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Trout in the Town Blog
08/01/2014 - 14:12
Perhaps topical given current examples of river-bed-transporting extreme flows happening in many parts of UK...
In a striking similarity to the widespread lack of understanding of the impacts of dredging on flood risk - the problems caused by weirs that choke off the downstream progression of river gravels, cobbles, sands etc. are not widely appreciated.
The presentation below gives a basic outlining of why everything that lives in our rivers depends on the capacity for river channels to continually transport (and periodically deposit) river bed material from the hilltops to the sea. If we work better with (rather than against) these processes it would be far better for both societal needs/costs and the natural world.
02/01/2014 - 16:05
Although extremely counter-intuitive upon first sight; trying to help wild populations of (for example) trout by boosting their reproduction actually has many more chances to go wrong than to actually help. There is a lot of coverage devoted to the various aspects of this in some of our WTT guidance pages (http://www.wildtrout.org/sites/default/files/library/Stocking_position_2012_final.pdf)
But as a really readily understandable example from the world of bird conservation; we can see just one of the prominent pitfalls of giving an artificial helping-hand to breeding success. The nub of it is that if the boosted numbers are made up of individuals that the natural environment would otherwise kill off; you risk actually pushing the population closer to extinction (or permanent reliance on human intervention). The story below and trout-specific research should give serious pause for thought to any club assuming that the best response to the Trout and Grayling Strategy (which will prohibit the stocking of fertile hatchery-bred trout from 2015) is to set up their own wild broodstock programme. Especially as the stripping of wild brood stock removes the natural reproduction that would otherwise have taken place(and which would, consequently, have been subjected to the relevant "filtering" by the actual environment in your river).
Story of the Black Robin: http://phenomena.nationalgeographic.com/2014/01/02/in-saving-a-species-you-might-accidentally-doom-it/
14/12/2013 - 19:07
29/10/2013 - 16:20
Had a great visit to and discussion around Shipley weir recently. Stephen Bottoms (who organised it all) has done a great blog post here:
23/10/2013 - 19:27
You can always have too much or too little of a good thing. When it comes to revetments (re-inforcements) of river-banks; there are a whole host of pitfalls.
On the one hand, many sections of river suffer from excessive grazing of the land surrounding them that leads to a dramatic reduction in the variety of bugs, plants as well as fish populations that can be supported. In addition, many rivers that run through towns and cities often pass through quite intensively used land upstream of urban reaches. The excessive inputs of fine silt and sand where bank-erosion is rampant and extensive often end up being accumulated in the engineered sections of channels in towns and cities. As well as causing maintenance problems,this can smother what may, otherwise, be perfectly good spawning gravel.
Conversely, where efforts to "lock" a river channel in one place are over-zealous; the result will be the strangulation of supply of spawning gravels and a variety of cobbles and other river-bed material. Each of the different diameters of gravels/cobbles/boulders that come from eroded banks form a unique and vital habitat either for particular species or particular stages within the lifecycle of a species.
So the optimum for biodiversity (and, consequently, the services that such biodiversity provides to society through flood risk attenuation, clean water that is more energy-efficient/cheaper to treat, protection against climate change in cities etc. etc. etc.) is somewhere in the middle.
When you add to this the fact that a large proportion of either ad-hoc or "one off" attempts to reinforce banks by installing a hard "skin" of some kind (perhaps concrete, corrugated sheet metal, rubble, boulders, gabion baskets etc.) often make the problem worse...then there seems to be a genuine need for short information pieces like the video above. Whilst at first sight, putting in a solid reinforcement along the face of a riverbank is totally logical; when the surrounding bank material is still as soft as ever - you really have to know what you are doing to make "hard" revetments work. The problem is that hard surfaces "reflect" or "bounce" fast current flows without taking any of the speed or "sting" out of them. When those currents career into the next soft bit of bank; they take away a huge chunk. To make matters worse, the angular surfaces of rocks or gabion baskets are brilliant at producing swirling whirlpool/eddying flows - especially during spates. These chew great big holes in soft bank material and are really good at eating the bank away so that flows can work their way BEHIND the revetment. Once this happens, the hard revetment actually squeeze and accelerate the flows between the soft bank and the inside of the reinforcement - this produces a much faster and much more dramatic rate of erosion than would exist if there was no revetment at all.
Contrast this with complex, bristly brash (especially thorn brash or coniferous brash which have impressive densities of twigs sticking out at a range of different angles)...this stuff actively absorbs and stops raging flood waters. The "braking" effect actually causes suspended sediment to drop out of the flood-water and accumulate amongst the branches. If grazing stock can be excluded, these rich river sediments can soon re-vegetate and start to knit together the banks. This is especially true if trees are allowed to re-colonise (as deep root structure then starts to perform a similar role to that of the initial brash installation. Even better than this, because such re-vegetated banks still allow a river to redistribute bank material; a sufficient supply of sediment is still maintained. So, even though you can always have too much of any good thing, using brash revetments can be a vital leg-up to re-establishing sections of healthy river-side vegetation.
They might even help to stop your fencing falling into the river and reduce your water-bills by reducing treatment costs.
Oh, and they are amazing cover for juvenile fish from feathered and finned predators!!
07/10/2013 - 19:20
The WTT borrowed the fabulous Emriver kit from Severn Rivers Trust during the 2013 CLA gamefair. It allowed us to set up a whole range of common river-channel scenarios in an accurate scaled-down simulation. Dredging was one of the things we modelled:
Dredging river-bed material is something that is of particular relevance to the urban/heavily-modified channel environment. As is so often the case with rivers, what seems like the obvious and correct thing to do can actually blow up in your face (or someone else's face several miles up or downstream!).
Just as intriguing are the potential knock-on impacts of ad-hoc dredging in rural environments in an effort to increase the capacity to drain land...
Nature abhors a vacuum and removing accumulated material from a river channel can have far-reaching and unintended consequences. The first principle effect is to increase the demand for eroded river-bed and river-bank material from upstream. This can dramatically increase the rate of erosion in upstream reaches.
It also rapidly leads to the re-filling of the dug-out channel... (back to square one, so you dredge again....)
The other major/worrying effect is the interruption of the transport of sediment downstream of the dredged reach. Whilst the bed material is being re-accumulated in the dredged area, there is far less material being supplied downstream. That "cutting off" of the sediment supply causes a net increase in the erosion downstream of the dredged reach as well. This is due to stopping the supply of material that would otherwise "patch up" and fill in eroded areas to produce a more stable dynamic equilibrium state. How many people who undertake dredging understand these processes? Is the move to "fast
-track" UK-landowners' ability to dredge their own streams with a much lower requirement for external assessment likely to create more problem than it solves?
In addition to all that, the video below might give an interesting perspective on dredging works for the purpose of flood-water conveyance in areas downstream of bridges and weirs in modified waterbodies...