Flow and sediment transport in an Indonesian tidal network

Abstract

The Berau river, situated in east Kalimantan (Indonesia), drains a relatively small catchment area and splits into several interconnected tidal channels. This tidal network connects to the sea. The sea is host to extremely diverse coral reef communities. Also the land side of the region is relatively pristine and biologically rich. The Berau river basin still has a relatively high rainforest cover, which was 50 - 60 % in 2007. Currently, large-scale deforestation is taking place in the catchment, especially since 2005. As the Berau region develops further and deforestation takes place, the sediment load in the Berau river is likely to increase. Increasing sediment loads may be an increasing threat for the coral reefs. The impact of an increasing sediment load on the coral reef ecosystem depends on the pathway of the sediment in the river and tidal network. Since the largest areal of the coral reefs is situated close to the northern branch, the threat is largest when sediment is expelled at this channel mouth. To investigate the impact of increasing sediment load on the coral reefs, the sediment distribution over the Berau river and the tidal network needs to be predicted. The flow and sediment dynamics in the Berau river are affected by the tides. Discharge observations in the river show that water flows both landward and seaward within a tidal cycle. The river discharge was obtained by averaging the discharge over a tidal cycle. The river discharge was 605 m3/s averaged over several months in 2007 and was maximally about 1400 m3/s. At the tidal junction where the Berau river first splits, observations of flow suggests that the tidally averaged water division depends on the tidal range. This is further investigated using an idealized numerical tidal junction model. For differences between the two channels at the sea side in depth, length and width decay, tides generally amplify unequal tidally averaged flow division that occurs in the case of river flow only. Tidally averaged flow into the deeper, shorter or on average wider channel increases with increasing tidal range. For roughness differences, however, the effect of tides is to enhance tidally averaged flow into the channel with higher roughness. The tidal effect partly cancels the preference for the channel with lower roughness in the case of only river flow. This tidal effect can be explained by evaluating the balance of Stokes transport and return discharge in each channel. At a more seaward located tidal junction, phase differences in flow between the channels were up to 2 hours, which is about one sixth of the dominant tidal period. Since the water level variation is similar at a tidal junction, these phase differences imply differences in Stokes transports per unit width. These differences play a substantial role in the tidally averaged flow division at the tidal junction. Also density differences have an effect on the flow division and sediment division. Especially the sediment division is also affected by flow circulation across the channels.