The day after we stop dredging: A world without sediment plumes?

7 - 9 November 2007, Ahoy' Rotterdam, The Netherlands

S.G.J. Aarninkhof1, W.F. Rosenbrand2, C. van Rhee3 and T.N. Burt4

Abstract

Dredging activities are a pre-requisite for the development of human welfare, coastal safety and economic profit. Nevertheless, the dredging industry is often criticized – and not seldom without any scientific justification – for having an adverse environmental impact, particularly through generation of sediment plumes during project implementation. Would the day after we stop dredging mark the onset of a world without sediment plumes?

To answer this question it is necessary to consider a wider range of natural and man-induced drivers of sediment plumes in delta areas. Would shipping activities cease the day after we stop dredging? Certainly not. In fact, propellor wash impacts during ship manoevring operations are likely to increase since vessels will face channels
and basins of decreased water depth. Also, would natural rivers stop discharging large quantities of fine sediment during periods of high water run-off? Again the answer is no. To assess the environmental benefits of an ‘idyllic’ world without dredging, it is necessary to evaluate the impact of maintenance dredging activities as compared to the impact of other, ongoing drivers of sediment plumes.

This paper presents recent progress in the framework of the TASS (‘Turbidity Assessment Software’) program, which aims at the development of a validated model to predict dredging-induced turbidity levels. A key component of this program is the execution of a series of large scale field trials to collect high-quality data that can be used for model validation purposes. Recent field trials carried out in Bremerhaven (2006) and Rotterdam (2007) resulted in valuable insight in optimal means to collect overflow samples for the quantification of overflow losses over a range of soil types, overflow configurations and environmental conditions. Moreover, it is expected that the Rotterdam (2007) field trial will help to assess the relevance of draghead plumes and propellor wash in view of dredging-induced turbidity, as well as the benefits of using a green valve. Both data sets will be used for TASS model validation and the identification of future model developments and research needs.

Although the TASS program focusses on dredging-induced turbidity increases, it should be noted that dredging is just one out of a series of processes that drive sediment plumes. These processes include natural events, shipping operations and fishing activities. An inventory of these processes suggests, at least qualitatively, that
the annual impact of these processes is of the same order of magnitude as dredging. Consequently, we conclude that ‘The day after we stop dredging’ will by no means mark the onset of a world without sediment plumes.

Keywords: Suspended sediment, sediment plume, environmental impact, TASS, field trials

1 Royal Boskalis Westminster nv, Hydronamic, PO Box 43, 3350 AA Papendrecht (The Netherlands), Phone +31-78-
6969415, Fax +31-78-6969555, S.G.Aarninkhof@boskalis.nl
2 Royal Boskalis Westminster nv, Dredging Development Department, PO Box 43, 3350 AA Papendrecht (The Netherlands), Phone +31-78-6969313, Fax +31-78-6969555, W.F.Rosenbrand@boskalis.nl
3 Van Oord Dredging and Marine Contractors BV, PO Box 8574, 3009 AN Rotterdam (The Netherlands), cvr@vanoord.com
4 HR Wallingford Ltd, Howbery Park, Wallingford, Oxon, OX10 8BA (UK), n.burt@hrwallingford.co.uk

Last update: 6 October 2009