14 Flood Potential of Polish Rivers
273
Every flood has a beginning in the processes acting in the atmosphere since it
is induced by intensive precipitation of rain or snow. In atmospheric processes, it
is visible certain cascade of processes acting in different time scales from cyclonic
circulation to convective cloud. Every scale of the atmospheric processes has a certain
dynamics and time of duration. Very intensive torrential rainfalls from convective
clouds cause flash floods which have limited spatial coverage on the other hand, rain
at the atmospheric front has a lower intensity but can affect large catchments. The
study of rain effectiveness in creating floods in Poland has shown that according
to a ratio of rain maximum to rain duration three types of rainfalls can be defined:
convective, frontal and wide convergence zones [9, 10].
The isolated system of convective clouds can produce the volume of rain which
can be calculated as a product of cloud area, rain duration, and height of precipitation.
The total volume of rain depending on the size of cloud systems can vary from 1.8 ×
10
5
− 2.1 × 10
7 m
3 at small convective clouds to 6.1 × 10
9 m
3 at meso-scale cloud
systems [11].
In Poland, rain amount is a function of air masses properties of different origin.
Dominating advection of air masses to Poland is from the Atlantic Ocean, a long
distance from the ocean creates conditions of air masses transformation on the way
over the continent. Additionally, the lowland relief of the country with a mountain
ranges of Sudety and Karpaty in the south do not induce high precipitation totals
by the orographic effect. This explains why in Poland maximum precipitation totals
compared to world data are considerably lower [11].
It has been shown that among catchment’s morphometric variables controlling
hydrologic response to the intensive rain the most important is the catchment area.
The area of the catchment is related to the spatial scale of meteorological processes
which are responsible for the creation of rain. According to the study of [12] and [13]
based on data from the period 1951–1990 the highest river run-off layer is created
in Poland from mountain catchments having an area less than 300 km
2 . For example
mountain catchment of upper Vistula at Skoczów profile was able to create the layer
of run-off of 275 mm. The river run-off layer considerably gets smaller with the
increase of the catchment area; this is especially visible in the longitudinal profile
of transit rivers. The increase of the river catchment area means also change in the
scale of rain inducing processes, from intensive convective cells to longer duration
and less heavy frontal rain.
In climatic conditions of Poland, high river run-off layer can be created by the
melting of the accumulated snow layer [13, 14]. The highest snow melting floods of
1970 and 1979 were observed in central Poland from the catchments of area around
1000 km
2 (Skrwa – 144 mm, Wkra – 123 mm). At the mountain catchments of Poland
snow melting floods were at the same time producing river run-off layer of less than
80 mm. This difference between lowland and mountain catchments can be explained
by the fact that in the mountains accumulated snow changes its physical state slowly
due to a diurnal pattern of thawing and freezing. The highest thawing floods occur
on the large lowland rivers Bug and Narew as well as in the lower and middle reach
of the Vistula river. Time of thawing floods occurrence at the lowland catchments is
273
Every flood has a beginning in the processes acting in the atmosphere since it
is induced by intensive precipitation of rain or snow. In atmospheric processes, it
is visible certain cascade of processes acting in different time scales from cyclonic
circulation to convective cloud. Every scale of the atmospheric processes has a certain
dynamics and time of duration. Very intensive torrential rainfalls from convective
clouds cause flash floods which have limited spatial coverage on the other hand, rain
at the atmospheric front has a lower intensity but can affect large catchments. The
study of rain effectiveness in creating floods in Poland has shown that according
to a ratio of rain maximum to rain duration three types of rainfalls can be defined:
convective, frontal and wide convergence zones [9, 10].
The isolated system of convective clouds can produce the volume of rain which
can be calculated as a product of cloud area, rain duration, and height of precipitation.
The total volume of rain depending on the size of cloud systems can vary from 1.8 ×
10
5
− 2.1 × 10
7 m
3 at small convective clouds to 6.1 × 10
9 m
3 at meso-scale cloud
systems [11].
In Poland, rain amount is a function of air masses properties of different origin.
Dominating advection of air masses to Poland is from the Atlantic Ocean, a long
distance from the ocean creates conditions of air masses transformation on the way
over the continent. Additionally, the lowland relief of the country with a mountain
ranges of Sudety and Karpaty in the south do not induce high precipitation totals
by the orographic effect. This explains why in Poland maximum precipitation totals
compared to world data are considerably lower [11].
It has been shown that among catchment’s morphometric variables controlling
hydrologic response to the intensive rain the most important is the catchment area.
The area of the catchment is related to the spatial scale of meteorological processes
which are responsible for the creation of rain. According to the study of [12] and [13]
based on data from the period 1951–1990 the highest river run-off layer is created
in Poland from mountain catchments having an area less than 300 km
2 . For example
mountain catchment of upper Vistula at Skoczów profile was able to create the layer
of run-off of 275 mm. The river run-off layer considerably gets smaller with the
increase of the catchment area; this is especially visible in the longitudinal profile
of transit rivers. The increase of the river catchment area means also change in the
scale of rain inducing processes, from intensive convective cells to longer duration
and less heavy frontal rain.
In climatic conditions of Poland, high river run-off layer can be created by the
melting of the accumulated snow layer [13, 14]. The highest snow melting floods of
1970 and 1979 were observed in central Poland from the catchments of area around
1000 km
2 (Skrwa – 144 mm, Wkra – 123 mm). At the mountain catchments of Poland
snow melting floods were at the same time producing river run-off layer of less than
80 mm. This difference between lowland and mountain catchments can be explained
by the fact that in the mountains accumulated snow changes its physical state slowly
due to a diurnal pattern of thawing and freezing. The highest thawing floods occur
on the large lowland rivers Bug and Narew as well as in the lower and middle reach
of the Vistula river. Time of thawing floods occurrence at the lowland catchments is