Cabin Road approximately 1600’ 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 just west of the FEMA regulated floodway but
within the 500-yr floodplain, per FEMA FIRM Maps 08069C0994F LOMR dated December 15,
2014. The floodplain and floodways lines are shown in Figure 1 on the next page.
Harmony 23 Preliminary Drainage Report
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Figure 1. Vicinity Map
Harmony 23 Preliminary Drainage Report
Page 3
1.3 Previous Studies
WEITZEL POND #1 Drainage Report for Harmony & Strauss Cabin
ODP Drainage Report for Harmony & Strauss Cabin
Harmony & Strauss Cabin Subdivision Filing No. 1 Convenience Shopping Center PDP
Final Drainage Report
North I-25 EIS, August 2011
Poudre River Drainage Basin Master Plan
1.4 Limitations of Future Projections
The analysis and proposed stormwater improvements contained in this study are for the H23
project area. Assumptions have been made to account for future development adjacent to the
site where developed flows and storm infrastructure from this project will influence future
designs. In addition, assumptions were made to account for interim and ultimate tail water
conditions downstream of this site. It is assumed that the outfall from the H23 site will connect
to a future stormwater system, which will ultimately daylight into a regional detention facility for
the 266-acre Harmony and I-25 development. This regional detention facility is also assumed to
be a non-potable irrigation pond. The final/ultimate water surface elevation in this future
regional detention pond will need to be designed to not affect upstream developments. A
master plan for the Harmony and I-25 development is in progress, but has not been completed
at the time of this report.
The grading and drainage plan provided herein assumes that the WEITZEL POND #1 project
has progressed and has filled the portion of the Existing Gravel Pond #1 shown in the WEITZEL
POND #1 plans. However, there will be no negative impacts to the H23 site if that project has
not been completed before H23 is complete. In addition, this report also assumes that the
Convenience Shopping Center will be constructed at some point in the future and that the Box
Elder Ditch will be realigned through the site prior to H23 construction.
For this project the downstream interim 100-yr WSEL in the interim regional detention pond (Ex.
Gravel Pond #1) is assumed to be a 4838.59, per the Convenience Shopping Center report.
This tailwater elevation was assumed to be the worst case scenario, and was used for the H23
design and calculations. The ultimate/final water surface elevation in the regional detention
pond is assumed to be equal to or lower than this elevation.
Stream stability analysis, open water and wetlands mitigation are beyond the scope of this
report and will be addressed separately or in a subsequent design report.
Harmony 23 Preliminary Drainage Report
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2.0 Historic/Existing Conditions
Historically this area was undeveloped grasslands, that was eventually utilized for farming.
The Boxelder Ditch which runs through the site was used for irrigation purposes. Today, this
site is no longer used for farming and has returned back to undeveloped grasslands. The site
generally slopes from the west to the east to the low point in Strauss Cabin Road. Slopes vary
from 1:1 on the west end to basically flat on the east.
According to the Natural Resources Conservation Service (NRCS) the project site is
characterized by Type A, B and D soils. The NRCS soils data for this site can be found in the
Appendix
The historic stormwater runoff rate from this site is based on the calculations provided in the
Convenience Shopping Center report, which was determined to be 0.165 cfs/acre (27.2 acres /
4.5 cfs).
There are currently a small amount wetlands, approximately 411 s.f., located on the project site.
As stated previously wetlands will be addressed separately in a subsequent design report if
required.
Harmony 23 Preliminary Drainage Report
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Figure 2. Outfall to the Poudre River
Harmony 23 Preliminary Drainage Report
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3.0 Developed Conditions Plan
3.1 Design Criteria for Hydrologic Analysis
The 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 a
Project Development Plan (PDP). 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.
The drainage basin presented in this report is less than 90 acres, therefore, the method used to
analyze drainage for this project will be the Rational Method. Runoff calculations for both the 2-year
and 100-year storms will be presented here.
The Rational Method is given by:
Q = C(Cf)IA where
Q is the maximum rate of runoff in cfs
C is the runoff coefficient
Cf is the runoff coefficient frequency factor adjustment
I is the rainfall intensity in inches per hour for a storm duration equal to the time of
concentration
A is the total area of the basin in acres
The runoff coefficient C has different values depending on the design storm recurrence interval.
The runoff coefficient is also dependent on land use or surface characteristics/imperviousness.
Runoff coefficients were assigned using Table RO-11 from the Fort Collins Stormwater Criteria
Manual. The frequency factor adjustment, Cf, varies depending on the storm frequency and can be
found in Table RO-12 of the Fort Collins Stormwater Criteria Manual.
The rainfall intensity is selected from Rainfall Intensity Duration Frequency Curves for the City of
Fort Collins (Tables RA-7 through RA-9 of the Fort Collins Stormwater Criteria Manual).
In order to utilize the Rainfall Intensity Duration Curves, the time of concentration is required. The
following equation is used to determine the time of concentration
tc = ti + tt where
tc is the time of concentration in minutes
ti is the initial or overland flow time in minutes
Harmony 23 Preliminary Drainage Report
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tt is the conveyance travel time in minutes
The initial or overland flow time is calculated with the UDFCD equation:
0.33
f
i S
t 1.87 1.1 * C D
C
3.2 Drainage Plan Development
The H23 project is proposed to be a multifamily site accompanied by parking areas, open
space, and public roadway improvements. The site and associated storm sewer system has
been designed to collect and convey a majority runoff from this project, including portions of the
fronting ½ of the surrounding public roadways to the two separate constructed wetland ponds
located onsite. Due to design constraints a small portion of runoff from the project site will be
collected and conveyed to an offsite detention pond where it will be detained and released.
For preliminary design calculations the site was divided into several larger basins. All “N”, “S”
and “U” basins described below will be further subdivided with final design.
Basin N-1, as shown on the Drainage Plan, is the northern portion of the project site which
includes open space, parking areas and building footprints. Runoff from this basin is conveyed
via curb & gutter into the storm sewer system which drains into to Pond A. The calculated water
quality capture volume (WQCV) from this basin will be treated in Pond A which is designed as a
constructed wetland pond.
Basin S-1 is the southern portion of the project site which includes open space, parking areas
and building footprints. Runoff from this basin is conveyed via curb & gutter into the storm
sewer system which drains into Pond B. Pond B which is designed as a constructed wetland
pond will treat the WQCV from this basin.
Basin S-2 incorporates the southern fronting portion of Strauss Cabin Road to the back of the
detached sidewalk. Runoff from this basin is conveyed via curb & gutter to a low point in
Strauss Cabin Road where it is conveyed via the proposed storm sewer system and into Pond
B. The WQCV from this basin will be treated in Pond B.
Basin U-1 incorporates fronting portions of Harmony Road and Strauss Cabin Road in addition
to a portion of the project site. Runoff from this basin is conveyed via curb & gutter to a low
point in Strauss Cabin Road north of Hacienda Drive, where it is conveyed via the proposed
storm sewer system south to the offsite ponds designed with the Convenience Shopping Center
Report. All storage for this basin will be handled in the offsite ponds.
Harmony 23 Preliminary Drainage Report
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Basin U-2 incorporates the middle fronting portion Strauss Cabin Road in addition to a portion of
the project site. Runoff from this basin is conveyed via curb & gutter to another low point in
Strauss Cabin Road just south of Hacienda Drive, where it is conveyed via the proposed storm
sewer system south to the offsite ponds designed with the Convenience Shopping Center
Report. All storage for this basin will be handled in the offsite ponds.
The 100-yr runoff volume for all “N” basins will be detained in Pond A, while all runoff for all “S”
basins will be detained in Pond B. The 100-yr runoff volume for all “U” basins will be detained in
the existing gravel ponds located offsite.
Additional offsite basins contributing to the H23 drainage design are described below.
Basin CSC-1 is an offsite basin contributing to the proposed storm sewer system located in
Strauss Cabin Road. This basin is a portion of the Convenience Shopping Center Basin N-2,
which has been subdivided with this project. Runoff from Basin CSC-1 will drain to a low point
on Strauss Cabin Road where it will be conveyed via the proposed storm sewer system
designed with the H23 project and into the existing gravel ponds. Detention and water quality
from this basin have already been accounted for in the Convenience Shopping Center Final
Drainage Report.
Basins O-1, O-2, and O-3 are offsite basins that will pass through the site. No water quality or
detention will be provided for these basins as they are not part of the subject property.
Basins O-1 and O-2 are the areas between the FCRID and the property boundary which
consists of gravel access road and undeveloped open space. Runoff from Basin O-1 will be
routed through Pond A, and runoff from Basin O-2 will be routed through Pond B.
Basin O-3 is the area just south of the project which consists of a small portion of undeveloped
open space and a portion of Strauss Cabin Road. Runoff from this basin is conveyed via curb &
gutter to a low point in Strauss Cabin Road where it will be routed through Pond B.
3.2.1 Street Capacity
Street capacities have been analyzed using the Urban Drainage UD-Inlet spreadsheet, version
4.01. Street capacity was analyzed along Strauss Cabin Road on the west side where ultimate
roadway improvements are proposed between Harmony Road and the south end of this project.
Table ST-2 and ST-3 in the City of Fort Collins Stormwater Criteria Manual state inundation
standards for the minor and major storms. These standards are summarized in the table below.
Harmony 23 Preliminary Drainage Report
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Table 1.
Street ID Street Classification Minor Storm (2-yr)
requirements
Major Storm (100-yr)
requirements
Strauss Cabin Road 2-Lane Arterial No curb-overtopping,
flow spread must
leave at least 12’ wide
travel path in each
direction
Depth of water must
not exceed bottom of
median gutter, 12”
max depth at gutter
flowline, flow must be
contained within
ROW.
For preliminary analysis the worst case scenario for Strauss Cabin Road was analyzed. It was
determined the streets meet the applicable requirements and will function below the allowed
capacities. The results of the street capacity analysis can be found with supporting calculations
in Appendix B. All other calculations will be provided with final design.
3.2.2 Inlet Design
Inlets have been analyzed using the Urban Drainage UD-Inlet spreadsheet, version 4.01. CDOT
Type R inlets are proposed to collect runoff and have been placed at the low points of the
basins where infall, vertical curb & gutter is located and have been analyzed for a “sump”
condition. These inlets have been sized to capture the 100-yr storm.
On-grade Type 13 inlets have been strategically placed within the cross pans in the parking lot
to intercept portions of runoff and direct it to the proposed storm sewer system. These have
been placed to reduce the amount or runoff that would flow down the drive isle, and to help
reduce inlet sizes further downstream.
The results of the inlet analysis will be provided at final design in Appendix B.
3.2.3 Storm Sewer Design
Storm sewer pipes have been analyzed using UDSEWER 2009, version 1.4. All storm pipes on
this project site are designed to collect and convey the 100-yr storm runoff. The pipes have all
been sized to ensure that the hydraulic grade lines remain below the flowline elevation at inlets
and below rim elevations at manholes.
The results of the storm sewer analysis will be provided at final design in Appendix C.
Harmony 23 Preliminary Drainage Report
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3.2.4 Channel Design
Proposed channels and swales at this project site have been analyzed using Bentley
Flowmaster V8i, Select Series 1 software. These swales have been designed to include 1’ of
freeboard above the normal 100-yr water depth in the channel. Where the 1’ criteria isn’t
possible, the swales have been checked to make sure they can pass at least 133% of design
flow.
Swale A-A is triangular swale located in Basin N-1 which conveys a portion of runoff within that
basin directly into Pond A.
The results of the channel design will be provided at final design in Appendix D.
3.2.5 Riprap Design
Riprap has been proposed at the outlet pipe to Pond B. Although the approximate groundwater
elevation is 4838 at this location and the pipe outlet and riprap are anticipated to be underwater
during an average day scenario, the groundwater in that pond may be lowered at any given time
due to future construction activities at this site, so riprap has been proposed in this area as a
ground stabilization measure.
The results of riprap sizing will be provided at final design in Appendix E.
All other pipes which discharge into the ponds onsite will discharge into concrete forebays.
These forebays are designed to dissipate energy in addition to providing sedimentation.
3.2.6 Detention Pond Design
The existing conditions for this area was analyzed with the Convenience Shopping Center
report to determine the allowable release rate from the site. The 2-yr historic discharge for this
area was determined to be 4.50 cfs for the 27.2 acres estimated in that report. This ends up
being a release rate of approximately 0.165 cfs/acre (4.5/27.2) which is the allowable release
rate used in this report.
There will be two ponds utilized on this site for water quality and detention. Pond A on the
northeast end of the site and Pond B on the south end of the site. Both are designed as
constructed wetland ponds to treat the WQCV. Both ponds will be supplied by groundwater to
help maintain vegetation and microorganisms. In addition, a portion of Pond A will be utilized
for wetland mitigation.
The Ex. Gravel Pond #2 in the Convenience Shopping Center Report will be utilized to detain
Harmony 23 Preliminary Drainage Report
Page 11
the 100-yr runoff from Basins U-1 and U-2. Per conversations with the City stormwater staff, the
existing gravel ponds can be used as a water quality feature. Therefore, the WQCV for these
two basins was not added into the total required volume being detained offsite in Ex. Gravel
Pond #2.
The UDFCD UD-BMP spreadsheet version 3.05 was used to calculate the WQCV for each of
the two constructed wetland ponds. Pond A is sized to collect the WQCV for all N basins, and
Pond B is sized to collect the WQCV for all S basins. The WQCV is designed to be released in
24-hr as recommend by UDFCD for a constructed wetland pond. In addition, the minimum
permanent pool and forebay volumes were also sized using this spreadsheet.
A mass-balance spreadsheet was used to calculate the overall 100-yr detention requirements
for this site. The required 100-yr detention volumes for Pond A and Pond B calculated from this
spreadsheet were determined to be 1.92 acre-feet, and 2.37 acre-feet respectively. An
additional 20% was added to these volumes, per City Criteria, resulting in a total 100-yr volume
of 2.30 acre-feet for Pond A and 2.84 acre-feet for Pond B. The total volume required in the
ponds based on City Criteria is the 100-yr plus the WQCV. Therefore, the required total volume
was determined to be 2.46 acre-feet for Pond A and 3.04 acre-feet for Pond B. The results for
these ponds are provided in Table 2.
Table 2.
Pond
ID
Contri-
buting
Area
(acres)
Ground-
water
Surface
Elevation
2-yr
Historic
Release
Rate
(0.165
cfs/acre)
100-yr
Detention
Volume
(120%)
(ac-ft)
WQCV
(ac-ft)
Total
Require
d
Storage
(ac-ft)
Total
Provided
Storage
(ac-ft)
100-yr
Water
Surface
Elevation
Pond A 9.76 4843.00 1.61 2.30 0.16 2.46 2.77 4847.75
Pond B 11.70 4839.00 1.93 2.84 0.20 3.04 3.05 4846.00
Total
Require
Harmony 23 Preliminary Drainage Report
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need to be modified. To simplify the design, the 9.0 cfs release rate from the spillway structure
will be maintained and both the 2-yr spillway crest (Ex. Elev.=4837.39) and the 100-yr spillway
crest (Ex. Elev.=4837.64) will be raised 0.10 feet each. Because this pond is situated within the
floodway, a no-rise analysis, previously completed with the Convenience Shopping Center
Report, will be updated and provided with final design.
The undeveloped areas adjacent to the site denoted as O-1, O-2, and O-3 will release
undetained through the site. This amount will be treated as a pass-through flow. The resulting
amount of stormwater (100-yr) passing from Pond A will be approximately 1.1 cfs and 5.9 cfs for
Pond B. The total 100-yr release from these undeveloped areas amounts to 7.0 cfs.
Outlet structures for Pond A and B will consist of 3 stages. A WQCV stage (24-hr drain time),
detention volume (2-yr historic release), and emergency overflow. An emergency overflow
stage was added due to site constraints discussed below. All ponds will have a minimum 1’ of
freeboard above the detention surface elevation.
The ponds for this site were located in areas to maximize developable area. Emergency
spillways for each pond have not been provided due to site constraints. Therefore, these ponds
don’t have a defined spillway. Instead Pond A is designed to pass a portion of the emergency
flow through the 100-yr overflow stage on the outlet structure. Any runoff which exceeds the
overflow capacity will pond up to the Boxelder Ditch (Elevation=4849) located along the east
side of this pond, where it will ultimately either spill into and or across this ditch to the low point
in Strauss Cabin Road. The difference between the detention surface elevation (4837.75) of
Pond A and the Boxelder Ditch is approximately 1.25’.
Similar to Pond A, Pond B will also have an emergency overflow stage on the outlet structure.
However, this outlet structure and outlet pipe will be sized to convey the 100-yr storm event. In
the event that the outlet pipe becomes clogged, runoff would back up into the parking lot and
around Bldg. G, until it can spill across the Box Elder Ditch and into Strauss Cabin Road. This
elevation is approximately 4849.50. All buildings within this area will be designed at an
elevation higher than the spill elevation to prevent flooding.
3.3 Low Impact Development
With the building coverage and parking areas, the amount of new imperviousness introduced to
this site will be approximately 60%. However, the grading design aims to minimize the “directly
connected impervious area” (DCIA) as much as possible. Strategies to minimize directly
connected impervious areas follow the Four-Step Process for Stormwater Quality Management,
as outlined in Volume 3 of the Urban Storm Drainage Criteria Manual. The four-step process is
shown below.
Harmony 23 Preliminary Drainage Report
Page 13
Figure 4. Four-Step Process for Stormwater Quality Management
Step 1: Runoff Reduction Practices for this project site are planned to include parking lot
drainage toward grassed swales located adjacent to parking lot areas to capture and route
stormwater drainage toward the two separate ponds located on the north and south ends of the
site. These grassed swales are intended to have meandering alignments to increase the
aesthetic and the time of concentration.
Step 2: BMP’s to Provide WQCV with Slow Release for this project site will employ LID
techniques such as constructed wetland ponds to act as a sedimentation basin by allowing for
the sediment, fines and pollution suspended in the stormwater to settle out into the groundwater
fed ponds. The ponds will be equipped with outlet structures to control the release and improve
water quality.
The City of Fort Collins criteria for low impact development, as outlined in the Stormwater
Criteria Manual in Volume 3, Chapter 3, are as follows:
No less than 75% of any newly developed or redeveloped area must be treated using
one or a combination of LID techniques,
OR
Harmony 23 Preliminary Drainage Report
Page 14
No less than 50% of any newly developed or redeveloped area must be treated using
one or a combination of LID techniques when a permeable pavement area covering at
least 25% of drivable surface area on private development property is constructed as
one of the components of the LID treatment techniques used on that site.
For this site, approximately 85% of the project will be treated using constructed wetland ponds,
and therefore, permeable pavers will not be required. The other 15% will be treated in the
massive gravel ponds located offsite.
Step 3: Stabilized Drainageways will come in the form of seeding, planting, energy dissipation
basins (forebays), and maintaining the constructed wetlands and grassed swales created for
this project site.
Step 4: Site Specific Source Control BMP’s will include things like proper maintenance and
sedimentation removal of the forebays and constructed wetland ponds.
The areas where the various LID techniques are planned to be utilized on the site are shown on
the drainage map. LID calculations are included in Appendix F.
3.4 Floodplain Management
This site is situated outside of any City of Fort Collins regulatory floodways and floodplains.
This project site sits within the FEMA FIRM Map 08069C0994F dated December 19, 2006. The
new LOMR (Case # 14-08-0580P, effective date 12-15-2014) that just received FEMA approval
corrected the floodway and floodplain lines to align with the original floodplain workmap.
Information provided in this LOMR was taken into consideration for this project. More
specifically, flood zone designations within this project site per the LOMR are as follows:
Flood Zone X, Shaded 500-YEAR FLOODPLAIN: Areas of 0.2% annual chance flood
hazard. Areas of 1% annual chance flood with average depth less than one foot or with
drainage areas of less than one square mile.
Flood Zone X, Unshaded: Areas determined to be outside the 0.2% annual chance
floodplain.
Chapter 10, Flood Prevention and Protection, of the Fort Collins Municipal Code provided
guidance and limitations for building and development within the floodway and floodplain.
Division 4 – Poudre River Flood Hazard Areas of this chapter provide specific standards for
development within the Poudre River floodway and flood fringe.
Harmony 23 Preliminary Drainage Report
Page 15
The “Limits of Development” as shown on the Grading and Drainage Plan are outside of the
half-foot floodway. However, we recognize that a majority of this area still resides within the
500-year floodplain. Land uses proposed within the “Limits of Development” do not include any
critical facilities, essential service critical facilities, hazardous materials facilities, government
services, or at risk population facilities. All 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.)
Ex. Gravel Pond #2 pond is situated within the floodway, a no-rise analysis, previously
completed with the Convenience Shopping Center Report, will be updated and provided with
final design.
4.0 Conclusion
The City of Fort Collins Stormwater Criteria Manual has been used to establish the criteria for a
developed condition runoff plan within the H23 project. This PDP drainage design report has
highlighted the following items:
Project site development is outside of the effective floodway boundary, but within the 500-yr
floodplain.
LID techniques are planned to be implemented within the site design including measures
that will help minimize directly connected impervious areas. These LID techniques will
include grassed swales and constructed wetland ponds.
Water quality and stormwater detention for the project will be provided in two separate
wetland ponds located onsite. These ponds have been sized to release the 100-yr storm at
the 2-yr historic release.
A small portion of this project will be detained offsite in Ex. Gravel Pond #2 located east of
the project.
All storm infrastructure has been sized to collect and convey the 100-yr storm event.
Harmony 23 Preliminary Drainage Report
Page 16
5.0 References
1. City of Fort Collins Stormwater Criteria Manual, Amendments to the Urban Drainage
and Flood Control District Criteria Manual, Adopted December 2011, Last Revision
April 2012.
2. Urban Drainage and Flood Control District Criteria Manual, Volume 3, August 2011
3. Federal Emergency Management Agency, 2006, Flood Insurance Rate Map, Larimer
County, Colorado, Map Number 08069C0994F
4. Federal Emergency Management Agency, 2006, Flood Insurance Rate Map, Larimer
County, Colorado, Map Number 08069C1013F
5. Natural Resources Conservation Services, 2013, Web Soil Survey: Soil Survey Area
Larimer County Area, Version 8, Dec 23, 2013
6. Colorado Division of Reclamation, Mining & Safety website,
http://mining.state.co.us/Pages/Home.aspx
7. Chapter 10 of the Fort Collins Municipal Code, http://colocode.com/fcmunihtml.html
8. LOMR, Case #14-08-0580P, Letter of Map Revision (LOMR-BD) Based on Better
Data on the Cache la Poudre River I-25 Divided Flow Path Southwest of Harmony
Road and Interstate 25, Anderson Consulting Engineers, Effective Date December
15, 2014.
APPENDIX A
Rational Method Hydrologic Analysis
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H23 Preliminary Drainage Report
City of Fort Collins
IDF Curves
Duration 2-yr 10-yr 100-yr
5 2.85 4.87 9.95
6 2.67 4.56 9.31
7 2.52 4.31 8.80
8 2.40 4.10 8.38
9 2.30 3.93 8.03
10 2.21 3.78 7.72
11 2.13 3.63 7.42
12 2.05 3.50 7.16
13 1.98 3.39 6.92
14 1.92 3.29 6.71
15 1.87 3.19 6.52
16 1.81 3.08 6.30
17 1.75 2.99 6.10
18 1.70 2.90 5.92
19 1.65 2.82 5.75
20 1.61 2.74 5.60
21 1.56 2.67 5.46
22 1.53 2.61 5.32
23 1.49 2.55 5.20
24 1.46 2.49 5.09
25 1.43 2.44 4.98
26 1.40 2.39 4.87
27 1.37 2.34 4.78
28 1.34 2.29 4.69
29 1.32 2.25 4.60
30 1.30 2.21 4.52
31 1.27 2.16 4.42
32 1.24 2.12 4.33
33 1.22 2.08 4.24
34 1.19 2.04 4.16
35 1.17 2.00 4.08
36 1.15 1.96 4.01
37 1.13 1.93 3.93
38 1.11 1.89 3.87
39 1.09 1.86 3.80
40 1.07 1.83 3.74
41 1.05 1.80 3.68
42 1.04 1.77 3.62
43 1.02 1.74 3.56
44 1.01 1.72 3.51
45 0.99 1.69 3.46
46 0.98 1.67 3.41
47 0.96 1.64 3.36
48 0.95 1.62 3.31
49 0.94 1.60 3.27
50 0.92 1.58 3.23
51 0.91 1.56 3.18
52 0.90 1.54 3.14
53 0.89 1.52 3.10
54 0.88 1.50 3.07
55 0.87 1.48 3.03
56 0.86 1.47 2.99
57 0.85 1.45 2.96
58 0.84 1.43 2.92
59 0.83 1.42 2.89
60 0.82 1.40 2.86
H23 Preliminary Drainage Report
From the Town of Timnath Design Criteria Manual and Construction Specifications
y = 2.5665e-0.021x
R² = 0.9633
y = 26.332x-0.53
R² = 0.9879
0.00
2.00
4.00
6.00
8.00
10.00
12.00
010203040506070
Rainfall Intensity (in/hr)
Storm Duration (min)
City of Fort Collins IDF Curves
2-yr
10-yr
100-yr
Expon. (2-yr)
Power (100-yr)
Appendix A
H23 Preliminary Drainage Report
City of Fort Collins
Rational Method Runoff Coefficients
Lawn, Heavy, <2% Slope 0.20
Lawn, Heavy, >7% Slope 0.35
Lawn, Heavy, 2-7% Slope 0.25
Lawn, Sandy, <2% Slope 0.10
Lawn, Sandy, >7% Slope 0.20
Lawn, Sandy, 2-7% Slope 0.15
Roofs 0.95
Streets: Permeable Pavers 0.30
Streets: Gravel 0.50
Streets: Paved 0.95
From Table 3-3 of the City of Fort Collins, Stormwater Criteria
Runoff
Coefficient
Appendix A
H23 Preliminary Drainage Report
C2 C10 C100
Streets: Paved 3.64 0.95
Lawn, Heavy, <2% Slope 4.30 0.20
Roofs 1.82 0.95
Total N Basins 9.76 Total % Imp. 55.0% Avg. 100-yr C100 = 0.77
Streets: Paved 4.60 0.95
Lawn, Heavy, <2% Slope 4.80 0.20
Roofs 1.46 0.95
Streets: Paved 0.64 0.95
Lawn, Heavy, <2% Slope 0.20 0.20
Roofs 0.00 0.95
Total S Basins 11.70 56.9% Avg. 100-yr C100 = 0.79
Streets: Paved 1.54 0.95
Lawn, Heavy, <2% Slope 0.50 0.20
Roofs 0.05 0.95
Streets: Paved 1.52 0.95
Lawn, Heavy, <2% Slope 1.44 0.20
Roofs 0.47 0.95
Total U Basins 5.52 64.6% Avg. 100-yr C100 = 0.86
Total 26.98 Total % Imp. 57.8% Avg. 100-yr C100 = 0.80
*Percent Impervious Values based on UDFCD Vol 1., Chapter 6, Table 6-3
C2 C10 C100
Streets: Paved 0.71 0.95
Lawn, Heavy, <2% Slope 0.22 0.20
Roofs 0.05 0.95
O-1 0.44 Lawn, Heavy, <2% Slope 0.84 0.20 20.0% 0.20 0.20 0.20 0.25
O-2 2.04 Lawn, Heavy, <2% Slope 0.84 0.20 20.0% 0.20 0.20 0.20 0.25
O-3 0.38 Lawn, Heavy, <2% Slope 0.84 0.20 20.0% 0.20 0.20 0.20 0.25
Rational Method Frequency Adjustment Factors
Min year Max year Frequency Factor, Cf
210 1
11 25 1.1
26 50 1.2
51 100 1.25
From Table 3-4 in the City of Fort Collins, SDDCCS
Table A-1a
N-1 9.76
Onsite Basin Composite Runoff Coefficients
Basin/ Sub-
Basin Area (ac) Attribute
55.0%
Attribute Area
(ac)
S-1 10.86 55.3% 0.62
Composite C
Frequency Adjusted Runoff Coefficients (C*Cf)
0.62 0.62 0.77
0.62
Runoff
Coefficient, C
Percent
Impervious
0.77 0.96
0.62 0.62 0.77
S-2 0.84 76.7% 0.77 0.77
U-1 2.09 76.3% 0.77 0.96
0.98
0.79
0.77 0.77
H23 Preliminary Drainage Report
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
N-1 0.62 0.62 0.77 190 2 10 10 7 615 2.2 3 3.4 13 13 10
S-1 0.62 0.62 0.77 66 2 6 6 4 1150 1 2 9.6 16 16 14
S-2 0.77 0.77 0.96 26 2 2 2 1 630 2.2 2.9 3.6 6 6 5
U-1 0.77 0.77 0.96 66 2 4 4 2 430 0.9 1.8 4.0 8 8 6
U-2 0.63 0.63 0.79 110 2 7 7 5 442 0.55 1.4 5.3 12 12 10
CSC-1 0.78 0.78 0.98 85 2 4 4 2 160 0.09 1.8 1.5 5 5 5
O-1 (EX. CONDITIONS
OFFSITE) 0.20 0.20 0.25 50 33 4 4 4 0 0 1 0.0 5 5 5
O-2 (EX. CONDITIONS
OFFSITE) 0.20 0.20 0.25 50 33 4 4 4 0 0.0 1.0 0.0 5 5 5
O-3 (EX. CONDITIONS
OFFSITE) 0.20 0.20 0.25 26 2 7 7 6 136 3.6 3.6 0.6 8 8 7
Overland Travel Time Time of Concentration
Basin Tc = Tov + Tt (min)
Table A-2
Basin Time of Concentration
Frequency Adj. Runoff Coefficients (C*Cf)
C2 C10 C100
Appendix A
H23 Preliminary Drainage Report
2 - year 10 - year 100 - year 2 - year 10 - year 100 - year 2 - Year 10 - Year 100 - Year 2 - Year 10 - Year 100 - Year
N-1 9.76 0.62 0.62 0.77 13 13 10 1.98 3.39 7.72 12.0 20.5 58.4
S-1 10.86 0.62 0.62 0.77 16 16 14 1.81 3.08 6.71 12.2 20.7 56.3
S-2 0.84 0.77 0.77 0.96 6 6 5 2.67 4.56 9.95 1.7 3.0 8.1
U-1 2.09 0.77 0.77 0.96 8 8 6 2.40 4.10 9.31 3.9 6.6 18.7
U-2 3.43 0.63 0.63 0.79 12 12 10 2.05 3.50 7.72 4.5 7.6 21.0
CSC-1 0.98 0.78 0.78 0.98 5 5 5 2.85 4.87 9.95 2.2 3.7 9.5
O-1 (EX. CONDITIONS
OFFSITE) 0.44 0.20 0.20 0.25 5 5 5 2.85 4.87 9.95 0.3 0.4 1.1
O-2 (EX. CONDITIONS
OFFSITE) 2.04 0.20 0.20 0.25 5 5 5 2.85 4.87 9.95 1.2 2.0 5.1
O-3 (EX. CONDITIONS
OFFSITE) 0.38 0.20 0.20 0.25 8 8 7 2.40 4.10 8.80 0.2 0.3 0.8
Total Bypass Flow 7.0
Time of Concentration, Tc (min)
Table A-3
Basin Peak Discharge
Basin Rainfall Intensity (in/hr) Peak Discharge (cfs)
Basin Area
(ac)
Frequency Adj. Runoff Coefficients
Appendix A
H23 Preliminary Drainage Report
2 - year 10 - year 100 - year 2 - year 10 - year 100 - year 2 - year 10 - year 100 - year 2 - year 10 - year 100 - year 2 - year 10 - year 100 - year
1 (Pond A Outlet) N-1 9.76 13 13 10 0.62 0.62 0.77 1.98 3.39 7.72 11.97 20.50 58.35 12.22 20.93 59.45
*2 (INLET-1B) Pond A Release, U-1 2.09 8 8 6 0.77 0.77 0.96 2.40 4.10 9.31 5.48 8.21 20.35 5.73 8.64 21.45
*3 (SDMH-1D) Pond A Release, U-1,
CSC-1 3.07 8 8 6 0.77 0.77 0.97 2.40 4.10 9.31 7.31 11.36 29.27 7.57 11.78 30.36
*4 (SDMH-1C) Pond A Release, U-1, U-
2 CSC-1 6.50 12 12 10 0.70 0.70 0.88 2.05 3.50 7.72 10.95 17.55 45.56 11.20 17.98 46.65
*5 (SDMH-1A) Pond A and B Release,
U-1, U-2, CSC-1 6.50 12 12 10 0.70 0.70 0.88 2.05 3.50 7.72 14.49 21.09 49.10 15.90 23.51 55.27
6 (Pond B Outlet) All S Basins 11.70 16 16 14 0.63 0.63 0.79 1.81 3.08 6.71 13.33 22.68 61.77 20.34 29.69 68.78
Table A-4
Attenuation of Peak Discharge
Time of Concentration, Tc (min) Weighted Runoff Coefficient Rainfall Intensity (in/hr) Peak Discharge (cfs)
Area (acres)
Contributing Basins &
Design Point Design Points
*Areas shown for these design points exclude the basin area contributing to Ponds A and B.
However, the release rates from these ponds have been added to the peak discharge column.
Peak Discharge + Offsite Bypass Flows (cfs)
Appendix A
APPENDIX B
Street Capacity and Inlet Analysis
Project:
Inlet ID:
Gutter Geometry (Enter data in the blue cells)
Maximum Allowable Width for Spread Behind Curb TBACK = 16.0 ft
Side Slope Behind Curb (leave blank for no conveyance credit behind curb) SBACK = 0.020 ft/ft
Manning's Roughness Behind Curb (typically between 0.012 and 0.020) nBACK = 0.020
Height of Curb at Gutter Flow Line HCURB = 6.00 inches
Distance from Curb Face to Street Crown TCROWN = 40.0 ft
Gutter Width W = 2.00 ft
Street Transverse Slope SX = 0.020 ft/ft
Gutter Cross Slope (typically 2 inches over 24 inches or 0.083 ft/ft) SW = 0.083 ft/ft
Street Longitudinal Slope - Enter 0 for sump condition SO = 0.000 ft/ft
Manning's Roughness for Street Section (typically between 0.012 and 0.020) nSTREET = 0.013
Minor Storm Major Storm
Max. Allowable Spread for Minor & Major Storm TMAX = 12.0 32.0 ft
Max. Allowable Depth at Gutter Flowline for Minor & Major Storm dMAX = 6.0 12.0 inches
Allow Flow Depth at Street Crown (leave blank for no) check = yes
MINOR STORM Allowable Capacity is based on Depth Criterion Minor Storm Major Storm
MAJOR STORM Allowable Capacity is based on Depth Criterion Qallow = SUMP SUMP cfs
ALLOWABLE CAPACITY FOR ONE-HALF OF STREET (Minor & Major Storm)
(Based on Regulated Criteria for Maximum Allowable Flow Depth and Spread)
H23
Inlet-01A
UD-Inlet_v4.01, Inlet-1A 11/22/2016, 11:16 AM
Design Information (Input) MINOR MAJOR
Type of Inlet Type =
Local Depression (additional to continuous gutter depression 'a' from 'Q-Allow') alocal = 3.00 3.00 inches
Number of Unit Inlets (Grate or Curb Opening) No = 3 3
Water Depth at Flowline (outside of local depression) Ponding Depth = 4.4 9.2 inches
Grate Information MINOR MAJOR
Length of a Unit Grate Lo (G) = N/A N/A feet
Width of a Unit Grate Wo = N/A N/A feet
Area Opening Ratio for a Grate (typical values 0.15-0.90) Aratio = N/A N/A
Clogging Factor for a Single Grate (typical value 0.50 - 0.70) Cf (G) = N/A N/A
Grate Weir Coefficient (typical value 2.15 - 3.60) Cw (G) = N/A N/A
Grate Orifice Coefficient (typical value 0.60 - 0.80) Co (G) = N/A N/A
Curb Opening Information MINOR MAJOR
Length of a Unit Curb Opening Lo (C) = 5.00 5.00 feet
Height of Vertical Curb Opening in Inches Hvert = 6.00 6.00 inches
Height of Curb Orifice Throat in Inches Hthroat = 6.00 6.00 inches
Angle of Throat (see USDCM Figure ST-5) Theta = 63.40 63.40 degrees
Side Width for Depression Pan (typically the gutter width of 2 feet) Wp = 2.00 2.00 feet
Clogging Factor for a Single Curb Opening (typical value 0.10) Cf (C) = 0.10 0.10
Curb Opening Weir Coefficient (typical value 2.3-3.7) Cw (C) = 3.60 3.60
Curb Opening Orifice Coefficient (typical value 0.60 - 0.70) Co (C) = 0.67 0.67
Low Head Performance Reduction (Calculated) MINOR MAJOR
Depth for Grate Midwidth dGrate = N/A N/A ft
Depth for Curb Opening Weir Equation dCurb = 0.20 0.60 ft
Combination Inlet Performance Reduction Factor for Long Inlets RFCombination = 0.42 0.87
Curb Opening Performance Reduction Factor for Long Inlets RFCurb = 0.67 0.94
Grated Inlet Performance Reduction Factor for Long Inlets RFGrate = N/A N/A
MINOR MAJOR
Total Inlet Interception Capacity (assumes clogged condition) Qa
= 5.3 34.0 cfs
Inlet Capacity IS GOOD for Minor and Major Storms(>Q PEAK) Q PEAK REQUIRED = 4.5 21.0 cfs
INLET IN A SUMP OR SAG LOCATION
CDOT Type R Curb Opening
H-Vert
H-Curb
W
Lo (C)
Lo (G)
Wo
WP
CDOT Type R Curb Opening
Override Depths
UD-Inlet_v4.01, Inlet-1A 11/22/2016, 11:16 AM
APPENDIX F
Detention Pond Design
H23 Preliminary Drainage Report
Detention Requirements Pond A - Major Storm (100-yr)
Mass Balance Method
0.77
1.61 cfs
9.76 acres
Detention
Requirements 1.92 acre-feet
City of Fort Collins
criteria (120%) 2.30 acre-feet
Time (min) Intensity (in/hr) Qin (cfs) Volume
in (ft
3
) Qout
(cfs) Volumeout
(ft
3
) Volume detained
(acre-feet)
5 9.95 74.776 22,432.87 1.61 483 0.504
10 7.72 58.017 34,810.41 1.61 966 0.777
15 6.52 48.999 44,099.19 1.61 1,449 0.979
20 5.60 42.085 50,502.14 1.61 1,932 1.115
25 4.98 37.426 56,138.54 1.61 2,416 1.233
30 4.52 33.969 61,143.67 1.61 2,899 1.337
35 4.08 30.662 64,390.23 1.61 3,382 1.401
40 3.74 28.107 67,456.44 1.61 3,865 1.460
45 3.46 26.003 70,207.00 1.61 4,348 1.512
50 3.23 24.274 72,822.29 1.61 4,831 1.561
55 3.03 22.771 75,144.48 1.61 5,314 1.603
60 2.86 21.493 77,376.50 1.61 5,797 1.643
65 2.70 20.291 79,135.06 1.61 6,281 1.673
70 2.60 19.540 82,065.98 1.61 6,764 1.729
75 2.50 18.788 84,546.00 1.61 7,247 1.775
80 2.40 18.036 86,575.10 1.61 7,730 1.810
85 2.30 17.285 88,153.30 1.61 8,213 1.835
90 2.20 16.533 89,280.58 1.61 8,696 1.850
100 2.05 15.406 92,436.96 1.61 9,662 1.900
110 1.90 14.279 94,240.61 1.61 10,629 1.919
120 1.75 13.152 94,691.52 1.61 11,595 1.908
130 1.60 12.024 93,789.70 1.61 12,561 1.865
140 1.40 10.521 88,378.75 1.61 13,527 1.718
150 1.20 9.018 81,164.16 1.61 14,494 1.531
160 1.15 8.642 82,967.81 1.61 15,460 1.550
170 1.10 8.267 84,320.54 1.61 16,426 1.559
180 1.05 7.891 85,222.37 1.61 17,392 1.557
Runoff Coefficient, C =
2-yr Release, Qout =
Area =
Appendix F
Project:
Basin ID:
Design Information (Input):
Width of Basin Bottom, W = ft Right Triangle OR…
Length of Basin Bottom, L = ft Isosceles Triangle OR…
Dam Side-slope (H:V), Zd = ft/ft Rectangle OR…
Circle / Ellipse OR…
Irregular X (Use Overide values in cells G32:G52)
MINOR MAJOR
Storage Requirement from Sheet 'Modified FAA': acre-ft.
Stage-Storage Relationship: Storage Requirement from Sheet 'Hydrograph': acre-ft.
Storage Requirement from Sheet 'Full-Spectrum': acre-ft.
Labels Water Side Basin Basin Surface Surface Volume Surface Volume Target Volumes
for WQCV, Minor, Surface Slope Width at Length at Area at Area at Below Area at Below for WQCV, Minor,
& Major Storage Elevation (H:V) Stage Stage Stage Stage Stage Stage Stage & Major Storage
Stages ft ft/ft ft ft ft
2
ft
2
User ft
3
acres acre-ft Volumes
(input) (input) Below El. (output) (output) (output) Overide (output) (output) (output) (for goal seek)
4843.00 (input) 18,220 0 0.418 0.000
WQCV 4844.00 4.00 0.00 0.00 21,150 19,685 0.486 0.452
4845.00 4.00 0.00 0.00 24,180 42,350 0.555 0.972
4846.00 4.00 0.00 0.00 27,312 68,096 0.627 1.563
4847.00 4.00 0.00 0.00 30,544 97,024 0.701 2.227
100-yr + WQCV 4847.75 4.00 0.00 0.00 33,035 120,866 0.758 2.775
4848.00 4.00 0.00 0.00 38,013 129,747 0.873 2.979
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
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Project:
Basin ID:
STAGE-STORAGE SIZING FOR DETENTION BASINS
4843.00
4844.00
4845.00
4846.00
4847.00
4848.00
4849.00
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50
Stage (ft. elev.)
Storage (acre-feet)
STAGE-STORAGE CURVE FOR THE POND
UD Detention Pond A VolCalc, Basin 11/22/2016, 11:50 AM
Sheet 1 of 3
Designer:
Company:
Date:
Project:
Location:
1. Baseflow
A) Is the permanent pool established by groundwater?
2. Surcharge Volume
A) Effective Imperviousness of Tributary Area, Ia
Ia = 55.0 %
B) Tributary Area's Imperviousness Ratio (i = Ia / 100 ) i = 0.550
C) Contributing Watershed Area Area = 9.760 ac
D) For Watersheds Outside of the Denver Region, Depth of Average d6 = in
Runoff Producing Storm
E) Design Concept
(Select EURV when also designing for flood control)
F) Water Quality Capture Volume (WQCV) Design Volume VWQCV
= 0.161 ac-ft
Based on 24-hour Drain Time
(VWQCV = (0.9 * (0.91 * i
3
- 1.19 * i
2
+ 0.78 * i) / 12 * Area)
G) For Watersheds Outside of the Denver Region, VWQCV OTHER
= ac-ft
Water Quality Capture Volume (WQCV) Design Volume
(VWQCV OTHER = (d
6*(VWQCV
/0.43))
H) User Input of Water Quality Capture Volume (WQCV) Design Volume VWQCV USER
= ac-ft
(Only if a different WQCV Design Volume is desired)
I) Predominant Watershed NRCS Soil Group
WQCV selected. Soil group not required.
J) Excess Urban Runoff Volume (EURV) Design Volume
For HSG A: EURVA = 1.68 * i
1.28
For HSG B: EURVB = 1.36 * i
1.08
For HSG C/D: EURVC/D = 1.20 * i
1.08
EURV = ac-f t
3. Depth of Surcharge WQCV DWQCV
= 1.0 ft
(Should not exceed 2 feet, required even if EURV is chosen)
4. Basin Shape L : W = 4.0 : 1
(It is recommended to have a basin length to width ratio between 2:1 and 4:1)
5. Permanent Pool
A) Minimum Permanent Pool Volume VPOOL
= 0.121 ac-ft
6. Side Slopes
A) Maximum Side Slope Above the Safety Wetland Bench Z = 4.00 ft / ft
(Horiz. dist. per unit vertical, should be no steeper than 4:1)
7. Inlet
A) Describe means of providing energy dissipation at concentrated
inflow locations:
DAP
TST, Inc. Consulting Engineers
Project:
Basin ID:
Design Information (Input):
Width of Basin Bottom, W = ft Right Triangle OR…
Length of Basin Bottom, L = ft Isosceles Triangle OR…
Dam Side-slope (H:V), Zd = ft/ft Rectangle OR…
Circle / Ellipse OR…
Irregular X (Use Overide values in cells G32:G52)
MINOR MAJOR
Storage Requirement from Sheet 'Modified FAA': acre-ft.
Stage-Storage Relationship: Storage Requirement from Sheet 'Hydrograph': acre-ft.
Storage Requirement from Sheet 'Full-Spectrum': acre-ft.
Labels Water Side Basin Basin Surface Surface Volume Surface Volume Target Volumes
for WQCV, Minor, Surface Slope Width at Length at Area at Area at Below Area at Below for WQCV, Minor,
& Major Storage Elevation (H:V) Stage Stage Stage Stage Stage Stage Stage & Major Storage
Stages ft ft/ft ft ft ft
2
ft
2
User ft
3
acres acre-ft Volumes
(input) (input) Below El. (output) (output) (output) Overide (output) (output) (output) (for goal seek)
4842.00 (input) 15,391 0 0.353 0.000
4843.00 4.00 0.00 0.00 18,220 16,806 0.418 0.386
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Project:
Basin ID:
STAGE-STORAGE SIZING FOR DETENTION BASINS
4842.00
4842.20
4842.40
4842.60
4842.80
4843.00
4843.20
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45
Stage (ft. elev.)
Storage (acre-feet)
STAGE-STORAGE CURVE FOR THE POND
UD Detention Pond A PermPool VolCalc, Basin 9/27/2016, 10:19 AM
H23 Preliminary Drainage Report
Detention Requirements Pond B - Major Storm (100-yr)
Mass Balance Method
0.79
1.93 cfs
11.70 acres
Detention
Requirements 2.37 acre-feet
City of Fort Collins
criteria (120%) 2.84 acre-feet
Time (min) Intensity (in/hr) Qin (cfs) Volume
in (ft
3
) Qout
(cfs) Volumeout
(ft
3
) Volume detained
(acre-feet)
5 9.95 91.968 27,590.36 1.93 579 0.620
10 7.72 71.356 42,813.58 1.93 1,158 0.956
15 6.52 60.264 54,237.92 1.93 1,737 1.205
20 5.60 51.761 62,112.96 1.93 2,317 1.373
25 4.98 46.030 69,045.21 1.93 2,896 1.519
30 4.52 41.778 75,201.05 1.93 3,475 1.647
35 4.08 37.711 79,194.02 1.93 4,054 1.725
40 3.74 34.569 82,965.17 1.93 4,633 1.798
45 3.46 31.981 86,348.11 1.93 5,212 1.863
50 3.23 29.855 89,564.67 1.93 5,792 1.923
55 3.03 28.006 92,420.76 1.93 6,371 1.975
60 2.86 26.435 95,165.93 1.93 6,950 2.025
65 2.70 24.956 97,328.79 1.93 7,529 2.062
70 2.60 24.032 100,933.56 1.93 8,108 2.131
75 2.50 23.108 103,983.75 1.93 8,687 2.188
80 2.40 22.183 106,479.36 1.93 9,266 2.232
85 2.30 21.259 108,420.39 1.93 9,846 2.263
90 2.20 20.335 109,806.84 1.93 10,425 2.282
100 2.05 18.948 113,688.90 1.93 11,583 2.344
110 1.90 17.562 115,907.22 1.93 12,741 2.368
120 1.75 16.175 116,461.80 1.93 13,900 2.355
130 1.60 14.789 115,352.64 1.93 15,058 2.302
140 1.40 12.940 108,697.68 1.93 16,216 2.123
150 1.20 11.092 99,824.40 1.93 17,375 1.893
160 1.15 10.629 102,042.72 1.93 18,533 1.917
170 1.10 10.167 103,706.46 1.93 19,691 1.929
180 1.05 9.705 104,815.62 1.93 20,849 1.928
Runoff Coefficient, C =
2-yr Release, Qout =
Area =
Appendix F
Project:
Basin ID:
Design Information (Input):
Width of Basin Bottom, W = ft Right Triangle OR…
Length of Basin Bottom, L = ft Isosceles Triangle OR…
Dam Side-slope (H:V), Zd = ft/ft Rectangle OR…
Circle / Ellipse OR…
Irregular X (Use Overide values in cells G32:G52)
MINOR MAJOR
Storage Requirement from Sheet 'Modified FAA': acre-ft.
Stage-Storage Relationship: Storage Requirement from Sheet 'Hydrograph': acre-ft.
Storage Requirement from Sheet 'Full-Spectrum': acre-ft.
Labels Water Side Basin Basin Surface Surface Volume Surface Volume Target Volumes
for WQCV, Minor, Surface Slope Width at Length at Area at Area at Below Area at Below for WQCV, Minor,
& Major Storage Elevation (H:V) Stage Stage Stage Stage Stage Stage Stage & Major Storage
Stages ft ft/ft ft ft ft
2
ft
2
User ft
3
acres acre-ft Volumes
(input) (input) Below El. (output) (output) (output) Overide (output) (output) (output) (for goal seek)
4839.00 (input) 8,417 0 0.193 0.000
WQCV 4840.00 4.00 0.00 0.00 10,896 9,657 0.250 0.222
4841.00 4.00 0.00 0.00 13,604 21,907 0.312 0.503
4842.00 4.00 0.00 0.00 16,778 37,098 0.385 0.852
4843.00 4.00 0.00 0.00 20,071 55,522 0.461 1.275
4844.00 4.00 0.00 0.00 23,406 77,261 0.537 1.774
4845.00 4.00 0.00 0.00 27,602 102,765 0.634 2.359
100-yr + WQCV 4846.00 4.00 0.00 0.00 32,629 132,880 0.749 3.051
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Project:
Basin ID:
STAGE-STORAGE SIZING FOR DETENTION BASINS
4839.00
4840.00
4841.00
4842.00
4843.00
4844.00
4845.00
4846.00
4847.00
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50
Stage (ft. elev.)
Storage (acre-feet)
STAGE-STORAGE CURVE FOR THE POND
UD Detention Pond B VolCalc, Basin 11/22/2016, 12:01 PM
Sheet 1 of 3
Designer:
Company:
Date:
Project:
Location:
1. Baseflow
A) Is the permanent pool established by groundwater?
2. Surcharge Volume
A) Effective Imperviousness of Tributary Area, Ia
Ia = 56.9 %
B) Tributary Area's Imperviousness Ratio (i = Ia / 100 ) i = 0.569
C) Contributing Watershed Area Area = 11.700 ac
D) For Watersheds Outside of the Denver Region, Depth of Average d6 = in
Runoff Producing Storm
E) Design Concept
(Select EURV when also designing for flood control)
F) Water Quality Capture Volume (WQCV) Design Volume VWQCV
= 0.198 ac-ft
Based on 24-hour Drain Time
(VWQCV = (0.9 * (0.91 * i
3
- 1.19 * i
2
+ 0.78 * i) / 12 * Area)
G) For Watersheds Outside of the Denver Region, VWQCV OTHER
= ac-ft
Water Quality Capture Volume (WQCV) Design Volume
(VWQCV OTHER = (d
6*(VWQCV
/0.43))
H) User Input of Water Quality Capture Volume (WQCV) Design Volume VWQCV USER
= ac-ft
(Only if a different WQCV Design Volume is desired)
I) Predominant Watershed NRCS Soil Group
WQCV selected. Soil group not required.
J) Excess Urban Runoff Volume (EURV) Design Volume
For HSG A: EURVA = 1.68 * i
1.28
For HSG B: EURVB = 1.36 * i
1.08
For HSG C/D: EURVC/D = 1.20 * i
1.08
EURV = ac-f t
3. Depth of Surcharge WQCV DWQCV
= 1.0 ft
(Should not exceed 2 feet, required even if EURV is chosen)
4. Basin Shape L : W = 4.0 : 1
(It is recommended to have a basin length to width ratio between 2:1 and 4:1)
5. Permanent Pool
A) Minimum Permanent Pool Volume VPOOL
= 0.149 ac-ft
6. Side Slopes
A) Maximum Side Slope Above the Safety Wetland Bench Z = 4.00 ft / ft
(Horiz. dist. per unit vertical, should be no steeper than 4:1)
7. Inlet
A) Describe means of providing energy dissipation at concentrated
inflow locations:
DAP
TST, Inc. Consulting Engineers
Project:
Basin ID:
Design Information (Input):
Width of Basin Bottom, W = ft Right Triangle OR…
Length of Basin Bottom, L = ft Isosceles Triangle OR…
Dam Side-slope (H:V), Zd = ft/ft Rectangle OR…
Circle / Ellipse OR…
Irregular X (Use Overide values in cells G32:G52)
MINOR MAJOR
Storage Requirement from Sheet 'Modified FAA': acre-ft.
Stage-Storage Relationship: Storage Requirement from Sheet 'Hydrograph': acre-ft.
Storage Requirement from Sheet 'Full-Spectrum': acre-ft.
Labels Water Side Basin Basin Surface Surface Volume Surface Volume Target Volumes
for WQCV, Minor, Surface Slope Width at Length at Area at Area at Below Area at Below for WQCV, Minor,
& Major Storage Elevation (H:V) Stage Stage Stage Stage Stage Stage Stage & Major Storage
Stages ft ft/ft ft ft ft
2
ft
2
User ft
3
acres acre-ft Volumes
(input) (input) Below El. (output) (output) (output) Overide (output) (output) (output) (for goal seek)
4838.00 (input) 6,220 0 0.143 0.000
4839.00 4.00 0.00 0.00 8,417 7,319 0.193 0.168
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Project:
Basin ID:
STAGE-STORAGE SIZING FOR DETENTION BASINS
4838.00
4838.20
4838.40
4838.60
4838.80
4839.00
4839.20
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18
Stage (ft. elev.)
Storage (acre-feet)
STAGE-STORAGE CURVE FOR THE POND
UD Detention Pond B PermPool VolCalc, Basin 9/27/2016, 10:19 AM
H23 Preliminary Drainage Report
Detention Requirements Offsite Pond - Major Storm (100-yr)
Mass Balance Method
0.79
0.91 cfs
5.52 acres
Detention
Requirements 1.12 acre-feet
City of Fort Collins
criteria (120%) 1.34 acre-feet
Time (min) Intensity (in/hr) Qin (cfs) Volume
in (ft
3
) Qout
(cfs) Volumeout
(ft
3
) Volume detained
(acre-feet)
5 9.95 43.390 13,016.99 0.91 273 0.293
10 7.72 33.665 20,199.23 0.91 546 0.451
15 6.52 28.432 25,589.17 0.91 820 0.569
20 5.60 24.420 29,304.58 0.91 1,093 0.648
25 4.98 21.717 32,575.18 0.91 1,366 0.716
30 4.52 19.711 35,479.47 0.91 1,639 0.777
35 4.08 17.792 37,363.33 0.91 1,913 0.814
40 3.74 16.309 39,142.54 0.91 2,186 0.848
45 3.46 15.088 40,738.59 0.91 2,459 0.879
50 3.23 14.085 42,256.15 0.91 2,732 0.907
55 3.03 13.213 43,603.64 0.91 3,006 0.932
60 2.86 12.472 44,898.80 0.91 3,279 0.955
65 2.70 11.774 45,919.22 0.91 3,552 0.973
70 2.60 11.338 47,619.94 0.91 3,825 1.005
75 2.50 10.902 49,059.00 0.91 4,099 1.032
80 2.40 10.466 50,236.42 0.91 4,372 1.053
85 2.30 10.030 51,152.18 0.91 4,645 1.068
90 2.20 9.594 51,806.30 0.91 4,918 1.076
100 2.05 8.940 53,637.84 0.91 5,465 1.106
110 1.90 8.286 54,684.43 0.91 6,011 1.117
120 1.75 7.631 54,946.08 0.91 6,558 1.111
130 1.60 6.977 54,422.78 0.91 7,104 1.086
140 1.40 6.105 51,283.01 0.91 7,651 1.002
150 1.20 5.233 47,096.64 0.91 8,197 0.893
160 1.15 5.015 48,143.23 0.91 8,744 0.904
170 1.10 4.797 48,928.18 0.91 9,290 0.910
180 1.05 4.579 49,451.47 0.91 9,837 0.909
Runoff Coefficient, C =
2-yr Release, Qout =
Area =
Appendix F
APPENDIX G
FIRM Maps and
Original Floodplain Workmap
Page 1 of 5 Issue Date: August 1, 2014 Effective Date: December 15, 2014 Case No.: LOMR-APP
14-08-0580P
Washington, D.C. 20472
Federal Emergency Management Agency
LETTER OF MAP REVISION
DETERMINATION DOCUMENT
COMMUNITY AND REVISION INFORMATION PROJECT DESCRIPTION
COMMUNITY
APPROXIMATE LATITUDE & LONGITUDE: 40.518, -104.995
SOURCE: USGS QUADRANGLE DATUM: NAD 83
City of Fort Collins
Larimer County
Colorado
COMMUNITY NO.: 080102
BASIS OF REQUEST
IDENTIFIER
NO PROJECT UPDATE
Cache La Poudre River I-25 Divided Flow Path
ANNOTATED MAPPING ENCLOSURES ANNOTATED STUDY ENCLOSURES
TYPE: FIRM* NO.: 08069C0994F DATE: December 19, 2006
TYPE: FIRM* NO.: 08069C1013F DATE: December 19, 2006
DATE OF EFFECTIVE FLOOD INSURANCE STUDY:
Enclosures reflect changes to flooding sources affected by this revision.
* FIRM - Flood Insurance Rate Map; ** FBFM - Flood Boundary and Floodway Map; *** FHBM - Flood Hazard Boundary Map
FLOODING SOURCE(S) & REVISED REACH(ES)
Cache La Poudre River I-25 Divided Flow Path - from approximately 110 feet downstream of County Road 36 to approximately 140 feet upstream of County Road 38
SUMMARY OF REVISIONS
Flooding Source Effective Flooding Revised Flooding Increases Decreases
Cache La Poudre River I-25 Divided Flow Path Zone AE Zone AE YES YES
Zone X (shaded) Zone X (shaded) YES YES
Floodway Floodway YES YES
* BFEs - Base Flood Elevations
DETERMINATION
This document provides the determination from the Department of Homeland Security's Federal Emergency Management Agency (FEMA) regarding
a request for a Letter of Map Revision (LOMR) for the area described above. Using the information submitted, we have determined that a revision to
the flood hazards depicted in the Flood Insurance Study (FIS) report and/or National Flood Insurance Program (NFIP) map is warranted. This
document revises the effective NFIP map, as indicated in the attached documentation. Please use the enclosed annotated map panels revised by
this LOMR for floodplain management purposes and for all flood insurance policies and renewals in your community.
This determination is based on the flood data presently available. The enclosed documents provide additional information regarding this determination. If you have any
questions about this document, please contact the FEMA Map Information eXchange toll free at 1-877-336-2627 (1-877-FEMA MAP) or by letter addressed to the LOMC
Clearinghouse, 847 South Pickett Street, Alexandria, VA 22304-4605. Additional Information about the NFIP is available on our website at http://www.fema.gov/nfip.
132942 PT202.BKR.14080580P.H20 102-I-C
Luis Rodriguez, P.E., Chief
Engineering Management Branch
Federal Insurance and Mitigation Administration
FLOODWAY DATA TABLE: 4
February 06, 2013
Page 2 of 5 Issue Date: Effective Date: Case No.: 14-08-0580P LOMR-APP
Washington, D.C. 20472
Issue Date: August 1, 2014 Effective Date: December 15, 2014
Federal Emergency Management Agency
LETTER OF MAP REVISION
DETERMINATION DOCUMENT (CONTINUED)
OTHER COMMUNITIES AFFECTED BY THIS REVISION
CID Number: 080101 Name: Larimer County, Colorado
AFFECTED MAP PANELS AFFECTED PORTIONS OF THE FLOOD INSURANCE STUDY REPORT
DATE OF EFFECTIVE FLOOD INSURANCE STUDY
FLOODWAY DATA TABLE: 4
TYPE: FIRM*
TYPE: FIRM*
NO.: 08069C0994F
NO.: 08069C1013F
DATE: December 19, 2006
DATE: December 19, 2006
This determination is based on the flood data presently available. The enclosed documents provide additional information regarding this determination. If you have any
questions about this document, please contact the FEMA Map Information eXchange toll free at 1-877-336-2627 (1-877-FEMA MAP) or by letter addressed to the LOMC
Clearinghouse, 847 South Pickett Street, Alexandria, VA 22304-4605. Additional Information about the NFIP is available on our website at http://www.fema.gov/nfip.
132942 PT202.BKR.14080580P.H20 102-I-C
Luis Rodriguez, P.E., Chief
Engineering Management Branch
Federal Insurance and Mitigation Administration
: February 6, 2013
Page 3 of 5 Issue Date: Effective Date: Case No.: 14-08-0580P LOMR-APP
Washington, D.C. 20472
Issue Date: August 1, 2014 Effective Date: December 15, 2014
Federal Emergency Management Agency
LETTER OF MAP REVISION
DETERMINATION DOCUMENT (CONTINUED)
COMMUNITY INFORMATION
APPLICABLE NFIP REGULATIONS/COMMUNITY OBLIGATION
We have made this determination pursuant to Section 206 of the Flood Disaster Protection Act of 1973 (P.L. 93-234) and in accordance with
the National Flood Insurance Act of 1968, as amended (Title XIII of the Housing and Urban Development Act of 1968, P.L. 90-448),
42 U.S.C. 4001-4128, and 44 CFR Part 65. Pursuant to Section 1361 of the National Flood Insurance Act of 1968, as amended,
communities participating in the NFIP are required to adopt and enforce floodplain management regulations that meet or exceed NFIP
criteria. These criteria, including adoption of the FIS report and FIRM, and the modifications made by this LOMR, are the minimum
requirements for continued NFIP participation and do not supersede more stringent State/Commonwealth or local requirements to which the
regulations apply.
We provide the floodway designation to your community as a tool to regulate floodplain development. Therefore, the floodway revision
we have described in this letter, while acceptable to us, must also be acceptable to your community and adopted by appropriate community
action, as specified in Paragraph 60.3(d) of the NFIP regulations.
COMMUNITY REMINDERS
We based this determination on the base (1-percent-annual-chance) flood discharges computed in the FIS for your community without
considering subsequent changes in watershed characteristics that could increase flood discharges. Future development of projects
upstream could cause increased flood discharges, which could cause increased flood hazards. A comprehensive restudy of your
community’s flood hazards would consider the cumulative effects of development on flood discharges subsequent to the publication of the
FIS report for your community and could, therefore, establish greater flood hazards in this area.
Your community must regulate all proposed floodplain development and ensure that permits required by Federal and/or
State/Commonwealth law have been obtained. State/Commonwealth or community officials, based on knowledge of local conditions and
in the interest of safety, may set higher standards for construction or may limit development in floodplain areas. If your
State/Commonwealth or community has adopted more restrictive or comprehensive floodplain management criteria, those criteria take
precedence over the minimum NFIP requirements.
We will not print and distribute this LOMR to primary users, such as local insurance agents or mortgage lenders; instead, the community
will serve as a repository for the new data. We encourage you to disseminate the information in this LOMR by preparing a news release for
publication in your community's newspaper that describes the revision and explains how your community will provide the data and help
interpret the NFIP maps. In that way, interested persons, such as property owners, insurance agents, and mortgage lenders, can benefit
from the information.
This determination is based on the flood data presently available. The enclosed documents provide additional information regarding this determination. If you have any
questions about this document, please contact the FEMA Map Information eXchange toll free at 1-877-336-2627 (1-877-FEMA MAP) or by letter addressed to the LOMC
Clearinghouse, 847 South Pickett Street, Alexandria, VA 22304-4605. Additional Information about the NFIP is available on our website at http://www.fema.gov/nfip.
132942 PT202.BKR.14080580P.H20 102-I-C
Luis Rodriguez, P.E., Chief
Engineering Management Branch
Federal Insurance and Mitigation Administration
Page 4 of 5 Issue Date: Effective Date: Case No.: 14-08-0580P LOMR-APP
Washington, D.C. 20472
Issue Date: August 1, 2014 Effective Date: December 15, 2014
Federal Emergency Management Agency
LETTER OF MAP REVISION
DETERMINATION DOCUMENT (CONTINUED)
COMMUNITY INFORMATION (CONTINUED)
We have designated a Consultation Coordination Officer (CCO) to assist your community. The CCO will be the primary liaison between
your community and FEMA. For information regarding your CCO, please contact:
Ms. Jeanine D. Petterson
Director, Mitigation Division
Federal Emergency Management Agency, Region VIII
Denver Federal Center, Building 710
P.O. Box 25267
Denver, CO 80225-0267
(303) 235-4830
STATUS OF THE COMMUNITY NFIP MAPS
We will not physically revise and republish the FIRM and FIS report for your community to reflect the modifications made by this LOMR
at this time. When changes to the previously cited FIRM panel(s) and FIS report warrant physical revision and republication in the future,
we will incorporate the modifications made by this LOMR at that time.
This determination is based on the flood data presently available. The enclosed documents provide additional information regarding this determination. If you have any
questions about this document, please contact the FEMA Map Information eXchange toll free at 1-877-336-2627 (1-877-FEMA MAP) or by letter addressed to the LOMC
Clearinghouse, 847 South Pickett Street, Alexandria, VA 22304-4605. Additional Information about the NFIP is available on our website at http://www.fema.gov/nfip.
132942 PT202.BKR.14080580P.H20 102-I-C
Luis Rodriguez, P.E., Chief
Engineering Management Branch
Federal Insurance and Mitigation Administration
Page 5 of 5 Issue Date: Effective Date: Case No.: 14-08-0580P LOMR-APP
Washington, D.C. 20472
Issue Date: August 1, 2014 Effective Date: December 15, 2014
Federal Emergency Management Agency
LETTER OF MAP REVISION
DETERMINATION DOCUMENT (CONTINUED)
COMMUNITY INFORMATION (CONTINUED)
A notice of changes will be published in the Federal Register. This information also will be published in your local newspaper on or about
the dates listed below and through FEMA’s Flood Hazard Mapping website at https://www.floodmaps.fema.gov/fhm/Scripts/bfe_main.asp.
LOCAL NEWSPAPER Name: The Fort Collins Coloradoan
Dates: August 8, 2014 and August 15, 2014
Within 90 days of the second publication in the local newspaper, a citizen may request that we reconsider this determination. Any request
for reconsideration must be based on scientific or technical data. Therefore, this letter will be effective only after the 90-day appeal period
has elapsed and we have resolved any appeals that we receive during this appeal period. Until this LOMR is effective, the revised flood
hazard determination information presented in this LOMR may be changed.
This determination is based on the flood data presently available. The enclosed documents provide additional information regarding this determination. If you have any
questions about this document, please contact the FEMA Map Information eXchange toll free at 1-877-336-2627 (1-877-FEMA MAP) or by letter addressed to the LOMC
Clearinghouse, 847 South Pickett Street, Alexandria, VA 22304-4605. Additional Information about the NFIP is available on our website at http://www.fema.gov/nfip.
132942 PT202.BKR.14080580P.H20 102-I-C
Luis Rodriguez, P.E., Chief
Engineering Management Branch
Federal Insurance and Mitigation Administration
15,100 3,600 11,992 1.3 4,820.7 4,820.7 4,821.4 0.7
18,988 901 2,871 1.6 4,826.7 4,826.7 4,827.6 0.9
C 20,738 385 446 6.0 4,833.5 4,833.5 4,833.9 0.4
D 21,993 1,011 1,277 2.1 4,836.7 4,836.7 4,837.7 1.0
E 23,278 1,004 4,622 1.0 4,837.1 4,837.1 4,838.0 0.9
F 24,268 1,000 3,848 1.2 4,837.3 4,837.3 4,838.2 0.9
G 24,993 900 890 5.3 4,841.1 4,841.1 4,841.3 0.2
H 25,893 1,000 3,163 1.5 4,842.2 4,842.2 4,843.2 1.0
I 27,193 700 1,154 4.1 4,843.9 4,843.9 4,844.7 0.8
J 27,903 600 726 6.5 4,849.9 4,849.9 4,850.4 0.5
K 29,603 796 3,402 1.4 4,850.3 4,850.3 4,851.3 1.0
L 30,103 451 692 6.8 4,853.3 4,853.3 4,853.6 0.3
1 Feet above Larimer-Weld County Line Road
FEDERAL EMERGENCY MANAGEMENT AGENCY
LARIMER COUNTY, CO
AND INCORPORATED AREAS
CACHE LA POUDRE
RIVER - I-25 DIVIDED
FLOW
TABLE 4
FLOODWAY
1-PERCENT ANNUAL CHANCE FLOOD
WATER SURFACE ELEVATION
FLOODWAY DATA
CACHE LA POUDRE RIVER -
INTERSTATE HIGHWAY 25 DIVIDED FLOW
FLOODING SOURCE
A
B
CROSS SECTION DISTANCE1 WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
REGULATORY
(NAVD 88)
INCREASE
(FEET)
WITHOUT
FLOODWAY
(FEET NAVD)
WITH
FLOODWAY
(NAVD 88)
REVISED
DATA
REVISED
DATA
4834
4831
4832
FLOODING I-25 DIVIDED EFFECTS FLOW
CHACHE LA POUDRE RIVER
c7
STAR GAZER DRIVE
ECLIPSE LANE
OBSERVATORY DRIVE
SOUTHERN CROSS
LANE
ROCK CREEK DRIVE
STEELHEAD STREET
PRECISION DRIVE
,
,
,
,
K
4850.3
4850
4850
4847
4843
,
,
,
,
G
4841.1
,
,
,
,
H
4842.2
,
,
,
,
I
4843.9
BROOKFIELD DRIVE
NORTHERN DRIVE LIGHTS
CINQUEFOIL LANE
4837.1
,
,
,
,
C
BIG DIPPER DRIVE
LITTLE DRIVE DIPPER
GALILEO DRIVE
FULL DRIVE MOON
,
,
,
,
J
,
,
,
,
J
4849.9
,
,
,
,
L
4853.3
4835
4833
4836
4834
4834
4843
4828
4831
4836
4844
4831
4832
BYREVISED AREA LOMR DATED
NOVEMBER 2008 14,
4853
4829
4830
4835
4837
4850
4837
4847
4837
4839
4841
4842
4843
4842
4841
4853
4850
REAVRIESAED
4836
4850
4837
4844
4839
,
,
,
,
I
4843.9
,
,
,
,
H
APPENDIX H
Excerpts from Referenced Reports
Harmony & Strauss Cabin Convenience Shopping Center
Subdivision Filing No. 1 PDP Final Drainage Report
Page 12
sites).
Release from these undeveloped areas amounts to 69.8 cfs. This amount will be treated as a
pass-through flow. The resulting amount of stormwater passing from the detention basin will be
approximately 78.8 cfs (9.0 + 69.8). Stormwater release from the detention basin will be
provided at the existing spillway at the southeastern finger of Existing Gravel Pond #2.
The required amount of stormwater detention for the combined CONVENIENCE SHOPPING
CENTER and WEITZEL POND #1 project areas is 9.64 acre-feet based on the Modified FAA
method for determining detention volumes, plus an additional 20% (City criteria), resulting in
11.57 acre-feet. The area of the Existing Gravel Ponds #1 and #2 will constitute the detention
basin for this project. The storage capacity for that pond is provided below in Table 3.
Table 3.
Pond ID Surface
Area
Groundwater Surface
Elevation
Detention Surface
Elevation
Total Storage
Ex. Gravel
Pond #1
11.0 acres 4838.23 4838.59 3.96 Ac-ft
Ex. Gravel
Pond #2
14.53 acres 4837.12 4837.64 7.61 Ac-ft
Total Required 11.57 Ac-ft
The required detention water depth in the ponds is 0.36’ and 0.52’, respectively and will require
that the existing spillway location top of bank to be raised to allow for detention capacity. In
order to construct fill in the floodway a no-rise certification will be required to prove that the new
elevations do not exceed those used in the approved LOMR 14-08-0580P (effective December
15, 2014). A spot elevation indicating 4835.5, located in the vicinity of the proposed improved
spillway, is shown on that workmap. The elevations on that workmap are shown in NGVD29
datum, thus the translated elevations to the NAVD88 datum would be 3.18’ higher, resulting in a
spot elevation of 4838.68 in this vicinity. This workmap is included in the Appendix for
reference.
In addition, the detention water surface elevation is still well below the Base Flood Elevations
(BFE) as published on the current FEMA maps, the worst case BFE in that area being 4841.
A WQCV calculation is 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. In addition, the permeable pavement areas proposed for this project site are
capable of storing approximately 0.41 ac-ft of WQCV within the sub-structure of those paved
areas.
Regardless, the calculated required WQCV for this site is 1.03 ac-ft, resulting in a total detention
11.57 Ac-ft
4837.12
4838.59
14.53 acres 4837.64
TAILWATER ELEVATION
USED FOR STRAUSS
CABIN ROAD STORM
SEWER SIZING
STORAGE REQUIREMENT FOR
CONVENIENCE SHOPPING CENTER
DESIGN ADDED TO STORAGE
REQUIREMENT FOR "U" BASINS PER
H23 DESIGN.
Harmony and Strauss Cabin
Subdivision Filing No. 1
Convenience Shopping Center
PDP Drainage Report
2 - year 10 - year 100 - year 2 - year 10 - year 100 - year 2 - Year 10 - Year 100 - Year 2 - Year 10 - Year 100 - Year
N-1 7.19 0.28 0.28 0.34 57 57 55 0.85 1.45 3.03 1.7 2.9 7.5
N-2 1.82 0.73 0.73 0.91 8 8 6 2.40 4.10 9.31 3.2 5.5 15.5
N-3 0.32 0.84 0.84 1.00 5 5 5 2.85 4.87 9.95 0.8 1.3 3.2
N-4 0.30 0.77 0.77 0.97 5 5 5 2.85 4.87 9.95 0.7 1.1 2.9
N-5 0.36 0.89 0.89 1.00 5 5 5 2.85 4.87 9.95 0.9 1.6 3.6
N-6 0.34 0.86 0.86 1.00 5 5 5 2.85 4.87 9.95 0.8 1.4 3.4
N-7 0.92 0.71 0.71 0.89 6 6 5 2.67 4.56 9.95 1.7 3.0 8.1
N-8 0.34 0.95 0.95 1.00 5 5 5 2.85 4.87 9.95 0.9 1.6 3.4
N-9 1.04 0.69 0.69 0.87 7 7 5 2.52 4.31 9.95 1.8 3.1 9.0
N-10 0.44 0.87 0.87 1.00 5 5 5 2.85 4.87 9.95 1.1 1.9 4.4
N-11 0.42 0.93 0.93 1.00 5 5 5 2.85 4.87 9.95 1.1 1.9 4.2
N-12 0.10 0.77 0.77 0.97 5 5 5 2.85 4.87 9.95 0.2 0.4 1.0
N-13 0.10 0.78 0.78 0.98 5 5 5 2.85 4.87 9.95 0.2 0.4 1.0
N-14 0.70 0.68 0.68 0.85 5 5 5 2.85 4.87 9.95 1.4 2.3 5.9
N-15 1.13 0.59 0.59 0.74 11 11 10 2.13 3.63 7.72 1.4 2.4 6.4
N-15 ULT 2.84 0.81 0.81 1.00 14 14 12 1.92 3.29 7.16 4.4 7.5 20.3
N-16 2.12 0.57 0.57 0.71 9 9 7 2.30 3.93 8.80 2.8 4.7 13.2
N-17 0.36 0.71 0.71 0.89 5 5 5 2.85 4.87 9.95 0.7 1.3 3.2
N-18 0.82 0.66 0.66 0.82 8 8 6 2.40 4.10 9.31 1.3 2.2 6.3
N-19 1.52 0.62 0.62 0.78 27 27 25 1.37 2.34 4.98 1.3 2.2 5.9
Total 28.5 48.7 128.5
PDP-N & S (EX
CONDITION FOR N
AND PROPOSED FILL
CONDITION FOR S)
59.02 0.20 0.20 0.25 45 45 43 0.99 1.69 3.56 11.7 20.0 52.5
PDP-S 36.95 0.20 0.20 0.25 50 50 48 0.92 1.58 3.31 6.8 11.7 30.6
O-1 (EX. CONDITIONS
OFFSITE) 27.2 0.20 0.20 0.25 69 69 68 0.82 1.40 2.86 4.5 7.6 19.4
O-2 (EX. CONDITIONS
OFFSITE) 69.3 0.20 0.20 0.25 90 90 88 0.82 1.40 2.86 11.4 19.4 49.5
Pass-through Flow = 49.5+20.3=69.8
Time of Concentration, Tc (min)
Table A-3
Basin Peak Discharge
Basin Rainfall Intensity (in/hr) Peak Discharge (cfs)
Basin Area
(ac)
Frequency Adj. Runoff Coefficients
Appendix A
O-1 (EX. CONDITIONS 27.2 0.20 0.20 0.25 69 69 68 0.82 1.40 2.86 4.5 7.6 19.4
OFFSITE)
2-yr Historic
Release Rate
(27.2 acres / 4.5
cfs = 0.165 cfs/
acre)
825' ROOKERY BUFFER
(FOR REFERENCE ONLY)
TAILWATER ELEVATION
USED IN H23 DESIGN
SPILLWAY TO BE MODIFIED
WITH H23 DESIGN
ALL RUNOFF FROM "U"
BASINS WILL BE DETAINED
IN EX. GRAVEL POND #2
H23 PROJECT SITE
BASIN N-2 TO BE SUBDIVIDED WITH H23
DESIGN (BASIN CSC-1). THIS BASIN WILL
NOW DRAIN TO TWO SEPARATE INLETS AS
STRAUSS CABIN ROAD HAS BEEN
REDESIGNED AND WILL NOW BE
CONSTRUCTED WITH H23 IMPROVEMENTS.
THESE INLETS HAVE BEEN DISCONNECTED
WITH THE H23 DESIGN TO HELP ALLEVIATE
PIPE CAPACITY AND SIZING CONSTRAINTS
DOWNSTREAM THROUGH THE
CONVENIENCE SHOPPING CETER SITE.
INSTEAD RUNOFF FROM THE FRONTING
HALF OF H23 WILL NOW BE ROUTED SOUTH
VIA THE PROPOSED STORM SEWER SYSTEM
WITHIN STRAUSS CABIN ROAD WHERE IT
WILL ULTIMATELY DISCHARGE INTO THE
EXISTING GRAVEL PONDS.
APPENDIX I
Proposed Drainage Maps
748 Whalers Way, Suite 200 Fort Collins, CO 80525 ideas@tstinc.com
970.226.0557 main 303.595.9103 metro 970.226.0204 fax www.tstinc.com
4842.2
,
,
,
,
F
4837.3
,
,
,
,
E 4837.1
,
,
,
,
D
4836.7
4833.5
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4843.8
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4844.1
ZONE AE
ZONE AE
c36
k25
PONDS DRIVE
COUNTY
ROAD 36 ROAD
Fort City Collins of
080102
Timnath Town of
080005
Town 080005 of Timnath
Unincorporated Larimer County Areas
080101
FRONTAGE INTERSTATE ROAD 25
c
38
Cache I-La 25 Divided Poudre River
Flow Path
T. 7 N.
T. 6 N.
La Cache Poudre
River
BoxelderDitch
Profile Baseline
NOTE: IS LOCATED MAP AREA WITHIN SHOWN TOWNSHIP ON THIS 6 NORTH PANEL
RANGE RANGE 68 68 WEST WEST. AND TOWNSHIP 7 NORTH
10
34
3
,
,
,
,
K
4850.3
BYREVISED LOMR AREA DATED JUNE 18, 2012
,
,
,
,
G
4841.1
Fossil Creek
Reservoir Inlet
HARMONY ROAD
SCALE
Map Projection:
NAD83 STATE PLANE Colorado North FIPS 0501 (Feet)
1 inch = 500 feet 1:6,000
Western Hemisphere; Vertical Datum: NAVD88
0 250000 500 1,Feet
0 75 150 300 Meters
NATIONAL FLOODPROGRAM INSURANCE FLOOD INSURANCE RATE MAP
AndCOLORADO LARIMER Incorporated COUNTY, Areas
PANEL 1013 OF 1420
Panel Contains:
COMMUNITY NUMBER PANEL SUFFIX
1013
1013
1013
F
F
F
FORT CITY OF COLLINS,
LARIMER COUNTY
TIMNATH, OF TOWN
080102
080101
080005
MAP NUMBER
MAP REVISED
08069C1013F
DECEMBER 19, 2006
VERSION N1U.MBER1.1.0
SPECIAL HAZARD FLOOD AREAS
OTHER FLOOD AREAS HAZARD OF
Without Base Flood Elevation (BFE)
With BFE or Depth
Regulatory Floodway
0.of 2% 1% Annual annual Chance chance Flood flood with Hazard, average Areas
depth areas of less less than than one one foot square or with mile drainage
Future Chance Conditions Flood Hazard 1% Annual
Area SeeLevee Notes. with Reduced Flood Risk due to
Zone A,V, A99 Zone AE, AO, AH, VE, AR
Zone X
Zone X
Zone X
4849.9
4850
ZONE AE
Fossil Creek Reservoir Inlet
ZONE AE
REAVRIESAED
NOTE: IS LOCATED MAP AREA WITHIN SHOWN TOWNSHIP ON THIS 6 NORTH, PANEL
RANGE RANGE 68 68 WEST WEST. AND TOWNSHIP 7 NORTH,
City of 080102 Fort Collins
Unincorporated Larimer County Areas
080101
9 10
33 34
3
4
c36
City of 080102 Fort Collins
T. T. 7 6 N. N.
H
ARMONY ROAD
SCALE
Map Projection:
NAD83 STATE PLANE Colorado North FIPS 0501 (Feet)
1 inch = 500 feet 1:6,000
Western Hemisphere; Vertical Datum: NAVD88
0 250000 500 1,Feet
0 75 150 300 Meters
NATIONAL FLOODPROGRAM INSURANCE FLOOD INSURANCE RATE MAP
AndCOLORADO LARIMER Incorporated COUNTY, Areas
PANEL 994 OF 1420
Panel Contains:
COMMUNITY NUMBER PANEL SUFFIX
0994
0994
F
F
FORT CITY OF COLLINS,
LARIMER COUNTY
080102
080101
MAP NUMBER
MAP REVISED
08069C0994F
DECEMBER 19, 2006
VERSION N1U.MBER1.1.0
SPECIAL HAZARD FLOOD AREAS
OTHER FLOOD AREAS HAZARD OF
Without Base Flood Elevation (BFE)
With BFE or Depth
Regulatory Floodway
0.of 2% 1% Annual annual Chance chance Flood flood with Hazard, average Areas
depth areas of less less than than one one foot square or with mile drainage
Future Chance Conditions Flood Hazard 1% Annual
Area SeeLevee Notes. with Reduced Flood Risk due to
Zone A,V, A99 Zone AE, AO, AH, VE, AR
Zone X
Zone X
Zone X
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
STAGE-STORAGE SIZING FOR DETENTION BASINS
H23
Check Basin Shape
Pond B CWP Permanent Pool Volume
UD Detention Pond B PermPool VolCalc, Basin 9/27/2016, 10:19 AM
SW Corner of Harmony Road and Strauss Cabin Road
November 21, 2016
H23 - Pond B
Design Procedure Form: Constructed Wetland Pond (CWP)
Riprap Pads
Choose One
Excess Urban Runoff Volume (EURV)
Choose One
A
B
C / D
Water Quality Capture Volume (WQCV)
Choose One
YES
NO
UD-BMP_v3.05 - H23 Pond B, CWP 11/21/2016, 4:51 PM
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
STAGE-STORAGE SIZING FOR DETENTION BASINS
H23
Check Basin Shape
Pond B
UD Detention Pond B VolCalc, Basin 11/22/2016, 12:01 PM
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
STAGE-STORAGE SIZING FOR DETENTION BASINS
H23
Check Basin Shape
Pond A CWP Permanent Pool Volume
UD Detention Pond A PermPool VolCalc, Basin 9/27/2016, 10:19 AM
SW Corner of Harmony Road and Strauss Cabin Road
November 21, 2016
H23 - Pond A
Design Procedure Form: Constructed Wetland Pond (CWP)
Concrete forebays and riprap pads
Choose One
Excess Urban Runoff Volume (EURV)
Choose One
A
B
C / D
Water Quality Capture Volume (WQCV)
Choose One
YES
NO
UD-BMP_v3.05 - H23 Pond A, CWP 11/21/2016, 4:44 PM
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
#N/A #N/A
STAGE-STORAGE SIZING FOR DETENTION BASINS
H23
Check Basin Shape
Pond A
UD Detention Pond A VolCalc, Basin 11/22/2016, 11:50 AM
U-2 3.43 57.5% 0.63 0.63 0.63
Composite C Frequency Adjusted Runoff Coefficients (C*Cf)
Table A-1b
Offsite Basin Composite Runoff Coefficients
CSC-1 0.98 77.5% 0.78 0.78 0.78
Basin/ Sub-
Basin Area (ac) Attribute
Attribute Area
(ac)
Runoff
Coefficient, C
Percent
Impervious
Appendix A
From the City of Fort Collins Storm Drainage Design Criteria and Construction Standards
Appendix A
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A mass-balance spreadsheet was used to calculate the overall 100-yr detention requirement for
Basins U-1 and U-2. The required 100-yr detention volume from these basins was calculated to
be 1.12 ac-ft. An additional 20% was added to the volume for a total volume of 1.34 ac-ft. This
volume was then added to the 11.57 ac-ft required per the Convenience Shopping Center
Report for a total volume of 12.91 ac-ft. Because Ex. Gravel Pond #1 has no available storage
left, the 1.34 ac-ft was added to Ex. Gravel Pond #2. The approximate surface area of this pond
was determined to be approximately 14.53 acres resulting in an increase of storage depth of
0.092 feet (1.37ac-ft / 14.53 acres). Because we will be adding additional storage volume in
this pond, the emergency spillway designed with the Convenience Shopping Center report will