Nevenka O`ani}, Public Water Management Enterprise

51000 Rijeka, G. Ciotta 17b, CROATIA

Josip Rubini}, Public Water Enterprise of Istria Catchment Area

52000 Labin, Zelenice 18, CROATIA

ANALYSIS OF HIDROLOGICAL CHARACTERISTIC OF KRI@ POTOK - A SMALL STREAMFLOW IN THE MOUNTAIN PART OF CROATIA

ABSTRACT

Hydrological characteristics of Kri` potok, a small streamflow in the mountain part of Croatia were considered. At the present time its energetic exploitation transfers a part of its water budget from the Danube to the Adriatic basin. Because reassignment of water is planned, this paper represents basic results of implemented hydrological analysis for the evaluation of water budget of planned accumulation.

The purpose of this paper was to represent primarily the parameters of high waters, for a small mountain catchment area with about 5,4 km2. Those parameters could be compared with same parameters of similar catchment areas on a wider area of the Danube basin and also surrounding catchment areas.

Simple methodological procedures described in present paper could be performed for other catchment also. The results of those analyses could be compared. That could be a basement for the regionalization of catchment areas on mountain part of Danube basin.

Key words:small mountain catchment area, water budget, snow cover, flood waves

1.0INTRODUCTION

The Kri` potok catchment with 5,4 km2 of area is located in the mountain part of the Croatian Danube basin in the vicinity with the catchment boundary of Adriatic basin. Kri` potok catchment area extends between 745 and 950 meters over sea level. Geologically it belongs to the Paleozoic and Triassic deposits. This area is low populated and it is well covered with forest vegetation. Figure 1 represents the location of the Kri` potok catchment area.

Average annual rainfall of Kri` potok catchment is 2705 mm, and the average annual temperature is 7,1 0C. Analyzed area is characterized with very intensive short rainfalls - especially those between few and 24 hours. Therefore, the extreme maximal daily rainfall could reach even 300 mm.

The snow cover has a special influence on the outflow regime of the Kri` potok catchment area. Average annual number of days with the snow cover on the local Lokve meteorological station is 97 (minimum 49 and maximum 122). The average annual height of snow cover is 79 cm. The maximal quantity of the retentioned water ever recorded (336 mm/m2) was at the height of the snow cover of 105 cm and its density of 320 kg/m3. Sudden episodes of snow thawing are connected as a rule with the occurrences of large rain. The quantities of water released by sudden snow thawing could be considerable. Because of slow melting and short time of the concentration of flood wave, such melting is not the cause of the extreme discharges. All maximal annual discharges were results of higher rainfall intensities. The quantities of discharge water from melted snow was not so significance.

Recordings of discharge were conducted at the pumping reservoir Kri` since 1962. At that point, water is transferring from the Danube basin to the Adriatic one. An overflow from the pumping reservoir is exceptional and it is happened only during flood waves. The balance of low-flow is followed over pumping regime and therefore hydrological data about water budget registration could be considered as confident.

Fig. 1:Location maps

Average annual discharge in 1963. - 1992 period is 0,313 m3/s. Monthly distribution analysis reveals November (0,513 m3/s) and December (0,511 m3/s) as months with maximal, and July (0,078 m3/s) as a month with minimal values.

2.0FLOOD WATER APPEARANCE ANALYSIS

Statistical analysis of probability of maximal discharge appearance reveals 21,5 m3/s; 24,6 m3/s, 28,6 m3/s and 31,7 m3/s for a return period of 10, 20, 50 and 100 years respectively. More complex goals are the determination of flood wave shape. Determination could be done in different ways. The simplest approach consists of application of some usual shape for particular area with the reduction of flood wave ordinates according to the ratio between the amounts of maximal discharge calculation. Less frequent approach is to conduct complex analysis of probability of characteristic flood wave shape appearance according to measured data.

The short and simple engineers’ analysis was performed at the Kri` potok profile. The goal of this analysis was to determine the most probable maximal flood wave shape. According to the 30-year period data there were 40 flood waves with peak discharge. Pumping of about 2,5 m3/s is a basic value after which overflow became, and it could also be considered as a base-flow of analyzed hydrographs. Therefore the analysis of flood wave shape was performed only for overflow amounts. Defining of the beginning and end time of overflow was performed for each flood wave. Hour and peak values of overflow drainage were also defined. Calculations of further parameters are performed from that analysis:

Tp- the rising time of flood wave (hour)

To- the recession time of flood wave (hour)

T - total duration of flood wave (hour)

Qmax- maximal discharge of flood wave (m3/s)

V - volume of a flood wave (m3)

- coefficient of flood wave shape which is calculated by equation:

g = V / (Qmax * T)

To determine mutual influence of each parameter during flood wave appearance, their regression analysis was performed. The stage of their correlative connection (“K”) was also determined. It was established that:

- volumes of flood waves and their maximal discharges are not heavily connected, but there is a sign of their connection (K=0,60)

- flood wave rising times and maximal discharges are not connected (K=0,19)

- total flood wave duration time and maximal discharges are not connected (K=0,13)

- flood waves rising and recession times are not so closely connected but there is a sign of their connection (K=0,46)

- water wave elevation times and theirs volumes are mutually dependent (K=0,86)

- total flood wave duration times and theirs volumes are mutually dependent (K=0,80)

- maximal discharges and theirs flood wave shape coefficients are not heavily connected, but there is a sign of their inverse dependence (K=-0,53)

- flood wave volumes and coefficients of their shape () are not related (K=-0,14)

- flood wave rising times and coefficients of their shape are not connected (K=-0,02)

Functional connections between certain parameters of flood waves are not strongly connected. Therefore, useful flood wave shape of maximal discharge for Kri` potok is determined according to the hypothetical unit shape made from average values of analyzed parameters (Table 1).

parameter / Qmax
(m3/s) / volume
(1000 m3) /  / Tp
(hour) / To
(hour) / T
(hour)
Average / 10,80 / 253,5 / 1181 / 6,4 / 15,4 / 21,8
St. dev. / 12,81 / 209,1 / 370,5 / 7,1 / 9,1 / 12,8
Cv / 0,59 / 0,82 / 0,31 / 1,12 / 0,59 / 0,59
Max. / 61 / 1222,1 / 2007 / 36 / 50 / 61
Min. / 6 / 64,8 / 460 / 1 / 5 / 6

Table 1.Basic results of the analysis of flood wave parameters (1963. -1992.)

Fig. 2:Mutual influence of maximal discharges and coefficients of flood wave shape from 1963. - 1992.

According to the values of flood wave parameters which were calculated in such way, one flood wave shows the closest value of flood wave shape coefficient ( = 1135) to the average one (= 1181). Other parameters of that flood wave were also close to other average values. The determination of the average flood wave was performed in that way that on real hydrograph secondary peak of a flood wave was reduced. Such adjustment permits its inclusion within calculated average values of the analyzed flood waves. Figure 3a presents flood wave of December, 7, 1976 and the flattened average flood wave shape. Figure 3b presents calculated flood waves for return periods of 5, 20 and 100 years.

a)

b)

Fig.3a:Flood wave of 07.12.1976. and flattened flood wave

3b:Calculated flood waves for return periods of 5, 20 and 100 years

3.0CONCLUSIONS

Irregular evaluation of high-water appearance, even on small catchments could raise some big problems. Such hydrological problems are frequently disregarded in everyday engineer practice. Even simple hydrological analysis performed for practical engineer assignment (as described in present paper) can be a valuable source of data. Such data could be a basement for obtaining a wider fund of information useful for further regional analysis of high-water appearance on small mountain catchment areas.