Impassible native surface road, northern Thailand, ca 2000
Impassible native surface road, northern Thailand, ca 2000

For nearly 20 years our group has been investigating the hydrological and geomorphological impacts of rural roads in SE Asia.  While road impacts have been reduced in many developed countries, nations across the developing world have been more tolerant of the potential negative consequences of roads systems, because they are vital to development.  We not only explore the extent of road impacts, but also ways to improve upon how roads are built.



The Dilemma of Mountain Roads

In a recent work Roy Sidle and I argue that while mountain roads and trails are proliferating throughout developing southeast Asia, their long-term consequences of associated landslides and surface erosion on downstream aquatic environments could be severe, but are largely unrecognized.

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Thailand Roads Project

Ending in 2004 the Thailand Roads Project investigated the interactions of various physical processes that allow unpaved roads to contribute disproportionately to basin-wide runoff and stream sediment in the 93.7-ha Pang Khum Experimental Watershed (PKEW) in northern Thailand. Many road sections in PKEW are constant sources of sediment entering the stream during most rain events because: (1) Horton overland flow is generated on the compacted surfaces after small depths of rainfall; (2) surface preparation processes, including vehicle detachment and maintenance activities, renew the supply of easily transportable surface sediment on inter- and intra-storm time scales; (3) erosion of the road surface is accelerated in locations where slopes are steep, overland flow distances are long, and/or vehicle usage is high; (4) surface runoff typically exits from the road directly into the stream. Owing to these collective processes, sediment delivery rate on PKEW roads is more than an order of magnitude higher than that on adjacent fields . Thus, unpaved roads appear to be on the same order of importance as agricultural lands in contributing sediment to the stream network, despite occupying a fraction of the total surface area in the basin. A more thorough assessment of linkages between all hillslope runoff/sediment sources and the stream network, however, is still needed to fully evaluate the relative impacts of roads versus those of agriculture practices in PKEW.

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Persistence of road runoff generation in a logged catchment: a case study from Malaysia

Excavation of roads into hillslope regoliths increases the propensity to generate high-velocity overland flow that potentially disrupt the natural timing of stream flows. Recovery of pre-road hydrological response is exceptionally slow because the only phenomena that promote deep infiltration of rain water are covering of the road surface with soil (either by rehabilitation or mass wasting) or the development of new soil via weathering, which includes the effects of colonizing vegetation. Natural recovery or rehabilitation techniques that simply hasten the emergence of vegetation on abandoned skid trails may reduce the propensity to generate Hortonian overland flow; however, subsurface flow would still likely be intercepted by the cutbank, thereby contributing to the persistence of hydro-geomorphological impacts of roads indefinitely. Thus, the key to reducing the long-term impacts associated with logging roads in some situations is efficient road network planning, which includes minimizing total road length – especially if roads are excised deeply into the hillslope profile.

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Feasibility of reducing stream sediment concentrations with riparian buffers

 Naturally occurring riparian wetlands, such as the one investigated herein, represent an affordable means of reducing road-generated sediment in upland areas of northern Thailand, but complementary conservation practices and buffer management and protection are required to ensure long-term functionality. Ensuring buffer effectiveness for larger flow volumes would require partitioning road runoff into isolated buffers and/or converting concentrated flow pathways into shallow unconcentrated overland flow. In general, restricting flow path distances to approximately 50 to 100 m on steep sections would be beneficial in reducing the velocity and total volume of flow into the buffer. All these measures would facilitate a reduction in road-generated sediment entering the riparian area and stream.

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Towards better design and management of tsunami evacuation roads

Abstract: Among the thousands of people killed or reported missing in Thailand during the 2004 Indian Ocean Tsunami were villagers in small communities on the Andaman Coast. A combination of factors contributed to loss of life, including the lack of defined evacuation routes. This vulnerability to tsunami attacks has recently been addressed with the demarcation of evacuation routes, along both well-maintained arteries and native surface (unpaved) roads. However, poor location design and irregular maintenance will reduce the lifetime that the latter can provide safe egress from remote coastlines. In this work we identified 10 major gullies and 18 landslides along a critical 0.5 km section of a tsunami evacuation road accessing a remote beach of the Andaman Coast in southern Thailand. Erosion rates from landslides and gullies were exceptionally high in the year following widening of the road. Importantly, the degradation features, landslides in particular, reduced the effectiveness of the road to serve as a safe passageway to escape future tsunamis or large storm surges. This study demonstrates that greater attention should be given to appropriate road location, design and maintenance in integrated programmes aimed at reducing tsunami vulnerability in remote coastal areas, not only on the Andaman Coast, but worldwide.

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A Road Erosion Modeling Approach

Splash erosion on roads is initially controlled by the removal of easily erodible material, followed by a dramatic reduction in sediment output associated with limited detachment from the resistant, highly compacted road surface. For situations where loose sediment is readily available, rain splash energy is less important to sediment detachment. If the loose layer is diminished (e.g., following an overland flow event) or protected by a "surface crust", splash energy is needed to detach material from the road surface. Equations in most physically based erosion models do not predict temporal variations in road sediment transport that result from the removal of a loose surface layer of finite depth. This work developed a strategy to treat this removal as changes in road erodibility (referred to as dynamic erodibility). We then coupled the approach with the KINEROS2 erosion algorithm to simulate time-varying sediment transport on unpaved mountain roads. 

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Other Publications (go to Pub Hub)

Cuo, L., TW Giambelluca, AD Ziegler, MN Nullet. 2008. The roles of roads and agricultural land use in altering hydrological processes in Nam Mae Rim watershed, northern Thailand. Hydrological Processes 22: 4339-4354.


Ziegler, AD, RA Sutherland. 2006. Effectiveness of a coral surfacing in reducing sediment production on unpaved roads, Schoffield Barracks, Oahu, Hawaii. Environmental Management 37: 98-110.


Cuo, L, TW Giambelluca, AD Ziegler, MA Nullet. 2006. Using Distributed-Hydrology-Soil-Vegetation Model to study road effects on stream flow and soil moisture. Forest Ecol & Manage 224: 81-94. 


Ziegler, AD, RA Sutherland, TW Giambelluca. 2001. Interstorm surface preparation and sediment detachment by vehicle traffic on unpaved mountain roads. Earth Surface Process & Landforms 26(3): 235-250.


 Ziegler, AD, RA Sutherland, TW Giambelluca.  2000. Runoff generation and sediment transport on unpaved roads, paths, & agricultural land surfaces in northern Thailand.  Earth Surf Process & Landforms 25: 519-534.





Ziegler AD (PI). 2009-2012. Landslide initiation mechanisms and fate of sediment in a headwater catchment in Thailand. NUS FASS Start-up grant (R-109-000-092-133) ($20,000): Thailand.


Ziegler AD (Co-PI, UHM), TW Giambelluca (PI UHM) (UHM), RA Sutherland (Co-PI , UHM). 2000-2003. Hydrological and Erosion Impacts of Road Networks vs. Agricultural Activities in Mountainous Tropical Watersheds.

National Science Foundation (EAR-0000546) ($239,743): northern Thailand.


Ziegler AD (UHM), TW Giambelluca (PI) (UHM), RA Sutherland (UHM). 1997–2000. Hydrologic change and accelerated erosion in mountainous tropical watersheds: the impact of rural roads. National Science Foundation (9614259) ($300,000): northern Thailand.