Functioning of small lowland reservoirs with pre-dam zone on the example of Kowalskie and Stare Miasto Lakes

The main problem described in the paper is performance of two-stage reservoirs. It means such a reservoir which is split into preliminary sedimentation zone and main storage located below. The first zone enables separation of fine sediment particles transported as bed-load or suspended load. The main reservoir located below the sedimentation zone is supplied with partially cleaner water. The purpose of the research is analysis and assessment of efficiency for the described approach taking into account the prevention from sediment accumulation and degradation of water quality in the main reservoir. For the purpose of this research, two lowland reservoirs are under consideration, because the problems mentioned are observed mainly in such kind of reservoirs. These are the Kowlaskie Lake in the Glowna river and the new Stare Miasto reservoir in the Powa river. The structure of both reservoirs is similar but other parameters differ significantly. The Kowalskie Lake is bigger reservoir with capacity equaling to 6.58.105 m3. The Stare Miasto reservoir is "only" 2.161.105 m3 large. The reservoir differ also in the magnitude of characteristic discharges. The analysis of hydraulic conditions is done by means of computer simulation. Their basis is HEC-RAS package and specially prepared water conditions scenarios. In both cases, the simulations presented in the paper are calculated for normal water level in the dam cross-section and maximum observed discharge as inflow to the reservoir. The scenarios applied differ in size of gate opening in the barrier, which split the reservoir into two parts. In the paper two cases are presented. These are minimum opening and full opening. In the first case, the water level in sedimentation zone increased above normal water level and the conditions for sedimentation of transported particles are better. The opening of gates causes lower differences in water levels between sedimentation zone and main reservoir part. The mixing of water is intensive, what is the effect of larger capacity of spillway gates. The analysis of water quality was done on the basis of repeated measurements of selected solutes concentrations in a number of points along the profile. The results show that the concentration of dissolved oxygen is greater in the main part of the reservoir. The concentrations of biogenic substances are significantly lower. Presented remarks proves, that the impact of preliminary sedimentation zone on the water quality and limitation of sediment accumulation in the main reservoir is positive. The primary conclusion drawn from the analyses is recommendation for broader use of the two-stage reservoirs. Such approach is effective and relatively cheap