Figure1: A topographic map of the Berau region in Kalimantan, Indonesia. At Tanjung Redeb two rivers join to form the Berau river that splits close to Batu-Batu in an estuarine channel network.
Introduction
The Berau river (Figure 1), East Kalimantan, Indonesia, discharges substantial amounts of water and sediment into the coastal zone. The river is an example of tropical rivers that drain a relatively small area with high relief, where the sediment delivery is disproportionally large (Milliman et al., 1999). The sediment loads in the Berau river have increased drastically over the past decades, mainly due to deforestation of parts of the hinterland. Part of the terrestrial sediment is transported as suspended load to the barrier reefs, where it may harm the coral reef ecosystems.
This PhD project focuses on the distribution of freshwater and suspended sediment over the Berau river and the estuarine channel network. The project of Ayi Tarya investigates the flow and suspended transport from the estuary mouths to the barrier reefs.
Figure 2: Satellite image of the Berau river and channel network; The distance from left to right is about 80 km.
Objectives
- To analyse the hydrological sensitivity of the Berau drainage basin and to estimate the impact for soil erosion on selected hill slopes.
- To determine and understand the tidally averaged suspended sediment transports from the Berau river through the estuarine channel network towards the shelf sea. This is investigated for spring and neap tides and varying river discharges.
Methods
The project can be divided into two parts. (1) In the first part the first objective is addressed. Discharge and suspended sediment concentration observations over several months at the head of the Berau river are analyzed. The likely changes in sediment yields with land use changes in the Berau drainage basin are estimated by modeling soil erosion on characteristic hillslopes using the Morgan, Morgan and Finney approach. (2) In the second part the distribution of riverine water and sediment over the Berau river and the estuarine channel network is examined (Figure 2). Observations of water levels, discharge, salinity, temperature and turbidity throughout the Berau delta have been made for several months (see photos 1 and 2). Additionally, transects were sailed at the main junctions of the channel network for both spring tide and neap tide. This gives the discharge division at the junctions for the particular tidal cycles. To gain understanding of the observations DELFT3D models will be set up. For example the sensitivity of discharge division at a junction to depth, roughness and length of the channels on the sea side are investigated. Further, the tidally averaged flow and suspended sediment transport through the estuarine channel network can be derived from the numerical modeling.
Results
At Gunung Tabur (see Figure 1) the mean water level shows a variation with the spring and neap tide cycle. Around spring tide the tidally averaged (subtidal) water level is higher than around neap tide (Figure 3 middle and top). This can be understood by the higher flow velocity variation at spring tide that gives higher frictional energy losses than at neap tide. To overcome the higher frictional losses at spring tide, an additional subtidal water surface gradient is established with respect to neap tide. This larger subtidal water surface gradient is responsible for a cumulatively increasing subtidal water level variation in the tidal river going landward (Figure 3 middle). Generally, when the river flow velocity is considerable with respect to the main tidal flow velocity amplitude, interaction of the river flow with the tidal flow generates substantial subtidal water level variation.
Figure 3: (top) The water level at Lighthouse2 (red, see Figure 1), Batu-Batu (blue) and Gunung Tabur (black) and (middle) their tidally averaged water level; (bottom) discharge (black) and subtidal discharge (green) at Gunung Tabur.
Figure 4: Discharge at the head of the channel network close to Batu-Batu on the eastside of the junction (blue) and on the southside (black) for a tidal cycle during spring tide.A positive discharge is seaward.
Future work
The distribution of freshwater and sediments over the estuarine channel network is largely determined by tidally averaged flow division at the junctions. At three of the first large junctions at the head of the channel network flow and suspended sediment division were measured over a tidal cycle during spring and neap tide.
Significant time differences occurred between the flow reversal in the channels on the seaside of the junctions (see for example the discharge at the junction close to Batu-Batu in Figure 4).
These time differences of the flow reversals are related to the tidally averaged flow division. To derive and understand the tidally averaged flow and suspended sediment division at junctions, conceptual hydrodynamic models of a junction and of the entire Berau channel network will be constructed.
Further info
Project period
2005-2010
Collaborating institute
Department of Oceanography, Faculty of Earth Sciences and Technology, Institut Teknologi Bandung (ITB).
Subproject of larger project
From river basin to barrier reef: physical, biological and socio-economic aspects of the Berau system (East Kalimantan Programme)
Funding
NWO-WOTRO
Please contact Frans Buschman for more info on this project.