Erosion in Several Large Gullies, Core Arboretum West Virginia University Jessica Gormont Submission for an Option II-Professional Studies Project in partial fulfillment of the requirements for the degree of Master of Science In Geology Department of Geology and Geography Morgantown, WV 2007 Committee Members: Steven Kite (Chair) Robert Behling Jonathan Weems Introduction The position and dimensions of natural gullies are created by the concentration of overland flow into topologic low spots (Zucca, 2006). When areas above a hillside are developed, it can decrease the ability of water to seep into the ground due to the addition of impervious materials such as pavement. These conditions are ideal for Horton overland flow (Luce, 2002). The impervious material can also cause a rerouting of the overland flow sending far more water over the surface of the hillside and into gullies than is natural (Nyssen, 2002). This is doubly true when culverts are added to the heads of the gullies, diverting the overland flow directly into the gullies. Utilization of natural gullies to remove unwanted water during the urbanization process may seem like effective planning. However, an increase in water volume in gullies can increase tractive force within the channel (Saldi-Caromile, 2004), which in turn causes rates of erosion to increase. Possible consequences of this erosional increase could include deepening of gullies and instability of adjacent hillsides, which could trigger landslides. This study examined four gullies within the Core Arboretum of West Virginia University (WVU) (fig. 1). In order from north to south, the gullies that were studied were Tennis Gully, located just beneath the middle section of the WVU tennis courts; Roof Gully, which diverts water from the roof of the Coliseum; Bathtub Gully, named for the size and shape of the gully beneath the Strausbaugh Trail bridge in the 1970s and 1980s (Weems, pers. comm.); and ZigZag Gully, named for the zigzag shape created by the intersection of the gully and a diversion ditch at the top of the hillside that diverts runoff from the Coliseum parking lot. The gullies are all located on the hillside adjacent to the Coliseum and all divert water from impervious surfaces. Culverts directing flow into these gullies have caused the natural gullies to widen and incise dramatically since development of the area above the Arboretum hillside (Weems, pers. comm.). 2 Figure 1 – Locations of Studied Gullies The increase in impervious surface area up slope and diversion of excess runoff down the gullies have caused increased instability of the Arboretum hillside, as evidenced by a large landslide 3 caused by improper water management in 1973 (Lessing and others, 1975). Sediment yield downstream has also increased, which may have contributed to the assimilation of nearby Granville Island into the floodplain in the lower Arboretum. The Study Area Arboretum History The area now occupied by the Core Arboretum was purchased by West Virginia University in 1948, but student research at this site goes back at least to the 1920s (Weems, 1992). Trail work in the arboretum started in 1949 but it was not until 1954 that the area was named the “West Virginia University Arboretum” and opened to the public (Weems, 1992). The WVU Physical Plant and Biology graduate students sporadically maintained the Arboretum until 1964, when the first groundskeeper was hired and maintenance became a continuous process (Weems, 1992). Figure 2- Core Arboretum Area in 1952 (West Virginia and Regional History Collection, West Virginia University Libraries) 4 Figure 3 – Core Arboretum Area in 2003 (United Stated Geological Survey) The Coliseum and adjacent parking lots were built in 1967, taking up about 6 hectares (15 acres) of what had been Arboretum property, including a pond and a display of exotic trees and shrubs (Weems, 1992). The culverts included in this study were most likely placed at that time. In 1975, the “West Virginia University Arboretum” became the “Core Arboretum”, named after Earl L. Core, a retired West Virginia University professor and Chair of the Biology Department, who had originally rallied for the creation of the Arboretum (Weems, 1992). Bedrock Geology Donaldson (1968) shows that shales are the most common rock type in the Arboretum (fig. 4). Sandstones occur in the Saltsburg Sandstone, but are most prominent in the Morgantown 5 Sandstone. The Morgantown Sandstone has thick beds and is composed of medium to coarse grains. Figure 4 – Stratigraphic Column of the Study Area (Donaldson, 1968) Limestones occur in several Arboretum stratigraphic units, including the “Pittsburg” Ewing limestone, Ames Limestone, Grafton-Birmingham shale, and Clarksburg limestone. Limestones in the study area are a bluish-gray and weather yellow-gray. Limestones are rarely thicker than 0.6 m (2 ft), are typically very argillaceous with gradational contacts with shales. The Ames Limestone contains marine fossils, whereas the other limestones contain non-marine fossils, including trace fossils, burrows, plants, and snails. There are two coal beds in the study area: the Harlem coal and the Elk Lick coal. These coals may contain clay partings. The Elk Lick coal has been mined in the Arboretum. The deepest gullying in the Arboretum is likely to occur in the most easily erodible rock underlying the hillside. Due do their low resistance, shales in the Portersville shale, Saltsburg Sandstone, “Pittsburg” Ewing limestone, Ames Limestone, Grafton-Birmingham shale, and 6 Clarksburg limestone, are the most easily eroded rocks in the Arboretum. These rocks are abundant in the Tennis, Roof, Bathtub, and ZigZag gullies, showing that the gully sites continue to be altered and are naturally unstable and prone to rapid incision in high-energy environments. Surficial Geology and Soils Willem van Eck (1975) stated that soils in the study area are mostly thin residual soils. The northern half of the Arboretum contains the Culleoka series, brown Ultic Hapludalfs, 0.5 m- 1 m (20 in-40 in) thick, with silt loam to silty clay loam textures. Culleoka soils in the center of the Arboretum are interspersed with the Upshur series, a reddish-brown to purple Typic Hapludalf with clay to silty clay textures. Willem van Eck (1975) mapped a Culleoka-Upshur complex on slopes near the south end of the Arboretum. Some gentle benches just below the Arboretum parking area contain Westmoreland series, another Ultic Hapludalf with textures and colors similar to the Culleoka series, but thicker. Colluvial deposits at the base of slopes contain the same textures and properties as the residual soils from which they originated (Willem van Eck, 1975). The Clarksburg series, a Oxyaquic Fragiudalf, also occurs here. These yellowish- brown to brown soils are commonly >1.5 m (>5 ft) thick and contain silty clay loam subsoil. The Culleoka and Clarksburg series are the main soil series adjacent to the Tennis, Roof, Bathtub, and ZigZag gullies, and both are composed largely of silty clays. Fine sands and silts are within the textural range that is most susceptible for detachment and transport (Lal, 1988) making the Culleoka and Clarksburg series very susceptible to gullying. The Research Problem Studies have shown that replacing natural vegetation with impervious surfaces can increase the soil erosion rate of adjacent slopes by an order of magnitude or more (Bracken, 2007). When impervious layers are added to the top of a hillside, they reduce the amount of 7 seepage into the ground, causing increased Hortonian overland flow (Luce, 2002). When the overland flow is routed into the heads of natural gullies through culverts, the amount of water that enters the gullies will be higher than prior to development. There are several affects of the increase of water into natural gullies. According to the Manning equation, V = (1.49 R 2/3 S ½)/n V = Velocity R = Hydraulic Radius S = Stream gradient (slope) N = Manning roughness coefficient, as the Manning roughness coefficient decreases, which would occur when changing a surface area from natural ground to pavement or another impervious material, flow velocity increases (Fetter, 2001). Also, as the depth of water in the gullies increases, tractive force also increases, as shown by the tractive force equation: τ = γ R S τ = Tractive Force γ = Specific Weight of Water R = Hydraulic Radius S = stream gradient (slope), allowing more sediment to be eroded and carried as bedload (Saldi-Caromile, 2004). As the velocity increases, the ability of the flow to erode the bed of the channel increases. According to the Hjulstrom's diagram (fig. 5), silty sized particles, such as those located in the majority of rock beds in the Arboretum, are the most easily eroded clast size in any given channel. Therefore, an increase of velocity and depth of the water in gullies in the Arboretum would cause an intensifying of erosion within gullies located on the hillside. 8 Figure 5 – Hjulstrom Diagram (Sauchyn, used with permission) This reaction appears to be occurring, as, since the construction of the Coliseum area in 1967, ZigZag, Bathtub, Roof, and Tennis gullies appear to have widened and incised rapidly, causing concern for the stability of the slope. Purpose The purpose of this study was to determine the geomorphological impact of the addition of impervious material and diversion of flow through culverts above a steep slope underlain by thin colluvial soils and shale-dominated bedrock. Objectives The objectives of this project are as follows: 1) Survey cross-sections, spaced at 10 m intervals along four large gullies in the Core Arboretum. 2) Estimate the total volume of sediment lost from the Arboretum gullies since incision began. 9 3) Create base-line gully geometry data for future studies, which may include determining futures rates of erosion. Methods Cross-sections for the Tennis, Bathtub, Roof, and ZigZag gullies were constructed at 10 m spacing, with a few longer increments where necessary due to inaccessibility (fig 6).