Floods of combined origin - rainfall and snow melt - in Romanian ...

ERB and Northern European FRIEND Project 5 Conference, Demänovská dolina, Slovakia, 2002
Floods of combined origin - rainfall and snow melt - in Romanian
small catchments
Pompiliu Mita 1 , Marinela Simota 1 , Valentina Ungureanu 1
In Romania the rainfall and snow melt originated floods (mixed floods) frequently
occur, in the mountainous rivers. There are years when water content of the snow cover
on the river basins is over the 500-600 mm. During the snowmelt periods, usually in the
springtime, when high amounts of rain superpose on the snowmelt process, intense
floods occur.
In the article, based on the analysis of the floods brought about by rainfall, snowmelt
and combinative action of these ones, recorded in the Iedut and Fintina Galbena
mountainous representative basins located in the western part of Romania, some results
concerning the main characteristics of these floods (depth of runoff, runoff coefficient,
and peak discharge) are presented. The assessment of the characteristics of the floods of
rainfall-snowmelt origin is a more complex operation than that to determining these
ones for the rainfall or snowmelt originated floods, separately considered. The cause of
it is the combinative and complex action of the snowmelt and rainfall upon the surface
runoff (discharges and volumes) at the river outlet. It is very difficult to precise the
portion of the rainfall or snowmelt contribution in the surface runoff, especially for
short time intervals. The rainfall intensifies the snowmelt process by changing the snow
pack metamorphism and by its heat flux. In the same time, the rain suffers a delay and
attenuation within the snow layer. However the both sources – rain and snowmelt –
contribute to the values of discharges and volumes of runoff.
The analysis of the characteristics of the rainfall-snowmelt-originated floods is made by
comparing the same elements obtained in the case when only rainfall or only snowmelt
produced the flood.
The relationships between the characteristics of flood wave – peak discharge, Q max
(m 3 /s), depth of runoff, h s (mm) and runoff coefficient, α – and the triggering factors
that generate the flood (rainfall amount, water resulted from snowmelt, air temperature,
soil moisture index) have been obtained for Iedut representative basin, separately in
case of the floods of rainfall and snowmelt origin.
Relationships for the floods of rain origin
The form of the relationships for rainfall flood origin is (Mita, Corbus, 2000):
h s = f(X, API 10 )
α = f(X, API 10 )
Q max = f(i N , T N , x)
where: X (mm) is the rainfall amount; API 10 (mm) is the soil moisture index calculated
from the rainfall during 10 days previously the flood occurrence; i N (mm/min) and T N
(minutes) are intensity and time interval of the rainfall core; x (mm) is the rain amount
between the beginning of the rain and the beginning of the rain core, depending on the
soil moisture index.
1 National Institute of Meteorology and Hydrology – Bucharest, Romania

ERB and Northern European FRIEND Project 5 Conference, Demänovská dolina, Slovakia, 2002
Relationships for the floods of snowmelt origin
The form of the relationships for snowmelt flood origin is (Mita and all, 1999):
h sz =f(h z ,h za )
α= f(h z ,h za )
Q max =f(i hz , T max , h za )
where: h sz (mm) is the depth of runoff from snowmelt; h z (mm) – water yielding by
snowmelt during a certain time interval; h za (mm) - the soil moisture index calculated
from the water yielded by snowmelt during 10 days previously the flood occurrence; i hz
(mm/hour) – intensity of the water yielding by snowmelt, T max ( o C) – daily maximum
air temperature.
The relationships for h sz and α are carried out both for the daily variation and for the
entire snowmelt season.
The estimation of the evolution of the water equivalent of the snow pack, E az (cm),
corresponding to entire basin area, was obtained using the measurements from the
points of the snow courses (5 days frequency) and from snow plots (daily frequency).
The daily water yielding by snowmelt, h z (mm), was calculated extracting the snow
water equivalent value for a given day from its corresponding value for the previously
day.
The analysis of the rain-snowmelt events in Iedut representative basin
As it is mentioned before, an important amount of rain falling upon an existing snow
pack amplifies the surface runoff.
The daily discharge variation having a form of successive waves resulting from
snowmelt and rain during the April 1980 at the Iedut representative basin are
represented in Figure 1. In this figure can observe that the daily variation of the surface
runoff from snowmelt is slower than that provoked by the combinative action of rain
and snowmelt and it follows with a some delay the evolution of the air temperature that
is the main cause of the snowmelt. The discharges commence to increase daily at 8 or 9
hour with a delay after the beginning of the rising of the air temperature. The maximum
discharge is recorded at 18 or 19 hour and after that, the discharges begin to decrease to
the minimum values around 8 or 9 hour in the next day.
When an intense rainfall occurs, the daily discharge hydrograph of the surface runoff
from snowmelt substantially changes and the surface runoff is amplified. This can be
observed in the Figure 1 between 19 and 23 April. For this time interval the
characteristics of the surface runoff from snowmelt have been calculated using the
above-presented relationships.
The flood wave during 19 – 21 April was generated by a rainfall of 30 mm and by
snowmelt water yielding of 9.9 mm. A flood wave that have been resulted only from a
rain of 30 mm and under the condition of the soil moisture index (API 10 ) of 60 - 70 mm
would have a depth of runoff (h s ) of 9.5 mm and a runoff coefficient (α) of 0.32.
A flood wave that have been resulted only from a snowmelt water yielding of 9.9 mm
and the same condition of the soil moisture, h za = 60 - 70 mm, would have a depth of
runoff ( hzs ) of 1.7 mm and a runoff coefficient (α) of 0.17. An overall depth of runoff of
11.2 mm have result from a simple addition of the both depths of runoff and a runoff

ERB and Northern European FRIEND Project 5 Conference, Demänovská dolina, Slovakia, 2002
coefficient of 0.28, that is less than the recorded depth of runoff of 12.8 mm and real
runoff coefficient of 0.32. These comparative values prove the role of the rain to
intensify the snowmelt processes.
Q (m3/s)
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
20
10
0
-10
-20
-30
-40
-50
-60
-70
Temperature ( o C), Rain (mm), Water equivalent (cm)
0
14 Apr 80 15 Apr
80
16 Apr
80
17 Apr
80
18 Apr
80
19 Apr
80
20 Apr
80
21 Apr
80
22 Apr
80
23 Apr
80
-80
rain Q snowmelt Q rain+snowmelt temperature water equivalent
Fig. 1. The evolution of runoff, air temperature, precipitation and snow water equivalent
in the Iedut representative basin during a snowmelt period
(14-23 April 1980).
1.8
1.6
1.4
1.2
Q (m 3 /s)
1
0.8
0.6
0.4
0.2
0
0
120
240
360
480
600
720
840
960
1080
1200
1320
1440
1560
1680
1800
1920
2040
2160
time (minuntes)
rain rain+snowmelt snowmelt
Fig 2. Flood waves - only from rain, only from snowmelt and combination of rain with
snowmelt during 19 – 21 April 1980.
Analysing comparatively the peak discharge values (figure 2) of the three categories of
flood waves - only from rain (Q max = 1.52 m 3 /s), only from snowmelt (Q max = 0.1 m 3 /s)

ERB and Northern European FRIEND Project 5 Conference, Demänovská dolina, Slovakia, 2002
and combination of rain with snowmelt (Q max = 0.56 m 3 /s) - can observed that contrary
to the depths of runoff corresponding to the three cases, the peak discharge resulted only
from rain is bigger than that resulted from rainfall superposed on the snowmelt or only
from the snowmelt. The explanation is the delay and the attenuation of the rain within
the snow layer.
Based on the analyses of recorded surface runoff resulted from combined action of rain
and snowmelt, in the Iedut representative basin, the relationships between the flood
wave characteristics (Q max , h s and α) and the generating factors (rain and snowmelt
intensity, air temperature and soil moisture index) will be achieved. Such relationships
obtained for small basins can be used for the maximum discharge and runoff volume
forecasts and the small basins can be considered the “warning” basins for the larger
basins (Diaconu, Stanescu, 1978).
References
Diaconu, C., Stanescu, V. Al., (1976) A mathematical model for flood wave forecasting
by means of warning basins. Hydrological Sciences Bulletin, XXI, No. 3,
Bucharest, Romania
Mita, P., Simota, M., Stancalie, G., Popovici, F., Catana, S., (1999) Some results
regarding the diminishing role of the forest in sediment runoff for snowmelt –
generate floods, Proceedings of the Symposium: Vegetation, Land Use and Erosion
Processes, Bucharest, Romania
Mita, P., Corbus. C., 2000 A model for determining flood waves in small basins up to
100 km 2 , Proceedings of the Liblice Conference: Catchment hydrological and
biochemical processes in the changing environment, sept. 1998, Czech Republic