The Response of Braided Planform Configuration to Flow Variations, Bed Reworking and Vegetation: the Case of the Tagliamento River, Italy

Morphological features of braided rivers (bars, channels and pools) experience major changes in area, shape and spatial distribution as a response to (i) the pulsation of discharge during a flood and (ii) the bed evolution induced by floods. In this work, at-a-station relationships between water level and planform configuration were investigated on the Tagliamento River, a large gravel-bed braided river in northeast Italy, over a 2-year study period comprising three bankfull events and several small-to-medium floods. The analysis was performed on two 1-km-long reaches, characterized by different riparian vegetation cover. Ground-based images with an hourly temporal resolution were acquired using software-controlled, digital cameras. Bars, channels, pools and vegetated patches were manually digitized on more than 100 rectified images. Sequences of constant-level images spanning the study period were used to quantify the impact of floods on the stability of at-a-station relationships and on the turnover rate of water bodies. The analysis shows that wetted area increased almost linearly with water level in both reaches. The average number of branches per cross-section peaked at intermediate flow levels, increasing from 2 at low flow up to 6–7. The number of branches displayed the largest fluctuations over time, with significant changes produced also by moderate floods. Turnover rates were high in both reaches, with more than 30% of wetted areas at low flow converting into bare gravel in less than 2 months. Vegetation colonization was found to limit the mobility of the low flow channels over time by concentrating the flow in fewer, deeper anabranches. The number of channels per cross-section was 30–40% less in the vegetated reach and the proportion of low flow water bodies in the same position after 12 months increased from 3% to 14%.