If you’re reading this, you probably love water-harvesting earthworks. When done right, they can start repairing damaged land right from the first rainfall after they are installed. When done wrong, they can waste a lot of money, or even put lives and property at risk.
Here are six things to keep in mind when planning earthworks for a site.
1. Slow it down
The point of harvesting water is grabbing it and holding it as long as you can before it escapes you by going off your site. The longer that water stays on site, the longer it is available to you. Whether you are choosing open storage, or directing it into the ground, your aim is to make it stay around longer.
It’s worth mentioning a curious misconception that some people have. Some folks have the idea that if you slow water on your site, you are taking it away from anyone downhill, leaving them with less water. This is not at all the case at all.
Allowing runoff rather than capturing it is generally a no-win situation. To regreen a water-starved place, would you encourage water to rush down off the hills as quickly as possible so that all the downhill people could have it? This, in essence, would be designing the land to make water travel out to the ocean as quickly as possible. Not only is this a recipe for erosion, it is a recipe for turning permanent rivers and streams into ephemeral ones that only flow during heavy rains.
With earthworks, you are rehydrating the landscape. Rehydrating the landscape means more water available more often for the people downstream. While they might see a temporary reduction in surface runoff in the short term, in the medium and long term they will have more groundwater available.
Nature will throw enough adversity at you without adding to the list by creating your own disaster. A good risk assessment will help you to make wise decisions. Here are some questions to keep in mind before bringing in the machines.
- How might this kill somebody?
- How might this damage property?
- What happens if there is a catastrophic failure?
Though you may be eager to install a particular feature on a site, safety might dictate that you walk away and leave the site untouched.
3. What does the land want to do?
You might have some exciting ideas about what you would like to see installed on a piece of land, but it is not going to work out very well if the earthworks do not suit the site you are working on.
A great perspective to take when deciding what to do is taking the land’s perspective. What does the land want to do? To answer that, consider what the environment was like before human intervention. That will give you some insight into the most optimal techniques to use on your site.
What the land will do depends on:
- Soil type and depth
- Local flora and fauna
Consider what the site would look like if it were just left alone for 40 years. How would it change over time?
Once you know the direction the land will naturally head, you can choose an approach that will steer it in that direction, rather than fight against what the land naturally wants to do. Nature rewards partners and punishes dictators.
4. Think like water
Bruce Lee wanted you to have a mind like water. I want you to think like water.
If you are water there are a few things you will do such as evaporate, or freeze, but the most important one with respect to earthworks is that you travel at 90° to contour. To put it another way, you flow downhill. (It’s true that if you are flowing through something, you will favour easily traversed mediums like sand over something denser like clay. In this case when travelling through soil, you might go at an angle less than 90o to contour. For planning purposes, however, 90° to contour is the rule.)
With this in mind, how will you, water, travel across the land? What path will you take? Check the land. There might be clues as to where you have travelled before – evidence of erosion, or of sediment building up. This can help you to make decisions of what to place where.
How much water will there be at one time? Not only do you need to know where the water does and doesn’t go, you also have to know how much there is at one time. How big do the water-harvesting systems need to be? What happens if you get more water than expected?
5. Know what earthworks do what jobs
There are a lot of choices when it comes to earthworks, choosing the right one for a site means that your efforts will go further
If you read your site correctly, it will go a long way towards determining what you can or should do on a site. Soil type, for instance, will determine what will be effective and what will not. As an example, ripping with a subsoiler such as a Yeoman’s plow can be very beneficial. It would be pointless to run one on sand or on lateritic soil, however. In the former case, the sand will just fall back in on itself. In the latter case, the land will flow and “heal” the ripping with the first rainfall (assuming you had the horsepower to get the power to break through the soil in the first case).
Next, you need to know what each type of earthwork does. For example, swales intercept water, sending it into the ground. They are great for sending runoff into the ground and helping trees to establish, but they won’t do much of anything at all for garden beds.
Do you want to get more water in the ground, or do you want open water storage? The answer will depend on your climate and what you are trying to do. If you were in an arid environment, you wouldn’t want a lot of open water storage, which would just be subject to evaporation. If you were in a humid temperate or tropical climate, open water could provide backup irrigation as well as opening space for aquaculture.
6. How much money do you have?
You can make a grand plan, but do you have the money to pay for it? You might have to implement things in stages. Hiring a tractor to pull a subsoiler is very affordable. Digging swales is affordable and has a high return on investment, if you are using them for the right thing in the right place. Installing a dam is a higher investment that will take considerable machine time, and thus more money.
You also have a choice of machines to work with. The hourly rate of a backhoe is cheaper than that of a bulldozer. If you are in a humid temperate environment, you can cut in a swale with a bulldozer in a small fraction of the time a backhoe could. If you were in a semi-arid environment with hard, lateritic soils, even if you could cut through with the dozer blade, the swale you made would be far too small and would just be erased with the first big rain event.
Over to you…
What approaches do you take when designing water-harvesting earthworks? What snags and solutions have you encountered with your projects?
Boguslaw Samborski says
Good Evening Mr. Barnes.
I must admit that when I was reading your article, I was impressed by its strategic and thoughtful approach.
I am about to become an owner of a 566ha forest land in NSW, Australia.
Thinking what to do with it, I started to calculate how much rain that land get every year. And with an average rainfall being about 1.2 to 1.6 meter per year, I calculated that it is a massive quantity 5.66km2 covered by 1.2m of water a year. Well, a lot gets evaporated because it is a subtropical area.
The forestland is not develop and can be described as native with a altitude difference of up to 40 meters over the area that is lets say about 2km length. that makes about 3% on average but in some places can be 2-3 times as much.
The trees a ranging from 1 year to 40 years of age and the forest land was not thinned out at all.
That left in places more tree leaves on the ground than grass.
As soon I started to think how to harvest that massive quantity of water by slowing its flow and holding it for a while until the next rain comes, I came across the state regulation of water harvesting. It shocked me because what was left for the farmer out of the massive rainfall is a quantity of water that is almost insignificant to the total volume. But luckily it relates to rainwater harvest in dams.
It does not describe water harvested by formation of hoops and forest land roads which would be reinforced on the lower side and build up to hold water. The key to those roads is that they have to be perfectly horizontal otherwise they will only enhance erosion of soil.
Small but numerous formations like that and very evenly spread over the area would be actually more beneficial that few large dams and they would not fall into government’s regulations and restrictions of water harvesting.
Water is a treasure . Soil without water has no value unless it contains some kind of minerals, sands or valuable rocks.
I would like to hear your comments and suggestions how best to improve the water harvesting in a forestland that I described
It is located at 768 Coaldale Road – Fortis Creek , about 22 km north of Grafton in NSW, Australia.
Hoops up to 20cm high at the lower level of slope, around the tree canopy – lets say 10m diameter across would reduce the run off down the slope of most rains with exception of very heavy ones. Reinforcement with rocks and sticks to prevent soil being taken down the slopes would be also perhaps a good idea – like the beavers do.
The whole ground on the slopes would be wetter for longer period of time and if the soil would be mulched with mulch made from processing very small regrowth (up to 30 mm diameter) covered by chips.
That management would reduce fire risk substantially, especially if larger regrowth – up to 100 mm would be processed to wood chips.
This is my retirement project.
The land has got about 7 species of native eucalyptus. I am planning to add teak as an experiment and would like to grow it in an extensive, mixed environment among the existing trees after thinning of the forestland.
I would like to read your comments and suggestions.
Douglas Barnes says
Hello Mr. Samborski,
Assuming that Google Maps put me in the right neighbourhood, your property is already full of water harvesting elements. The trees that you have on site are doing the job of assisting in water infiltration on the site. If you are going to build access roads through the property, then your idea of placing them on contour will help. With a road, the removal of trees will be necessary. Otherwise, I would not disturb or remove the tree cover for the sake of earthworks. If you are looking for open water storage, you could put in a dam, which would necessitate an access road. Otherwise, I would disturb the forest as little as possible.
I can see what looks to be either an ephemeral stream, or a first order stream (on the Strahler stream ordering method) flowing into Fortis Creek. You could put in some gabion check dams there, but for anything more substantial, I would recommend an engineer.
I think I catch what you are saying regarding the hoops. On site, have a look for evidence of active erosions. Start at the highest point in the landscape where it is evident. These erosive sites will be keys to where to concentrate your efforts. As it is now, however, the trees are doing an excellent job of rainwater harvesting.
I hope that helps you out some.
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