Modelling Performance of a Tile Drainage System Incorporating Mole Drainage.

Modelling performance of a tile drainage system incorporating mole drainage P. Tuohy1, J. O’ Loughlin1, O. Fenton2 1Animal and Grassland Research and Innovation Centre, Teagasc, Moorepark, Fermoy, Co. Cork, Ireland; 2Environment Research Centre, Teagasc, Johnstown Castle, Wexford, Co. Wexford, Ireland. Corresponding author: Pat Tuohy; phone: +353-25-42366; e-mail: patrick.tuohy@teagasc.ie. ABSTRACT. Mole drain performance is known to vary temporally and spatially due to variations in soil properties, installation conditions, mole channel integrity and weather patterns. The objective of the present modeling study was to investigate the performance impacts of variations in mole integrity and design during a range of rainfall event scenarios. A finite element software package, SEEP/W, was used to model a field site having (System 1) subsurface tile drains with gravel aggregate( 10 - 50 mm) (0.9 m depth, 15 m spacing) and intersecting mole drains (0.6 m depth, 1.4 m spacing). A site Met. station and end-of-pipe flow meters provided rainfall and discharge data from which the model could be calibrated. The calibrated model showed close agreement between modeled and observed subsurface discharge in the validation period (Coefficient of mass residual = 0.12, index of agreement = 0.94, Model Efficiency = 0.74). The model was then used to evaluate the impact of a range of alternative designs namely; System 2: tile drains only; System 3 and System 4: both similar to System 1 but with the ks of the mole drained layer decreased/increased respectively to mimic changes in mole drain integrity/effectiveness.. These systems were analyzed across a range of rainfall scenarios. The designs modeled exhibited similar relative behavior in all simulated rainfall scenarios. Systems 1 and 4 consistently outperformed Systems 2 and 3 in terms of average and peak discharge and watertable control capacity. Across rainfall events, System 2 (without mole drains) was the least effective and was seen to decrease drain discharge by an average of 63% and reduce mean watertable depth by an average of 72% relative to Systems 1 and 4. With high resolution data the software could be calibrated to cater for other drainage system and climate change scenarios.