Figure 1: The Berau Delta and Barrier Reef System are located along the East Kalimantan coast in Indonesia.

Introduction

The Berau Delta and Barrier Reef System are located along the East Kalimantan coast in Indonesia (Figure 1). The sediment loads carried by 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 coral reef ecosystems. Coral reefs are complex, dynamic, and sensitive systems. Slight changes in the reef environment may have effects on the health of coral reefs. Therefore it is essential to understand and predict how the turbid water form the Berau river mixes over the shelf region and how this influences the dynamics in the coral reef area.

Objectives

The main aim of this project is to understand the hydrodynamics, turbulence characteristics, mixing of fresh water and transport of suspended sediment that are important for the coral reef system in the Berau inner shelf.

Figure 2: The drag coefficient inferred from observations with an ADCP in Fishing Platform (A) and 3 locations at reef environment (B,C,D).

Methods

Local moorings
To understand hydrodynamics and upwelling in the barrier reef a fixed ADCP (RDI 1200-kHz ) was deployed at three locations: Derawan Island, West Masimbung and East Masimbung. To characterize the Berau river plume a fixed ADCP and a Seabird SBE-19 CTD were placed at Fishing Platform. The data of the four locations is used to characterise and understand vertical profiles of velocity, turbulence and density.

Oceanographic surveys
Vessel mounted ADCP transects were sailed at a deep passage between Derawan Island and Masimbung reef, and a shallow passage between Masimbung and Pinaka reef. The main aim of measuring along these transects is to characterize tidal and subtidal flow patterns over a channel transect and to determine the exchange processes between the continental shelf and the ocean.

Hydrodynamic modeling
A hydrodynamic model is set-up to interpret the data and to understand the hydrodynamic processes at the regional scale of the Berau inner shelf and on the local scale of the barrier reef. The regional model aims to investigate tidal and subtidal flow patterns and how fresh water and fines are mixed and transported at the Berau Inner Shelf. The small scale model results are used to understand process of upwelling along the steep reef slope and to model the boundary layer structure above the rough coral reef. The results will also be used to understand and quantify the exchange of water and sediments between the inner shelf and the ocean.

Figure 3: Amplitude contours every 0.1 m (red line) and Phase contours every 5 degrees (blue line) of the M2 constituent. Simulation without barrier reef (left panel) and with barrier reef (right panel).

Results

A barotropic model was set up to study the character of the barotropic tide and the tide-induced mean circulation patterns at the scale of the Berau inner shelf. In addition, the model results are used to study the effect of the barrier reef on the propagation of the tide. It turns out that the barrier reef influences the hydrodynamics only at the scale of the reef and that the shelf-wide propagation of the tide is only marginally influenced by the presence of the reef. On the other hand, analysis of the ADCP data reveals that typical drag coefficients in the reef passage should be taken a factor 5 larger than typical values that are found on sandy shelfs (Figure 2). The M2 tide is the dominant constituent. It propagates in the cross-shelf direction and increases in amplitude when it reaches the land (Figure 3). In the shallow parts of the shelf a mean current is modelled that compensates for the mass flux induced by the tide (Figure 4). At the transition from the shallow part to the inner shelf a mean current is generated that might transport sediment and fresh water towards the reef. This mean current can be understood by taking a careful look at the advective tidal processes and is cause by the shoaling and refraction of the tidal wave over the shallow shelf area.

Future work

• Paper 2 : Freshwater dispersion at the Berau Continental Shelf.
  Aim : to establish and understand baroclinic processes at the Berau Inner Shelf governing river plume behavior.
• Paper 3: Residual flow through a barrier reef at the shelf-edge.
  Aim: to understand subtidal flow patterns through barrier reef passages and over shallow reef flats based on data and model simulations. To understand the upwelling along a steep reef slope.
• Paper 4: Transport of terrigenous sediment towards the Berau barrier reef complex.
  Aim: to understand the mechanisms responsible for the transport of terrigenous sediment
  towards the Berau barrier complex.

Figure 4: Residual depth averaged tide induced current. Simulation without barrier reef (left panel) and with barrier reef (right panel).

Further info

Project period
2006-2010

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 Ayi Tarya for more info on this project.