a) Are these criticisms important?
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.”