Agricultural Drainage Design
Evaluating and designing surface and subsurface drainage systems for row crop farmland in the NC Coastal Plain — where the difference between a productive field and a flooded one often comes down to a few inches of grade and a well-placed outlet.
Background
Eastern North Carolina sits in the Coastal Plain — flat, poorly drained soils with a high water table and seasonal flooding that can knock out significant portions of a crop if drainage isn't actively managed. Good agricultural drainage isn't just a productivity issue; it's an economic one.
This project involves evaluating the existing drainage infrastructure on an 85-acre row crop field, identifying problem areas through field survey and soil assessment, and designing improvements — a mix of open ditch grading, surface water management, and potential tile drain installation.
Approach
The project started with a field visit: walking the site after a rain event to map low spots, pond areas, and poor outlet locations. Elevation data was collected using GPS and verified against LiDAR. Soil borings were done at several locations to assess depth to restrictive layer and soil texture — key variables for tile drain design.
Open ditch grades were calculated using Manning's equation to size channel cross-sections for the design storm runoff. A surface water management plan was developed to address low spots through precision land grading. Tile drain spacing was calculated using the Hooghoudt equation for the dominant soil series.
"In the Coastal Plain, an inch of grade change across a hundred acres can mean the difference between a productive field and standing water for two weeks."
Key Details
- Field size: approximately 85 acres; dominant soil series — Roanoke and Altavista loams (Coastal Plain Ultisols).
- Depth to restrictive layer averaged 30–38 inches — within the workable range for subsurface tile drain installation.
- Three primary problem zones identified: two interior low spots and a poorly graded ditch outlet.
- Recommended tile drain spacing of 60 ft for the dominant soil type based on Hooghoudt calculation.
- Open ditch regrading designed for a 10-year, 24-hour storm event using NC Coastal Plain flow coefficients.
- All grading design drafted in AutoCAD Civil 3D with plan/profile sheets and earthwork volumes calculated.
Field Photos
Outcomes & Status
Field assessment is complete and drainage deficiencies are documented. Open ditch grading design is finalized. The subsurface tile drain layout and specifications are currently in progress.
The final design package will include plan sheets, earthwork quantities, material specs, and a brief design narrative suitable for review by NRCS or a drainage contractor.