Bank Processes and Revetment Erosion of a Large Lowland River: Case Study of the Lower Tisza River, Hungary

Abstract

Stone and concrete revetments are widely constructed to control bank erosion and thus stabilize river banks. The consequences include accelerated erosion at unrevetted downstream channel sections and in-channel incision at revetted sections. The studied section of the Tisza River (Hungary) was revetted along 49% of its banks in the 20th century with stepped-block and placed-rock revetments. We therefore aimed (1) to study the effects of revetment constructions on channel processes and (2) to evaluate the state and collapse of revetments using a DEM of the channel and ADCP measurements. In the late 19th century, the river had an equilibrium meandering channel, with alternating eroding and aggrading banks. Currently, erosional processes dominate. Thus, 65% of the total channel length is affected by bank erosion. The revetments reduced the active lateral erosional processes only temporarily, as 58% of the revetments, mainly placed-rock, are already damaged. The flow characteristics at the revetted sections were found to depend on the rate and type of revetment collapse. Large pools developed in front of the revetments, playing an important role in initiating their erosion. The placed-rock revetments can collapse by slides or by stones falling into the pool one-by-one. In this case, a knickpoint develops, propagating upstream. Thus, the collapse and accelerated bank erosion also propagate upstream. The increased hazard created by the failure of the revetments requires the re-planning of bank stabilization practices.

Active point bar development and river bank erosion in the incising channel of the lower Tisza river, Hungary

“…This suggests that the processes of erosion within this section was not necessarily controlled only by the hydrological regime, although according to Hickin (1974), floods control most channel forming processes including bank erosion. The erosion within this section was probably controlled by high flow energy generated from high velocities due to the revetment, which was even distinguishable at low stage (Kiss et al, 2019). This increased flow energy was transferred to the downstream end of the revetted section where it was dissipated.…”

“…10 km) was reduced to just 1 km by constructing artificial levees. To protect the artificial levees in the 20 th century, the construction of revetments was necessary, which accelerated the channel incision and transferred the bank erosion to the unrevetted downstream sections (Kiss et al, 2019). The absolute water stage at Csongrád is 1394 cm (least stage: -357 cm and highest stage: 1037 cm).…”

“…In its natural state, before the regulations, the sinuosity of the channel was high (with up to 30 meander bends), point-bars appeared in every bend, and the channel was relatively shallow (6-8 m). Along the entire Lower Tisza in the late 19 th , 47 point-bars existed (in 52.3 km length), but nowadays, their number (20) and total length (4.7 km) have reduced considerably (Kiss et al, 2019). For this study, point-bars which were not dredged, and develop in a quasi-natural way were selected (Fig.1).…”

“…At Ányás, the high and near-vertical banks failed mainly through circular slips and toppling failure (Kiss et al, 2019). This is usually through saturation of the bank material which lowers its angle of repose, 13 (1) 2019.…”

“…The river as a result has transformed from a meandering river to an incising meandering one (incision has lowered base level while maintaining the meandering pattern with high bank walls), as it strives to re-establish a new equilibrium after the river training ( Kiss et al, 2008;Amissah et al, 2018). In recent times, some studies have shown that although the flood hazards have reduced due to higher flood velocities, there are threats to artificial levees due to increased lateral erosion (Kiss et al, 2019). Further research on the factors generating bank instability have also established the high magnitudes and various types of bank erosion in the Lower Tisza (Kis and Lóczy, 2018).…”