Erosional dynamics, sediment sources and designing strategies to reduce downstream sedimentation and flooding in oil palm terrain in Sabah (Malaysian Borneo).

Abstract

Amongst the adverse consequences of oil palm land-use in the wet tropics are high rates of hillslope and catchment erosion and enhanced downstream channel change, sedimentation and flooding.  These consequences are being exacerbated by some features of current and predicted future climatic change (notably increases in magnitude-frequency-intensity of large rainstorms) and the spread of oil palm to cover greater proportions of landscapes and into steeper terrain.  Although there is growing awareness of these problems within the oil palm industry including the adoption of some conservational measures, such as maintenance of ground covers of low vegetation and piles of palm fronds, retention or restoration of riparian forest strips (or strips of dense low vegetation) and (in steep terrain) of engineered terraces and roadside ditches), the effectiveness of such measures has largely been assumed rather than systematically assessed.  Furthermore, erosion studies have been of catchment sediment yields covering establishment and the early years of oil palm plantation and within-catchment erosion and sediment sources and the mature and old-age phases of the oil plantation cycle have received little attention.  These research gaps form the focus of this paper. It uses results of hydrological components of the long-term (since 2011) multi-catchment Stability of Altered Forest Ecosystems (SAFE) programme investigating impacts of multiple phases of selective logging and conversion to oil palm in the steep headwaters areas of the Kalabakan, Brantian and Segama catchments in eastern Sabah, Malaysian Borneo.  The focus is on a small mature (> 20 year-old) oil palm catchment, but with comparisons with multiple-logged and primary forest catchments. Results are reported for (1) suspended sediment (turbidity) and streamflow dynamics and magnitudes at catchment gauging stations instrumented with depth and turbidity sensors and Campbell data-loggers, (2) soil erosion rates from networks of erosion bridge sites,  (3) channel size and change at networks of repeat-measurement cross-sections, and (4) within-storm observations and measurements of suspended sediment concentrations of road ditches and overland flow.  The findings are used to assess the relative importance of sources of sediment within the oil palm catchment.  The results suggest that erosion from the oil palm slopes and from roadside ditches of the dense road/track network characteristic of oil palm terrain, although significantly higher than from forested slopes, are not the main sources of sediment, which appear to be (1) enlargement of valleyside ephemeral channels by road ditch runoff, as well as (2) erosion of unsurfaced tracks and roads and (3) fluvial erosion.  Possible practical ways to reduce storm sediment and streamflow peaks from oil palm terrain (and thereby reduce downstream sedimentation and flooding problems) are discussed. These include ways of reducing delivery of road runoff to valleyside ephemeral channels and erodible sections of the road/track system by directing road runoff into vegetated soakaways on oil palm slopes.