The turbidity of the Severn is caused by the energetic, scouring effect of the tides on the estuary bed. A Barrage, by reducing the energy of this effect, will clarify the water of the Estuary, which will have a beneficial effect on the amenity value of Severnside, and will also improve the bacteriological load of the water, since the turbidity prevents Ultra Violet light from penetrating the water and slows up the sterilising of sewage-derived pathogens. This is good.

However, the particles which were previously sustended as turbidity will be deposited onto the estuary bed. Year by year the River Severn will add more silt. Soil Erosion

This will build up, and unless removed, will eventually turn the body of water behind the barrage into a marsh. This is not so good, except for marsh-dwelling life forms.

This is a major problem, and when studied in a similar situation in Newfoundland, was so severe it caused the idea of a barrage to be dropped.

Removal by dredging will carry energy costs, so prevention or minimisation needs to be researched.

Organic agriculture produces a better soil structure than conventional agriculture, and topsoil loss due to rain and flood is less on organic farms.

Therefore, all agriculture in the Severn catchment area should be turned over to organic as the Barrage is being built. This is a challenge, but is achievable because of the many other advantages of organic agriculture, not least, the reduced need for energy intensive fertilisers and pesticides.

Second, siltation in the barrage basin can be reduced by creating silt deposition fields in the freshwater part of the Severn.

Some fields would be set aside as flood areas. Their rich topsoil is skimmed off so that they lie about a foot below the mean river level. The topsoil is taken (at an energy cost) to areas upstream where soil is poor or thin (soil erosion is a major, but undicscussed, problem). River water is run over the flood field, especially at times of heavy rain when soil load is high. Over a few days, the suspended soil particles are precipitated out in the still water. The clarified water is released to the river, and the cycle is repeated until the filed is up to its previous level, when it can be skimmed off to be returned to areas of topsoil loss, and the process repeated.

There is a flood risk with every river. At present the risk is reasonably well managed, although the Environment Agency is not well funded for improving flood defences, and local authorities persist in building on flood plains.
However, global warming will increase the risk both as a result of increased frequency and intensity of storms, and also as a result of sea level rise.
This threatens the security of all the low-lying and, including human settlements along Severnside in the long term, perhaps in three or four generations.

It might be that in the long term, the ability of a Barrage to prevent the loss of these lands and settlements will be of even more value than the energy that the Barrage would generate.
