Braided Channels Case Study

The central Platte River in Nebraska, USA, has undergone substantial channel narrowing since basin settlement in the mid-nineteenth century. Many researchers have studied the causes of channel narrowing and its implications for endangered species that use wide, shallow channel segments with barren sandbars. As a result, changes in metrics such as unvegetated channel width have been studied. With few exceptions, these measures are estimated from aerial imagery without mention of error in relation to actual channel conditions and/or investigator bias. This issue is not unique to central Platte River studies, as a general lack of commentary is apparent regarding the direct comparison of channel planform characteristics interpreted from aerial imagery relative to those measured in the field. Here we present a case study where data collected by the Platte River Recovery Implementation Program was used to make multiple comparisons using three years of field-measured unvegetated channel widths and those photointerpreted from aerial imagery. Widths were interpreted by three investigators, who identified similar widths in almost all cases. Photointerpretation from imagery collected during the fall resulted in unvegetated width estimates that were more consistent with field measurments than estimates derived using imagery collected in June. Differences were attributed to three main factors: (1) influences of discharge on photointerpretation of unvegetated channel width; (2) increases in vegetative cover throughout the growing season; and (3) resolution of imagery. Most importantly, we concluded that photointerpretation of unvegetated widths from imagery collected during high flows can result in significant over estimation of unvegetated channel width.

1. Introduction

[2] Braided rivers are characterized by a network of unstable anabranches separated by ephemeral bars. One fundamental measure of braided channel pattern characteristics is braiding intensity, which defines the multiplicity of channels in a braided river at a given time and discharge or the complexity of the anabranch network. Previous studies of braided river planform have developed a number of different measurements and indices for braiding intensity [e.g., Brice, 1964; Howard et al., 1970; Rust, 1978; Mosley, 1981; Germanoski and Schumm, 1993] that have been assessed in detail by Egozi and Ashmore [2008].

[3] Braiding intensity measurements usually include all anabranches apparent at any one time in, for example, an aerial photograph of the river. A braiding index based on all these visible channels is referred to here as “total braiding intensity” (BIT) [see also Egozi and Ashmore, 2008]. BIT is defined as the number of wetted channels counted and averaged over a number of cross sections (rather than all channels, both wet and dry), and consequently is sensitive to the flow level at which the measurement is made. Alternatively, braiding intensity may be conceived as referring to the network of channels that is transporting bed material load, i.e., that portion of the network that is actively involved in the channel morphodynamics at a given time and flow level [Ashmore, 1991a, 2001]. This is referred to as “active braiding intensity” (BIA) and computed in a similar way to BIT.

[4] Observations in physical models of braided channels [Ashmore, 1991a, 2001; Bertoldi et al., 2006] suggest that at any given time, only a subset of the total channels are actually transporting bed material and actively forming the braided pattern and river morphology, i.e., BIA is always less than BIT and the remaining channels convey water and wash load, but no bed load. The implication is that the braided channel network observed at a given time forms progressively over time by shifting of a few active channels rather than by simultaneous development of all channels.

[5] Total and active braiding intensity both correlate positively with discharge and stream power [Howard et al., 1970; Mosley, 1981; Ashmore, 1991a; Robertson-Rintoul and Richards, 1993] on the basis of both observations of natural rivers and on physical model experiments. Ashmore [1991a] found that BIA in physical models reached an upper limit beyond which further increases in stream power had no further effect on BIA.

[6] Fractal analysis of different braided morphologies has shown that within a given river, channel pattern characteristics are self similar over a limited range of scales [Sapozhnikov and Foufoula-Georgiou, 1996, 1997; Walsh and Hicks, 2002]. In addition, braiding intensity remains fairly stable because with increasing discharge inundation of islands occurs at a rate compensating for the wetting of new channels [Walsh and Hicks, 2002]. Furthermore, Sapozhnikov and Foufoula-Georgiou [1997] suggested that there is also internal dynamic scaling so that the same processes operate over a range of scales within the system from which it is inferred that small-scale processes are controlled by larger-scale processes within the system. This provides some general conceptual physics that sets a context for the development of braiding, but it does not provide insight into actual mechanisms responsible for this behavior nor the nature of the overall controls of braiding intensity and the active channel network.

[7] We hypothesize that 1) braiding intensity (both total and active) is a regime or equilibrium property of a river which is adjusted to the imposed flow regime with both BIT and BIA having upper limits imposed by the available discharge or energy. 2) Total braiding intensity develops progressively as a consequence of the instability of the (less extensive) active channel network. Neither of these two fundamental aspects of braided river morphology and processes have been confirmed by systematic experimentation and linked into an overall picture of braided pattern development and bar-scale “unit processes” [e.g., Ashmore, 1991b, 1993; Ferguson, 1993]. Here we report the results of physical model experiments used to investigate these ideas by (1) simultaneously measuring BIT and BIA at different channel-forming discharge to establish whether there is a consistent relationship between them and with total discharge and stream power; (2) observing the temporal adjustment and development of BIT and BIA to step increase in channel-forming discharge to assess whether an equilibrium value is reached and how quickly this occurs; and (3) mapping the channel pattern development over time to describe the processes by which the active and total channel system develops over time to produce the observed braided channel network morphology.

[8] After explaining the experimental procedure the paper discusses the overall response of braid indices to step increases in discharge and shows the differing adjustment times for BIT and BIA and the trend toward a stable value of both indices and of the ratio of the two. The paper then describes the morphodynamic processes involved in the adjustment and the development of BIT by progressive migration and avulsion of the active channel(s). The discussion shows the similarity with other recent analyses of braided river planform properties in relation to discharge and stream power, and the likely link to bifurcation dynamics and asymmetry. The paper ends with concluding remarks, which emphasize the importance of the main active channel in dominating the dynamics of the braided channel pattern.

2. Experimental Methods

2.1. Physical Modeling

[9] Physical models, while not necessarily exact scale models of a particular river or reach, have been used extensively in the study of braided river morphodynamics [e.g., Schumm and Khan, 1972; Hong and Davies, 1979; Ashmore, 1982, 1991a; Ashmore and Parker, 1983; Federici and Paola, 2003; Bertoldi and Tubino, 2005]. This study uses a physical model and controlled set of experiments to test the ideas raised in the previous section. In a physical model both BIA and BIT can be observed simultaneously under the same conditions of grain size distribution, stream power and sediment supply and at a discharge known to have formed the observed channel pattern, with minimal historical effect on the morphology. This eliminates some of the uncertainties associated with field-based investigations of braided channel patterns. Also, the use of physical models provides a high temporal resolution of channel pattern change on an accelerated time scale relative to natural streams.

[10] The physical model experiments were conducted in a flume 3 m wide, 18 m long, and 0.3 m deep and filled with a 0.15 m thick layer of sediment (Figure 1). The whole flume can be tilted to adjust the slope between values of 0.5 and 2.5 percent. Water is pumped into the flume at a rate of 1–3 ls−1. The sediment was a mixture of grain sizes between 0.1 and 8 mm with D50 of 1.2 mm and D90 of 3.6 mm (Table 1). Sand leaving the end of the flume was recirculated, along with some water, from the tail tank and fed back into the flume (recirculation time from tail tank to sediment feed is about 25 s) via a vibrating (to aid feeding of wet sand) mesh tray that drained the remaining water so that only sand was fed back to the flume. In this way the sediment feed rate varies “naturally” and over the long-term matched the sediment transport rate in the model (for more details see Egozi and Ashmore [2008]).

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[11] There were three consecutive experiments each with a different constant channel-forming discharge but identical slopes of 0.015 (Table 1). Therefore, total stream power differs between the experiments because of differences in discharge alone. The experiments were run in sequence of progressively increasing discharge: 1.4 ls−1 (experiment 7); 2.1 ls−1 (experiment 8), and 2.8 ls−1 (experiment 14). Prior to the first experiment, the sediment in the flume bed was mixed and leveled, and a straight channel with a trapezoidal cross section was cut with top width of 0.5 m and depth of 0.015 m deep along the center of the flume. The dimensions of this channel were calculated to just accommodate the planned discharge without going “overbank.” The bed was not reflattened between the successive experiments so that the channels in experiments 8 and 14 developed from the river topography at the end of the previous experiment to simulate the adjustment to increased channel-forming discharge in an established braided pattern. The three experiments, each at a different constant discharge, were run for approximately equal time (∼70 hours; Table 1). The duration of the experiments was designed to provide time sufficient for adjustment of the braiding intensity to the prevailing discharge (phase 1) and for subsequent variation in channel pattern to establish a long-term average braiding intensity at that discharge (phase 2) and so observe the interaction between total and active braided networks.

2.2. Data Collection

[12] The experimental data were collected in a 12 m long section of the flume beginning 5 m downstream of the entrance so as to minimize any entrance and sediment feed effects on the channel pattern (Figure 1). This 12 m length is more than 10 times the average wetted width of the braided channels (Table 1), which provides sufficient length to average out local variations and sampling effects on braided pattern and braiding intensity measurements [Egozi and Ashmore, 2008]. Data on channel pattern development were collected along the study reach over the ∼70 hours of run time by direct observation and interpretation of vertical photographs.

[13] Near-vertical overhead images of the planform were captured throughout the experiments by two wide-angle digital cameras (Olympus 5060) mounted 3 m above the flume (Figure 1). Each camera covered the full width of the flume and a length of approximately 5.4 m, with 0.5 m overlap between the two cameras. The cameras were controlled remotely by computer using the program PTC camera controller (Pine tree computing, 2003, available at https://secure3.ntwebb.com/pinetreecomputing.com/default.asp) and captured images at 15 min intervals. The images were then ortho rectified to eliminate image distortion, using ERDAS Imagine Orthobase Pro V8.5.1 (for more details see Egozi and Ashmore [2008]). The ortho images were used for mapping channel pattern development throughout each experiment.

[14] Braiding indices were measured by direct observation every hour, at which time the total number of channels per cross section (BIT), and the number of active channels (i.e., channels transporting bed sediment) per cross section (BIA) were recorded. Thirteen cross sections were established 1 m apart along the study reach and a laser level line was used to mark each section during sampling. The observations were done twice, once from each side of the flume, to assure accuracy of observations of active braiding intensity across the 3m wide flume. A channel was defined as a path of flowing water with definable boundaries (Figures 1b and 1c). An active channel was defined as a channel in which movement of bed material was observed at successive cross sections during the time of observation (less than 1 min). Typically the channel network consisted of one main channel and a number of smaller “secondary anabranches,” with a range of dimensions carrying flow but not necessarily bed load (Figures 1b and 1c). The main active channel (MACh) is readily identified because it is substantially wider with larger discharge and velocity (Figure 1b) than any of the secondary anabranches (Table 2). The water in the flume was completely clear so that bed particle movement was easy to determine and was checked with tracer particles [e.g., Pyrce and Ashmore, 2003]. Fluorescent painted particles of the finest size found in the bed material mixture were seeded in anabranches which were identified as nonactive. These anabranches were then illuminated with a UV lamp to confirm that no movement occurred. We prefer this direct observation to the indirect (but spatially continuous) threshold depth (dye intensity) method of Gran and Paola [2001] which is based on uniform grain size and threshold depth and needs extensive calibration. The direct method also provides results comparable with those of Ashmore [1991a] and Bertoldi et al. [2006].

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3. Results

3.1. Relationship Between Braiding Intensity and Channel-Forming Discharge

[15] When grouped for each experiment, the hourly BIT and BIA data show distinct differences in channel pattern between the three runs, with both indices showing higher means and greater variation and range with increasing total discharge (Figure 2 and Table 1). The modal values of BIT (or BIA) are 2(1), 2–3(1), and 3–5(2) in experiments 7 (Qchf = 1.4 ls−1), 8 (2.1 ls−1) and 14 (2.8 ls−1), respectively. Maximum BIT values are more than twice the maximum BIA values. Only occasionally, at individual cross sections, was BIA equal to BIT and this occurred only when BIT was 3 or less. These results from the systematic experiments confirm, over a range of channel-forming discharges, the previous preliminary observations [e.g., Stojic et al., 1998; Ashmore, 2001] that the braided channel network is only partially active at any given time and location.

3.2. Time Development of Braiding Intensity

[16] In each experiment, BIT increased progressively from the beginning of the experiment (phase 1) until reaching a stable value (phase 2) but with substantial temporal variability even in phase 2 (Figure 3). The initial increase in BIT in experiment 7 is because of the formation of the braided pattern from an initial straight channel. In experiments 8 and 14, this increase in braiding intensity is a response to the step increase in discharge and consequent channel pattern adjustment. In experiments 7 and 8, there is a fairly clear divide, marked by a BIT maximum, between the phase 1 of increasing BIT and the subsequent stable phase 2. A t test on the means indicates that mean BIT of the first phase of the experiment is significantly different from mean BIT of the second phase in each experiment (p < 0.005 for experiments 7 and 8; p < 0.05 for experiment 14). The trends in experiment 14 are not as clear as they are in experiments 7 and 8 perhaps because the braided channel pattern in experiment 14 was affected by the main active channel being against the wall of the flume periodically for a significant amount of time during the experiment.

[17] At the beginning of phase 1, in experiments 8 and 14, BIT values are lower than those measured at the end of experiments 7 and 8, respectively. This is because the higher discharge drowns the channel network formed at the lower discharge, so submerging bars and merging channels, until a more extensive channel network develops to accommodate the higher discharge.

[18] The time to reach the stable value varied among experiments and was shorter at higher discharges. It took approximately 40, 36 and 20 hours to reach a stable mean BIT in experiments 7, 8 and 14, respectively (Figure 3). The longer time for development in experiment 7 is partly because of the necessity to develop a braided pattern from the initial straight channel and also because bed load transport rates are very low, as indicated by BIA averaging less than 1 so that bed load movement was discontinuous along the flume (Figure 3). This discontinuous transport almost disappeared in experiment 8 and did disappear in experiment 14.

[19] Active braiding intensity developed very quickly and stabilized within a few hours of the beginning of each experiment (Figure 3). This is a clear contrast with the development of BIT. The consequence is that for most of the duration of each experiment BIA was effectively constant. BIA values measured in our experiment fall within the range of values measured by Ashmore [1991a] (Figure 4

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