Damages from floods is one of the most devastating natural disasters and flood control measures have been constructed for decades to prevent flooding of important areas. Structural flood control has effects on the natural river system, and can have large effects on river ecosystems if embankments, dredging and channelisation is used to prevent flooding. Utilising natural flood control such as natural storage in flood plains are an option to the engineered protection systems. This task will look into nature based flood control systems and apply them to try to mitigate flooding in a test case in the Lærdal river. The Lærdal city centre in the lower part of the river is exposed to flood and flood control measures in the lower part are costly and will cause large impacts in the river channel. A hydraulic model is made for the lower part of the river (Seguin Garcia, MSc, 2019) and this project will extend that model as far as detailed bathymetry data is available and then use this model to evaluate options for flood control.
The main question is if we can utilise upstream flood plains to dampen floods in the lower part of the river. This will be done through the following steps:
- Investigate if high flow data exists for the river that can be used to calibrate the hydraulic model for high flood episodes
- Identify embankments holding back floods in the upper part of the river and install overflow weirs in these to flood the old floodplains during the design flood.
- Evaluate the effect of these measures on the flooding at the river outlet.
- Evaluate the flow on the floodplains to avoid high velocity events that may cause erosion and damages to the flood plains
- Evaluate the cost incurred by flooding existing farm land and infrastructure upstream versus flooding the city centre downstream.