Under the weather

By: Andrew Steens

The recent brush with Cyclone Oma turned into a bit of a fizzer, which was fortunate as my niece’s wedding was timed for the day it was supposed to hit. But the moderate amount of rainfall that the associated weather pattern caused was just what the gardener ordered for parched lawns and dry gardens.

The advent of cyclone season does cause a little nervousness, though. Here on the flat peat soils of Point Wells, we are susceptible to temporary surface flooding, particularly if there is a king tide. Other parts of northern Rodney can also flood, as long-time residents of Omaha, Whangaripo and Kaipara Flats all know well. In fact, given the right conditions, just about any property can flood. Water has an uncanny ability to find the lowest point or become dammed by some stray object.

With intense rain events becoming more common, property owners are well advised to be looking at drainage on their properties and the surrounds. Next time there is a decent amount of rain, grab your umbrella and gummies and head outside. Areas that are pooling in a moderate amount of rain may become flood issues when there is intense rain.

There are many methods of dealing with poor drainage and flooding. For example, gardeners can grow vegetables and even ornamentals in raised beds. I have all my gardens raised about 15-20cm above ground level. This avoids roots becoming waterlogged and oxygen deficient, which in turn helps the plants withstand winter cold stress and disease. In this situation my paths act as drains, and as the surface flooding drains away relatively quickly, this is not a problem.

Flat lawns, paths or driveways can be sloped slightly with a 1 to 2 per cent gradient so that water runs off. If this is not possible, then an alternative is to cut a swale alongside the lawn or path. Simply put, a swale is a very shallow drain that is usually still grassed. The angle of the cut can be so low that on casual inspection the area seems still flat, but the swale is enough for water to collect and move away.

Sub-surface drains such as perforated PVC pipes, clay drainage tiles or gravel drains are very effective for combating soggy soils. These are typically laid a few centimetres below the soil surface at between 2 per cent or even up to 5 per cent gradient. Mesh ‘socks’ or geo-textile mats placed over the drain are helpful where soil or tree roots are likely to block the drain over time. Larger drainage volumes can be dealt with by bigger pipes, overlaid with gravel.

A surface drain is almost unbeatable at moving volume, although care needs to be taken in the design to ensure that there is enough slope, so standing water doesn’t attract mosquitoes and is safe for children. Commercially available surface drain channels with grate tops are relatively expensive, but very effective and relatively easy to install. A low-cost alternative, but much less attractive, is to just cut a drain into the ground with a digger or spade. However, this only works where the soil is stable enough to form a wall without collapsing.

Next time it rains heavily, it would be a good idea to also wander out onto the road (look both ways first!) and see how the neighbourhood drains are working. You might be surprised to see how much they can back-up. Perhaps they need re-digging or cleaning, or maybe they have been badly designed. A call, or several, to Council is a good idea to reduce the risk of this becoming the weak link in your drainage plan. If all else fails – the ducks will love it!


Andrew Steens

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