How to move the earth
When you have enormous amounts of water moving over your block, good earthworks keep everything calm.
Good earthworks that can ‘calm’ your block
Earthworks are all about dealing with water, says veteran digger driver Kevin Adamson. Earthworks must be shaped to shed water before its powerful erosive force can gather momentum. Shed water “little and often” is Kevin’s pithy advice. It’s not rocket science either, says sustainable home and landscaping consultant, and lifestyle block owner, Daniel Woolly.
“It’s common sense, but that seems to be increasingly uncommon. Conventional ways of managing water on roads, and in the landscapes, often see water as a problem to be offloaded and measures are taken to get it away someplace else as quickly as possible.”
He believes in a different approach, based on best-practice permaculture design. It means creating a landscape where water travels over the longest path possible, on the lowest gradient, dispersing over the widest possible area.
Daniel and his wife Bena Denton have learned a lot from their own experience with the steep, wet, bare block they bought in 2011. Their house faces north and is tucked into the landscape to protect it from the area’s strong winds. It overlooks terraces and swales created to form a sheltered orchard space.
The couple has a lot of water to manage. Taranaki is known for its high rainfall, up to 2m a year. The house sits below the driveway, which enters the property at a low point in the road. It collects “crazy quantities” of runoff from their uphill neighbour. With two young children and a business they run from home, the couple wanted to be sure their driveway would be passable, no matter how heavy the rain.
Eight years and lots of learning later, Daniel says two principles stand out when it comes to protecting soil against erosion from water.
1. Design for failure.
2. Understand your levels. To achieve this, Daniel likes to use water structures. These are earthworks that hold, move, infiltrate, and release water. On his block, it includes swales, level sills, drains, culverts, and soak holes.
All of these structures will fail at some point, says Daniel.
“That’s not a problem if overland flow can be designed into the system. Even drier parts of the country are seeing more extreme storms; we need to create earthworks with the capacity to handle larger water inflows.”
Daniel and Bena’s driveway is a good example of designing redundancy into a system. The original was constructed by a major roading company, and you’d think that would be a good thing.
No, says Daniel.
“They were wildly incompetent, which was a great way for me to learn.”
In its second iteration, the driveway snakes along the land’s contour and has the lowest possible gradient, falling just 1.5m over its 180m length.
Runoff from the neighbour’s property, above the driveway, is collected in an open soak hole that can hold 100,000 litres; it can fill after just an hour of heavy rain.
It empties through two culverts stacked on top of each other. The top culvert only starts working when the first is completely submerged or blocked.
The culverts feed into what Daniel calls a ‘koru swale,’ an ingenious water structure he designed (pictured above). The water falls from the culvert into the ‘round’ part of the koru. It hits still water, which dissipates the force. The now calmer water disperses through the wide ‘tail’ of the koru. From there it gently flows over a level sill, down the hill.
Constructing these earthworks took careful thought but no earthmoving machines. It was created mostly by hand.
“Before we built the koru swale, there would be deep erosion cuts every time it rained (pictured at left). Now there’s none,” says Daniel.
Rainfall runs into a drain on the high side of the driveway. The two lowest areas of the driveway are the weak points, so Daniel created a 2m-wide, level sill to protect them. It’s like a reverse speed hump, slightly below the driveway’s
All of these structures will fail at some point.
surface and level along the downhill edge. It’s lined with filter fabric and covered with gravel.
The sill is another level of redundancy. If it was ever overwhelmed with rain, says Daniel, the water could only scour out the gravel because the earth base of the driveway is protected by the fabric. Another option would be to concrete this section, but it hasn’t proved necessary.
From the sill, water moves over the lip of the level spillway. The filter fabric – now covered in grass – extends over this lip and down to the bottom of the slope. The grass flattens out when large amounts of water move over it, springing back up and continuing to grow when it’s dry.