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Debris Plug (debris + plug)
Selected AbstractsLinking upstream channel instability to downstream degradation: Grenada Lake and the Skuna and Yalobusha River Basins, MississippiECOHYDROLOGY, Issue 3 2009Sean J. Bennett Abstract Unstable fluvial systems are characterized by actively migrating knickpoints, incising channel beds, failing banks, and recruitment of large woody debris and it would appear that river corridors downstream of these processes would be adversely affected or impaired because of higher fluxes of sediment and other riverine products. In north-central Mississippi, the Yalobusha River is one such system and the characteristics of two downstream locations are examined to explore this geomorphic linkage between upstream instability and downstream degradation. For the large woody debris plug along the Yalobusha River, it is found that (1) the deposit is composed mostly of sand covered with a veneer of silt and clay, (2) agrichemicals and enriched concentrations of elements are prevalent, and (3) excessive sedimentation and wood accumulation have forced river flow entirely out-of-bank. For Grenada Lake, it is found that (1) the impounded sediment is predominantly clay, (2) agrichemicals and elements observed throughout the reservoir show no spatial variation, (3) little difference exists in the amount and quality between the sediments deposited in Skuna and Yalobusha River arms, and (4) only a small fraction of the reservoir's storage capacity has been lost because of sedimentation. While excessive sedimentation and large woody debris recruitment have had a marked affect on stream corridor function in the area of the debris plug, the high sediment loads associated with the unstable portions of the Yalobusha River and their associated products have not been communicated to Grenada Lake. The fish consumption advisories within Grenada Lake and its tributaries due to bioaccumulated trace elements and agrichemicals, appear to be independent of the pervasive river channel instability occurring upstream. Copyright © 2009 John Wiley & Sons, Ltd. [source] Processes and forms of an unstable alluvial system with resistant, cohesive streambeds ,EARTH SURFACE PROCESSES AND LANDFORMS, Issue 7 2002Andrew Simon Abstract As a response to channelization projects undertaken near the turn of the 20th century and in the late 1960s, upstream reaches and tributaries of the Yalobusha River, Mississippi, USA, have been rejuvenated by upstream-migrating knickpoints. Sediment and woody vegetation delivered to the channels by mass failure of streambanks has been transported downstream to form a large sediment/debris plug where the downstream end of the channelized reach joins an unmodified sinuous reach. Classification within a model of channel evolution and analysis of thalweg elevations and channel slopes indicates that downstream reaches have equilibrated but that upstream reaches are actively degrading. The beds of degrading reaches are characterized by firm, cohesive clays of two formations of Palaeocene age. The erodibility of these clay beds was determined with a jet-test device and related to critical shear stresses and erosion rates. Repeated surveys indicated that knickpoint migration rates in these clays varied from 0·7 to 12 m a,1, and that these rates and migration processes are highly dependent upon the bed substrate. Resistant clay beds of the Porters Creek Clay formation have restricted advancement of knickpoints in certain reaches and have caused a shift in channel adjustment processes towards bank failures and channel widening. Channel bank material accounts for at least 85 per cent of the material derived from the channel boundaries of the Yalobusha River system. Strategies to reduce downstream flooding problems while preventing upstream erosion and land loss are being contemplated by action agencies. One such proposal involves removal of the sediment/debris plug. Bank stability analyses that account for pore-water and confining pressures have been conducted for a range of hydrologic conditions to aid in predicting future channel response. If the sediment/debris plug is removed to improve downstream drainage, care should be taken to provide sufficient time for drainage of groundwater from the channel banks so as not to induce accelerated bank failures. Published in 2002 John Wiley & Sons, Ltd. [source] | ||||||||||