Summary:

Detailed recommendations formulated by European Commission in 2007 introduce new regulations regarding the continuity of river channels. The existing hydraulic structures should be equipped with fish ladders in order to keep the fish restitution conditions and the new hydro-technical structures should assure tech-nical solutions that do not cause any change in hydro-morphological state. Ac-cording to the Fisheries Institute the following conditions of biological stability should be followed to keep the biological stability conditions for diadromic fishes:
- an alluring stream criterion,
- a terminal velocity criterion,
- a criterion of admissible fish ladder filling,
- a criterion of admissible loss Δh for the transom ladder discharge.
The paper presents results of numerical modeling of 400-meter sector of
a river that includes coated weir, small hydrostation and a fish transom ladder. The action taken concentrated on such formation of the discharge in the ladder and modification of the bed configuration in the hydro mouth that creates the alluring stream. Numerical modeling results were compared to results of calculations by empirical formula.