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<br />- <br /> <br />.' <br /> <br />. <br /> <br />Erosion and Deposition by Debris Flows in Mountainous <br />Channels on North Fork Mountain. Eastern West Virginia <br /> <br />Daniel A. Cenderelli and J. Steven Kite' <br /> <br />ABSTRACT <br />A study of four debris-flow impacted channels in the North Fork <br />Mountain area of eastern West Virginia shows that the geomorphic effect <br />of debris flows on channel morphology is variable in different reaches <br />impacted by the debris flow. Erosion is the dominant geomorphic process <br />in the upper two-thirds of the debris-flow track, whereas deposition <br />dominates the lower one-third. The geomorphic effect of the debris-flow <br />event extends beyond the terminus of the debris flow as 20 to 40 percent <br />of the sediment generated by the debris flow is incorporated into <br />floodwaters immediately downstream of the terminus. This influx of <br />sediment increased the erosiveness of the floodwaters. which in turn <br />caused intense scouring. <br />INTRODUCTION <br />Catastrophic storms playa significant role in initiating debris flows, <br />transporting sediment downslope, and influencing the morphology of <br />channels in mountainous regions. During sediment transport, debris flows <br />produce a complex distribution of deposits and eroded surfaces throughout <br />the channel. Debris flow is one of the most important processes <br />influencing channel morphology in mountainous regions (Hack and Goodlett, <br />1960; Williams and Guy, 1973; Campbell, 1975; Benda, 1990; Wohl and <br />Pearthree. 1991). Despite the importance of debris flows on channel <br />morphology, the distribution of deposits and eroded surfaces associated <br />with debris flows is poorly understood. Further. the distribution and volume <br />of sediment eroded and deposited during a debris. flow event has not been <br />adequately quantified. <br />The North Fork Mountain area in eastern West Virginia is an ideal <br />setting for addressing these concerns because two catastrophic rainfall <br />events (June 17-1 B, 1949 and November 3-5, 1985) have initiated <br />numerous large debris flows on steep slopes of the mountain. Four of the <br /> <br />1West Virginia University, Department of Geology and Geography, PO Box <br />6300, Morgantown, WV, 26506-6300, <br /> <br />772 <br /> <br />- <br /> <br />. <br /> <br />. <br /> <br />EROSION & DEPOSITION-W. VIRGINIA <br /> <br />773 <br /> <br />(Iebris flows were selected for detailed study: Austin R~n and Kisamore <br />Run from the 1949 storm event; Twin Run an? Gravel Lick Run from the <br />, 985 storm event. The primary focuses of thiS paper .are to ~resent the <br />lhstnbution and volume of sediment erode.d and deposited dunng these <br />,.vents and to evaluate the impact of debns flows on channel morphology. <br />CHARACTERISTICS OF NORTH FORK MOUNTAIN DEBRIS FLOWS <br />.. Based upon the distribution of eroded surfaces and depOSits, the <br />llf>bos.flow impacted channels can be divided into four zones: an upper <br />failure zone, a middle transport zone, a lower deposition zo.ne. and a ~cour <br />1(lIU~ Immediately beyond the debris-flow terminus. The failure zone 15 the <br />,If~a III which sediment initially fails. generating the debris flow. ~he <br />Ir ansport zone is dominated by erosion of the channel as t.he de.brls-flow <br />'Ilass travels downslope. The deposition zone is the area In which .the <br />m<llority of the debris.f1ow sediment is deposited. The scour zone IS <br />l()~ated immediately downstream of the debris.flow deposition. zone and is <br />I haracterized by extensive erosion of the channel floor by sedlment.laden <br />tloodwaters. <br />FQ.ilure zone . <br />The failure zones are steep and situated in hollows and/or planar Side <br />slopes. This zone is marked by a distinct vertical scarp along margins of <br />Ihe lailure. The failure surfaces are 0.25 m to 2.5 m below the ground <br />surlace of adjacent undisturbed slopes. The failure zones have average <br />qr<ldients ranging from 280 to 310 and lengths ranging from 150 m to 240 <br />111 Based upon sediment immediately adjacent to the failure zone of each <br />debriS flow, the sediment that failed was a clast.supported bouldery. <br />diamicton with minor amounts of fine sediment. The volume of sediment <br />(>foded in the failure zone of each debris flow ranges from 2.274 m3 to <br />7.497 mJ. <br />TUHlSDort zone . <br />The debris flows that initiated in the failure zones of Austin Run, <br />Kisamore Run, and Twin Run propagated downslope through the preeXisting <br />channel systems. The transport zones have lengths ranging from 1,450 m <br />to 1.900 m and include first., second., or third.order channel segments <br />With average gradients ranging from 90 and 120. A transport zone did not <br />develop in Gravel Lick Run because of an abrupt change in gradient. and <br />lIow direction between the failure zone and the third-order channel It <br />Intersecterl The sudden decrease in gradient and sharp bend reduced the <br />momentum of the debris. flow mass, inhibiting further transport, and <br />causing deposition. <br />The transport zone is distinguished by scoured channel slopes and <br />bottoms. During transport, the debris flows eroded sediment and <br />vegetation from channel side slopes and bottoms. exposing fresh outcrops <br />01 bedrock or diamicton. The depth of erosion in this zone varied between <br />0.10 m to 2.50 m. The volume of sediment eroded in the transport zone of <br />t!ach debris flow ranges from 5.079 m3 to 9,502 m3, whereas the volume <br />of sediment deposited in this zone ranges from 492 m3 to 1,595 m3.. . <br />Comparison of erosional and depositional volumes indicates that erosion IS, <br /> <br />l <br /> <br />