Water Harvesting Earthworks “Design To Reality” . Part 2
How to determine a starting point for surveying:
For me the starting point to survey is often a dam for high water storage.
Dams are a major investment and it’s a must to have specific characteristics to construct the dam particular to that site.
What I look for is the shortest dam wall possible to hold the most amount of water while staying as high as possible.
These major features dictate the height you work with. We call it the ‘high-water mark’. Its basically where water will sit when the system is full.
The height of the high-water mark will give us our free board height for the dam wall and the spillway height. That height can change depending on available material to construct the dam wall to that height and above that even for our free board on the dam wall.
Hard-surface runoff areas
Roofs, Roads, Concrete, Pavement, Paths and Rocky out-crops.
Below hard surface run-off areas are a great place to start any earthworks because you know you have ‘x ‘amount of water coming in with ‘x’ amount of rain you receive. Is this a good thing, or something to be concerned about?
Culverts on roads or drive-ways are a key feature to look for.
Why is that? They concentrate the water flow causing huge erosion problems below them. They are basically a shot gun!
Look at the valley above the culvert and then looking below you will see clear signs of the effects – i.e. volumes of water at high speed.
In a sense we have a moral obligation to turn the problem into the solution. (I always get excited to find these features on a new site!)
Observation is key! You can see clear signs of water moving through the system when we train ourselves to see these patterns.
(Just like a tree flagging from a prevailing wind).
The pattern I’m looking for is organic matter, silt, sand and gravel deposits. These don’t show on a contour map but are great indicators of water movement.
As a designer we can grab these opportunities and invite them into the system.
Table drains only catch half the water from a road as generally there is a crown in the road to shed water, but again,
some BASIC MATH…
10-meter x 3 meters with 100 millimetres of rain = 3000 litres. When you take out 15% for evaporation and surface tension for a gravel road you will be harvesting 2550 litres.
(That’s a lot of water coming off the road and worthy of earthworks to harvest the surplus runoff to storage and soak into the landscape!)
If this is the highest feature giving the site the most water, that’s my starting point of surveying, and that’s where I take my first height from and transfer it around the landscape ideally into a dam.
So, you could say I have two starting points when I’m designing and on the job installing major earthworks on a site. We always need to keep an open mind to new opportunities that are presented.
(There are so many variables!)
Key points in the landscape
Not all landscapes have key points to work from, but even if you are in the right landscape, not on all sites you work will you find a keypoint for a dam!
But if the site you are designing does have keypoints, then height is the central theme of the design for me.
Its where we can store the most water high in the landscape that will be keeping cost down for the client.
We take a height at the keypoint and transfer that exact height out either side of the valley, that starts to create a footprint of the Dam.
This height dictates any earthworks associated with that system as I generally install back-flood swales to key point dams. Alternatively, you may want to install a diversion drain, a road swale or a gradient road.
(The dam should be built before considering this level line to be set in concrete!)
One of the easiest ways I think to get a mental picture is just to think of water when the system’s full. Where does water sit? Where does water exit?
Why? Because this start point of your level line is going to be your spillway height AND it’s going to dictate your freeboard height.
Now that we have the water you don’t want to let it go.
We want it to perform as many duties as possible. We want it soaking into as much landscape as possible.
How? We have it overflow at our desired point to be picked up by another series of earthworks that puts it into storage systems, and the surplus into soakage’s or just into soakage systems, but we just need to identify the best positions to spill that water.
At times there are significant volumes of water moving over these spillways, so correct placement is essential!
(I hope you can start to see my design theme reasoning. It’s the storing of water in dams as high as possible, with as little earthworks as possible and as economically as possible.)
My aim first with any design is always the designing of water into landscapes. In the end, water is life and life’s most basic requirement.
Scale is a issue we need to be smart with design, doing the least amount of work for the greatest good.
Justifying our choice of placement and the amount of earthworks is to me, very important. I rarely put in a swale that’s not part of a broader plan to direct the surplus water to dams. There are many options to consider before I put in swales as a restoration practice.
Having taught and worked on various projects extensively within Australia and internationally, such as Morocco, Jordan, Palestine and New Caledonia, David Spicer has covered a broad array of different soil types, topography and climatic zones.
David is a valued member of the Permaculture Sustainable Consulting team headed up by Geoff Lawton.
He is a master of practical and logical mainframe permaculture design which allows him to give his extensive experience of life and the cost involved to change a site. He has majored in the design water harvesting and storage earthworks which frames all regenerative farming, fo more information you can see his work on docspicepermaculture.com
David has the distinction of being Registered Teacher #5 with the Permaculture Research Institute of Australia.
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