The Occurrence of Small-Scale Debris Fans in Sandstone Landscapes of the Central Appalachians

Alluvial fans occur in diverse landscapes, and represent critical sites of sediment routing in mountainous watersheds. These radiating, wedge-shaped deposits of sediment are commonly associated with high-energy floods or debris flows. Fan storage within the fluvial system favors conditions where sediment supply exceeds transport capacity. Although the alluvial fan literature is voluminous, traditional research is spatially biased towards the arid southwestern United States. Fans in the central Appalachians provide a valuable record of humid-regime process, climate change, and debris-flow recurrence interval; however, relatively few studies have been completed in this region. In addition, Appalachian fans occur in hazard-prone footslopes that are sites of increasing development. Fan analysis provides a fundamental tool for regional hazards assessment. This task requires a necessary understanding of sediment delivery mechanisms, fan morphology, and geomorphic conditions conducive to fan occurrence.

Kochel (1990) and Mills (2000) provided summaries of humid temperate fans in the Appalachian region. Debris fans are those composed of poorly sorted mixtures of gravelly sediment (clay to boulder size) deposited by debris flows emanating from regolith-covered hillslopes during storm events. Fans may lie dormant for hundreds to thousands of years between debris-flow events, which can result in a significant hazard for unknowing residents who build their mountain homes on these potentially hazardous surfaces. Such landforms are ubiquitous throughout the central Appalachian region, and are associated with low-order drainage basins in erosion-dominated landscapes. Debris fans are characterized by small-scale (<1 sq. km), irregularly shaped deposits that commonly occur at valley intersections. Although restricted in areal dimension compared to fault-bounded basins of the southwestern U.S., low-order (small valley width) to high-order (larger valley width) tributary junctions provide the optimum topographic expansion necessary for fan deposition in mountainous watersheds (Figure 1).

Classic arid-climate fans occur at the intersection between upland drainage basins and tectonically-controlled mountain fronts (Figure 1). Streamflow expansion at the edge of the front results in an unconfined fan shaped deposit. Episodic deposition, faulting, and fluvial incision result in abandonment of fan surfaces and creation of a complex mosaic of morphostratigraphic units. Fan surfaces are typically segmented with varying longitudinal slopes along tectonically-active mountain fronts. Arid-environment fans are very sensitive to climate change and depositional processes have been modulated by variable precipitation levels throughout the Quaternary. By comparison, small-scale debris fans in the central Appalachians occur primarily within the upland drainage basin proper, rather than at well defined mountain fronts (Figure 2). Thus, debris fan location is largely a function of basin morphometry (geometric characteristics of the master channel and its tributary network) in forested mountain watersheds.

To further understand the controls on debris fan occurrence in the Appalachians, detailed geomorphic mapping was conducted at three watersheds set in sandstone landscapes of the Appalachian Plateau and Valley and Ridge provinces. These study sites include the Fernow Experimental Forest, Tucker County, West Virginia; the North Fork basin, Pocahontas County, West Virginia; and the Little River basin, Augusta County, Virginia (Figure 3). Fan map units are delineated according to fan surface morphology and height above channel grade. Debris fans are classified as either simple or compound. Simple fans are characterized by single map-unit types, whereas compound fans include complex map patterns with inset fan-terrace relationships. Fan terraces represent preserved segments that are otherwise dissected by tributary channels following deposition. Figure 4 illustrates the variety of fan types and shapes recognized at the study areas. The most notable observation is that these small-scale debris fans lack the classic semi-conical shape of arid fans. Fans commonly display a width-to-length ratio greater than 1.0 relative to the principal transport direction. Limited growth in the axial direction is largely controlled by narrow valley widths of the receiving channel. Feeder channels incise higher-level fan terraces, with sediment delivery to lower segments, or directly into the master tributary. Debris fans are most commonly preserved at tributary junctions, with greater than 75% occurring at the intersections of first or second order channels with higher order trunk streams. Site tallies of simple and compound fans reveal that the Little River has a significantly higher percentage of compound-inset fans compared to the Fernow or North Fork (Table 1).

Based on the above analysis, the optimum conditions for fan preservation include: (1) high drainage density (total stream length divided by drainage area), (2) high tributary junction frequency, (3) high valley-width expansion rates, (4) steep low-order channels, (5) wide high-order channels, and (6) steep, colluvial hillslopes prone to debris flow. In addition, greater valley-bottom area available for sediment storage, increases the residence time of fan deposits on the landscape. For example, 1949 debris-flow deposits at the Little River were unaffected by record discharges in a subsequent 1985 flood event. Many fans are armored with coarse bouldery debris that will likely remain in storage indefinitely until broken down by weathering. This observation, coupled with the high number of compound fans and terraces, suggests that valley-bottom deposits at the Little River are much older than those at the Fernow or North Fork.

Geomorphology is that branch of the geosciences that is positioned squarely at the crossroads of human systems and dynamic surface processes. Important climate and debris-flow records are most certainly preserved in central Appalachian debris fans. Fan-accommodation mechanisms outlined in this study provide a research model for locating watersheds with high fan-preservation potential. In addition to providing a record of catastrophic events in Earth history, understanding debris fan occurrence is critical for landslide risk assessment in mountainous areas associated with increasing population growth and property development.

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