Tuesday, 22 November 2016

St Aubin: Changing Hydrodynamics and Land Reclamation

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.

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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