Freiburger Schriften zur Hydrologie

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Band/volume 23: Koch A. (2007):

Modellierung der hydrodynamischen Wechselwirkungen einer Flusslandschaft

The European Water Framework Directive introduces, besides the claim for a good quality of European water bodies, new holistic views and approaches which were not applied in German water resources management up to that point. These new views and approaches include the use of the river basin as the geographical and administrative basis for water management and the use of ecological and biological parameters to define objectives. These parameters can be seen as a combination of different biotic and abiotic parameters describing and influencing the riverine habitat. Particularly the use of biotic and abiotic parameters leads to the demand for new tools based on different fields of research to determine the impact of measures inside the river floodplain. In this context it is no longer valid to consider the dynamics of groundwater and surface water and the land ecosystems dependent on these dynamics as independent from each other.
The objective of this study was to create an expert system which is able to represent the essential hydrodynamic processes and to determine the impact of measures inside the river floodplain on the good ecological state of our river systems based on abiotic indicators. Regarding the complex flow pattern in natural floodplains this information is required in a high spatial resolution. Due to the complex dependencies of the ecology inside the river floodplain a modular structure for the expert system was selected which will allow to integrate new important indicators in the future, identified by a dialogue between different fields of research.
The expert system therefore consists of three main parts: a base platform as a basis for the integration of different modelling systems, a hydrodynamic component for the simulation of the interaction between groundwater and surface water and a component for the evaluation of modelling results and the creation of abiotic indicators for landscape development.
The core of the base platform SEDAT-C consists of system managers providing the infrastructure for managing different kinds of modules. Everything beyond this functionality is implemented in task specific modules. These modules deliver the infrastructure for storing, manipulating and displaying geospatial and time depending data. In addition to that, an abstraction layer for the communication between the different components of the expert system is also included.
The expert system WETLANDS is built on top of the infrastructure provided by the base system. It includes modules for pre- and post processing as well as modules for controlling the combined simulation of ground- and surface water and the generation of abiotic indicators. The pre-processing is done inside the MESH module. Time-consuming computations are done here to speed up the succeeding simulation. These are computations of parameter sets describing the spatial interdependency between the two finite element-meshes of the groundwater and surface water models. This is necessary because the expert system allows for different spatial distributions between these two models. Inside the module SIM, the combined simulation of ground- and surface water is controlled and the exchange flow between these models is computed. For this two different numerical finite-element models are used. The two models (software) – MEADFLOW for the two-dimensional surface flow and FEFLOW for the three-dimensional groundwater flow – were not developed as a part of this research project. They are adopted by the expert system as existing and approved models. For this purpose, shell-programs where developed enabling the expert system to control and to exchange data with these models without the need of changing the model code base.
Regarding the complex flow pattern in natural floodplains and the small scales for which information is required, most recent developments in two-dimensional (2D) surface water modelling were included in the expert system. The generation of abiotic indicators for landscape development, done by the AIL module, is again based on this two-dimensional (2D) surface water modelling.
The applicability of the developed expert system was tested and proved using two different model test areas. The first test area located at the upper Weser river basin was used to calibrate and validate the model approach using measured field data. The data was collected using groundwater probes with data loggers which where placed at six groundwater stations inside the test area. Further information about groundwater levels was available from several other groundwater stations belonging to a local water supplier. For the second test area located at the middle Elbe river basin an existing model application from an independent
survey of the impact of a planned dike relocation on the groundwater tables was available for model validation.
The dike relocation scenario of the second test area was also used for the exemplary generation of abiotic indicators of landscape development. A new approach was developed creating head and velocity functions for each node inside the model boundary based on simulation results using a wide range of boundary flow values. These functions are used to transform a recorded long-time series of flow into a node-based long-time series of head/velocity. The long-time series are then used to derive the statistical parameters describing the hydrodynamic of the surface water at a high spatial resolution by using abiotic performance indicators. These indicators are oriented to the European water framework directive and thus more focused on ecological demands than on demands given by the water resources management like flood duration etc.
In the conclusion the need for further research based on the created expert system is discussed. The need for further research arises from the necessary combination of the technical model-based work of this study on the one hand and field observations on the other with the aim to create mathematical descriptions of dependencies between the derived abiotic indicators and important species inside the river floodplain. For this a comprehensive dialogue between biology, ecology, hydrology and hydraulics has to take place.
Furthermore it is recommended to extend the existing expert system by combined models of water balance, transport of solid matter to the river system, sedimentation and erosion in the river system, and – of large importance – water quality in the river system. Regarding the demands of flood management, the expert system can be extended by combining the surface and groundwater models with quantitative rainfall forecast and a detailed hydrologic model to be used for flood forecast.