Sudden heavy rain and flooding

Not only bushfire, but another threat to safety

Late in November 2017 I had been doing research about rainfall events and the effects on water flow of the stream at our place. I’ve been recording rainfall most days in a garden diary since buying here in early 2006 as well as temperatures. Sometimes heavy rains have coincided with photos of the stream and the flows.
Generally we have noticed over the 12 years we’ve been here that the catchment is very responsive. Some heavy rain will be followed within a few hours by a rise in the level of the stream. A real cloudburst, say of 20 mm in an hour will show up within a short time of a few hours. This is not the effect of spring water, it is surface water flowing down hillsides and into the stream.
On Saturday, 7 September 2013, the day of a Federal election, I spent the morning at the Balingup Primary School handing out electoral material. Late morning large clouds gathered and we had a torrential downpour that lasted maybe an hour or so. When I returned home by about 1 pm I checked out the stream once the rain had stopped. It was running at the highest I have seen it, with water being almost over the bridge.

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In this flash flood after heavy rain, the level was about 1m above the usual level. It was lapping up to the bridge. A few minutes before it was even higher.

See first photo, taken at 1.23 pm. The second photo shows water roaring out of two medium sized pipes.

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Water was also pouring out from two other points, namely via the spillway under the bridge, and a channel nearer the hillside that goes under the track to the hill. See third and fourth photos for the latter. See how gravel has started to be washed off the track and down its sides. Water can be very destructive when it floods.

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Then from near the bridge I videod the scene, taken at 1.26 pm. The level had been even higher, it had gone over the track leading up the hill. Note the rather worried-looking dog, Joanna, who wasn’t very happy about the flood.

With the video, note the sheer volume of water going through, it pushed a wooden platform/deck into the stream, so it has power.


I checked today with one of our farming neighbours as to whether this sudden rise of at least a metre within an hour or so of the downpour might be due to a dam release. She was adamant that upstream from them the owners did not do dam releases to any extent, the flow was due to water streaming down off the hillsides of surrounding farms and hurtling down to the course of the stream and the wetland now known as 155 Grimwade Road.My 2013 diary tells me that at 9 am the next day 28 mm had fallen in the previous 24 hours. From memory nearly all the rain fell over the short period about noon on Saturday.

Are short, heavy showers frequent here in Balingup that could cause this sort of stream rise? On 10 February this year 24 mm was in the gauge at 9 am, followed by 14 mm the next day. On 20 July there was 29.5 mm in the gauge at 9 am. This small sample shows we can get heavy showers in a short timeframe.

Conclusion: with a heavy downpour now and only the overflow pipes in the dams newly built upstream from us, with a pipe needing to be manually opened by opening a valve to let water come out from the bottom of the dam and the small side pipe to cope with the flow, the water could potentially go over the top of the dam. The water in the dams is quite near the crests, much higher than that recommended by dam experts which is to have a gap (the freeboard) of at least 600mm, better 1 m, from the water level to the crest. If water does go over the crest, erosion can bring about a breach to the dam.
It may also be too much for the new culvert pipe under the track to the farming neighbours and cut off the two farms west of the new dams.

Dams need to be engineered for purpose, taking into account likely weather events, eg heavy showers in a short time-frame. Expert advice is needed to build dams that are structurally sound and will not risk the safety of neighbours either up- or down-stream. Perhaps more standardised rules and regulations are needed in this area to reduce risk of dam failure.

As for us downstream, how would we fare if the new dams breached after heavy rain??

Lower Dam, 28 November, 2017.  IMG_7003

Lower Dam showing water level near the top of the dam walls. It is recommended that freeboard (the distance between the water level and the top of the dam walls) be at a minimum of 600 mm, better, 1 m.