Monitoring Small Scale Developments in Bedforms and Bedload Transport

The paper presented here examines the utilization of a digital camera in monitoring the evolution of cluster microforms in a laboratory flume. Two types of experiments were conducted to examine the spacing of self organized cluster microforms and their effects on bedload transport. The use of the digital video camera in concurrence with other analysis tools allowed for the examination of cluster spacing under varying flow conditions, accurate continuous bedload sampling (bedload samples at less than 1/sec), and the examination of integration and disintegration of particles into and out of individual clusters. Utilization of the digital camera proved useful and accurate in the lab setting. Introduction Small-scale cluster bedforms, or microforms, are prevalent features in gravel-bed rivers. An understanding of the mechanics associated with them is needed for a more comprehensive understanding of river processes, as well as the development of accurate predictive tools for bed roughness characteristics, stage-discharge relations, bedload transport, and implementation of restoration plans. Bedform structures are one component of the nonlinear system of gravel-bed rivers (Kuhnle 1996, Muller and Gyr 1996, Gomez and Phillips 1999, Lanzoni and Tubino 1999). Bedforms are produced by flow structure and entrainable sediment; once developed, they in turn influence sediment transport and the surrounding flow field. The complexity of these interactions and the inability to account for them has lead to site specific resistance and bedload formulas, which produce predictions with large errors and degrees of variability (Brayshaw 1984, Gomez and Church 1989, Rickenmann 2001). One hindrance in the study of microtopography and bedload transport is the fact that bed evolution occurs during high flow events, making it difficult to make real-time measurements of bedload and observations during the formation and disintegration process. Most field studies conducted on evolution of clusters have been limited to observations of tagged particles before a runoff event and then again after normal flow conditions have returned (Billi 1988, de Jong 1991, Mikos and Escorza 2000). The purpose of this paper is to examine the use and analysis of digital video in monitoring cluster microform development and bedload transport. Background information and further analysis and results of the fundamental aspects of the study on cluster development and effects on bedload transport can be found in Strom et al. (2002). However, a short description of the test setup will be presented here to better help the read understand the conditions of the tests and analysis of the digital video. Copyright ASCE 2004 HMEM 2002 Setup The experimental runs were conducted in a flume at the Albrook Hydraulic Laboratory of Washington State University. The flume is a 22 m long, 0.9 m wide, water recirculating flume capable of attaining slopes of up to 13% by means of motor-driven screw jacks. Figure 1 is a schematic of the flume and experimental set up. The test section, which is 4.2 m in length, was located 17m downstream of the headbox to ensure that fully developed flow conditions were present during the experiments. The digital video camera was mounted on a rolling trolley located above the test section (Fig. 1). Two types of experiments where carried out, one for monitoring the development and spacing of cluster microforms under varying flow conditions and the second for determining the effects that clusters had on bedload transport. For the first type of experiments, i.e. cluster development and spacing, pre-set marks for trolley position were used so that the camera position could be quickly changed to ensure that the whole test section was captured and that camera positions were constant from placement to placement. To monitor bedload transport, the trolley was placed in one location for the duration of the test so as to capture some of the clusters located in the downstream end of the test section as well as the exit of particles out of the test section. Green spherical glass particles of 8mm diameter were used as active layer sediment for the development of clusters. This was done atop a uniform roughness bed comprised of clear 8mm spherical particles packed as tightly as possible (Fig. 2, shows active layer sediment, green clusters, atop uniform roughness bed of clear particles). Figure 1. Flume and digital video camera setup Cluster Spacing Analysis One objective of the test was to determine if clusters formed with any kind of pattern regularity with respect to each other. That is, do clusters have, in general, a predictable spacing in relation to each other for varying flow conditions. To quantify cluster spacing at each of the bed shear stress conditions that clusters where found to form under (Table 1), the following procedures Copyright ASCE 2004 HMEM 2002 where followed. First, still images of the bed for each flow condition where captured from the digital video with the program PhotoDV. The pictures where then opened in Adobe Photoshop for quantification of cluster spacing. Test τ* S h Q Ubulk Re Fr u* Rep B/h (%) (m) (m/s) (m/s) (m/s) (m/m) 1.25τ*cr 0.01 0.2 0.063 0.0233 0.416 2.78E+04 0.53 0.035 247 14.1 1.5τ*cr 0.012 0.2 0.076 0.0335 0.5 3.91E+04 0.57 0.039 271 11.7 1.75τ*cr 0.014 0.2 0.089 0.0417 0.526 4.76E+04 0.56 0.042 294 10.0 2.0τ*cr 0.016 0.2 0.103 0.0518 0.565 0.00E+00 0.56 0.045 316 8.6 Table 1. Flow conditions for which cluster where found to form Cluster spacing in relation to each other was described by longitudinal spacing λx, transverse spacing λy, and an angle of orientation αc (Fig. 2). For consistency, this spacing was defined from the stoss tip of one cluster to that of the next (where the “stoss” is the front of a cluster). The overall shape and alignment of clusters was similar to other reports (Brayshaw 1984, de Jong 1991, Hassan and Reid 1990) in that they aligned and elongated in the flow direction, and have an offset diagonal spacing (Fig. 2).