River flooding has become a major issue in the last decades, placing at risk wide portions of the planet. Modeled scenarios are the most helpful tools to reduce flood hazard; nevertheless they can lead to significant over- or underestimation of flood discharge and peak flood level if field attributes, especially floodplain roughness, are not carefully surveyed and reasonably included in their design. Besides, floodplains provide optimal habitats for invasive species, which can form dense stands, influencing roughness and flow velocity considerably. The floodplains of the Tisza River and its tributaries (Hungary) are highly invaded by invasive plants, however their existence is not considered during flood modeling. Among these plants Amorpha fruticosa has the greatest impact and creates impenetrable shrubbery on the floodplain, having 5-10 branches sprouting from its stump. The aim of this study is to give a precise and field-based estimation of vegetation density and floodplain roughness of different vegetation categories along the floodplain of the Tisza River, to overcome inaccuracies caused by the use of reference values. Through the Parallel Photographic Method, digital images are used to create photo mosaics and derive accurate estimates of vegetation density and floodplain roughness. Further, the role of invasive A. fruticosa in increasing vegetation density of the floodplain is also investigated. Results confirm a considerable underestimation of vegetation density and floodplain roughness (average difference: 0.04-0.06) by using simple reference values, as invasive Amorpha contributes for 55% of vegetation density on average, and on abandoned fields it constitutes 100% of the woody vegetation. These values suggest that modeled flood water levels can be remarkably underestimated if density of invasive species is not considered. The results of this study stress the importance of including field-based assessments of vegetation characteristics and the influence of invasive species in river modeling, to generate realistic predictions and address effective flood hazard strategies.
