HomeMy WebLinkAboutHOWES SIX TWO NINE - PDP210008 - DOCUMENT MARKUPS - ROUND 3 - DRAINAGE REPORT
Preliminary Drainage and Erosion Control Report for
Howes Six Two Nine
Fort Collins, Colorado
June 6, 2021
Prepared for:
Carlson Land Development
Blake Carlson
14570 Clay St.
Broomfield, CO 80023
Prepared by:
301 N. Howes St. Suite 100
Fort Collins, Colorado 80521
Phone: 970.221.4158
www.northernengineering.com
Project Number: 1590-002
This Drainage Report is consciously provided as a PDF. Please consider
the environment before printing this document in its entirety. When a hard
copy is necessary, we recommend double-sided printing.
June 6, 2021
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, CO 80521
RE: Preliminary Drainage and Erosion Control Report for
Howes Six Two Nine
Dear Staff:
Northern Engineering is pleased to submit this Preliminary Drainage and Erosion
Control Report for your review. This report accompanies the combined Preliminary &
Final Plan submittal for the proposed Howes Six Two Nine Development.
This report has been prepared in accordance with the Fort Collins Stormwater Criteria
Manual (FCSCM) and serves to document the stormwater impacts associated with the
proposed Howes Six Two Nine project. We understand that review by the City of Fort
Collins is to assure general compliance with standardized criteria contained in the
FCSCM.
If you should have any questions as you review this report, please feel free to contact
us.
Sincerely,
Northern Engineering Services, Inc.
Austin Snow, PE Danny Weber, PE
Project Engineer Project Manager
Howes Six Two Nine Preliminary Drainage Report June 6, 2021
Carlson Development Table of Contents
Table of Contents
I. General Location and Description ........................................................................ 1
II. Drainage Basins and Sub-Basins .......................................................................... 3
III. Drainage Design Criteria ..................................................................................... 4
IV. Drainage Facility Design ..................................................................................... 7
V. Conclusions ......................................................................................................... 9
VI. References ......................................................................................................... 10
Tables and Figures
Figure 1 – Vicinity Map .................................................................................... 1
Figure 2 – Aerial Photograph .......................................................................... 2
Figure 3 – FEMA Firmette (Map Number 08069C0979H) ............................... 3
Appendices
Appendix A – Hydrologic Computations
Appendix B – Hydraulic Computations
Appendix C – Water Quality/Lid Design Computations
Appendix D – Erosion Control Report
Appendix E – USDA Soils Report
Map Pocket
DR1 – Drainage Exhibit
Howes Six Two Nine Preliminary Drainage Report June 6, 2021
Carlson Development Page 1 of 11
I. General Location and Description
A. Location
1. Vicinity Map
Figure 1 – Vicinity Map
2. The Howes Six Two Nine project site is located within the North half of Lot 2 and
the South half of Lot 3 of Block 93 of the Harrison subdivision, City of Fort
Collins, County of Larimer, State of Colorado.
3. The project site (refer to Figure 1) is bordered on the North and South by parking
lots for multifamily residences; to the West by an inverted-crown alleyway; to
the East by the public right-of-way for South Howes.
B. Description of Property
1. The Howes Six Two Nine development is comprised of ±0.43 acres.
Howes Six Two Nine Preliminary Drainage Report June 6, 2021
Carlson Development Page 2 of 11
2. The site is currently occupied by one two-story brick building planned to remain,
a garage adjacent the alleyway which is planned for removal, as well as various
concrete sidewalk and pavers, and an approximately 4240 square-foot asphalt
parking area connected to the alley.
Figure 2 – Aerial Photograph
3. The existing groundcover consists of grasses, concrete, pavers, asphalt, and the
existing structures’ rooftops. The existing on-site runoff generally drains
Westward into the inverted crown alleyway and Eastward to South Howes on
shallow grades (e.g., <2.00%). From there, the drainage follows existing City of
Fort Collins stormwater infrastructure.
4. According to the United States Department of Agriculture (USDA) Natural
Resources Conservation Service (NRCS) Soil Survey website:
(http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx),
the site consists primarily of Fort Collins loam (Hydrologic Soil Group C).
5. The proposed development will consist of two townhome buildings (connected
above-ground) containing a total of 7 single-family attached units. Other
proposed improvements include: an enlarged asphalt parking lot, walkways and
a courtyard, and landscaping.
Howes Six Two Nine Preliminary Drainage Report June 6, 2021
Carlson Development Page 3 of 11
6. The project site is located in the Campus North sub-district of the Downtown
District. The proposed use (Multi-family, 8-units or less/building) is an approved
use in the Downtown District, subject to Type 1 Administrative Review.
7. No significant offsite flows are directed into the site. Off-site flows are prevented
from entering the site by existing topography.
8. The site is not required to provide on-site detention as it is located in the Old
Town Basin and is increasing total impervious area by less than 5,000 square feet.
Additionally, the standard water quality requirement for this site has been
provided for in the City’s Udall water quality facility. This site will meet the
City’s on-site LID treatment requirements via a bioswale included in the design.
C. Floodplain
1. The subject property is not located in a FEMA or City regulatory floodplain
according to FEMA Firm No. 08069C0979H, effective on 05/02/2012.
2.
Figure 3 – Existing Floodplain(s)
II. Drainage Basins and Sub-Basins
A. Major Basin Description
Howes Six Two Nine is located within the City of Fort Collins Old Town major
drainage basin. Specifically, the project site is situated in the lower center of this
Howes Six Two Nine Preliminary Drainage Report June 6, 2021
Carlson Development Page 4 of 11
major drainage basin. This basin is located in north-central Fort Collins and has a
drainage area of approximately 2,120 acres. The entire basin is urbanized and
generally drains from west to east. Most of the water from the Old Town basin
drains to the Poudre River, just to the east.
B. Sub-Basin Description
1. The outfall for the project site is the Cache la Poudre River, to the east.
2. The subject site can be defined with three (3) historical sub-basins that
encompass the entire project site.
3. The existing site runoff drains to four different places, including the alley to the
west, the parking lot to the north of the site, low areas within the site itself, or
east to the Howes gutter.
4. The project site does not receive notable runoff from off-site properties.
III. Drainage Design Criteria
A. Optional Provisions
There are no optional provisions outside of the FCSCM proposed with the Howes
Six Two Nine project.
B. Stormwater Management Strategy
The overall stormwater management strategy employed with the Howes Six Two
Nine project utilizes the “Four Step Process” to minimize adverse impacts of
urbanization on receiving waters. The following is a description of how the
proposed development has incorporated each step.
Step 1 – Employ Runoff Reduction Practices. The first consideration taken in trying
to reduce the stormwater impacts of this development is the site selection itself. By
choosing an already developed site with public storm sewer currently in place, the
burden is significantly less than developing a vacant parcel absent of any
infrastructure.
The Howes Six Two Nine project aims to reduce runoff peaks, volumes and
pollutant loads from frequently occurring storm events (i.e., water quality (i.e., 80th
percentile) and 2-year storm events) by implementing Low Impact Development
(LID) strategies. Wherever practical, runoff will be routed across landscaped areas or
through an infiltration gallery. These LID practices reduce the overall amount of
impervious area, while at the same time Minimizing Directly Connected Impervious
Areas (MDCIA). The combined LID/MDCIA techniques will be implemented, where
Howes Six Two Nine Preliminary Drainage Report June 6, 2021
Carlson Development Page 5 of 11
practical, throughout the development, thereby slowing runoff and increasing
opportunities for infiltration.
Step 2 – Implement BMPs that Provide a Water Quality Capture Volume (WQCV)
with Slow Release. The efforts taken in Step 1 will help to minimize excess runoff
from frequently occurring storm events; however, urban development of this
intensity will still have stormwater runoff leaving the site. The primary water quality
treatment will occur off-site at the City’s Udall Water Quality facility.
Step 3 – Stabilize Drainageways. The project will pay one-time stormwater
development fees, as well as ongoing monthly stormwater utility fees, both of which
help achieve Citywide drainageway stability. Additionally, LID BMPs provide
downstream
Step 4 – Implement Site Specific and Other Source Control BMPs. This step
typically applies to industrial and commercial developments.
C. Development Criteria Reference and Constraints
1. The subject property is part of the Old Town basin drainage master plan.
2. The site plan is constrained on the east by a public street, on the west by an alley,
and by existing development on the north and south.
D. Hydrological Criteria
1. The City of Fort Collins Rainfall Intensity-Duration-Frequency Curves, as
depicted in Figure 3.4-1 of the FCSCM, serve as the source for all hydrologic
computations associated with the development. Tabulated data contained in
Table 3.4-1 has been utilized for Rational Method runoff calculations.
2. The Rational Method has been employed to compute stormwater runoff utilizing
coefficients contained in Tables RO-11 and RO-12 of the FCSCM.
3. The Rational Formula-based Federal Aviation Administration (FAA) procedure
has not been utilized for detention storage calculations since detention is not
required for the project.
4. Two separate design storms have been utilized to address distinct drainage
scenarios. The first event analyzed is the “Minor,” or “Initial” Storm, which has a
2-year recurrence interval. The second event considered is the “Major Storm,”
which has a 100-year recurrence interval.
Howes Six Two Nine Preliminary Drainage Report June 6, 2021
Carlson Development Page 6 of 11
E. Hydraulic Criteria
1. The drainage facilities proposed for the Howes Six Two Nine project are
designed in accordance with criteria outlined in the FCSCM and/or the Mile
High Flood District’s (MHFD) Urban Storm Drainage Criteria Manual.
2. As stated in Section I.C.1, above, the subject property is not located in any FEMA
or City floodplains.
F. Floodplain Regulations Compliance
1. As previously mentioned, this project is not be subject to any floodplain
regulations.
G. Modifications of Criteria
No formal modifications are requested at this time. However, staff has determined
that detention will not be required with this project if the increase in impervious area
is less than 5,000 square-feet in total.
H. Conformance with Water Quality Treatment Criteria
City Code requires that 100% of runoff from a project site receive some sort of water
quality treatment. As per staff guidance, his project will utilize the Udall water
quality facility to treat the runoff emanating from the site. Additionally, the site will
feature LID treatment for at least the required treatment area. Due to the physical
constraints associated with an project of this nature/size and the prohibition of
providing water quality facilities within the public right-of-way, there are some
small, narrow areas around the perimeter of the project that cannot be captured. The
uncaptured areas tend to be narrow strips of landscaped area between the property
line and buildings or curbs.
While these small areas will not receive formal water quality treatment, most areas
will still see some treatment as runoff is directed across through the landscaped
areas or LID system before outletting to the storm sewer off-site.
I. Conformance with Low Impact Development (LID)
The project site will conform with the requirement to treat a minimum of 75% of the
project site using a LID technique. Please see Appendix C for LID design
information, table, and exhibit(s). As shown in the LID table provided in the
appendix, 86.4% of the proposed site impervious area will receive LID treatment,
which exceeds the minimum required.
J. Sizing of LID Facilities
Howes Six Two Nine Preliminary Drainage Report June 6, 2021
Carlson Development Page 7 of 11
Bioswale
1. The bioswale was sized by first determining the required treatment area for the
system (75% of new/modified impervious area). New impervious area equals
approximately 4,874 square feet, so the required treatment area equals
approximately 3,656 square feet.
2. After preliminary placement was identified, the contributing area was delineated
in order to determine if the treatment area requirement in 1., above, was being
met.
3. Once it was determined that the treatment area requirement was met, the
bioswale was sized using the method described in UDFCD/MHFD Treatment
BMP Fact Sheet T-03, referenced in Appendix C (LID Implementation Manual) of
the FCSCM.
4. A volume calculation utilizing the contributing area flow rate into the bioswale
and the calculated release rate through the filter area was completed as per Fact
Sheet T-03.
IV. Drainage Facility Design
A. General Concept
1. The main objective of the Howes Six Two Nine drainage design is to maintain
existing drainage patterns, while not adversely impacting adjacent properties.
2. No notable off-site runoff passes directly through the project site.
3. A list of tables and figures used within this report can be found in the Table of
Contents at the front of the document. The tables and figures are located within
the sections to which the content best applies.
4. Drainage for the project site has been analyzed using thirteen (13) drainage sub-
basins, designated as sub-basins A through G and H-C. The drainage patterns
anticipated for the basins are further described below.
Sub-Basin A
Sub-basin A encompasses approximately 4.5% the total site area. This sub-basin
is comprised primarily of asphalt parking/drive and landscaped area. The sub-
basin will drain to the alley located along the west property line and be captured
by an area drain which will then convey runoff from the basin through the storm
drain system off-site. Historically, approximately 360 square feet of impervious
area drains to the alley. The proposed development will increase the impervious
area draining to the alley by 262 square feet to approximately 622 square feet.
Howes Six Two Nine Preliminary Drainage Report June 6, 2021
Carlson Development Page 8 of 11
Since the increase is less than 350 square feet, as per City staff, drainage analysis
of the alley’s conveyance is not required.
Sub-Basin B
Sub-basin B encompasses approximately 40.6% the total site area. This sub-basin
is comprised primarily of roof area, concrete flatwork, an asphalt parking lot,
and landscaped area. Flows for the sub-basin will drain to a curb-cut in the
southeast corner of the parking lot that flows into Sub-Basin F and the LID
system.
Sub-Basin C
Sub-basin C encompasses approximately 18.4% the total site area. This sub-basin
is comprised primarily of roof area, concrete flatwork and landscaped areas. The
sub-basin will sheet flow to an area inlet which will then convey flows to the off-
site storm sewer in Howes.
Sub-Basin D
Sub-basin D encompasses approximately 5.3% the total site area. This sub-basin
is comprised primarily of landscaped area and roof area. Flows from the sub-
basin will flow to an area inlet which will convey runoff from the basin into the
off-site storm sewer.
Sub-Basin E
Sub-basin E encompasses approximately 16.5% the total site area. This sub-basin
is comprised primarily of roof area. Flows from the sub-basin will flow via
gutters into Sub-Basin F and the LID system.
Sub-Basin F
Sub-basin F encompasses approximately 8.5% the total site area. This sub-basin is
comprised primarily of landscaped area. Flows from the sub-basin will enter the
LID system (bioswale) which has been designed to capture the major flows from
the contributing area and features an underdrain conveying flows to the off-site
storm sewer in Howes. An area drain is also featured to collect flows greater than
the major event. Since this Sub-Basin receives flows from multiple other Sub-
Basins, it has been designed to overflow into the Sub-Basin C outlet, an area inlet
that leads directly to the off-site storm sewer, in case of a long-return period
event or clogging in the LID system.
Sub-Basin G
Sub-basin G encompasses approximately 6.2% the total site area. This sub-basin
is comprised primarily of roof area, concrete flatwork and landscaped areas.
Flows from this sub-basin cannot be captured due to existing topography at the
property boundary and will flow off-site to the parking lot to the north or the
gutter in Howes, following historical drainage patterns.
Howes Six Two Nine Preliminary Drainage Report June 6, 2021
Carlson Development Page 9 of 11
A full-size copy of the Drainage Exhibit can be found in the Map Pocket at the
end of this report.
B. Specific Details
1. Since detention is not required with this site, the existing impervious area has not
been considered in determining allowable release from the property.
2. An allowable release rate was not determined for this project due to an increase
in impervious area less than 5,000 square feet from historical.
3. The FAA method was not used to size the on-site detention volume for quantity
detention since detention is not required.
4. Storm infrastructure was sized to the major event flows from contributing areas
as per the FCSCM.
V. Conclusions
A. Compliance with Standards
1. The design elements comply without variation and meet all LID requirements.
2. The drainage design proposed with the Howes Six Two Nine project complies
with the City of Fort Collins Master Drainage Plan for the Old Town Basin.
3. There are no FEMA regulatory floodplains associated with the development.
4. The drainage plan and stormwater management measures proposed with the
project are compliant with all applicable State and Federal regulations governing
stormwater discharge.
B. Drainage Concept
1. The drainage design proposed with this project will effectively limit potential
damage associated with its stormwater runoff. The project will not detain for the
pervious area converted to impervious areas to release at the 2-year existing rate
during the 100-year storm as per staff instruction, however, LID will be
implemented to treat new impervious area.
2. The development will not impact the Master Drainage Plan recommendations for
the Old Town major drainage basin.
Howes Six Two Nine Preliminary Drainage Report June 6, 2021
Carlson Development Page 10 of 11
VI. References
City of Fort Collins Landscape Design Guidelines for Stormwater and Detention
Facilities, November 5, 2009, BHA Design, Inc. with City of Fort Collins Utility
Services.
Fort Collins Stormwater Criteria Manual, City of Fort Collins, Colorado, as adopted by
Ordinance No. 159, 2018, and referenced in Section 26-500 of the City of Fort Collins
Municipal Code.
Soils Resource Report for Larimer County Area, Colorado, Natural Resources
Conservation Service, United States Department of Agriculture.
Urban Storm Drainage Criteria Manual, Volumes 1-3, Mile High Flood District,
Denver, Colorado, Revised April 2018.
Project:
Calc. By:
Date:
Description
Surface Area
(ft2)
Percent
Impervious
Impervious
Area ( ft2)
Rooftop 2,431 90%2,188
Concrete 1,040 100%1,040
Asphalt 4,200 100%4,200
Pavers - 40%-
Gravel 482 40%193
Landscaping 10,828 2%217
Total 18,981 7,837
Description
Surface Area
(ft2)
Percent
Impervious
Impervious
Area ( ft2)
Rooftop 6,300 90%5,670
Concrete 1,497 100%1,497
Asphalt 5,308 100%5,308
Pavers - 40%-
Gravel - 40%-
Landscaping 5,876 2%118
Total 18,981 12,593
4,755 Difference in Impervious Area (ft 2)
Developed Impervious Areas
Historic Impervious Areas
IMPERVIOUS AREA CALCULATIONS
629 Howes
A. Snow
April 5, 2020
Bioretention T-3
November 2015 Urban Drainage and Flood Control District B-5
Urban Storm Drainage Criteria Manual Volume 3
The following steps outline the design procedure and criteria, with Figure B-1 providing a corresponding
cross-section.
2. Basin Storage Volume: Provide a storage volume based on a 12-hour drain time.
Find the required WQCV (watershed inches of runoff). Using the imperviousness of the tributary
area (or effective imperviousness where LID elements are used upstream), use Figure 3-2 located
in Chapter 3 of this manual to determine the WQCV based on a 12-hour drain time.
Calculate the design volume as follows: 𝑉 = �WQCV12�𝐴 Equation B-1
Where:
V= design volume (ft3)
A = area of watershed tributary to the rain garden (ft2)
3. Basin Geometry: UDFCD recommends a maximum WQCV ponding depth of 12 inches to
maintain vegetation properly. Provide an inlet or other means of overflow at this elevation.
Depending on the type of vegetation planted, a greater depth may be utilized to detain larger
(more infrequent) events. The bottom surface of the rain garden, also referred to here as the filter
area, should be flat. Sediment will reside on the filter area of the rain garden; therefore, if the
filter area is too small, it may clog prematurely. If the filter area is not flat, the lowest area of the
filter is more likely to clog as it will have a higher sediment loading. Increasing the filter area
will reduce clogging and decrease the frequency of maintenance. Equation B-2 provides a
minimum filter area allowing for some of the volume to be stored beyond the area of the filter
(i.e., above the sideslopes of the rain garden).
Note that the total surcharge volume provided by the design must also equal or exceed the design
volume. Where needed to meet the the required volume, also consider the porosity of the media at 14
percent. Use vertical walls or slope the sides of the basin to achieve the required volume. Sideslopes
should be no steeper than 4:1 (horizontal:vertical).
AIAF02.0=
Equation B-2
Where:
AF= minimum (flat) filter area (ft2)
A = area tributary to the rain garden (ft2)
I = imperviousness of area tributary to the rain garden (percent expressed as a decimal)