The Allan Lindsey Center - Stormwater Management Design Report 052518

May 25, 2018

STORMWATER MANAGEMENT
DESIGN REPORT
FOR

THE ALLAN LINDSEY CENTER
17,483-SF RETAIL DEVELOPMENT
WAXHAW PARKWAY
WAXHAW, NORTH CAROLINA

BY
OAK ENGINEERING
828 EAST BOULEVARD
CHARLOTTE, NC 28203
NC FIRM LICENSE: P-1742

NLPSEAL
REIN EENG
33996
O
N
S
RA
IS
OTORE
H
RFO
C
L
IAAN05/25/2018

May 25, 2018

TABLE OF CONTENTS

I. NARRATIVE

INTRODUCTION
STORMWATER MANAGEMENT REQUIREMENTS
EXISTING SITE CONDITIONS
PROPOSED SITE CONDITIONS
PROPOSED BMP DESIGN
CONCLUSIONS

II. MAPS AND FIGURES

HYDROCAD MODEL ROUTING DIAGRAM
DRAINAGE AREA MAPS
TOWN OF WAXHAW RAINFALL DATA
SOILS MAP
FEMA FIRM MAP

III. HYDROCAD MODEL OUTPUT

HYDROCAD: 1-INCH STORM EVENT (NCDEQ SWM)
HYDROCAD: 2YR-24HR STORM EVENT (TOWN OF WAXHAW)
HYDROCAD: 10YR-24HR STORM EVENT (TOWN OF WAXHAW)

IV. HYDROFLOW STORM SEWERS MODEL OUTPUT

HYDRAFLOW STORM SEWERS:10YR-24HR STORM EVENT

May 25, 2018
INTRODUCTION
Moody Group Commercial Real Estate Services, LLC is proposing the construction of a new commercial/retail facility in the
Town of Waxhaw. The proposed improvements will include a 18,483-sf building with and approximately 73 vehicular
parking spaces to serve the facility. A driveway connection will be made to Waxhaw Parkway. State stormwater management
is required for all proposed and future BUA. The Town of Waxhaw requires peak control of the 2yr and 10yr-24hr storm
events.

STORMWATER MANAGEMENT REQUIREMENTS
Town of Waxhaw Stormwater Management Requirements
1. Must provide peak control for the 2-yr and 10-year, 24-hour storm events
NCDEQ Stormwater Management Requirements
1. Must provide a minimum of 85% average annual removal for total suspended solids (TSS) for all impervious areas.

EXISTING SITE CONDITIONS
The total site acreage is 3.01acres and is predominantly wooded with rolling terrain (2% to 50% - average slope of 13%).
The site is bounded to the north and west by Waxhaw Parkway, to the south by rural farmland, and to the east by an existing
retail facility. The entire site drains from a high point at the northeast property corner east, ultimately leaving the site in a box
culvert under Waxhaw Parkway (tributary to Twelvemile Creek) at the northwestern property corner.

Figure 1: Existing Drainage Patterns (Source: Town of Waxhaw GIS)

May 25, 2018

Figure 2: Pre Development Drainage Area (HydroCAD Model)

Soils on the site are entirely Tarrus soils (HSG-B).

Figure 3: Hydrologic Soils Group map of the site (WebSoil Survey)

See the Maps & Figures section of this report for Drainage Area and Soils Maps.

May 25, 2018

PROPOSED SITE CONDITIONS
As previously mentioned, the proposed improvements will include a new 18,483-sf commercial/retail facility and
approximately 73 vehicular parking spaces. The proposed improvements will create approximately 1.25 acres of built-upon
area (BUA). All new BUA will drain to the proposed BMP via the proposed storm drainage system. An additional 0.72
acres of open space (grass cover) will also be detained in the BMP. The total drainage area to the proposed BMP is 1.97
acres at 63.5% impervious. Approximately 1.08 acres of the site will not reach the BMP (undetained). (For the purpose of
the pre/post comparison, the discharge from the BMP outfall will be combined with the discharge from the undetained
drainage area and compared to the discharge of the predevelopment drainage area).

Figure 4: Post Development Drainage Area to Sand Filter (HydroCAD Model)

Figure 6: Post Development Drainage Undetained HydroCAD Model)

PROPOSED BMP DESIGN
The Sand Filter is designed to treat 3.05 acres at 63.5% impervious for water quality and water quantity control as outlined
in this report. The latest version (2017) of the NCDEQ BMP Design Manual was used to design the sand filter.

May 25, 2018

PRE/POST SUMMARY TABLE Storm Event Predeveloped Drainage Area (cfs) HydroCAD Node 1S Post Developed Area to BMP (cfs) HydroCAD Node POST Sand Filter HydroCAD Node SF WSE / Discharge Post Undetained (cfs) HydroCAD Node 11S Combined (Total) Discharge (cfs) HydroCAD Node 12P 1.0” 0.00 0.49 576.98 / 0.08 0.00 0.08
2yr-24hr 1.00 7.09 579.54 / 0.20 0.69 0.87
10yr-24hr 4.48 11.99 580.96 / 0.29 2.18 2.41
25yr-24hr 7.26 15.06 581.32 / 1.28 3.29 3.56
50yr-24hr 9.73 17.53 581.45 / 4.88 4.25 7.84
100yr-24hr 12.49 20.10 581.61 / 11.51 5.31 16.48

CONCLUSIONS
The Town of Waxhaw and NCDEQ Stormwater requirements are met in the proposed design.

May 25, 2018

MAPS & FIGURES
HYDROCAD MODEL ROUTING DIAGRAM
DRAINAGE AREA MAPS
TOWN OF WAXHAW RAINFALL DATA
SOILS MAP
FEMA FIRM MAP

11S
POST (UNDETAINED)
POST
POST DEVELOPED
AREA TO SF
PRE
PREDEVELOPED SITE
8P
SAND FILTER
12P
TOTAL POST
Routing Diagram for LINDSEY CENTER
Prepared by OAK Engineering
HydroCAD® 10.00-20 s/n 10066 © 2017 HydroCAD Software Solutions LLC
Subcat Reach Pond Link
GVTPEDTPEDR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WWAXHAW PARKWAYSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS
SS
SS
SS
SS
SS SS SS SS SS SS SSSDSD SDSDSDSDSD
SD
SD
S
D
SD
SD
SD
SD
SD
SDSDSDSDSDSDSDSDSD
SDSDSDSDSDSD
SDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSD 8'X14'OVAL PIPESDSDSDR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WSUITE EOFFICE3,107 S.F.O O O
OOSSSSSSSSSSSSSSW W
MMW W W W W WLOADING ZONE571
5755805805855905955985
9
8598597596 597597
598
59
8
599600601602603604605606607608609610611610605600605
603
602
601
600
599599 5995985975965955945935925915905895885875
8
65855845
8
35825825
8
2
5
8
2
5
8
0
580
582582580
575
57
7
58
0
585
590
595
596 595
590
585
580
577
599600601602603604605606607608601602603604605606607608609599595590598595590R/WPOST TOSAND FILTER1.97 ACRES (63.5% IMPERVIOUS)CN: 84.5POST (UNDETAINED)1.08 ACRESCN: 59.6POST (UNDETAINED)1.08 ACRESCN: 59.6PREDEVELOPMENTDRAINAGE AREA3.05 ACRESCN: 84.5OUTFALL: CULVERT UNDER WAXHAW PKWY
GVTPEDTPEDR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WWAXHAW PARKWAYSDSDSDSDSD SD SD
SD
SD
SD
SDSD
SDSDSDSDSDSDSDSD
SDSDSDSDSDSDSD
SDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WR/WSUITE EOFFICE3,107 S.F.O O O
OOLOADING ZONE571575580580
5855905955985
9
8598597596 597597
598
598
599600601602603604605606607608609610611610605600605
603
602
601
600
599599 5995985975965955945935925915905895885875
8
65855845
8
3582582582582580580582582580575577580585590595
596 595
590
585
580
577
599600601602603604605606607608601602603604605606607608609599595590598595590TO DI70.13 ACRESTO CB60.16 ACRESTO CB50.10 ACRESTO CB40.34 ACRESTO CB30.33 ACRESTO CB20.29 ACRESTO CB10.13 ACRESTO CB80.12 ACRES
3.5 Design Frequency
_____________________________________________________________________

Design Description Design Storm
Frequencies
3.5.1 Storm system pipes 10 yr.*
Ditch systems 10 yr.*
Culverts (subdivision streets) 25 yr.
Culverts (thoroughfare roads) 50 yr.

* Except as required by section 18.10.5, Storm Drainage Not Contained in Street Rights of Way,
of the UDO.
_____________________________________________________________________

Rainfall In addition to the following rainfall intensities (Table 3-3), the most recent
Intensity Atlas 14 or other acceptable sources may be used for hydrologic analysis
3.5.2

Hydrologic Soil Group—Union County, North Carolina
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
5/16/2018
Page 1 of 4386556038655803865600386562038656403865660386568038657003865720386556038655803865600386562038656403865660386568038657003865720522490522510522530522550522570522590522610522630522650522670522690522710522730
522490 522510 522530 522550 522570 522590 522610 522630 522650 522670 522690 522710 522730
34° 56' 1'' N 80° 45' 14'' W34° 56' 1'' N80° 45' 3'' W34° 55' 56'' N
80° 45' 14'' W34° 55' 56'' N
80° 45' 3'' WN
Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 17N WGS84
0 50 100 200 300
Feet
0 15 30 60 90
Meters
Map Scale: 1:1,200 if printed on A landscape (11" x 8.5") sheet.
Soil Map may not be valid at this scale.
MAP LEGEND MAP INFORMATION
Area of Interest (AOI)
Area of Interest (AOI)
Soils
Soil Rating Polygons
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Soil Rating Lines
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Soil Rating Points
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Water Features
Streams and Canals
Transportation
Rails
Interstate Highways
US Routes
Major Roads
Local Roads
Background
Aerial Photography
The soil surveys that comprise your AOI were mapped at
1:24,000.
Warning: Soil Map may not be valid at this scale.
Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil
line placement. The maps do not show the small areas of
contrasting soils that could have been shown at a more detailed
scale.
Please rely on the bar scale on each map sheet for map
measurements.
Source of Map: Natural Resources Conservation Service
Web Soil Survey URL:
Coordinate System: Web Mercator (EPSG:3857)
Maps from the Web Soil Survey are based on the Web Mercator
projection, which preserves direction and shape but distorts
distance and area. A projection that preserves area, such as the
Albers equal-area conic projection, should be used if more
accurate calculations of distance or area are required.
This product is generated from the USDA-NRCS certified data as
of the version date(s) listed below.
Soil Survey Area: Union County, North Carolina
Survey Area Data: Version 17, Oct 3, 2017
Soil map units are labeled (as space allows) for map scales
1:50,000 or larger.
Date(s) aerial images were photographed: Jun 14, 2015—Feb 8,
2017
The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
imagery displayed on these maps. As a result, some minor
shifting of map unit boundaries may be evident.
Hydrologic Soil Group—Union County, North Carolina
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
5/16/2018
Page 2 of 4
Hydrologic Soil Group
Map unit symbol Map unit name Rating Acres in AOI Percent of AOI
TbB2 Tarrus gravelly silty clay
loam, 2 to 8 percent
slopes, moderately
eroded
B 0.6 15.6%
TbC2 Tarrus gravelly silty clay
loam, 8 to 15 percent
slopes, moderately
eroded
B 3.4 84.4%
Totals for Area of Interest 4.0 100.0%
Description
Hydrologic soil groups are based on estimates of runoff potential. Soils are
assigned to one of four groups according to the rate of water infiltration when the
soils are not protected by vegetation, are thoroughly wet, and receive
precipitation from long-duration storms.
The soils in the United States are assigned to four groups (A, B, C, and D) and
three dual classes (A/D, B/D, and C/D). The groups are defined as follows:
Group A. Soils having a high infiltration rate (low runoff potential) when
thoroughly wet. These consist mainly of deep, well drained to excessively
drained sands or gravelly sands. These soils have a high rate of water
transmission.
Group B. 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 coarse texture.
These soils have a moderate rate of water transmission.
Group C. Soils having a slow infiltration rate when thoroughly wet. These consist
chiefly of soils having a layer that impedes the downward movement of water or
soils of moderately fine texture or fine texture. These soils have a slow rate of
water transmission.
Group D. Soils having a very slow infiltration rate (high runoff potential) when
thoroughly wet. These consist chiefly of clays that have a high shrink-swell
potential, soils that have a high water table, soils that have a claypan or clay
layer at or near the surface, and soils that are shallow over nearly impervious
material. These soils have a very slow rate of water transmission.
If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is
for drained areas and the second is for undrained areas. Only the soils that in
their natural condition are in group D are assigned to dual classes.
Hydrologic Soil Group—Union County, North Carolina
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
5/16/2018
Page 3 of 4
FEMA May 2 5, 20 18¯
No rth Carol ina F loo dpl ai n Mapp ing Prog ram

May 25, 2018

SAND FILTER DESIGN

SOUTH SAND FILTER NCDENR 2017 BMP MANUAL
COMPUTE WATER QUALITY VOLUME
Drainage Area =1.97 ac
% Impervious =63.50 %
Curve Number =84.50
USE SIMPLE METHOD Rv=0.05+0.009(I)
0.6215 IN/IN
P=1.0 IN
S =1.834
PModified =1.79
WQV =4,445 ft3
WQV Adj = 3,334 ft3
DETERMINE FILTER BED & SEDIMENTATION BASIN SURFACE AREA
h MaxFilter =
Assume h MaxFilter
h MaxFilter =5.00 ft Min. (As + AF)=2,487 ft2
Min. Area for Sedimentation Chamber Min. Area for Sand Filter
AS = 0.66(WQVadj)
AS =2,200 ft2 Af =287 ft2 Elev
Bottom Area Used =724 ft2 (577)Bottom Area Used =315 ft2 576.75
Top Area Used =6,913 ft2 (582)Top Area Used =2,884 ft2 580
13,854 ft3 5,911 ft3
VERIFY VOLUMES
Voltotal =( As + Af ) x hMaxFilter
Voltotal =19,765 ft3 COMPARE TO WQVadj OK
SIZE UNDERDRAIN SYSTEM
Determine capacity of Under Drain System
Af = WQv x df / [ ( k x Hf +Df) x Tf ]
Where:
Af =251 ft2 surface area of filter bed
df =1.5 ft filter bed depth
k =4 ft/day coefficient of permeability media
hf =2.50 ft average height of water above filter bed, (Hmax / 2)
Tf =1.66 Days (40 Hours, 1.66 Days))
5.0 Ft Max. Depth of Storage Volume
Af Provided=315 ft2 surface area provided
Q at Full Depth =0.063 cfs
Q at Avg Depth =0.039 cfs
Time to Drain=31.66 hrs
Safety Factor of 10 0.390 cfs
3.5 ft/day x 1.66 days x (Avg Head + 1.5 ft)Af =WQV x 1.5 ft
WQV Adj (ft3)
As (ft2) + Af (ft2)
Compute Perforation Capacity
Pipes Length 4 Holes/Row
# of Perforations =1 28 4 =224 3/8" Dia. Holes spaced 6" O.C.
50% of Perforations =112 Holes
Capacity of One Hole =CA(2gh)^0.5
0.007 cfs/Hole
Total Capacity =0.828 cfs
Capacity of Filter Media =0.390 cfs
Perforation Capacity =>
Media
Capacity OK
BOUYANCY CALCULATIONS
Top of Structure =581.25
Bottom of Structure =574.20
Width of Box =5.00 ft (O.D.)
Length of Box =5.00 ft (O.D.)
BF=VOLstruct x densityH2O (62.4 lb/cf)
lbs
Weight of Structure (see calcs, next page)
lbs
Weight of Concrete = 145-62.4(lb/cf) = 87.6 lb/cf
SF=1.25
3,525 lbs
42.7 cf
L=5 ft Thickness Reqd=1.707 ft
W=5 ft 20.5 in
OS-1
10,998
10,223
Weight of anchor required:
Volume of anchor required:
Anchor Dimensions:
Page 2 of 2
Type II 24-hr 1" Storm Rainfall=1.00"LINDSEY CENTER
Prepared by OAK Engineering
Page 4HydroCAD® 10.00-20 s/n 10066 © 2017 HydroCAD Software Solutions LLC
Time span=0.00-120.00 hrs, dt=0.05 hrs, 2401 points
Runoff by SCS TR-20 method, UH=SCS, Weighted-CN
Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method
Runoff Area=1.080 ac 0.00% Impervious Runoff Depth=0.00"Subcatchment 11S: POST (UNDETAINED)
Flow Length=588' Tc=9.6 min CN=59.6 Runoff=0.00 cfs 0.000 af
Runoff Area=1.970 ac 63.45% Impervious Runoff Depth=0.16"Subcatchment POST: POST DEVELOPED AREA TO SF
Tc=5.0 min CN=84.5 Runoff=0.49 cfs 0.027 af
Runoff Area=3.050 ac 0.00% Impervious Runoff Depth=0.00"Subcatchment PRE: PREDEVELOPED SITE
Flow Length=588' Tc=9.6 min CN=55.0 Runoff=0.00 cfs 0.000 af
Peak Elev=576.98' Storage=273 cf Inflow=0.49 cfs 0.027 afPond 8P: SAND FILTER
Outflow=0.08 cfs 0.027 af
Inflow=0.08 cfs 0.027 afPond 12P: TOTAL POST
Primary=0.08 cfs 0.027 af
Total Runoff Area = 6.100 ac Runoff Volume = 0.027 af Average Runoff Depth = 0.05"
79.51% Pervious = 4.850 ac 20.49% Impervious = 1.250 ac
Type II 24-hr 2yr-24hr Rainfall=3.55"LINDSEY CENTER
Prepared by OAK Engineering
Page 4HydroCAD® 10.00-20 s/n 10066 © 2017 HydroCAD Software Solutions LLC
Time span=0.00-120.00 hrs, dt=0.05 hrs, 2401 points
Runoff by SCS TR-20 method, UH=SCS, Weighted-CN
Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method
Runoff Area=1.080 ac 0.00% Impervious Runoff Depth=0.54"Subcatchment 11S: POST (UNDETAINED)
Flow Length=588' Tc=9.6 min CN=59.6 Runoff=0.69 cfs 0.048 af
Runoff Area=1.970 ac 63.45% Impervious Runoff Depth=2.02"Subcatchment POST: POST DEVELOPED AREA TO SF
Tc=5.0 min CN=84.5 Runoff=7.09 cfs 0.332 af
Runoff Area=3.050 ac 0.00% Impervious Runoff Depth=0.36"Subcatchment PRE: PREDEVELOPED SITE
Flow Length=588' Tc=9.6 min CN=55.0 Runoff=1.00 cfs 0.092 af
Peak Elev=579.54' Storage=8,398 cf Inflow=7.09 cfs 0.332 afPond 8P: SAND FILTER
Outflow=0.20 cfs 0.332 af
Inflow=0.87 cfs 0.380 afPond 12P: TOTAL POST
Primary=0.87 cfs 0.380 af
Total Runoff Area = 6.100 ac Runoff Volume = 0.472 af Average Runoff Depth = 0.93"
79.51% Pervious = 4.850 ac 20.49% Impervious = 1.250 ac
Type II 24-hr 10yr-24hr Rainfall=5.19"LINDSEY CENTER
Prepared by OAK Engineering
Page 4HydroCAD® 10.00-20 s/n 10066 © 2017 HydroCAD Software Solutions LLC
Time span=0.00-120.00 hrs, dt=0.05 hrs, 2401 points
Runoff by SCS TR-20 method, UH=SCS, Weighted-CN
Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method
Runoff Area=1.080 ac 0.00% Impervious Runoff Depth=1.39"Subcatchment 11S: POST (UNDETAINED)
Flow Length=588' Tc=9.6 min CN=59.6 Runoff=2.18 cfs 0.125 af
Runoff Area=1.970 ac 63.45% Impervious Runoff Depth=3.49"Subcatchment POST: POST DEVELOPED AREA TO SF
Tc=5.0 min CN=84.5 Runoff=11.99 cfs 0.574 af
Runoff Area=3.050 ac 0.00% Impervious Runoff Depth=1.08"Subcatchment PRE: PREDEVELOPED SITE
Flow Length=588' Tc=9.6 min CN=55.0 Runoff=4.48 cfs 0.274 af
Peak Elev=580.96' Storage=15,585 cf Inflow=11.99 cfs 0.574 afPond 8P: SAND FILTER
Outflow=0.29 cfs 0.574 af
Inflow=2.41 cfs 0.698 afPond 12P: TOTAL POST
Primary=2.41 cfs 0.698 af
Total Runoff Area = 6.100 ac Runoff Volume = 0.972 af Average Runoff Depth = 1.91"
79.51% Pervious = 4.850 ac 20.49% Impervious = 1.250 ac
Type II 24-hr 25yr-24hr Rainfall=6.21"LINDSEY CENTER
Prepared by OAK Engineering
Page 4HydroCAD® 10.00-20 s/n 10066 © 2017 HydroCAD Software Solutions LLC
Time span=0.00-120.00 hrs, dt=0.05 hrs, 2401 points
Runoff by SCS TR-20 method, UH=SCS, Weighted-CN
Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method
Runoff Area=1.080 ac 0.00% Impervious Runoff Depth=2.03"Subcatchment 11S: POST (UNDETAINED)
Flow Length=588' Tc=9.6 min CN=59.6 Runoff=3.29 cfs 0.182 af
Runoff Area=1.970 ac 63.45% Impervious Runoff Depth=4.45"Subcatchment POST: POST DEVELOPED AREA TO SF
Tc=5.0 min CN=84.5 Runoff=15.06 cfs 0.730 af
Runoff Area=3.050 ac 0.00% Impervious Runoff Depth=1.64"Subcatchment PRE: PREDEVELOPED SITE
Flow Length=588' Tc=9.6 min CN=55.0 Runoff=7.26 cfs 0.417 af
Peak Elev=581.32' Storage=17,746 cf Inflow=15.06 cfs 0.730 afPond 8P: SAND FILTER
Outflow=1.28 cfs 0.730 af
Inflow=3.56 cfs 0.912 afPond 12P: TOTAL POST
Primary=3.56 cfs 0.912 af
Total Runoff Area = 6.100 ac Runoff Volume = 1.329 af Average Runoff Depth = 2.61"
79.51% Pervious = 4.850 ac 20.49% Impervious = 1.250 ac
Type II 24-hr 50yr-24hr Rainfall=7.03"LINDSEY CENTER
Prepared by OAK Engineering
Page 4HydroCAD® 10.00-20 s/n 10066 © 2017 HydroCAD Software Solutions LLC
Time span=0.00-120.00 hrs, dt=0.05 hrs, 2401 points
Runoff by SCS TR-20 method, UH=SCS, Weighted-CN
Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method
Runoff Area=1.080 ac 0.00% Impervious Runoff Depth=2.59"Subcatchment 11S: POST (UNDETAINED)
Flow Length=588' Tc=9.6 min CN=59.6 Runoff=4.25 cfs 0.233 af
Runoff Area=1.970 ac 63.45% Impervious Runoff Depth=5.22"Subcatchment POST: POST DEVELOPED AREA TO SF
Tc=5.0 min CN=84.5 Runoff=17.53 cfs 0.858 af
Runoff Area=3.050 ac 0.00% Impervious Runoff Depth=2.14"Subcatchment PRE: PREDEVELOPED SITE
Flow Length=588' Tc=9.6 min CN=55.0 Runoff=9.73 cfs 0.545 af
Peak Elev=581.45' Storage=18,535 cf Inflow=17.53 cfs 0.858 afPond 8P: SAND FILTER
Outflow=4.88 cfs 0.858 af
Inflow=7.84 cfs 1.090 afPond 12P: TOTAL POST
Primary=7.84 cfs 1.090 af
Total Runoff Area = 6.100 ac Runoff Volume = 1.635 af Average Runoff Depth = 3.22"
79.51% Pervious = 4.850 ac 20.49% Impervious = 1.250 ac
Type II 24-hr 100yr-24hr Rainfall=7.89"LINDSEY CENTER
Prepared by OAK Engineering
Page 4HydroCAD® 10.00-20 s/n 10066 © 2017 HydroCAD Software Solutions LLC
Time span=0.00-120.00 hrs, dt=0.05 hrs, 2401 points
Runoff by SCS TR-20 method, UH=SCS, Weighted-CN
Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method
Runoff Area=1.080 ac 0.00% Impervious Runoff Depth=3.21"Subcatchment 11S: POST (UNDETAINED)
Flow Length=588' Tc=9.6 min CN=59.6 Runoff=5.31 cfs 0.289 af
Runoff Area=1.970 ac 63.45% Impervious Runoff Depth=6.05"Subcatchment POST: POST DEVELOPED AREA TO SF
Tc=5.0 min CN=84.5 Runoff=20.10 cfs 0.993 af
Runoff Area=3.050 ac 0.00% Impervious Runoff Depth=2.71"Subcatchment PRE: PREDEVELOPED SITE
Flow Length=588' Tc=9.6 min CN=55.0 Runoff=12.49 cfs 0.689 af
Peak Elev=581.61' Storage=19,564 cf Inflow=20.10 cfs 0.993 afPond 8P: SAND FILTER
Outflow=11.51 cfs 0.993 af
Inflow=16.48 cfs 1.282 afPond 12P: TOTAL POST
Primary=16.48 cfs 1.282 af
Total Runoff Area = 6.100 ac Runoff Volume = 1.970 af Average Runoff Depth = 3.88"
79.51% Pervious = 4.850 ac 20.49% Impervious = 1.250 ac
Luke Bugenske, PE Date:May 16, 2018
GTW Date:May 16, 2018
Step 1.
1.6
24
0.0
Min TW (Fig. 8.06a)
8.7
5.2
Step 2.
Step 3.
Minimum TW Maximum TW
0.5 0.1
11 10
6 6
W = Do + La W = Do + .4La
13 6
Step 4.
Minimum TW Maximum TW
Max Stone Diameter, dmax (Inches): 9 2
Apron Thickness (Inches): 14 10
Step 5.
Rip Rap Apron Design
Determine the maximum Stone Diameter. Dmax = 1.5 X d50
Riprap d50, (ft.)
Minimum apron length, La (ft.) **
Apron width at pipe outlet (ft.)
Apron shape
Apron width at outlet end (ft.)
Project Name:
Drainage Specialist:
Checked By:
Based on the tailwater conditions determined in Step 1, enter Figure 8.06a or Figure 8.06b and
determine the d50 rip rap size and minimum apron length (La). The d50 size is the median
stone size in a well-graded rip rap apron.
Determine the apron width at the pipe outlet, the apron shape, and the apron width at the outlet
end from the same figure used in Step 2.
Tailwater depth (Feet)
Tailwater Method To Be Used
Discharge (cfs)
Velocity (ft./s)
** - Minimum Apron Length Is 10 Feet per CLDS 20.23
Determine the Apron Thickness, Ta = 1.5 X dmax
Fit the rip rap apron to the site by making it level for the minimum length La . Extend the
apron farther downstream and along the channel banks until stability is assured. Keep apron
as straight as possible and align it with the flow of the receiving stream. Make any necessary
alignment bends near the pipe outlet so that the entrance into the receiving stream is straight.
Some locations may require lining of the entire channel cross section to assure stability. It
may be necessary to increase the size of the rip rap where protection of the channel side slopes
is necessary. Where overfalls exist at pipe outlets or flows are excessive, a plunge pool should
be considered.
REEP-LRC Premier Development
Total Drainage Area (Acres):
Determine the tailwater depth from the channel characteristics below the pipe outlet for the
design capacity of the pipe. If the tailwater depth is less than half the outlet pipe diameter, it is
classifies as minimum tailwater conditions. If the tailwater is greater than half the pipe
diameter, it is classified as maximum tailwater conditions. Pipes that outlet onto wide flat
areas with no defined channel are assumed to have a minimum tailwater condition unless
reliable flood stage elevations show otherwise.
Rational Method for Flow
Outlet pipe diameter, Do (in.)
** - Minimum Apron Thickness Is 10 Inches per CLDS 20.23
Return to Main Worksheet
Figure 8.06a: Design of outlet protection from a round pipe flowing full, minimum
tailwater condition (Tw<0.5 diameter)
42 inch 22 ft 26 ft 11 ft 19 inch SEE PLAN