River morphology: what runoff from storm-water drains can show us

On holiday on Phillip Island in January of 2012, I went for a walk along a beach the day after a downpour, and before the tide had come in.  Some way along the beach, I noticed a stormwater outlet, and the results of runoff from the night before.    It occurred to me that here was a nice example of a newly formed stream which provides some insights into the nature of the form of long-established rivers.  The water had gone, but its effects had not.

What could be seen was the action of a rapid, and fairly brief, flow of water on sand, a substrate that is easily eroded and mobilised.  Notice the meander or curving nature of this ‘stream’ as it makes its way downhill over the relatively flat beach (see above).  Notice also that the ‘outside bend’ on the left, has formed a ‘cliff’, whereas the ‘inside bend’ is flat and smooth (see below).  The speed of the current in rivers is greatest on the outside bend and this is where most of the erosion takes place.  The speed of the current is slowest on the inside bend and this is where deposition happens.

The picture below shows a close-up of the sand on the outside bend of the river and clearly shows the turbulence (or chaotic flow) that typically occurs on the outside bend of rivers, where corkscrew-like flow (hydrocoidal flow) picks up sand and deposits it on the opposite shore, creating a beach or point bar.

In normal rivers, over time, the outside bend erodes and the inside bend deposits, and the river becomes wider, as does the floodplain.  The width of all of this is ultimately limited by the geology and topography of the landscape.  The meander also tends to migrate downstream.  A river will tend to continue to meander back and forth across the floodplain, but if the meander gets wide enough, it can become disconnected from the river at low flows and forms a billabong or oxbow lake.  These will usually be reconnected during high flows.

So it just goes to show you what you can learn from an early-morning walk along the beach.  And the beauty of it is that this new river only lasted a few more hours, until the tide came in and obliterated it.

You can’t go past Luna Leopold’s “A View of the River” 2006 (Reprint, Harvard University Press . ISBN 0-674-01845-1), for an in-depth description of all aspects of river form and function.  Brilliant stuff.

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