The comparison of possibilities for demolition or rehabilitation of hydraulic structures

Study on evaluation the technical state of dams, the impact on ichthyofauna, socioeconomic benefits, and the results of cost–benefit analyses was performed in 2021–2022. A list of dams whose demolition would improve the ecological state of rivers and have the least negative impact from a socioeconomic point of view was compiled. A more detailed analysis of demolition/reconstruction possibilities was performed for the 15 dams with the highest position in the list. Analysed under two scenarios. Scenario No. 1 – the pond is drained, the existing surplus water fall spillway is demolished, and part of the sludge accumulated in the pond is removed; compensatory measure – a bridge is installed at the culvert place and a small water pond (for recreation) is formed. Scenario No. 2 – the pond is not drained, the existing dam/ surplus water fall spillway is repaired, and a new fish pass is installed. A guide screen or fish barrier is installed. Costs for implementing scenario No. 2 were obtained higher than according to scenario No. 1 in 5 out of 15 investigated objects, while in the remaining 10 it was the opposite. The total costs mainly depend on total area of ponds (important for calculation amount of sludge removal (for implementing scenario No. 1 and 2)), costs for demolition (scenario No. 1) or repairing of existing hydraulic structures (scenario No. 2) and costs for installing compensatory measures (scenario No. 1) or fish pass (scenario No. 2).