Numerical modeling to rainfall infiltration into planting-soil-crushed-stone green belt

Abstract

To verify the rationality of the theoretical analytical method to solve the rainfall infiltration into planting-soil-crushed-stone green belt, the finite element model of the rainfall infiltration into unsaturated planting soil is established. The water storage skin is used to model the infiltration and accumulation of rainwater on the upper boundary of the model. The lower boundary is saturated and drained, and the both sides of the model are impervious. Firstly, the infiltration characteristics of road rainwater under uniform rainfall conditions are modeled by the finite element model. Compared with the theoretical analytical solution, the finite element model is proved to be right. Secondly, the infiltration of road rainwater into planted soils is calculated by the finite element model under the condition of 2-year recurrence period and 3-hour design rainstorm of Shijiazhuang city. The beginning time of the surface water, the rainwater depth of stopping rainfall and the coefficient of rainfall runoff are 0.75 h, 13.6 cm and 0.24, respectively. The analytical results of uniform rainfall method are 0.72 h, 14.4 cm and 017, respectively. The results of the both methods are basically identical. Thirdly, the finite element model is employed to calculate the beginning time of the surface water under the condition of different pore pressure at the lower boundary and upper boundary. The results show that with the pore pressure descending at the lower boundary, the beginning time of the surface water rises up. With the pore pressure dropping at the upper boundary, the beginning time of the surface water increases. When the pore water pressure is 0 kPa at the lower boundary and the initial pore water pressure at the upper boundary are -6 and -12 kPa, the beginning time of the surface water are 45 and 50 min, respectively. Obviously the beginning time of the surface water is different. When the initial pore water pressure is -6 kPa at the upper boundary and the pore water pressure at the lower boundary are 0, -1, -2 and -3 kPa, the beginning time of the surface water are 45, 45, 46 and 47 min, respectively. The lower boundary condition has little effects on the beginning time of the surface water. When the permeability coefficient is 6.5×10^-9 m/s and the pore water pressure of ground soil are 0, -4.5, -9, -13.5 and-18 kPa, the finite element model is used and the beginning time of the surface water are 54, 54, 55, 55 and 56 min, respectively. Correspondingly the surface water depths are 11.9, 11.7, 11.5, 11.4 and 11.3 cm, respectively. With the pore water pressure in ground soil decreases, the beginning time of the surface water slightly increases, and the surface water depth descends. As a whole, the pore water pressure in ground soil has little effect on the beginning time and the depth of the surface water. When the pore water pressure in ground soil is 0 kPa and the permeability coefficients are 6.5×10^-9 and 6.5×10^-7 m/s, the beginning time and the depth of the surface water by the finite element model are 54 and 11.9 cm, respectively. The permeability coefficient of the ground soil has little effect. Based on above analysis, our main conclusions are: Uniform rainfall intensity and theoretical analytical method can be used to calculate the beginning time and the depth of the surface water. The lower boundary condition has little effects on the beginning time of the surface water and water depth. The saturated permeability coefficient and the initial water content of the ground soil have no significant influence on the beginning time of the surface water and water depth. It is recommended to employ the analytical solution to analyze the rainwater infiltration in to planting-soil-crushed-stone green belt.

Keywords

• city water supply and sewage engineering
• planting-soil-crushed-stone green belt
• finite element model
• boundary conditions
• rainfall pattern