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Reduction of Stream Sediment Concentration by a Riparian Buffer: Filtering of Road Runoff in Disturbed Headwater Basins of Montane Mainland Southeast Asia

^a Department of Geography, University of Hawaii, 2424 Maile Way, 445 Saunders Hall, Honolulu, HI 96822
^b Geography Department, National University Singapore, Singapore
^c Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan
^d Ministry of Natural Resources and Environment, Chiang Mai, Thailand
^e Division of Soil and Land Conservation, Department of Land Development, Bangkok, Thailand

View larger version (45K): [in a new window]   Fig. 1. Location of the Pang Khum experiment site in northern Thailand.

View larger version (104K): [in a new window]   Fig. 2. The 93.7-ha Pang Khum Experimental Watershed (PKEW) and location of the 2.5-ha PKEW Noi subbasin. Long-term rainfall data are collected at Station 401; Station 405 is where PKEW discharge is measured at a v-notch weir in Loei Stream. B1, B2, and B3 represent additional locations where road runoff could be directed into riparian buffers.

View larger version (145K): [in a new window]   Fig. 3. (a) On-road runoff draining directly into the stream at a low-water bridge in the Pang Khum Experimental Watershed (PKEW). (b) The steep, 60-m subsection of the observed road where severe erosion is perpetual. (c) Road runoff exiting the measurement flume at the base of the monitored section; flow rate exceeds 8 L s–1. (d) Exit point at the channel head for water diverted directly past the riparian buffer. (e) Flume and drainage ditch constructed at the base of the 165-m monitored road section. (f) The PVC pipe used to direct runoff past the 30-m riparian buffer during the NO BUFFER events.

View larger version (61K): [in a new window]   Fig. 4. Experimental setup in the 2.5-ha Pang Khum Experimental Watershed (PKEW) Noi. (a) The photograph was taken in 2002 from light aircraft at 500 m above the surface. (b) Exploded view of principal road measurement area shown in (c), including locations of the road, fillslope, banana patch, riparian buffer, and channel head. (c) Circles labeled "R" and "S" refer to discharge and sediment measurement locations at the base of the 165-m road section and in the stream channel, respectively; the tipping bucket rain gauge is located at Station 406. (d) Transect shows the flow path sequence from the road, down the fillslope, through the banana patch, into either the riparian buffer (BUFFER treatments) or through the PVC pipe (NO BUFFER), and into the stream channel. Note: only panel (a) is to scale.

View larger version (22K): [in a new window]   Fig. 5. Discharge (Qi) versus suspended sediment concentration (Ci) for (a) BUFFER events (closed circles); (b) NO BUFFER events (open circles); and (c) both BUFFER (closed circles) and NO BUFFER events (open circles) superimposed on one graph. The fitted lines are the power regression curves of Eq. [1] (p < 0.001, r2adj = 0.3, n = 59) and Eq. [2] (p < 0.001, r2adj = 0.4, n = 101), which are listed in the lower right corner of panels (a) and (b). The "+" symbols signify baseflow values.

View larger version (19K): [in a new window]   Fig. 6. For BUFFER and NO BUFFER events, the following: (a) comparison of total stream sediment (Ss) with total road runoff (Qr) and (b) comparison of maximum measured stream concentration (Csmax) and road runoff. The "x" in panel (b) is an estimation of maximum concentration for Event 3, had we not disturbed the wetland before the rainfall event (determined via regression). For various magnitudes of road runoff during the BUFFER events only, the following: (c) the reduction of total stream sediment (Ss) versus that of road runoff, calculated as Ss = (Sr – Ss)/Sr x 100%; and (d) the reduction in maximum stream concentration (Csmax) versus that of road runoff, calculated as Csmax = (Crmax – Csmax)/Crmax) x 100%. Abbreviations are explained in Table 2. Numbers in all panels refer to the storms listed in Table 2.

View larger version (22K): [in a new window]   Fig. 7. The following data were measured during event 7: (top) 1-min rainfall intensities; (middle) discharge rates for road and stream runoff; and (bottom) suspended sediment concentrations for road and stream runoff. Measurement locations for road and stream runoff are shown in Fig. 4. The dashed line highlights the threshold road runoff volume (10–13 L s–1) for which the filtering effect of the riparian buffer is compromised.