P. Mita, C. Corbus & S. Matreata
National Institute of Hydrology and Water Management, Bucuresti - Ploiesti 97, 013686 Bucharest, Romania.
Corresponding author: Pompiliu Mita, email: pompiliu.mita@hidro.ro
ABSTRACT
The main objective of this paper was the elaboration of a flood forecast method on rivers with surfaces larger than 500-600 km2 by using data from representative basins (R.B.). It is considered that the existence of such basins within certain larger river basins allows a faster and more precise elaboration of flood forecasts. The approach relies on the fast determination in the R.B. of the main elements of the flood wave (starting moment, increase time, maximum discharge, depth of runoff) based on relations obtained between these elements and precipitation characteristics. For the determination of the flood wave elements in the ungauged small basins, the synthesis relations of these elements as a function of event rainfall and basin characteristics are used. For large rivers, the knowledge of the flood wave elements in small rivers that form in the end the large river basins is very useful.
Keywords : representative basins, flood analysis, hydrological modelling
Introduction
Major floods occurred in the last years on the Romanian rivers and many institutions in the country have been involved in the development of mitigation strategies. The main goal of this study was to find some mitigation methods for the flooding effects. The proposed flood forecast method is part of this effort.
The elaboration of a flood forecast method for big rivers, with areas larger than 500 - 600 km2, uses the data from representative basins (R.B.), which are small basins, covering less than 100 km2. The R.B. are used as warning basins.
The paper has the following goals:
• Establish a methodology for the determination of the flood wave elements from hydrometrical stations.
• Elaborate synthesis relations for the determination of the flood wave elements for ungauged river basins.
• Establish a method for the determination of the flood wave hydrograph in sub-basins and their aggregation and routing in the forecast points in the main rivers.
It is considered that the existence of such warning basins within certain big rivers basins enables a faster and more precise elaboration of flood forecasts. The fast determination in the R.B. of the main elements of the flood wave (starting moment, increase time, maximum discharge, runoff depth) based on relations obtained between these elements and precipitation characteristics is taken into consideration. But the authors are aware that the flood forecast elaborated for small rivers is not sufficient alone due to the very quick reaction of small basins to the precipitation factor, most of the times in a few hours. In these conditions, even when information is rapidly transmitted, the possibility to take the necessary measures is minimal. However, for large rivers, the knowledge in advance of the flood wave elements on small rivers that make up large river basins is very useful.
Methodology
The calculation methodology of flood forecasts using data from representative basins relies on 6 steps, which will be presented here below with an example of the Crisul Alb river basin.
Topological modelling: In the first step a schematic representation of the way in which flow into a river basin is formed, called topological modelling of the basin, is realized. This modelling takes into account the fact, that a successive integration process on the slope and riverbed forms the runoff. The topological modelling of river basins supposes the division of the basin and hydrographic network into homogeneous units, considering certain criteria. The river basin will be divided into homogeneous areas (sub-basins) from the point of view of the variability of factors that condition runoff: topography, vegetation, soils and geology (Fig. 1).
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Fig. 1: Topological modelling of the Crisul Alb river basin.
The hydrographic network will have discrete sectors, taking into account the following criteria: the homogeneity of hydraulic and morphometric characteristics of the low-flow channel and the flood channel;
runoff type (one-dimensional or two-dimensional); the degree of stability of the riverbed (Fig. 2).
1. R. Crisul Alb - conf. Valea Laptelui
4. R.B. Valea Satului - Criscior
6. R.B. Criscior - Baia de Cris
7. R.Ralita - conf. Crisul Alb
8. R.Baldovin - conf. Crisul Alb
10. R.Obarsia - conf. Crisul Alb
12. R.Halmagel - conf. Crisul Alb
16. R.B. Obarsia - av. Sighisoara
13. R.V. cu Lazuri - conf. Crisul Alb
14. R.Tacasel - conf. Crisul Alb
17. R.Zimbru - conf. Crisul Alb
19. R.Sebis - conf. Crisul Alb 2. R.Valea Satului - conf. Crisul Alb
3. R.Sipot - conf. Crisul Alb
5. R.Luncoi - conf. Crisul Alb
11. R.B. Baia de Cris - av. Obarsia
20. R.B. Gurahont - Bocsig 9. R.Vata - conf. Crisul Alb
15. R.Sighisoara - conf. Crisul Alb
18. R.Chisindia - conf. Crisul Alb F = 105 km2
F = 45 km2
Fig. 2: Flood routing scheme for the Crisul Alb river, reach spring - Bocsig (Topological modelling of the Crisul Alb hydrographic network).
The topological modelling of river basins and of their hydrographic network also takes into account the following elements: quality and quantity of available data, the purpose of the modelling and required accuracy, type and importance of the reservoirs that influence runoff.
Morphohydrographical characterisation: The second step is the determination of morphohydrographical characteristics (area, length, mean altitude, basin slope, river slope, etc) and the characteristics of the environment (soil, relief, vegetation) for all sub-basins. The example on Fig. 3 is for R.B. Moneasa. These characteristics were also determined for all R.B.
R.B.
R.B. MoneasaMoneasa Physical map Vegetation map
Fig. 3: The determination of morphohydrographical characteristics for all sub-basins.
The purpose of the determination of these characteristics for all small river basins was to transmit (extrapolate) data (flood wave elements) from the R.B. to the other river basins, which have similar runoff conditions. When these basins have different characteristics than the R.B. from the respective river basin, data from the R.B. from another river basins will be used.
Relation between flood wave elements and precipitation characteristics: The relation between the flood wave elements (maximum discharge Qmax; the increase duration/time tcr; decrease duration/time td) and precipitation characteristics (quantity, duration, intensity) obtained at hydrometric stations (the third step) are presented for certain hydrometric stations within the Moneasa representative basin (Crisul Alb basin).
Fig. 4a shows the relation between maximum discharge and precipitation characteristics for the Ranusa hydrometric station: precipitation intensity ip (mm/min), total rainfall duration Tp (min), the antecedent precipitation recorded 10 days before (computed with the API model, API10 in mm). For this hydrometric station the following values were determined: Qmax = 69 m3/s when ip = 1.2 mm/min, Tp = 90 min and API10
= 20 mm.
Fig. 5a shows the relation between the increase duration tcr (min) and total duration of the rainfall Tp (min) for the sub-basins of the Moneasa R.B.