Pompiliu Miţă, Marinela Simota, Valentina Ungureanu, Simona Matreata
National Institute of Meteorology and Hydrology,Bucharest, Romania
ABSTRACT
In this paper, the differences between the main characteristics (runoff depth, peak discharge) of rainfall-snowmelt originated floods and those corresponding to flood waves generated only by rainfall or only by snowmelt are analysed. The comparison has been made considering that the flood wave parameters have been determined under comparable conditions of water inflow and of previous precipitation amounts.
Keywords flood of mixed origin, synthetic flood wave, runoff depth
INTRODUCTION
In Romania, rainfall and snowmelt originated floods (floods of mixed origin) frequently occur on rivers in mountainous regions. There are years in which the water content of the snow cover in these river basins exceeds 500-600 mm. During the snowmelt period, usually in the spring when high amounts of rainfall superpose the snowmelt process, severe floods occur. In this paper, several results concerning two main characteristics of these floods – runoff depth and peak discharge – brought about by rainfall, snowmelt and combination of these two processes are presented. The issue has proven to be difficult for conditions when the rainfall intensifies the snowmelt process by its heat flux and by changing the snow peak metamorphism.
At the same time, rainfall suffers a delay and attenuation through the snow cover. Both sources – rain and snowmelt – exert a certain influence upon the volumes and discharges, both from the quantitative point of view and from the point of view of the evolution in time.
DATA
The paper is based on data recorded at the Iedut and Fantana Galbena representative basins, situated in the western part of Romania. These basins have provided data on the variation of the snow cover (depth, density, water equivalent) in the basins, and also on the recorded flood waves generated by rainfall, snowmelt or their combined action. For all three types of floods, the main characteristics of the flood waves (runoff depth and peak discharge) have been separately analysed. In the case of the two studied basins, the relationships between the flood characteristics and the factors determining them have also been established.
METHODOLOGY
To point out the particularities of rainfall and snowmelt originated floods (mixed origin), a comparative method has been chosen. The main characteristics of flood waves (runoff depth and peak discharge) were determined in the case of floods of mixed origin precisely recorded at the gauging stations, and compared with values of these elements determined in the case of “synthetic” flood waves generated only by rainfall or only by snowmelt. They have been called “synthetic” because these floods have not been recorded at the gauging station. The synthetic flood waves have been created on the basis of flood wave characteristics: peak discharge, increasing duration, decreasing duration, volume. These characteristics were determined from synthesis relations (Mita and Stancalie, 1996; Mita et al., 1999). Such relations are presented in Figs 2 and 3.
To achieve this comparison, the following steps were taken:
1. The values of the flood wave characteristics (runoff depth and peak discharge) have been determined for each flood of mixed origin analysed at the gauging station. These floods have been generated by water inflow (P+hz), where P is a rainfall amount and hz the water amount coming from snowmelt.
2. For each of the floods of mixed origin a corresponding pair of “synthetic” floods has been created, generated only by rainfall or only by snowmelt (Fig 1).
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
0 100 200 300
400 500 600 700
800 900 1000 1100 1200 1300 140
0
1500 1600 1700 180 0
1900 2000 2100 220
0 time (minuntes)
Q (m3/s)
rain rain+snowmelt snowmelt
Fig 1: The flood wave of mixed origin (rainfall + snowmelt) recorded at the gauging station Stana de Vale during 19 – 21 April 1980 and the synthetic flood waves only from rainfall and only from
snowmelt.
The synthetic flood waves have been created using the same values of the water inflow (P+hz) which generated the flood wave of mixed origin, i.e. the same precipitation, P(mm), in the case of the synthetic flood wave produced by rainfall, and the same water amount from snowmelt, hz, in the case of the synthetic flood wave produced by snowmelt. A comparison has been made between the values of the runoff depth and the peak discharge corresponding to the flood wave of mixed origin (hsM, Qmax M) and the sum of the values of these characteristics corresponding to the two synthetic flood waves: hsp and Qmax p in case of the flood waves produced by rainfall, and hsz and Qmax,zin the case of the flood waves produced by snowmelt.
The following relationships were used for the floods of rainfall origin:
Qmax p = f(ip, Tp) tcr = f(Tp) td = f(Ttot, iXtot) hsp = f(p, API10) where:
Qmax,p – peak discharge of the rainfall originated flood (m3/s); Tp – time interval between the beginning of the rainfall and the end of the rainfall core (min); ip – intensity of the rainfall for the time interval Tp (mm/min); tcr – flood wave increase duration (min); td – flood wave decrease duration (min); Ttot – total duration of rainfall (min); iXtot – intensity of rainfall for Ttot (mm/min); hsp – runoff depth of the rainfall originated flood wave (mm); P – amount of rainfall that generates the flood (mm); API10 – the soil moisture index calculated from rainfall during 10 days prior to the flood occurrence (mm).
These relationships are presented in Fig 2.
Fig 2: Relationships derived for floods of rainfall origin (explanation of the symbols is given in the text above).
The form coefficient has been adopted for each flood as a function of the precipitation distribution between 0.2-0.3.
For the floods of snowmelt origin, the following relationships were applied:
hsz = f(hz, API10) Qmax,z = f(ihz, API10) where:
hsz – runoff depth of the snowmelt originated flood wave (mm); hz – water yield of snowmelt during a certain time interval (mm); Qmax,z – peak discharge of the snowmelt originated flood (m3/s); ihz – intensity of the water produced by snowmelt (mm/hour); API10 – the soil moisture index calculated from the rainfall and water yield of snowmelt during 10 days prior to the flood occurrence (mm).
These relationships are presented in Fig 3.
Fig 3: Relationships derived for floods of snowmelt origin (explanation of the symbols is given in the text above).
In the case of the increase and decrease duration of the flood wave, there also exist syntheses showing their dependency on the diurnal duration of the snowmelt process and on its intensity, in accordance with the air temperature variation. Such syntheses have been used to create the “synthetic” floods generated only by snowmelt.
RESULTS
By establishing the values of the flood characteristics (runoff depth and peak discharge) in the case of the floods of mixed origin and of the “synthetic” floods, a comparison of the respective parameters for the three types of floods could be made.
Runoff depth
The values of the runoff depth determined in the case of the flood waves of mixed origin, hsM (mm), generated by certain precipitation amounts, P (mm), and certain water amounts produced by snowmelt, hz (mm), were compared with the sum of the values of the runoff depth obtained separately from synthetic flood waves: hsp given by rain and hsz (mm) given by snowmelt (hsp + hsz).
In all the cases it was found that hsM> hsp+hsz. The higher values of the runoff depth in the case of mixed floods, hsM, in comparison with the sum (hsp+hsz) are justified through the acceleration of the snowmelt process, which is a function of the precipitation fallen on the snow surface.
The difference hsM- (hsp+hsz.) has been calculated for all mixed floods. The analysis has shown that this difference, ∆ hsM, favouring the mixed floods, depends on the share of the rainfall amount (P) within the total water inflow generating the mixed wave (P+hz); β = P/(P+hz).
In order to prove this fact, both the ∆ hsM difference and the β ratio have been determined (in percentage).
Thus:
∆ hsM(%) = [hsM - (hsp+hsz)]/ (hsp+hsz)*100 β (%)= P/(P+hz)*100
In the relationship ∆hSM(%)=f(β, P+hz) presented in Fig 4, it can be noticed that the highest values of
∆hSM(%) are encountered in the area of the values β = 55 - 75%, irrespective of the total water inflow amount (P+hz).
Consequently, the difference between the values of the runoff depth in the case of mixed floods (hsM) and the sum of the runoff depths in the case of the “synthetic” floods (hsp+hsz.) is the largest when the share of rainfall (P) in total water inflow (P+hz) varies between 55% and 75%. This means that in the case of such values of the rainfall share its influence upon snowmelt is the largest.
In Fig 4, a decrease of ∆ hsM(%) values as β approaches 100% can also be observed. This means that when there is a significant increase in the rainfall share β, the water inflow coming from snowmelt (hz) decreases considerably.
Finally, values of the rainfall share of β = 100% are reached when hz=0 and the value ∆hSM(%)=0. Because snow does not exist any more, there are no supplementary yields coming from its melting.
∆ hsM(%) also decreases in the region with low values of the β parameter. This means that due to the rainfall share decrease, the accelerating effect of the snowmelt process also decreases. Thus, for values of β=0%
values of ∆hsM=0% will result because factors affecting the snow depth do not exist.
The same relationship points out the fact that in absolute value ∆ hsM does not record significant variations in the conditions of the total water inflow variation (P+hz). This means that rainfall has a certain potential with respect to snowmelt process acceleration to induce a higher variation of the water equivalent in the snow depth.
Peak discharge
The comparative analysis of mixed floods (Qmax M) with the peak discharges of the “synthetic” floods generated only by rainfall (Qmax P) and only by snowmelt (Qmax Z), has proved to be more complex than in the case
of the runoff depth analysis. This is due to many factors, affecting not only the peak discharge value but also the time of its occurrence for the mixed floods. It is worth mentioning some of these factors: quantity and intensity of the rainfall over the snow, snow cover characteristics (depth, density, water equivalent), and others.
In addition to this, the rainfall nucleus and the maximum intensity of the snowmelt are recorded at different moments. Rainfall can be observed at any time during the day, while the maximum intensity of snowmelt is recorded only after the maximum air temperature has been reached. Within this context, the peak discharge recorded for mixed floods, (Qmax M), has not been compared to the sum of the peak discharges corresponding to both “synthetic” floods, (Qmax P+Qmax Z).
Within this analysis, the values Qmax P were first determined from the synthesis relationships, for the same characteristics of the rainfall P existing in the case of the flood wave of mixed origin (amount, intensity, the moment of recording of the nucleus). In the case of the synthetic flood wave from snowmelt a certain discharge, QZ, which corresponded to the time of occurrence of the maximum discharge Qmax P was considered. Thus, Qmax M has been compared to the sum (Qmax P+QZ). Under these conditions, (Qmax M) was smaller than the sum (Qmax P+QZ).
For the comparative analysis, the reducing coefficient of (Qmax M) with respect to the sum (Qmax P+QZ) was defined: ∆Qmax M(%).
100 ) /(
] )
[(
(%)
max max maxmax
= + − + ⋅
∆ Q
MQ
PQ
ZQ
MQ
PQ
ZThis coefficient has been correlated, as in the case of the runoff depth, with the β parameter.
In Fig 5 the relationship ∆Qmax M(%) = f(β P+hz) is shown.
Fig 4: The realationship ∆hSM= f(β, P+hz). Fig 5: The realationship ∆Qmax,M = f(β, P+hz).
It is worth to emphasise the fact that the relationship refers to a more restricted domain of the total water inflow variation (P+hz): between 40 and 60mm. The analysed rainfall intensity has varied between 0.15 and 0.25 mm/min. As the relationship has shown, the biggest attenuation of Qmax M is recorded in the range β = 60%-80%. This can be explained by the fact that at such rainfall values (amounts, intensity), the synthetic flood wave has a much higher discharge than the mixed floods. (It is taken into account that water inflow P + hz = 40 - 60 mm and the rainfall share β > 60%, P > 25 mm). For β > 80%, the values of Qmax P
increase as the rain increases. To a certain degree the values of Qmax M will also increase, being determined
more by precipitation than by snowmelt. The result will be a decrease of ∆Qmax M. For β = 100%, Qmax M = 0, because the discharge will be generated only by rainfall. In the case in which the rainfall share decreases towards zero, a significant decrease of the ∆Qmax M parameter can be noticed. This parameter may have negative values at rainfall shares less than 30-40%. The explanation is that at such rainfall shares, the rainfall amount is quite small, with a little more than 10 mm (in the relationship in Fig 5, the values P+hz = 40 - 60 mm).
At such precipitation amounts and intensities (0.15 - 0.20 mm/min) the discharges are very small. In spite of this, the snowmelt originated discharges, at rainfall shares of 60-70%, and hz>25 mm, are relatively high.
They will condition values of the mixed flood peak discharge to be higher than the sum resulting from discharges of both synthetic hydrographs. Because the rainfall originated discharge will not be taken into account, but the rainfall amount, even small, will accelerate the snowmelt process, causing a higher discharge value.
CONCLUSIONS
The comparative analysis of the main characteristics of rainfall–snowmelt originated floods (depth of runoff and peak discharge) with those of flood waves originated from rainfall or snowmelt only, considering the same generating parameters (water inflow and soil moisture index), shows an increase of the runoff depth and a decrease of the peak discharges of the mixed flood waves.
The results of this analysis are very important for flood forecasting, giving the possibility to quantitatively estimate the characteristics of mixed floods, especially the depth of runoff, in small mountain river basins which can be considered "warning basins" for larger basins in a flood forecasting model (Diaconu and Stanescu, 1976).
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.
Miţă, P., Stancalie, G. (1996) Methods of computing of the variation of the characteristics of the snow cover and runoff generated by snowmelting.. Ed. Centre d'Etudes et de Recherces Eco-Geographiques.
CEREG, Strasburg - France.
Miţă, 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.