At the beginning of this year I had the pleasure of designing and implementing on a property in Leongatha, Victoria with my mate Dan Palmer.

Whilst designing the water harvesting systems, we were acutely aware of the trend towards a drier landscape, whilst still dealing with periods of problematic waterlogging and leaching. 

This next generation of Permaculture designers has the wonderful benefit of being able to learn from a whole range of experienced designers and practitioners. Dan and I have done this, and the system we created definetely involves a fair bit of cross fertilization. 

The swale/drainage system I have described below allows us to catch water in our drains and infiltrate it as a normal swale will do. But through design the same trench can quickly become a catchment for the dam or a drain to prevent waterlogging and leaching with a couple of other tricks thrown in too. Hope you enjoy, Cam

FIGURE A

An explanation of some of the major features.

1. Top of swale mound level all the way. When your trench is full, this prevents the water spilling over at any point and washing away your mound. You must ensure that the top is a good distance above the level sill spillway to prevent this occurring. Also ensure that you wrap the mound around at the end of the swale and the sides of any crossing pipes. When the swale is full, this prevents the water from creeping around the side and eroding the mound and downhill area.
2. Cut at the front edge dead level. As water rises within a swale, it obviously does so dead level. It is also obvious that the water will soak into the loosened soil of the mound faster than it will the undisturbed soil below the cut. Hence, if the cut at the front edge of the swale (the point of contact between original ground and the mound) undulates even slightly certain areas of the mound will become far wetter than others.
3. 1:500 fall on base of the trench. This fall gives us 2 main capabilities: 1. To divert water to a dam as it’s first function; 2. To drain the slope and prevent waterlogging in periods of very high rainfall
4. Adjustable pipe. A side view of this can be seen in Figures C & D in which a pipe leads water from the trench, underneath the mound, and sheds it down hill. Using 90mm PVC, the end of the pipe inside the trench can be fitted with an unglued 90 degree bend with a short upright. The length of this upright will determine the height you want water to fill to in the swale before it trickles out (Figure C). During periods of heavy rain, where water logging is a concern, because it is unglued the bend can be laid down on the floor of the trench, effectively converting what was a swale in to a drain (Figure D). 
5. Trench/Dam division. This acts as a barrier preventing the dam level from dropping as the swale area infiltrates. This can occur if the division is fitted with a pipe which can be closed off. The pipe should be baffled and compacted in properly. This divider can become a useful access point.
6. Trickle pipe for dam. A tee with an open upright. The open upright should be set just below the height of the spillway. This is advantageous when the dam is filled to capacity with a continuous low flow entering from above. The trickle pipe allows the water to exit the dam through a pipe rather than flowing over the spillway. Small constant flows over a spillway are often the most damaging as they create rivulets. It is this compromised area which will be eaten out and magnified greatly during the next large event over the spillway. Maintaining a spillway with a nice wide horizontal surface, free from defects or obstructions is essential.
7. One way valve. This allows water to flow from the trench into the dam. If desired, both will fill at the same rate. The benefit of a one way valve is that as the swale soaks into the ground, the valve in conjunction with the divider [A5] will hold the water in the dam to the maximum level desired.
8. Level sill spillway. A wide flat surface. The size of the spillway required will be determined by the amount of catchment above (Construction of Small Earth Dams by K. Nelson is a good reference for determining this). It should always be constructed on original ground. If it’s built out of fill it will disappear the first time water spills over. The spillway can be positioned at any point along a swale, but ideally should be situated on a ridge. This causes the water to fan out rather than concentrating as an erosive force in a valley.

FIGURE B

Situation: Dam level is low. With it’s 1 in 500 fall towards the dam, the trench first acts as a catchment. The water flows through the one way valve [A7] and spills out the end into the dam.

FIGURE C

Situation: Adjustable pipe [A4] in upright position. Once the dam has filled to the height of the bottom of the trench, the entire length of the trench including the dam will rise as one. Effectively this is one large swale. It will continue to rise until it reaches a point it can exit. This first point is the adjustable pipe which exits the trench [A4]. (large flows are considered in Figure F)

FIGURE D

Situation: Adjustable pipe [A4] in lowered position. The trench is now effectively a drain. This can be very useful in climates where waterlogging or leaching can become a problem. The water can be caught in the trench and diverted sideways, effectively drying out the slope below and preventing it becoming waterlogged. The beauty of the adjustable pipe is we can control the amount of water we are infiltrating into the soil. Hence, during drier periods, we can choose to capture every valuable drop and store it in the soil for the use of tree crops.   

FIGURE E

Situation: Option to fill swale from dam. If the dam is filled to capacity and exiting the trickle pipe, rather than it flowing downhill and away from our site, we can choose to store it in the soil. By opening the one way valve we can send the dam water up the swale, allowing it to infiltrate for the use of the tree crops below.

FIGURE F

Situation: Large inflow from above. When the inflow is too large for the trickle pipe to handle, the entire body of water will overflow at the level sill spillway [A8]. You must ensure that the trench/dam division [A5] is higher than the spillway. However, you must also ensure that this divider is sufficiently lower than the top of the swale mound [A1] to ensure it doesn’t wash over and erode away.