The costs and benefits of the implemented flood protection measures were calculated for particular events with different return period: 20, 50, 100 and 500-year flood. The estimated total cost amounts to 145.94 Million euro (2013) and includes: total cost of the realisation of the flood control system (144.4 Million EUR, 2013), installation costs per event (0.65 Million euro, 2013), annual maintenance and storage costs (0.89 Million euro, 2013). The costs did not include damage to the underground. The estimated benefits include avoided damage costs in Million euros of:
- Residential buildings, ranging between 332 (Q20) and 1,971 (Q500);
- Infrastructure and industrial buildings, ranging between 124 (Q20) and 613 (Q500);
- Equipment, ranging between 42 (Q20) and 254 (Q500);
- Citizen´s evacuation, cleaning and other costs, ranging between 42 (Q20) and 74 (Q500);
- Environmental and cultural assets, ranging between 38 (Q20) and 57 (Q500).
Costs caused despite the flood control system are estimated to range between -410 (Q20) and -966 (Q500) Million euros. Total avoided damage costs are (in Million EUR) are therefore between 168 (Q20) and 2,003 (Q500).
If costs and benefits are compared for each event, it is possible to see that benefits are greater than costs for the event with a return period of 50 years (Q50) and more. Benefits are slightly greater than costs also for flood events Q20. However, flood events with a return period of 20 years could appear more times than once in the 80 years (life span of the measures), which would make the comparison negative.
Even if a scenario where there would be just one event of Q50 (or events with longer return period, i.e. Q100 and Q500) during the expected life span of the measures (about 80 years) is considered, the benefits would still overweight the costs, also despite the annual maintenance and storage costs. Based on calculations it is possible to argue that if at least one event of Q50/100/500, or Q20 with a combination with another event (Q20/50/100/500) or any other combination of these occur, the flood control system investment will return.
In the case of Prague an implementation of grey infrastructure including flood barriers was essential in order to effectively protect the city centre. Based on the performed cost-benefits analysis it proved to be a very effective investment. However, there are still possibilities to adopt green and blue measures on small streams (e.g. in Rokytka or Botič).