Sediment and nutrient retention in reservoirs of small hydro-power plants

Abstract

Dams segment river channels into dammed and undammed reaches causing both changes in the integrity of free-flow conditions and disturbance of the hydrological regime including downstream transport of both sus pended sediments and nutrients. To learn more about the impact of small dams on these phenomena, the investigations were carried out in Lithuania and involved 17 small hydropower plant dams with heights from 2.25 to 14.50 m in four rivers of the 3rd to 4th order. Two beaver-built dam cascades in open drains of the 2nd order were used for comparison. The concentrations of suspended solids, total nitrogen, and total phosphorus there were studied both in each reservoir and river upstream from the reservoir and downstream from the dam. It was revealed that all dams and reservoirs changed the natural regimes of suspended solids trapping them and the finest particles therein. As a result, the percentage of the particles < 0.01 mm and amount of total nitrogen and total phosphorus in the reservoir bed substrates increased several times in comparison to that in the river bed upstream from reservoir. In the rivers below the dams, due to increased velocity and turbulence of flow as water drained off via turbines, the finer particles were scoured out from bed substrates resulting in both the progress of armouring of river channel beds and some increase in concentrations of suspended solids and total phosphorus. Certain consideration concerning environmental aspects related with the subject of the study is presented.