How to: Water Harvesting Earthworks "Design To Reality"
Updated: Jan 6, 2019
So, you've been contacted by a client and you've discussed the client’s brief, you’ve started to look at the contour map, aerial images, whatever data you can find on the site and about the site, and with the client brief in mind always considering WATER IS LIFE you set to the task of patterning the landscape using functional forms.
You start to look at what's the most economical way I can hold water in the landscape, move water around the landscape passively and make it perform as many duties as possible before it leaves the site.
Next is to develop the Mainframe design theme. The big part of that theme is looking for high water storage sites and so we take that approach of looking at our contour map to take into account where the highest possible spot I think we can safely store water on the site in dams?
(Always considering how much catchment area is above the potential dam site or is there any hard-surface run-off areas above the dam site?)
Reader's will understand catchment area, but hard surface run-off areas aren't so well utilised and it's just a bit of pattern recognition to identify when you look at a new site.
Identifying hard surface run off areas. These hard surface run-off areas with a bit of design thinking can be harvested and brought into our water harvesting systems. At times it does require good observation skills to identify them, but there are generally clues for the observer.
There are many examples of hard surface run-off areas, sometimes called ‘hard-ware.’ Think of your roof, a road, any compacted surface or a rock outcrop. All run off water.
Gravel roads runs off 85% of the water that hits the surface, concrete areas 100% minus whatever evaporates, and your roof 100%.
These areas can represent a significant volume of water to be harvested and any designer should pay careful attention to these hardware runoff areas.
BASIC MATH: 1 meter x 1 meter x 1 millimetre = 1 litre
(Consider roughly $3 a bottle if bought in a shop!)
Site survey is critical...so, does the design fit?
Surveying the site...
This is one of the most critical parts of the design to installation process, surveying the site to know that we can harvest water in Swales, Roads and Terraces, directing the surplus water toward dams.
Things that we're looking at when we are surveying the levels out in readiness for the earthworks...
Difficult slope/s not seen on the contour map. We must survey a line out to see if it’s possible. Ask ourselves if it is safe to do the earthworks on that slope for the site and any machines utilised. Consider whether there are any rear or endangered species at that height, or there could be a rock out-crop that would require us to adjust the Height of the level we are working with.
(That's why I like to say, “Design to reality!”)
Why? Because when we start to survey out heights and we survey around the landscape, we start find out (in reality) how we can pick water up from one catchment area or from some hard surface run-off and bring it around all on one level and put it into the dam.
At times it’s just not possible to do this on just one level. We must walk the water into the landscape by placing a swale or some other impoundment to hold the water than over-flow at the desired point with our spillway to be pick up by the next swale line lower down the slope and into a dam before it over-flows.
By surveying the landscape it often highlights other opportunities that you didn't see within the subtleties of the landscape itself.
Landscape form is hard to see with strong day light. If you want to look at landscape to get the best representation or image, early morning or late afternoon, or a full moon is the best from my experience.
It's a very soft light you’re after and it will show you some things in the landscape that you will not see in bright daylight, which of course open to presenting other opportunities.
A level line can be very deceptive and the contour map is just the guide.
- Remember, don’t get discouraged by your design not fitting on the landscape! Adapt it. I always do!
- Your design is merely the theme. Don’t get caught up on imposing your design on a site. Allow the design to evolve as new information becomes available, and don’t trust your eye.
I have done many jobs that I've looked at a level line and thought
(that can't be right), but it is. Double checking your heights is always a good practice.
I always request photos from my clients after large rain events when I know the system is working, and it is working with often very subtle height differences imperceptible by the human eye!
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 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 our selves 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 = 2550 litres minus the 15% for evaporation and surface tension if gravel road.
(That’s a lot of water coming off the road and worthy of earthworks to harvest run the surplus to storages 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 a 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 all sites you work on are you going to find a keypoint for a dam!
But if the site you are designing does have keypionts, then height it 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.
It is at times 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.