Modeling performance of a tile drainage system incorporating mole drainage

Abstract

Abstract. In fine textured low permeability soil profiles, mole drainage is used as a supplementary measure to subsurface tile drains. However, performance is known to vary temporally and spatially due to variations in soil properties, installation conditions, mole channel integrity and weather patterns. A model study is presented whereby a finite element software package, SEEP/W, has been calibrated and validated for a field site having (System 1) subsurface tile drains with gravel aggregate (0.9 m depth, 15 m spacing) and intersecting mole drains (0.6 m depth, 1.4 m spacing) on the west coast of Ireland. The calibrated model showed close agreement between modeled and observed subsurface drainage (index of agreement = 0.89) and predicted cumulative subsurface drainage volumes 12% higher than observed in the validation period. The model was then used to evaluate the impact of a range of alternative drainage designs namely; System 2: tile drains only; System 3: Similar to System 1 with k s of the mole drained soil layer decreased to mimic a reduction in mole drain integrity/effectiveness and System 4: Similar to System 1 with k s of the mole drained soil layer increased to mimic improved soil disturbance and fissuring during installation. These systems were analysed under the calibration (Event A) and validation (Event B) dataset rainfall scenarios as well as notional rainfall scenarios; the “fixed rainfall” scenario (Event C), a rainfall rate of 2 mm/h applied to all systems for 50 hours and the “historical rainfall” scenario (Event D), the annual average (30 year) daily values for the area (taken as the average monthly total divided by the number of days) applied over a year. 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. System 2 (without mole drains) was the least effective and was seen to decrease drain discharge by an average of 54% and reduce mean watertable depth by an average of 62% relative to Systems 1 and 4 across rainfall events.