Modelling of water and sediment transport over grassed areas

Grassed areas, such as grass filter strips (GFS) and grassed swales, have been used extensively for the protection of surface water against erosion and polluted runoffs. A mathematical model of sediment transport in runoff over grass has been developed, allowing an assessment of the sediment removal efficiency of GFS and swales for non-submerged flow conditions. The model is one-dimensional and simulates two main processes: (1) generation of runoff and (2) sediment transport. The modified Green - Ampt model is used for assessment of infiltration, while a kinematic wave model is used for simulation of overland flow. A transport equation is used for modelling of sediment movement through grass, where sediment deposition is assessed by the method developed at the University of Aberdeen. The model does not simulate particle trapping in a pond (which may develop upstream from a GFS), re-suspension of particles already deposited, or infiltration of particles into soil. For a known particle size distribution of inflow sediment, the model is capable of predicting the particle size distribution of the outflow sediment. The model was developed for modelling single rain events. However, it could be also applied for a sequence of rain events assuming that the initial soil wetness is constant. An example of model application in an urban area is presented in the paper, along with sensitivity analysis of the main model results.