HomeMy WebLinkAboutLEGACY AUTO - PDP - PDP120001 - REPORTS - DRAINAGE REPORTFINAL DRAINAGE AND EROSION
CONTROL REPORT
LEGACY AUTO
FINAL DRAINAGE AND EROSION
CONTROL REPORT
LEGACY AUTO
Prepared for:
WG Architects
P.O. Box 270788
Fort Collins, CO 80528
Prepared by:
North Star Design
700 Automation Drive, Unit I
Windsor, Colorado 80550
(970) 686-6939
March 20, 2013
Job Number 264-06
March 20
Wes Lam
City of F
700 Woo
Fort Coll
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TABLE OF CONTENTS
TABLE OF CONTENTS ............................................................................................................... iii
1. GENERAL LOCATION AND DESCRIPTION
1.1 Location ...................................................................................................................1
1.2 Description of Property ............................................................................................1
2. DRAINAGE BASINS AND SUB-BASINS
2.1 Major Basin Description ..........................................................................................2
2.2 Sub-Basin Description .............................................................................................2
3. DRAINAGE DESIGN CRITERIA
3.1 Regulations ..............................................................................................................3
3.2 Development Criteria Reference and Constraints ....................................................3
3.3 Hydrologic Criteria ..................................................................................................4
3.4 Hydraulic Criteria ....................................................................................................4
4. DRAINAGE FACILITY DESIGN
4.1 General Concept .......................................................................................................4
4.2 Specific Flow Routing .............................................................................................4
4.3 Drainage Summary ..................................................................................................5
5. EROSION CONTROL
5.1 General Concept ......................................................................................................6
5.2 Specific Details ........................................................................................................6
6. CONCLUSIONS
6.1 Compliance with Standards ....................................................................................6
6.2 Drainage Concept .....................................................................................................7
7. REFERENCES ....................................................................................................................8
APPENDICES
A Vicinity Map
B Hydrologic Computations
C Hydraulic Calculations
D Water Quality and Detention Pond Calculations
E Figures and Tables
1
1. GENERAL LOCATION AND DESCRIPTION
1.1. Location
The Legacy Auto project is located in north Fort Collins. This site is located in a portion
of the southeast quarter of Section 35, Township 8 North, Range 69 West of the 6th P.M.,
City Of Fort Collins, County Of Larimer, State Of Colorado. See the vicinity map
located in Appendix A.
This project is bounded on the north by the Larimer/Weld Canal (also known as Eaton
Ditch), on the east by North College Avenue, on the south by an existing building, and on
the west by an existing building.
1.2. Description of Property
The proposed project site consists of approximately 3.06 acres of land. The project site
consists of the existing Legacy Auto facility, associated drives, walks and landscaping.
Access to the existing Legacy Auto parking lot is from North College Avenue. The
property is currently used for commercial uses that include the sales and service of farm
equipment.
The proposed site is located on Federal Emergency Management Agency (FEMA) Flood
Insurance Rate Map number 08069C0977G with an effective date of June 17, 2008.
FEMA does not have a defined floodplain within the project boundary, however there is a
floodplain defined along the Larimer / Weld Canal, north of this property. A portion of
the FEMA floodplain is shown on the FIRMette that is located in Appendix D of this
report. The boundaries of this site are not located within the City of Fort Collins or
FEMA designated floodplains.
The majority of stormwater from the development of this site is routed to the southwest
to the proposed onsite detention pond where water quality and detention is provided.
There are no offsite flows entering the boundaries of this site.
Existing onsite vegetation consists of native grasses and various landscaping. It is
proposed to provide detention for Basins 2 and 3 and then pipe detained stormwater to
the existing inlet located near the southeast corner of the site. Existing slopes onsite are
2
flat and range from approximately 0.0% to 2.0% and generally slopes to the southwest.
Stormwater from this site is ultimately conveyed to the Poudre River.
The site has been divided into 4 drainage basins to better define the area that will drain
north to the Larimer/Weld Canal, to the detention pond and to the existing and proposed
inlets located in the southeast corner of the site. Detention volume calculations are based
on detaining the 100 year developed storm and releasing runoff into the existing storm
sewer at the specified 0.2 cfs/acre runoff rate. The detention volume has been calculated
using the FAA method as described in the City of Fort Collins Drainage Criteria manual.
Due to site constrains detention can only be provided for Basins 2 and 3. Basin 1 will
remain undeveloped and runoff will be allowed to sheet flow north to the Larimer/Weld
Canal in historic flow routes. Basin 4 has been minimized to the greatest extent possible
but because of existing grades (existing walk along College, existing driveway to the
southeast) cannot be captured and conveyed to the proposed detention area and will be
allowed to flow to the existing storm inlet located near the southeast corner of the site in
historic flow routes.
Water quality is required for this development, for Basin 1 water quality will be provided
by the conveyance of runoff through the grass buffer to the Larimer/Weld Canal, for
Basins 2 and 3 water quality will be provided by the use of a water quality extended
detention, and due to site constraints the small asphalt area in Basin 4 will sheet flow to
the existing inlet in historic flow routes.
2. DRAINAGE BASINS AND SUB-BASINS
2.1. Major Basin Description
This site is located within the City of Fort Collins Dry Creek Basin which states that
water quality and detention is required for all new developments. The detention pond has
been designed to detain the 100 year developed storm event with a release rate equal to
0.20cfs/acre. The calculated detention pond release rate is 0.52 cfs. Historic drainage
patterns have been maintained to the greatest extent possible and with the development of
this site the overall drainage will be an improved.
2.2. Sub-basin Description
This site is divided into four drainage basins where the north portion sheet flows to the
Larimer/Weld Canal and the majority of the remaining development is routed to the
3
southwest corner of the site to the proposed detention pond. Due to site constraints there
is a small portion of the site in the southeast corner that will sheet flow to the existing
inlet.
See Section 4 of this report for specific details for each sub-basin.
3. DRAINAGE DESIGN CRITERIA
3.1. Regulations
Drainage for this site has been designed to meet or exceed the “City of Fort Collins Storm
Drainage Design Criteria Manual” specifications. Where applicable, the criteria
established in the “Urban Storm Drainage Criteria Manual” (UDFCD), 2001 has been
used.
3.2. Development Criteria Reference and Constraints
Stormwater runoff from this site has been routed to conform to the requirements of the
City for Fort Collins Stormwater Department. Detention is required for this site and is
being provided for the majority of the development. Runoff for the remaining site is
allowed to drain in historic flow routes. The grass buffer along the north property line
will slow runoff and promote settling of the suspended solids prior to runoff being
conveyed into the Larimer / Weld Canal.
Due to site constraints, a variance is requested to allow the detention pond to have 0.45’
of freeboard from the calculated 100 year water surface elevation to the proposed top of
berm instead of the 1.0’ that is typically required. The existing and proposed buildings
are located a minimum of 1.5’ above the calculated 100 year water surface elevation and
will therefore be protected from localized flooding in the event the pond reaches capacity.
The detention pond has been designed to detain the 100 year developed storm event
without overtopping. If the detention pond reaches capacity, then any additional
stormwater will be routed the future public street along the southern property boundary
and once fully constructed it is anticipated this additional runoff will be conveyed to
Willox Court or College Avenue. The spillway for the pond shall be along the south
embankment of the pond adjacent to the future public street. If the outlet structure
becomes completely clogged the entire 100 year runoff overtops the sidewalk to the
public street and runoff will flow in historic flow routes.
4
3.3. Hydrologic Criteria
Runoff computations were prepared for the 2 year and 10 year minor storm events and
100 year major storm event utilizing the Rational Method. All hydrologic calculations
associated with the basins are included in Appendix B of this report. Water quality has
been provided by utilizing the extended detention method recommended in the “Urban
Storm Drainage Criteria Manual”. See Appendix D for water quality calculations.
3.4. Hydraulic Criteria
All hydraulic calculations within this report have been prepared in accordance with the
City of Fort Collins Drainage Criteria, Urban Area Drainage Criteria Manual and are
included in Appendix C.
4. DRAINAGE FACILITY DESIGN
4.1. General Concept
Runoff from this site historically sheet flows to all 4 sides of the development. With the
construction of these proposed improvements onsite stormwater will be allowed to flow
in historic flow routes. The majority of the site drains to the southwest corner to the
proposed detention pond. Detained stormwater will be released at 0.20 cfs/acre (0.49cfs)
and conveyed in storm piping to the existing inlet located near the southeast corner of the
development. Historic flow routes are maintained to the greatest extent possible.
4.2. Specific Flow Routing
A summary of the drainage patterns within each basin is provided in the following
paragraphs.
Basin 1 is located along the north portion of the site. This basin will remain undeveloped
and will consist of native vegetation. This basin and existing landscaping is preserved for
a wildlife corridor as required by the Colorado Department of Wildlife. Runoff from this
basin sheet flows north to the Larimer/Weld Canal. Water quality for this basin will be
provided by runoff flowing through the established vegetation prior to being conveyed to
the canal.
Basin 2 is located in the central portion the site. This basin includes roof area for the
existing and proposed buildings, asphalt drives, sidewalks, gravel storage area and
landscaping. Stormwater runoff will sheet flow to low points where runoff is intercepted
5
and conveyed to the detention pond. Water quality is provided for this basin by a
sediment trap and a “Snout” inlet hood located within the detention pond outlet structure.
Runoff is detained and routed in storm pipe to the existing inlet that is located near the
southeast corner of the site.
Basin 3 is located along the eastern property boundary and contains a portion of the
existing and proposed buildings, asphalt drives, sidewalks, and landscaping. Runoff in
this basin is routed to the south in the proposed curb and gutter where runoff is
intercepted by a proposed inlet and conveyed west to the proposed detention pond.
Water quality is provided for this basin by a sediment trap and a “Snout” inlet hood
located within the detention pond outlet structure. Runoff is detained and routed in storm
pipe to the existing inlet that is located near the southeast corner of the site.
Basin 4 is located along the east edge of the site which includes the existing concrete
sidewalk along College and the adjacent landscape area. The Basin is also located in the
southeast corner of the site and includes a portion of the existing building, existing
asphalt drive and sidewalk. This basin has been reduced in size to the greatest extent
possible. Runoff from this basin sheet flows south to the existing inlet in historic flow
routes. Due to site constraints water quality is not provided for this basin.
4.3. Drainage Summary
Drainage for this site was designed to conform to the City of Fort Collins stormwater
requirements. Water quality is provided for the majority of the site which includes basins
1, 2, and 3. Water quality is provided by routing runoff through either a grass buffer or
through the detention pond and providing extended detention. Due to site constraints
detention and water quality is not provided for basin 4 and allowed to flow in historic
flow routes. Stormwater will be detained in the pond and released through the outlet
structure to the existing storm system located along the west side of College Avenue.
Drainage facilities located outside of the right-of-way will be maintained by the owners
of the property. During construction the pond and outlet structure shall be visually
inspected at a minimum of once a week for collected sediment and cleaned if the depth of
sediment is greater than 4 inches. After construction and once vegetation is considered
established, then the outlet structure shall be visually inspected once a month until a
maintenance schedule can be created. The maximum allowable time between visual
inspections shall be 6 months.
6
5. EROSION CONTROL
5.1. General Concept
This site lies within the Moderate Rainfall and Wind Erodibility Zone per the City of Fort
Collins. Potential also exists for tracking of mud onto existing streets which could then
wash into existing storm systems. See the drainage and erosion control plan for location
and measures taken to control the movement of sediment offsite to the public streets and
existing storm sewer system.
5.2. Specific Details
To limit the amount of silt leaving the site, several erosion control measures shall be
implemented during construction. All inlets (existing and proposed) shall be protected
with gravel filters or wattles. Wattles will be placed in the existing curb and gutter and
other areas where deemed appropriate and silt fence shall be installed along the property
boundaries. The detention pond will act as a sediment trap during construction. All
construction traffic shall access this site by way of a vehicle tracking pad. The vehicle
tracking pad shall be located wherever construction traffic access the public roads. If
mud is tracked on public roads then it shall be removed from the street on a daily basis.
During demolition and site construction, all disturbed areas are to be kept in a roughened
condition and watered to reduce wind erosion. If the area remains bare for longer than 30
days and is not being worked on, temporary vegetation is required and shall be planted to
help reduce the potential for soil movement. The detention pond and outlet structure shall
be used to collect sediment during construction and prior to final seeding of the detention
pond, collected sediment shall be removed and the pond shall be graded to final design
elevations.
6. CONCLUSIONS
6.1. Compliance with Standards
All computations completed within this report are in compliance with the City of Fort
Collins Drainage and Erosion Control Reference Manuals.
7
6.2. Drainage Concept
The proposed drainage concepts presented in this report, and on the construction plans,
adequately provide for stormwater quality treatment for the proposed improvements.
Conveyance elements have been designed to convey required flows to the proposed
detention pond and for minimal maintenance.
If, at the time of construction, groundwater is encountered, a Colorado Department of
Health Construction Dewatering Permit will be required.
8
7. REFERENCES
1. City of Fort Collins, “Storm Drainage Criteria Manual”, (SDCM), dated March
1999.
2. Urban Drainage and Flood Control District, “Urban Storm Drainage Criteria
Manual”, Volumes 1 and 2, dated June 2001, and Volume 3 dated September
2001.
3. City of Fort Collins Floodplain maps found on the City of Fort Collins web site.
Supplied by the City of Fort Collins, printed on October 19, 2009
A
APPENDIX A
VICINITY MAP
B
APPENDIX B
HYDROLOGIC COMPUTATIONS
North Star Design, Inc.
700 Automation Drive, Unit I
Windsor, CO 80550
LOCATION: Legacy Auto
PROJECT NO: 264-06
COMPUTATIONS BY: TDS
DATE: 3/20/2013
Recommended % Impervious from Urban Storm Drainage Criteria Manual
%
Impervious
Roofs: 90%
Streets and Parking (Asphalt) 100%
Concrete Drives and Walks 90%
Packed Gravel: 40%
Landscaped Areas (Clayey soil, 0-1% slopes): 0%
SUBBASIN TOTAL TOTAL ROOF ASPHALT CONCRETE GRAVEL LANDSCAPE % RUNOFF RUNOFF RUNOFF RUNOFF
ID AREA AREA AREA AREA AREA AREA AREA Impervious COEFF. COEFF. COEFF. COEFF.
(ac.) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (sq.ft) C2 C5 C10 C100
historic site 3.06 133,162 2,500 12,978 350 20393 96,941 18% 0.16 0.25 0.34 0.55
1 0.21 9,342 0 0 0 0 9,342 0% 0.04 0.15 0.25 0.50
2 2.00 86,955 8,960 23,649 6,968 20350 27,028 53% 0.36 0.41 0.47 0.61
3 0.59 25,840 3,140 13,640 3,210 0 5,850 75% 0.54 0.57 0.61 0.71
4 0.18 7,780 0 2,111 3,915 0 1,754 72% 0.51 0.55 0.59 0.69
2+3 2.59 112,795 12,100 37,289 10,178 20,350 32,878 58% 0.39 0.44 0.50 0.63
RUNOFF COEFFICIENTS & % IMPERVIOUS
Flow_legacy.xls Cval
North Star Design, Inc.
700 Automation Drive, Unit I
Windsor, CO 80550
LOCATION: Legacy Auto
PROJECT NO: 264-06
COMPUTATIONS BY: TDS
DATE: 3/20/2013
SUB-BASIN DATA INITIAL / OVERLAND TIME TRAVEL TIME / GUTTER OR CHANNEL FLOW tc CHECK FINAL REMARKS
(ti)(tt) (URBANIZED BASIN) tc
DESIGN SUBBASIN Area C5 Length Slope ti Length Slope Chan. Cv Vel. tt tc = Total L tc=(l/180)+10
POINT ID (ac) (ft) (ft/ft) (min) (ft) (ft/ft) Type (ft/s) (min) ti + tt (ft) (min) (min)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)
historic site historic site 3.06 0.25 200 0.020 17.5 550 0.015 D 10 1.2 7.5 25.0 750 14.2 14.2
1 1 0.21 0.15 50 0.050 7.2 50 0.020 D 10 1.4 0.6 7.8 100 10.6 7.8
2 2 2.00 0.41 80 0.050 6.6 550 0.005 F 20 1.4 6.5 13.1 630 13.5 13.1
3 3 0.59 0.57 30 0.020 4.2 350 0.020 F 20 2.8 2.1 6.2 380 12.1 6.2
4 4 0.18 0.55 10 0.020 2.5 130 0.050 F 20 4.5 0.5 3.0 140 10.8 5.0
EQUATIONS: Channel Type Cv
tc = t
i + tt
A 2.5
B 5.0
ti = [0.395 (1.1 - C
5 ) L
0.5
] / S
1/3
C 7.0
D 10.0
tt = L/V V = C
v*S
0.5
E 15.0
F 20.0
final tc = minimum of t
i + tt
and urbanized basin check
minimum tc = 5 minutes
TIME OF CONCENTRATION
Paved
Grassed Waterway
Bare Ground
Short Pasture/ Lawn
Tillage / Field
Heavy Meadow
Flow_legacy.xls TOC5
North Star Design, Inc.
700 Automation Drive, Unit I
Windsor, CO 80550
LOCATION: Legacy Auto
PROJECT NO: 264-06
COMPUTATIONS BY: TDS
DATE: 3/20/2013
DIRECT RUNOFF TOTAL Inlet Capacity REMARKS
DESIGN SUBBASIN A C2 tc iQ2 Q2 Total
POINT ID
(ac) (min) (in/hr) (cfs) (cfs) (cfs)
historic site historic site 3.06 0.16 14.2 2.44 1.2 1.2
1 1 0.21 0.04 7.8 3.08 0.0 0.0
2 2 2.00 0.36 13.1 2.52 1.8 1.8
3 3 0.59 0.54 6.2 3.32 1.1 1.1
4 4 0.18 0.51 5.0 3.50 0.3 0.3
Q = C i A
Q = peak discharge (cfs)
C = runoff coefficient
i = rainfall intensity (in/hr) from IDF curve
A = drainage area (acres)
RATIONAL METHOD PEAK RUNOFF
(2-YEAR)
Flow_legacy.xls Q2
North Star Design, Inc.
700 Automation Drive, Unit I
Windsor, CO 80550
LOCATION: Legacy Auto
PROJECT NO: 264-06
COMPUTATIONS BY: TDS
DATE: 3/20/2013
DIRECT RUNOFF TOTAL Inlet Capacity REMARKS
DESIGN SUBBASIN A C10 tc iQ10 Q10 Total
POINT ID
(ac) (min) (in/hr) (cfs) (cfs) (cfs)
historic site historic site 3.06 0.34 14.2 4.00 4.1 4.1
1 1 0.21 0.25 7.8 5.54 0.3 0.3
2 2 2.00 0.47 13.1 4.13 3.9 3.9
3 3 0.59 0.61 6.2 6.28 2.3 2.3
4 4 0.18 0.59 5.0 6.85 0.7 0.7
Q = C i A
Q = peak discharge (cfs)
C = runoff coefficient
i = rainfall intensity (in/hr) from IDF curve
A = drainage area (acres)
RATIONAL METHOD PEAK RUNOFF
(10-YEAR)
Flow_legacy.xls Q10
North Star Design, Inc.
700 Automation Drive, Unit I
Windsor, CO 80550
LOCATION: Legacy Auto
PROJECT NO: 264-06
COMPUTATIONS BY: TDS
DATE: 3/20/2013
DIRECT RUNOFF TOTAL Inlet Capacity REMARKS
DESIGN SUBBASIN A C100 t
c iQ100
Q100 Total
POINT ID
(ac) (min) (in/hr) (cfs) (cfs) (cfs)
historic site historic site 3.06 0.55 14.2 6.41 10.7 10.7
1 1 0.21 0.50 7.8 8.13 0.9 0.9
2 2 2.00 0.61 13.1 6.62 8.1 8.1
3 3 0.59 0.71 6.2 8.80 3.7 3.7
4 4 0.18 0.69 5.0 9.30 1.1 1.1
Q = C i A
Q = peak discharge (cfs)
C = runoff coefficient
i = rainfall intensity (in/hr) from IDF curve
A = drainage area (acres)
RATIONAL METHOD PEAK RUNOFF
(100-YEAR)
Flow_legacy.xls Q100
C
APPENDIX C
HYDRAULIC CALCULATIONS
D
APPENDIX D
DETENTION POND CALCULATIONS
Northstar Design, Inc
700 Automation Drive, Unit I
Windsor CO, 80550
(970) 686-6939
LOCATION: Legacy Auto
ITEM: Pond 1
COMPUTATIONS BY: TDS
SUBMITTED BY: North Star Design, Inc
DATE: 3/20/2013
From Urban Storm Drainage Criteria Manual, June 2001
(Referenced figures are attached in Appendix D)
Use 40-hour brim-full volume drain time for extended detention basin
Water Quality Capture Volume = WQCV = (required storage/12)*(tributary drainage area)
MAJOR Trib. % Imperv. Req. Storage WQCV DWQ req. vol req. area/row
BASIN area (in. of runoff) WQCV *1.2 (in
2
/row)
(ac) from Fig. SQ-2 (ac-ft) (ft) (ac-ft) from Fig. EDB-3
Pond 1 2.59 58.00 0.230 0.050 1.55 0.059 0.22
Required Storage = 1 * ( 0.91 * I^3 - 1.19 * I^2 + 0.78 * I )
WQ outlet sizing for NORTH pond
From Figure 5,
for 9/16 in. hole diameter
Area of hole = 0.24 in2
# of columns = 1
Area provided/row = 0.24 in2
Area Required/row = 0.22 in2
Use 1 column of 9/16" diam holes
WATER QUALITY POND OUTLET SIZING
DetPond.xls WQCV 1
MINIMUM DETENTION VOLUME REQUIRED
FAA METHOD
LOCATION: Legacy Auto
PROJECT NO: Pond 1
COMPUTATIONS BY: TDS
SUBMITTED BY: North Star Design, Inc
DATE: 3/20/2013
Equations: A trib. To pond = 2.59 acre
QD = CiA C100 = 0.63
Vi = T*CiA = T*QD Developed C*A = 1.63 acre
Vo =m*QPO*(60*T) QPO = 0.52 cfs (0.2 cfs/acr
S = Vi - Vo tc = 18 min
Rainfall intensity from Larimer County Area 1 IDF Curve
Storm Rainfall QD Vol. In Outflow Vol. Out Storage Storage
Duration, T Intensity, i (cfs) Vi Adjustment Vo SS
(min) (in/hr) (ft3) Factor, m (ft3) (ft3) (ac-ft)
5 9.95 16.2 4871 1.00 155 4715 0.108
10 7.72 12.6 7558 1.00 311 7247 0.166
20 5.60 9.1 10965 0.95 591 10375 0.238
30 4.52 7.4 13276 0.80 746 12530 0.288
40 3.74 6.1 14646 0.73 901 13745 0.316
50 3.23 5.3 15811 0.68 1057 14754 0.339
60 2.86 4.7 16800 0.65 1212 15588 0.358
70 2.62 4.3 17955 0.63 1368 16588 0.381
80 2.38 3.9 18641 0.61 1523 17118 0.393
90 2.22 3.6 19561 0.60 1678 17882 0.411
100 2.05 3.3 20070 0.59 1834 18236 0.419
110 1.93 3.1 20785 0.58 1989 18795 0.431
120 1.80 2.9 21147 0.58 2145 19002 0.436
130 1.60 2.6 20364 0.57 2300 18064 0.415
140 1.40 2.3 19189 0.56 2455 16733 0.384
150 1.20 2.0 17622 0.56 2611 15012 0.345
160 1.15 1.9 18014 0.56 2766 15248 0.350
170 1.10 1.8 18308 0.55 2922 15386 0.353
180 1.05 1.7 18503 0.55 3077 15427 0.354
Required Storage Volume: 19002 ft3
0.436 acre-ft
Northstar Design, Inc
700 Automation Drive, Unit I
Windsor CO, 80550
(970) 686-6939
Proposed Detention Pond - Stage/Storage
LOCATION: Legacy Auto
PROJECT NO: Pond 1
COMPUTATIONS BY: TDS
SUBMITTED BY: North Star Design
DATE: 3/20/2013
V = 1/3 d ( A + B + sqrt(A*B))
where V = volume between contours, ft3
d = depth between contours, ft
A = surface area of contour
Pond 1
Required Water Quality = 0.059 acre-feet
REQ. 100 YR DETENTION VOL. = 0.436 acre-feet
Total Volume Required = 0.496 acre-feet
Surface Incremental Total
Stage Area Storage Storage
(ft) (ft2) (ac-ft) (ac-ft)
4981 4981.50 0
4982.00 8807 0.034 0.034
4982.12 9524 0.025 0.059 <-- WQ VOL
4983.00 14781 0.268 0.302
4983.55 15958 0.195 0.496 <-- 100 yr HWEL (TOTAL)
4984.00 16914 0.169 0.665
DetPond.xls Stage-Storage 1
100yr ORIFICE PLATE SIZING
Detention Pond Outlet Sizing
(100 yr event)
LOCATION: Legacy Auto
PROJECT NO: 215-04
COMPUTATIONS BY: ROB
SUBMITTED BY: North Star Design, Inc.
DATE: 3/20/2013
Submerged Orifice Outlet:
release rate is described by the orifice equation,
Qo = C
oAo
sqrt( 2g(h-Eo))
where Qo = orifice outflow (cfs)
Co = orifice discharge coefficient
g = gravitational acceleration = 32.2 ft/s
Ao = effective area of the orifice (ft
2
)
Eo = greater of geometric center elevation of the orifice or d/s HGL (ft)
h = water surface elevation (ft)
Qo = 0.52 cfs
outlet pipe dia = D = 12.0 in
Invert elev. = 4981.50 ft
Eo = 4981.65 ft
h = 4983.55 ft - 100 yr WSEL
Co = 0.7
solve for effective area of orifice using the orifice equation
Ao = 0.067 ft
2
= 9.6 in
2
orifice dia. = d = 3.50 in
Check orifice discharge coefficient using Figure 5-21 (Hydraulic Engineering )
d / D = 0.29
kinematic viscosity, u = 1.22E-05 ft
2
/s
Reynolds no. = Red = 4Q/(pdu) =
1.85E+05
Co = (K in figure) = 0.70 check
Use d = 3.50 in
A o = 0.067 ft
2
= 9.62 in
2
Q max = 0.52 cfs
DetPond.xls
Legacy Auto
Emergency Overflow Spillway Sizing
LOCATION: Legacy Auto
PROJECT NO: 215-04
COMPUTATIONS BY: ROB
SUBMITTED BY: North Star Design, Inc.
DATE: 3/20/2013
Equation for flow over weir
Q = CLH3/2
where C = weir coefficient = 3.1
H = overflow height
L = length of the weir
Spillways will be designed with flow depth, H = 0.5 ft
Size the spillway assuming that the pond outlet is completely clogged.
North Detention & Water Quality Pond
Q (100) = 10.00 cfs
Spill elev = 4983.55 ft 100 yr WSEL = 4983.55 ft
Min top of berm elev.= 4984.00 ft
Act. Top of berm elev.= 4984.00
Wil Weir length th required: i d
L = 10.76 ft
Use L = 11.0 ft
v = 1.33 ft/s
DetPond.xls
E
APPENDIX E
FIGURES AND TABLES
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