MARINET presents evidence that offshore dredging will cause beach erosion

Presented by: Pat Gowen and Mike King:-
Marine Information Network for Friends of The Earth (MARINET)

The East Anglian Region Offshore Dredging companies are continually telling coastal erosion concern action groups that the removal of large amounts of material from the offshore seabed along extensive areas of our coastline does not cause beach draw-downdraw down The process by which tides and wave motion remove (draw down) material from a beach and pull it out to sea. A sandy beach experiencing draw down is thus denuded of its sand. The process can be natural (i.e. winter storms) or can be artificially caused (e.g. aggregate dredging, whereby the dredging of sand and gravel offshore causes sand to be drawn down from the beach in order to replace the material which has been dredged).; this is because they insist that there is no movement of the seabed material between the shoreline and offshore dredged areas — consequently offshore coastal dredging does not accelerate coastal erosion.

The dredging companies also state that these vast dredged depressions in the sea-bed (five metres deep in new areas and up to ten metres deep in re-dredged areas) do not infill from the surrounding sea-bed, the sea-bed is not mobile and can not eventually cause draw-down of adjacent coastline beaches.

The offshore dredging companies also claim that they are helping to combat coastal erosion by replenishing beaches (at a handsome profit) with the silt/sand residue from their offshore dredging operations.

MARINET, in liaison with the study organisations mentioned below, submit the following detailed study documents which include selective tests analysis results and provide the evidence (which the offshore dredging companies claim does not exist) that both of the above claims made by the dredging companies are false, and that offshore dredging along the East Anglian coastline is accelerating coastal erosion at an alarming rate.

Evidence : The following evidence confirms that it is the in-fill of the offshore dredged pits from the surrounding seabed which causes beach draw-down.

The mechanism of the lowering of beaches and shorelines caused by offshore dredged areas is established in the recent SANDPIT Report which can be viewed at: — To find the Report on the Sandpit website, click ‘Progress Reports’ on the left menu and then the report will be found as ‘Scientific Report of Year Two (size 7.1 Mb)’

This document is the Second Sandpit Report “SCIENTIFIC REPORT OF SANDPIT PROJECT, APRIL 2003 — APRIL (2004 YEAR 2) MAY 2004, SANDPIT EC FIFTH FRAMEWORK PROJECT No. EVK 3-2001-00056” which emanates from a large group of independent European Scientists concerned with the impact of Offshore Aggregate Dredging.

The report gives extensive tests and computer analysis results which confirm that offshore dredged areas (depressions/pits) eventually cause draw-down of adjacent beaches.

This report quotes: “Large-scale mining pits will have a significant impact on the near-field and far-field (up to the coast) flow and wave patterns; the flow velocities inside the pit will be reduced and the wave heights may also be reduced, depending on the depth of the pit. As a consequence, the sand transport capacity inside the pit will decrease and sediments will settle in the pit area, resulting in deposition. Thus, the pit will act as a sink for sediments originating from the surrounding areas and depending on the local flow and wave patterns. Hence, erosion of the sea floor will take place in the (immediate) surrounding of the pit.”

Deeper water brought about by seabed lowering increases the wave height and hence erosion, and a lowered offshore seabed means a greater beach slope, therefore a more rapid gravitational movement of mobile sand and shingle from the beach and shoreline to offshore and eventually to the created pit.

Evidence: The following evidence confirms that the replenishment of eroded beaches by offshore dredged silt/sand is a waste of time because this unwanted/surplus material is easily washed away by tides and wave action.

The processes involved in the selective grain sorting of natural beach sand by tides and wave action has been studied by extensive sampling and analysis of sand grain samples from the beaches, seabed and offshore sand bars along the Oregon (USA) coastline.

The results of this sampling and analysis are detailed in the study report: “PROCESSES OF SELECTIVE GRAIN TRANSPORT AND THE FORMATION OF PLACERS ON BEACHES”
by: Paul D. Komar, College of Oceanography, Oregon State University Corvallis, Oregon 97331 USA.
and Chi Wang, Sandong College of Oenology, Quindao, the Peoples Republic of China.

This document, using test results and analysis of the grain samples taken from beaches, offshore sand bars (placers) and the offshore sea-bed, concludes that:

  1. The natural beach sand grains which have been sorted by tides and wave action and comprise (96%) heavy dense materials such as ilmenite.
  2. The offshore banks and offshore coastal area seabed comprise mostly lighter grain materials such as quartz and feldspar and contain only 6% heavy mineral grains.

This report quotes:
“Beach face samples were taken at many locations from the beaches of the Oregon coast when sand was being eroded from the upper beach and transported to offshore-bars since the process of grain-selective sorting leading to placer formations were probably most active at this time. The landward most sample contained 96% heavy minerals, this concentration systematically decreased offshore to a 6% heavy mineral content in the seaward most sample (54m offshore).”

The evidence from the results of sampling analysis in this report makes sense (even to non scientific personnel) as everyone is familiar with the gold panning technique: water is continually added and sluiced around the pan by circular motion, this results in the lighter grain materials being washed off the top of the pan and heavier grain materials (such as gold) sinking to the bottom of the pan. The sluicing action of the gold prospectors pan is a miniature version of the wave action washing and grading beach sand grains.

The dredged sand/silt from offshore which the dredging companies provide for beach replenishment comprises mostly (96%) lighter quartz and feldspar grains which are quickly washed away by tides and wave action and consequently the exercise is a waste of time as it only gives short term benefit — except for the positive publicity it gives the offshore dredging companies and the money they make by having to repeat this practice annually.

The above evidence and other statements in this report also support the Second Sandpit Report findings to prove that beach sand grains are mobile, transported offshore by wave and tide action and that the seabed sand/material is also mobile and infills the offshore dredged areas.

The first page of this report can be viewed here as a large pdf file 2Mb. The full report is available from the British Library

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