C05 Attach 11 - Septic Calcs and Soils Reports.pdf
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- Attached to
- GAOA Sherando Recreation Area Rehabilitation, George Washington-Jefferson National Forest
- Solicitation number
- 1240LT26Q0089
- Issued by
-
Department of Agriculture Forest Service
About this file
This document is a comprehensive engineering report for septic tank and drainage field sizing for a bathhouse rehabilitation project at Sherando Recreation Area Upper White Oaks Loop Campground in North Carolina.
The project involves designing a wastewater treatment system to handle a daily flow of 1,450 gallons per day (GPD) from 14 luxury campsites at 100 GPD each and 1 sewer hookup at 50 GPD. The septic tank sizing calculation specifies a minimum tank volume of 2,900 gallons. The drainage field design incorporates 14 trenches, each 90 feet long and 3 feet wide, spaced 9 feet center-to-center, with a total trench length of 1,214 feet and total piping length of 1,595.3 feet. The system utilizes a siphon dosing approach (Siphon 417 from Fluid Dynamic Systems, Inc. and Kistner Concrete Products "G" Type Dosing Chamber) with a 150 gpm average flow rate and 5.09-minute dose time. The design includes 14 lateral lines with 21 orifices spaced 4.3 feet apart, using 0.1875-inch orifices for regulated flow distribution. A 1,547-gallon denitrification tank with 850.85 gallons effective volume provides 14.1-hour hydraulic retention time to achieve 85-90% nitrate reduction, reducing influent concentrations of approximately 40 mg/L to 4-6 mg/L before soil dispersal. Groundwater modeling demonstrates a 4.0:1 dilution ratio that reduces treated effluent nitrate concentrations to 1.20 mg/L at the project boundary, below the 5 mg/L regulatory limit. Water mounding calculations estimate a maximum rise in water table of 0.366 feet. Site approvals from Augusta County Health Department specify drainfield trench bottoms at 21 inches depth for the primary area and 24 inches for the secondary area, with percolation rates of 50 min/inch estimated in texture group III soils classified as Craigseville fine sandy loam alluvial series. Constant-head borehole permeameter testing conducted January 22-23, 2026, at six test holes (A-F) confirms saturated hydraulic conductivity ranging from 50.9 cm/day to 1,400+ cm/day, supporting the design feasibility.
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Sherando Recreation Area Rehabilitation Upper White Oaks Loop A Bathhouse Sanitary Calculations Septic Tank and Drainage Field Sizing
Septic Tank Sizing Loading Load 1 14 Luxury Campsites GPD per Load 1 100 GPD Load 2 1 Sewer Hookup GPD per Load 2 50 GPD Load 3 GPD per Load 3 0 GPD Load 4 GPD per Load 4 0 GPD
Daily Flow 1450 GPD Minimum Tank Volume 2900 gallons
Drain Field Sizing Width of Trench 3 ft Perc Rate 50 min/in log(y)=2.00+0.008(Perc Rate ) y= 251.1886 ft2/100 gals Trench Area (TA ) 3642.235 ft2
TA=DF*(y/100)
Trench Length 1214.078 ft
6 Lines 202.3464 linear ft each 8 Lines 151.7598 linear ft each 10 Lines 121.4078 linear ft each 12 Lines 101.1732 linear ft each 14 Lines 86.71989 linear ft each 90 feet
Conveyance Line to Tee 128 ft Tee to First Lateral (W) 26 ft Tee to First Lateral (E) 42.15 ft Manifold (W) 78.9 ft Manifold (E) 60.2 ft Laterals 1260 ft
Total 1595.3 ft
Total Length of Piping from Siphon including
Page 1 of 6 Sizing
Septic Tank and Drainage Field Sizing
Dose Calcs Header Size 3 inch Lateral Size 1.25 inch
0.104 feet Manifold Size 4 inch Number of Lines 14 Length of Line 90 ft Volume Inside Dist. Piping 10.73787 cu.ft
80.32997 gal
Dose Min 562 gal Check: Area = 3780 sq ft Dose Volume = 75 cu.ft Liquid Depth = 0.020 ft Liquid Depth = 0.24 inches
Dose Max 803 gal Check: Area = 3780 sq ft Dose Volume = 107 cu.ft Liquid Depth = 0.028 ft Liquid Depth = 0.34 inches
Daily Design Flow = 1450 gal/day Percolation Rate = 50 min/inch Application Rate = 193 sq.ft/100 gals Number of Doses per Day = 2 Doses
Total Capacity = 3917.1 gal/day Max Effluent per Dose = 1958.5 gal/dose Dose Volume = 725 gal/dose
Is this between Min Dose and Max Dose? Yes!
Siphon Sizing Round Rectangle Tank Radius 35 in Tank Length 144 in Tank Height 60 in Tank Width 72 in Tank Volume 230907.1 cu inches Tank Height 22 in
133.6 cu ft Tank Volume 228096 cu inches
999.6 gal 132 cu ft
987.492 gal Volume per inch 16.7 gal/inch Volume per inch 44.886 gal/inch Inches of Drawdown = 43.51762 in 16.15202959 inches Dose for above inches of drawdown =
Volume for Drawdown 763.06 gals Drawdown, h 17 inch Use siphon 417 from Fluid Dynamic Systems, Inc and Kistner Concrete Products "G" Type Dosing Chamber
Siphon Flow Q(siphon, avg) 150 gpm Dose Time 5.09 minutes Velocity(manifold pipe) 3.8 fps Velocity (Header pipe) 3.4 fps Velocity (Lateral pipe) 2.8 fps
Page 2 of 6 Dose & Siphon
Septic Tank and Drainage Field Sizing
Orifice Calcs
Dose Volume 763.06 gals Q(siphon,avg) 150 gpm Siphon Head 17 inch Lateral size 1.25 inches From 12VAC5-610-940 Orifice Size (min) 0.1875 inches From 12VAC5-610-940 Orifice Size (max) 0.25 inches From 12VAC5-610-940 Selected Orifice Size 0.1875 inches Selected Orifice Size 0.015625 feet Number of Trenches 14
Flow per Trench 10.71 gpm
Orifice Flow Calc Q(orifice) 0.0011355 cfs
0.51 gpm C 0.62 for PVC A(orifice) 0.000192 ft2 g 32.2 ft/s2 h 1.42 ft
Number of Orifices 21 orifices Orifice Spacing 4.3 ft
Page 3 of 6 Orifice Calcs
Septic Tank and Drainage Field Sizing
Headloss Across Laterals
Inputs Q(siphon, avg) 150 gpm 0.33418 cfs Number of Laterals 14 Lateral Length 90 ft Lateral Pipe ID 1.25 in Hazen-Williams C 150 Operating head at manifold 17 in
Velocity Calculations Q(per lateral) 10.71 gpm Q(avg, per lateral) 5.36 gpm Lateral Pipe ID 0.104 ft2
Area of Lateral 0.00852 ft2
Q(per lateral) 0.024 cfs Velocity(Lateral) 2.8 fps
Headloss Calculations Headloss 0.29 ft Headloss 3.46 in Head(Operating) 1.42 ft Head(at end) 1.13 ft Q(ratio, first to last) 1.12
Page 4 of 6 Headloss in Lats
Septic Tank and Drainage Field Sizing
Treatment Calculations Flow 1450 gpd Flow 60.4 gal/hour Denitrification Tank Volume 1547 gal Porosity of Woodchips 0.55 Effective Volume 850.85 gal Hydraulic Retention Time 14.1 hours TN (of typical Effluent) 40 mg/L
Nitrate Reduction Percantage (Low) 85% Nitrate Reduction Percantage (Mid) 90% Nitrate-N After Reduction (Denitrification Reactor Effluent, Low) 6 mg/L
Nitrate-N After Reduction (Denitrification Reactor Effluent, Mid) 4 mg/L
In the case of 6 mg/L, we want to show that groundwater mixing reduces the Nitrate-N to an acceptable level at the boundary.
KSAT 10.98 ft/day Boundary 15.5 ft
Elevation Drop across that distance 3.65 ft Hydraulic Gradient, i 0.24 Flow Area (conservative assumption) 300 ft2
Groundwater Flow rate, QGW=KiA 775.68 ft3/day Groundwater Flow rate 5802.891 gal/day Dilution Ratio (QGW/QFlow) 4.0 :1 Worst-case Treated Effluent 6 mg/L
Cboundary 1.20 mg/L
Conservative groundwater flow calculations demonstrate sufficient dilution between the drainfield and project boundary to reduce treated effluent nitrate concentrations to below the 5 mg/L regulatory limit.
Published studies of woodchip denitrification reactors demonstrate nitrate reduction efficiencies generally ranging from 80% to 95% within retention times of 6 to 24 hours. Using conservative performance assumptions of approximately 85% to 90% nitrate removal, influent nitrate-N concentrations of approximately 40 mg/L are expected to be reduced to approximately 4 to 6 mg/L prior to soil dispersal.
Page 5 of 6 Treatment Calcs
Septic Tank and Drainage Field Sizing
Water Mounding Calculations
Parameter Value Units Description Daily Flow 1450 gal/day Total Effluent Flow Number of Trenches 14 count Total Number of Trenches Trench Length 90 ft Length of Each Trench Trench Width 2.1042 ft Width of Each Trench Trench Spacing (Center-to-Center) 9 ft Distance Between Trenches KSAT (K) 5.49 in/hr Saturated Hydraulic Conductivity Specific Yield 0.2 dimensionless Duration 1 days
Calculations Total Infiltration Area 2651.3 ft2 How much bottom area is available for effluent dispersal Flow 193.84 ft3/day Total Wastewater Entering System Recharge Rate (R) 0.0731 ft/day Effluent applied per unit area (ft/day) KSAT 0.0191 ft/day Hydraulic conductivity in consistent units Half Field Length (x) 45 ft Geometry for mound estimation Total Field Width 117 ft Geometry for mound estimation Half Field Width (y) 58.5 ft Geometry for mound estimation Estimated Mound Height 0.366 ft Estimated Maximum Rise in Water Table
*This spreadsheet uses the Hantush equation as a basis for these calculations.
Page 6 of 6 Water Mounding
| 05 Septic Design Calculations-Upper White Oak Loop A Bathhouse.pdf |
| 06 Sherando Lake Upper Loop Campground Bathhouse Site-Soil Eval_8-5-2024.pdf |
| 07 Sherando Lake upper loop Bath house Soil Summary & Descriptions_10-10-2025.pdf |
| 08 Sherando Lake Upper Loop Campground KSAT results_2-1-2026.pdf |
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