Article

Distribution and residence times of large woody debris along South River, Shenandoah Valley, Virginia

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Abstract

Numerous studies on large woody debris (LWD) have focused on forested mountain streams located in the Pacific Northwest. Wood in these streams typically form log jams that have a profound impact on stream morphology, promoting channel stability and forcing pools. However, studies are sparse on larger rivers where LWD occurs primarily as individual pieces. Even less is known about the residence times of LWD in these settings. This study focuses on the use of dendrochronology to determine the rates of LWD recruitment and LWD residence times. Located in the Shenandoah Valley of Virginia, the South River is a fourth order stream with a drainage area of over 600 km2 at its confluence with the South Fork Shenandoah River. The riparian zone is dominated by agriculture. Agricultural land use creates a distribution of LWD unlike that of forested streams, consisting primarily of isolated pieces and small jams versus larger jams. Four species of trees are dominant at South River: sycamore (Platanus occidentalis), silver maple (Acer saccharinum), boxelder (Acer negundo) and black walnut (Juglans nigra). Average diameters of LWD are 32 cm with lengths averaging 6-7 m. At the South River, LWD has no influence on channel morphology, but wood traps fine-grained sediment (storing 16% of the river's annual load) and associated contaminants making it the most significant mechanism for storing clay and silt within the channel perimeter. Sixty to seventy-five percent of LWD occurs in pools, while 10-20% occurs in riffles. Most of the wood falls from adjacent banks by both natural processes and agricultural practices. Preliminary dating results indicate that LWD reside in the channel a minimum of 15 years.

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... This general trend can be observed in numerous situations, despite the fact that each river has a peculiar distribution depending on the characteristics of the forested area, such as forests that are pristine or harvested, old-growth or young-growth (Sedell et al., 1988). Several site observations in North-America (Sedell et al., 1988;Hess, 2007;Cadol and Wohl, 2010), Europe (Van Sickle and Gregory, 1990;MacVicar and Piégay, 2012;Lucía et al., 2015;De Cicco et al., 2016) and Central America (Cadol and Wohl, 2010) indicated that the highest number of observations of woody debris is for those pieces of length typically between 10% and 20% of the longest elements. ...
... In natural rivers, distributions of these lengths are typically skewed towards smaller sizes (Blersch and Kangas, 2014). For the definition of a suitable function representing the lengths distribution, published data by Hess (2007) was used. The author measured the length of 156 debris on the South River (Virginia, US). ...
... Through maximum likelihood estimation (MLE) regression analysis, values of µ and σ for the data by Hess (2007) were computed as µ=-1.3039 and σ=0.7367. ...
Thesis
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The accumulation of large wood debris at bridge piers obstructs the flow, producing increased upstream water levels, large horizontal structural loadings, and exacerbated scour. These effects have frequently been held responsible for the failure of a large number of bridges around the world, as well as for increased risk of flooding of adjacent areas. Yet, little is known about the formation and growth of these debris piles. This thesis is aimed at deciphering the whole life of debris accumulations through an exhaustive set of 732 experiments in which debris elements were individually introduced into a flume and accumulated at a pier model downstream. In all experiments the growth of debris accumulations was observed to stop at a critical stage, after which the jam is removed from the pier by the flow. This condition typically coincides with the time when the dimensions of the accumulations are maxima. The values of the jam maximum size display a clear dependence on flow characteristics and debris length distribution, whilst other variables (such as pier diameter, debris diameter, debris density, water depth, pier shape) have shown much weaker effects. For a given debris length, accumulations are wide, shallow, and long at low flow velocities but become narrower, deeper, and shorter with increasing velocities. A comparison of accumulations formed with debris of uniform and non-uniform size distributions has revealed that the former can be up to 2.5 times wider than the latter. The effect of the shape of debris pieces was also studied by using cylindrical dowels, unbranched sticks and single-branched sticks. The maximum size of debris piles formed by idealised cylindrical debris is smaller than that of jams formed by natural wood of irregular shape. Experiments with branched debris resulted in jams significantly smaller and less stable than those with nonbranched sticks. On the base of the experimental results, a mechanistic theoretical model of idealised jam geometry and a reduced set of dynamic actions was developed through conservation of angular momentum. The resulting system of ODEs was studied in the phase plane, which revealed that the failure of the accumulation depends on both planar asymmetry and ratio between the length of the jam and the extension downstream of the pier, defined as tail. The former is necessary for any jam to fail, and higher asymmetries lead to less stable jams; the latter provides stability for large tails and small lengths, but yields instability when the ratio is reduced. Results from this thesis will pave the way for practical applications in bridge engineering and flood risk assessments, and inform future research about debris jams at bridge piers.
... A typical example ( Figure 2) shows a deposit that is approximately 5 m wide, 10 m long, with a maximum thickness of 80 cm. The fi gure also illustrates characteristic LWD accumulations along the South River, which typically occur as isolated logs and small jams along the margins of the channel (Hess, 2007). ...
... Third, we can independently determine a maximum date for sediment in one of the of the FGCM deposits that we cored (H2A) because this deposit was created in the lee of living trees leaning into the fl ow. Tree cores obtained using an increment borer provide a maximum age for this deposit of 24 years (Hess, 2007), indicating that all the sediments must have accumulated after 1963. Finally, if the samples formed before 1963, then all the samples must have been deposited within a 5-year period from 1955 to 1960. ...
... We speculate that the formation of a FGCM deposit begins with the accumulation of LWD. Along the South River, LWD is signifi cantly smaller than the channel width, and as a result, the distribution of wood is governed by channel morphology (Hess, 2007), unlike smaller headwater channels, where the wood itself controls pool and riffl e spacing and other elements of fl uvial morphology (Abbe and Montgomery, 1996;Gurnell et al., 2002;Webb and Erskine, 2003). Once LWD has accumulated along the river banks, hydraulic processes control the development of FGCM deposits. ...
Article
Previously undocumented deposits are described that store suspended sediment in gravel-bedded rivers, termed ‘fine-grained channel margin’ (FGCM) deposits. FGCM deposits consist of sand, silt, clay, and organic matter that accumulate behind large woody debris (LWD) along the margins of the wetted perimeter of the single-thread, gravel-bed South River in Virginia. These deposits store a total mass equivalent to 17% to 43% of the annual suspended sediment load. Radiocarbon, 210Pb and 137C dating indicate that sediment in FGCM deposits ranges in age from 1 to more than 60 years. Reservoir theory suggests an average turnover time of 1·75 years and an annual exchange with the water column of a mass of sediment equivalent to 10% to 25% of the annual sediment load. The distribution of ages in the deposits can be fitted by a power function, suggesting that sediment stored in the deposits has a wide variety of transit times. Most sediment in storage is reworked quickly, but a small portion may remain in place for many decades. The presence of FGCM deposits indicates that suspended sediment is not simply transported downstream in gravel-bed rivers in agricultural watersheds: significant storage can occur over decadal timescales. South River has a history of mercury contamination and identifying sediment sources and sinks is critical for documenting the extent of contamination and for developing remediation plans. FGCM deposits should be considered in future sediment budget and sediment transport modeling studies of gravel-bed rivers in agricultural watersheds. Copyright © 2010 John Wiley & Sons, Ltd.
... Debris lengths in rivers are usually nonnormally distributed (Blersch & Kangas, 2014) and skewed toward smaller sizes. In order to produce a length distribution that mimics these observed characteristics, published data (Hess, 2007) of debris lengths found in the South River (Virginia, United States) were used to fit a log-normal probability density function: ...
Article
Full-text available
The accumulation of large wood debris around bridge piers obstructs the flow, producing increased upstream water levels, large horizontal structural loadings and flow field modifications that can considerably exacerbate scour. These effects have frequently been held responsible for the failure of a large number of bridges around the world, as well as for increased risk of flooding of adjacent areas. Yet, little is currently known about the time evolution and processes responsible for the formation and growth of these debris piles. This paper is aimed at deciphering the whole life of debris accumulations through an exhaustive set of 570 experiments in which debris elements were individually introduced into a flume and accumulated at a pier model downstream. Our findings show that in all experiments the growth of accumulations is halted at a critical stage, after which the jam is removed from the pier by the flow. This condition typically coincides with the time when the dimensions of the accumulations are maxima. The values of the accumulation maximum size display a strong dependence on flow characteristics and debris length distribution. On the other hand, other variables have shown much weaker effects on the geometry of the accumulations. For a given debris length, accumulations are wide, shallow and long at low flow velocities but become narrower, deeper and shorter with increasing velocities. A comparison of results of accumulations formed with debris of uniform and nonuniform size distributions has revealed that the former can be up to 2.5 times wider than the latter.<br/
... The density and size of trees in the riparian zone is also correlated with rates of bank erosion (Allmendinger et al., 2005;Pizzuto and Meckeln burg, 1989). We used a system developed by Hess (2007) and specifi cally calibrated for the South River to visually classify each image of every site into one of the following categories: 0 trees/400 m 2 , 1-2 trees/400 m 2 , 3-4 trees/400 m 2 , and >5 trees/ 400 m 2 . (For additional discussion of the visual classifi cation method, see the Data Repository.) ...
Article
Full-text available
A recent hypothesis suggests that fluvial processes in areas of the eastern United States are strongly influenced by the demise of colonial mill dams, rather than reflecting a quasi-equilibrium adjustment to the current hydrologic and sediment regime. We evaluated the control of colonial mill dams on twentieth and twenty-first century bank erosion rates on the South River, Virginia, through studies of historical aerial photographs, historical documents, hydrologic and climatic records, and hydraulic modeling. Historical sources and aerial photographs document eight colonial mill dams along the study reach in the early twentieth century; all but one of these dams disappeared in the 1950s, and the last was breached by 1976. From initially low values between 1937 and 1957, mean bank erosion rates increased by more than a factor of 2 after 1957, remaining high through 2005. Accelerated bank erosion rates cannot be explained by changes in storm intensity, the frequency of freeze-thaw cycles, or by changes in the density of riparian trees. Hydraulic modeling suggests that mill dams reduced velocities of the 5 yr flood through similar to 80% of our study reach. By considering the timing of mill dam loss, the spatial extent of backwater influence, and the locations of our study sites, we find that the loss of mill dams explains the observed trends in bank erosion rates at 9 (and possibly 10) of our 14 monitoring sites. These results support the hypothesis that the demise of mill dams has been an important influence on fluvial processes in the region.
Article
We developed a conceptual model and suspended sediment budget for a 38 km reach of the fifth-order South River, Virginia, for the past 75 yr. Bedrock, terraces, and alluvial fans confine 64% of the channel's lateral boundaries, while bedrock exposures impose vertical confinement along 37% of the channel. Bedrock exposures in the bed separate pools and riffles developed in gravelly bed material, create unusual kilometerlong pools, and divide the study area into a gently sloping upstream reach and a steeply sloping downstream reach. Bedrock exposures upstream and downstream of an alluvial monitoring site limit changes in bed elevation (documented by scour chains and repeat surveys) by flows with up to 10 yr return periods. Fifty-seven islands (features rarely mentioned in previous studies), mostly created by avulsive floodplain incision, occur in the study reach. Rates of bank retreat, likely moderated by bedrock exposures, have modal values of only a few centimeters per year, while floodplain growth by lateral accretion is negligible. Overbank deposition dominates the sediment budget, but the areal of the extent of the floodplain is currently being reduced by bank erosion and channel widening. The South River stores 2.5% of its annual suspended sediment load per kilometer of downstream transport, demonstrating that suspended sediment storage along partly confined, mixed bedrock-alluvial rivers can be equivalent to storage along fully alluvial rivers. The future evolution of the South River will likely be controlled by bank stabilization designed to control mercury loading into the channel from erosion of contaminated floodplain sediments.
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