The physical world

Sea-level rise, with all of its projected adverse effects, is beneficial when it comes to counteracting the shallowing of estuaries in disturbed catchments.

Other physical processes might also stall the inevitable, perhaps even indefinitely in some circumstances. For instance, as an estuary becomes shallower, small waves kicked up by winds become more effective at lifting sediments – grain by grain – off the bed and mixing them into suspension in the water column. Once suspended in this way, sediments can be dispersed and possibly flushed from the estuary by tidal currents, thus slowing down the rate at which the estuary grows shallower. 'What regulates sedimentation in estuaries?' describes how this works, and also gives a broader overview of waves and currents and how they do their business in the different parts of an estuary.

Read our Water & Atmosphere article 'Waves in shallow water'

Read our Water & Atmosphere article 'What regulates sedimentation in estuaries?' (PDF 1.2 MB)

It’s a good trick, really: the shallower the estuary becomes, the better it gets at flushing itself of sediments. Still, it cannot always work, otherwise we wouldn’t have those Coromandel cases just mentioned. We might ask ourselves this: should we leave the ultimate fate of our estuaries to these little waves which may or may not be their ultimate saviour? Perhaps not, when the cause of the problem – increased sediment runoff from the land – is decidedly manageable.

Those small waves kicked up by the wind – “wind chop” – actually do a lot more than most people realise (including scientists, at least until recently). In deeper waters, they just upset boaties, but in the shallower waters around the estuary fringes they create a different type of havoc. 'The dance of the turbid fringe' explains how, and also explains some of the implications for dispersal of contaminants such as heavy metals and pesticides. These “fringe processes” are currently the subject of intense research, for they may hold a key to understanding and predicting how our estuaries are going to continue to change in this next, possibly crucial, phase of their lives in the face of rising sea level and continued sediment runoff from the land.

Read our Water & Atmosphere article 'The dance of the turbid fringe'

Small waves scour these sandflats of mud, and build these ripples while they are at it. [Malcolm Green]
'Waves in shallow water' explains how waves in estuaries are generated and what they do. [Malcolm Green]