A study which surveyed San Francisco Bay and tributaries
in California noted that:
“About 95 percent of the "reclaimed" land used for
recreation is in Suisun Bay. This land, reclaimed in the late 1800's for
agricultural use was reflooded due to subsidence and failure of levees during
the period between 1914 and 1943. “
“The Corps of Engineers in the ‘Technical Report on
Barriers,’ (1963) called attention to the problems and dangers of continued
filling of the bay and marshlands to reclaim land. Two of the pertinent
paragraphs from the report are quoted below:”
"Basic conflicts between issues of grave technical
consequences and explosive public concern characterize Bay Area reclamation. To
a greater degree than in any other groupings of problems facing the area, high
level decisions will have to be made on policy formulation that once by
physical implementation will become irrevocable."
"Reclamation by reducing the
tidal prism will increasingly alter the natural functioning of the Bay System -
changing tidal ranges, tidal currents, tidal phasing. There will result new
sedimentation and shoaling patterns in all the bays... Imperceptible though the change may be resulting from a single project of small size,
slow encroachment through the years, ends in irremediable physical effects."
Now St Aubin’s Bay is not an estuary as such. An estuary is
a partially enclosed, coastal water body where freshwater from rivers and
streams mixes with salt water from the ocean. But as the land reclamation has been
increasingly moving out into the bay, the basic of St Aubin has become more
enclosed, and more like a tidal inlet.
A tidal prism is the volume of water in an estuary or inlet
between mean high tide and mean low tide, or the volume of water leaving an estuary
or inlet at ebb tide. Since tidal prism is largely a function of area of open
water and tidal range, it can be changed by alterations of the basin area.
Land reclamations influence the cross-shore distribution of
hydrodynamic energy because the tidal prism is reduced and the cross-shore
profile generally becomes steeper. A reduction of the tidal prism leads to
smaller tidal flow velocities in the tidal channels.
The key factor here is that any kind of land reclamation
directly changes the coastline and topography. It can strongly disturb the
whole natural system and induce an imbalance, for example reducing tidal
volume, the water flushing rates and the capacity of flow carrying sediments.
Large-scale reclamation projects can cause the changes of topography
and water exchange, which can affect the hydrodynamic directly and
significantly.
With St Aubin’s Bay, the reclamation blocks off the water
passing through to Havre des Pas, which results in a tidal chocking.
Significantly, this would lead us to expect that the high water levels are
driven up.
As one of my correspondents noted:
“The creation of the reclaimed land at the eastern end of
St. Aubin's Bay effectively blocked the Small Roads channel between the Harbour
and Elizabeth Castle, which previously allowed water to 'escape' on the
outgoing tide. The situation now is that an eddy current is created in the bay
so that nutrient-rich water is trapped and circulates over a series of tides,
rather than being washed away. I believe this was all predicted in various
studies prior to the land reclamation.”
As the reclaimed land cuts off parts of water exchange, the
current directions in the Bay would be expected to move parallel to the coastal
line. In such a situation, the maximum flood and ebb current velocities. In
other words, the La Collette reclamation which reduces tidal flows and tidal
flushing.
One of the surveys to look at this indirectly was the
“Review of the current ecological status of the SE
coast Jersey Ramsar site” (2009). While this looked mostly
at land east of La Collette, it also considered the impact of the land
reclamation on tidal flows.
“The port area at St Helier has already been subject to
major changes as a result of reclamation, and our visit in July allowed us to
assess the ecological status of the Ramsar site east of La Collette during the
summer and compare with the results of observations made in 1998.
This area is
not only less biodiverse than the remainder of the Ramsar site, but also the
quality of the biotopes has declined over the past decade... This has almost
certainly resulted from a combination of factors including, exposure to poorer
water quality (combined outfalls / urban storm water), the high intensity of
beach usage near to St Helier and reduced tidal flushing resulting from
successive reclamations. “
And they also note that:
“The main changes in the area as a whole were the
accumulation of fine sediment (muddy sand to sandy mud) along the boulders
placed around the reclamation area and on the lower shore – which would
normally result from reductions in speed of tidal flows.”
What we expect from reduced flows? Studies suggest that seawater
purification declines, and the frequency and intensity of algal bloom increases.”
Sea hydrodynamic conditions are closely related to coastline
shape.
Reclamation in St Aubin’s Bay has changed the shape, and changing the marine environment.
Reclamation in St Aubin’s Bay has changed the shape, and changing the marine environment.
As far as I have been able to ascertain, there is only one
study which has looked at this, and that before much reclamation took
place. However, it may prove interesting. It is now available
online because I asked for it to be made available via an FOI request - it was delivered to the States, after all.
Falconer, R.A. and Kolahdoozan, M., “Mathematical Model
Study of Tidal Currents, Flushing and Water Quality Indicators Around St.
Aubin’s Bay, Jersey”, Report for the States of Jersey, April 1998
As I recognise from my hydrodynamics course, what they are doing or appear
to be doing, is testing a model using numerical methods (the coarse grid and the
fine grid) and comparing the parameters in their equations with some actual data
to check how accurate the model is. If it fits mostly the grid points, it should
be able to predict the movements outside of the grid points.
“A mathematical model study has been undertaken to predict the hydrodynamic
conditions around the coast of Jersey at a coarse grid scale and, in particular,
around St. Aubin’s Bay at a fine grid scale (ie. 100 m). In addition, the
concentration distributions of bacterial and nutrient level indicators have been
predicted in St. Aubin’s Bay and the surrounding waters, including: total and
faecal colifonns, faecal streptococci, dissolved available inorganic nitrogen
(DAIN) and phosphorus (DAIP).”
And it confirms what we know to be the case:
“The bounded nutrient level predictions confirmed that the Bellozanne STW
was the main source of DAIN and DAIP concentration levels in St. Aubin’s Bay.
“
Although it is interesting that the “Weighbridge catchment area” also plays a part.
And this is interesting:
“This suggests that the basin exchanges well with the adjacent offshore waters and the enclosed fluid is extensively flushed out by the tide and the alongshore current during each tide. However, on being flushed out of the Bay, much of the efflux fluid mass is advected eastwards on leaving the Bay.”
But that was 1998! There is now a large “arm” of reclaimed land going right out to sea from the power station outwards, which may well be blocking the eastward flow or at least curtailing it.
It would be useful, I think, for an updated study to be
conducted along those lines.
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