HomeMy WebLinkAboutFLATS @ THE OVAL - FDP - 26-08/A - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTCONCLUSIONS
COMPLIANCE WITH STANDARDS
This Final Drainage and Erosion Control Report for The Oval Flats is in general compliance with
the City of Fort Collins Storm Drainage Design Criteria and construction Standards.
REQUEST FOR VARIANCE(S)
None at this time.
DEVELOPED STORM WATER CONDITIONS
OVERVIEW
In general, the majority of the developed stormwater runoff drains to an on -site detention pond
"that will outfall into an existing storm drain system in South Howes Street.
SUB -BASIN DESCRIPTIONS
The project site is divided into eight on -site developed stormwater basins (Refer to the Final
Drainage and Erosion Control Plan, Back Pocket).
A 1 and A2: These basins encompass most of the proposed developed area which is comprised
of mostly hardscape. They drain to the proposed on -site detention pond.
B 1 thru B4: These four basins are situated around the perimeter of the project site and drain
offsite without being detained. The undetained off -site runoff was accounted for in the design of
our on -site detention pond.
RF1: This basin consists of roof area for the proposed building. The roof drainage will be
connected directly to the proposed storm drain infrastructure using 10" and 12" A2000 PVC
pipe. With this submittal, we provided inverts for this system.
STORM DRAIN SYSTEM DESIGN
With this submittal we designed a single on -site storm drain system. This system will connect to
an existing 12" clay storm drain pipe in South Howes Street. Appendix B presents capacity
calculations for the 12" and 15" A2000 PVC storm drain pipes that we propose to construct.
With this submittal we provided additional design information for the storm drain infrastructure,
including: hydraulic grade line calculations and plan and profiles.
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1 STORM WATER DETENTION
According to the Old Town Master Drainage Plan, Section R3. Redevelopment Criteria (Refer to
Appendix C for the excerpt) and with respect to the required storage volume, the proposed
redevelopment can take credit for the existing on -site impervious area. The required storage
volume was estimated with the FAA Methodaccording to the following explanation.
We took credit for the existing impervious surface area, t0.45 acre. The remainder of the
existing site, f0.38 acre of pervious surfaces was used to size the on -site detention pond. More
specifically, the on -site detention pond was sized to detain the percentage of the existing
pervious area that we expect to be converted into impervious surface area.
IN With respect to the detention pond sizing calculation presented in Appendix B, we
conservatively assumed all the existing pervious surface areas would be converted to impervious
area. This assumption is reflected in the 2-year and 100-year runoff coefficients (C): 0.93 and
10 1.00, respectively. The total release rate, 0.27cfs, is based on the 2-year release rate from the
pervious surface area in each of the existing condition basins (i.e., EX-1, EX-2 and EX-3).
Formula: Basin2_year: CiA
EX-12_yedr : (0.25)(2.86)(0.01)=0.007cfs
IN EX-22_y,,, : (0.25)(2.86)(0.26)=0.19cfs
EX-32_yeyr : (0.25)(2.86)(0.11)=0.078cfs
Notes:
1. Runoff coefficient (C) of 0.25 to describe the compacted existing soils
2. Assumed Tc= 5.00min for each basins pervious surface area; therefore, i=2.86 in/hr
The 100-year release rate from the on -site detention pond is t 1.75cfs. This is less than the 100-
year peak runoff from existing basins EX-2 and EX-3, 1.72cfs and 0.88cfs, respectively. The
difference is that the existing runoff drains to the surface whereas the release from the pond will
drain directly to the existing 12" storm drain system in South Howes Street.
The proposed detention pond is located in the parking area. During the 100-year storm event, the
detained runoff will pond to the maximum allowable depth of 1.00' at elevation 4998.40. The
lowest adjacent proposed building F.F.=4999.50; therefore, the building is protected from
' inundation by ponded water during the 100-year storm event in accordance with Section 9.4 of
,+ the SDDC.
The emergency spillway elevation is set at 4998.50. Stormwater that passes through the spillway
will drain east and into South Howes Street. The street section that we outfall into is at the
upstream end of the basin; therefore, sufficient street capacity should be available.
The on -site detention pond will drain through (2)-CDOT Concrete Inlets, Type 13. The inlets
will drain through the proposed 12" storm drain pipe (upsized to 15" within the South Howes
aStreet R.O.W.). The release rate from the inlets will be controlled by a 5114" vertical orifice plate.
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HYDROLOGY AND HYDRAULIC CRITERIA
DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS
The calculation methods presented in this report conform to the standards set forth in the City of
Fort Collins Storm Drainage Design Criteria and Construction Standards (SDDC) and, in
certain cases, Volumes I through 3 of the UDFCDs Urban Storm Drainage Criteria Manual
14 (Manual).
HYDROLOGY CRITERIA
The Rational Method was used to estimate the developed runoff.
For this preliminary submittal, the existing and developed composite runoff coefficients were
calculated using Table 3-3, Rational Method Runoff Coefficients for Composite Analysis.
The rainfall intensity data for the hydrologic calculations was read from the City of Fort Collins
Rainfall Intensity -Duration -Frequency Table, Figure 3-1a and Figure 3-1b.
HYDRAULIC CRITERIA AND CALCULATIONS
With this submittal we provided inlet sizing calculations for the proposed Type 13 inlets (Refer
4! to Appendix B), emergency spillway sizing calculations, orifice plate sizing calculations and
hydraulic grade line calculations.
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WETLANDS IMPACTS
There are no existing wetlands which the proposed redevelopment will disturb.
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wash construction vehicles before they leave the site and enter the public right-of-way. A BMP
such as this is a good way to avoid the costs associated with street cleaning or accidentally
discharging large amounts of sediment into other storm drain systems.
When in doubt, choose methods that prevent pollution rather than ones that force clean-up at the
downstream end just before the stormwater enters the receiving waters. In other words, the most
efficient construction site BMPs are those that prevent erosion from happening.
FINAL STABILIZATION
Final stabilization is reached when all soil disturbing activities at the site have been completed,
and uniform vegetative cover has been established with a density of at least 70 percent of pre -
disturbance levels or equivalent permanent physical erosion reduction methods have been
employed.
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• Silt Fence (SF): a woven synthetic fabric that filters runoff. The silt fence is a temporary
barrier placed at the base of a disturbed area. This BMP should be installed prior to the
start of any earth disturbing activity and remain in place and in an operational and
maintained condition until final stabilization of the tributary area is achieved.
NON-STRUCTURAL PRACTICES
The following information about non-structural practices was excerpted from Volume 3 - Best
Management Practices, Urban Drainage and Flood Control District (UDFCD).
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Nonstructural BMPs are intended to prevent or reduce the contamination of stormwater runoff.
They are applicable to a variety of different sources or activities. By reducing pollutant
generation, adverse water quality impacts are reduced from potential pollutant sources.
Preventing and controlling the sources of these pollutants requires a change in behavior.
Because non-structural BMPs are so closely tied to people's activities and behavior, their
effectiveness depends on people's attitude, their willingness to accept information, and their
willingness to put BMPs into practice.
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Two main objectives of non-structural BMPs are to:
1. Reduce or eliminate the pollutants that impact water quality at their source, thus reducing
the need for structural control requirements.
2. Address water quality concerns that are not cost effectively handled by structural
controls.
Options:
1. Illicit Discharge Controls (UDFCD, Page NS-16)
2. Good Housekeeping (UDFCD, Page NS-20)
3. Preventative Maintenance (UDFCD, Page NS-22)
4. Spill Prevention and Response (UDFCD, Page NS-27)
OTHER CONTROLS
Many other potential pollutant sources will exist during construction. Some of these include:
exposed and stored soils, tracking of mud and debris from construction equipment to adjacent
:. roadways, outdoor material storage, vehicle maintenance and fueling, on -site waste disposal,
LL! equipment washing and concrete washout.
Some of the items listed above can be managed by designating an area of the site where the
activity takes place. For example, materials storage, waste disposal and vehicle maintenance and
fueling can occur within an enclosed and secure area. The area can be enclosed with temporary
cyclone fencing and accessed through a gate with a lock.
Soil stockpiles should be kept below the locally mandated maximum height and surrounded with
silt fencing. If the storage of these soils is going to be for an extended period of time, then
consider temporarily seeding the soil to minimize soil loss to wind and water erosion.
The off -site tracking of mud and debris is easily controlled by installing Vehicle Tracking
Control (VTC). This is a stabilized stone pad located at points of ingress and egress on a
construction site. It is designed to reduce the amount of mud transported onto the public roads by
construction traffic. If the site is extraordinarily muddy, then consider designating an area to
STORM WATER QUALITY
GENERAL
The Colorado Department of Public Health and Environment (CDPHE) states in the Rationale
Stormwater Discharges Associated with Construction Activity General Permit in Colorado that,
under the framework of the NPDES and CDPS stormwater regulations, a discharge permit is
required for construction activities.
Projects with l acre -or more of earth disturbance (Phase II) require a CDPS permit, which
involves the completion and submittal of a "General Permit for Stormwater Discharges
Associated with Construction Activity".
All projects involving an earth disturbance require a Stormwater Management Plan (SWMP).
The SWMP is required as a permit condition, must reflect current site conditions, must be
available for review by state inspectors, and is available as a public document.
The goal of the SWMP is to describe appropriate controls and measures to improve water quality
by reducing pollutants in stormwater discharges and ensure compliance with the requirements of
the stormwater permit. The SWMP must be completed and implemented at the time the project
breaks ground, and revised if necessary as construction proceeds to accurately reflect the
conditions and practices at the site.
TEMPORARY STRUCTURAL AND NON-STRUCTURAL PRACTICES
Structural controls reduce the amounts of pollutants that migrate off the urban landscape by
reducing runoff and providing facilities to remove pollutants from stormwater. This type of Best
Management Practice (BMP) includes physical processes ranging from diversion structures to
silt fences to retention ponds. Physical BMPs are generally simple, low cost and effective if they
are correctly installed and properly maintained.
Nonstructural BMPs prevent or limit the entry of pollutants into stormwater at their source
through operational or managerial techniques.
STRUCTURAL PRACTICES
The following are examples of temporary BMPs that may need to be installed and maintained in
order to control on -site erosion and prevent sediment from migrating within the project site and
off -site during construction.
Mulching (MU): plant residues applied to the soil surface to prevent erosion by
protecting the soil surface fromraindrop impact and to foster the growth of vegetation
(e.g., temporary seeding) by increasing the available moisture. This BMP should be
applied in conjunction with the temporary seeding after the interim or final design grade
has been reached and permanent stabilization is still a ways off. Mulching and temporary
seeding are BMPs that help prevent erosion from occurring. When properly installed and
maintained, they can improve the performance and decrease the maintenance and/or
replacement interval of other BMPs such as gravel inlet protection or straw wattles.
STORM WATER BASINS AND DRAINAGE PATTERNS
The subject site is located in the Old Town major drainage basin which is located in north -central
Fort Collins and has a drainage area of approximately 2,120 acres, including approximately 400
acres of the Colorado State University campus. The entire basin is urbanized, with some
development dating back to the late 1800s. Generally, the basin drains from west to east. It
receives some runoff water from the Canal Importation Basin directly west of Old Town. Most
of the water from Old Town drains to the Poudre River, just to the east.
Street flooding is a common occurrence in Old Town. In recent years, several capital projects,
such as the Howes Street, Locust Street and Oak Street Outfall projects, have vastly improved
storm drainage in Old Town.
The existing development sub -divided into three drainage basins (Refer to the Existing
Conditions Drainage Plan, Back Pocket). The existing conditions peak runoff in each basin was
estimated for the 2- and 100-year storm events (Refer to Appendix A for calculations).
- EXI: This basin is located along the west side of the subject site. It is comprised of some
roof area, concrete walks and patios, and an asphalt parking lot.
` The existing runoff in this basin flows overland and into the existing alley that neighbors
�a the project along its west property line. The runoff in the alley drains north to Myrtle
Street. In Myrtle Street, the runoff flows east and into an existing combination inlet at the
southwest corner of South Howes Street and Myrtle Street (Refer to Appendix C for an
exhibit -from the City of Fort Collins GIS Utility Mapping).
- EX2: This basin is located along the east side of the project site. It is comprised of some
roof areas, concrete walks, and grassy areas.
The existing runoff in this basin flows overland and into South Howes Street which
drains south to Myrtle Street and then to same inlet as the runoff from basin EX1
EX3: This basin is located along the south side of the existing project site. The excess
surface runoff in it flows overland and into West Laurel Street. In the street, the
stormwater flows east, into the existing curb and gutter and into an existing combination
inlet at the northwest corner of West Laurel Street and South Howes Street. This inlet
connects to the 12" clay storm drain pipe in South Howes Street via a 12" PVC pipe.
The subject site is not affected by off -site runoff.
We are not aware of existing drainage studies for the subject site or surrounding properties that
include information which affects our current work.
The project site is not located within a FEMA designated 100-year floodplain per Map Number
IN 08069CO979G (Effective date: June 17, 2008).
DEVELOPMENT PROPOSAL
The Oval Flats encompasses 0.83 acre of land are and is anticipated to be a mixed -use
development. The subject site is currently zoned C-C (Community Commercial).
When complete, the development will consist of approximately 47,040 ft2 of residential space,
4,896 ft2 of commercial space and 3,018 ft2 to be utilized as a common meeting space. The
residential space is targeted at undergraduate and graduate CSU students and will have 98
bedrooms in 41 units. The individual units will be a mix of 1, 2, 3 and 4 bedroom units.
Additional improvements include utility connections to provide the project site with domestic
water, sanitary sewer, and storm drain service.
The project will be constructed in a single phase.
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BACKGROUND
SITE LOCATION
The Oval Flats mixed -use redevelopment is located at 306 West Laurel Street, Fort Collins,
Colorado 80521 (Refer to Appendix C for a copy of the Plat).
4 The subject site is located in the Northeast Quarter of Section 14, Township 7 North, Range 68
West of the 6`s P.M., City of Fort Collins, County of Larimer, State of Colorado.
4 The Oval Flats is bordered to the north by existing residential development, to the south by West
Laurel Street; to the east by South Howes Street; and to the west by an existing alley.
EXISTING SITE DESCRIPTION
�] The subject site is comprised of t0.83 acre and is developed. It contains the former EAE
�! Fraternity house which was abandoned in 2006 after the fraternity was barred from Colorado
State University.
The existing improvements include the EAE fraternity house, several concrete walkways, an
asphalt parking lot along the west side of the site, and sub -surface utilities. The existing
vegetation consists of unmaintained grass and many large trees and shrubs.
The existing developed runoff flows from the site in three different directions: into South Howes
Street, into West Laurel Street or into the existing alley along grades in the neighborhood of
1.00% to 3.50%.
In general, the on -site soils consist of Fort Collins Loam, 1 to 3 percent slopes which are NRCS
HSG `13' soils (Refer to Appendix C for a copy of the NRCS soils map). A description of the soil
type follows:
Group 8: Soils having a moderate infiltration rate when thoroughly wet. These consist
chiefly of moderately deep or deep, moderately well drained or well drained soils that
have moderately fine texture to moderately course texture. These soils have a moderate
rate of water transmission.
A soils report (Soilogic #: 07-,1088) was completed by Soilogic, Inc. on November 19, 2007.
This report contains the results of a complete geotechnical subsurface exploration as well as
pertinent geotechnical engineering recommendations. According to the findings presented in the
aforementioned report, the on -site soils below the topsoil layer consist of USCS soil group CL.
These are inorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays;
and lean clays. This information does not match the soils information from the NRCS web -site.
However, it should be considered more reliable/accurate since it is based on site specific soils
testing.
The sites existing developed runoff ultimately drains into the Poudre River. This condition will
not change with the proposed improvements.
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PE CERTIFICATION AND SEAL
This report for the final design of the The Oval Flats was prepared by me or under my direct
supervision in accordance with the provisions of the City of Fort Collins Storm Drainage Design
Criteria and Construction Standards. I understand that the City of Fort Collins and its designated
city authority do not and will not assume liability for drainage facilities designed by others.
Stamp and Signature at Final
Registered Professional Engineer
State of Colorado
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TABLE OF CONTENTS
Tableof Contents.............................................................................................................................
i
PE Certification and Seal................................................................................................................
Background.....................................................................................................................................
ii
1
SiteLocation................................................................................................................................
1
ExistingSite Description.............................................................................................................
DevelopmentProposal....................................................................................................................
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2
Storm Water Basins and Drainage Patterns....................................................................................
3
StormWater Quality.......................................................................................................................
General.........................................................................................................................................
4
4
StructuralPractices...................................................................................................................
4
Non -Structural Practices...........................................................................................................
OtherControls..........................................................................................................................
5
5
FinalStabilization........................................................................................................................
6
WetlandsImpacts............................................................................................................................
Hydrology and Hydraulic Criteria..................................................................................................
7
8
Development Criteria Reference and Constraints.......................................................................
8
HydrologyCriteria....................................................................................................................... 8
Hydraulic Criteria and Calculations............................................................................................ 8
StormWater Detention...................................................................................................................
9
Developed Storm Water Conditions.............................................................................................
Overview....................................................................................................................................
10
10
Sub -basin Descriptions..............................................................................................................
10
StormDrain System Design......................................................................................................
Conclusions...................................................................................................................................
10
I I
Compliancewith Standards.......................................................................................................
I 1
Requestfor Variance(s).............................................................................................................
11
References.....................................................................................................................................12
APPENDIX A - HYDROLOGIC CALCULATIONS
EXISTING CONDITION
DEVELOPED CONDITION .
APPENDIX B - HYDRAULIC CALCULATIONS
STORM DRAIN SYSTEM DESIGN
INLET DESIGN
DETENTION POND DESIGN
APPENDIX C- EROSION AND SEDIMENT CONTROL
APPENDIX D - BACKGROUND INFORMATION
PLAT
NRCS SOILS MAP
OLD TOWN MASTERPLAN DESIGN GUIDELINES
FEMA MAPPING INFORMATION
GENERAL INFORMATION
Back Pocket: Preliminary Drainage and Erosion Control Plan, Sheet 6 of 13
FINAL
DRAINAGE AND EROSION CONTROL REPORT
THE OVAL FLATS
FORT COLLINS, COLORADO
Prepared for:
OVAL FLATS, L.L.C.
3003 EAST HARMONY ROAD, SUITE 300
FORT COLLINS, COLORADO 80528
Submitted on:
March 3, 2009
Prepared by:
Jim Sell Design, Inc.
153 West Mountain Avenue
Fort Collins, Colorado 80524
Phone 970.484.1921
FAX 970.484.2443