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UNESCO-IHE Institute for Water Education, Delft (2011)

Morphodynamic effects of dam construction in the Tuul River (Mongolia)

Sukhbaatar, C

Titre : Morphodynamic effects of dam construction in the Tuul River (Mongolia)

Auteur : Sukhbaatar, C

Etablissement de soutenance : UNESCO-IHE Institute for Water Education, Delft

Grade : Master of Science (MS) 2011

A dam will be constructed on the Tuul River, which is one of the biggest rivers in the northeast of Mongolia. This dam, which has multiple purposes, was planned by the Water Agency of Mongolia and Ministry of Nature and Environment in 2008. After the dam is constructed, short and long-term morphological changes will take place in the river downstream and upstream of the dam. This thesis studed the effects of the planned dam on the Tuul River morphology and more in general on the river environment. The Tuul River is one of the biggest rivers in the northeast of Mongolia. The river originates from the confluence of the Nergui River and the Namya River which flow from the Khan Khentiin Nuruu Mountain which is 2000 m above sea level. The largest tributaries of the Tuul River are the Khagiin-gol River (159 km) and the Terelj River (59 km). The Tuul River is the largest tributary of the Orkhon River, which flows into the Selenge River. The Tuul River is 704 km long and its drainage area is 50400 km3(Gombo and Magsar, 2004). The Tuul River flows across Ulaanbaatar city, which is the Mongolian capital and the largest city. The population of Ulaanbaatar is one million. The drinking and industrial water is extracted from 175 wells, which are located along the Valley of Tuul River. The study reach is the part of the river downstream of the planned dam and is 5.8 km long. In this area the river splits in two channels that join after 2 km. Due to dam construction, one of the two channels may become dominant, changing the characteristics of this river reach. The effects will be felt also by the floodplain environment. The outputs of the study are future morphological short and medium-term changes. The output has been derived from a Delft 3D morphological model. Three scenarios (A, B and C) have been considered : without dam, with dam and sediment bypass, as well as with dam and no sediment bypass. The simulations cover periods of 1, 5 and 10 years. The output of this thesis is a comparison of sedimentation and erosion trends, river pattern and water levels between scenarios A, B and C. Since the river is gravel-bed, the presence of the dam will increase bed armouring, which result in a reduction of erosion rates if sediment is bypassed. If all sediment is kept inside the reservoir (no by pass) the river downstream will be subject to erosion. Reduction of water flow in spring will result in lowered ground water levels in the period of vegetation growth. This might strongly affect the floodplain environment.

Sujets  : dam construction ; morphodynamics ; river morphology ; Mongolia


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