Culverts designed 20 years ago have often an inadequate conception in terms of potential use by fauna. They are often barriers to wildlife movement. The older generation of culverts shall be examined, their fauna compatibility assessed and the best and most cost-effective techniques to retrofit described.
The objective of the project is recording and analysis of current remediation techniques for water passages to remove potential obstacles to the aquatic and terrestrial fauna. Further aim of the project is to provide definition of the most efficient techniques and their evaluation in terms of the cost-benefit ratio. Finally the improvement proposals for existing and new proposals for remediation techniques will be created.
The work includes literature review, interviews with engineers and technical experts in the field of fauna, representative review of existing water passages on the (national) road network in Switzerland and the selection of representative sample examples. The derived recommendations will ensure, that the proposed solutions the water passages Fulfill its hydraulic engineering and faunas specific function, are technically feasible and financially viable engineering.
The final report of the project focuses mainly on culverts with a diameter of more than 1 m and on small animals, for which such culverts can play a role in connectivity. The results are based on a review of literature, field studies, interviews of experts and specialized departments. A broad consultation was arranged.
The minimal needs of different fauna categories can be summarized as follows:
- For fish the drop at passage outlet should be no higher than 5 - 10 cm (maximal height for small and juvenile fish). The current should be variable and there should be some low velocity places inside the culvert. Furthermore a natural bottom substrate is recommended.
- Minimal width of a ledge for terrestrial animals: 20 cm for otter and polecat; 40 cm for reptiles, amphibians, martens and mouse; 60 cm for beaver, fox and badger.
- Minimal height from the bottom of the ledge to the top of the culvert: 40 cm for reptiles, martens and mouse; 60 cm for beaver, otter, polecat, amphibians, fox and badger. For round culverts, only half of the shoulder has to fulfil these demands.
The report can be used as a decision support. Several flow diagrams are developed to help planners find the most cost-effective solutions in retrofitting existing structures to improve connectivity. An overview of different solutions and their cost-effectivity is given taking into account construction and maintenance costs.
The final chapters address monitoring of success and describe examples in the canton Aargau, giving a description of potential measures.