Sunday 20 November 2016

Stabilising Slopes

With the current hiatus in finishing the build, (is a house ever finished?), I can now start considering the immediate problem of stabilising the large areas of sloping, rock strewn bare hillside on the site.

There are basically two choices - Terracing or not terracing.

The advantage of the traditional terracing approach using, say, stone walls is that not only is it attractive and in keeping with the hillside  but it also helps water run off by allowing rainfall to seep into the "flattened" areas of the soil at the top of the walls.

The cost of building such retaining walls however is high since to build even the most basic suitable dry stone walls, that allow the water to seep through, requires both skill and heavy labour.

I have had to have a couple of such terraces created at the rear of the house in order to stem the flow of water down the hillside and divert it away from the house. Unfortunately there has been no rain to consolidate the newly built walls and a herd of wild mountain goats decided to take their summer vacation on our hillside and, whilst looking great clambering across the rocky terrain, have damaged the unconsolidated dry stone walls in several places. An early priority, therefore, has to be the erection of a goat proof fence between the house and the mountain to deny such destructive access or an awful lot of "Goat from the oven" on the menu.

In view of the cost I have been considering other options to terracing with dry stone walls in order to slow the steady erosion of the hillside.

For large areas of sweeping hillside it is possible to dig a series of swales across the hillside on contour and pile the spoil on the downhill sides of the swales to form berms. These berms can be planted with suitable deep rooted trees and shrubs. (see below) The overflow from the uppermost swale can be run, via, say, a rock based mini stream bed, to run down and form an inlet to the swale on the next contour level and so on down the hillside. This system can then be made more sophisticated by introducing simple wooden "gates" at the overflows to hold water longer in particular swales as required.

The above approaches will slow the natural erosion and allow suitably positioned trees, shrubs and plants to further reduce the water and soil run off. Correct plant selection is vital as it is only the deeply rooted trees, shrubs and plants that will produce the required stabilising effect upon the hillside. Perennials that have deep root systems, well rooted grasses and small and medium shrubs perform this function well. For Crete all species planted on an exposed, south facing hillside, as is my case, will also need to be drought tolerant as there is normally very little rain from May to October and summer temperatures are high.

Given, that the larger the root system the better, trees are obviously an essential component of any plans with their extensive root systems. Large plants, shrubs and trees should be planted vertically not pointing out from the hillside. Planting vertically makes each plant into a mini terrace by building a small half wall into the hillside that helps retain the water and allows it to soak into the soil around the plant. Using smaller plants and ground cover simply planted flat with the hillside to fill in between the major stabilising species.

Even when using drought tolerant species there will still be a need to irrigate for the first couple of years until growth has been established. This is best done by drip irrigation as regular hand watering on large steep slopes is not realistic. I therefore need to plan how best to irrigate any slopes that are planted up using the limited amount of water I am able to store over the winter and early spring, which is when the spring water supply significantly exceeds our household needs.

A typical view across the site showing the range of slopes that need stabilising.

Another view of some of the slopes to be stabilised.

A couple of newly built  dry stone terraces on the hillside above the house.

Awaiting the rains to settle and stabilise the dry stone wall. Note the water system overflow pipe moving any surplus water away from the rear of the house.

Looking west along new drystone wall.

Looking west along same dry stone wall, as above, after the mountain goats have "summered" on the plot!!

Another view of goat damage.

Self explanatory!