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<br />.....-- <br /> <br /> <br /> <br />. <br /> <br />774 <br /> <br />HYDRAULIC ENGINEERING '94 <br /> <br />by far, the dominant geomorphic process in the transport zone. <br />DSDosition zone <br />The deposition zone in each of the debris flows is characterized by <br />multiple lobate boulder deposits, boulder terraces, boulder levees, tree <br />levees, and isolated boulders. The deposition zones of Kisamore Run, Twin <br />Run, and Gravel lick Run are located in third.order channels. The <br />deposition zone of Twin Run terminates at the junction of a tourth.order <br />channel. In Austin Run. the deposition zone is situated at the end of a <br />second~order channel and at the beginning of a third~order channel. The <br />deposition zones for each debris flow are relatively narrow with lengths <br />ranging from 500 m to aoo m and widths ranging from 25 m to 60 m. The <br />deposition zones have average gradients ranging from 50 to go. The <br />thickness of the deposits in the deposition zone are variable and range from <br />0.5 m to 2.0 m. The volume of sediment deposited in this lone ranges <br />from 2.083 m3 to 10.543 m3. Eighty.six to one. hundred percent of the <br />total volume of sediment deposited by the debris flows occurs in the <br />deposition zone. <br />The volume of sediment deposited in the deposition zone was less <br />than the total volume of sediment eroded in the failure and transport zones. <br />This difference is probably the result of erosion of the debris.flow deposits <br />by flood streamflow or hyperconcentrated flow immediately after deposition <br />(Pierson, 19aO; aenda. 1990; Cenderem and Kite, 1993), Channel. in the <br />deposition zones are evidence of this post-depositional erosion. <br />Scour zone <br />Channels located immediately downstream of the deposition zones In <br />each debris flow are intensely scoured. Tributaries upstream of the <br />deposition zone and channels that join the scoured channel show little <br />evidence of extensive erosion. This suggests that the erosive power 01 the <br />floodwaters in channels downstream of the deposition zones were <br />increased significantly by an influx of sediment generated by the debris <br />flow. Minor deposition, primarily as thin, clast.supported cobble and <br />boulder sheet deposits, occurs in the scour zone. <br />The scour lanes of Austin Run, Kisamore Run, and Gravel Lick Run <br />are situated in third-order channels, while the Twin Run scour zone is <br />situated in a fourth-order channel segment. The scour zones have slopes <br />ranging from 30 to 80 and lengths ranging from 270 m to 1440 m. The <br />volume of sediment eroded in the scour zones was quantified only in AUSllr <br />Run and Gravel Lick Run. Approximately 3,616 m3 and 694 m3 of sedimer~ <br />were eroded from the steep valley walls of Austin Run and Gravel Lick Run <br />respectively. Erosion was most extensive on valley side slopes at bedrock <br />constrictions. On Austin Run, this sediment was either deposited on the <br />surface of a pre-1949 debris fan or flushed into the South Branch potomac <br />River. Much of the sediment eroded in the Gravel Lick Run scour ;zone was <br />deposited on the channel floor in this lone. <br />The distribution of sediment durino the debris-flow event <br />To better understand the geomorphic effects of debris flows on <br />channel morphology, a sediment budget was developed to evaluate the <br /> <br />r <br /> <br />.775 <br /> <br />EROSION & DEPOSITION-W. VfRGlNlA <br /> <br />spdllal distribution of sediment eroded and deposited during a debris. flow <br />Slient. Erosional and depositional sediment volumes determined for each <br />debriS flow were summed at 100 m intervals and plotted as a function of <br />dIstance (Fig. 1 L Each diagram shows a nonuniform distribution of erosion <br />dnd depOSition associated with each debris flow. The most intense erosion <br />~nd deposition ,in each debris flow is in the failure and deposition zones, <br />respectively (Fig, 1), However, the total volume of sediment eroded in the <br />Hansport zones of Austin Run. Kisamore Run, and Twin Run is greater than <br />lhe volume of sediment eroded in the failure 20nes. <br />The sediment distribution graphs show that the upper two~thirds of <br />Austill Run, Kisamore Run, and Twin Run are dominated bV erosion (Fig. 1). <br />fhe IflIUauon of debris flows in the failure zone and subsequent passage <br />through the channel system in the transport zone causes degradation in <br />4000 <br /> <br />2000 <br />o <br /> <br />,2000 <br /> <br /> <br />II oroslon <br /> <br />.- <br /> <br />0---+-01-5.. <br /> <br />.4000 <br /> <br /> <br />I ~~~~~':: .1 <br />I ~E~~<::..I <br /> <br /> <br />15 <br />> fiOOO Austin Roo <br />'000 <br />2000 <br />o <br />-2000 <br />-'000 <br />-0000 <br /> <br /> <br /> <br />T <br /> <br />l J 0 1 ~ <br /> <br />o <br /> <br />500 <br /> <br />1000 1500 2000 2500 <br />Distance downslream (meters) <br /> <br />~ It/Ie 1, Histograms Showing the distribution of eroded and deposited <br />~"'l,rnent on Austin Run, Kisamore Run, Twin Run, and Gravel lick Run. <br />"1' faIlure (F), transport (Tl, deposition 10), and Scour IS) zones for each <br />'''1If15 flow are delineated on their respective diagrams. <br /> <br />3000 <br /> <br />3500 <br /> <br />--- <br />