Braided channels are characterized as being dynamic geomorphic agents whose behavior is somewhat unstable and unpredictable. Vegetation can provide stability to this unstable behavior, constraining lateral migration rates and providing resistance to flow. In this thesis I investigate the relationship between vegetation and braided channels to better understand what controls braided versus meandering channel morphology. More specifically, I am interested in the effects of vegetation including roughness and cohesion, how much vegetation is required to have an influence, and the effect of aggradation to predict the future of the braidplain on the Pasig-Potrero and Sacobia Rivers at Mount Pinatubo, Philippines.
The eruption of Mount Pinatubo in 1991 deposited 5-6 cubic kilometers of pyroclastic flow deposits onto the flanks of the volcano. I am studying the Pasig-Potrero and Sacobia Rivers on the east flank of Mount Pinatubo as vegetation becomes re-established in the braidplain. Vegetation was absent in the valley bottom for the first decade following the eruption due to extremely high sediment transport rates and rapid reworking of the braidplain. In the last three years vegetation has begun to persist in the braidplain through the rainy season.
Research procedures included a combination of field work and cellular numerical modeling. Field work was comprised of 76 1x1 meter plots to characterize vegetation growth, 181 root strength measurements, 15 root density samples, 52 Wolman pebble counts, and surface sediment samples for grain size analysis. A fiber bundle model, RipRoot, devised by Pollen and Simon (2005) was utilized to obtain values for added cohesion due to roots present on streambanks. Roughness was quantified for different vegetation growth scenarios. A cellular numerical model was devised, based on Murray and Paola (2003), to test field data and gain more understanding of the relationship between vegetation and channel dynamics. Water and sediment were routed through a 200x42 cellular matrix based on slope and stream power, respectively. Vegetation was added as an impedance to sediment transport, which can also be thought of as an increase in bank strength.
Both braidplains range in with from 300-500 meters, with an average of 10-50 braids, 0.1-20.0 meters wide, in cross-section. Vegetation growth occurs in patterns categorized as sparse, dense and clumpy comprised of a mix of grasses, vines, forbs and woody trees. Both stem diameter and height increased from sparse to clumpy to dense vegetation, with an average diameters of 5.25 mm in sparse to 9.92 mm in dense plots and average stem heights from 0.72 m in sparse to 2.44 m in dense plots. Roughness calculations show that vegetation decreases flow velocities by an estimated 3-12%. Values obtained from RipRoot show that dense vegetation adds 8.21-12.31 kPa of cohesion to streambanks while sparse vegetation adds 0.13-0.29 kPa. This added cohesion creates stable streambanks, which otherwise are considered unstable. Cellular model results run with the effect of vegetation show more organization of flow, seen as a decrease in total channel width and an increase in channel depth. Channel width decreases with increasing vegetation density. Channel width increases with increasing sediment transport rates, lending the idea of a sediment transport threshold that must be overcome for vegetation growth to occur on the braidplain. This observation is compatible with field observations showing a lack of vegetation growth in the braidplain while fluvial aggradation rates were high. In all, field data and observations, coupled with model results show that vegetation increases bed roughness and increases bank strength, effects that are consistent with evolution to a single-thread channel pattern.
University of Minnesota M.S. thesis. June 2011. Major: Geological sciences. Advisor: Dr. Karen Gran. 1 computer file (PDF); vii, 78 pages, appendices A-E.
Dunn, Emily O"Donnell.
Incipient vegetation recovery and its effect on braided channel morphology at Mount Pinatubo, Philippines..
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