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  • Author: István Fehérváry x
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Abstract

The very dense floodplain vegetation on the artificially confined floodplains results in decreased flood conveyance, thus increase in flood levels and flood hazard. Therefore, proper floodplain management is needed, which must be supported by vegetation assessment studies. The aims of the paper are to introduce the method and the results of riparian vegetation classification of a floodplain area along the Lower Tisza (Hungary) based on automatized acquisition of airborne LiDAR survey. In the study area 15x15 m large training plots (voxels) were selected, and the statistical parameters of their LiDAR point clouds were determined. Applying an automatized parameter selection and 10-fold cross-validation he most suitable decision tree was selected, and following a series of classification steps the training plots were classified. Based on the decision tree all the pixels of the entire study area were analysed and their vegetation types were determined. The classification was validated by field survey. On the studied floodplain area the accuracy of the classification was 83%.

Abstract

Along the Lower Tisza River (Hungary) the water level of the floods reached new record stages in 1998 and 2006, resulting in 80 cm increase in the peak flood level since the “great flood of 1970”. Due to the gradual weakening of the levee-system caused by the several long-lasting floods, the question has arisen, that as in case of a levee breach or failure how would it modify the hydrological parameters of the river. The aim of the research is to create a hydrological model to analyse the effects (as stage reduction, slope and stream power) of two different levee breaches: one happening before the peak of the flood and another at the time of the flood level. The simulated levee breaching happened on the Tisza River at Mindszent, and the data-set of the 2006 flood was used for the modelling (at that time no levee failure happened in Hungary, and it was the greatest flood in history).

In the simulation the levee was broken at a point, where the channel is very close and intensively eroding, thus there is a real risk of a levee failure. If the levee would be broken a well defined area (reservoir) would be flooded, surrounded by the secondary levees and the rim of the high floodplain. During the simulation the HEC-RAS 4.1. ArcGIS 10.1 and HEC-GeoRAS software were applied.

The greatest changes in the hydrology of Tisza occurred in the cross section where the levee breached, though the effects propagated upstream and downstream too. Due to the water outflow from the Tisza the greatest stage reduction effect was 1.54±0.1 m. The slope conditions changed too, as it increased from 4 cm/km to 6.5 cm/km in the upstream reach, while downstream of the failure point it decreased from 3.5 cm/km to 1.9 cm/km. At the same time the stream power increased from 4 W/m to 5.5 W/m in the upstream section, while it decreased from 3.5 W/m to 1.5 W/m in the downstream reach. Comparing the results of the simulations at different stages (one at the highest stage and one at 1.0 m lower stage) it seems that the hydrological parameters did not change considerably (1%), though in a case of a levee failure at higher the reservoir reached the maximal water level sooner, though less water was stored in it, as the fall of the river was continuous

Abstract

Flood conveyance of floodplains is significantly influenced by the riparian vegetation cover, since vegetation affects flow velocity, therefore has a considerable impact on flood height and rate and pattern of sedimentation. However, climate change promotes the spread of invasive species, and their rapid growth results in dense vegetation stands, thus they have a significant impact on floodwater hydraulics. The aims of the present study are (1) to analyse the long-term changes in land-use and vegetation density on the Lower Tisza River, (2) to evaluate the role of the invasive Amorpha fruticosa in increasing vegetation density, and (3) to model the effect of dense floodplain vegetation on flood level and flood conveyance. Long-term (1784-2017) changes of land-use suggest that in natural conditions the study area was occupied by wetlands (92%), thus water covered the area for almost the whole year. In the 19th century, after levee constructions the wetlands were replaced by meadows and pastures (94%), then by the end of the 20th century planted and riparian forests replaced these land-covers. As a result, the mean roughness (0.14) of the floodplain has increased threefold until the early 21st century. Today forests are invaded by Amorpha fruticosa, which increases the vegetation density by 3% in riparian forests, by 23% in forest plantations, and by up to 100% in abandoned pastures and a rable lands. According to the results of HEC-RAS (Hydrologic Engineering Center’s River Analysis System) and CES (Conveyance Estimation System) models, if floodplain vegetation was managed and Amorpha fruticosa was cleared from the floodplain, peak flood level would decrease by 15 cm. Due to dense vegetation, the flood conveyance decreased by 4-6%, and the presence of Amorpha fruticosa reduced the flood flow velocities by 0.014-0.016 m/s. Accordingly, clearance of the floodplain from Amorpha fruticosa would have positive effects on flood protection, since peak flood stages would decrease and flood waves would shorten.