Bank erosion - a matter of balance
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!!