Flooding drainage

Flooding may not mean more drains

(Golf Course News International, Feb/Mar 2005)

How does it happen? After even light rains there can be more water in localised areas than the measured rainfall. Water run-off is grossly underestimated and during cool wet winters low evaporation rates means prolonged soggy areas. Though underground water from springs and seepage zones will always be a problem on undulating land, it is the damage caused by water run-off that is usually the main concern. Not only is there disruption to play but persistent saturated conditions lead to deterioration of grass cover that takes time and effort to rectify.

Water run-off will collect somewhere

Prolonged collection of water in isolated areas is simply due to the fact that it is unable to enter the soil and cannot get away. However, this water has in most cases comes from higher ground. Stopping water run-off on to greens, tees and fairways must be the first approach and diversion of this water before it reaches the facility is vital before any meaningful drainage provisions can be considered.

Water by nature of its fluid and cohesive properties will always flow where there is a continuous film and there is a positive gradient with minimal resistance to hold the water back. On a concrete surface even a thin film of water on the surface can result in 80-100% water runoff with minimal gradient. On golf courses bunkers fill and puddles develop even when run-off is as little as 20 to 30%. Long gradients accumulate water run-off and a positive gradient but one changing from steep to shallow will cause collection at the change in grade.

Grass cover by virtue of its resistance and the irregularities in the soil micro-level can hold back between 4 and 8 mm of initiating rainfall – even with a significant gradient. In fact researchers in Australia maintain that a grassed sports pitch surface must have at least 1 1/2 % gradient to permit surplus water run-off under high intensity rainfall conditions. In the UK a 2 to 4% fall is often needed before there is significant water run-off where good grass cover retains significant water under lower intensity rainfall.

The intensity of rainfall

It is the short heavy shower that generally causes the damage. The rate of the falling rain, termed the intensity, determines whether it will penetrate the soil or be lost as water run-off

In a study of rainfall of daily rainfall over 10 years at the Wisley Royal Horticultural Gardens the number of rain days between the mid winter months of October and April was found to be 15 to 20 days in the month. Within the average number of 16.5 rain days per month 15.1 of these measured less than 10mm in a 24 hour day. On 8.7 days the daily rainfall measured less than 2 mm in a day. Since there is slow but positive evaporation and water is able to infiltrate into the soil as well as be retained in the grass cover and soil depressions, the intensity of rainfall needs to be significant before water run-off will occur.

Water run-off can result in flooding and estimates of rainfall intensities and storm duration have been studied in different locations throughout the UK. A significant fact is that greatest intensities are limited to short duration downpours of 20 to 30 minutes. In this period the highest intensity rainfall measured over a return period of 20 years does not generally exceed 30mm in one hour and between 20 and 25 mm in a 30 minute period. In the south east of England the publication, produced by HR Wallingford and known as the Flood Estimation Handbook, estimates the highest rainfall of 30 minutes duration over a 10 year return period to be 21mm.

Reasonable provisions

Estimating rainfall and duration of downfalls is of vital concern in the design of dams where failure can lead to considerable structural and human loss. In the design of grassed areas there are not the same dangers. Catchment areas too are much smaller and so provisions for high probability rainfall intensity and storm duration can hardly be warranted economically where expected loss would be minimal. In addition to the resistance grass cover gives to water flow and the slow but positive infiltration into the soil, adequate provisions can be made for the safe diversion and collection of water run-off.

The provision to cope with surplus water run-off is therefore a matter of gauging the consequences of expected high intensity rainfall in respect of the result to both in the local condition as well as adjacent ‘downstream’ facilities. Cultural treatments to improve infiltration and reduce compaction are of little help when the rainfall rate exceeds the infiltration rate of the soil. On existing problem areas there are only two alternatives – cut-off drains or the installation of swales at suitable intervals.

Where gradients are too shallow to promote run-off, only an intense matrix of slit drains or moling will remove surplus water. Over long gradients that accumulate water run-off, cut-off drains or swales at suitable intervals are the only means of removing this water. Eventually cut-off drains silt over but the judicious use of shallow swales in the design layout is successful in diverting surplus water – and architects can utilise them to great advantage in the aesthetic quality of the design. They do not generally need to be deeper than 150mm and with of side slopes varying from 1:5 to 1:10 they can be insignificant in the earthworks layout. In fact they do not cause much disruption to construct and can be quickly and relatively inexpensively installed and reseeded. With swales concentrating and directing water flow, ditches and streams must safely convey this surplus water to suitable collection areas – be it a wetland, pool, dam or river.

Conclusion

There is no quick solution to dealing with water run-off. However, careful monitoring of developing wet areas, planning the correct treatments and then systematically undertaking the required measures can avoid a lot of grief in the future. It does not take much lost revenue from closure of the course to warrant the expense in overcoming wet areas. Special thought too must be given to the outfall as high flow rates at the time of heavy rains can be very erosive. Where there is no outfall the construction of wetlands or attenuating storage trenches can serve to cope with peak flow. While the law does not require provisions to prevent the natural surface water flow from high ground to lower adjacent property any measure to increase this rate of flow is of concern to the Environment Agency.

Gordon Jaaback

July 2005