Geotechnical Testing Journal, Vol. 27, No. 2 Paper ID GTJ11503 Available online at: www.astm.org H. Rahardjo, 1 T. T. Lee, 2 E. C. Leong, 3 and R. B. Rezaur 4 A Flume for Assessing Flux Boundary Characteristics in Rainfall-Induced Slope Failure Studies ABSTRACT: Rainfall-induced slope failure studies and slope hydrological behavior studies often require an assessment of the flux boundary characteristics of a slope. However, quantification of the flux boundary conditions across a slope surface with respect to rainfall, runoff, and infiltration is difficult. This paper introduces the design and field installation of a flume for high resolution monitoring of runoff data from a small catchment, particularly for rainfall-induced slope failure studies. The features of the flume include reliable and continuous runoff measurement at high resolution, options for accommodating various flow rates as dictated by the slope size, and portability. The flume can be used to quantify the flux boundary conditions required for seepage analyses associated with rainfall-induced slope failure studies. KEYWORDS: rainfall, runoff, infiltration, flume, depth probe, data logger, rainfall-induced slope failures Introduction Slope failures can be attributed to several factors such as ge- ological features, topography, vegetation, climatic conditions, or a combination of these factors (e.g., Jiao et al. 1999; Elliott and Buleka 1999). These factors vary with geographical location. In the tropical region shallow landslides often occur due to heavy rainfall and infiltration (Lim et al. 1996; Toll et al. 1999). During dry periods, high matric suctions can exist in unsaturated residual soils and these suctions contribute to the shear strength of the soil. During wet periods, infiltration of rainwater into a residual soil slope may affect slope stability by increasing the water content of the soil, which in turn results in a decrease in the matric suction. As a result, the additional shear strength provided by the matric suction can decrease to a critical limit to trigger a shallow landslide (Fredlund and Rahardjo 1993; Rahardjo et al. 1995). Therefore, it is of vital importance to characterize the infiltration characteristics and assess the relative amount of infiltration into a residual soil slope for appropriate management strategy of slope stability. Furthermore, application of seepage and slope stability analyses requires knowl- edge of the flux boundary conditions at the soil-atmosphere inter- face, an understanding of the infiltration amount and its contribution to the water content, pore-water pressure changes in the slope, as controlled by the soil physical properties, and climatic conditions. Received July 11, 2002; accepted for publication June 16, 2003; published February 9, 2004. 1 Associate Professor and Vice Dean, School of Civil and Environmental Engineering, Nanyang Technological University, Blk N1, #1A-02, Nanyang Avenue, Singapore 639798, chrahardjo@ntu.edu.sg. 2 Formerly postgraduate student, School of Civil and Environmental Engi- neering, Nanyang Technological University, Singapore 639798. 3 Associate Professor, School of Civil and Environmental Engineering, Nanyang Technological University, Blk N1, #1C-80, Nanyang Avenue, Singapore 639798, cecleong@ntu.edu.sg. 4 Research Fellow, School of Civil and Environmental Engineering, Nanyang Technological University, Blk N1, #B4-06, Nanyang Avenue, Singapore 639798, crezaur@ntu.edu.sg. However, direct measurement of infiltration under field condi- tions in conjunction with rainfall and runoff is difficult, and the cost involved for installation of routine infiltration measuring de- vices can be considerably high. Therefore, in most field studies of rainfall-runoff-infiltration, infiltration rates or amounts are often derived from runoff measurements as the differences between rain- fall and runoff rates, assuming other losses to be negligible (e.g., Premchitt et al. 1992; Tsaparas et al. 2002b). Other problems associ- ated with runoff and infiltration measurements for rainfall-induced slope failure studies are permanent masonry flume construction, which is costly and impairs the aesthetic view of urbanized slopes. Apart from rainfall-induced slope failure studies, runoff measure- ments from small experimental plots have also been found to be important for a number of hydrological studies, such as seasonal changes of infiltration rates in scrubland on limestone (e.g., Cerda 1997); post-fire runoff and erosion characteristics of shrubland and grasslands (e.g., Johansen et al. 2001); infiltration and hill slope hydrology (e.g., Guebert and Gardner 2001); and runoff generation and routing on slopes (e.g., Nicolau 2002). For both project-specific and basic hydrologic data recording, it would be desirable to have a low-cost, portable, flexible, reliable, yet rugged runoff-measuring device. This paper presents the design, installation, and test results of a portable flume for the study of runoff and infiltration processes in slopes. Equipment and Methodology Flume Design and Fabrication The flume design was guided by several considerations necessary to meet the objectives: 1. It should enable monitoring runoff from different catchment sizes; 2. It should enable runoff measurements at sufficiently high fre- quency to capture the rising and falling limbs of a hydrograph, accurate enough to derive the infiltration rates; Copyright C  2004 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. 145