Case Study Wertach and Lech – 05.05.2017

Wertach and the Lech Pilot Sites- Field Trip to Wertach river-05.05.2017

1. Introduction

General presentation of the study site(s) Both study site rivers Wertach and Lech are originating in the northern Alps near the border between Austria and Germany, and are then running in roughly north direction through Bavaria towards the Danube River. Thereby, the Wertach represents a tributary of the Lech, and joins it in the city center of Augsburg (Fig. 1).

Figure 1 Location of the two study rivers in Germany. In orange, the Wertach River, in red the Lech River. (Graphics: M. Carolli, IGB).

Both rivers originally have been typical pre-Alpine rivers with high sediment load. The seasonal discharge regime is shaped by snow melt in spring. This snowmelt regime may be superimposed by sudden floods which may appear after heavy rains (orographic lift rains) occurring with northerly winds at the northern rim of the Alps.

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In both rivers, restoration projects have been conducted in some reaches, which are partially continuing up to date. These two restoration projects ‘Licca liber’ and ‘Wertach Vital’ serve as case studies within the HyMoCARES project.

2. Hydromorphological restoration/management

Lech river: The goal is to stabilise and restore the river mouth of the Danube and Lech river. Damages from the canalisation therefore have to be eliminated. If possible, the Lech river will be approached to its natural situation. It requires a stable river bed and preservation if not improvement in flood safety. The aquatic ecology must be strengthened to secure a “good ecological condition” following the WRRL. Besides ecological arrangements sites for people’s leisure and recovery will be developed. The study examines two parts of the river (river-km 17 - 37,3 & river-km 46,95 – 56,7).

Wertach river: Most importantly, the nearby inhabitants of the Wertach river have to be reliably secured from destroying floods. To prevent the river from further erosion, measures have to be taken to reduce the force of its water. Building a dynamic and continuous river system will help generating a larger habitat for both animals and plants. The „Wertachauen”, as an urban recreation area, are planned to be naturally designed and accessible for everyone. Planning wise, the project perimeter has been divided into three parts (Wertach vital I, II & III).

Monitoring activities

General objectives of the monitoring program

Lech: - Stabilization of the river bed - River ecology strengthening - Improvements in flood safety - Establishment of recreation areas

Wertach: - Chutes to secure the river bed - Sole-protection arrangements with breakwater stones - River broadening with natural development up to a barrier line - Bank slope flattening - Restoration and elevation of flood-preventing dikes - Building of fish ladders at the “Ackermann” weir and the “Inningen” barrage - Flooding of alluvial forests to raise the groundwater level again

Figure 2: Wertach vital plan at Ackermannwehr with fish pass, river bed protection and residual water chutes; source : (http://www.wwa-don.bayern.de/hochwasser/hochwasserschutzprojekte/wertachvital/umsetzung/wertach_vital_zwei/index.htm)

Figure 3: Ackermannwehr and fishpass of the Wertach river in detail

3. Case Study Site short descriptions/images

The first meeting point is the above (Figure 2 & 3) shown Ackermannwehr, where the realized reach of Wertach vital I and the planning of Wertach vital meet. The Alteration of the Wertach river with its surroundings increased the flood problematics and also channel incision until 1999 a flood event occurred with very large channel erosion rates. To fix the Problem of channel erosion and for the reason of flood protection the Project Wertach vital was called to life.

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Figure 4 : Overview of the field trip and stops at the Wertach river

Upstream of the meeting point the Wertach section with channel widening and gravel bar formation begins (Figure 5). The gravel bars are fixed with riprap to achieve a certain river pattern. Because of the lacking dynamics, the brush and vegetation have to be removed from the gravel bars every year to maintain the characteristics.

Figure 5 : Fixed gravel bars of the Wertach river.

Figure 6 : Loogwood debris and macro structures that have been installed in the Wertach, new logwood (back left) is installed in certain periods of time.

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Figure 7 : Groine in the upstream unit, with the sediment depot across the river.

To maintain the active gravel layers a sediment depot has been put in the upstream part of the project. This depot is built with a groine with the erosive force of the water as a driver. Because both banks are depots after a certain time, the groine can be switched to change the sides of the depot of which grave lis eroded.