Inundation Dynamics in Braided Floodplains: Tagliamento River, Northeast Italy

Abstract

The relationships among water level, inundated area, and shoreline dynamics were investigated in a bar-braided and an island-braided floodplain of the Tagliamento River in northeast Italy. Ground-based surveys with a differential global positioning system (aGPS) unit were used to delineate all aquatic–terrestrial interfaces (shorelines) in the active floodplain at different water levels. Despite complex inundation patterns, a highly significant (P < 0.00001) linear relationship between water level and arcsine square root of inundated area was found in both reaches (y = 0.49x + 0.07). A highly significant (P < 0.00009) second-order polynomial relationship occurred between water level and shoreline length (y = 87.83 − 65.85x² + 169.83x). Using these relationships as simple predictive models, we converted several years of water-level data into predictions for degree of inundation and shoreline length. The plot of the simulated degree of inundation strongly resembled the actual hydrograph. Complete inundation of the active floodplains occurred one or two times per year; however, the degree of inundation at lower water levels was highly dynamic during most of the year. Simulated shoreline length averaged 171 m ha⁻¹ (13.6 km km⁻¹), with a maximum of 197 m ha⁻¹ (15.6 km km⁻¹) occurring during periods with intermediate water levels. The corresponding values determined with GPS were somewhat higher, with an average value of 181 m ha⁻¹ (14.4 km km⁻¹) and a maximum of 214 m ha⁻¹ (16.3 km km⁻¹). During major flood events, actual shoreline length decreased to 28 m ha⁻¹ (2.1 km km⁻¹). Braiding index and upstream surface hydrologic connectivity were positively related to water level, whereas total area of isolated water bodies was negatively related to water level. The number of nodes remained high most of the time during the 2-year study period.