Kinsey Run Debris Flow
Volumetric Sediment Analysis for Kinsey Run Debris Flow
Triggered During an Extreme Storm Event, Madison County, Virginia
MAZZA, Nilah A. and WIECZOREK, Gerald F., U.S. Geological Survey, National Center, MS 955, Reston, Virginia, 20192
WARNING: Provisional Data, Subject to Revision
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Volumetric calculations indicate that the Kinsey Run debris flow deposited or redistributed approximately 5.7 x 104 m3 of very poorly sorted debris to a maxium depth of 1.1 m in Madison County, Virginia on June 27, 1995. Over 1000 debris flows occurred during an intense storm that stalled over the Blue Ridge Mountains and produced over 30 inches of rain in 16 hours with local intensities exceeding 4 in/h. Wet antecedent conditions also contributed to the triggering of the Kinsey Run debris flow which failed as a landslide 2.5 km upslope from the fan deposit. The debris flow rapidly reached velocities of 8-20 ms-1 eroding up to 0.6 m of material from the ground surface and incorporating large volumes of channel fill, prehistroic debris-flow deposits and vegetation. Erosion and deposition occurred in surges, frequently truncating and reincorporating fresh deposits. Deposits included coarse particles, including boulders up to 7.1 x 6.9 x 1.8m, supported by a fine grained matrix of clay- to sand-sized particles.
The study area was mapped using a total station. Survey coordinates were graphed using Alacarte 3.0 (a geologic interface for ArcInfo) and combined with data from aerial photographs flown one month after the storm to estimate the amount of material deposited from a single debris-flow event. Aerial photographs were used to map fan morphology before modification from natual processes and human manipulation.
The Blue Ridge Mountains are very susceptible to landslides and debris flows triggered by storm events. It is probable that another storm of equal magnitude would cause more debris-flow events. Characteristics of debris flows are important to understand in this region because events of this magnitude are plausible within a recurrence interval of tens of years.
Figure 1
Figure 1 Sequence of paleo debris-flow deposits exposed by stream incision of Kinsey Run. At least three separate prehistoric debris-flow deposits overlay a gray, orgnaic-rich, clayey silt layer dated at approximately 34,500 yrs B.P. The June 27, 1995, Kinsey Run debris flow (from left side of photo) temporarily blocked Kinsey Run at this site on the lower part of the debris flow fan. (Photo by Scott Eaton).
Figure 2
Figure 2 Swath of trees and other vegetation cleared by Kinsey Run debris flow. The velocity of debris flow on the upper portion of the debris-flow fan was estimated to have reached a maximum velocity of 24 m/s (53 mph) based on calculation of superelevation (denoted on aerial photo as site A). At site B shown in this photograph, the velocity of the flow reached 20 m/s (45mph). The surface of the flow tilted almost 17 degrees as it turned this bend with higher edge of the flow on the right side of channel. Further downslope on the fan, progressively slower velocities of 16 m/s (36 mph) and 8 m/s (18 mph) were calculated using superelevation measurements (illustrated on aerial photo as sites C, and D). (Photo by Nilah Mazza).
Figure 3
Figure 3 Toppled tree with large root bound boulder was incorporated and carried along by the debris flow. Large trees stripped from hillsides were deposited on fans or carried along by flooding streams. (Photo by Nilah Mazza).
Figure 4
Figure 4 Large boulders and tree trunks deposited on the lower portion of Kinsey Run debris fan. As the flow emerged from the upper steep narrow channel the flow widened across the much flatter topography of the fan. The flow achieved a maximum width of about 200m across near this location on the lower portion of fan. Boulders were supported by a matrix of clayey silt. Debris flow deposits reached a maximum thickness of about 1.1m. (Photo by Nilah Mazza).
Figure 5
Figure 5 Near vertical cut resulting from lateral erosion of hillside by Kinsey Run debris flow. Periglacial stratigraphy of stratafied slope wash consisting of silt with rocky fragments interpreted to be solifluction deposits from hillside adjacent to lateral edge of debris-flow fan. Sample from basal layer of either pine or hemlock was radiocarbon dated at approximately 25,000 BP. (Photo by Scott Eaton).
Figure 6
Figure 6 Infrared photograph of Kinsey Run debris flow in the vicinity of Graves Mill, Virginia taken in August of 1995. Field evidence suggested at least three pulses or surges of debris flows. Two landslide source areas are evident in the photo; the third debris-flow pulse was probably mobilized during subsequent flooding in the channel. The first and largest of the debris-flow pulses, orginiated from the western-most of the two source landslides, and travelled about 2.5 km to temporarily block Kinsey Run in the lower center part of photograph. Section lines A,B,C, and D indicate where velocities were measured using superelevation criteria.
Figure 7
Figure 7 Large granitic boulder carried and deposited on the fan of Kinsey Run debris flow. The largest boulder moved by the June 27, 1995 debris flows at Kinsey Run was 7.1x 6.9 1.8 m. Larger boulders, with weathered and lichen-covered surfaces, from previous debris flow events are evidence of repeated debris-flow activity. (Photo by Nilah Mazza)
Reference
MAZZA, Nilah A. And WIECZOREK, Gerald F., 1997, A volumetric sediment analysis for Kinsey Run debris flow, triggered during an extreme storm event, Madison, County, Virginia., Geological Society of America, North Eastern Section, Abstracts with Programs, v. 29, n. 1, p. 64
Contact Information
Any questions or comments contact Gerry Wieczorek
