Geoscience Reference
In-Depth Information
of water into the left and right fork occurs on the gravel bank during the flood. In
cross-section III, most probably at floodwater subsidence, a considerable amount of
bed material was accumulated on the left fork, whereas the right branch was lowered
by over 1.5 m.
Cross-sections VI and III are situated at the most sore point of the back water
formed by the narrowing between the bridge abutments 75 m apart. This is not
much considering the floodwater, the more so as also the two bridge pillars are
positioned in the riverbed (and one more on the left bank). This might have been the
reason why the Białka (cross-section VI) formed a tight meander currently posing a
hazard for the bank near the left abutment. In this place, the bank was scoured along
40 m and to a depth of 13 m. The right arm carrying also a considerable portion of
flow was separated by a formed midstream gravel bank about 40-m wide.
New river channel developed in cross-section II in the middle part of the
riverbed and the main current of the Białka shifted into it. Measurements of the
cross-sections and in situ reconnaissance reveal small changes of the bottom con-
figuration in cross-section I, which is justified by the river step effect.
The fact of lowering river channel in the majority of studied cross-sections is
symptomatic. It may evidence a disturbance of hydrodynamic balance in the Białka
and a diminished supply of bedload from the upper cross-sections (Krzemie´
2006
).
Both acquisition of bed material and river control works are conducted on the
Białka River, like on many other rivers in Europe (Wy˙ga et al.
2008
). Moreover,
an invisible process of changes in the catchment management occurs, diminishing
the bed material supplies (Wy˙ga et al.
2010
; Wy˙ga
2008
).
Analysis of the cross-sections made before and after floodwater passing show
that they do not illustrate the beginning and end of a single erosion or accumula-
tion process. The Białka River is a considerably complicated system, in which
dynamic structures, e.g., in a midstream gravel bank, form depending on the
current flow and riverbed pattern, after the discharge and new arms develop
(see cross-sections IV-VI).
5.2 Results of 2D Modeling and Interpretation
There was no passing of the current from the right to the left bank in the computer
simulation process between cross-sections V and VI, and in result the modeled area
of the deep erosion occurred erroneously in the right part of the cross-section. In
cross-section III, erosion of the right part of the riverbed was computed correctly,
but the area of accumulation existing in the left part of the riverbed did not appear in
the model. In cross-section VI, the arrangement of streams of the modeled flow
caused not only the scour of the right bank gravel bar but also an incision appeared
there, not registered in nature. No phenomenon of midstream gravel bank forming
or left bank scouring appeared either. In cross-sections I and II, the results of
modeling are satisfactory or correct.
