Cleaning out a charged downpipe rainwater harvesting system

Our raingardens and tank have been operating for over three years now.   I have been too busy to do the stocktake on performance that I have been intending to do for the last 6 months, but even without doing so, it is clear that the impressive growth of the grapevine this summer has substantially increased evapotranspiration losses from the back raingarden, contributing to improved performance of the system.  But that story is for another day.

Today, I’m writing about another job I had been intending to do for a couple of summers now: flushing out the charged downpipes.  I tried to do it last summer, but was unprepared for the difficulty of removing the cap at the downslope point of the charged system.  This summer, having secured the right tool, I have succeeded.  Given the difficulties I struck, I thought it would be worth documenting how I’ve done it.

Advice that I have heard is that charged systems should be flushed every year, so I was getting increasingly concerned that 3 years was too long.  But, it seems, in our inner-city environment without a lot of overhanging vegetation (other than an annoying ornamental grapevine that grows up into our gutters at the back from next door), the build-up of sediment has only been moderate over 3 years, so I’m thinking that I’ve lucked on about the right frequency of flushing for our system.  Here’s what I did. 

[Before I start, I should emphasise that a major motivation behind all of the following is to make sure that all of the water from the downpipes and the flushed water stays on the property rather than being allowed to drain to the street drainage, which would, of course, negate the primary purpose of our system.]

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1. Emptying the downpipes.  Summer is the time for this job because I want the front raingarden to be empty enough to receive the water sitting in the downpipes.  To drain them, I attached a hose to the charged system outlet and fed it into the  inspection well in the raingarden.  I let this drain slowly overnight.   Ideally by the time I got to the next step, the flow would have been reduced to a trickle, but an annoying, unexpected 0.4 mm of rain in the morning meant the raingarden had almost filled before the downpipes were completely empty.
2. Preparing a trench to receive the flushing water.  With the downpipes emptied, I still needed a place for the flushing water to drain to. So I dug a trench to direct the water to the front garden, and keep it away from the house.  Without the ideal equipment to hand, I improvised, and used the market umbrella cover as a makeshift gutter to keep water from the house.  The still-emptying downpipes gave me a chance to check that my levels were right to allow the water to drain to the garden.
3. Removing the cap from the outlet.  This is where I got stuck last year, but this year, I sourced a cap remover, a lever with a cloth strap on the end that you tighten around the cap to more easily remove the cap. It has also been brought to my attention that removing caps like this is much more difficult if there is still a head of water behind them(hence step 1).
4. Flushing the system.  I placed a hose down the pipe that empties into the tank and turned it on maximum pressure.  This took a surprising minute or so to make its way to the open outlet at the other end of the house, where it carried some fine mud in dribs and drabs.  Not a huge amount.  After the flush filled up the trench, I let it drain and repeated the flush.  This time the flushed water ran clean.
5. Cap back on and wait for it to rain again.