Sources of suspended sediment within a tropical catchment recovering from selective logging

Abstract

Abstract:Quantification of the source of suspended sediments generated by selective forestry activities is central to the development of sustainable forestry guidelines. This assessment is hampered by a dearth of available studies, particularly from the tropics. This study involved the monitoring (at 10 s intervals) of surface discharge and turbidity from 15 contributory areas of a 44 ha catchment. The catchment is located within lowland dipterocarp rain forest on Borneo Island, in a region recovering from the first episode of selective timber removal that took place some 5 years previously. Both the within-storm dynamics of the sediment flux and the time-integrated sediment yields were analysed to link the source landforms to the catchment behaviour.A 10 year rainfall event of 167 mm occurred during the monitoring period and triggered a debris slide and several log-culvert collapses along the area's main timber haulage road. The sampling design captured this event's dynamics and allowed lumped catchment response to be traced to the new landforms. During the 1 day period of the 10 year event, some 33 t of suspended sediment were transported from one debris slide, comprising a significant proportion of the 105 t discharging from the whole catchment, which itself constituted 40% of the annual yield of 592 t km 2 . The contributory areas with only ephemeral waterflows, including former haulage roads and tracks, generated relatively little sediment during this 10 year event or in other storms. This work suggests that though some sediment sources recover from the impacts of forest road construction and harvesting, collapse of roadfill materials or more local log-culvert failure persists for several years after harvesting. Sustainable forestry guidelines that do not focus on ameliorating these persistent instabilities may not significantly mitigate the geomorphic impacts of conventional, selective harvesting.

Estimating Costs and Effectiveness of Upgrades in Forestry Best Management Practices for Stream Crossings

“…Anthropogenic factors related to road construction and management are more readily manipulated. The literature provides considerable insight regarding erosion and sedimentation consequences of road location [9][10][11], road grade [12][13][14], road density [15], road area [16,17], road infiltration rates [18], road surfacing conditions [19,20], traffic levels [5], expected road duration of use [21], road templates including cut and fill slopes [22,23], adequacy of drainage structures [24], and road connectivity to streams [25,26]. These considerations capture the challenges addressed by forest road and skid trail best management practices (BMPs).…”

Soil pipe distribution and hydrological functioning within the humid tropics: a synthesis

“…3. An 8-cm diameter pipe which supplies all of the flow to weir E6 in the Baru catchment (and then drains into a 60-cm diameter pipe: Chappell et al, 2004a) has a time constant of approximately 1 h and hence is comparable to that of the third-order Baru stream (or W8S5 stream). This pipe outlet only produces flow irregularly and is located towards the top of the gully head at a height of 1Ð2 m. This contrasts with the W3 pipe of Sayer et al (2004) with its slower time constant, which discharges close to the base of the gully head, and hence close to the level which is regularly saturated (i.e.…”

Sediment Discharge through Buffer Zones in a Tropical Rainforest of Peninsular Malaysia

“…Soil physical properties including bulk density, total porosity, coarse porosity, fine porosity, and soil water retention were measured after collecting undisturbed cylindrical surface soil samples (400 cm 3 ) from four erosion-accumulation plots in the buffer zones. Samples were collected from eight points, including four points in the undisturbed area and four points in the area of accumulation (hereafter "accumulated area").…”

“…Several studies have been conducted in Sabah, Malaysia, relating to the discharge of sediment following selective logging 3,9,10,11,31 . However, in these areas, the most negative ecological impact of sediment discharge was not from tree felling, but from the construction of bulldozer paths 31 .…”