Leaky Weirs on a Spillway
Background (you can skip this section to get to the insights)
A few weeks ago at Whispering Pines, disaster struck. The gate valve on the dam sheared off. To cut a long story short, the dam is now virtually empty. To fix the pipe, we need to have no water in the dam so that the pipe is dry enough to work with. To do this, we need to minimise the water going into the dam. Because water continually overflows on our neighbours dam higher up in the catchment, we have been trying to use siphons to bring down the level of this upstream dam and provide ourselves with a buffer to get our dam dry enough. Our first attempt had 1 L/s coming out of the siphon. Our second attempt had an extra 2 L/s coming out. With 3 L/s total coming out of the dam, we were able to stop it from spilling. For a few hours, at least. Then, we had a massive downpour. 70mm of rain over a couple of days meant that our siphons simply couldn’t keep up. Back to the drawing board. The problem with our original plan was that although we had the siphons dropping the level of the dam, the water from the siphons was still flowing into our dam. So, when we felt the time was right, we could stop the siphons and we would then have to empty our dam with a dirty water pump, then work on the pipe. Our new plan, executed in the last couple of days, was to have longer pipes which completely bypassed our dam, pushing the water into the spillway of our dam and flow down the creek. This way, so long as our neighbour’s dam level was lower than its spill point, we would have (almost) no water going into our dam. This would make it easier to remove the excess water and provide us more time to work on the pipe. After installing some longer siphons, we now have about 3.5 L/s flowing down our spillway. Owing to a large flood event in the 2000s, there is a headwall cut on our spillway which we are careful not to exacerbate. This unique set of conditions provided me with a great testing ground for what I learnt at a recent Mulloon Institute bootcamp at Kernot.
Key Insights
By constructing a series of leaky weirs in the flow line, we are able to hold the water higher in the landscape and slow the water down. In our context, with our fertile soils, holding the water back also allows time for the water to seep into the soil. Today, after setting up a second siphon flowing at about 1.5 L/s, I did some work to build some leaky weirs. I’ll share three key insights that I found when I was building them and seeing how they work.
1. Serving a similar purpose to a pressure reducing valve in a reticulation system
When I did some work designing the reticulation system for Blackwood Valley, there were some paddocks which had a large drop from the header tank. It was recommended that we reduce the water pressure because we would be exceeding the ratings on the pipe. We ended up using a pressure reducing valve for this purpose, but another option was to place a smaller tank lower on the landsacpe, and allow the water to sit in that tank before moving further down, effectively resetting its momentum at that point. It occurred to me today that this is exactly what a leaky weir is doing. Instead of allowing the water to flow straight down the middle of the flowline, gaining velocity and momentum as it goes, a leaky weir resets this momentum, de-energising the water and reducing the risk of erosion. Remembering from the Natural Sequence Farming course, water can be the most destructive thing when it is moving at velocity, or the most life-giving thing when it is still. As managers, we can work towards allowing it to express that life-giving force. (Although that sentence sounded a little too airy fairy for me).
2. Amazing how quickly water settles and makes an aesthetic change to the landscape.
Something I was amazed by was just how quickly water settles out sediment. The water coming out of these siphons was crystal clear because it was being taken out of the top of a nice dam, but when you’re constructing the leaky weir you are moving dirt and stirring up soil. This creates a murky, muddy puddle. But because the construction is built to keep the water more still, the sediment eventually falls out and the weir turns into this beautiful crystal clear pond. It made me think of the many rivers and streams you see driving around that are brown with sediment from upstream erosion. If we managed our landscape using leaky weirs and allowed this sediment to drop out, how much better could our water be, even in the muddiest of creeks?
3. Learning by doing: How much you can learn in a short period of time when interacting.
A third insight was the amount you learn by simply having a crack. My first attempt was to grab a shovel full of dirt and place it in the middle of the flow line, thinking that this would slow the flow. It may have slowed the flow somewhat, but eventually my shovel full of dirt was dissolved and being carried down the creek as the water continued to gain momentum. Clearly I needed something more solid, so I placed a piece of wood there. But even that began to look unstable, so I stuck some sticks in the ground as pegs for my wooden barrier. The water seems to flow quite fast over this barrier so I find some rocks to disperse the flow further. Sure, many of these concepts were taught in the Mulloon Institute Bootcamp that I took a few weeks ago, but there is nothing like working with the real thing to understand on a visceral level exactly why things need to be constructed in a certain way.
Conclusion
And so there you have it. The opportunity to take something you have learned in a classroom and apply it in the field is something I love about regenerative agriculture. These concepts have allowed me to take an annoying issue like a dam being emptied, and find some opportunities to learn about landscape function and rehydration.
Until next time.