DROP INLET SPILLWAYS IN LITHUANIA

摘要

Drop inlet spillway, DIS, (usually called ,,shaft’’ in Lithuania) is the most popularrnspillway type in our country. They are built at 274 (77,6 %) hydroschemes of 353rnregistered (which create a reservoir area ≥ 5 hectares). The design discharge of 1...3rn% probability of our DISs varies from several up to 300 m~3/s, the overflow head is inrnlimits of 0,5...2 m, and the height of dams concerned reaches 21 m. Prevailing droprninlet shape in plan is quadrangular and hexagonal, in profile - straight vertical.rnSeveral special types of drop inlets are proposed to receive more compact andrneconomic structure. Geometric proportions and hydraulic parameters of those typesrnwere optimized by hydraulic simulations in our laboratory. On the drop inlet crestrnsmall piers and bar screens, in the front wall - bottom outlets are provided. Thernconduit is made of precast reinforced concrete pipes of 1...4 parallel lines. The stillingrnbasin width is constant or increasing downstream, the side walls are vertical. Variousrnenergy dissipators are used in stilling basins, including the ,,comb’’-type dissipator.rnTransition to the downstream apron usually is of straight wall type; then therntrapezoidal channel follows.rnConstruction of small DISs was carried out by local teams of land reclamationrnbuilding organizations; bigger DISs were build by specialized hydrotechnical units.rnNowadays, after 20-25 years from the start of extensive DIS commissioning, one canrnsay that, in general, our DISs are good and reliable spillways. Of course, sometimesrnunexpected hydraulic phenomena take place; several DISs (usually built by localrnteams) should be repaired; several DOSs, alas, are out of service, together withrndams and reservoirs. More and more attention to ageing DISs is to be paid.