Reduction of Road-Generated Stream Suspended Sediment by a Riparian Buffer — case study from northern Thailand Alan Ziegler University of Hawaii (UHM) Junjiro Negishi (National University Singapore) Roy Sidle (Kyoto University) Pornchai Preechapanya (MNRE, Thailand) Thomas Giambelluca, Ross Sutherland (UHM)

Field Site PKEW 1996 – pres Pang Khum Thailand 50 km NNW of Chiang Mai population 350+ MONTANE MAINLAND SE ASIA

? secondary regrowth ++ ROADS ===

Alter Hydrological function - Δ timing of storm runoff - water droughts/excesses PROBLEM WITH Accelerate erosion processes - elevate stream SSC - sedimentation downstream - associated ecological impacts

Question If managers in the most developed countries can not solve many of their road-related impacts, what can we expect to do in regions where resources are limited?

FIELD EXPERIMENT QUANTIFY the reduction in stream sediment by altering flow of road runoff through a wetland: i.e., NATURAL BUFFER Dominant species: Fimbristylis aphylla Zoll. ex Steud. (Cyperaceae) Period: Rainy season of 2002 (18 events) Site: 2.5-ha sub-basin in PKEW (PKEW Noi) Feature: 165-m road section draining into a 30-m riparian buffer

PHYSICAL CHARACTERISTICS OF PKEW elevation: m rainfall: mm y -1 LANDUSE 72% forest, secondary vegetation 27% agriculture 1% roads, paths, dwellings (Landuse 93 ha)

PKEW: Pang Khum Experimental Watershed DESIGN 2 TREATMENTS: (BUFFER & NO BUFFER) MEASUREMENTS: (RF, Q, SSC) PKEW NOI Zero-order basin Area = 2.5ha

EXPERIMENTAL SETUP 165-m road section & RO collection ditch Bypass through 30-m buffer Road RO > 50,000 mg/L Bypass exit Fimbristylisaphylla

FIELD DATA BUFFER (no pipe) 8 Events RF T = 1-31 mm SS T = <1–160 kg SSC T = mg L -1 SSC max = mg L -1 ROAD SSC max = 6000 – mg L -1 NO BUFFER (pipe) 10 Events RF T = 1-23 mm SS T = 2–212 kg SSC T = mg L -1 SSC max = mg L -1 ROAD SSC max = – mg L -1

SEDIMENT RATING CURVES BUFFER NO BUFFER Baseflow C i = Q i {BUFFER} C i = Q i {NO BUFFER}

PRINCIPAL RESULT Reduction in STREAM SSC by the 30-m buffer 30-85% (storm-dependent) METHOD: used the BUFFER rating curve to re-calculate SSC at every logged Q for 10 NO BUFFER events [crude simulation] [index]

Specific Factors 1.Buffer infiltrability 2.Physical properties of the buffer 3.Antecedent soil moisture 4.Storm characteristics 5.Slope angle 6.Nature of the overland flow— SUOF versus COF CONCLUSION ? INEFFECTIVE ? Road RO > threshold flow  8 L s -1 RF = 90 mm h -1 ; < 4% annual RF total CASES: Steep slopes | ISSF | ++ RF Must limit road runoff volume (e.g., drains at 50 m intervals) 30-m buffer is effective for m road sections

Direct evidence how natural buffers are affordable strategies for reducing sediment inputs to streams Limit slope angles Limit flow path distances Proper hillslope locations Resurfacing material Control traffic Routine maintenance Experience: should be combined with other recognized strategies: Reduce overland flow Reduce sediment availability

thanks Alan Ziegler | 2002 Posse: Junjiro Negishi (National University Singapore) Roy Sidle (Kyoto University) Pornchai Preechapanya (MNRE, Thailand) Thomas Giambelluca, Ross Sutherland (UHM)

Rainfall, discharge, and sediment variables for 18 monitored events

Index of buffer efficiency 1 Determined using Eq. 1 instead of Eq. 2 in the calculation of suspended sediment at each logged Q value during the storm. 2 ΔC s_event = (Ĉ s_event - C s_event )/ C s_event

Alteration of Hydrological Flowpaths

ROADS ? UPPER BASIN Interstorm Hysteresis in SSC 15:30 16:00 15:45 16:15 17:00 16:30 16:45 clockwise hysteresis 17:45 ROADS ?

We determined sediment concentrations by filtering samples ( L) through pre-weighed, pre-ashed, 0.7-μm Whatman glass filters (Φ = 0.47mm) using a 60-psi vacuum pump and NALGENE™ polysulfone filter holders. After drying at 105  C for 24 h, we weighed the filters to determine the mass of total suspended material.