Worst case scenario and climate adaptation
The local authorities are also responsible for flood protection and are required to formulate a climate adaptation plan for flood prevention. Consequently, flood mapping is an integral part of the web-based scenario tool. A simple user interface supports ‘worst case’ scenario formulations which include factors such as:
- choice of season
- initial conditions of river and basin
- upstream inflow
- rainfall
- roughness associated with a specified maintenance level
Specifically with respect to climate change, authorities are required to carry out extreme flow condition scenarios corresponding to 5-, 10-, 20-, 50- and 100-year return periods for present conditions (2010) and future conditions (2050). The model outputs – in terms of maximum depth inundation maps – are used in flood risk assessments with the aim of outlining flood prone areas to be protected through climate adaptation measures.
For Gudenå River, the upper reaches receive relatively large inflows from the upstream basin. As per current practices, lake water levels need to be maintained at constant levels by releasing water rather than storing excess water. The capacity to discharge flood water at Silkeborg city will be limited by cross sectional dimensions of the downstream river and the flow resistance due to aquatic vegetation. Further downstream, the Tange Lake created for hydropower purposes offers very limited protection for downstream reaches due to its limited volume and the release operation aiming at maximising power production, not to offer flood protection.
Results show that for extreme flows, the water level rise in the upstream city of Silkeborg and associated flooding is restricted to areas near the lake shore. Further downstream
however, maximum water levels and frequencies suggest that the most vulnerable houses on the flood plain may become uninhabitable unless investments are made to prevent climate change-induced deterioration.