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Influence of a large woody debris obstruction on three-dimensional flow structure in a meander bend
Geomorphology 51 (2003) 159 – 173 www.elsevier.com/locate/geomorph
Influence of a large woody debris obstruction on three-dimensional flow structure in a meander bend Melinda D. Daniels a,*, Bruce L. Rhoadsb a
Department of Geography, University of Connecticut, Storrs, CT 06269, USA b Department of Geography, University of Illinois, Urbana, IL 61801, USA
Received 10 August 2001; received in revised form 17 January 2002; accepted 18 October 2002
Abstract A field experiment has been conducted to assess the influence of a large woody debris (LWD) obstruction on threedimensional flow through a meander bend of a small stream in east central Illinois. Previous studies in unobstructed meander bends have shown that flow through a curved channel should develop a coherent three-dimensional structure characterized by large-scale helical motion. Many meander bends are complicated by naturally occurring persistent obstacles, such as LWD, that have the potential to profoundly disrupt flow structure. The results of this study show that the LWD obstruction systematically influences the three-dimensionality of flow through the bend, particularly the position of the high-velocity core and the development of helicity. The high-velocity core is positioned in the center of the channel upstream of and near the bend apex, but as flow approaches the LWD, it is steered toward the inner bank by the obstruction. Evolving helicity in the upstream portion of the bend is amplified by abrupt turning of the flow induced by the LWD. As the flow moves past the LWD, helicity diminishes rapidly and may even reverse its pattern of rotation. The net effect of the LWD obstruction is to reduce near-bank velocities along the outer bank downstream of the bend apex—a critical locus for bank erosion in meander bends. Given the persistence of the LWD obstruction, it probably has an important local influence on bend migration and evolution. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Large woody debris; Obstruction; Helicity
1. Introduction The fluvial dynamics of meandering rivers have been the focus of numerous process-based field studies over the last 25 years. In particular, the structure of flow-through meander bends has garnered considerable attention (Hey and Thorne, 1975; Dietrich and * Corresponding author. Department of Geography, University of Connecticut, 215 Glenbrook Road, U-4148, Storrs, CT 062694148, USA. Tel.: +1-860-486-2117; fax: +1-860-486-1348. E-mail address: [email protected] (M.D. Daniels).
Smith, 1983; Thorne and Rais, 1984; Thorne et al., 1985; Markham and Thorne, 1992). The interest in flow structure derives from the recognition that this structure, via its influence on bed-material transport, determines patterns of bank erosion, channel geometry and the evolution of channel planform (Hooke and Harvey, 1983; Rhoads and Welford, 1991). Based on physical considerations, Thomson (1876) argued that flow through a curved channel should develop a coherent three-dimensional structure characterized by large-scale helical motion. This structure is the result of a local imbalance between the centrifugal
0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 5 5 5 X ( 0 2 ) 0 0 3 3 4 - 3
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force and the counteracting pressure-gradient force associated with super-elevation of the water surface along the outer bank of the curved channel (Dietrich, 1987). Field measurements of downstream and crossstream velocity components in meander bends show that orientations of two-dimensional velocity vectors change systematically over depth, a pattern consistent with helical motion (Hey and Thorne, 1975). This pattern, however, is confined mainly to the central region of the flow. Near the outer bank, a small, counter-rotating helical cell is produced by flow stagnation (Thorne and Rais, 1984). Moreover, in many meander bends, the presence of a well-developed point bar directs flow along the inner bank outward over the entire flow depth (Dietrich and Smith, 1983). The result is three distinct zones of fluid motion in the crossstream plane: (i) outward flow over the point bar, (ii) large-scale, curvature-induced helicity in the central portion of the channel and (iii) a small, counter-rotating helical cell along the outer bank (Markham and Thorne, 1992). Advection of downstream momentum outward toward the outer bank and downward toward the bed by helical motion leads to submergence of the core of high downstream velocity over the outer half of the channel cross-section. The location of the high-velocity core near the bed produces large, near-bed shear stresses that promote scouring of the bed, i.e., pool development, in this portion of the cross-section (Dietrich, 1987). Although flow-through meander bends has been intensively investigated, most field studies have focused on relatively simple meander bends that are free from major obstructions. Many meander bends are complicated by naturally occurring persistent obstacles, such as large woody debris (LWD), that have the potential to profoundly disrupt flow structure. LWD influences river dynamics by functioning as a resistance element, thereby modifying flow and sediment transport, patterns of erosion and deposition and, potentially, planform development (Keller and Swanson, 1979; Robinson and Beschta, 1990; Gregory et al., 1994; Assani and Petit, 1995; Fetherston et al., 1995; Gippel, 1995; Gurnell et al., 1995; Montgomery et al., 1995; Richmond and Fausch, 1995; Abbe and Montgomery, 1996; Myers and Swanson, 1997; Gurnell and Sweet, 1998). Past work has shown that LWD can be a dominant control of stream geomorphology in watersheds with forested riparian corridors (Keller and Swanson, 1979; Maser and Sedell,
1994; Fetherston et al., 1995; McKenny et al., 1995), but detailed, process-based investigations of the effects of LWD on the fluvial dynamics of meander bends are lacking. Thorne and Furbish (1995) assessed the influence of dense bank vegetation on flow through a meander bend and found that this material lowered velocities near the outer bank and led to repositioning of the high-velocity core away from this bank. Because the study relied on onedimensional velocity measurements, the effects of vegetation on flow three-dimensionality had to be inferred from visualization of flow patterns using streamers. Even in unobstructed bends, the threedimensionality of the flow has been inferred from at most two-dimensional (downstream and cross-stream) measurements of velocity components (Dietrich and Smith, 1983; Thorne and Rais, 1984; Thorne et al., 1985). Recent advances in measurement technology now permit the collection of 3-D velocity data in field settings (Lane et al., 1998). As a result, 3-D flow structure through meander bends now can be measured directly rather than being inferred from 2-D data. Direct characterization of flow structure in three dimensions is especially important where the path of the flow changes rapidly over space relative to the orientation of the stream channel (e.g., Rhoads and Sukhodolov, 2001)—a situation that, as this study demonstrates, can be produced by LWD obstructions. This paper examines the influence of LWD on time-averaged flow structure in a meander bend along a small agricultural stream in the Midwestern United States. The study is based on 3-D velocity measurements, thereby providing a fully 3-D representation of the flow structure. The results of this research illustrate the extent to which LWD can affect flow structure in bends. Studies of this type represent a first step toward determining the ensemble of process interactions between LWD and bend dynamics.
2. Field site Madden Creek is a low-gradient (0.00185) tributary of the Sangamon River in east central Illinois (Fig. 1). The 42-km2 watershed of Madden Creek lies in the Bloomington Ridged Plain physiographic region of Illinois, which is characterized by nearly flat or gently sloping till plains crossed by broad, low end moraines.
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Fig. 1. (A) Map of the field site showing the location of study reach. (B) Aerial photograph of Madden Creek showing the location of the study site. (C) Detailed topographic map of the study site showing location of measurement cross-sections (numbered lines) and location of LWD obstruction (shaded area) (arbitrary datum of 10.0 m).
These landscape features formed during the late Wisconsin subepisode when the Lake Michigan lobe of the Laurentide ice sheet reached this area (Hansel and Johnson, 1992). Madden Creek originates on the Champaign Moraine in northern Piatt County and flows south across a broad, gently sloping till plain to the Sangamon River. Prior to European settlement in the mid-1800s, east central Illinois was a wet prairie. The growth of agriculture in the region involved extensive land drainage, including installation of subsurface drainage tiles,
the construction of open ditches, and the channelization of existing streams (Rhoads and Herricks, 1996). Today, over 50% of stream reaches in the Sangamon basin are channelized (Mattingly et al., 1993). Land use in the Madden Creek watershed is predominantly agricultural, and the stream has been affected both directly and indirectly by agricultural activity. The University of Illinois Map and Geography Library’s collection of aerial photography for Madden Creek extends to 1946 and shows that many sections of the creek have been straightened and channelized
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over the past 55 years. Despite the severity of human impacts in this system, some stream reaches have not been straightened since the 1940s. The study site for this research project is a meander bend located within an approximately 2.5-km-long section of Madden Creek that is freely meandering and has a narrow, forested riparian corridor (Fig. 1). Historical aerial photography indicates that the meander and immediately adjoining reaches have not been affected by channelization or significant natural planform changes since the 1940s. A review of the General Land Office (GLO) survey records for this site indicate that, at the time of the surveys in 1821 and 1822, some mature trees were present near the channel of Madden Creek. This information is consistent with the current status of the riparian zone, which consists of a narrow band of mature trees flanking the channel. The meander bend selected for this study contains a LWD obstruction consisting of a living tree and root bowl attached to the outer bank just past the apex of the meander bend (Fig. 2). The tree extends upright
out of the channel and the curved trunk and root bowl have trapped several other pieces of LWD, producing a complex obstruction that extends into the channel at nearly a right angle to the outer bank. Roots also extend away from the obstruction into the channel from the root bowl. The LWD structure is not an impermeable obstacle and some water passes through it, especially near the outer bank where the connection consists of open root structures anchored in the bank material. Within the bend, the channel has an average bankfull width (Wb) of approximately 9 m and a bankfull depth of 1.2 – 1.4 m (Fig. 1). The dimensionless curvature of the bend (r/Wb, where r is radius of curvature) is approximately 2.0, a value that lies at the low end of the range of dimensionless curvature for natural channels and that generally is associated with high rates of meander migration (Hickin and Nanson, 1975; Nanson and Hickin, 1986; Hudson and Kelsel, 2000). At sub-bankfull stages, the geometry of the channel varies spatially. A large point bar protrudes
Fig. 2. Photograph of the study site looking downstream toward the LWD obstruction. Wading rod holding the ADV is visible upstream of the obstruction.
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into the channel from the inner bank, which confines flow adjacent to it at sub-bankfull stage. Downstream of the point bar, the thalweg (or zone of maximum channel depth) turns abruptly toward the inner bank near the LWD obstruction. Bed material throughout the reach consists of bimodal sand and gravel.
3. Collection, processing and analysis of field data Field data on 3-D velocities were collected at Madden Creek on July 2 and 9, 1998. The data for these two dates reveal nearly identical patterns of fluid motion and only data for July 9, the most detailed set of measurements, are reported in this paper. Velocity measurements were obtained using an acoustic Doppler velocimeter (ADV) at five cross-sections aligned orthogonally to the direction of the channel centerline (Fig. 1). The five cross-sections are distributed around the bend with four cross-sections upstream of the LWD obstruction and one downstream of it. Velocities were measured at six to eight verticals within each cross-section and at two to eight points within each vertical. The total number of sample points within each cross-section ranged from 29 to 43. During the period of 3-D velocity measurements, the water stage at the study site was monitored using two ultrasonic sensors connected to a Campbell 21 data logger. The stage at the beginning of the period of measurements was 9.10 m, which corresponds to a halfbankful event with flow widths of 3.75 m at crosssections 4– 6, 4.75 m at cross-section 7 and 6.10 m at cross-section 8. Stage remained relatively constant, rising gradually by 0.03 m over the 8-h measurement period. The ADV measures downstream (U), vertical (W) and cross-stream (V) velocities in a sampling volume < 0.25 cm3 located f 5 cm away from the probe head. Because values of 3-D velocity components depend on the orientation of the sensor, maintaining consistency of sensor alignment is important. To ensure that the measurements were obtained within a fixed frame of reference at each cross-section, the ADV was mounted on a custom-built wading rod that, in turn, was attached to a steel cable stretched tautly between iron pipes at each end of the crosssection (e.g., Rhoads and Sukhodolov, 2001) (Fig. 2). This system ensured that the X-axis of the sensor
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was perpendicular to the cross-section and the Y-axis of the sensor was parallel to the cross-section. The wading rod also was plumbed prior to each measurement to ensure that the Z-axis of the sensor coincided with the vertical plane of the cross-section. This positioning system also precluded the need for an operator to stabilize the wading rod, eliminating the possibility of operator-induced unsteadiness of the probe during measurements. Once plumbed, the wading rob was left in place while the operator moved away from the unit a distance sufficient to avoid any disruption of the flow field near the ADV probe (Fig. 2). Three-dimensional velocities were sampled for 60 s at a rate of 25 Hz. In some cases, interruption of the acoustic signal by floating debris or by intermittent intersection of the acoustic beam with the channel bed produced periods of excessive noise or large spikes in the velocity records. These situations were noted in the field and the sampling duration was increased to obtain an uninterrupted 60-s record of clean data. Postprocessing of the velocity data was performed with Explore ADV software developed by Nortek USA. Any individual spikes resulting from acoustic noise were removed from the data set using a 3r filter in the Explore ADV software, where r is the standard deviation of the time series of velocities. The occurrence of such spikes was rare. Most data sets contained no spikes and those that did had only one or two spikes. In all cases, application of the 3r filter removed the spikes, yet changed the timeaveraged velocity by at most a few millimeters per second. Stationarity of the times series was assessed by dividing each series into 10 equally spaced intervals and then cumulating a running mean for the 10 intervals. If the data are stationary, the cumulated mean gradually should converge on a constant value as the number of intervals included in the computation of the mean increases. All means, except those for time series with low values of timeaveraged downstream velocity (U < 0.10 m s 1), exhibited convergence of means over the 10 intervals. Means for low-velocity locations may not be entirely accurate but provide a reasonable basis for comparing rates of fluid motion relative to rates for other portions of the flow. Values of depth-averaged downstream (U) and cross-stream (V) velocity for each vertical were used
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to compute the magnitude (MUV) and orientations (qxy) of depth-averaged velocity vectors: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi U2 þ V2
MU V ¼
hxy ¼ tan1
V U
ð1Þ
ð2Þ
where U¼
n X
! Ui Dh =H
ð3Þ
i¼1
V ¼
n X
! Vi Dh =H
ð4Þ
i¼1
and where n is the number of velocity measurements in a vertical, Dh is the increment of depth associated with a point measurement and H is the flow depth. Values of U were used to generate contour plots of downstream velocity. Contouring of the downstream velocity field was performed manually because application of computer-contouring algorithms based on various interpolation routines, including kriging, did not accurately represent the strong spatial anisotropy of some velocity fields. Patterns of V and W were superimposed on the contour plots as vectors of cross-stream/vertical velocity components. The magnitude (MVW) and vertical orientation (hUZ) of these vectors are: MV W ¼
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi V2 þ W2
hU Z ¼ tan1
W U
urement cross-sections, a factor that can mask helicity on plots of V–W velocity vectors (Rhoads and Kenworthy, 1995, 1998, 1999). As a result, the helicity assessment procedure (HAP) developed by Rhoads and Sukhodolov (2001) was used to facilitate evaluation of the 3-D structure of time-averaged flow in the reach. The HAP examines relations among lateral (hXY) and vertical (hXZ) angles of 3-D velocity vectors to determine whether large-scale helical motion exists within the flow. The vertical orientation of the crossstream plane ( Y– Z) usually is aligned with the influence of gravitational forces—a condition that has been met in this study by ensuring that the sensor is plumb relative to the channel bed. Thus, absolute values of hXY define the vertical motion of the flow in the crossstream plane as depicted on the V –W vector plots. However, the horizontal alignment of the cross-section can be defined by various criteria—each of which produces different values of V (Dietrich and Smith, 1983; Markham and Thorne, 1992; Rhoads and Kenworthy, 1999). Whereas values of V are sensitive to rotation of the cross-section about a vertical axis, differences among values of hXY are independent of the frame of reference and, therefore, provide an objective method for assessing helicity within a complex flow. The HAP procedure uses five criteria to identify helical patterns from the vector angle differences: (i) a difference of at least 5j between the crossstream (hXY) vector angles calculated for near-surface and near-bed velocity vectors, (ii) systematic changes
ð5Þ
ð6Þ
Lateral vector orientations are determined by the sign of V ( + V to the left looking upstream, V to the right looking upstream). An aim of this study was to explore the influence of LWD at Madden Creek on any coherent three dimensionality of the flow, especially the helical motion found in most meander bends. Field observations and preliminary analysis of depth-averaged velocity vectors indicated that flow at some locations was highly skewed relative to the orientation of the meas-
Fig. 3. Spatial pattern of depth-averaged velocity vectors.
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Fig. 4. Patterns of downstream velocity components (contours) and cross-stream/vertical velocity components (vectors) at cross-sections 4 – 8 (looking upstream). Shaded areas indicate zones of separated or stagnant flow.
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Fig. 5. Plots of hxy (column A) and hxz (column B) for measurement cross-sections. Bolded numbers are values conforming with the criteria of helicity assessment procedure and are indicative of helical rotation of the flow.
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Fig. 5 (continued ).
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in cross-stream (hXY) vector angles over depth, (iii) the presence of similar systematic patterns of change in cross-stream (hXY) vector angles over depth at a minimum of two adjacent verticals, (iv) occurrence of similar patterns of change in cross-stream (hXY) vector angles over depth in corresponding portions of a minimum of two consecutive cross-sections and (v) the orientation of 3-D vectors downward ( hXZ) toward the bed and upward ( + hXZ) toward the surface on opposite sides of the presumed helix within the set of cross-sections over which it is defined (Rhoads and Sukhodolov, 2001).
4. Spatial patterns of velocity components Depth-averaged vectors of downstream and crossstream velocity components show that the spatial path of the flow generally conforms with the planform geometry of the meander bend from the entrance (cross-section 4) to immediately downstream of the bend apex (cross-section 6) (Fig. 3). Over this distance, the largest vectors occur in the center of the channel. As flow approaches the LWD obstruction (cross-section 7), vectors are directed strongly towards the inner bank. The largest vector still occurs in the center of the channel, but vector magnitudes decrease abruptly toward the outer bank. Downstream of the LWD (cross-section 8), vector magnitudes are greatest along the inner bank, where flow is directed past the woody debris. In contrast, vector magnitudes are small along the outer bank in the lee of the LWD. Plots of downstream velocity reveal a well-defined, high-velocity core just beneath the water surface in the center of the channel within the bend (crosssections 4 – 6) (Fig. 4). As flow approaches the LWD, the size of the core expands; but its magnitude decreases by about 10 cm s 1 (cross-section 7), mainly because much of the flow is directed obliquely to the alignment of the cross-section (Fig. 3). A region of stagnant, recirculating fluid exists along the outer bank of the channel immediately upstream of the LWD obstruction, and a zone of separated flow exists along the inner bank where the channel widens abruptly (cross-section 7). Downstream of the LWD, a complex spatial pattern of U develops with the highest velocities immediately adjacent to the inner
bank and a region of stagnant recirculating flow in the center of the channel (cross-section 8). A distinct shear layer characterized by a strong lateral gradient in U separates the region of stagnant fluid from the high-velocity zone. A weak secondary maximum of downstream velocity exists adjacent to the outer bank in the lee of the LWD. Vectors of downstream and cross-stream velocity components, referred to here as secondary vectors, are directed uniformly inwards at the entrance to the bend (cross-section 4) (Fig. 4). Depth-averaged vectors at this location are oriented parallel to the local channel direction (Fig. 3); thus, the systematic orientations of secondary vectors represent slight net skewing of the flow relative to the cross-section alignment, rather than net flow toward either bank. The vertical orientation of vectors at the bend entrance suggests slight net downward fluid motion over the outer portion of the channel and upward movement over the inner portion. As flow moves from the entrance around the bend apex, the composite pattern of vector orientations is suggestive of counter-clockwise rotation of the fluid (cross-sections 5 and 6) (Fig. 4). Vectors near the surface are oriented outward, whereas vectors near the bed are oriented inward. Pronounced upward motion is evident over the inner portion of the channel (cross-sections 5 and 6), and strong downward motion develops over the deepest portion of the flow near the bend apex (cross-section 6). Secondary vectors upstream of the LWD obstruction are uniformly and strongly directed inward, indicating net motion of the fluid towards the inner bank (cross-section 7) (Fig. 4). Downstream of the LWD obstruction (cross-section 8), secondary vectors are directed uniformly inwards. Most vectors are parallel with the water surface; however, near the inner bank the vectors are oriented sharply downward, suggesting that strong downwelling occurs as flow deflected inward by the LWD obstruction collides with the inner bank.
5. Spatial patterns of helical motion The helicity assessment procedure (HAP) reveals the total spatial extent of helical motion through the meander bend. Between the bend entrance and the bend apex, at least two adjacent verticals in each
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cross-section exhibit systematic changes in values of hXY over depth. At the bend entrance (cross-section 4), values of hXY at three adjacent verticals increase from the surface to the bed by between 9j and 14j, indicating that the flow near the bed is directed more toward the inner bank than the flow near the surface (Fig. 5). This pattern of systematic deviation in vector angles cannot be discerned from the plot of secondary vectors, where it is masked by slight net skewing of the flow relative to the cross-section alignment (Figs. 3 and 4). On the inner half of the cross-section, values of hXZ increase from the surface to the bed by between 5j and 10j, indicating that flow near the bed is moving upward more than flow near the surface. The pattern of vector angles at cross-sections 5 and 6 confirm that helical motion becomes well developed as flow moves around the bend apex. Values of hXY change systematically from negative angles at the surface or within the center of the flow to positive angles at the bed over several verticals at both crosssections (Fig. 5). The magnitude of total angular deviation over depth increases from 6j to 12j at cross-section 5 to 14j to 31j at cross-section 6. These trends indicate that flow near the surface is directed toward the outer bank while flow near the bed is directed toward the inner bank. Values of hXZ over the inner portion of cross-section 5 increase systematically over depth by 1– 3j, indicating that flow near the bed is moving upward relative to flow near the surface. At cross-section 6, values of hXZ are negative over the outer portion of the channel and strongly positive near the bed over the inner portion of the channel—a pattern consistent with counterclockwise helical motion of the flow. The HAP shows that despite net inward movement of the flow as it approaches the woody debris (crosssection 7, Figs. 3 and 4), strong helical motion persists within the flow. At cross-section 7, values of hXY increase systematically either from near the surface or within the middle of the flow toward the bed at several adjacent verticals (Fig. 5). The total angular deviation over depth ranges from 33j to 61j—a range greater than that for cross-sections 4 – 6. Values of hXZ are negative over the central and outer portion of the channel and positive near the bed and at the vertical adjacent to the lateral separation zone along the inner bank. Together, these patterns of vector angles are
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consistent with counterclockwise rotation of fluid within a flow that is highly skewed in relation to the cross-section orientation. No distinct pattern of vector angle changes is apparent downstream of the LWD obstruction (cross-section 8), indicating that helical motion in the flow decays rapidly as it passes the LWD obstruction (Fig. 5). Values of hXY at the two verticals closest to the inner bank do decrease systematically over depth by 10 –20j, a pattern opposite that associated with counterclockwise motion. This pattern, along with strong downward motion along the inner bank ( hXZ) and upward motion immediately adjacent to the inner bank (+ hXZ), suggests that helical motion characterized by clockwise rotation may exist within the flow along the inner bank. No systematic patterns of hXY or hXZ can be discerned within the shear layer adjacent to the high-velocity core or within the regions of separated or low-velocity flow in the lee of the LWD obstruction.
6. Discussion The results show that the LWD obstruction within the meander bend at Madden Creek has a pronounced influence on the structure of flow through this bend. Patterns of depth-averaged vectors suggest that between the bend entrance and the bend apex flow streamlines follow a path that is roughly parallel to the direction of local channel curvature (Fig. 6). The curvature of flow streamlines generates evolving counterclockwise helicity in this portion of the bend that intensifies in the downstream direction, eventually manifesting itself in the pattern of secondary vectors within the cross-stream plane (Fig. 6). The development of helical motion accounts for submergence of the high velocity core beneath the water surface. This motion provides a mechanism for advective transport of high-momentum fluid toward the bed and low-momentum fluid toward the surface, resulting in a maximum downstream velocity in the central portion of the cross-sectional flow field (e.g., Thorne et al., 1985). As flow along the outer bank approaches the LWD obstruction, it stalls due to the damming effect of the LWD. Although variations in water surface topography at the site are too small for reliable measurements
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Fig. 6. Conceptual model illustrating the pattern of flow streamlines and helical motion in the LWD-obstructed bend at Madden Creek. Dark shading indicates position of the LWD obstruction, and light shading corresponds to regions of separated or stagnant flow. Crosssections showing patterns of secondary flow in the cross-stream plane are also shown. The dashed pattern of secondary motion for cross-section 7 corresponds to the dashed reorientation of this crosssection as shown on the map of the bend. The dashed pattern for cross-section 8 indicates weak helical motion.
of elevation differences, super-elevation of the water surface most likely occurs immediately upstream of the obstruction. Flow moving against this adverse pressure gradient will decelerate rapidly, resulting in flow stagnation (Fig. 6). Local mounding of the water surface along the outer bank also will produce a crossstream pressure gradient that can turn flow laterally towards the inner bank. The development of lateral flow separation along the inner bank (Fig. 6), a region of low pressure caused by an abrupt change in the channel boundary, will enhance the cross-stream pressure gradient. The combined influence of the LWD obstruction and channel geometry on the water surface topography and cross-stream pressure gradient steers the flow around the obstruction.
Steering of the flow past the LWD obstruction results in an abrupt increase in streamline curvature (Fig. 6). This increase in curvature, combined with enhanced vortex stretching as flow accelerates through the narrow cross-sectional area adjacent to the obstruction, intensifies counterclockwise helical motion. The helicity, despite its strength, cannot be detected directly in the pattern of secondary vectors due to substantial skewing of the flow direction relative to the orientation of the cross-stream plane (Fig. 6). It can, however, be readily detected in the pattern of 3-D vector angles. Downstream of the LWD obstruction, the highvelocity core is positioned along the inner bank. Patterns of 3-D vector angles suggest that clockwise helical motion may develop within the flow along this bank, perhaps due to reversal in curvature of streamlines as the flow passes through the opening adjacent to the LWD obstruction (Fig. 6). The vector-angle data, however, do not satisfy the HAP criteria; further evidence is needed to determine whether spiral motion exists in the downstream flow. Behind the bulk of the obstruction, a region of separated flow develops, much like that in the lee of a boulder or other large obstacle (Fig. 6). The LWD, however, is not an impervious solid and some flow manages to move through the obstruction as revealed by the weak secondary maximum in downstream velocity adjacent to the outer bank. Flow through this side of the obstruction may be enhanced by a strong downstream pressure gradient associated with super-elevation of the water surface along the outer bank of the channel upstream of the obstruction. The flow structure at the Madden Creek site differs from other field and experimental studies of flowthrough unobstructed meander bends, which show that the high-velocity core shifts toward the outer bank as flow moves through the bend (Jackson, 1975; Bathurst et al., 1979; Dietrich and Smith, 1983; Geldof and de Vriend, 1983; Alphen et al., 1984; Frothingham, 2001). The lack of an outward shift in the high-velocity core upstream of the bend apex at Madden Creek is not unusual. Outward shifting of the maximum-velocity core is the product of redistribution of momentum by helical motion and by topographic steering of the flow around the point bar (Dietrich and Smith, 1983; Nelson and Smith, 1989). In the absence of topographic steering, the
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high-velocity core will not shift markedly toward the outer bank until well downstream of the bend apex. Topographic steering by a point bar in equilibrium with flow conditions will produce an abrupt outward shift of the high-velocity core, but this shift still occurs downstream of the bend apex (Nelson and Smith, 1989). Thus, not surprisingly, the high-velocity core at Madden Creek remains centered in the channel upstream of the bend apex. Neither helical motion nor topographic steering would be expected to redistribute momentum rapidly enough to affect an outward shift of the high-velocity core within this portion of the bend. The velocity data suggest that helical motion develops in the flow between the bend entrance and the bend apex, but the cross-stream vector data do not reveal substantial lateral forcing of the flow toward the outer bank by the point bar (Fig. 4). Although the bend does contain a well-developed point bar, at this stage, cross-sectional asymmetry is minor, thereby diminishing the influence of topographic steering. The lack of substantial topographic steering probably explains why the high-velocity core at cross-section 6, which is located slightly downstream of the bend apex, remains near the center of the channel. The main discrepancy between flow structure in unobstructed bends and the pattern of flow observed at Madden Creek occurs downstream of the bend apex in the vicinity of the LWD obstruction. In unobstructed bends, the core of high velocity often is situated adjacent to the outer bank downstream of the bend apex and may in some cases be submerged beneath the water surface near the toe of the outer bank (Thorne et al., 1985). The result is a zone of high, near-bed shear stress capable of mobilizing sediment and eroding the outer bank of the channel (Thorne and Rais, 1984). In contrast, flow structure downstream of the bend apex at Madden Creek is characterized by a zone of stagnation along the outer bank that extends to the base of this bank and a highvelocity core located in the center of the channel (Fig. 4). This pattern of flow should serve to protect the outer bank by producing small or negligible bed shear stresses along the bank toe—a factor of considerable importance in determining bank erosion rates (Thorne, 1982). The findings of this study are generally consistent with past work on the influence of bank vegetation on bend flow, which showed that the presence of
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vegetation reduces velocities near the outer bank and confines the filament of maximum velocity to the center of the channel (Thorne and Furbish, 1995). The results here differ from this past work in that the LWD obstruction at Madden Creek acts like a partial dam or weir that steers the flow away from the outer bank, rather than just increasing flow resistance along this bank. Whereas Thorne and Furbish (1995) found that bank vegetation diminishes helicity by reducing the cross-stream pressure gradient, this study has shown that steering of the flow around the LWD obstruction intensifies helical motion, presumably by locally enhancing the cross-stream pressure gradient. Although details of flow structure in the vicinity of the LWD obstruction may change with increases in stage, visual observations of near-bankfull conditions at the field site indicate that at this stage the obstruction, which has a height equal to about 60 – 70 % of the depth of the bankfull channel, still deflects the flow strongly away from the outer bank. By modifying the flow around it, the LWD obstruction (which is quite persistent) probably serves as an anchor for bend development. Aerial photos of the reach show minor extension of the bend over the last few decades as the channel upstream of the LWD has migrated laterally, whereas the location of the channel at the LWD obstruction has remained fixed. The apparent LWD-induced stabilization of channel morphology at the study site is consistent with past studies of obstruction-anchored channel features, which showed that quasi-permanent obstructions greatly influenced the positions of bar and pool channel features as well as anchored channel planform locally (Lisle, 1986). Overall, the LWD obstruction at the study site should reduce the net rate of lateral migration because if the bend were not obstructed maximum rates of bank erosion most likely would occur downstream of the bend apex at the current location of the obstruction. On the other hand, deflection of flow around the LWD may cause increased erosion along the inner bank of the channel. Flow past the obstruction has produced a zone of well-developed scour adjacent to the inner bank (Fig. 1). Moreover, the trend of the inner bank abruptly shifts inward downstream of the obstruction, resulting in sudden widening of the flow. This characteristic appears to be a fairly recent development within the reach based on visual inspection of historical aerial photographs.
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7. Conclusions This study has contributed to a process-based understanding of flow through meander bends containing LWD by documenting the 3-D flow structure in a meander bend with a LWD obstruction. The results show that the LWD obstruction within the meander bend at Madden Creek has a pronounced influence on the structure of flow through this bend. Major findings include the following: (i) Depth-averaged flow upstream of the LWD obstruction generally follows the path of the channel, but as flow approaches the obstruction downstream of the bend apex, it is steered abruptly toward the inner bank. (ii) Downstream velocity fields upstream of the obstruction are characterized by a well-defined, submerged high-velocity core in the center of the channel. The core expands but decreases in magnitude where flow is deflected around the LWD. Downstream of the obstruction, a complex pattern of downstream velocity develops with the highest velocities adjacent to the inner bank and separated, recirculating flow in the lee of the LWD. (iii) Patterns of secondary-velocity vectors and 3-D vector angles suggest that helical motion develops in the flow between the bend entrance and the bend apex. Steering of the flow past the LWD obstruction results in an abrupt increase in streamline curvature and enhanced vorticity as flow accelerates through the narrow, crosssectional area adjacent to the obstruction, intensifying helical motion. Helicity decays as flow moves past the LWD toward the inner bank and exits the bend. Given the diversity of LWD structures and meander morphologies, the results of this particular field study are unlikely to be widely generalized. Additional process-based studies of other configurations of LWD are needed to quantify the range of effects that such obstructions can have on flow-through bends. Ideally, other studies should investigate the influences of the scale of LWD in relation to the size of the stream channel, placement of LWD in relation to channel planform, the internal structure of LWD
obstructions, and the persistence of obstructions relative to the time scale of river dynamics. Such studies hopefully will define the general factors that control complex interactions between LWD and fluvial processes. Field-based research of this type should also be combined with numerical modeling of interaction between fluvial processes and LWD to develop a predictive understanding of the influence of specific types of obstructions on river dynamics. Acknowledgements The authors are grateful to Kelly Frothingham, Marta Graves, Perry Cabot, Matt Ladewig and Kristin Jaburek for their help and assistance in collecting the field data. We would also like to thank Tom Lisle for helpful comments that improved the manuscript. Funding for this research was provided by the U.S. Environmental Protection Agency, Water and Watersheds Program (R82-5306-010) and the University of Illinois Research Board. References Abbe, T.B., Montgomery, D.R., 1996. Large woody debris jams, channel hydraulics and habitat formation in large rivers. Regulated Rivers: Research and Management 12, 201 – 221. Alphen, J.S.L.J., Blocks, P.M., Hoekstra, P., 1984. Flow and grainsize pattern in a sharply curved river bend. Earth Surface Processes and Landforms 9, 513 – 522. Assani, A.A., Petit, F., 1995. Log-jam effects on bed-load mobility from experiments conducted in a small gravel-bed forest ditch. Catena 25, 117 – 126. Bathurst, J.C., Thorne, C.R., Hey, R.D., 1979. Secondary flow and shear stress at river bends. Journal of the Hydraulics Division: Proceedings of the American Society of Civil Engineers 105 (HY10), 1277 – 1295. Dietrich, W.E., 1987. Mechanics of flow and sediment transport in river bends. In: Richards, K. (Ed.), River Channel Environment and Process. Basil, Blackwell, London, UK, pp. 179 – 227. Dietrich, W.E., Smith, J.D., 1983. Influence of the point bar on flow through curved channels. Water Resources Research 19 (5), 1173 – 1192. Fetherston, K.L., Naiman, R.J., Bilby, R.E., 1995. Large woody debris, physical process, and riparian forest development in montane river networks of the Pacific Northwest. Geomorphology 13, 133 – 144. Frothingham, K.M., 2001. Geomorphological processes in meandering and straight reaches of an agricultural stream in East Central Illinois: relations to aquatic habitat. PhD dissertation thesis, University of Illinois, Urbana.
M.D. Daniels, B.L. Rhoads / Geomorphology 51 (2003) 159–173 Geldof, H.J., de Vriend, H.J., 1983. Distribution of main flow velocity in alternating river bends. Special Publication of the International Association of Sedimentologists 6, 83 – 93. Gippel, C.J., 1995. Environmental hydraulics of larger woody debris in streams and rivers. Journal of Environmental Engineering 121 (5), 388 – 395. Gregory, K.J., Gurnell, A.M., Hill, C.T., Tooth, S., 1994. Stability of the pool-riffle sequence in changing river channels. Regulated Rivers: Research and Management 9, 35 – 43. Gurnell, A.M., Sweet, R., 1998. The distribution of large woody debris accumulations and pools in relation to woodland stream management in a small, low-gradient, stream. Earth Surface Processes and Landforms 23, 1101 – 1121. Gurnell, A.M., Gregory, K.J., Petts, G.E., 1995. The role of coarse woody debris in forest aquatic habitats: implications for management. Aquatic Conservation: Marine and Freshwater Ecosystems 5, 143 – 166. Hansel, A.K., Johnson, W.H., 1992. Fluctuations of the Lake Michigan Lobe during the late Wisconsin subepisode. Sveriges Geologiska Undersokning 81, 133 – 144. Hey, R.D., Thorne, C.R., 1975. Secondary flows in river channels. Area 7, 191 – 195. Hickin, E.J., Nanson, G.C., 1975. The character of channel migration on the Beatton River, northeast British Columbia, Canada. Geological Society of America Bulletin 86, 487 – 494. Hooke, J.M., Harvey, A.M., 1983. Meander changes in relation to bend morphology and secondary flows. Special Publications of the International Association of Sedimentologists 6, 121 – 132. Hudson, P.F., Kelsel, R.H., 2000. Channel migration and meanderbend curvature in the lower Mississippi River prior to major human modification. Geology 28 (6), 531 – 534. Jackson, R.G., 1975. Velocity-bed-form – texture patterns of meander bends in the lower Wabash River of Illinois and Indiana. Geological Society of America Bulletin 86, 1511 – 1522. Keller, E.A., Swanson, F.J., 1979. Effects of large organic material on channel form and fluvial processes. Earth Surface Processes 4, 361 – 380. Lane, S.N., Biron, P.M., Bradbrook, K.F., Chandler, J.H., Crowell, M.D., McLelland, S.J., Richards, K.S., Roy, A.G., 1998. Threedimensional measurement of river channel flow processes using acoustic doppler velocimetry. Earth Surface Processes and Landforms 23 (13), 1247 – 1267. Lisle, T.E., 1986. Stabilization of a gravel channel by large streamside obstructions and bedrock bends, Jacoby Creek, northwestern California. Geological Society of America Bulletin 97 (8), 999 – 1011. Markham, A.J., Thorne, C.R., 1992. Geomorphology of gravel-bed river bends. In: Billi, P., Hey, R.D., Thorne, C.R., Tacconi, P. (Eds.), Dynamics of Gravel-Bed Rivers. Wiley, New York, pp. 433 – 457. Maser, C., Sedell, J.R., 1994. From the Forest to the Sea: The Ecology of Wood in Streams, Rivers, Estuaries, and Oceans. St. Lucia Press, Delray Beach, FL. 200 pp. Mattingly, R.L., Herricks, E.E., Johnston, D.J., 1993. Channelization and levee construction in Illinois: review and implications for management. Environmental Management 17, 781 – 795. McKenny, R., Jacobson, R.B., Wertheimer, R.C., 1995. Woody
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vegetation and channel morphogenesis in low-gradient, gravelbed streams in the Ozark Plateaus, Missouri and Arkansas. Geomorphology 13, 175 – 198. Montgomery, D.R., Grant, G.E., Sullivan, K., 1995. Watershed analysis as a framework for implementing ecosystem management. Water Resources Bulletin 31, 369 – 386. Myers, T., Swanson, S., 1997. Variability of pool characteristics with pool type and formative feature on small Great Basin rangeland streams. Journal of Hydrology 201, 62 – 81. Nanson, G.C., Hickin, E.J., 1986. A statistical analysis of bank erosion and channel migration in western Canada. Geological Society of America Bulletin 97, 497 – 504. Nelson, J.M., Smith, J.D., 1989. Flow in meandering channels with natural topography. In: Ikeda, S., Parker, G. (Eds.), River Meandering. Water Resources Monograph, American Geophysical Union, Washington, DC, pp. 69 – 102. Rhoads, B.L., Herricks, E.E., 1996. Naturalization of headwater streams in Illinois: challenges and possibilities. In: Brookes, A., Shields, F.D. (Eds.), River Channel Restoration: Guiding Principles for Sustainable Projects. Wiley, London, pp. 331 – 367. Rhoads, B.L., Kenworthy, S.T., 1995. Flow structure at an asymmetrical stream confluence. Geomorphology 11, 273 – 293. Rhoads, B.L., Kenworthy, S.T., 1998. Time-averaged flow structure in the central region of a stream confluence. Earth Surface Processes and Landforms 23, 171 – 191. Rhoads, B.L., Kenworthy, S.T., 1999. On secondary circulation, helical motion and Rozovskii-based analysis of time-averaged two-dimensional velocity fields at confluences. Earth Surface Processes and Landforms 24, 369 – 375. Rhoads, B.L., Sukhodolov, A., 2001. Field investigation of threedimensional flow structure at stream confluences: 1. Thermal mixing and time-averaged velocities. Water Resources Research 37, 2393 – 2410. Rhoads, B.L., Welford, M.R., 1991. Initiation of river meandering. Progress in Physical Geography 15, 127 – 156. Richmond, A.D., Fausch, K.D., 1995. Characterisics and function of large woody debris in subalpine Rocky Mountain streams in northern Colorado. Canadian Journal of Fisheries and Aquatic Sciences 52, 1789 – 1802. Robinson, E.G., Beschta, R.L., 1990. Coarse woody debris and channel morphology interactions for undisturbed streams in southeast Alaska, U.S.A. Earth Surface Processes and Landforms 15, 149 – 156. Thomson, J., 1876. On the origin of winding rivers in alluvial plains. Proceedings of the Royal Society 25, 5 – 8. Thorne, C.R., 1982. Processes and mechanisms for river bank erosion. In: Hey, R.D., Bathurst, J.C., Thorne, C.R. (Eds.), GravelBed Rivers. Wiley, Chichester, UK, pp. 227 – 271. Thorne, S.D., Furbish, D.J., 1995. Influences of course bank roughness on flow within a sharply curved river bend. Geomorphology 12, 241 – 257. Thorne, C.R., Rais, S., 1984. Secondary current measurements in a meandering river. In: Elliott, C.M. (Ed.), Rivers ’83. American Society of Civil Engineers, New Orleans, LA, pp. 675 – 686. Thorne, C.R., Zevenbergen, L.W., Pitlick, J.C., Rais, S., Bradley, J.B., Julien, P.Y., 1985. Direct measurements of secondary currents in a meandering sand-bed river. Nature 315, 746 – 747.
Influence of a large woody debris obstruction on three-dimensional flow structure in a meander bend Melinda D. Daniels a,*, Bruce L. Rhoadsb a
Department of Geography, University of Connecticut, Storrs, CT 06269, USA b Department of Geography, University of Illinois, Urbana, IL 61801, USA
Received 10 August 2001; received in revised form 17 January 2002; accepted 18 October 2002
Abstract A field experiment has been conducted to assess the influence of a large woody debris (LWD) obstruction on threedimensional flow through a meander bend of a small stream in east central Illinois. Previous studies in unobstructed meander bends have shown that flow through a curved channel should develop a coherent three-dimensional structure characterized by large-scale helical motion. Many meander bends are complicated by naturally occurring persistent obstacles, such as LWD, that have the potential to profoundly disrupt flow structure. The results of this study show that the LWD obstruction systematically influences the three-dimensionality of flow through the bend, particularly the position of the high-velocity core and the development of helicity. The high-velocity core is positioned in the center of the channel upstream of and near the bend apex, but as flow approaches the LWD, it is steered toward the inner bank by the obstruction. Evolving helicity in the upstream portion of the bend is amplified by abrupt turning of the flow induced by the LWD. As the flow moves past the LWD, helicity diminishes rapidly and may even reverse its pattern of rotation. The net effect of the LWD obstruction is to reduce near-bank velocities along the outer bank downstream of the bend apex—a critical locus for bank erosion in meander bends. Given the persistence of the LWD obstruction, it probably has an important local influence on bend migration and evolution. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Large woody debris; Obstruction; Helicity
1. Introduction The fluvial dynamics of meandering rivers have been the focus of numerous process-based field studies over the last 25 years. In particular, the structure of flow-through meander bends has garnered considerable attention (Hey and Thorne, 1975; Dietrich and * Corresponding author. Department of Geography, University of Connecticut, 215 Glenbrook Road, U-4148, Storrs, CT 062694148, USA. Tel.: +1-860-486-2117; fax: +1-860-486-1348. E-mail address: [email protected] (M.D. Daniels).
Smith, 1983; Thorne and Rais, 1984; Thorne et al., 1985; Markham and Thorne, 1992). The interest in flow structure derives from the recognition that this structure, via its influence on bed-material transport, determines patterns of bank erosion, channel geometry and the evolution of channel planform (Hooke and Harvey, 1983; Rhoads and Welford, 1991). Based on physical considerations, Thomson (1876) argued that flow through a curved channel should develop a coherent three-dimensional structure characterized by large-scale helical motion. This structure is the result of a local imbalance between the centrifugal
0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 5 5 5 X ( 0 2 ) 0 0 3 3 4 - 3
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force and the counteracting pressure-gradient force associated with super-elevation of the water surface along the outer bank of the curved channel (Dietrich, 1987). Field measurements of downstream and crossstream velocity components in meander bends show that orientations of two-dimensional velocity vectors change systematically over depth, a pattern consistent with helical motion (Hey and Thorne, 1975). This pattern, however, is confined mainly to the central region of the flow. Near the outer bank, a small, counter-rotating helical cell is produced by flow stagnation (Thorne and Rais, 1984). Moreover, in many meander bends, the presence of a well-developed point bar directs flow along the inner bank outward over the entire flow depth (Dietrich and Smith, 1983). The result is three distinct zones of fluid motion in the crossstream plane: (i) outward flow over the point bar, (ii) large-scale, curvature-induced helicity in the central portion of the channel and (iii) a small, counter-rotating helical cell along the outer bank (Markham and Thorne, 1992). Advection of downstream momentum outward toward the outer bank and downward toward the bed by helical motion leads to submergence of the core of high downstream velocity over the outer half of the channel cross-section. The location of the high-velocity core near the bed produces large, near-bed shear stresses that promote scouring of the bed, i.e., pool development, in this portion of the cross-section (Dietrich, 1987). Although flow-through meander bends has been intensively investigated, most field studies have focused on relatively simple meander bends that are free from major obstructions. Many meander bends are complicated by naturally occurring persistent obstacles, such as large woody debris (LWD), that have the potential to profoundly disrupt flow structure. LWD influences river dynamics by functioning as a resistance element, thereby modifying flow and sediment transport, patterns of erosion and deposition and, potentially, planform development (Keller and Swanson, 1979; Robinson and Beschta, 1990; Gregory et al., 1994; Assani and Petit, 1995; Fetherston et al., 1995; Gippel, 1995; Gurnell et al., 1995; Montgomery et al., 1995; Richmond and Fausch, 1995; Abbe and Montgomery, 1996; Myers and Swanson, 1997; Gurnell and Sweet, 1998). Past work has shown that LWD can be a dominant control of stream geomorphology in watersheds with forested riparian corridors (Keller and Swanson, 1979; Maser and Sedell,
1994; Fetherston et al., 1995; McKenny et al., 1995), but detailed, process-based investigations of the effects of LWD on the fluvial dynamics of meander bends are lacking. Thorne and Furbish (1995) assessed the influence of dense bank vegetation on flow through a meander bend and found that this material lowered velocities near the outer bank and led to repositioning of the high-velocity core away from this bank. Because the study relied on onedimensional velocity measurements, the effects of vegetation on flow three-dimensionality had to be inferred from visualization of flow patterns using streamers. Even in unobstructed bends, the threedimensionality of the flow has been inferred from at most two-dimensional (downstream and cross-stream) measurements of velocity components (Dietrich and Smith, 1983; Thorne and Rais, 1984; Thorne et al., 1985). Recent advances in measurement technology now permit the collection of 3-D velocity data in field settings (Lane et al., 1998). As a result, 3-D flow structure through meander bends now can be measured directly rather than being inferred from 2-D data. Direct characterization of flow structure in three dimensions is especially important where the path of the flow changes rapidly over space relative to the orientation of the stream channel (e.g., Rhoads and Sukhodolov, 2001)—a situation that, as this study demonstrates, can be produced by LWD obstructions. This paper examines the influence of LWD on time-averaged flow structure in a meander bend along a small agricultural stream in the Midwestern United States. The study is based on 3-D velocity measurements, thereby providing a fully 3-D representation of the flow structure. The results of this research illustrate the extent to which LWD can affect flow structure in bends. Studies of this type represent a first step toward determining the ensemble of process interactions between LWD and bend dynamics.
2. Field site Madden Creek is a low-gradient (0.00185) tributary of the Sangamon River in east central Illinois (Fig. 1). The 42-km2 watershed of Madden Creek lies in the Bloomington Ridged Plain physiographic region of Illinois, which is characterized by nearly flat or gently sloping till plains crossed by broad, low end moraines.
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Fig. 1. (A) Map of the field site showing the location of study reach. (B) Aerial photograph of Madden Creek showing the location of the study site. (C) Detailed topographic map of the study site showing location of measurement cross-sections (numbered lines) and location of LWD obstruction (shaded area) (arbitrary datum of 10.0 m).
These landscape features formed during the late Wisconsin subepisode when the Lake Michigan lobe of the Laurentide ice sheet reached this area (Hansel and Johnson, 1992). Madden Creek originates on the Champaign Moraine in northern Piatt County and flows south across a broad, gently sloping till plain to the Sangamon River. Prior to European settlement in the mid-1800s, east central Illinois was a wet prairie. The growth of agriculture in the region involved extensive land drainage, including installation of subsurface drainage tiles,
the construction of open ditches, and the channelization of existing streams (Rhoads and Herricks, 1996). Today, over 50% of stream reaches in the Sangamon basin are channelized (Mattingly et al., 1993). Land use in the Madden Creek watershed is predominantly agricultural, and the stream has been affected both directly and indirectly by agricultural activity. The University of Illinois Map and Geography Library’s collection of aerial photography for Madden Creek extends to 1946 and shows that many sections of the creek have been straightened and channelized
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over the past 55 years. Despite the severity of human impacts in this system, some stream reaches have not been straightened since the 1940s. The study site for this research project is a meander bend located within an approximately 2.5-km-long section of Madden Creek that is freely meandering and has a narrow, forested riparian corridor (Fig. 1). Historical aerial photography indicates that the meander and immediately adjoining reaches have not been affected by channelization or significant natural planform changes since the 1940s. A review of the General Land Office (GLO) survey records for this site indicate that, at the time of the surveys in 1821 and 1822, some mature trees were present near the channel of Madden Creek. This information is consistent with the current status of the riparian zone, which consists of a narrow band of mature trees flanking the channel. The meander bend selected for this study contains a LWD obstruction consisting of a living tree and root bowl attached to the outer bank just past the apex of the meander bend (Fig. 2). The tree extends upright
out of the channel and the curved trunk and root bowl have trapped several other pieces of LWD, producing a complex obstruction that extends into the channel at nearly a right angle to the outer bank. Roots also extend away from the obstruction into the channel from the root bowl. The LWD structure is not an impermeable obstacle and some water passes through it, especially near the outer bank where the connection consists of open root structures anchored in the bank material. Within the bend, the channel has an average bankfull width (Wb) of approximately 9 m and a bankfull depth of 1.2 – 1.4 m (Fig. 1). The dimensionless curvature of the bend (r/Wb, where r is radius of curvature) is approximately 2.0, a value that lies at the low end of the range of dimensionless curvature for natural channels and that generally is associated with high rates of meander migration (Hickin and Nanson, 1975; Nanson and Hickin, 1986; Hudson and Kelsel, 2000). At sub-bankfull stages, the geometry of the channel varies spatially. A large point bar protrudes
Fig. 2. Photograph of the study site looking downstream toward the LWD obstruction. Wading rod holding the ADV is visible upstream of the obstruction.
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into the channel from the inner bank, which confines flow adjacent to it at sub-bankfull stage. Downstream of the point bar, the thalweg (or zone of maximum channel depth) turns abruptly toward the inner bank near the LWD obstruction. Bed material throughout the reach consists of bimodal sand and gravel.
3. Collection, processing and analysis of field data Field data on 3-D velocities were collected at Madden Creek on July 2 and 9, 1998. The data for these two dates reveal nearly identical patterns of fluid motion and only data for July 9, the most detailed set of measurements, are reported in this paper. Velocity measurements were obtained using an acoustic Doppler velocimeter (ADV) at five cross-sections aligned orthogonally to the direction of the channel centerline (Fig. 1). The five cross-sections are distributed around the bend with four cross-sections upstream of the LWD obstruction and one downstream of it. Velocities were measured at six to eight verticals within each cross-section and at two to eight points within each vertical. The total number of sample points within each cross-section ranged from 29 to 43. During the period of 3-D velocity measurements, the water stage at the study site was monitored using two ultrasonic sensors connected to a Campbell 21 data logger. The stage at the beginning of the period of measurements was 9.10 m, which corresponds to a halfbankful event with flow widths of 3.75 m at crosssections 4– 6, 4.75 m at cross-section 7 and 6.10 m at cross-section 8. Stage remained relatively constant, rising gradually by 0.03 m over the 8-h measurement period. The ADV measures downstream (U), vertical (W) and cross-stream (V) velocities in a sampling volume < 0.25 cm3 located f 5 cm away from the probe head. Because values of 3-D velocity components depend on the orientation of the sensor, maintaining consistency of sensor alignment is important. To ensure that the measurements were obtained within a fixed frame of reference at each cross-section, the ADV was mounted on a custom-built wading rod that, in turn, was attached to a steel cable stretched tautly between iron pipes at each end of the crosssection (e.g., Rhoads and Sukhodolov, 2001) (Fig. 2). This system ensured that the X-axis of the sensor
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was perpendicular to the cross-section and the Y-axis of the sensor was parallel to the cross-section. The wading rod also was plumbed prior to each measurement to ensure that the Z-axis of the sensor coincided with the vertical plane of the cross-section. This positioning system also precluded the need for an operator to stabilize the wading rod, eliminating the possibility of operator-induced unsteadiness of the probe during measurements. Once plumbed, the wading rob was left in place while the operator moved away from the unit a distance sufficient to avoid any disruption of the flow field near the ADV probe (Fig. 2). Three-dimensional velocities were sampled for 60 s at a rate of 25 Hz. In some cases, interruption of the acoustic signal by floating debris or by intermittent intersection of the acoustic beam with the channel bed produced periods of excessive noise or large spikes in the velocity records. These situations were noted in the field and the sampling duration was increased to obtain an uninterrupted 60-s record of clean data. Postprocessing of the velocity data was performed with Explore ADV software developed by Nortek USA. Any individual spikes resulting from acoustic noise were removed from the data set using a 3r filter in the Explore ADV software, where r is the standard deviation of the time series of velocities. The occurrence of such spikes was rare. Most data sets contained no spikes and those that did had only one or two spikes. In all cases, application of the 3r filter removed the spikes, yet changed the timeaveraged velocity by at most a few millimeters per second. Stationarity of the times series was assessed by dividing each series into 10 equally spaced intervals and then cumulating a running mean for the 10 intervals. If the data are stationary, the cumulated mean gradually should converge on a constant value as the number of intervals included in the computation of the mean increases. All means, except those for time series with low values of timeaveraged downstream velocity (U < 0.10 m s 1), exhibited convergence of means over the 10 intervals. Means for low-velocity locations may not be entirely accurate but provide a reasonable basis for comparing rates of fluid motion relative to rates for other portions of the flow. Values of depth-averaged downstream (U) and cross-stream (V) velocity for each vertical were used
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to compute the magnitude (MUV) and orientations (qxy) of depth-averaged velocity vectors: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi U2 þ V2
MU V ¼
hxy ¼ tan1
V U
ð1Þ
ð2Þ
where U¼
n X
! Ui Dh =H
ð3Þ
i¼1
V ¼
n X
! Vi Dh =H
ð4Þ
i¼1
and where n is the number of velocity measurements in a vertical, Dh is the increment of depth associated with a point measurement and H is the flow depth. Values of U were used to generate contour plots of downstream velocity. Contouring of the downstream velocity field was performed manually because application of computer-contouring algorithms based on various interpolation routines, including kriging, did not accurately represent the strong spatial anisotropy of some velocity fields. Patterns of V and W were superimposed on the contour plots as vectors of cross-stream/vertical velocity components. The magnitude (MVW) and vertical orientation (hUZ) of these vectors are: MV W ¼
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi V2 þ W2
hU Z ¼ tan1
W U
urement cross-sections, a factor that can mask helicity on plots of V–W velocity vectors (Rhoads and Kenworthy, 1995, 1998, 1999). As a result, the helicity assessment procedure (HAP) developed by Rhoads and Sukhodolov (2001) was used to facilitate evaluation of the 3-D structure of time-averaged flow in the reach. The HAP examines relations among lateral (hXY) and vertical (hXZ) angles of 3-D velocity vectors to determine whether large-scale helical motion exists within the flow. The vertical orientation of the crossstream plane ( Y– Z) usually is aligned with the influence of gravitational forces—a condition that has been met in this study by ensuring that the sensor is plumb relative to the channel bed. Thus, absolute values of hXY define the vertical motion of the flow in the crossstream plane as depicted on the V –W vector plots. However, the horizontal alignment of the cross-section can be defined by various criteria—each of which produces different values of V (Dietrich and Smith, 1983; Markham and Thorne, 1992; Rhoads and Kenworthy, 1999). Whereas values of V are sensitive to rotation of the cross-section about a vertical axis, differences among values of hXY are independent of the frame of reference and, therefore, provide an objective method for assessing helicity within a complex flow. The HAP procedure uses five criteria to identify helical patterns from the vector angle differences: (i) a difference of at least 5j between the crossstream (hXY) vector angles calculated for near-surface and near-bed velocity vectors, (ii) systematic changes
ð5Þ
ð6Þ
Lateral vector orientations are determined by the sign of V ( + V to the left looking upstream, V to the right looking upstream). An aim of this study was to explore the influence of LWD at Madden Creek on any coherent three dimensionality of the flow, especially the helical motion found in most meander bends. Field observations and preliminary analysis of depth-averaged velocity vectors indicated that flow at some locations was highly skewed relative to the orientation of the meas-
Fig. 3. Spatial pattern of depth-averaged velocity vectors.
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Fig. 4. Patterns of downstream velocity components (contours) and cross-stream/vertical velocity components (vectors) at cross-sections 4 – 8 (looking upstream). Shaded areas indicate zones of separated or stagnant flow.
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Fig. 5. Plots of hxy (column A) and hxz (column B) for measurement cross-sections. Bolded numbers are values conforming with the criteria of helicity assessment procedure and are indicative of helical rotation of the flow.
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Fig. 5 (continued ).
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in cross-stream (hXY) vector angles over depth, (iii) the presence of similar systematic patterns of change in cross-stream (hXY) vector angles over depth at a minimum of two adjacent verticals, (iv) occurrence of similar patterns of change in cross-stream (hXY) vector angles over depth in corresponding portions of a minimum of two consecutive cross-sections and (v) the orientation of 3-D vectors downward ( hXZ) toward the bed and upward ( + hXZ) toward the surface on opposite sides of the presumed helix within the set of cross-sections over which it is defined (Rhoads and Sukhodolov, 2001).
4. Spatial patterns of velocity components Depth-averaged vectors of downstream and crossstream velocity components show that the spatial path of the flow generally conforms with the planform geometry of the meander bend from the entrance (cross-section 4) to immediately downstream of the bend apex (cross-section 6) (Fig. 3). Over this distance, the largest vectors occur in the center of the channel. As flow approaches the LWD obstruction (cross-section 7), vectors are directed strongly towards the inner bank. The largest vector still occurs in the center of the channel, but vector magnitudes decrease abruptly toward the outer bank. Downstream of the LWD (cross-section 8), vector magnitudes are greatest along the inner bank, where flow is directed past the woody debris. In contrast, vector magnitudes are small along the outer bank in the lee of the LWD. Plots of downstream velocity reveal a well-defined, high-velocity core just beneath the water surface in the center of the channel within the bend (crosssections 4 – 6) (Fig. 4). As flow approaches the LWD, the size of the core expands; but its magnitude decreases by about 10 cm s 1 (cross-section 7), mainly because much of the flow is directed obliquely to the alignment of the cross-section (Fig. 3). A region of stagnant, recirculating fluid exists along the outer bank of the channel immediately upstream of the LWD obstruction, and a zone of separated flow exists along the inner bank where the channel widens abruptly (cross-section 7). Downstream of the LWD, a complex spatial pattern of U develops with the highest velocities immediately adjacent to the inner
bank and a region of stagnant recirculating flow in the center of the channel (cross-section 8). A distinct shear layer characterized by a strong lateral gradient in U separates the region of stagnant fluid from the high-velocity zone. A weak secondary maximum of downstream velocity exists adjacent to the outer bank in the lee of the LWD. Vectors of downstream and cross-stream velocity components, referred to here as secondary vectors, are directed uniformly inwards at the entrance to the bend (cross-section 4) (Fig. 4). Depth-averaged vectors at this location are oriented parallel to the local channel direction (Fig. 3); thus, the systematic orientations of secondary vectors represent slight net skewing of the flow relative to the cross-section alignment, rather than net flow toward either bank. The vertical orientation of vectors at the bend entrance suggests slight net downward fluid motion over the outer portion of the channel and upward movement over the inner portion. As flow moves from the entrance around the bend apex, the composite pattern of vector orientations is suggestive of counter-clockwise rotation of the fluid (cross-sections 5 and 6) (Fig. 4). Vectors near the surface are oriented outward, whereas vectors near the bed are oriented inward. Pronounced upward motion is evident over the inner portion of the channel (cross-sections 5 and 6), and strong downward motion develops over the deepest portion of the flow near the bend apex (cross-section 6). Secondary vectors upstream of the LWD obstruction are uniformly and strongly directed inward, indicating net motion of the fluid towards the inner bank (cross-section 7) (Fig. 4). Downstream of the LWD obstruction (cross-section 8), secondary vectors are directed uniformly inwards. Most vectors are parallel with the water surface; however, near the inner bank the vectors are oriented sharply downward, suggesting that strong downwelling occurs as flow deflected inward by the LWD obstruction collides with the inner bank.
5. Spatial patterns of helical motion The helicity assessment procedure (HAP) reveals the total spatial extent of helical motion through the meander bend. Between the bend entrance and the bend apex, at least two adjacent verticals in each
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cross-section exhibit systematic changes in values of hXY over depth. At the bend entrance (cross-section 4), values of hXY at three adjacent verticals increase from the surface to the bed by between 9j and 14j, indicating that the flow near the bed is directed more toward the inner bank than the flow near the surface (Fig. 5). This pattern of systematic deviation in vector angles cannot be discerned from the plot of secondary vectors, where it is masked by slight net skewing of the flow relative to the cross-section alignment (Figs. 3 and 4). On the inner half of the cross-section, values of hXZ increase from the surface to the bed by between 5j and 10j, indicating that flow near the bed is moving upward more than flow near the surface. The pattern of vector angles at cross-sections 5 and 6 confirm that helical motion becomes well developed as flow moves around the bend apex. Values of hXY change systematically from negative angles at the surface or within the center of the flow to positive angles at the bed over several verticals at both crosssections (Fig. 5). The magnitude of total angular deviation over depth increases from 6j to 12j at cross-section 5 to 14j to 31j at cross-section 6. These trends indicate that flow near the surface is directed toward the outer bank while flow near the bed is directed toward the inner bank. Values of hXZ over the inner portion of cross-section 5 increase systematically over depth by 1– 3j, indicating that flow near the bed is moving upward relative to flow near the surface. At cross-section 6, values of hXZ are negative over the outer portion of the channel and strongly positive near the bed over the inner portion of the channel—a pattern consistent with counterclockwise helical motion of the flow. The HAP shows that despite net inward movement of the flow as it approaches the woody debris (crosssection 7, Figs. 3 and 4), strong helical motion persists within the flow. At cross-section 7, values of hXY increase systematically either from near the surface or within the middle of the flow toward the bed at several adjacent verticals (Fig. 5). The total angular deviation over depth ranges from 33j to 61j—a range greater than that for cross-sections 4 – 6. Values of hXZ are negative over the central and outer portion of the channel and positive near the bed and at the vertical adjacent to the lateral separation zone along the inner bank. Together, these patterns of vector angles are
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consistent with counterclockwise rotation of fluid within a flow that is highly skewed in relation to the cross-section orientation. No distinct pattern of vector angle changes is apparent downstream of the LWD obstruction (cross-section 8), indicating that helical motion in the flow decays rapidly as it passes the LWD obstruction (Fig. 5). Values of hXY at the two verticals closest to the inner bank do decrease systematically over depth by 10 –20j, a pattern opposite that associated with counterclockwise motion. This pattern, along with strong downward motion along the inner bank ( hXZ) and upward motion immediately adjacent to the inner bank (+ hXZ), suggests that helical motion characterized by clockwise rotation may exist within the flow along the inner bank. No systematic patterns of hXY or hXZ can be discerned within the shear layer adjacent to the high-velocity core or within the regions of separated or low-velocity flow in the lee of the LWD obstruction.
6. Discussion The results show that the LWD obstruction within the meander bend at Madden Creek has a pronounced influence on the structure of flow through this bend. Patterns of depth-averaged vectors suggest that between the bend entrance and the bend apex flow streamlines follow a path that is roughly parallel to the direction of local channel curvature (Fig. 6). The curvature of flow streamlines generates evolving counterclockwise helicity in this portion of the bend that intensifies in the downstream direction, eventually manifesting itself in the pattern of secondary vectors within the cross-stream plane (Fig. 6). The development of helical motion accounts for submergence of the high velocity core beneath the water surface. This motion provides a mechanism for advective transport of high-momentum fluid toward the bed and low-momentum fluid toward the surface, resulting in a maximum downstream velocity in the central portion of the cross-sectional flow field (e.g., Thorne et al., 1985). As flow along the outer bank approaches the LWD obstruction, it stalls due to the damming effect of the LWD. Although variations in water surface topography at the site are too small for reliable measurements
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Fig. 6. Conceptual model illustrating the pattern of flow streamlines and helical motion in the LWD-obstructed bend at Madden Creek. Dark shading indicates position of the LWD obstruction, and light shading corresponds to regions of separated or stagnant flow. Crosssections showing patterns of secondary flow in the cross-stream plane are also shown. The dashed pattern of secondary motion for cross-section 7 corresponds to the dashed reorientation of this crosssection as shown on the map of the bend. The dashed pattern for cross-section 8 indicates weak helical motion.
of elevation differences, super-elevation of the water surface most likely occurs immediately upstream of the obstruction. Flow moving against this adverse pressure gradient will decelerate rapidly, resulting in flow stagnation (Fig. 6). Local mounding of the water surface along the outer bank also will produce a crossstream pressure gradient that can turn flow laterally towards the inner bank. The development of lateral flow separation along the inner bank (Fig. 6), a region of low pressure caused by an abrupt change in the channel boundary, will enhance the cross-stream pressure gradient. The combined influence of the LWD obstruction and channel geometry on the water surface topography and cross-stream pressure gradient steers the flow around the obstruction.
Steering of the flow past the LWD obstruction results in an abrupt increase in streamline curvature (Fig. 6). This increase in curvature, combined with enhanced vortex stretching as flow accelerates through the narrow cross-sectional area adjacent to the obstruction, intensifies counterclockwise helical motion. The helicity, despite its strength, cannot be detected directly in the pattern of secondary vectors due to substantial skewing of the flow direction relative to the orientation of the cross-stream plane (Fig. 6). It can, however, be readily detected in the pattern of 3-D vector angles. Downstream of the LWD obstruction, the highvelocity core is positioned along the inner bank. Patterns of 3-D vector angles suggest that clockwise helical motion may develop within the flow along this bank, perhaps due to reversal in curvature of streamlines as the flow passes through the opening adjacent to the LWD obstruction (Fig. 6). The vector-angle data, however, do not satisfy the HAP criteria; further evidence is needed to determine whether spiral motion exists in the downstream flow. Behind the bulk of the obstruction, a region of separated flow develops, much like that in the lee of a boulder or other large obstacle (Fig. 6). The LWD, however, is not an impervious solid and some flow manages to move through the obstruction as revealed by the weak secondary maximum in downstream velocity adjacent to the outer bank. Flow through this side of the obstruction may be enhanced by a strong downstream pressure gradient associated with super-elevation of the water surface along the outer bank of the channel upstream of the obstruction. The flow structure at the Madden Creek site differs from other field and experimental studies of flowthrough unobstructed meander bends, which show that the high-velocity core shifts toward the outer bank as flow moves through the bend (Jackson, 1975; Bathurst et al., 1979; Dietrich and Smith, 1983; Geldof and de Vriend, 1983; Alphen et al., 1984; Frothingham, 2001). The lack of an outward shift in the high-velocity core upstream of the bend apex at Madden Creek is not unusual. Outward shifting of the maximum-velocity core is the product of redistribution of momentum by helical motion and by topographic steering of the flow around the point bar (Dietrich and Smith, 1983; Nelson and Smith, 1989). In the absence of topographic steering, the
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high-velocity core will not shift markedly toward the outer bank until well downstream of the bend apex. Topographic steering by a point bar in equilibrium with flow conditions will produce an abrupt outward shift of the high-velocity core, but this shift still occurs downstream of the bend apex (Nelson and Smith, 1989). Thus, not surprisingly, the high-velocity core at Madden Creek remains centered in the channel upstream of the bend apex. Neither helical motion nor topographic steering would be expected to redistribute momentum rapidly enough to affect an outward shift of the high-velocity core within this portion of the bend. The velocity data suggest that helical motion develops in the flow between the bend entrance and the bend apex, but the cross-stream vector data do not reveal substantial lateral forcing of the flow toward the outer bank by the point bar (Fig. 4). Although the bend does contain a well-developed point bar, at this stage, cross-sectional asymmetry is minor, thereby diminishing the influence of topographic steering. The lack of substantial topographic steering probably explains why the high-velocity core at cross-section 6, which is located slightly downstream of the bend apex, remains near the center of the channel. The main discrepancy between flow structure in unobstructed bends and the pattern of flow observed at Madden Creek occurs downstream of the bend apex in the vicinity of the LWD obstruction. In unobstructed bends, the core of high velocity often is situated adjacent to the outer bank downstream of the bend apex and may in some cases be submerged beneath the water surface near the toe of the outer bank (Thorne et al., 1985). The result is a zone of high, near-bed shear stress capable of mobilizing sediment and eroding the outer bank of the channel (Thorne and Rais, 1984). In contrast, flow structure downstream of the bend apex at Madden Creek is characterized by a zone of stagnation along the outer bank that extends to the base of this bank and a highvelocity core located in the center of the channel (Fig. 4). This pattern of flow should serve to protect the outer bank by producing small or negligible bed shear stresses along the bank toe—a factor of considerable importance in determining bank erosion rates (Thorne, 1982). The findings of this study are generally consistent with past work on the influence of bank vegetation on bend flow, which showed that the presence of
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vegetation reduces velocities near the outer bank and confines the filament of maximum velocity to the center of the channel (Thorne and Furbish, 1995). The results here differ from this past work in that the LWD obstruction at Madden Creek acts like a partial dam or weir that steers the flow away from the outer bank, rather than just increasing flow resistance along this bank. Whereas Thorne and Furbish (1995) found that bank vegetation diminishes helicity by reducing the cross-stream pressure gradient, this study has shown that steering of the flow around the LWD obstruction intensifies helical motion, presumably by locally enhancing the cross-stream pressure gradient. Although details of flow structure in the vicinity of the LWD obstruction may change with increases in stage, visual observations of near-bankfull conditions at the field site indicate that at this stage the obstruction, which has a height equal to about 60 – 70 % of the depth of the bankfull channel, still deflects the flow strongly away from the outer bank. By modifying the flow around it, the LWD obstruction (which is quite persistent) probably serves as an anchor for bend development. Aerial photos of the reach show minor extension of the bend over the last few decades as the channel upstream of the LWD has migrated laterally, whereas the location of the channel at the LWD obstruction has remained fixed. The apparent LWD-induced stabilization of channel morphology at the study site is consistent with past studies of obstruction-anchored channel features, which showed that quasi-permanent obstructions greatly influenced the positions of bar and pool channel features as well as anchored channel planform locally (Lisle, 1986). Overall, the LWD obstruction at the study site should reduce the net rate of lateral migration because if the bend were not obstructed maximum rates of bank erosion most likely would occur downstream of the bend apex at the current location of the obstruction. On the other hand, deflection of flow around the LWD may cause increased erosion along the inner bank of the channel. Flow past the obstruction has produced a zone of well-developed scour adjacent to the inner bank (Fig. 1). Moreover, the trend of the inner bank abruptly shifts inward downstream of the obstruction, resulting in sudden widening of the flow. This characteristic appears to be a fairly recent development within the reach based on visual inspection of historical aerial photographs.
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7. Conclusions This study has contributed to a process-based understanding of flow through meander bends containing LWD by documenting the 3-D flow structure in a meander bend with a LWD obstruction. The results show that the LWD obstruction within the meander bend at Madden Creek has a pronounced influence on the structure of flow through this bend. Major findings include the following: (i) Depth-averaged flow upstream of the LWD obstruction generally follows the path of the channel, but as flow approaches the obstruction downstream of the bend apex, it is steered abruptly toward the inner bank. (ii) Downstream velocity fields upstream of the obstruction are characterized by a well-defined, submerged high-velocity core in the center of the channel. The core expands but decreases in magnitude where flow is deflected around the LWD. Downstream of the obstruction, a complex pattern of downstream velocity develops with the highest velocities adjacent to the inner bank and separated, recirculating flow in the lee of the LWD. (iii) Patterns of secondary-velocity vectors and 3-D vector angles suggest that helical motion develops in the flow between the bend entrance and the bend apex. Steering of the flow past the LWD obstruction results in an abrupt increase in streamline curvature and enhanced vorticity as flow accelerates through the narrow, crosssectional area adjacent to the obstruction, intensifying helical motion. Helicity decays as flow moves past the LWD toward the inner bank and exits the bend. Given the diversity of LWD structures and meander morphologies, the results of this particular field study are unlikely to be widely generalized. Additional process-based studies of other configurations of LWD are needed to quantify the range of effects that such obstructions can have on flow-through bends. Ideally, other studies should investigate the influences of the scale of LWD in relation to the size of the stream channel, placement of LWD in relation to channel planform, the internal structure of LWD
obstructions, and the persistence of obstructions relative to the time scale of river dynamics. Such studies hopefully will define the general factors that control complex interactions between LWD and fluvial processes. Field-based research of this type should also be combined with numerical modeling of interaction between fluvial processes and LWD to develop a predictive understanding of the influence of specific types of obstructions on river dynamics. Acknowledgements The authors are grateful to Kelly Frothingham, Marta Graves, Perry Cabot, Matt Ladewig and Kristin Jaburek for their help and assistance in collecting the field data. We would also like to thank Tom Lisle for helpful comments that improved the manuscript. Funding for this research was provided by the U.S. Environmental Protection Agency, Water and Watersheds Program (R82-5306-010) and the University of Illinois Research Board. References Abbe, T.B., Montgomery, D.R., 1996. Large woody debris jams, channel hydraulics and habitat formation in large rivers. Regulated Rivers: Research and Management 12, 201 – 221. Alphen, J.S.L.J., Blocks, P.M., Hoekstra, P., 1984. Flow and grainsize pattern in a sharply curved river bend. Earth Surface Processes and Landforms 9, 513 – 522. Assani, A.A., Petit, F., 1995. Log-jam effects on bed-load mobility from experiments conducted in a small gravel-bed forest ditch. Catena 25, 117 – 126. Bathurst, J.C., Thorne, C.R., Hey, R.D., 1979. Secondary flow and shear stress at river bends. Journal of the Hydraulics Division: Proceedings of the American Society of Civil Engineers 105 (HY10), 1277 – 1295. Dietrich, W.E., 1987. Mechanics of flow and sediment transport in river bends. In: Richards, K. (Ed.), River Channel Environment and Process. Basil, Blackwell, London, UK, pp. 179 – 227. Dietrich, W.E., Smith, J.D., 1983. Influence of the point bar on flow through curved channels. Water Resources Research 19 (5), 1173 – 1192. Fetherston, K.L., Naiman, R.J., Bilby, R.E., 1995. Large woody debris, physical process, and riparian forest development in montane river networks of the Pacific Northwest. Geomorphology 13, 133 – 144. Frothingham, K.M., 2001. Geomorphological processes in meandering and straight reaches of an agricultural stream in East Central Illinois: relations to aquatic habitat. PhD dissertation thesis, University of Illinois, Urbana.
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