Silt laden Severn water - all this fine silt will be deposited if the flow is forcibly reduced

Dr Graham Daborn’s report on silting

06.29.08

Here’s an excerpt from Dr Graham Daborn’s report study done for a barrage proposal.

a) Are these criticisms important?

Our experience in the Bay of Fundy system, the Miramichi (Canada), the Humber and Severn Estuaries (UK) and the work of colleagues in the Netherlands and around the world shows that the fine sediments that dominate suspensions in the water column of macrotidal estuaries do not behave in any way like non-cohesive sediments (e.g. sands) that engineers have traditionally modelled. Their settlement rate depends upon: particle size, salinity of the water, temperature, the mineralogy, the organic content, and the presence and activity of biological factors such as bacteria and phytoplankton. Once settled on the bottom, these sediments continue to display entirely unique properties. If allowed to dry out, as occurs in intertidal locations, the sediments may become extremely resistant to erosion, especially when low tide occurs in the middle of the day. In Minas Basin, for example, we found that over a two month period in summer the stability of the mudflat increased by several orders of magnitude because the sediments were held together by mucus secretions from benthic diatoms, bacteria, molluscs and worms. This is a seasonal phenomenon that causes sediments to settle and stay on the bottom, building up a bank that may accumulate many centimetres over the summer. When the sediments were examined in traditional ways in an engineering laboratory, it was found that they could be up to 80 times more resistant to erosion than one would have predicted on the basis of their grain size. Biological factors are recognised as playing extremely important roles .

Potentially, such accumulations would quickly fill up an estuary, but in our region winter conditions are harsh enough to remove much of the summerís accumulation.

In order to understand this behaviour we need to know in detail: sediment concentrations, sediment type (mineralogy and grain size), organic content, current velocities, shear velocities, turbulence, wave height and period, diatom concentrations and growth rates, invertebrate types and densities , and important vertebrates such as fish and birds that have major effects on benthic invertebrates. None of this information seems to have been available to or acquired by the consultants that prepared the CER. Consequently, I suggest that it is impossible at this time to make any judgement beyond pure guesswork about the effect of the barrages, the channel and the filling/discharging operations that would be involved in building this project. To make even a preliminary estimate of sediment fate will require a field campaign covering a full four seasons, and aimed at making the kinds of measurements indicated above, followed by a thorough modelling exercise.

b) To illustrate the importance of such information, I rely on some of our own experience.

In Atlantic Canada we have constructed three dams across macrotidal estuaries that are comparable in size to those proposed for Doctors Creek: The Petitcodiac Causeway (about 1955), the Annapolis Causeway (1960) and the Windsor Causeway (1970). In all three cases, the dams diminished the water velocities that had prevailed previously. Two of these, at Petitcodiac and Windsor, caused massive deposition of sediments on the seaward side of the dam as well as accumulation in the lake/headpond above the dam. At Windsor, accumulation on the seaward side during the first year occurred at rates of up to 15 cm/month until it reached the mean high water level (i.e. about 8 m in thickness). Now, after 28 years, the new mudflat is still growing down the estuary; and is causing increasing problems for shipping at the port of Hantsport, some 11 km away from the dam. The Annapolis Causeway had the opposite effect: it has accumulated a large amount of sediment upstream of the dam which is steadily filling in the headpond, but appears to have induced accelerated erosion on the seaward side that threatens one of Canadaís most important historic sites.

In all three cases, construction of the dams caused massive deposition of suspended sediments, but why the differences?

The answer relates to the source of the sediment. At Petitcodiac and at Windsor, the major sources of sediments were from the seaward side, generated by the erosion of sea cliffs subjected to tides of up to 16m range. The sediments accumulating behind the dam were mostly derived from agriculture-induced erosion upstream. At Annapolis, most of the sediment was derived from upstream, and, prior to building of the causeway, this was maintained in suspension by a tidal system that oscillated from 5 to 9 m in range. When the dam was built, all upstream sediment tended to settle out as flows decreased in the lower estuary, and got trapped in the headpond. In addition, any sediment in water brought in from the sea that passed through the open fishway in the dam also got trapped upstream. This had the effect of creating a sediment deficit in the tidal water on the seaward side, and thus caused the greater erosion occurring along the shore of the Annapolis Basin.”

2 comments so far

[…] 11.52am: Canada’s Bay of Fundy, like the Severn estuary, has one of the world’s highest tidal ranges. It too has long been the focus of those seeking to build a barrage. A small 20MW barrage across the mouth of the Annapolis River has been in place since the 1980s. There have been a couple of incidents over the years of whales getting trapped in the sluices, but environmental damage caused by silting seems to be the principle concern of a larger scale barrage across the whole bay. The Save Our Severn campaign (seemingly inactive since the government said in 2010 that it would not giving public funding to a barrage) made much of the research of Dr Graham Daborn, a researcher at the Acadia Centre for Estuarine Research in Canada, into the effects of silting. But in a 2008 study, Daborn wrote that much still has to be learned: Our experience in the Bay of Fundy system, the Miramichi (Canada), the Humber and Severn Estuaries (UK) and the work of colleagues in the Netherlands and around the world shows that the fine sediments that dominate suspensions in the water column of macrotidal estuaries do not behave in any way like non-cohesive sediments (e.g. sands) that engineers have traditionally modelled. Their settlement rate depends upon: particle size, salinity of the water, temperature, the mineralogy, the organic content, and the presence and activity of biological factors such as bacteria and phytoplankton. Once settled on the bottom, these sediments continue to display entirely unique properties… […]



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