What can we learn from the rains
(Turf Business, January/February 2008)
Standing water is everywhere – but January is always a wet month and floods are expected in many parts of the country. All low lying areas are at risk and expanses of water are evident on much of the flat ground. A study of the aftermath gives us valuable information from which to make plans for improvement.
Surface water finding exit
The passage of surface water flow becomes an interesting observation at the end of a rainy season. Water run-off is always localised with water finding the easiest route of exit. It is only when outfall restrictions cannot cope with the surface flow that flooding occurs. It is at these times that the benefit of a topographical study becomes of immense value. Routing surface water sounds simple but it is often neglected or calculations of water flow quantities are underestimated. The most important consideration must be the maintaining of adequate outfall locations. In low lying areas this becomes a difficulty and options can be limited to either boring down to permeable horizons or establishing a wet land sufficient to cope with all the surface water generated.
At times of heavy rains the marking out of low points on site becomes invaluable. Besides highlighting wet areas they serve to create the best route of reticulation needed in drainage provisions. Though exits do not become immediately apparent the direction of surface water flow can only be properly planned with a detailed contour plan. The construction of shallow mowable swales directs water flow to safe outfall locations. These could be ponds, streams or ditches. The need for a ditch reticulation or extension of it becomes very apparent after a rainy season and on the golf course they are invaluable at times of flooding with the swales bringing surplus water to a safe outfall.
At times of continual rain there can be temporary restriction in outfall locations or there is just not the provision to cope with excessive water volumes. Where these situations could arise, swales and ditches designed to very shallow gradients also offer the opportunity for temporary storage and water loss by evaporation and percolation into the soil over time. With this capacity peak flow off the site can be controlled to a degree. Swale installation can also enhance overall design creating mounding and undulations with excavated material. In diverting and concentrating surface water flow swales enable surrounding areas to remain firm and playable as well as maintaining sustainable conditions for growth in these areas.
Run-off is very apparent on pathways and roads. These areas deserve special study as they deteriorate rapidly if surface water is not catered for. This becomes readily apparent after rain. The limitations in construction are highlighted with increased usage in wet weather. Furthermore, it is common to overlook the potential of pathways and roads to become waterways. Cambering road surface helps maintain firm ground in the centre but accumulating water flow must have regular exit out of harms way. Intervals between exits must remove water flow before it becomes erosive and the distance between them is a function of the slope and the porosity of construction.
Where run-off is restricted
Areas holding water soon result in deteriorating plant growth. This can result from a number of factors that become evident after considerable rain over an extended period. Limiting water penetration can be due to the excessive accumulation of thatch and organic matter at the surface which acts as an insulating material. This becomes readily apparent after persisting rain and highlights the need for reduction. Notably too is the effect of compaction from foot traffic on wet ground. On sports pitches particularly this is considerable and confirms the need for decompaction with the vertidrain or ‘earthquake’ when the soil has partially dried out.
The depth of grass cover relative the gradient is a prime factor in withholding surface water and it is fairly well established that gradients of at least 1:70 are needed to promote run-off on playing fields. In addition, on sloping ground it is often overlooked that even a decreasing slope can cause a problem with surface water accumulation – there is the build-up of water off a steeper gradient that just cannot be despatched on a flatter gradient. This always becomes apparent after rain.
Where topsoil is permeable and becomes saturated there could be an underlying hard pan – this may be as a result of compaction below the level of surface aeration and cultivation. The persistence of this condition must lead to this possibility and the remediation necessary.
At the end of the winter months much can be learnt from the condition of sports pitches and how they have performed after continual use – often in wet conditions. Surface drainage installations need annual assessment. Besides over-seeding and sand dressing, slit drains may have become silted at the surface with inadequate attention in the past. There may be a need to install sand injected grooves to penetrate through the impermeable surface – this condition reinforces the vital need for regular annual sand dressings to keep the surface slit drains open.
The physical composition of the root-zone depth of topsoil gives an idea of the infiltration potential. Compaction and thatch may be overcome but the inherent soil and the gradient become limiting factors that can only be remedied with complete reconstruction. On golf greens with improved root zones these two factors are prime concerns too together with underground drainage installed. Where surplus water cannot be removed from low points anaerobic conditions develop and grass cover can weaken and die. However, on sports pitches with indigenous topsoil on a slowly draining subsoil, weakened growth gives rise to invading weed and moss growth.