Kösters, Frank

Schubert, Reiner

Zorndt, Anna

English

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Conference Poster, Author´s PostprintKösters, Frank; Schubert, Reiner; Zorndt, AnnaSystematic assessment of the impact of a planned channeldeepening on estuarine physics, the example of the Weserestuary (North Sea)Verfügbar unter/Available at: https://hdl.handle.net/20.500.11970/108944Vorgeschlagene Zitierweise/Suggested citation:Kösters, Frank; Schubert, Reiner; Zorndt, Anna (2018): Systematic assessment of the impactof a planned channel deepening on estuarine physics, the example of the Weser estuary(North Sea). Poster präsentiert bei: PECS 2018, Physics of Estuaries and Coastal SeasMeeting, October 15-19, 2018, Galveston, TX.Standardnutzungsbedingungen/Terms of Use:Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichendenNutzungsbedingungen getroffen wurden. Damit ist sowohl die kommerzielle Nutzung als auch das Teilen, dieWeiterbearbeitung und Speicherung erlaubt. Das Verwenden und das Bearbeiten stehen unter der Bedingung derNamensnennung. Im Einzelfall kann eine restriktivere Lizenz gelten; dann gelten abweichend von den obigenNutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte.Documents in HENRY are made available under the Creative Commons License CC BY 4.0, if no other license isapplicable. Under CC BY 4.0 commercial use and sharing, remixing, transforming, and building upon the materialof the work is permitted. In some cases a different, more restrictive license may apply; if applicable the terms ofthe restrictive license will be binding.Systematic assessment of the impact of a planned channel deepening on estuarine physics, the example of the Weser estuary (North Sea)Frank Kösters, Reiner Schubert and Anna Zorndt Federal Waterways Engineering and Research Institute (BAW), GermanyContact:   Dr. rer. nat. Frank Kösters  ·  Hydraulic Engineering in Coastal Areas  ·  Phone: 0049 (40) 81908-356  ·  frank.koesters@baw.deIntroductionMany estuaries experience strong anthropogenic pressure as they contain approach channels for eco-nomically important harbors. At the same time, they represent ecologically valuable habitats which need to be protected in order to ensure a proper func-tioning of their ecosystem services. Therefore, the careful assessment of the environmental impact of human interventions such as a channel deepening has become increasingly important.The planned adaption of the Weser Estuary (German Bight, North Sea) is taken as an example to illustrate challenges for the interpretation of numerical mod-elling results in order to obtain reliable predictions for adaption related changes in hydrodynamics and salinity.An Environmental Impact Assessment (EIA) for large coastal engineering projects usually contains a detailed numerical modelling study comparing the present-day situation (base-line) with the future situation including the planned channel adaption (scenario).The numerical model applied here was UnTRIM in a high resolution 3D set-up.MethodsFigure 3: Flow chart of the hydrodynamic modelling part of an EIA study.MechanismsResultsFigure 5: Typical landscape and agricultural land use in the Weser estuary. Figure 4: Hovmöller diagram of salinity in along stream direction (right) and   forcing due to changing water levels (left) and river runoff (middle).Changes due to the channel deepening in tidal high water may be different for spring and neap tides or the effects on salinity may be different for different hydrological situations. In the past commonly a rep-resentative spring-neap-cycle for specific scenarios (e.g. low or high river runoff) was chosen in order to assess adaption related changes. This approach was extended in order to overcome the need for speci-fying certain scenarios but rely on natural variability itself.As the quantification of adaption-related changes is the basis for ecological or financial compensation measures in the following EIA, they need to be highly reliable and specific (“one number”). As an example, farmers may get compensation for the adverse ef-fect of higher salinity in riverine water which is used for feeding their cattle.The underlying estuarine physics of the effects of a channel deepening are well understood, but quanti-fying changes due to the channel deepening alone is less straightforward. This becomes especially evident when trying to identify changes due to anthropogenic interventions based on measurements in highly vari- able natural systems, e.g. for salt intrusion.Figure 1:  Morphology of the Weser estuary (German Bight,    North Sea). Numbers denote the distance along the   main shipping channel.Figure 2: Planned increases of design   depths of the Weser   (two separate adaptions of the   Lower Weser and Outer Weser)   in along stream direction.Research QuestionThe general physical mechanisms of a channel deepening consist among other things of a reduction of effective roughness. Less flow resistance yields an in-crease of tidal energy input into the estuary and results in higher current veloci-ties, a stronger intrusion of saline water and frequently an increased accumula-tion of fine sediments.For the EIA of the Weser Estuary, the numerical modeling results cover a full hydrological year with its natural variability. In order to get a robust estimate for the mean value of adaption related changes, the 5%- and 95%-percentiles of the whole year are computed which serve as upper and lower bound of the mean. Interestingly, the results suggest, that adaption related changes of some tidal characteristic values such as tidal high water are largely in-sensitive to the specific hydrological situation (spring vs. neap tide). Therefore the results can be also seen as a corroboration of the previ-ous approach where estimates were based on shorter model simulations.Figure 6: Schematic changes due to the channel deepening for current velocity (upper) and salinity (lower)Figure 7: Time-series of max. salinity per tide at Weser-km 55 over   a hydrological year for the baseline togehther with mean,   5%- and 95%-percentile. The location is in the mixing zone   of the Weser.Figure 8: Time-series of adaption related changes (scenario - base  line) of max. salinity per tide at Weser-km 55. Note the   robust changes over most of the time covered.Figure 9: Robust predictions of changes in salinity based on   numerical modelling results (red) for use in the EIA.Numerical modelling studies for Environmental Im-pact Assessments (EIA) for large coastal engineering projects usually rely on relatively short modelled time series (i.e. a spring-neap cycle). This requires a care-ful definition of relevant scenarios to be considered (e.g. low river runoff). This approach was extended here by modelling a full year and analyzing natural variability instead of specific scenarios.From these data robust predictions for channel adap-tion related changes in hydrodynamics have been obtained based on higher statistical percentiles. On the one hand this may be an approach for future studies as the dependence on specific scenarios is reduced. On the other hand, the results suggest that anthropogenic changes (of average conditions) are not very sensitive to specific hydrological conditions.Conclusions

Systematic assessment of the impact of a planned channel deepening on estuarine physics, the example of the Weser estuary (North Sea)

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Systematic assessment of the impact of a planned channel deepening on estuarine physics, the example of the Weser estuary (North Sea)

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