| Summary: | Käesolevas magistritöös on Ahja jõel Saesaare paisu likvideerimist käsitletud antud kohas võimaliku arengustsenaariumina, mis lahendaks kalade rändeprobleemi ja võimaldaks taastuda Ahja jõe looduslikul ökosüsteemil, mida laiemas mõttes loetakse pinnavee hea seisundi küsimuses riiklike keskkonnaeesmärkide saavutamist toetavaks tegevuseks. Põhiline töö eesmärk on pakkuda välja insenertehniline lahendus Saesaare paisu lammutamiseks (paisutuse täielikuks likvideerimiseks) ja paisu asukohas looduslähedase kärestiku taastamiseks. Töös on antud uurimuslik ülevaade Ahja jõe ja Saesaare hüdrosõlme hetkeolukorrast. Kirjandusallikate ja arhiivimaterjalide alusel on täpsustatud Saesaare hüdrosõlme rajamise eelset Ahja jõe looduslikku olukorda Saesaares. Kogutud andmetele tuginedes on vaadeldud taastatava kärestiku kahte lahendusvarianti - omaaegse kärestiku taastamine (variant 1) ja asenduskärestiku rajamine (variant 2), koos eelisvariandi valikuga ja sellele projektlahenduse, sh kärestiku ehitusjooniste, koostamisega. Eelisvariandina on käsitletud töös asenduskärestikku variant 2. Projekteeritud looduslähedase kärestiku pikkuseks on 250 m, languks ~1%. Projekteeritud on kolm kärestiku alamvarianti: üheharuline (2-A), kaheharuline pika saarega (2-B1) ja kaheharuline lühikese saarega (2-B2) kärestik. Paisu lammutamine ja kärestiku rajamine on projektlahendusega kavandatud etapiviisiliselt. Esiteks toimub liigveelaskme konstruktsioonide lammutamine ja kärestiku parempoolse haru väljaehitamine (1. etapp). Seejärel olenevalt kärestiku alamvariandist pinnaspaisu ja hüdroelektrijaama rajatiste osaline või täielik eemaldamine ning kärestiku vasakpoolse haru väljaehitamine (2. etapp). Ehitustööd on ette nähtud läbi viia ajutiste pinnasest tõkketammide varjus. Lammutatava liigveelaskme ette on paisjärve veetaseme ohutuks alandamiseks (peale liigveelaskme eemaldamist ja kärestiku parempoolse haru kujundamist) projekteeritud ajutine lukustuvatest betoonplokkidest pais, mille järkjärgulise lammutamise teel saab alandada kontrollitult veetaset paisjärve põhjani. 1. etapis on ehitusaegse vooluhulga läbilaskmiseks ette nähtud kasutada täiendatud olemasolevaid hüdroelektrijaama rajatisi, tulva korral ka ümberkujundatavat liigveelaskme haru ennast - selleks on haru ülaveepoolse tõkketammi kehandisse projekteeritud avariiülevool. 2. etapis, paisjärve allalaskmise järel, on ette nähtud jõevool mööda juhtida kärestiku väljaehitatud parempoolse haru kaudu, tagades seejuures ehitustehnikale ülepääs sängist ajutise torusilla abil. Töö raames on dimensioneeritud ajutisi veeläbilaskerajatisi (avariiülevool, torusild) ning koostatud ehitusaegsete rajatiste põhimõttelist tehnilist lahendust kajastavad ehitusjoonised. This Master’s thesis deems the removal of Saesaare dam on Ahja River the potential development scenario for the location in question, as it would solve the problem of fish migration and enable the natural ecosystem of Ahja River to recover. In a broader sense, this is considered an activity that supports achieving national environmental targets regarding the issue of the good condition of surface water. The main objective of this thesis is to propose an engineering solution for the demolition of Saesaare dam (complete removal of the dam and impounded lake) and for the restoration of close to nature rapids at the site of the dam. This thesis provides an exploratory overview of the current situation of Ahja River and the hydro-engineering complex of Saesaare. The natural condition of Ahja River at Saesaare prior to the construction of the hydro-engineering complex is investigated based on literature and archival material. Based on the collected data, two solutions are considered for the restoration of the rapids - the restoration of the former rapids (version 1) and the construction of replacement rapids (version 2) - with a selection of a preferred version and design of a technical solution, incl. technical drawings of rapids, for the chosen version. The preferred version developed in this thesis is version 2 (replacement rapids). The length of the designed rapids is 250 m; the bottom slope is approximately 1%. Three subvariants of the rapids were designed: one-branch rapids (2-A), two-branch rapids with a long island (2-B1) and twobranch rapids with a short island (2-B2). The demolition of the dam and the construction of rapids are planned to be performed in phases. Stage 1 includes the demolition of the spillway structures and the construction of the right branch of the rapids. Stage 2 includes the partial or complete (depending on the subvariant of the rapids) removal of the earth dam and hydropower plant structures, as well as the construction of the left branch of the rapids. The construction works are planned behind the soil cofferdams. A temporary dam of interlocking concrete blocks will be placed in front of the spillway to be demolished to safely lower the water level in the impounded lake (after the spillway has been removed and the right branch of the rapids has been shaped). The existing upgraded structures of the hydropower plant will be used to redirect flow during the construction works in Stage 1. In the event of a flood, the reconstructed spillway branch will also be used - for this purpose has been emergency overfall designed to the body of the headwater cofferdam. In Stage 2, after the impounded lake is drained, the flow is redirected through the completed right branch of the rapids, while ensuring a crossing over the completed riverbed for the construction technology by using a temporary pipe bridge. The temporary structures (emergency overfall, pipe bridge) have been dimensioned and the technical drawings with the principle technical solutions of the structures for the duration of the construction works have been created.
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