Water and sediment distribution at lowland river junctions: the Mahakam Lakes region
09 / 2007 - 01 / 2014
River junctions are nodal points where river channels confluence or diverge. Over the past decades much research has been focused on channel junctions in braided rivers. These are morphologically highly dynamic alluvial environments, which occur typically in steep upstream river reaches. Physical analyses of channel junctions in lowland rivers are relatively few. Especially bifurcation processes controlling the water and sediment distribution over lowland distributaries are not well understood. The distribution function of river bifurcations is captured in nodal point relations used for one-dimensional morphodynamical models, which relate water and sediment flow rates. These relations can be used to analyze the stability of a bifurcation, which can be defined as the likeliness that both the downstream branches remain open. The present objective is to include more physical processes in existing nodal point relations, and to analyse the effect of local morphodynamic changes on the stability of a bifurcation. It is proposed to analyze river junctions in the Mahakam Lakes region, which is an extremely flat area that seldom reaches 10 m above mean sealevel. As a result of the small bed level gradient the Mahakam River is meandering with a tendency to anastomose. On opposite sides the Mahakam is connected to about 30 shallow lakes, which modulate the water discharge. Although no evidence exists to date, it is assumed that the Mahakam Lakes alternately feed and drain the Mahakam, preventing extreme floods or low flow conditions. Accordingly, river junctions may alternately be regarded as a bifurcation (divergence) and a confluence. The functioning of the channel junctions in the Mahakam Lake region will be analysed ultimately aiming to establish the vulnerability of the hydraulic connection between the lakes and the Mahakam River, which discharges into the economically important Mahakam Delta. The proposed methods include field measurements, satellite radar remote sensing for observation of water surface elevation and width, theoretical analysis and morphodynamic modelling. The field campaign encompasses bathymetric surveys, installation of water level gauges and ADCP measurements of flow and sediment transport. A two-dimensional morphodynamic model will be used after calibration to simulate morphometric changes of the bifurcations between the Mahakam river and the connecting channels to the three main lakes. The nodal point relations will be used in the network model setup in WP10 of the research cluster.