Great Smoky Mountains-Southern Appalachians
The Southern Appalachian Mountains are characterized by complex relief but moderate orography (maximum elevation around 2000 m (Prat and Barros, 2010a)). The region experiences humid continental climate with strong orographic effects. These effects serve to increase the spatial and temporal variability of rainfall, causing potential widespread flooding and landslides (Wooten et al., 2008; Tao and Barros, 2013a and 2013b). In the warm season, from spring through early fall, major weather systems include westerly mesoscale convective systems and fronts, southerly and easterly tropical depressions, and localized convective activity, and thus storm precipitation is characterized by higher intensity and shorter duration (< 24 hours) as compared to the cold season, when intensities are low but duration can last several days. The region is experiencing intense growth of specialized agriculture in the valleys, and sedimentation has become a recent priority in terms of water quality.
Prat and Barros (2010a) showed there are significant differences in rainfall microphysics in the region depending on the nature of the storm system, and location where DSD data were collected. They showed that the right-hand side of observed valley DSDs is “heavier” than that of DSDs in adjacent ridges for the same event, suggesting enhanced drop coalescence between ridge and valley locations. Wilson and Barros (2014) observed that the diurnal cycle of light rainfall is related to the diurnal cycle of fog occurrence, “with mid-day peaks concurrent with valley fog, and evening peaks concurrent with radiation fog”. Their detailed analysis of raingauge, radar profilers, and disdrometers showed intermittent periods of very intense rainfall in valleys and sheltered ridge locations concurrent with dense fog and/or cap clouds. Using modelling experiments, the seeder-feeder mechanism is one pathway that may explain the enhancement of rainfall events in the presence of this low level moisture.
Pigeon River Basin
The Pigeon River Basin (PRB) is found in the Southern Appalachians in western North Carolina and eastern Tennessee. We focus on the North Carolina section of the basin, which contains all the high elevation locations in the basin. The PRB is composed of three headwater catchments, namely: the Big Creek Basin (BCB), the Cataloochee Creek Basin (CCB) and the Jonathan Creek Basin (JCB) in North Carolina. Cataloochee Creek is a small tributary to the Pigeon River and has a drainage area of 128 km2. The BCB and JCB have drainage areas of about 95 km2 and 172 km2, respectively. The three headwater catchments are heavily forested and are characterized by steep slopes. In recent years, the JCB has witnessed significant land-use and land-cover (LULC) change due to increased urbanization.
The Pigeon River Basin is underlain by crystalline-rock aquifers comprising crystalline metamorphic and igneous rocks covered by an extensive mantle of unconsolidated material consisting of saprolite, colluvium, alluvium, and soil (Trapp Jr. and Horn, 1997; Miller, 1999). The colluvial deposits are mainly found on the hillsides due to rock weathering and are highly susceptible to landslides. Substantial alluvial deposits appear along streams and are built over time due to sediment transport in the streams. The dominant soil types are Edneyville–Chestnut complex soil, Plott fine sandy loam, Wayah sandy loam, and eroded Wayah loam soil (Allison et al., 1997). The climate for the study area is subject to moisture-rich winds from the Gulf of Mexico and westerly mesoscale convective systems in the warm season, whereas westerly and northwesterly flows govern most of winter weather activities. Previous research has shown that the orographic rainfall enhancement is very strong, on the order of 60% at ridge compared to valley locations (Prat and Barros, 2010). The rainfall threshold for debris flows based on the historical record is 125mm over a 24 h period (Witt, 2005a). However, recent observations such as during the July event studied here indicate that such rainfall can accumulate in periods of less than 90 min (Prat and Barros, 2010; Tao and Barros, 2013). Existing landslide hazard risk assessments indicate that most of the area of the Pigeon River Basin is highly unstable, especially the headwater catchments (Witt, 2005b, a).