HomeMy WebLinkAbout20060447 Ver 2_Preliminary Stormwater Plan_20080826O CD
WITHERS RAVENEL
Olde Towne Rec. Center
Preliminary Stormwater Management Plan
Raleigh, North Carolina
Prepared By:
WITHERS & RAVENEL, INC
111 MacKenan Drive
Cary, North Carolina 27511
WR Project # 02080510
July 2008
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INTRODUCTION
Olde Towne Recreation Center is a ±14.48-acre, site located in the City of Raleigh, North Carolina.
The site is located within the Neuse River Basin. The purpose of this study is to document
calculations of the pre- and post-development peak stormwater discharges for the 2-, and 1o-year
storm events as well as the BMP sizing calculations and Nitrogen Export rates for the site.
Figure 1 Site Outlined in Red on Aerial Photography
Per City of Raleigh regulations the site will be required to mitigate post-development flows to less
than or equal to the pre-development flows in the 2-, and so-year 24-hour storm events. The site will
also be required to reduce nitrogen export rates to less that 1o.o lbs/ac/yr. A single extended
detention wetland is proposed to satisfy The City of Raleigh detention requirements and the nitrogen
export requirements.
Currently, the site and surrounding drainage area are primarily open space and woods, and contain
predominantly Hydraulic Soil Group B soils.
AUG E 6 2006
OENR -WATER QUALI'JY
WET, 4ND 8T09MWATER BRANCH
01de Towne Rec. Cent. i W&R Project 2o80510
Pond Report July 2oo8
• METHODOLOGY
The stormwater study was conducted using the natural drainage features as depicted by field
survey, LIDAR topography (i-ft contours) and proposed development within the drainage areas.
The scope of work included the following analyses:
• Hydrolos;y
? Simulation of the 2-year, zo-year, and loo-year rainfall events for the Raleigh area
? Formulation of the 2-year, io-year, and Soo-year flood hydrographs for the pre- and post-
development drainage areas
• Hydraulic
? Routing the 2-year, io-year, and ioo-year flood hydrographs for post development runoff
through the proposed extended detention wetland.
• The results of the hydrology calculations are used in the hydraulic analyses. The hydraulic design
requires the development of stage-storage and stage-discharge functions for the wetland. The
rainfall/runoff hydrographs, stage-storage and stage-discharge functions have been compiled to
• create a routing computer simulation model using Haestad Methods PondPack vio.o software. This
PondPack model was then used to assess the impact of the peak discharges for the design rainfall
events.
• HYDROLOGY
The SCS Method was used to develop runoff hydrographs in the Raleigh area. The 24-hour duration
• rainfall curves are determined using the NOAA Atlas 14 rainfall data for Raleigh-Durham International
Airport for the 2-year, io-year, and ioo-year storm events. This method requires three basic
parameters: a curve number (CN), time of concentration (Q, and drainage area.
Curve numbers were based on soil type and land use. Soil types were delineated from the USDA
Web Soil Survey. Land use for existing conditions was based on the most recent Wake County aerial
photographs. Post development land use is based on the proposed site plan.
Times of concentration were calculated using methods described in the SCS publication "Urban
Hydrology for Small Watersheds, TR-55" and based on Wake County topographic maps and field
• survey data. The post-development time of concentration was estimated at 5 minutes due to the
effect of piped drainage systems.
The curve number data, time of concentration calculations and PondPack output summaries are
• included in the appendices of this report.
HYDRAULICS
• Computer simulated reservoir routing of the 2-year, io-year, and loo-year design storms utilized
stage-storage and stage-discharge functions. Stage-storage functions were derived from the
proposed contours for the wetland. A non-linear regression relation for surface area versus
elevation was derived for the detention basin. This relation estimates the incremental volume of the
basin to the stage or elevation of that basin. Stage-discharge functions were developed to size the
outlet structure for the wetland.
• 01de Towne Rec. Cent. 2 W&R Project 20805-10
• Pond Report July 2oo8
CONCLUSIONS
Based on the routing study, the post development peak discharge from the entire site after
improvements is less than pre-development levels for the 2-year, and 1o-year design storms. The
attached sheets summarize the preliminary peak attenuation results and general design
information.
Summary of Results
Storm Event Pre-Development
cfs Post-Development*
cfs Difference (cfs)
2-Year 5.72 5.73 0.01
io-Year 19.77 16.63 -3.14
Extended Detention
Wetland inflow (cfs) Outflow (cfs) WSE (ft)
2-Year 29.66 o.6 195-72
1o-Year 6.61 9.911 196.28
loo-Year 65.26 2.68 196.78
Pond Outlet Summary: 5'x 5' Inlet Box, Weir Crest @ 196.oo, Top of Dam @ 197.50
6" Diameter Orifice @ 195.00, 2" Diameter Inverted PVC,@194.00
36" Diameter Outlet Barrel Invert out =190.30'
* Note: Post Development flows include 3.37 acres of off-site area.
01de Towne Rec. Cent. 3 MR Project 2o80510
Pond Report July 2008
Background Information
Pond Pack Results
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Post to Wetland
Predev-Site
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Pre-Out
Pond 10
Post Bypass
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Post-Out
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Table of Contents
Table of Contents
********************** MASTER SUMMARY **********************
Watershed....... Master Network Summary ............. 1.01
****************** DESIGN STORMS SUMMARY *******************
RDU NOAA 14 Desi Design Storms ...................... 2.01
RDU NOAA 14 Desi 2
Design Storms ...................... 2.02
********************** TC CALCULATIONS *********************
POST BYPASS..... Tc Calcs ........................... 3.01
POST TO WETLAND Tc Calcs ........................... 3.03
PREDEV-SITE..... Tc Calcs ........................... 3.05
********************** CN CALCULATIONS *********************
POST BYPASS..... Runoff CN-Area ..................... 4.01
POST TO WETLAND Runoff CN-Area ..................... 4.02
PREDEV-SITE..... Runoff CN-Area ..................... 4.03
*********************** POND VOLUMES ***********************
POND 10......... Vol: Elev-Area ..................... 5.01
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Table of Contents
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Table of Contents (continued)
******************** OUTLET STRUCTURES *********************
Outlet 1........ Outlet Input Data .................. 6.01
Composite Rating Curve ............. 6.04
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Type.... Master Network Summary Page 1.01
Name.... Watershed
File.... K:\08\08-C)510\080510-Olde Towne - The Residence Ctr\H-H\O1deTowneRec.ppw
MASTER DESIGN STORM SUMMARY
Network Storm Collection: RDU NOAA 14 Desi
Total
Depth Rainfall
Return Event in Type RNF ID
------------
2 ------
3.4400 ----------------
Time-Depth Curve ----------------
RDU NOAA 2yr
10 4.9800 Time-Depth Curve RDU NOAA 10yr
100 7.3700 Time-Depth Curve RDU NOAA 100yr
MASTER NETWORK SUMMARY
SCS Unit Hydrograph Method
(*Node=Outfall; +Node=Diversion;)
(Trun= HYG Truncation: Blank=None; L=Left; R=Rt; LR=Left&Rt)
Max
Return HYG Vol Qpeak Qpeak Max WSEL Pond Storage
Node
------ ID
-------
---- Type
---- Event
------ ac-ft Trun
---------- -- hrs
--------- cfs
-------- ft
-------- ac-ft
---------
-
POND
10
IN
POND
2
1.368
12.1000
29.66 -
-
POND 10 IN POND 10 2.446 12.1000 46.61
POND 10 IN POND 100 4.245 12.1000 65.26
POND 10 OUT POND 2 .845 15.0000 .64 195.72 .914
POND 10 OUT POND 10 1.922 12.3500 9.91 196.28 1.242
POND 10 OUT POND 100 3.720 12.2000 42.68 196.78 1.547
POST BYPASS AREA 2 .362 12.1000 5.73
POST BYPASS AREA 10 .900 12.1000 16.11
POST BYPASS AREA 100 1.971 12.1000 30.73
POST TO WETL AND AREA 2 1.368 12.1000 29.66
POST TO WETL AND AREA 10 2.446 12.1000 46.61
POST TO WETL AND AREA 100 4.245 12.1000 65.26
*POST- OUT JCT 2 1.207 12.1000 5.73
*POST- OUT JCT 10 2.822 12.1000 16.63
*POST- OUT JCT 100 5.691 12.1500 66.04
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Type.... Master Network Summary Page 1.02
Name.... Watershed
File.... K:\08\08-0510\080510-Olde Towne - The Residence Ctr\H-H\OldeTowneRec.ppw
MASTER NETWORK SUMMARY
SCS Unit Hydrograph Method
(*Node=Outfall; +Node=Diversion;)
(Trun= HYG Truncation: Blank=None; L=Left; R=Rt; LR=Left&Rt)
Return HYG Vol Qpeak Qpeak Max WSEL
Node ID
--------------- Type
-- -- Event ac-ft Trun hrs cfs ft
*PRE-OUT --
JCT ------
2 ---------- --
.475 ---------
12.2000 -------- --------
5.72
*PRE-OUT JCT 10 1.320 12.1500 19.77
*PRE-OUT JCT 100 3.092 12.1500 41.61
PREDEV-SITE AREA 2 .475 12.2000 5.72
PREDEV-SITE AREA 10 1.320 12.1500 19.77
PREDEV-SITE AREA 100 3.092 12.1500 41.61
Max
Pond Storage
ac-ft
------------
S/N: Bentley Systems, Inc.
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Type.... Design Storms
Name.... RDU NOAA 14 Desi
Page 2.01
File.... K:\08\08-0510\080510-Olde Towne - The Residence Ctr\H-H\O1deTowneRec.ppw
Title... Project Date: 6/26/2008
Project Engineer: DLee
Project Title: Watershed
Project Comments:
DESIGN STORMS SUMMARY
Design Storm File,ID = RDU NOAA 14 Desi
Storm Tag Name = 2
Data Type, File, ID
Storm Frequency
Total Rainfall Depth
Duration Multiplier
Resulting Duration
Resulting Start Time
Time-Depth Curve RDU NOAA 2yr
2 yr
3.4400 in
1
23.9904 hrs
.0000 hrs Step= .0833 hrs End= 23.9904 hrs
Storm Tag Name = 10
Data Type, File, ID = Time-Depth Curve RDU NOAA 10yr
Storm Frequency = 10 yr
Total Rainfall Depth= 4.9800 in
Duration Multiplier = 1
Resulting Duration = 23.9904 hrs
Resulting Start Time= .0000 hrs Step= .0833 hrs End= 23.9904 hrs
Storm Tag Name = 100
Data Type, File, ID = Time-Depth Curve RDU NOAA 100yr
Storm Frequency = 100 yr
Total Rainfall Depth= 7.3700 in
Duration Multiplier = 1
Resulting Duration = 23.9990 hrs
Resulting Start Time= .0000 hrs Step= .0833 hrs End= 23.9990 hrs
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Type.... Design Storms Page 2.02
Name.... RDU NOAA 14 Desi Event: 2 yr
File.... K:\08\08-0510\080510-Olde Towne - The Residence Ctr\H-H\OldeTowneRec.ppw
Storm... RDU NOAA 2yr Tag: 2
DESIGN STORMS SUMMARY
Design Storm File,ID = RDU NOAA 14 Desi
Storm Tag Name = 2
Data Type, File, ID = Time-Depth Curve RDU NOAA 2yr
Storm Frequency = 2 yr
Total Rainfall Depth= 3.4400 in
Duration Multiplier = 1
Resulting Duration = 23.9904 hrs
Resulting Start Time= .0000 hrs Step= .0833 hrs End= 23.9904 hrs
Storm Tag Name = 10
Data Type, File, ID = Time-Depth Curve RDU NOAA 10yr
Storm Frequency = 10 yr
Total Rainfall Depth= 4.9800 in
Duration Multiplier = 1
Resulting Duration = 23.9904 hrs
Resulting Start Time= .0000 hrs Step= .0833 hrs End= 23.9904 hrs
Storm Tag Name = 100
Data Type, File, ID = Time-Depth Curve RDU NOAA 100yr
Storm Frequency = 100 yr
Total Rainfall Depth= 7.3700 in
Duration Multiplier = 1
Resulting Duration = 23.9990 hrs
Resulting Start Time= .0000 hrs Step= .0833 hrs End= 23.9990 hrs
SIN: Bentley Systems, Inc.
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Type.... Tc Calcs
Name.... POST BYPASS
Page 3.01
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........................................................................
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TIME OF CONCENTRATION CALCULATOR
........................................................................
........................................................................
Segment #1: Tc: User Defined
Segment #1 Time: .0830 hrs
------------------------------------------------------------------------
Total Tc: .0830 hrs
Calculated Tc < Min.Tc:
Use Minimum Tc...
Use Tc = .0833 hrs
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Type.... Tc Calcs
Name.... POST BYPASS
Page 3.02
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------------------------------------------------------------------------
Tc Equations used...
------------------------------------------------------------------------
___= User Defined ______----- ________________
Tc = Value entered by user
Where: Tc = Time of concentration
SIN: Bentley Systems, Inc.
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Type.... Tc Calcs
Name.... POST TO WETLAND
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........................................................................
TIME OF CONCENTRATION CALCULATOR
........................................................................
........................................................................
Segment #1: Tc: User Defined
Segment #1 Time: .0830 hrs
------------------------------------------------------------------------
-------------------------
Total Tc: .0830 hrs
Calculated Tc < Min.Tc:
Use Minimum Tc...
Use Tc = .0833 hrs
S/N: Bentley Systems, Inc.
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Type.... Tc Calcs
Name.... POST TO WETLAND
Page 3.04
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------------------------------------------------------------------------
Tc Equations used...
------------------------------------------------------------------------
User Defined --------
T c = Value entered by user
Where: Tc = Time of concentration
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Type.... Tc Calcs
Name.... PREDEV-SITE
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........................................................................
TIME OF CONCENTRATION CALCULATOR
........................................................................
........................................................................
Segment #1: Tc: TR-55 Sheet
Mannings n .1000
Hydraulic Length 100.00 ft
2yr, 24hr P 3.6000 in
Slope .074000 ft/ft
Avg.Velocity .42 ft/sec
Segment #2: Tc: TR-55 Shallow
Segment #1 Time: .0660 hrs
Hydraulic Length 274.00 ft
Slope .037000 ft/ft
Unpaved
Avg.Velocity 3.10 ft/sec
Segment #2 Time: .0245 hrs
-----------------------------------------------------------------------
Segment #3: Tc: TR-55 Channel
Flow Area 16.0000
Wetted Perimeter 12.00
Hydraulic Radius 1.33
Slope .019000
Mannings n .0500
Hydraulic Length 532.00
sq.ft
ft
ft
ft/ft
ft
Avg.Velocity 4.98 ft/sec
Segment #3 Time: .0297 hrs
------------------------------------------------------------------------
Total Tc: .1202 hrs
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Type.... Tc Calcs
Name.... PREDEV-SITE
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------------------------------------------------------------------------
Tc Equations used...
------------------------------------------------------------------------
___= SCS TR-55 Sheet Flow ____
Tc = (.007 * ((n * Lf)**0.8)) / ((P**.5) * (Sf**.4))
Where: Tc = Time of concentration, hrs
n = Mannings n
Lf = Flow length, ft
P = 2yr, 24hr Rain depth, inches
Sf = Slope, a
___= SCS TR-55 Shallow Concentrated Flow =-----------------
Unpaved surface:
V = 16.1345 * (Sf**0.5)
Paved surface:
V = 20.3282 * (Sf**0.5)
Tc = (Lf / V) / (3600sec/hr)
Where: V = Velocity, ft/sec
Sf = Slope, ft/ft
Tc = Time of concentration, hrs
Lf = Flow length, ft
SIN: Bentley Systems, Inc.
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Type.... To Calcs
Name.... PREDEV-SITE
Page 3.07
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___= SCS Channel Flow
R = Aq / Wp
V = (1.49 * (R**(2/3)) * (Sf**-0.5)) / n
To = (Lf / V) / (3600sec/hr)
Where: R
Aq
Wp
V
Sf
n
To
Lf
Hydraulic radius
Flow area, sq.ft.
Wetted perimeter, ft
Velocity, ft/sec
Slope, ft/ft
Mannings n
Time of concentration, hrs
Flow length, ft
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Type.... Runoff CN-Area
Name.... POST BYPASS
Page 4.01
File.... K:\08\08-0510\080510-Olde Towne - The Residence Ctr\H-H\O1deTowneRec.ppw
RUNOFF CURVE NUMBER DATA
..........................................................................
..........................................................................
Impervious
Area Adjustment Adjusted
Soil/Surface Description
-
---------- - CN
--- acres
--------- °C %UC
----- ----- CN
------
-----------------
----
Impervious Areas - Paved parking to 98 .350 98.00
Woods - good 55 1.720 55.00
Open space (Lawns,parks etc.) - Goo 61 5.890 61.00
COMPOSITE AREA & WEIGHTED CN --->
....................................
....................................
....
.... 7.960
..........
..........
............
............ 61.33 (61)
.............
.............
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Type.... Runoff CN-Area
Name.... POST TO WETLAND
Page 4.02
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RUNOFF CURVE NUMBER DATA
..........................................................................
..........................................................................
Impervious
Area Adjustment Adjusted
Soil/Surface Description
-------------------------------- - CN
--- acres
--------- %C %UC CN
----- ----- ------
Impervious Areas - Paved parking to 98 3.390 98.00
Open space (Lawns,parks etc.) - Goo 61 3.130 61.00
Off-Site Impervious 98 1.930 98.00
Off-Site Open 61 1.440 61.00
COMPOSITE AREA & WEIGHTED CN --->
....................................
....................................
....
.... 9.890
..........
.......... 80.90 (81)
.........................
.........................
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Type.... Runoff CN-Area
Name.... PREDEV-SITE
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RUNOFF CURVE NUMBER DATA
..........................................................................
..........................................................................
Impervious
Area Adjustment Adjusted
Soil/Surface Description
-
---------- - CN
--- acres
--------- °C %UC CN
----- ----- ------
--------------------
-
Open space (Lawns,parks etc.) - Goo 61 4.757 61.00
Woods - good 55 8.632 55.00
Woods - grass combination - good 58 1.091 58.00
COMPOSITE AREA & WEIGHTED CN --->
....................................
....................................
....
.... 14.480
..........
.......... 57.20 (57)
.........................
.........................
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Type.... Vol: Elev-Area
Name.... POND 10
Page 5.01
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Elevation Planimeter Area Al+A2+sgr(Al*A2) Volume Volume Sum
(ft)
------------ (sq.in) (sq.ft)
--------------------- (sq.ft)
----------------- (ac-ft)
--------- (ac-ft)
------
194.00
----- 21529
0
.000 -------
.000
195.00 ----- 23451 67449 .516 .516
196.00 ----- 25429 73300 .561 1.077
197.00 ----- 27464 79320 .607 1.684
POND VOLUME EQUATIONS
* Incremental volume computed by the Conic Method for Reservoir Volumes.
Volume = (1/3) * (EL2-EL1) * (Areal + Areal + sq. rt. (Areal*Area2) )
where: ELI, EL2 = Lower and upper elevations of the increment
Areal,Area2 = Areas computed for ELI, EL2, respectively
Volume = Incremental volume between ELI and EL2
S/N: Bentley Systems, Inc.
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Type.... Outlet Input Data
Name.... Outlet 1
Page 6.01
File.... K:\08\08-0510\080510-Olde Towne - The Residence Ctr\H-H\O1deTowneRec.ppw
REQUESTED POND WS ELEVATIONS:
Min. Elev.= 194.00 ft
Increment = .10 ft
Max. Elev.= 197.00 ft
OUTLET CONNECTIVITY
---> Forward Flow Only (UpStream to DnStream)
<--- Reverse Flow Only (DnStream to UpStream)
<---> Forward and Reverse Both Allowed
Structure No. Outfall E1, ft E2, ft
Inlet Box RO ---> CO 196.000 197.000
Orifice-Circular 00 ---> CO 195.000 197.000
Culvert-Circular CO ---> TW 190.300 197.000
TW SETUP, DS Channel
SIN: Bentley Systems, Inc.
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Type.... Outlet Input Data
Name.... Outlet 1
Page 6.02
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OUTLET STRUCTURE INPUT DATA
Structure ID = RO
Structure Type
------------ = Inlet Box
------
# of Openings ------------
= 1 ------
Invert Elev. = 196.00 ft
Orifice Area = 25.0000 sq.ft
Orifice Coeff. _ .600
Weir Length = 20.00 ft
Weir Coeff. = 3.000
Transition Elev. = 197.00 ft
Transition Ht. _ .00 ft
Crossover Elev. = 196.00 ft (weir = orifice)
K, Reverse = 1.000
Mannings n = .0000
Kev,Charged Riser = .000
Weir Submergence = No
Structure ID = 00
Structure Type = Orifice-Circular
------------------------------------
# of Openings = 1
Invert Elev. = 195.00 ft
Diameter = .5000 ft
Orifice Coeff. _ .600
SIN: Bentley Systems, Inc.
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Type.... Outlet Input Data
Name.... Outlet 1
Page 6.03
File.... K:\08\08-0510\080510-Olde Towne - The Residence Ctr\H-H\O1deTowneRec.ppw
OUTLET STRUCTURE INPUT DATA
Structure ID = CO
Structure Type = Culvert-Circular
-------------
-----------------
No. Barrels ------
= 1
Barrel Diameter = 3.0000 ft
Upstream Invert = 190.30 ft
Dnstream Invert = 190.00 ft
Horiz. Length = 60.00 ft
Barrel Length = 60.00 ft
Barrel Slope = .00500 ft/ft
OUTLET CONTROL DATA...
Mannings n = .0130
Ke = .5000
Kb = .007228
Kr = .5000
HW Convergence = .001
(forward entrance loss)
(per ft of full flow)
(reverse entrance loss)
+/- ft
INLET CONTROL DATA...
Equation form = 1
Inlet Control K = .0098
Inlet Control M = 2.0000
Inlet Control c = .03980
Inlet Control Y = .6700
T1 ratio (HW/D) = 1.158
T2 ratio (HW/D) = 1.304
Slope Factor = -.500
Use unsubmerged inlet control Form 1 equ. below T1 elev.
Use submerged inlet control Form 1 equ. above T2 elev.
In transition zone between unsubmerged and submerged inlet control,
interpolate between flows at T1 & T2...
At Tl Elev = 193.77 ft ---> Flow = 42.85 cfs
At T2 Elev = 194.21 ft ---> Flow = 48.97 cfs
Structure ID = TW
Structure Type = TW SETUP, DS Channel
------------------------------------
FREE OUTFALL CONDITIONS SPECIFIED
CONVERGENCE TOLERANCES...
Maximum Iterations= 40
Min. TW tolerance = .01 ft
Max. TW tolerance = .01 ft
Min. HW tolerance = .01 ft
Max. HW tolerance = .01 ft
Min. Q tolerance = .00 cfs
Max. Q tolerance = .00 cfs
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7/8/2008
Type.... Composite Rating Curve
Name.... Outlet 1
Page 6.04
File.... K:\08\08-0510\080510-Olde Towne - The Residence Ctr\H-H\OldeTowneRec.ppw
***** COMPOSITE OUTFLOW SUMMARY ****
WS Elev, Total Q Notes
-------- -------- ----- --- Converge
-------------------------
Elev. Q TW E lev Error
ft
-------- cfs
------- f
----- t +/-ft
--- Contributing Structures
194.00
.00
Free -----
Outfall --------------------------
(no Q: RO,OO,CO)
194.10 .00 Free Outfall (no Q: RO,OO,CO)
194.20 .00 Free Outfall (no Q: RO,OO,CO)
194.30 .00 Free Outfall (no Q: RO,OO,CO)
194.40 .00 Free Outfall (no Q: RO,OO,CO)
194.50 .00 Free Outfall (no Q: RO,OO,CO)
194.60 .00 Free Outfall (no Q: RO,OO,CO)
194.70 .00 Free Outfall (no Q: RO,OO,CO)
194.80 .00 Free Outfall (no Q: RO,OO,CO)
194.90 .00 Free Outfall (no Q: RO,OO,CO)
195.00 .00 Free Outfall (no Q: RO,OO,CO)
195.10 .02 Free Outfall O0,C0 (no Q: RO)
195.20 .09 Free Outfall OO,CO (no Q: RO)
195.30 .19 Free Outfall OO,CO (no Q: RO)
195.40 .31 Free Outfall OO,CO (no Q: RO)
195.50 .47 Free Outfall O0,C0 (no Q: RO)
195.60 .56 Free Outfall O0,C0 (no Q: RO)
195.70 .63 Free Outfall OO,CO (no Q: RO)
195.80 .70 Free Outfall O0,C0 (no Q: RO)
195.90 .76 Free Outfall OO,CO (no Q: RO)
196.00 .82 Free Outfall O0,C0 (no Q: RO)
196.10 2.77 Free Outfall RO,OO,CO
196.20 6.29 Free Outfall RO,OO,CO
196.30 10.82 Free Outfall RO,OO,CO
196.40 16.18 Free Outfall RO,OO,CO
196.50 22.27 Free Outfall RO,OO,CO
196.60 28.99 Free Outfall RO,OO,CO
196.70 36.28 Free Outfall RO,OO,CO
196.80 44.12 Free Outfall RO,OO,CO
196.90 52.45 Free Outfall RO,OO,CO
197.00 61.20 Free Outfall RO,OO,CO
S/N:
Bentley PondPack (10.00.027.00)
2:41 PM
Bentley Systems, Inc.
7/8/2008
Appendix A
Index of Starting Page Numbers for ID Names
A-1
----- 0 -----
Outlet 1... 6.01, 6.04
----- P -----
POND 10... 5.01
POST BYPASS... 3.01, 4.01
POST TO WETLAND... 3.03, 4.02
PREDEV-SITE... 3.05, 4.03
----- R -----
RDU NOAA 14 Desi... 2.01, 2.02
----- W -----
Watershed... 1.01
SIN: Bentley Systems, Inc.
Bentley PondPack (10.00.027.00) 2:41 PM 7/8/2008
BMP Sizing
Project Name: Olde Towne Rec. Cent.
City/State: Raleigh, NC
Project #: 2080510
Date: 30-Jul-08
Table 1.1 Surface Area to Drainage Area Ratio for Permanent Pool Sizing for 85% Pollutant
Removal Efficiencv in the Piedmont
% Impervious
Cover Permanent Pool Depth (feet)
3.0 4.0 5.0 6.0 7.0 8.0 9.0
10 0.59 0.49 0.43 0.35 0.31 0.29 0.26
20 0.97 0.79 0.70 0.59 0.51 0.46 0.44
30 1.34 1.08 0.97 0.83 0.70 0.64 0.62
40 1.73 1.43 1.25 1.05 0.90 0.82 0.77
50 2.06 1.73 1.50 1.30 1.09 1.00 0.92
60 2.40 2.03 1.71 1.51 1.29 1.18 1.10
70 2.88 2.40 2.07 1.79 1.54 1.35 1.26
80 3.36 2.78 2.38 2.10 1.86 1.60 1.42
90 3.74 3.10 2.66 2.34 2.11 1.83 1.67
Source: NCDENR/DWQ Stormwater Best Management Practices, Table 10-2, July 2007
STORMWATER BMP 1 - STORMWATER WETLAND (40% TN Removal)
Land Use Area
(ac) % IA Imp. Area
(ac)
Impervious 5.32 98 5.2
Open Space 4.57 0 0.0
Totals 9.89 5.2
Total % Impervious Surface Area = 53.8%
Surface Area of Permanent Pool:
Assumed depth = 3 feet
SA/DA ratio = 2.19 % From Table 1.1
Minimum pond surface area (SA) = (DA * SA/DA ratio)/100
SA = 0.216 acres
9430 sq. ft.
Normal pool elevation = 194.0 feet
Surface area provided = 0.494 acres
21529 sq. ft.
1-Inch Runoff Volume Calculation
Using the runoff volume calculations in the "Simple Method" as described by Schueler (1987)
Rv = 0.05 + 0.9(I) where, Rv = Runoff coefficient, in./in.
= Percent impervious
Rv = 0.53 in./in.
Total runoff volume from 1-inch precipitation:
Runoff volume, S = (Design rainfall) (Rv) (Drainage Area)
S = 0.440 acre-ft
19175 cu. ft.
Water quality pool elevation= 195.00 feet
Storage volume provided = 0.510 acre-ft
22216 cu. ft.
Project Name: Olde Towne Rec. Cent.
i /State: Raleigh, NC
STORMWATER BMP - WETLAND (40% TN Removal)
Orifice Calculator
Q =Cd*A*(2gh)^(1/2)
Variables
WQ Volume:
Head:
Draw down time:
0.51 Acre-ft
1.00 ft
83 hrs
22215.6 cf
0.5 ft
298800 s
Orifice Area =
0.021837 sq. ft
Orifice Diameter = 2.000947 in
3.144566 sq. in
Project #: 2080510
Date: 9-j u 1-08
Constants
g = 32.2 ft/sz
Cd = 0.6
USE 2 INCH DIAMETER ORIFICE
Nitrogen Calculations
Nitrogen Control Plan
Commercial / Industrial / Residential Sites with Known Impervious Area
Project Title: Olde Towne Rec. Site
PartL Riparian Buffers
Area includes riparian buffers? [ ] No [ X ] Yes [ ] Exempt
If yes, [ X ] 50 foot [ ] 100 foot
River Basin? [ X ] Neuse [ j Cape Fear
Basis for Exemption
Show buffers on site plan.
Part II. Nitrogen Calculations (Method 2, Appendix C):
a. Site Information
Total Area of Property incl. R/W 14.48
Denuded Area 14.48
Impervious Area incl. R/W 3.91
Managed open space 8.85
Protected open space 1.72
b. Pre-development loading:
Type of Land Cover
Permanently protected undisturbed open
space (forest, unmown meadow)
Permanently protected managed open space
(grass, landscaping, etc.)
Impervious Area
TOTAL
Nitrogen Loading Rate (Ibs/ac/yr) _
c. Post-development loading;
acres
acres
acres
acres
acres
TN Export
Area TN export coeff from use
(acresl (Ibs/ac/vrl (Ibs/vrl
14.48 0.6 8.69
0 1.2 0
0 21.2 0
14.48 8.69
0.60
TN Export
Area TN export coeff from use
lacresl Hhs/ac/vrl (Ihs/vrl
Type of Land Cover
Permanently protected undisturbed open
space (forest, unmown meadow)
Permanently protected managed open space
(grass, landscaping, etc.)
Impervious Area
TOTAL
Nitrogen Loading Rate (Ibs/ac/yr) _
1.72 0.6 1.03
8.85 1.2 10.62
3.91 21,2 82.89
14.48 94.54
6.53
Proposed BMP(s) Wetland
Nitrogen Load after BMPs = 4.21 Ibs/ac/yr
Nitrogen Load offset by Payments = 0.61 Ibs/ac/yr Estimated Buy Down = $7,456
Net change in on-site N Load = 2.93 Ibs/ac/yr
•
•
Nitrogen Control Plan
Commercial / Industrial / Residential Sites with Known Impervious Area
Towne
Project Name: Olde Towne Rec Site
Project #: 2080510
TOTAL TRACT
Total Area 60,7. T3. sqft. = 14.48 ac.
Impervious sgft. = 3.91 ac.
Natural Open Space Area. ` 2855 966- sgft. = 8.85 ac.
Managed Open Space Area: ? ' '74,43> sqft. = 1.72 ac.
Estimated Pre-Development TN Loading
Date: 7/10/2008
Computed by: DBH
Type of Land Cover Acres TN Export Coeff. TN EXPORT (Ibs/yr)
(Ibs/ac/yr)
Permanently Protected Undisturbed Area 14.48 0.6 8.69
Permanently Protected Managed Area 0.00 ' 1.2 0.00
Impervious Surface .0.00 21.2 0.00
Totals 14.48 0.6 8.69
Estimated Post-Development TN Loading
Drainage Area I:
Type of Land Cover Acres TN Export Coeff. TN EXPORT (Ibs/yr)
(Ibs/ac/yr)
Permanently Protected Undisturbed Area 0.00 0.6 0.00
Permanently Protected Managed Area 3.13 1.2 3.76
Impervious Area 3,39 21.2 71.87
Totals 6.52 75.62
Estimated UMN Keducllon Tor VVetland: 4u
Estimated Drainage Area I Nitrogen Loading Reduction: 30.25 Ibs/yr
Drainage Area II:
Type of Land Cover Acres TN Export Coeff.
(Ibs/ac/yr) TN EXPORT (Ibs/yr)
Permanently Protected Undisturbed Area 1.72 0.6 1.03
Permanently Protected Managed Area 5.89 1.2 7.07
Impervious 0:35 21.2 7.42
Totals 7.96 15.52
Estimated rsMr reduction: u io
Estimated Drainage Area II Nitrogen Loading Reduction: 0.00 Ibs/yr
Total Area: 14.48 Acres
Estimated Project Pre-BMP TN Export: 91.14 Ibs/yr
Estimated Project BMP TN Removal: 30.25 Ibs/yr
Estimated Project TN Load Rate After BMPs: 4.21 Ibslac/yr
Estimated Project Composite Nitrogen Loading Rate Reduction: 33.19 %
Estimated Project Buy Down: $7,455.82