Operational modelling for pollution control in

coastal areas: experiences from continuous

operation in the Elbe estuary

I. Nohren* & K. Duwe**

*Institutfur Meereskunde, Universitat Hamburg Troplowitzstr. 7,

D-22529 Hamburg, Germany

**HYDROMOD Scientific Consulting GbR Bahnhofstr. 52,

D-22&&0 #We/, Germany

Abstract

The high spatial and temporal variability of dynamical processes in aquaticenvironments requires appropriate monitoring and forecasting methods and strate-gies. The combination of necessarily spotlike measurements with precise circu-lation and transport models is the basic idea of operational modelling to copewith these requirements. Under the EUREKA-EUROMAR umbrella the OP-MOD project (Operational MODelling of Regional Seas and Coastal Waters)has been performed by 11 European institutions. Existent modelling and mea-surement techniques were combined into a flexible operational tool for a widerange of environmental monitoring, navigational support, decision-making, andpollution control tasks. Major objectives of OPMOD systems are the routinemonitoring of application areas as well as shortterm forecasting in cases of nat-ural or man-made hazards like oilspills or storm surges.

In 1994 an OPMOD system became operational on a routine basis for the tidalestuary of the Elbe river and the dynamically complex port region of the Cityof Hamburg. Experiences of the routine operation of this system are describedin this paper. Furthermore the system is used as a contribution to the MAST 3project PROMISE (Pre-Operational Modelling in the Seas of Europe).

1 Motivations for Operational Modelling

In most shelf, coastal and estuarine areas as well as in many limnic regions thespatial and temporal variability of the dynamical processes requires appropriatemonitoring and forecasting methods and strategies. Typical coherency scales forexample for circulation and transport processes are in the range of 10 to 100 me-ters in the horizontal space domain and from some minutes to some hours in the

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220 Water Pollution

time domain, respectively. Even quasisynoptic and intensive measurements can-not resolve these processes. Therefore fully three-dimensional, non-linear prog-nostic models are necessary. Furthermore transport processes and the predictionof water levels, currents, waves, spill-behaviour, and various other processes canbe described by numerical models on a high quality level.

The combination of continuously operating monitoring networks and numer-ical models to an operational system is a powerful tool for the observation andshortterm forcast of the dynamical situation and distribution of hydrographicand environmental parameters in the area of investigation.

2 General Concept

The OPMOD system was designed as a flexible and modular concept whichallows the adaptation to many applications in a large range of objectives inmarine and more generally aquatic technology, monitoring, and management byrelatively simple modifications or extensions. Besides the operational approachit also allows off-line operation for case studies and specific analysis. Additionaland external models and software can be applied to the internal system-conceptand incorporated upon user's specifications.

The combined system consists of autonomous measurement stations, tele-metric equipment for data transfer, computer periphery, and the correspondingmodel, display, and technical software. The scientific software includes the cur-rent and transport calculations and the correlation of measurement and modeldata within the system.

Additionally software for data acquisition, data handling, and data represen-tation have been developed to improve the respective model's performances anflexibilities.

The results produced by the system may act as a database for the identifi-cation of data gaps and measurement requirements allowing the optimization ofsurvey procedures, tracklines, and station spacing positioning as well as preciselevel corrections of bathymetric and other survey data.

The development work was carried out in the EUREKA framework within theproject OPMOD. which commenced in 1989 as a cooperation of 11 participantsfrom 8 countries. It aims at applications from regional seas to river systems andlakes.

3 Model Characteristics and Requirements

The core of the system is the current model based on a three-dimensional prog-nostic baroclinic FD-scheme to solve the nonlinear Navier-Stokes equations (seeDuwe et al. [1]). The semi-implicit numerical algorithm uses the Boussinesqapproximation, is a multi-level scheme in the vertical dimension and can resolvevertical turbulence from constant approximation to full 3D A; —e—scheme approx-imations. The system is driven by a few representative measurements providedby continuously processing field stations or monitoring networks. These measure-ment stations require small maintenance efforts combined with longterm stability

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Water Pollution 221

and high resolutions. In the case of forecasts boundary conditions are derivedfrom operational meteorological models and efficient hydrological prognosis (e.g.tidal constituents).

The circulation model resolves the appropriate coherency scales and showslongterm stability and operability. It calculates the actual state as well as short-term forecasts of the governing parameters and processes (e.g. waterlevels, cur-rents, salinity, temperature, density).

Besides the circulation model additionally designed spill and transport modulscan be invoked upon necessity which are fed by the results of the circulationmodel. For the transport either finite differences (flux corrected transport) orLagrangian tracer techniques are used which offer a high accuracy and showthe necessary flexibility for individual definitions of items and characteristics ofsubstances (e.g. hydro-chemical and biological interactions, decay and accumu-lations, sedimentation and erosion, etc.). They are either incorporated into thesystem or operating in stand-alone units which are connected via data links.

The models have been verified in a number of very different applications incoastal waters, shelf seas, and rivers (see e.g. Nohren et al. [2], Pfeiffer et al. [4],Siindermann et al. [5]).

4 Area of Application

In 1992 the German OPMOD system became operative with a real applicationto the tidal estuary of the Elbe river and the dynamically complex port regionof the City of Hamburg.

The morphology is rather complicated with large tidal creeks and flats, inter-sected by some islands and sands and a deep navigational channel leading fromthe North Sea to Hamburg. The dynamical processes are governed by the tideyielding complex co-oscillations and non-linear deformations of the tidal wavewhen progressing riverup. In the outer regions local wind-induced circulationpatterns may occur. The brackish water zone is characterized by partially andtemporarily strong baroclinic influences (i.e. density influenced flow patterns).The upper part of the area includes the Hamburg Port which is located in andaround a branching and re-merging area of the river forming complex systems ofstrongly tidally influenced port bases, junction channels, and waterways.

5 The Basic Model System and Information

Network

A three-dimensional baroclinic circulation model covers the area of the tidallyinfluenced part of the river Elbe from Geesthacht to the river mouth west ofCuxhaven. The horizontal resolution of the model is 250 m and the averagevertical one 1.5 rn. At the seaward boundary values from the North Sea/GermanBight forecast model system of the German Hydrographic Authority are applied.Wind forcing is taken from the meteorological forecast models operated by theGerman Meteorological Office.

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222 Water Pollution

In this basic model a barotropic model is embedded covering the area ofHamburg port. The higher horizontal resolution of 50 m is required to resolveand simulate the complex circulation in the river branches and canals as well asthe narrower river sections east of Hamburg.

If not provided by the basic model with 250 m horizontal grid the input datafor the current model (waterlevel, water temperature, wind, etc. for boundaryconditions and verification purposes) are supplied from the measurement stationsof the port authorities and of the Environmental Authority of Hamburg. Thelatter is operating five monitoring stations on the river Elbe and in the harbourarea evaluating water quality by measurement of temperature, oxygen content,alkalinity, conductivity, and radioactivity.

Additional connections exist to the monitoring devices of the measurementsystem established in the EUREKA-EUROMAR MERMAID project in the Elberiver. In this context a significant improvement in the reliability of links withthe external measurement data and model forecast sources was experienced bythe extensive use of network facilities both internally (workstations connectedvia ethernet) and externally (internet connection).

Generally the transport and spill models are driven by the results of thecirculation models. Lagrangian schemes which have the advantage of low nu-merical diffusivity and give better possibilities for formulation and incorporationof sources, sinks, and intra-ensemble interaction are used. Turbulent diffusion,wave action, and other parameterized formulae or bulk parameters are usuallyrepresented by Monte Carlo schemes and random walk methods. For more detailssee e.g. Pfeiffer and Duwe [4].

The transport models can be operated either as an integral part of the systemor as stand-alone systems on personal computers with the input data transferredor accessible via a local network. They are invoked either automatically or onuser's request and control whenever required.

6 System Control

A major part of the OPMOD systems are peripherical programs and softwaretools for configuration, installation, maintenance, communication (intra-systemand with the user), real-time and multi-process control, automatic data pro-cessing, automatic control of the model's performance and quality. The generalsystem operation control is depicted in figure 1. The programs and system toolsare usually not accessible and generally not interesting for the user. However,the system provides user shells as interfaces and menues which are graphicallysupported.

Thereby user-defined operations like online displays, animations, plotting,archiving, further evaluation of data and model results, setup of transport modelsare controlled.

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Water Pollution

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224 Water Pollution

7 Experiences from Routine Operation

Intensive research and optimization work of numerical schemes and models, pro-cess and case studies have been done and are still ongoing in other project andresearch activities to support the development of the OPMOD system. Testscomprised the reaction of the model to noisy non- or only slightly smoothed dataas they might be delivered by the interfaced measurement stations. Other modeltests comprised the long-term stability and predictability of the model by hind-casting longer (some weeks to months) periods. This is an essential requirementfor operational applications. The circulation models fully proved thereby theirlong-term stability, predictability, and operability. The simulation of estuarinedynamics in the Elbe estuary has shown a nature-like behaviour of the brackishwater zone and temperature variation in the area (see e.g. figures 2 and 3). Thequality of the model results was in the range of the error margins of field mea-surements and shortterm meteorological forecasts. Figures 4 and 5 depict thecorresponding distributions of current speeds whose detailed and reliable fore-cast is essential for determining turbulent mixing and dispersion as well as forsedimentation/erosion investigations.

8 Conclusion and Outlook

The OPMOD system for the Elbe estuary is able to produce routinely and perma-nently the actual state as well as daily 24 hours forecasts of relevant hydrographicparameters for the area of application. Thus the system has proven its reliabilityas a tool for continuous monitoring of the aquatic environment. It has a widerange of applications e.g. the monitoring and management of surface waters (syn-optic information, forecasts, control and monitoring of effluents, determination oforigin of detected oil spills), application in the offshore industry (prediction andhindcasts of spills, safety and risk analysis, etc.), or for navigators (e.g. on-lineinformation for ship's operation, integration into electronic mapping). Due to itsmodular design the system offers high flexibility to the requirements of individualusers and allows the integration of specially applied modules and the couplingwith monitoring networks already in operation or with models and evaluationprocedures made available by other institutions.

The OPMOD system is envisaged to include other German coastal and estu-arine areas and can be included into larger scale monitoring and forecast areascovering the German Bight and the Southern North Sea. The connection with thelarger-scale operational model of the North Sea of the Bundesamt fur Seeschif-fahrt und Hydrographie (BSH) has already provided useful experiences to enablea generalization of the approach to the coupling of small- and larger-scale modelsystems within GOOS activities.

Furthermore the experiences made within the OPMOD project are a valu-able contribution to the MAST 3 project PROMISE (Pre-operational Modellingin the Seas of Europe). This project aims at collecting high resolution compre-hensive data sets which will be used to test pre-operational models including allaspects from data links, real-time monitoring devices and national data centers

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Westliche Unterelbe

Figure 4: Example of an output: Current speed distribution in the surface layer closeto high water; same area and time as in fig. 2 (darker colours denote higher velocities).

Westliche Unterelbe

Oste

Figure 5: Example of an output: Current speed distribution in the surface layer closeto low water; same area and time as in fig. 3.

Transactions on Ecology and the Environment vol 14, © 1997 WIT Press, www.witpress.com, ISSN 1743-3541

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Westliche Unterelbe

Oste

Figure 2: Example of an output: salinity distribution in the surface layer close to highwater; area near Cuxhaven (lighter colours denote brackish water).

Westliche Unterelbe

Oste

Figure 3: Example of an output: salinity distribution in the surface layer close to lowwater; same area as in fig. 2.

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Water Pollution 227

and improve these model systems. Main focus of PROMISE is to quantify therates and scales of exchange of sediment between the coast and the near-shorezone and, thereby, to enable subsequent application of this experience to othercoastal areas and for broader management applications.

The model system is applied to the Sylt-R0m0 Bight and the coastal regionsnear Holderness (UK) in the North Sea.

Acknowledgements

The authors would like to thank the German Ministry of Research and Technol-ogy for funding relevant parts of the German OPMOD project. We would liketo express our thanks to the Environmental Authority of Hamburg, the FederalAgency of Hydrography Hamburg, the German Meteorological Office and theport authorities for for their support with data.

References

[1] Duwe, K. C., R. Hewer, and J. O. Backhaus, Results of a semi-implicit two-step method for the simulation of markedly nonlinear flows in coastal seas,Continental Shelf Research, Vol 2, No. 4, pp. 255-274, 1983.

[2] Nohren, I, K. D. Pfeiffer, K. C. Duwe, E. Jasiriska A. Walkowiak, Three-Dimensional Modelling of Current and Transport Processes in the OdraEstuary. In: Gambolati et al. (eds.), Computational Methods in Surface Hy-drology. Proc. of the 8th Int. Conf. on Comp. Methods in Water Resources,Venice, June 1990, pp. 107 - 112, Springer 1990.

[3] Pfeiffer, K. D. and K. C. Duwe, Modelling of Environment and Water Qual-ity Relevant Processes with Combined Eulerian and Lagrangian Models. In:Gambolati et al. (eds.), Computational Methods in Surface Hydrology. Proc.of the 8th Int. Conf. on Comp. Methods in Water Resources, Venice, June1990, pp. 113 - 117, Springer 1990.

[4] Pfeiffer, K. D. and K. C. Duwe, Three-Dimensional Modelling of Tides,Storm Surges, and River Flow in the Elbe, Tapi, and Pearl River Estuar-ies. Storm Surges, River Flow, and Combined Effects. A Contribution toUNESCO-IHP project H-2-2, pp. 65 - 74, Hamburg, 1991.

[5] Sundermann, J., K. C. Duwe, I. Nohren, K. D. Pfeiffer, Y. Longfei, Strategyof Current and Transport Modelling in Different Estuaries. In: Michaelis etal. (eds.), Estuanne Water Quality Management (Monitoring, Modellingo,nd Research), Coastal and Estuarine Studies, pp. 25 - 34, Springer 1990.

Transactions on Ecology and the Environment vol 14, © 1997 WIT Press, www.witpress.com, ISSN 1743-3541