Water Safety Measures for Your Backyard (Strategic Water Reserve for Any Garden Endeavor) Part 3
Don’t build a death trap.
Bodies of water above your head can hold tons of liters of water, which equals tons of kilograms. If their walls break, you are going to be smashed and washed away. In a home garden, people usually don’t deal with this dimension of swales, water tanks, ponds, and dams, but you could face blowouts in your system. Catching water to create a strategic water reserve is crucial for success, but it can be the thing that destroys your life if you don’t have the safety measures that prevent all that.
The safety measures are spillways, and here is how to install them yourself.
This is Part 3 of ‘Strategic Water Reserve for Any Garden Endeavor’, which is about a Water Flywheel. Link to Part 1and link to Part 2.
In this edition, in 5 minutes or less:
#1 After a dam burst or a 100-year rain event, the flood surge is dangerous because the engineers didn’t place safety measures downstream.
#2 How to calculate and place spillways
#3 How creating non-erosive water flow is an asset
After a dam burst or a 100-year rain event, the flood surge is dangerous because the engineers didn’t place safety measures downstream.
Spillways allow water to exit downstream, directed to a planned runway in the form of a gentle slope where it doesn’t pick up speed or cause erosion. The wall is not a safety measure, but the spillways and the slope where the water flows are, plus water pacifying features downstream.
No water-holding element should be designed where one failure would mean catastrophe. So, by placing multiple safety measures, you would rule out, by design, the possibility of a critical failure. That can seem logical, but it is something that is not practiced in the real world. Because of my background in medicine, I can attest that it even happens in health, so much so that one of the leading causes of death and harm is iatrogenic (meaning caused by medical treatment or intervention itself). That is relevant because it shows how we, as humans, create and put ourselves under avoidable danger, which needs to be avoided.
The causes of iatrogenic and other self-imposed avoidable harm are multiple and go far beyond ignorance and lack of skill. Actually, we have enough technology and skill, but as the systems become more complex and we couple that with the human element and our unconsciousness, we fall into avoidable death traps.
Never create a death trap by designing water bodies with correct safety measures:
- Walls properly built
- Spillway of the right size and leveled
- Gentle slope where the water from the spillway flows in a non-erosive way (sheet flow)
- Place other water pacifying features downstream accompanied by their right-sized spillways and gentle-sloped water runways afterward. Some are:
- Check dams - Small barriers across waterways that slow flow and create pools
- Riffle-pool sequences - Rock formations that create turbulence and energy loss
- Constructed wetlands - Vegetated areas that absorb and slow water flow
- Buffer strips - Vegetated zones along waterways that reduce velocity
- Gabion walls - Wire mesh containers filled with rocks that slow and filter water
- Baffles - Deflecting structures that redirect and slow water flow
- Swales - Our multifunctional tool
How to calculate and place spillways
First, identify the area where precipitation collects and flows into a pond, swale, water tank, etc (known as the catchment area). By knowing the catchment area, you can discover the total volume of water that will flow through the system by:
- Catchment area (m²) × Height of rain that falls (mm) in 24 hours of the biggest possible rain event ÷ 86400 (to discover the flow of water in seconds) = Maximum liters of water received by the system in a 24-hour event that must be drained per second
Very important warning: If you are connecting multiple ponds, swales, or water tanks to other ones, or if you are placing a swale downstream to pacify the flow of water, the catchment area of the downstream swale is its catchment area + any other catchment area from upstream structures.
I want to stress that if the spillway fails, meaning that the volume of water that it must drain exceeds its capacity, you will face danger because the water level will rise in the swale and spill over its crest without a controlled flow of water, causing destruction of the wall, erosion, and flood surge. In conclusion, the spillway must be able to handle the whole volume of water, which means that it is best practice to overestimate if in doubt (and double or triple check).
So, the maximum liters of water received by the system in a 24-hour event that must drain per second determines the size of the spillway.
For example, you may get the value of 3 liters per second that your spillway must handle in the biggest possible rain event. The second part of the question is how to figure out the size of it to handle this volume.
The spillway will be like an indentation in the wall of the swale, with length, width, and depth. Our teacher Geoff Lawton taught us an infallible trick to easily determine its size using 1L water bottles as a gauge. In this case, simply place three 1L water bottles side by side to create the right-size spillway that drains 3L of water per second. To finish it off, it is essential to make the spillway exactly level. Last but not least, prepare a gentle slope to maintain proper non-erosive water flow (sheet flow).
A slope of 3-5% gradient (the elevation drops 0.03-0.05 meters for every 1 meter of horizontal distance) is perfect. Everything exceeding 5%, you will need to place water pacifying features to dissipate energy downstream.
If water builds speed again, you set the trap once more.
How creating non-erosive water flow is an asset
Swales stop the water and take out their destructive force. Once the water leaves the swales by the spillways, you have to create a runway for it on a gentle slope so that it continues to be in a pacified state.
Geoff Lawton says that after a rain event when all your swales are charged up, you should be able to walk alongside the flow of water. That is how slow the velocity of the flowing water should be to not wash away soil. That also guarantees that in an extreme event, like a flood, you are safe. In that circumstance, instead of soil being washed away from your garden, this incoming water would be dropping soil at your place that it washed away from somewhere else. Not only will you have safety, but it will also harvest soil for you.
A swale is not placed isolated from a Whole Site (Garden) Design and it is not complete in itself. Leveled spillways and proper slope make up the swale anatomy, and together they accomplish multiple jobs. Swales are tree planting systems par excellence, but also work exceptionally in other ways to stabilize the site.
Safety, by pacifying water and maintaining non-erosive water flow, is also their job.
Before, you were vulnerable to building yourself a death trap.
Now you have full control of your safety. Both having a strategic water reserve that makes you drought-proof, and installing spillways to prevent water blowouts. Furthermore, you understand how to pacify fast-running water.
You can size your swales, ponds, dams, and water tanks, but you must build an adequately sized spillway with proper water pacifying features.
