The rock manual - Dredging Engineering Research Laboratory

The Day After We Stop Dredging: A World Without Sediment Plumes? 15STEFAN G.J. AARNINKHOFTHE DAY AFTER WE STOP DREDGING:A WORLD WITHOUT SEDIMENT PLUMES?ABSTRACTDredging activities are a pre-requisite forthe development of human welfare, coastalsafety and economic profit, yet thedredging industry is often criticised forhaving an adverse environmental impact,particularly through generation of sedimentplumes during project implementation.Would the day after we stop dredgingmark the onset of a world withoutsediment plumes? To answer this questiona wider range of natural and humaninduceddrivers of sediment plumes in deltaareas should be considered. Wouldshipping activities cease the day after westop dredging? Would natural rivers stopdischarging large quantities of finesediment during periods of high waterrun-off? To assess the environmentalbenefits of an “idyllic” world withoutdredging, the impact of maintenancedredging activities as compared to theimpact of other, ongoing drivers ofsediment plumes must be evaluated.The research presented here reflects recentprogress in the framework of the TASS(Turbidity Assessment Software) programme,which involves a series of largescalefield trials to collect high-quality datathat can be used for model validationpurposes. Recent field trials in Bremerhaven(2006) and Rotterdam (2007) resulted invaluable insight in optimal means to collectoverflow samples for the quantification ofoverflow losses over a range of soil types,overflow configurations and environmentalconditions.Moreover, the Rotterdam (2007) field trialis expected to help to assess the relevanceof draghead plumes and propeller wash inview of dredging-induced turbidity, as wellas the benefits of using a green valve. Bothdata sets will be used for TASS modelvalidation and the identification of futuremodel developments and research needs.Although the TASS programme focusses ondredging-induced turbidity increases, itshould be noted that dredging is just oneout of a series of processes that drivesediment plumes. These processes includenatural events, shipping operations andfishing activities. An inventory of theseprocesses suggests, at least qualitatively,Above: Dredging operations often generate no moreincreased suspended sediments than are naturallypresent. Above a clear boundary forms where a riverwith high-levels of suspended sediments meets anocean environment with low-level suspended sediments.that the annual impact of these processesis of the same order of magnitude asdredging. The author wishes to acknowledgethe important contributions to thisresearch by W.F. Rosenbrand ofRoyal Boskalis Westminster nv, DredgingDevelopment Department, C. van Rhee ofVan Oord Dredging and Marine ContractorsBV and T.N. Burt, recently retired fromHR Wallingford Ltd., UK as well as thefunding by Stichting SpeurwerkBaggertechniek (SSB) and Fonds CollectiefOnderzoek as part of crow. The paperwas originally presented at the CEDADredging Days in November 2007 and waspublished in the conference proceedings.It is reprinted with permission in a slightlyrevised and updated version.INTRODUCTIONDredging activities are a pre-requisite forthe development of human welfare, coastalsafety and economic profit. Nevertheless,the dredging industry is often criticised– and not seldom without any scientificjustification – for having an adverseenvironmental impact, particularly throughgeneration of sediment plumes duringproject implementation. Would the day