Understanding river dynamics of the Ayeyarwady River, Myanmar How dynamic behaviour contributes to adapting the river morphology for navigational purposes

Abstract

The Ayeyarwady River in Myanmar is an important navigational trading route and contributes a large part to the Myanmar economy. Almost all of the rivers drainage basin lies within the country borders. It flows from the Himalayan mountain range in the north to the Andaman Sea in the south. It is a melt water and rain fed river with most of the discharge flowing between May and October. Water levels between winter and summer (monsoon period) vary up to 10 metres. This variation in water levels and the dynamic morphology of the river create navigational problems. Often water depths are not sufficient and the channel thalweg – which is the best navigational route – is continuously changing. Its behaviour is unpredictable and navigational management teams find difficulties anticipating on these changes. This research is part of the Deltares contribution to the Ayeyarwady Integrated River Basin Management plan (AIRBM), commissioned by a partnership between the Netherlands and Myanmar. The project team is assigned the task to update an existing River management master planand to propose two river engineering pilots. The Deltares project area is situated between the important harbour of Mandalay and the city Nyaung-U near touristicBagan.In this area the Chindwin River joins the Ayeyarwady River. This research focuses on the river reach below the confluence point, which is dynamically braiding and shows many variations in its planform. The main objective is to determine how the dynamic behaviour of the river can contribute to adapting the river morphology for navigational purposes. Satellite images were used to describe and visualise the morphologic changes of the river in the past 15 years. Simulations with Delft3D-FLOW contributed to a better understanding of these processes and to review the effect of upstream changes on downstream sections. A data analysis of the water level and discharge datasets provided information on the hydrology of the river. Sediment size and sediment discharge for the past 35 years were reviewed. Also a field trip in Myanmar contributed to analysing the active shift of the river. In this report an overview is provided of the morphodynamic changes of the river within the research area. Inmost of the research area the river has two parallel channels except for the sinuous reach at Nyaung-U. The island in between the two channels is formed by formation of braiding bars and consequent stabilisation by vegetation. At the point where the two channels confluence, the confluence angle and discharge ratio of the two channels determines the main flow direction downstream and with that the downstream river planform. In the research area there are two types of river banks – the resistant river banks and the non-resistant floodplains. The channels along the resistant river banks are confined and stabilized, with higher flow velocities and larger water depths attracting the major part of the discharge. At locations with two channels which are both not confined, bar migration is more active and the channel thalweg is difficult to determine. For river engineering,structures should be used which are adaptable to the ever-changing river dynamics, preferably even on a 5-year time scale. When adjusting the planform of one section of the river, the morphological effects will be visible for more than 50 kilometres downstream. It is recommended to start the first Deltares project pilot in the upstream parts of the river and review its upstream hydrologic and downstream morphologic effects with model simulations combined with past planform changes. Current research and model simulations are not of sufficient quality to guarantee the effect of river management plans. To improve the quality of these models, hydrologic data with higher accuracy is required.