Conservation, Courses/Workshops, Dams, Earth Banks, Gabions, Irrigation, Land, Limonia, Rehabilitation, Roads, Storm Water, Surveying, Swales, Water Harvesting — by Steve Grace May 12, 2011
The sun works on an 11 year cycle over which it radiates heat at varying levels upon the earth. The cycle is observed by counting the frequency and placement of sunspots visible on the sun. Currently we are at a peak of the cycle whereby the sun is radiating a maximum amount of heat and energy. This means increased evaporation off the oceans’ waters and therefore increased precipitation over our lands. When the sun moves towards its less generative stage of the cycle, less evaporation occurs, which means less precipitation and impending dry conditions.
And so the rains have come down upon Zaytuna farm — 111mm in 5 days. The dams are full to the freeboard, the swales are soaking in the sediment, the spillways are spilling, the swivel pipes are swivelling, the soil is having a regeneration party, and the plants are just hangin’ out doing their thing.
And the earthworks have been stopped in their tracks….
Everyone was looking forward to seeing the excavator in action – but you can’t argue with the elements and in the end it was just too wet to get the excavator into an operational position.
So we had to revert to the resources available and build our own ‘miniature mainframes’, digging away with our hands in the dirt to apply the new skills we had developed over the five day course. Earthworks is easy – in theory….
Valley, ridge-point and saddle dams, all with level sill spillways and adjoining swales, along with gabions and limonia systems, were carved out by hand with varying degrees of success. It turns out it’s not so easy to make water do exactly what you want it to, but all the design elements are now firmly engrained and the course has very thoroughly covered all the aspects of a successful implementation of sustainable earthworks.
Understanding the flow of water across the land is the core of a fully sustainable system. It is essential to create a mainframe infrastructure that maximises the use of water between the moment it enters our site through to the moment it departs. This is the main objective of earthworks and we can achieve it by slowing down the flow of water through the thoughtful arrangement of various water catchment and distribution methods. Dams are designed to hold water on site, allowing it to be stored for various uses including irrigation, anaerobic breakdown, soil rehabilitation, thermal mass and aquaculture.
The construction of a stable dam wall is critical to the success of the dam and this is achieved by cutting out a large keyway trench that runs through the centre of the wall. After the precious top soil has been first removed and carefully stored away for later use, the keyway is then filled with clay material that is dug up from the dam water catchment area and heavily compacted until the entire core of the wall is set, fully compacted with good, water-impervious, clay soil. Before completely filling the keyway it is always a good idea to dig a narrower adjacent trench to house a dribbler pipe fitted with baffle plates, which runs from the catchment area through the wall and out, to allow for excess water to be relieved in times of heavy rainfall. The core is then further compacted upon using additional clay soil if available or otherwise good quality sub soil. The top soil is then laid out once again over the top to seal the deal. The catchment area of the dam is then sealed through compaction. A level spillway is created which is designed as an overflow system once the water level reaches a specific height, preventing water from reaching the top of the dam wall and thus compromising the integrity of the wall.
Often designed in direct connection with dams, swales are trenches cut in to the ground along contour, so that the water is soaked slowly and at a constant level into the landscape, creating a self -irrigating, nutrient rich tree planting zone, which rapidly increases soil regeneration and prevents soil erosion. Pipes can be used to great effect across both dam and swale systems to increase the control of water across the site. Piped crossings allow water to keep flowing below access paths and swivel joints are useful when connected to the ends of standard pipes to allow for the diversion of water in specific directions.
Gabions are stone walls built across flowing water ways and are designed to hold back the water flow, creating silt beds which generate a high density of nutrients that are slowly released through the water passage into the surrounding landscape.
A Limonia is an ephemeral lake system formed by building an earth wall around a existing large hard surface such as a mountain base, large rock or a car park in an urban environment. The wall prevents penetration of runoff from the object creating a temporary lake, which proceeds to soak into the soil producing a rich propagation area.
Locating appropriate dam and swale positions on a topographic map as well as surveying the area to mark out the perimeters of these positions and making calculations of predicted water catchments and dam areas were other important skills that were learnt during the course.
Access to the site is another central aspect covered during the course. It is favourable to establish access that is on contour where possible or otherwise located along ridgelines. This creates safer, less erosive access ways that also maximise water runoff into pit and pipe systems that act to harvest surplus water and direct it to good use across the site.
In the end the rain proved beneficial in that we had a lot more time to cover all the course material in finer detail, rather than racing around chasing after the machine.
So now we are accredited Swalians, ready to go out and conduct reconstructive earth surgery across a land that is in dire need of some TLC.
- PRI Ridge Point Dam Earthworks
- Letters from Sri Lanka – The World’s Largest Water Harvesting Earthworks Project