Forestry-related erosion and slope instability:
Spatially-significant effects of selective tropical forestry on water, nutrient and sediment flows: a modelling-supported review
There is an increasing desire to improve the environmental sustainability of timber harvesting within tropical natural forests. These efforts are, however, hampered by the dearth of robust interpretations of the impacts of selective (non-clearfell) forms of tropical forestry on water, nutrient and sediment flows at meaningful, landscape-scales. This review, therefore, seeks to (a) assimilate the results of studies of selective forestry impacts on catchment-scale, ecohydrological flows in tropical natural forests (as opposed to tropical plantation forests), and (b) examine the value of data-based (time-series) modelling in the assimilation of the most reliable case studies.
The findings indicate that very few studies have measured catchment-scale water, nutrient or sediment flows within natural tropical forests undergoing selective logging. Most of the studies that do exist have been undertaken within one country - Malaysia. Statistical modelling and numerical modelling (using Data-Based-Mechanistic or 'DBM' approaches) of the limited data that does exist, indicate that: (i) catchment water-yield is increased but by less than a factor of two, (ii) the rate of migration of rainfall through a catchment to a river and hence the 'river responsiveness' may be increased, but only slightly and for a short period, (iii) nutrient flows increase by a factor of 1 to 6 in the harvesting year, (iv) river sediment flows are significantly increased by a factor of 2 to 50, though more precise explanation of this range remains unclear, (v) the effect of natural cycles (e.g., El Niņo Southern Oscillation) in the rainfall may exert as strong an influence on the river records as the selective forestry activities, (vi) differences in the physical impact of different forms selective forestry remain poorly quantified at the landscape-scale, and (vii) the rate of recovery of the catchment system from selective forestry impacts, in terms of water-yield, nutrient loss and sediment delivery, also remains poorly quantified at the landscape-scale.
Further catchment-scale studies to support, scientifically, explanations of the differential impacts of different selective forestry operations in tropical natural forests, particularly on sediment delivery where the greatest relative changes are seen, are urgently needed. Further, the extension of such water, nutrient and sediment studies over 10 to 30 years is needed to quantify the recovery and hence environmental sustainability of such natural forest systems within the tropics. Data-based modelling approaches, such as DBM, may assist further rainforest catchment studies by allowing us to (i) undertake quality assurance of the existing and new river data, and (ii) extract selective forestry impacts from natural cycles and trends. Once such data-based modelling approaches have indicated that changes have taken place, then process studies (even undertaken at small scales) and more process-based models would be useful in further developing guidelines of more sustainable forestry operations.
Chappell, N.A., Nik, A.R., Yusop, Z., Tych, W and Kasran, B. 2004. Forests, Water and People in the Humid Tropics,
Bonell M. and Bruijnzeel, L.A. (Eds), Cambridge University Press, Cambridge.
Understanding the affect of rain-storms and selective forest management on riverflow and eroded sediment flows: A statistically acceptable modelling approach
Considerable field research into the effect of forestry and agroforestry operations on soil erosion has taken place in South-East Asia over the last 40 years (e.g., Bruijnzeel, 1990). Recently developed, physics-based models of soil erosion and resultant sediment flow - 'SSflux' (e.g., Favis-Mortlock et al., 1996) 'promise' an ability to simulate the site-specific relationships (between rainfall characteristics, physical properties and forestry operations) contained within these field studies. Clearly, a capability to separate the effects of specific forestry operations from other site factors would aid in the identification (and demonstration) of 'best' or 'most sustainable' forestry practices (see e.g., Bruijnzeel, 1982; Abdul Rahim Nik et al., 1997). Model calibration of a single-output-variable combined with limited catchment data on soil-geological properties does, however, mean that such physics-based models can give highly uncertain predictions; though this is rarely quantified in rigorous uncertainty analyses (Beven, 1996). As a result, such approaches can sometimes give a false sense of understanding of the processes or likely scenarios.This presentation demonstrates how the mobilisation and transport of suspended-sediments within multi-scale catchments can be modelled without the use of complex (high order) and hence uncertain, model formulations. The modelling uses rainfall, waterflow and sediment data collected within the Baru Experimental Catchment of the Ulu Segama Forest Reserve - which is part of Yayasan Sabah timber concession in Sabah, Malaysian Borneo. Some of the localised erosional-landforms within the catchment relate to selective forestry operations that took place some five years prior to the collection of the model data-set. The results of a Data-Based-Mechanistic (DBM) modelling approach, which combines physically-based understanding with model-structure identification based on transfer-functions and objective statistical inference (Young and Beven, 1994), are presented here, though conceptual models (e.g., TOPMODEL) have also been applied to the same data-series (e.g., Chappell et al., 1998). Model realisations of the 'rainfall to riverflow' and 'rainfall to SSflux' behaviour of five of the nested, contributory areas of the Baru Experimental Catchment indicate (i) that these systems exhibit relatively low-order behaviour that can be modelled with a few (and hence relatively certain) model parameters (Chappell et al., in press), (ii) that parametric differences between the different contributory areas can be (tentatively) explained in process-based terms, and (iii) that the exploratory modelling approach adopted is capable of identifying new processes and properties that should be measured to give better explanation. With this initial success - more complex model formulations that retain a similar degree of statistical credibility are being developed. The importance of road-landslides to the understanding and successful modelling of even low-order erosional systems was, however, very clear.
Chappell, N.A., Chotai, A. and Bidin, K. 1999. Proceedings of the CFFPR conference 'Water: Forestry and Landuse Perspectives', Forestry Research Institute of Malaysia (FRIM), Kepong, 31 March to 1 April 1999.
Minimising the hydrological impact of forest harvesting in Malaysia's rain forests
A process of formal certification of forest harvesting practices undertaken within Malaysia's rain forests is ongoing. The criteria* and indicators* used to assess the sustainability* of these practices within the Malaysian environment ('MC&I') specifically address measures to minimise damage to the hydrological system.
This study details those MC&I and associated benchmarks (or Standards of Performance*) that help minimise hydrological impacts. Ability to meet the hydrological benchmarks during forest certification* was examined using a case study of an independent re-assessment of a Forest Management Unit* (i.e., Selangor State FMU) certified as 'sustainably managed'. Lastly, the consistency of the benchmark standards with current understanding of tropical forest hydrology was addressed, with strengths and weaknesses being identified.
Thang H.C., and Chappell, N.A. 2004. Forests, Water and People in the Humid Tropics,
Bonell M. and Bruijnzeel, L.A. (Eds), Cambridge University Press, Cambridge.
Source of suspended-sediment within a tropical catchment recovering from selective logging
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 second 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 tonnes of suspended sediment were transported from one debris slide comprising a significant proportion of the 105 tonnes discharging from the whole catchment, which itself constituted 40 % of the annual yield of 592 t km-2 yr-1. 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 would suggest that while 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.
Chappell, N.A., Douglas, I., Hanapi, J.M., and Tych, W. 2004. Hydrological Processes, 18, 685-701.
Inta-catchment dynamics of water and sediment
Understanding the changing behaviour of water and sediment flows from points on a hillslope to a whole 'experimental catchment' is critical to the identification of the hydrogeomorphic effects of specific land-use practices. Such terrain manipulations may be associated with forestry, agricultural, or urban development. This paper utilises a novel modelling technique (that allows the data to indicate the form and complexity of model structure) to characterise rainfall-waterflow and rainfall-sediment flow within a small tropical catchment. This experimental catchment is recovering from the complex patchwork of disturbance associated with the practices of 'selective timber harvesting'. Differences in the dynamic behaviour at scales ranging from 0.1 ha slope plots to the 44.1 ha catchment can be captured by the DBM-modelling undertaken. Explanation of differences in sediment mobilisation with scale change, can be seen to be closely associated with changing waterflow behaviour, in terms of responsiveness and process (i.e., the balance of infiltration-excess and return-flow pathways). These analyses provide: (a) the basis for a conceptual model of the relationship between intra-catchment hydrogeomorphic scales, and (b) a demonstrable linkage between forestry-induced landforms and landscape-scale behaviour.
Chappell, N.A., Tych, W., and Douglas, I., 2001. Trans., Japanese Geomorphological Union, 22(4), C38.
Understanding the affect of rain-storms and selective forest management on riverflow and eroded sediment flows: A statistically acceptable modelling approach
Considerable field research into the effect of forestry and agroforestry operations on soil erosion has taken place in South-East Asia over the last 40 years (e.g., Bruijnzeel, 1990). Recently developed, physics-based models of soil erosion and resultant sediment flow - 'SSflux' (e.g., Favis-Mortlock et al., 1996) 'promise' an ability to simulate the site-specific relationships (between rainfall characteristics, physical properties and forestry operations) contained within these field studies. Clearly, a capability to separate the effects of specific forestry operations from other site factors would aid in the identification (and demonstration) of 'best' or 'most sustainable' forestry practices (see e.g., Bruijnzeel, 1982; Abdul Rahim Nik et al., 1997). Model calibration of a single-output-variable combined with limited catchment data on soil-geological properties does, however, mean that such physics-based models can give highly uncertain predictions; though this is rarely quantified in rigorous uncertainty analyses (Beven, 1996). As a result, such approaches can sometimes give a false sense of understanding of the processes or likely scenarios.This presentation demonstrates how the mobilisation and transport of suspended-sediments within multi-scale catchments can be modelled without the use of complex (high order) and hence uncertain, model formulations. The modelling uses rainfall, waterflow and sediment data collected within the Baru Experimental Catchment of the Ulu Segama Forest Reserve - which is part of Yayasan Sabah timber concession in Sabah, Malaysian Borneo. Some of the localised erosional-landforms within the catchment relate to selective forestry operations that took place some five years prior to the collection of the model data-set. The results of a Data-Based-Mechanistic (DBM) modelling approach, which combines physically-based understanding with model-structure identification based on transfer-functions and objective statistical inference (Young and Beven, 1994), are presented here, though conceptual models (e.g., TOPMODEL) have also been applied to the same data-series (e.g., Chappell et al., 1998). Model realisations of the 'rainfall to riverflow' and 'rainfall to SSflux' behaviour of five of the nested, contributory areas of the Baru Experimental Catchment indicate (i) that these systems exhibit relatively low-order behaviour that can be modelled with a few (and hence relatively certain) model parameters (Chappell et al., in press), (ii) that parametric differences between the different contributory areas can be (tentatively) explained in process-based terms, and (iii) that the exploratory modelling approach adopted is capable of identifying new processes and properties that should be measured to give better explanation. With this initial success - more complex model formulations that retain a similar degree of statistical credibility are being developed. The importance of road-landslides to the understanding and successful modelling of even low-order erosional systems was, however, very clear.
Chappell, N.A., Chotai, A. and Bidin, K. 1999. Proceedings of the CFFPR conference 'Water: Forestry and Landuse Perspectives', Forestry Research Institute of Malaysia (FRIM), Kepong, 31 March to 1 April 1999.
The role of extreme events in the impacts of selective tropical forestry on erosion during harvesting and recovery phases at Danum Valley, Sabah
Ten years' hydrological investigations at Danum have provided strong evidence of the effects of extremes of drought, as in the April 1992 El Nino southern oscillation event, and flood, as in January 1996. The 1.5 km(2) undisturbed forest control catchment experienced a complete drying out of the stream for the whole 1.5 km of defined channel above the gauging station in 1992, but concentrated surface flow along every declivity from within a few metres of the catchment divide after the exceptional rains of 19 January 1996. Under these natural conditions, erosion is episodic. Sediment is discharged in pulses caused by storm events, collapse of debris darns and occasional landslips. Disturbance by logging accentuates this irregular regime. In the first few months following disturbance, a wave of sediment is moved by each storm, but over subsequent years, rare events scour sediment from bare areas, gullies and channel deposits. The spatial distribution of sediment sources changes with time after logging, as bare areas on slopes are revegetated and small gullies are filled with debris. Extreme storm events, as in January 1996, cause logging roads to collapse, with landslides leading to surges of sediment into channels, reactivating the pulsed sediment delivery by every storm that happened immediately after logging. These effects are not dampened out: with increasing catchment scale. Even the 721 km(2) Sungai Segama has a sediment yield regime dominated by extreme events, the sediment yield in that single day on 19 January 1996 exceeding the annual sediment load in several previous years. In a large disturbed catchment, such road failures and logging-activity-induced mass movements increase the mud and silt in flood waters affecting settlements downstream. Management systems require long-term sediment reduction strategies. This implies careful road design and good water movement regulation and erosion control throughout the logging process.
Douglas, I., Bidin, K., Balamurugam, G., Chappell, N.A., Walsh, R.P.D., Greer, T., and Sinun, W. 1999. Phil. Trans. Roy. Soc. Lond. B., 354, 1749-1761
Parsimonious modelling of water and suspended-sediment flux from nested-catchments affected by selective tropical forestry
The ability to model the suspended-sediment flux (SSflux) and associated waterflow from terrain affected by selective-logging is important to the establishment of credible measures to improve the ecological sustainability of forestry practices. Recent appreciation of the impact of parameter uncertainty on the statistical credibility of complex models with little internal-state validation supports the use of more parsimonious approaches such as Data-Based-Mechanistic (DBM) modelling. The DBM approach combines physically-based understanding with model-structure identification based on transfer-functions and objective statistical inference. Within this study, these approaches have been newly applied to rainfall-SSflux response. The dynamics of the sediment system, together with the rainfall-riverflow system, were monitored at five, nested contributory areas within a 44-ha headwater region in Malaysian Borneo. The data series analysed covered a whole year at a 5-minute resolution, and were collected during a period some 5-6 years after selective timber harvesting had ceased. Physically-based and statistical interpretation of these data was possible given the wealth of contemporary and past hydro-geomorphic data collected within the same region.
The results indicated that parsimonious, three-parameter models of rainfall-riverflow and rainfall-SSflux for the whole catchment describe 80 and 90 % of the variance, respectively, and that parameter changes between scales could be explained in physically meaningful terms. Indeed, the modelling indicated some new conceptual descriptions of the riverflow- and sediment-generation systems. An extreme rainstorm having a 10-20-year return-period was present within the data-series and was shown to generate new mass movements along the forestry roads that had a differential impact on the monitored contributory areas. Critically, this spatially-discrete behaviour was captured by the modelling and may indicate the potential utility of DBM approaches for (a) predicting the differential effect of alternative forestry practices, (b) estimating uncertainty in the behaviour of ungauged areas and (c) forecasting riverflow and SSflux in terrain with temporal changes in rainfall regime and forestry impacts.
Chappell, N.A., McKenna, P., Bidin, K., Douglas, I., and Walsh, R.P.D. 1999. Phil. Trans. Roy. Soc. Lond. B., 354, 1831-1846.
Correlation of physicochemical properties and sub-erosional landforms with aggregate stability variations in a tropical Ultisol disturbed by forestry operations
The stability of soil aggregates against water erosion is a largely unstudied factor in research on the spatially-distributed impacts of tropical forestry. Soil strata with particularly weak aggregates are likely to be the focus for erosional activity whether buried or exposed by either natural processes or forestry activities. Understanding the location and likely cause of such intrinsic instability would, therefore, allow better spatially-distributed parameterisation of erosion models. This study has aimed to identify the range in aggregate stability within a region of Bornean Ultisol disturbed by forestry operations, and to identify the bulk soil properties associated with such stability variations. The sites sampled were subject to a range of denudational processes including piping, rilling and landslide-triggered erosion. Soil profiles with rates of erosion in excess of 10 mm a(-1) were shown to have less aggregate stability (as characterised by the rainfall simulation survival index (RSSI)) than those with no visible signs of erosion. Further, large differences in aggregate stability between soil horizons of the same profile were observed and seen to be statistically correlated with the bulk soil properties of organic carbon and clay content (i.e. stabilising agents) and the dispersing agent of exchangeable sodium percentage (ESP) at sites undergoing erosion. Organic carbon appeared to be the most important governing factor, accounting for 56% of the variance in the aggregate stability. The presence of strongly and weakly expanding 2 : 1 clays within the soils may have been an additional de-stabilising factor. Reduced porosity was also observed at eroding sites with weak soil aggregates. The properties of EC25 and sodium adsorption ratio (SAR) were not correlated with the RSSI or the presence of soil erosion, Identification of the role of organic carbon, clay and ESP in the stability of these tropical forest aggregates is important in focusing future, more intensive research on the spatial parameterisation of models to simulate forestry impacts on erosion.
Chappell, N.A., Ternan, J.L., and Bidin, K. 1999. Soil and Tillage Research, 50, 55-71.
Upscaling suspended-sediment flows in disturbed rain forest terrain: role of localised new sources
Modelling the impacts of complex distributions of sediment producing landforms on catchment-scale sediment behaviour requires an understanding of the role of (a) scale, and (b) new sediment-sources. This paper first examines the variations in sediment flux from 11 land-units ranging in size from the 0.1 ha 'landform-scale' to the 44 ha 'experimental-catchment'. Second, the work demonstrates the role of new sediment-sources with high sediment yields (sometimes triggered by extreme hydro-meteorological conditions) on behaviour at the landform to catchment scale. The analysis is based on 5-minute datalogged waterflow and suspended-sediment (calibrated turbidity) data from 11 weir and tipping-bucket gauging stations monitored over the period 1 July 1995 to 30 June 1996. Detailed work on sub-grid scale denudation and erodibility patterns was also undertaken. The study catchment is located within an area of tropical Borneo, where selective timber harvesting in 1988-89 left a mosaic of active erosional landforms including road gullies, eroding skid trails, mass movements and disturbed channel-heads.
Analysis of the time-series data indicated that the impact of localised road-culvert collapses and rotational landslides propagated through to the catchment-scale. Mass movements at new locations appeared part way through the time-series, giving rise to marked changes in the pattern of sediment flux within the catchment (compared to areas where gully or sheet-wash erosion dominated). This localised behaviour is likely to give rise to non-stationarity in sediment behaviour at the lumped catchment scale. Furthermore, not all of the mass movement events observed were related to extreme hydro-meteorological conditions and each had locally complex stability/erodibility patterns.
These conclusions lead the authors to investigate the value of transfer-functions (not requiring the definition of geomorphic parameters, such as stability/erodibility and topography) in identifying spatial and temporal patterns in suspended-sediment flux.
Chappell, N.A., McKenna, P., Bidin, K., Douglas, I., and Walsh, R.P.D., 1998. Proceedings of the 3rd International Conference on GeoComputation, University of Bristol, 17-19 September 1998. (ISBN 0-9533477-0-2).
