DEVELOPMENT OF A RIVER FLOOD DEFENCE CONCEPT AT THE LAHN/GERMANY

Frank Toensmann and Tobia s Lang

Dept. of Hydraulic and Water Resources Engineering,

University of Kassel, Kurt-Wolters-Straße 3, D-34109 Kassel

Tel.: +49 561 804 2749, Fax: +49 561 3952,

E-Mail: wawi@wasser.bauingenieure.uni-kassel.de

Abstract: The Lahn project is one of about 150 projects, which is worked on in the frame of the EU-program “INTERREG IIC Rhine-Meuse-Activities (IRMA)”. Based on the strategies and proceedings directions, which were published by the Länderarbeitsgemeinschaft Wasser (LAWA) in 1995, a concept for decentral flood defence for the Hessian river Lahn is developed.The hydrological effectivity will be computed for the systems conditions and three development scenarios. Based on this, a design proposal will be worked out, which, in turn, is going to be put in concrete terms for eight sub- and three example-projects.

Keywords: river flood defence concept, decentral flood defence, river basin model, Lahn/Germany

1  INTRODUCTION

Trigger points for this project were the christmas floods in 1993 and 1995 in the Rhine-Meuse-catchment area. In the frame of the INTERREG IIC – initiative of the European Union the operational program IRMA was developed. IRMA is an abbreviation for INTERREG – Rhine-Meuse-Activities. The program contains about 150 flood defence projects in the Netherlands, Belgium, Germany, France and Switzerland. The national confinancier is the Hessisches Ministerium für Umwelt, Landwirtschaft und Forsten (Hessian Ministry of Environment, Agriculture and Forestry) with its program “natural oriented streams”.

The Lahn project consists of the scientific work, described in this paper, and of the design for three example-projects and its sub-projects, which are planned and put into order in close co-operation with the scientific work. Members of the scientific work group are the Bundesanstalt für Gewässerkunde (Federal Institute of Hydrology), the Justus-Liebig-University Gießen, the Technical University Darmstadt, the Technical University Braunschweig and the University of Kassel.

2  STRATEGIES AND PROCEEDINGS DIRECTIONS

According to LAWA (1995) three strategies for flood defence concepts, as listed in tab. 1, exist. These are Natural flood retention, Technical flood defence, and Extensive flood precautions. The strategies comprise instructions for measures, e.g. building precautions: building, living and dwelling adapted to possible flood risks.

In the publications and articles dealing with future flood protection (LAWA 1995;Hess. Ministerium 1999; Plate 2000; Samuels 2000; LAWA 2000) emphasis is put on the fact that the main point of interest in a flood defence concept should lie in the strategies of natural retention and extensive flood precautions. The technical flood defence, i.e. mainly dams and flood control reservoirs built between 1945 and 1980, should stay in the background due to low public acceptance.

                      Table 1  Strategies and proceeding directions

With its major project “Retention Register”, the state of Hesse mainly supports the water retention in rivers and flood plains. The focus is on the redevelopment and activation of retention areas. This also includes relocation of dikes and the reduction of peak discharge and discharge volume by means of engineering measures, regulation of morphology and initial planting.

Sometimes another classification is made, namely decentral, semicentral and central retainers. The decentral or mini retainers are part of the “natural flood retention”-strategy, while the semicentral or small and central retainers (dams, flood control reservoirs) are part of the technical flood defence-strategy, (tab. 1.), (Assmann et al. 1996; Tönsmann 2000a).

According to these principles, flood defence concepts have already been realised in a few small catchment areas (Tönsmann 1997; Bauer 1999; Tönsmann et al. 2000). However, so far there is no systematic investigation and planning for a middle-sized catchment area, as it is currently done in the Lahn project.

3  METHODS OF CONCEPT DEVELOPMENT

The following methods (models and procedures) are at our disposal:

l        Models for the determination of spatial and temporal distribution of statistical and maximum precipitation

l        river basin models

l        methods of water level computation

l        benefit/cost analyes

l        evaluations of environmental assessment

With these methods a flood defence concept — as shown in Fig.1 — can be designed.

Fig. 1  Course of planning for a flood defence concept

The first step is to apply the entrance data of the precipitation to a river basin model in order to calculate the actual condition. Water level computations are used to determine the flood plains appertaining to the statistical flood events. Useful is a comparison with past flood events or with the flood plains established by the authorities. The first step of the benefit/cost analysis is to determine the damages of past events in order to be able to find out the damaging potential for the flood plains of rarer flood events as well. Based on this data, the outline for the planned flood defence project has to be laid down. According to the planned objective, the main point of interest will either lie in the natural flood retention, the technical flood defence or extensive flood precautions. If engineering measures are required, one should start with a technical pre-feasibility study, sketch out different variants, and evaluate the respective costs. By means of an iterative procedure, the hydrological and hydraulic efficiency for the different variants will be examined and the benefit/cost ratio determined. Hydrological efficiency is the size of reduction of peak discharge and wave volume, hydraulic efficiency is the change of water level height. An assessment of environmental compatibility will be carried out in order to determine economically sensible variants. Here, too, the planning procedure will pass various iteration loops in order to bring forth the variant with the least possible remaining risk. Based on this, compensation measures will be suggested and a proposal for implementation prepared.

4  APPLICATION ON THE RIVER LAHN

4.1  Planning

The above-mentioned methods are also used in the Lahn-project. However, in a catchment area of 5,964 km², not all methods could be applied in similar depth to the entire area. Therefore, the focus of the work lies on the computation of different system cases for the catchment area with the river basin model in the line of the “natural flood retention”-strategy. For the following system conditions calculations are carried out: actual situation 1998, situation 2010 without flood defence measures, rainwater retainers in urban areas, unsealing of areas respectively infiltration of rainwater, retainers in side valleys, relocation of dikes, activation of retention in the flood plain, restoration, change of land use outside of populated areas, sub-projects, increase of precipitation-heights and potential natural condition.

The computations are made by the Leichtweiß-Institute at the Technical University Braunschweig/Germany with the program NAXOS, developed there. Part of the system cases are models (Leitbilder), which are known from ecological development concepts (DVWK 1996).

In Fig. 2 the potential leitbild is differentiated from the integral leitbild. The potential leitbild means “an approach to natural development, which would only be accessible, if there were no limitations of social and economic reasons”. It is therefore a maximum integral leitbild, which, in the river basin model is tested with the system case “potential natural situation” for its hydrological efficiency. The integral leitbild will give the probable realisable situation in three scenarios.

In the leitbilds, the parts catchment, floodplain and river are differentiated. The leitbilds will be developed for catchment areas (ecoregions), which differ mainly in morphology and topography (broadness of the valley, longitudinal slope).

The water level computations, the benefit/cost analysis and the evaluation of the environmental assessment cannot be applied to the whole catchment area, their application is restricted to the sub- (depending on investigations during the planning process) and example

Fig. 2  Function and position of guiding principles and development objectives in planning of ecological concepts (after DVWK 1996).

projects (fig.3). The three example-projects are:

(1) Kleebach catchment area near Gießen-Allendorf

(2) Floodplain of the Lahn between Niederweimar and Staufenberg

(3) Lahnloop near Wehrda.

The achieved results should be transferred to other parts of the Lahn catchment area. This, however, is difficult, the scale problems should only be pointed out here (DVWK 1997).

4.2  Implementation

In eight sub-projects the implementation has already started (Tab. 2). The outline of the projects makes clear that these projects are examples of the “natural flood retention”.-strategy. The most interesting projects are the reactivation of the furcation channels near Sterzhausen and the relocations of dikes, which give a good idea of future flood protection measures (Lang et al. 2000). A remaining problem is the fact that all sub-projects are supposed to be finished in December 2001. An extension will be given only under special circumstances.

                     Table 2  Selected sub-projects in the catchment area of the Lahn river in Hesse

、

Fig. 3  Catchment area of the Lahn river and position of sub- and example projects.

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