Riparian Vegetation Density Mapping of an Extremely Densely Vegetated Confined Floodplain

Abstract

The most crucial function of lowland-confined floodplains with low slopes is to support flood conveyance and fasten floods; however, obstacles can hinder it. The management of riparian vegetation is often neglected, though woody species increase the vegetation roughness of floodplains and increase flood levels. The aims are (1) to determine the branch density of various riparian vegetation types in the flood conveyance zone up to the level of artificial levees (up to 5 m), and (2) to assess the spatial distribution of densely vegetated patches. Applying a decision tree and machine learning, six vegetation types were identified with an accuracy of 83%. The vegetation density was determined within each type by applying the normalized relative point density (NRD) method. Besides, vegetation density was calculated in each submerged vegetation zone (1–2 m, 2–3 m, etc.). Thus, the obstacles for floods with various frequencies were mapped. In the study area, young poplar plantations offer the most favorable flood conveyance conditions, whereas invasive Amorpha thickets and the dense stands of native willow forests provide the worst conditions for flood conveyance. Dense and very dense vegetation patches are common in all submerged vegetation zones; thus, vegetation could heavily influence floods.

Increased Riparian Vegetation Density and Its Effect on Flow Conditions

“…Due to the centurial changes in riparian vegetation cover along the Tisza River [76], today, the mean vegetation density is quite high (NRD median : 0.007-0.051) in the area, especially in the lowest inundation zone, in the Amorpha thickets [78]. Our NRD results provided very similar data to the results of point-like density measurements performed by Sándor [83] and Delai et al [75] in the same area.…”

“…The vegetation is quite dense in the selected section of the Lower Tisza at Mindszent. According to our previous measurements [76][77][78], within a century, the meadows were replaced by planted (43%) and riparian forests (31%), and only 13% of the original meadows and plough fields remained. Moreover, as the study area is located very close to the town of Mindszent, fruit and vegetable gardens were on the floodplain but, due to their gradual abandonment, invasive plants colonised them.…”

“…Along the Tisza River (catchment area: 157,200 km 2 ; length: 962 km) in Central Europe (Figure 1), the 5-10-km-wide natural floodplain was confined by artificial levees to 1-4 km in the mid-19th Century [34,77,78]. Ever since, the floodplain has been gradually altered by semi-natural processes.…”

“…Ever since, the floodplain has been gradually altered by semi-natural processes. The original wetlands quickly disappeared after the river regulation works, as the channel incised by 3-4.6 m (max 45% increase), and the originally six-month-long floods shortened to 1-3 months [34,[76][77][78]. The wetlands were replaced by meadows and plough fields, but by the late 20th Century, even these traditional land uses were abandoned as intensive animal husbandry moved to the villages, and the increasing flood levels destroyed the crops; thus, the farmers ended the cultivation of their lands.…”

“…Data are needed pertaining to riparian vegetation density to test our hypothesis, but only point-like measurements (applying quadrates) have been carried out to estimate the vegetation density and roughness [75][76][77][78]. Due to the lack of detailed data on vegetation density, the exact hydrological consequences of riparian vegetation changes are not known.…”