Water control is important both to help keep the material moist and to allow drainage.
Bad drainage will result in the surface being inconsistent for the horse to move on. A fast draining arena which is made of fibre sand or waxed sand will drain sufficiently well that the surface is effectively unaffected by water.
If natural drainage is used and the soil conditions are suitable, the ground must be even with some cross slope, typically less than 1% up to 2%. In some case the arena is designed with a camber (=highest in the middle, sloping to the sides) to achieve drainage. The design of an arena with a camber introduces the risk that the cushion depth will vary significantly through routine maintenance. This is because the harrow will tend to grade away the ridge of surface material in the middle. The possible thinning layer of surface material can cause changes in functional properties as well as blending of surface material and underlying layer material. This is less of an issue for an area with a simple cross fall across the surface, although attention must always be paid to the development of shallow areas.
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Stone House Farm, Handcross Road, Horsham, Sussex RH13 6NZ