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Continuous rainfall-runoff modelling of railway embankment steep slopes
Rainfall induced erosion, and related sedimentation issue, on steep slopes (batters) of railway embankments in Queensland is causing increased maintenance costs. Grass cover establishment is being employed to combat the erosion problems as it slows down runoff, the roots increase infiltration and the foliage decreases the impact of raindrops on the soil surface. The objective of this research is to investigate grass cover effects on runoff on railway formation steep slopes to help quantify the scale of the problems. Long-term monitoring of runoff and soil loss is an expensive exercise, so limited field trials have been carried out to calibrate a 1-D distributed rainfall-runoff model that takes into account the initial moisture. It consists of the Saint Venant continuity and momentum equations for overland flow, and a modified Green-Ampt model for infiltration on steep slopes. The experimental batter plots, each 10 m wide and with different grass cover percentages, were established at the Gregory Erosion project field trials site near Blackwater, Queensland, Australia. Rainfall and runoff were monitored at 1 minute intervals. It is observed that the average values of the saturated hydraulic conductivity (Ks) for 0%, 50% and 100% grass cover are 0.3, 5.03 and 22.56 mm/hr respectively, and in general runoff decreases exponentially with grass cover percentage. The model has successfully predicted runoff from the plots, with the Nash Sutcliffe Efficiency values varying between 0.50 and 0.83. With the aid of an antecedent moisture parameter which varies with grass cover percentage, continuous simulation of runoff can be carried using long records of observed fine timescale rainfall or derived from a stochastic rainfall model.