Sand dune ecology

Transcription

We're here at Plage de Tronoan in the west coast of Brittany to have a look at the dune system here and to think about how the dunes are formed right up to how they're managed and how people can have an effect on them dunes, well first of all, to a dune formed you need three things to be in place first of all you need sand because out sand there won't be a dune you also need the space to put the sand, so the accommodation space and then you need the wind the weather conditions to be able to pick up the sand to move it and to deposit it and the way that the sand is moved is by three different methods or mechanisms the first one, if the wind is strong enough, it can actually pick the sand up and blow it along in suspension, something we call suspended transport however, if the wind isn't strong enough to keep it in suspension it may be able to get the grains to hop up into the air to bounce into the ground again and they might hop up again a leap-frog sort of motion across the beach and up into the dunes that's something we call saltation and the third mechanism by which sand can move is when the wind is just strong enough to move it along the ground as a blanket, if you like, something we call surface creep now when the sand is moving by any one of those three mechanisms it will only come to stop and be deposited when the wind velocity that is transporting it decreases and that can occur when the sand encounters an obstacle like a strandline at the back of the beach composed of driftwood, seaweed, maybe the odd dead dolphin, or so, and the sand can then be deposited in the lee side. and sometimes also on the upwind side of the object to start creating an accumulation of sand something we call a shadow dune and at this point the shadow dune doesn't have any vegetation on it it's just sand around a particular piece of debris but if the sand accumulates enough and it gets above the influence of the waves then vegetation can actually start to colonise that shadow dune to produce what we then call an embryo dune and an embryo dune can be a very small round, circular accumulation of sand with a bit of tufty vegetation on it centered on a piece of debris but eventually, if it's along a strandline, these individual embryo dunes through continued accumulation of sand and the increased trapping ability of the vegetation they can actually start to coalesce and to merge along the strandline, so you have these merging, coalescing embryo dunes which eventually form a linear dune feature and it occurs right at the front of the dunes and because of that we call it the foredune now once a foredune has been created you can have another strandline deposited by waves in front of it and the whole process can start all over again producing another foredune in front of the older foredune and in this way, if sediment supply is constant and continuous then we can have a whole number, a whole series of foredunes growing out seaward from the original position and we call that process dune progradation so the dunes can prograde out towards the sea if the sediment availability is high enough now once that happens the old foredune that is left behind becomes what we call a hind dune it's behind the foredune so we call it a hind dune and the foredune is the one at the front now, eventually with progradation, those hind dunes are going to become more and more ever distant from the beach and the source of sediment so they effectively become starved of sediment when that happens they are vulnerable to incision and disturbance, perhaps by rabbits burrowing by goats or sheep grazing, or indeed by tourists walking across the beach across the dunes rather to the beach and incising pathways as we have here when that happens the wind can actually get into those pathways and start to undercut the dune on either side, releasing the sand from underneath the vegetation which, in this case, we have marram grass and sedges the sand is liberated and can be blown further inland and in this way these hind dunes, the sand can be transported from the dune ridge to behind the fragments that are left so the whole dune can be reorientated from what was parallel to the coast to actually become perpendicular to the coast - at right angles and when that happens we call them secondary dunes because they are no longer in their original orientation and, of course, when they are in their original orientation we call them primary dunes because they are in their primary position so by looking from the air at a dune system the dunes that are still parallel to the coast are primary dunes those that have been reorientated through sand starvation and blow-out formation and undercutting though deflation by the wind they become perpendicular, and they are secondary dunes so, there's a whole evolutionary process going on there with sand dunes