Suspended sediment load in the tidal zone of an Indonesian river

Abstract

Abstract. Forest clearing for reasons of timber production, open pit mining and the establishment of oil palm plantations generally results in excessively high sediment loads in tropical rivers. The increasing sediment loads pose a threat to coastal marine ecosystems, such as coral reefs. This study presents observations of suspended sediment loads in the Berau River (Kalimantan, Indonesia), which debouches into a coastal ocean that is a preeminent center of coral diversity. The Berau River is relatively small and drains a mountainous, still relatively pristine basin that receives abundant rainfall. In the tidal zone of the Berau River, flow velocity was measured over a large part of the river width using a horizontal acoustic Doppler current profiler (HADCP). Surrogate measurements of suspended sediment concentration were taken with an optical backscatter sensor (OBS). Averaged over the 6.5 weeks covered by the benchmark survey period, the suspended sediment load was estimated at 2 Mt yr −1 . Based on rainfall-runoff modeling though, the river discharge peak during the survey was supposed to be moderate and the yearly averaged suspended sediment load is most likely somewhat higher than 2 Mt yr −1 . The consequences of ongoing clearing of rainforest were explored using a plot-scale erosion model. When rainforest, which still covered 50-60 % of the basin in 2007, is converted to production land, soil loss is expected to increase with a factor between 10 and 100. If this soil loss is transported seaward as suspended sediment, the increase in suspended sediment load in the Berau River would impose a severe stress on this global hotspot of coral reef diversity.

Wind forcing controls on river plume spreading on a tropical continental shelf

“…The southern branch is expected to be of minor importance for the transfer of river water to the sea. The freshwater discharge of the Berau river varies between 100 and 2000 m 3 s −1 , averaging roughly 500 m 3 s −1 [ Buschman et al ., ; Hoitink et al ., ; Buschman et al ., ]. A coral reef area, located near 118.2 degrees west and between 2.0 and 2.4 degrees north, is located at the shelf break.…”

Use of ADCPs for suspended sediment transport monitoring: An empirical approach

“…The bed is probably the source of interference in view of transducer side-lobe effects but we remain uncertain-in any event, the signal appears to be spurious beyond 60 m. This outcome compromises hADCP in-beam calibration measurements beyond that distance, consequently we confine attention to a beam-averaged calibration covering the first 60 m. In this sector, relatively quiet water carries silt and fine sand with limited size variation over the season and in the vertical (Figure 9), and a small range in concentration ( Figure 10), making the accessible sampling volume relatively ideal for consistent hADCP sediment detection. (Buschman et al [2012] have previously described low cross-sectional variation in suspended sediment concentration in a silt-clay dominated river.) Figure 6.…”

“…( Buschman et al . [] have previously described low cross‐sectional variation in suspended sediment concentration in a silt‐clay dominated river. )…”

Resilience of River Deltas in the Anthropocene

“…However, because light passing through water is both attenuated as a function of water depth and color, and scattered by particles, measurements are limited to less than 6 m, where turbidity is affected by sediment concentrations of 0.2-9.0 mg/L (Gao, 2009). Considering that suspended-sediment concentrations in most delta systems regularly exceed 100 mg/L (Eidam et al, 2019;Buschman et al, 2012;Hale et al, 2019;Falcini et al, 2012;Walker & Hammack, 2000), optical sensing of the water-sediment interface in deltaic systems is rarely possible below a few decimeters. This limitation severely hinders the ability to detect rapid changes in shallow bathymetry, which can include erosion and deposition exceeding 1 m over a single flood or storm event (Jaramillo et al, 2009;Hale et al, 2014;Khan et al, 2013;Shaw & Mohrig, 2014).…”