Water Movement Through The Soil

    As logic would dictate, soil particle size affects the rate at which water moves through soil.  For coarse sand, if dyed water is dripped onto the surface, it can be seen to move rapidly downwards due to gravity (i.e. efficient drainage).  In practice, however, natural turf facilities are invariably constructed using a layer of finer-grained material over coarse.  The effect of this on water movement is counter-intuitive in that water moves readily into the fine-grained  material but will not move from the fine pores into the coarser pores in the material below due to surface tension effects.  This phenomenon can be used to advantage in areas of the world where ‘water conservation’ is a priority by creating a ‘suspended water table’ below a playing surface to provide a reservoir of stored water.
 


Figure 4 Efficient water movement through coarse sand, a hydraulic barrier between fine and coarse sand, and a suspended water table in a typical natural turf construction.

    Whilst this would appear to provide a panacea to optimising water availability, in many parts of the world precipitation exceeds evapotranspiration for a significant period of the year.  In these areas, this type of construction is often associated with excessive water content in the vicinity of the playing surface and the knock-on effect this has on turf quality and surface conditions (insufficient oxygen for the grass roots and wet, soft surface conditions).
 
    The effect of this can be exacerbated further by another often ignored phenomenon termed ‘capillary rise’.  For example, if the base of a column of sand/soil mix typically used in natural turf constructions is placed in water, water will rise up through the soil, against gravity, to a height of between 150 to 350 mm depending on the size of the pores in the material (the finer the pores, the higher the rise).  The building industry is more familiar with this process and refers to it as ‘rising damp’.  The implication of this for natural turf constructions is that if a suspended water table is too near the surface, water will rise up through the profile towards the surface and compromise playability and turf quality.


Figure 5 Capillary rise, testing the capillary rise characteristics of construction materials, capillary rise above a suspended water table.

This also has implications for traditional approaches to natural turf drainage systems which are based on the incorporation of coarse media (or a perforated pipe) into the soil profile in the belief that water will move readily from the fine pores in the soil into the coarse pores in the drain.  In practice, water moving down from the surface will positively avoid moving into the coarse pores space in the drain.  Typical drain installations comprise a perforated pipe in the base of a gravel-filled trench which is topped off with a shallow sand, or sand/soil mix, layer.  This causes water to be held tightly in the fine pores of the upper layer thereby preventing the downward movement of excess surface water through the coarse gravel and into the pipe.



Figure 6 Fundamental flaws in surface drainage design

Whilst this type of drain design would be effective in controlling a rising water table, the dominant requirement for natural turf is usually the rapid removal of surface water.