Cherry_Point_Crossing_Stormwater_Pipe_Sizing_Narrative.pdf

Stormwater Pipe Sizing Narrative

Beaufort County Pritcher Point Stormwater Improvements

Prepared for Beaufort County Beaufort County, SC

February 27, 2026

P.O. Box 381

Bluffton, SC 29910

843.837.5250 (T)

843.837.2558 (F)

WE Project: 170601

Table of Contents

Stormwater Narrative

Appendix A - Site Exhibits

Appendix B – Curve Number Calculations

Appendix C – Pipe Sizing Calculations

Project: Beaufort County Pritcher Point Stormwater Improvements Beaufort County, SC

Description The 91.5‑acre parcel owned and maintained by Beaufort County is currently bisected by an existing drainage ditch that ultimately conveys runoff to the Okatee River. To maintain access to the rear portion of the property, a new crossing is proposed across this ditch. The parcel (Beaufort County TMS #R600 013 000 0061 0000) is located parallel to Pritcher Point Road.

The site is bordered by Heffallump Road to the north, the Beaufort County Animal Campus and US-170 to the west, Pritcher Point Road to the south, and an unnamed tributary to the Okatee River along the eastern boundary. The land is undeveloped and vacant, with pine logging operations occurring at the rear of the property.

Existing Conditions The property is predominantly wooded and undeveloped. Stormwater currently flows into and through the existing ditch running through the center of the site, ultimately discharging toward the Okatee River.

Demolition & Tree Removal Minimal tree removal is anticipated to accommodate the proposed stormwater crossing. Specific tree species and sizes will be identified during survey activities. The design will be developed to avoid impacts to significant or specimen trees to the extent practicable.

Post-Development Conditions The proposed improvements include installation of a 60-inch reinforced concrete pipe (RCP) to maintain uninterrupted flow within the ditch, along with a gravel-stone surface crossing above the pipe to facilitate access. No utilities are proposed as part of this project, as the site is unoccupied.

Proposed Site Drainage The existing ditch bisects the property, dividing it nearly in half. To maintain access to the rear portion of the site, a ditch crossing is necessary.

An existing 30-inch pipe extending from the Malind Bluff subdivision appears to initiate the ditch system, which widens naturally moving northeast. The drainage areas contributing to the ditch upstream of the proposed 60-inch pipe were delineated and analyzed for the 100-year storm.

These areas were combined and modeled using the Rational Method within an AutoCAD Express model to appropriately size the culvert.

30” Pipe Flowing Full = 42.85 cfs Property Flowing to Crossing = 60.57 cfs Total Flow to 60” Pipe = 103.42 cfs

The selected 60-inch pipe appears to be adequately sized to ensure no constriction of existing flow and to avoid creating upstream flooding or backwater conditions.

Access Property access is available via a left turn from Pritcher Point Road, directly behind the Beaufort County Animal Services building.

Wetlands No wetlands are located within the proposed disturbance area.

Soils Based on the Web Soil Survey (USDA Natural Resources Conservation Service), the soil found onsite is Bladen fine sandy loam (Bd), Capers association (CE), Coosaw loamy find sand (Cs), Nemours fine sandy loam (NeA), Tomotley loamy fine sand (To),Wahee fine sandy loam (Wa), and Yemassee loamy fine sand (Ye) classified as HSG A/B/C/D.

Erosion Control The Erosion Prevention and Sediment Control Plan will be presented in one Erosion Prevention and Sediment Control Phase. This includes the initial installation of perimeter controls, sediment control BMPs, the implementation of internal stormwater management BMPs, and the stabilization of the site.

Flood Zone The project area is located within FEMA Flood Insurance Rate Map (FIRM) Panel 45013C0260G and includes Zone AE (Base Flood Elevation 11 feet) and Zone X (0.2% annual chance flood hazard). Due to topographic relief and the presence of well-draining sandy soils, no known flooding concerns currently exist on the site or are anticipated as a result of the proposed improvements.

Maintenance Responsibilities Beaufort County, as the property owner, will be responsible for maintaining all improvements in accordance with Beaufort County and SCDES requirements and guidelines.

Appendix A Site Exhibits

This map was created using Latapult | www.latapult.com | Bigger Data Faster Maps Stronger Decisions

Pritcher Point Drainge Area

02/27/2026

Parcels

Channel Report

Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Feb 27 2026

30 Inch Pipe FULL

Circular Diameter (ft) = 2.50

Invert Elev (ft) = 12.40 Slope (%) = 0.93 N-Value = 0.012

Calculations Compute by: Known Depth Known Depth (ft) = 2.50

Highlighted Depth (ft) = 2.50 Q (cfs) = 42.85 Area (sqft) = 4.91 Velocity (ft/s) = 8.73 Wetted Perim (ft) = 7.85 Crit Depth, Yc (ft) = 2.19 Top Width (ft) = 0.00 EGL (ft) = 3.68

0 1 2 3 4

Elev (ft) Section

11.50

12.00

12.50

13.00

13.50

14.00

14.50

15.00

Reach (ft)

Appendix B Curve Number Calculation

WinTR-55 Current Data Description

--- Identification Data ---

User: DKL Date: 2/27/2026

Project: Pritcher Point Drainage Units: English

SubTitle: Areal Units: Acres

State: South Carolina

County: Beaufort_NOAA_B

Filename: O:\01-Projects\170601-Beaufort County-Pritcher Pt Rd Stormwater Repair\02-Design\02-Reports\Stormwater\TR55\170601-TR55 Flow from Woods.w55

--- Sub-Area Data ---

Name Description Reach Area(ac) RCN Tc

Area to Di Outlet 19.5 57 0.666

Total area: 19.50 (ac)

--- Storm Data --

Rainfall Depth by Rainfall Return Period

2-Yr 10-Yr 25-Yr 50-Yr 100-Yr -Yr -Yr

(in) (in) (in) (in) (in) (in) (in)

4.5 6.9 8.4 9.7 11.0 .0 .0

Storm Data Source: User-provided custom storm data

Rainfall Distribution Type: Type III

Dimensionless Unit Hydrograph: <standard>

WinTR-55, Version 2.00.00 Page 1 2/27/2026 11:01:27 AM

DKL Pritcher Point Drainage

Beaufort_NOAA_B County, South Carolina

Storm Data

Rainfall Depth by Rainfall Return Period

2-Yr 10-Yr 25-Yr 50-Yr 100-Yr -Yr -Yr

(in) (in) (in) (in) (in) (in) (in)

4.5 6.9 8.4 9.7 11.0 .0 .0

Storm Data Source: User-provided custom storm data

Rainfall Distribution Type: Type III

Dimensionless Unit Hydrograph: <standard>

Watershed Peak Table

Sub-Area Peak Flow by Rainfall Return Period or Reach 100-Yr

Identifier (cfs)

SUBAREAS

Area to Di 60.57

REACHES

OUTLET 60.57

Hydrograph Peak/Peak Time Table

Sub-Area Peak Flow and Peak Time (hr) by Rainfall Return Period or Reach 100-Yr

Identifier (cfs)

(hr)

SUBAREAS

Area to Di 60.57

12.46

REACHES

OUTLET 60.57

Sub-Area Summary Table

Sub-Area Drainage Time of Curve Receiving Sub-Area

Identifier Area Concentration Number Reach Description

(ac) (hr)

Area to Di 19.50 0.666 57 Outlet

Total Area: 19.50 (ac)

Sub-Area Time of Concentration Details

Sub-Area Flow Mannings's End Wetted Travel

Identifier/ Length Slope n Area Perimeter Velocity Time

(ft) (ft/ft) (sq ft) (ft) (ft/sec) (hr)

Area to Di

SHEET 100 0.0200 0.800 0.525

SHALLOW 1159 0.0200 0.050 0.141

Time of Concentration 0.666

Sub-Area Land Use and Curve Number Details

Sub-Area Hydrologic Sub-Area Curve

Identifier Land Use Soil Area Number

Group (ac)

Area to DiWoods (fair) A 10.1 36

Woods (fair) D 9.4 79

Total Area / Weighted Curve Number 19.5 57

Appendix C Pipe Sizing Calculations

Culvert Report

Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Feb 27 2026

60 Inch RCP

Invert Elev Dn (ft) = 7.00 Pipe Length (ft) = 32.00 Slope (%) = 0.31 Invert Elev Up (ft) = 7.10 Rise (in) = 60.0 Shape = Circular Span (in) = 60.0 No. Barrels = 1 n-Value = 0.012 Culvert Type = Circular Concrete Culvert Entrance = Square edge w/headwall (C) Coeff. K,M,c,Y,k = 0.0098, 2, 0.0398, 0.67, 0.5

Embankment Top Elevation (ft) = 15.00 Top Width (ft) = 30.00 Crest Width (ft) = 45.00

Calculations Qmin (cfs) = 103.42 Qmax (cfs) = 103.42 Tailwater Elev (ft) = Normal

Highlighted Qtotal (cfs) = 103.42 Qpipe (cfs) = 103.42 Qovertop (cfs) = 0.00 Veloc Dn (ft/s) = 8.55 Veloc Up (ft/s) = 8.73 HGL Dn (ft) = 9.96 HGL Up (ft) = 10.01 Hw Elev (ft) = 11.46 Hw/D (ft) = 0.87 Flow Regime = Inlet Control

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0

Elev (ft) Hw Depth (ft) Profile

6.00 -1.10

7.00 -0.10

8.00 0.90

9.00 1.90

10.00 2.90

11.00 3.90

12.00 4.90

13.00 5.90

14.00 6.90

15.00 7.90

16.00 8.90

Reach (ft)

Embankment

32.00 Lf of 60(in) @ 0.31 %

Hw

HGL