CHAPTER ONE

Background of Study

1.1 Introduction

Surface runoff and soil water percolation are closely associated with rainfall and melting of snow, or glaciers. Soil inability to absorb excess stormwater and meltwater due to heavy rainfall, high melt rate of snow and glacier, soil saturation, impervious resulting from surface sealing or pavement, etc., do lead to surface runoff [10]. Surface runoff is the major cause of soil erosion and surface water pollution. In urban areas, runoff is the main cause of flooding which may damage properties and infrastructures including loss of life [23]. In order to alleviate the unpleasant effects of surface runoff, several proactive measures are needed to boost soil absorption of stormwater and meltwater. These measures may include minimizing impervious surfaces in urban areas, adopting soil erosion and flood control programs, etc.

Moreover, percolation describes the downward flow rate of the stormwater or meltwater within the soil [6]. Water percolation in the soil contributes to the formation of groundwater aquifers which serves as a freshwater storage that can be utilized during droughts when surface water supplies are reduced. Generally, soil is regarded as a porous media; the soil loose sediments like sand and gravel are porous and permeable.

It can hold water and allows water to flow through [5]. While the amount of porosity in a soil depends on its mineral content and structure, the rate of water percolation depends on soil permeability (i.e. the size of the soil pore spaces and how the pores are connected). For instance, sandy soils have large well connected pores and higher permeability than the clay soils [17]. The use of mathematical models to tackle the menace of surface runoff and enhance the soil water percolation for the formation of groundwater aquifers has attracted the attention of several scientists and researchers [7, 22, 4, 21, 20, 15]. For soil with weak permeability, the relationship between the flow rates and the pressure gradient would be practically linear based on the Brinkman form of Darcy law, while this relationship may be nonlinear for soil with strong permeability (Darcy–Forchheimer law) [25, 2, 18]. Bristow and Horton [3] theoretically investigated the influence of surface mulch soil water flow and heat transfer. The effects of temperature gradient on the soil water flow were studied by Gurr et al. [9]. Numerical results on soil water flow and heat transfer rate together with soil-atmosphere interaction was reported by Fetzer et al. [8]. In all the above studies, it is observed that mathematical model of soil-runoff interface at the continuum scale where water and energy fluxes are highly dynamic are often magnified. This may lead to inaccuracy in the result obtained.

In this present study, the biomechanics of surface runoff and its interaction with soil water percolation is numerically examined. The Darcy-Brinkman-Forchheimer nonlinear model for porous medium coupled with appropriate energy equation is employed in order to analysis the soil percolation rate while the model representing.

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