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Title: Non-Darcy flow of water through woodchip media

item GHANE, EHSAN - The Ohio State University
item Fausey, Norman
item BROWN, LARRY - The Ohio State University

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/7/2014
Publication Date: 10/2/2014
Publication URL:
Citation: Ghane, E., Fausey, N.R., Brown, L.C. 2014. Non-Darcy flow of water through woodchip media. Journal of Hydrology. 519:3400-3409.

Interpretive Summary: Agricultural production generates off-site air and water pollutants. One significant pollutant is nitrogen, an important plant nutrient. Soluble nitrogen moves off site and into streams with drainage water. Woodchip bioreactors are an emerging technology intended to reduce the delivery of soluble nitrogen in subsurface (tile) drainage water. Design of effective and efficient bioreactors requires knowledge of the water transmitting properties of the woodchips. The objective of this study was to determine intrinsic permeability of woodchips. While water flow through woodchips has been assumed to be laminar, this study concludes that the linear flow assumption is inadequate for describing water flow through woodchips. This information is helpful to engineers and technical assistance agencies that design bioreactors for environmental protection.

Technical Abstract: A denitrifying bioreactor is a system where a carbon substrate (commonly woodchips) is used to reduce nitrate concentration in water flow. Knowledge of intrinsic permeability of woodchip media in different types of this system is of great importance for design and modeling. For many years, water flow through woodchips has been assumed to be laminar without proper validation. The objective of this study was to determine intrinsic permeability of woodchips in the laboratory and in the field while applying the valid governing flow equation. Laboratory results revealed that Darcy’s law does not apply for the majority of the hydraulic gradients forcing flow through fresh and old woodchip media. However, Forchheimer’s equation described the flow pattern sufficiently using a quadratic equation. Comparison between fresh and old degraded woodchips excavated from a denitrification bed indicated that old woodchips had significantly lower intrinsic permeability compared to fresh woodchips. Old woodchips also showed stronger deviation from Darcy’s law at the same flow rate compared to fresh woodchips. Forchheimer and Darcy’s in-situ coefficients were determined and used to predict flow rate in a denitrification bed. Model evaluation statistics resulted in a better flow prediction for Forchheimer than for Darcy’s equation when comparing to measured flow rate. In conclusion, the linear flow assumption was found to be inadequate for describing water flow through woodchips in a denitrification bed and laboratory experiments.