HomeMy WebLinkAboutDrainage Reports - 10/17/2017August 22, 2017
Mr. Dan Mogen
City of Fort Collins Stormwater
700 Wood Street
Fort Collins, CO 80522-0580
RE: Final Drainage Memo for Living Oaks
Dear Dan,
INTlRwlBT c O N SULTING GROUP
City of Fort Collins Ap rov d Plans
Ap�►� br
Dafe: t0/i 20:t-7
I am pleased to submit for your review and approval, this Final Drainage Memo for the Living
Oaks development. I certify that this memo for the drainage design was prepared in accordance
with the criteria in the City of Fort Collins Storm Drainage Manual.
Location and Description of Property
The Living Oaks development is located in the Southwest Quarter of Section 12, Township 7
North, Range 69 West of the Sixth Principal Meridian, in the City of Fort Collins, Larimer
County, Colorado.
The project site is located at the southwest corner of East Oak Street and Mathews Street. The
property consists of 4,620 sf of land and will consist of a new building and a drive aisle /
hardscape area behind and tucked under the upper floors of the building. The site currently has
two driveways, both will be removed and only one will be replaced (so the net impervious area in
the right of way will be reduced).
The site generally slopes in an easterly direction at approximately 0.5% to 1.0%. The land is
currently vacant.
According to FEMA Panel 08069CO979H there are no mapped FEMA Floodways on this
property.
The proposed development lies within the Old Town Master Drainage Plan. The outfall for this
site is the existing storm sewer system in Oak Street.
Off Site Flows
The survey indicates that the portion of the sidewalk (not under the roof overhang) directly south
of the property drains into the site. The roof to the south appears to drain south (based on
downspout locations). Additionally a portion of the property to the west also drains into the site,
1218 W. ASH, STE. A, WINDSOR, COLORADO 80550
TEL. 970.674.3300 - FAX 970.674.3303
although it appears to be extremely flat, so there is a chance that no runoff actually leaves that
site. The off site flows are minimal and the grading plan for Living Oaks accounts for these
flows onto the site in the spillway calculation at the driveway to Mathews. This small area of
imperviousness was also included in the 3:1 ratio check of run-on area to the proposed pavers.
If and when either these properties redevelop, they will need to capture their water and this off
site flow will be eliminated.
Development Criteria Reference and Constraints
The runoff from this site has been routed to conform to the requirements of the City Stormwater
Department. Because the increase in impervious area for this site is not greater than 5,000 sf,
onsite detention is not required. Water quality treatment for 50% of the site is provided for in the
Udall Natural Area water treatment facility. City LID requirements are in place which require
50% water quality treatment onsite. The onsite water quality will be accomplished through the
use of a pervious pavement system that includes 12" of storage in the voids below the pavers. ,
Runoff reduction practices (LID techniques) are required. No less than fifty percent of any newly
added impervious area must be treated using one or a combination of LID techniques. The
project adds 4,280 sf of new impervious area (including the pervious pavers). Using the pervious
paver LID technique, all of new impervious area in Basin A will be treated (75% of the overall
site) which exceeds the 50% requirement.
No less than twenty five percent of any newly added pavement areas must be treated using a
permeable pavement technology. The project adds 1,640 sf of new pavement area in the on side
driveway. This project will incorporate 1,000 sf of pervious pavers which is 61% of the newly
added pavement. This meets the required 25%.
Please refer to the attachments for LID calculations and information.
Directly Connected Impervious Area (DCIA) Discussion
Urban Drainage and Flood Control District (UDFCD) recommends a Four Step Process for
receiving water protection that focuses on reducing runoff volumes, treating the water quality
capture volume (WQCV), stabilizing drainageways and implementing long-term source controls.
The Four Step Process applies to the management of smaller, frequently occurring events.
Step 1: Employ Runoff Reduction Practices
To reduce runoff peaks, volumes, and pollutant loads from urbanizing areas, implement Low
Impact Development (LID) strategies, including Minimizing Directly Connected Impervious
Areas (MDCIA).
Runoff from the drive aisle and roof drain directly into a pervious pavement system thereby
slowing runoff and also promoting filtration.
Step 2: Implement BMPs that Provide a Water Quality Capture Volume with Slow Release
2
Once runoff has been minimized, the remaining runoff shall be treated through the pervious
pavement system. The pervious pavement system allows for stormwater filtration.
Step 3: Stabilize Drainageways
Natural Drainageways are subject to bed and bank erosion due to increases in frequency,
duration, rate and volume of runoff during and following development. Downstream
drainageways have already been designed and built for the development of this parcel.
Step 4. Implement Site Specific and Other Source Control BMPs
Proactively controlling pollutants at their source by preventing pollution rather than removing
contaminants once they have entered the stormwater system or receiving waters is important
when protecting storm systems and receiving waters. It will be the responsibility of the
contractor to develop a procedural best management practice for the site during construction.
After construction is completed, the only likely pollutant source would be vehicles using the
driveway. This potential pollutant would be treated with the pervious pavers.
Drainage Facility Design
The study has been divided into three drainage basins, A, B and OS. Basin A consists of the
south side of the building and drive aisle. The roof in this area will drain south to downspouts
which will drain over the top of the pervious pavement system. The system has an underdrain
and will outfall into the inlet at the corner of the intersection.
There is a ridgeline on the roof and the north 13' of the roof will drain to the north (Basin B).
There are two roof drains that will come out of the north wall. The project will build sidewalk
chases under the Oak Street walk and these flows will enter Oak Street through two new curb
openings.
Basin OS (although very small) is accounted for with the run-on ratio design of the pavers.
Basin A and OS will reach the paver system and underdrain. In the event that these are
overwhelmed, stormwater will spill directly east through the driveway and into Mathews Street
to the east. The lowpoint of the driveway is at elevation 4979.76 and the finished floor and
lowest opening of the building is 4980.50. This elevation difference is only 9" and the standard
from a spillway to a finished floor is 12". Calculations are included showing that the
overtopping depth at the lowpoint is 1-1/2" and a variance has been requested for the lack of
freeboard (attached).
Please refer to the attachments for the drainage plan.
Conclusions
All computations that have been completed within this report are in compliance with the City of
Fort Collins Storm Drainage Design Criteria Manual.
3
The proposed drainage concepts presented in this report and on the construction plans adequately
provides for stormwater quantity and quality treatment of proposed, impervious areas.
Conveyance elements have been designed to pass required flows and to minimize future
maintenance.
If, at the time of construction, groundwater is encountered, a Colorado Department of Health
Construction Dewatering Permit will be required.
I appreciate your time and consideration in reviewing this submittal. Please call if you have any
questions.
Sincerely,
t
Erika Schneider, P.E.
Colorado Professional
Engineer No. 41777
4
Reviewed By:
Michael Oberlander, P.E., LEED AP
Colorado Professional
Engineer No. 34288
R C H I T E C T
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I. ANYDAMAGED UI' CURD GtER AND 510EWA" ,IjSTING PRIOR TOCONSTRUCTION. ASWELL AS STREETS,
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EXPENSE PRIORTO THE ACCEPTANCE OF COMPLETED IMPROVEMENTS ANOiOR PRIORTO THE ISSUANCE OF THE
FIRST CERTIFICATE OF OCCUPANCY.
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Stormwater
Alternative Corn pi lanceNaria rice Application
City of Fort Collins Water Utilities Engineering
Engineer Name Michael Oberlander Phone 970=460-8471
Street Address 1218 West Ash, Suite A
City Windsor State CO Zip 80550
Owner Name Robert L, Jr and Laurie P Davis Phone 619-200-8993
Street Address 722 W Mountain Ave
City Fort Collins State CO Zip 80521
Project Name Living Oaks
Project/Application Number from Development Review (i.e. FDP123456) PDP170009
Legal description and/or address of property 221 East Oak
Description of Project Four-plex building on 4,600 SF lot (SW Comer of Mathews and Oak).
1 V driveway on south side of building with 12' drive cut to Mathews.
Existing Use (check one): 0 residential t w non-residential 0 mixed -use r!J vacant ground
Proposed Use (check one): t�' residential IC) non-residential n mixed -use G other
If non-residential or mixed use, describe in detail
Section C: Alternative ComplianceNariance Information
State the requirement from which alternative compliance/variance is sought. (Please include
applicable Drainage Criteria Manual volume, chapter and section.)
pg 68194(H) Vol 2, Chap 10, Storage, 3.3.3 (b) Spillway Freeboard Is to be 12" above spillway crest
What hardship prevents this site from meeting the requirement?
The driveway connection at Mathews Is the location where stormwater would spill out of the pavers
and Into Mathews. Access from this same point In the driveway is needed up to the building (12' N).
The building can't elevate and still gain access to the garage doors becusse the site Is so narrow.
What alternative is proposed for the site?
Attach separate sheet H necessary
If the pavers failed and water topped to the east out the drive, the 100 yr depth would be 1-1/2" (see
calcs). This WSEL Is 7" below the garage doors on this side of the building. This is an adequate
factor of safety for the building and 12" of freeboard from this point Isn't physically possible.
Attach separate sheet It necessary
page 2
The owner agrees to comply with the provisions of the zoning ordinance, building code and all other
applicable sections of the City Code, Land Use Cade, City Plan and all other laws and ordinances
affecting the construction and occupancy of the proposed building that are not directly approved by
this variance. The owner understands that if this variance Is approved, the structure and its occupants
may be more susceptible to flood or runoff damage as well as other adverse drainage Issues.
Signature of
044400
The engineer hereby certifies that the above information, along with the reference plans and project
descriptions Is correct. 1
Signature of
7_1q—/
PE STAMP
Date complete application submitted: -7/1 2ok7
Date of approvalidenlal: 8%iro 2t7 17 Variance [l,,approved 10 denied
Staff ustifi tion/no: tes/condition - n t% r- S S1� / Q n
-A4- LAr,.,.11 f a �o
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' ! r+ o i /. i i n C loo Gr 0 .t
• Approved by:
Entered in UtllityFlle Database
atabas yes ono
Worksheet for 12" SIDEWALK CHASE
Protect Description
Row Element: Rectangular Channel
Friction Method: Manning Formula
Solve For Discharge
Input Data
Roughness Coefficient:
0.013
Channel Slope.
0,02000
ft/ft f T_ 1
Q
Normal Depth
0.40
ft �� *4
Bottom Width.
1.00
ft
Results
Discharge:
2.37
/ ,, /
ft,/s �— fA cxd5 `�CI,,y
` I
Flow Area
040
ft2
Wetted Perimeter:
1.80
ft
Top Width
1.00
ft
V
Critical Depth:
0.56
ft 1 WV•+
Critical Slope:
0.00813
ft/ft CAASE,
Velocity:
5.93
ft/s \
CO, CYS
Velocity Head:
0.55
ft SAS, t.�, /\
Specific Energy:
Fronde Number:
0.95
ft
T
Flow Type:
Supercritical
GVF Input Data
Downstream Depth.
0.00
ft
Length:
000
ft
Number Of Steps.
0
GVF Output Data
Upstream Depth
0.00
ft
Profile Description.
N/A
Profile Headloss
0.00
ft
Downstream Velocity:
0.00
ft/s
Upstream Velocity
0.00
ft/s
Normal Depth:
0.40
ft
Critical Depth:
0.56
ft
Channel Slope:
0.02000
ft/ft
Critical Slope:
O.00B13
ft/ft
Rating Table for 12" SIDEWALK CHASE
Protect Description
Flow Element:
Friction Method.
Solve For.
Input Data
Roughness Coefficient-
Channel Slope.
Bottom Width:
Rectangular Channel
Manning Formula
Discharge
0.013
0,02000
1 00
Attribute
Minimum
Maximum
Increment
Normal Depth
(ft)
0.00
0.40
0.10
Normal Depth
Discharge
Velocity
Flow Area
Wetted
Top Width
Perimeter
000
000
030
1,00
010
0.31
308
0 10
1.20
1 00
020
088
4.42
020
14C
1.00
0.30
1.59
5.30
0.30
1.60
1.00
0.40
2 37
5.92
C 4'
1 80
1 00
ft/ft
It
Worksheet for Emergency Overflow Spillway
Project Description
Flow Element:
Irregular SectionQ
//� �D�
Friction Method:
Manning Formula
Solve For:
Normal Depth
I
Input Data
Channel Slope
0.00400
ittft
Discharge
0.go
ft/s
Options
Current Roughness Weighted Meth(
ImprovedLotters
Open Channel Weighted Roughnes:
ImprovedLotters
Closed Channel Weighted Roughne
Hortons
Results
Roughness Coefficient:
Water Surface Elevation
Elevation Range
Flow Area:
Wetted Perimeter
Top Width:
Normal Depth.
Critical Depth:
Critical Slope.
Velocity:
Velocity Head.
Speck Energy:
Froude Number.
Flow Type:
Segment Roughness
Start Station End Station Roughness
CDeffdent
(0+00. 79.90; (C+2C, 80.50) 0.013
Section Geometry
Station Elevation
0+00 79 90
0+02 79.85
0.013
D 10Z, /{^
- r
79.88
4o
5f:h t..� TO
ft
79.76 to 80 50 ft
r
0.78
ft'
12.40
ft
12.40 r>r= t4 splu , ft
0.00638
ft/ft
1.15
f/s
0.02
It
0.14
ft
0.80
Subcritical
Worksheet for Emergency Overflow Spillway
Station Elevation
0+08 79.76
0+14 7990
0+19.9 80. 10
0+20 80.50
LID Table
50% On -Site Treatment by LID Requirement
New Impervious Area
Required Minimum Impervious Area to be Treated
Treatment #1- Pervious Pavers
Impervious Area to Treatment #1 (Basin A)
4,280
2,140
1,000
3,200
sf
sf
sf
sf
Total Area Treated
3,200
sf
Actual % On -Site Treated by LID
75 %
25% Porous Pavement Requirement
New Driveway Pavement Area (including area under roof)
Required Minimum Area of Pervious Pavement
Area of Pervious Pavers Provided
1,640
410
1,000
sf
sf
sf
Actual % of Pervious Pavement Provided
61 %
3:1 Ratio Requirement
Impervious Area to Pervious Pavers - INCLUDING BASIN OS EXISTING IMPERVIOUS
Total Pervious Pavement Area
2,931
1,000
sf
sf
Ratio
2.9
20 IO O 20
SCALE: 1" - 20'
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50%On-SFte Treatment by UD Requirement
New Impervious Area
Required Minimum Impervious Area to be Treated
Treatmental-Pervious Pavers
Impervious Amato Treatment RS (Basin A)
4,280 sf
2,140 sf
1,DOO sf
3200 sf
Total Area Treated
3,200 sf
Actual %On-SiteTreatedh UD
75%
25%Porous Pavement Requirement
New Driveway Pavement Area(includingarea under roof)
Required Minimum Area of Pervious Pavement
Area of Pervious Pavers Provided
1,640 sf
410 sf
1,0D0 sf
Actual %of Pervious Pavement Provided
61 %
11 Rado Requirement
Impervious Areato Pervious Pavers- INCLUDING BASIN OS EXISTING IMPERVIOUS
Total Pervious Pavement Area
2,931 sf
1,000 sf
Ratio
2.9
LIVING OAKS
LID EXHIBIT
IMPERVIOUS AREA
TREATED BY LID
AREA OF
PAVER SECTION