This paper reviews available models for estimating surface erosion and sediment delivery to streams from unsealed roads. It summarises current progress and identifies directions for ongoing research and model development. The paper provides a framework for assessing road erosion and sediment delivery models and it includes an overview of road erosion and sediment delivery processes and how they are commonly represented in models. Seven road models are reviewed in terms of their representations of erosion and sediment delivery processes, assumptions, application and limitations. While simple models are thought to be more useful and easily applied for land management purposes, more complex models provide a basis for building and consolidating scientific knowledge. This article reveals some of the limitations and needs of existing road erosion models. These include limitations of their ancestor hillslope erosion models, the imbalance between representation of erosion processes versus sediment delivery, a lack of representation of subsurface flow interception and the lack of model testing and uncertainty analysis. One of the most fundamental limitations to developing improved models of road erosion and delivery is access to data of an appropriate standard.

Excess delivery of land-based sediments is an important control on the overall condition of nearshore coral reef ecosystems. Unpaved roads have been identified as a dominant sediment source on St John in the US Virgin Islands. An improved understanding of road sediment production rates is needed to guide future development and erosion control efforts. The main objectives of this study were to: (1) measure sediment production rates at the road segment scale; (2) evaluate the importance of precipitation, slope, contributing area, traffic, and grading on road sediment production; (3) develop an empirical road erosion predictive model; and (4) compare our measured erosion rates to other published data. Sediment production from 21 road segments was monitored with sediment traps from July 1998 to November 2001. The selected road segments had varying slopes, contributing areas, and traffic loads. Precipitation was measured by four recording rain gauges.Sediment production was related to total precipitation and road segment slope. After normalizing by precipitation and slope, the mean sediment production rate for roads that had been graded within the last two years was 0·96 kg m The normalized mean sediment production rate for road segments that had been abandoned for over fifteen years was only about 10 per cent of the mean value for ungraded roads. Sediment production was not related to traffic loads. Multiple regression analysis led to the development of an empirical model based on precipitation, slope to the 1·5 power, and a categorical grading variable.The measured and predicted erosion rates indicate that roads are capable of increasing hillslope-scale sediment production rates by up to four orders of magnitude relative to undisturbed conditions. The values from St John are at the high end of reported road erosion rates, a finding that is consistent with the high rainfall erosivities and steep slopes of many of the unpaved roads on St John. Other than paving, the most practical methods to reduce current erosion rates are to minimize the frequency of grading and improve road drainage.

Landsliding is a complex process that modifies mountainscapes worldwide. Its severe and sometimes long-lasting negative effects contrast with the less-documented positive effects on ecosystems, raising numerous questions about the dual role of landsliding, the feedbacks between biotic and geomorphic processes, and, ultimately, the ecological and evolutionary responses of organisms. We present a conceptual model in which feedbacks between biotic and geomorphic processes, landslides, and ecosystem attributes are hypothesized to drive the dynamics of mountain ecosystems at multiple scales. This model is used to integrate and synthesize a rich, but fragmented, body of literature generated in different disciplines, and to highlight the need for profitable collaborations between biologists and geoscientists. Such efforts should help identify attributes that contribute to the resilience of mountain ecosystems, and also should help in conservation, restoration, and hazard assessment. Given the sensitivity of mountains to land-use and global climate change, these endeavors are both relevant and timely.

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[1] Geomorphic processes play an important role in the transfer and storage of carbon within steep mountainous terrain. Among these, mass wasting stands out because of its impact on above-and below-ground carbon pools and its potential for releasing or sequestering carbon. A combined remote-sensing and GIS modeling approach was used to quantify the amount and spatial redistribution of modern organic carbon mobilized by mass wasting activity in a tropical mountain setting. The study focused on a population of hundreds of shallow, translational landslides triggered by Hurricane Mitch (1998) on seven watersheds draining the southern flank of the Sierra de Las Minas mountain range (SLM) in central-eastern Guatemala. Results illustrate that mass wasting contributed to the transfer of 43 Â 10 4 MgC, or 3%, of the pre-event C in above-ground vegetation and soils for an equivalent carbon flux rate of 0.08-0.33 MgC ha À1 y À1 , depending on whether we consider Hurricane Mitch to be a landslide-triggering event with a 20-year or an 80-year recurrence interval. While 30% of this carbon was delivered to hillslopes or first-order streams with a presumed high potential for long-term sequestration, the remaining 70% was delivered to higher-order streams with unknown carbon retention capabilities. Therefore, the ultimate fate of the carbon released by landsliding is very uncertain, but depending on the proportion sequestered by colluvial deposits, the recurrence interval of landslide-triggering events, and the rate of ecosystem recuperation at the landslide failure sites, mass wasting could be either a net source or sink of carbon. In a simulated setting based on the SLM study results in which all carbon transferred by landslides from all tropical mountains of the globe is released to the atmosphere, it would represent an amount equivalent to 1%-11% of the global carbon currently being released by the burning of fossil fuels. Meanwhile, in a projected scenario where a significant proportion of the carbon transferred by landslides is retained within sedimentary deposits, sequestration rates would equal 2%-19% of the residual land sink.Citation: Ramos Scharrón, C. E., E. J. Castellanos, and C. Restrepo (2012), The transfer of modern organic carbon by landslide activity in tropical montane ecosystems,

Abstract:Unpaved roads are believed to be the primary source of terrigenous sediments being delivered to marine ecosystems around the island of St John in the eastern Caribbean. The objectives of this study were to: (1) measure runoff and suspended sediment yields from a road segment; (2) develop and test two event-based runoff and sediment prediction models; and (3) compare the predicted sediment yields against measured values from an empirical road erosion model and from a sediment trap. The runoff models use the Green-Ampt infiltration equation to predict excess precipitation and then use either an empirically derived unit hydrograph or a kinematic wave to generate runoff hydrographs.Precipitation, runoff, and suspended sediment data were collected from a 230 m long, mostly unpaved road segment over an 8-month period. Only 3-5 mm of rainfall was sufficient to initiate runoff from the road surface. Both models simulated similar hydrographs. Model performance was poor for storms with less than 1 cm of rainfall, but improved for larger events. The largest source of error was the inability to predict initial infiltration rates.The two runoff models were coupled with empirical sediment rating curves, and the predicted sediment yields were approximately 0Ð11 kg per square meter of road surface per centimetre of precipitation. The sediment trap data indicated a road erosion rate of 0Ð27 kg m 2 cm 1 . The difference in sediment production between these two methods can be attributed to the fact that the suspended sediment samples were predominantly sand and silt, whereas the sediment trap yielded mostly sand and gravel. The combination of these data sets yields a road surface erosion rate of 0Ð31 kg m 2 cm 1 , or approximately 36 kg m 2 year 1 . This is four orders of magnitude higher than the measured erosion rate from undisturbed hillslopes. The results confirm the importance of unpaved roads in altering runoff and erosion rates in a tropical setting, provide insights into the controlling processes, and provide guidance for predicting runoff and sediment yields at the road-segment scale.

Unpaved roads may induce adverse effects on downstream water resources by increasing suspended sediment concentration (SSC). This study documents the localized impacts on stream SSC of six unpaved road–stream crossings in the rural Guabiroba River Catchment, in southern Brazil. Results demonstrated that SSC values downstream of road–stream crossings was between 3.5 and 10 times higher than upstream SSC at all third‐ and fourth‐order stream locations. However, downstream values were statistically undistinguishable from those collected upstream of road–stream crossings at fifth‐order sampling sites. These findings suggest that localized road effects on stream SSCs are scale‐dependent in that these are important for low‐order headwater streams yet undetectable for their higher order counterparts. Findings point to the importance of low‐order stream crossings in increasing SSC and the need to further explore the role of unpaved roads as agents of water quality degradation in agriculturally active rural settings.

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