Rioja-Logroño PROBLEMS OF THE DYNAMICS OF SOME ROMANIAN RIVER CHANNELS

El presente trabajo pretende discutir la dinamica de algunas secciones transversales de varios rios situados bajo diferentes condiciones de caudal, carga de sedimentos, tamano de los depositos del lecho, tipo de cauce e intervencion antropica. En cortos intervalos de tiempo caracterizados por elevadas crecidas y ciclos estacionales, se ha comprobado una alternancia de agradacion e incision cuya amplitud puede ir mas alla de 1-1,5 m. Para largos intervalos de tiempo (anos) destacamos la alternancia de fenomenos de agradacion y degradacion con una amplitud de hasta 2-3 m. e incluso mas. Evaluamos tambien las condiciones de la estabilidad morfologica de los cauces, obteniendo exponentes de la descarga en una estacion con un relativamente amplio espacio de variacion.


SUMMARY
Our present work is meant to discuss the dynamics in sorne cross-sections of severa!rivers situated in different conditions: discharge and load, grain size channel deposits, slope, channel pattern, anthropic intervention.
For the short intervals of time characteristics for high floods and season cycles, there has been noticed an alternation of "fill" and "scour" on an amplitude which may go beyond 1-1,5 m.
For the longer intervals of time (years) we noticed the alternation of the aggradation and degradation phenomena with a maximal amplitude up to 2-3 m. and even more.

Introduction
Romania's territory has a surface of 237.500 km 2 anda river network with a totallength of more than 60.000 km.This is, almost exhaustively, tributary to the Danube.Sorne specific conditions control the dynamics of these channels, namely: the position of the territory being in temperate-continental dimate (with different nuances), an evident leveling of morphoclimatic conditions from the steppes to the alpin belt, inclusively; the variety of lithological structure; recent crustal movements with a yearly amplitude of about 10 mm.
( + 6 mm./year in the northern-central part of the Eastern Carpathians, and -4 mm/year in Tara Birsei); a complex anthropic influence in morphogenesis (dam-building, r~ver regulation, irrigation and urban needs, etc.).
The flow regime is, with few exceptions, characterized by high floods well marked in every season (DIACONU et al., 1971); about 85% of the yearly flow volume is produced in the mountaineous area, which covers 30% of the country's territory.In the extra-carpathians area there are important drainage basins (sorne having a surface of over 3.000 km 2 ) in which the rivers may dry up at least one time in 2-3 years.In the multi-yearly flow variation one can identify, on the main rivers, a certain cyclic reccurence at about 9-12 years.

Anterior researchs
The channel dynamics of Romanía rivers was the aim of many specialists in reclamations problems and many geographers already for a long time.Thus, in the XVIII-th century, on the occasion of elaborating de'sings for the rivers Dimbovita, Sabar and of the Bega canal, the first thorough examination had beeri made.We won't make a history of such researchs.We can, though, remark the peculiar attention payed in the last 25-30 years when the bases of field experimental researchs had been larged.We shall mention sorne of the contributions that are based on long series of field measurements (URZI-CEANU- ROSCA, 1971ROSCA, , 1973;;DIACONU et al, 1962;DIACONU, 1971;HÍNCU, 1976;BÁTUCÁ, 1978BÁTUCÁ, , 1979;;SEIBULESCU et al., 1970;BON-DAR & PAPADOPOL, 1973;ICHIM & RÁDOANE, 1980).
The investigations made up to now show that, in the complex of conditions presented by Rumania, we are in a re&ion with intense channel proces-ses.Important lateral shifting are mentioned, especially on the big sub-carpathian and extra-carpathian valleys.To identify sorne evolutional tendencies we took under study more cross-sections situated in different physiographical conditions (table 1).Cross-section dynamics in short times (problems of scour and fill processes).
Analysing the exemplified channel cross-sections, we mainly took into account the changing of the talweg elevation as to refference plan considered the zero plan of the rod.
Reworking daily .measurementsmade in periods up to 18 years, we learned that the amplitude of the bed hight variation, in short time, had cometo 1.5-2 m., and sometimes even over 3m.It is an amplitude which is comparable to that which Leopolod at al. (1964) gave for channels with semi-arid conditions.In our case, such fluctuations are the expression of climatic continentalism, with influence on the high flood flow.No matter their genetical signification, there fluctuations introduce wide scatter of the correlation points on the basis of which the regime equations are obtained, a fact that could invalidate, in sorne cases, the attempt to define on this way the hydraulic geometry of a given cross-section (KNIGHTON,) 1977: Radonne et al. 1980).
The mecanism of producing the scour-fill phases, in one and same section, is generally known.But the problems of succesion of such phases alongside the river remains an unanswered questions.The investigations performed up to new give different answers to this problems, answers that are often con-tradictory: LANE & BORLAND (1954), LEOPOLD et al. (1964), COLBY (1964), ANDREWS (1979).In Romanía, URZICEANU-ROSCA (1971) noticed that with the flood of may-june, 1970 "in channels, heavy erosion processes were produced on the whole length of the river" (p.194).At the same time, the total volume of the alluvial deposits brought by the Dan u be into the Black Sea was much more reduced that the volume resulted from river-section scour, without adding the volume of the slope sediments.Thus the following question arises: where were the great quantities of alluvial deposits resulted from river-section scour accumulated?As to such phenomena noticed in other parts, different opinions have been expressed.We think that in short time (minutes,.hours,days or seasons) the channel dynamics, at least on certain reachs, represents a succession of situations between scour-fill: scour is dominant during high flood, and fill during lower flow.W e do not consider this observations as a fully satisfactory answer.The cause of not exactly knowing the mentioned phenomena is due to the lack of detailed researchs made in long period of time on the longitudinal profile.

Cross-section dynamics in long time (problems of aggradation and degradation processes)
To appreciate long time channel dynamics (MACKIN, 1948, p. 471), refers to "periods of many years") we took into account the defining of the grade condition for sorne Romanian rivers.To this purpose we analysed: The daily data regarding channel bed elevation and discharge by means of filtering method.Thus, we obtained 30 days and 6 month-moving means which may graphically be represented by means of sinusoidal curves.This phenomenon is common both for natural and for man-made cross-sections (fig. 1, 2, 3).
The balance of scour and fill processes determined for 600 time cross sections of the Birlad river channel.The obtained results permitted thes following conclusions: In •[ong time, with the same cross section, a river channel bed presents an altenation of degradation cycles with aggradation cycles.This reality is clearly expressed by calculating the scour-and-fill balance in a long period of time.In table 2, we exemplify a model of balance calculation, in which we chose three moments of the Birlad river channel dynamics in the period 1969-1978: negative balance, zero and positive.The cycle length was, on an average, of 11 years: between 1958 and 1969 there was a degradation cycle, and between 1969 and 1979 there was an aggradation cycle.

The long term fluctuations of channel bed dynamics almost truely expresses the flow fluctuations, in which the cycles of high flow approximately coincide with the aggradation cycles and the cycles of low flow with degradation
cycles.This is one more proof that the cyclicity of natural phenomena, often mentioned in scientific literature (DIACONU, 1973;ROVENTA & NICO-LAE, 1975) also has repercussion long term channel dynamics.We are not intended to have a special discussion on the mechanism of cycle producing, their spacial development, with the part played by load, and others.We only wont to show that the three statements may offer a real image on the manifestation of the grade condition for sorne Romanian rivers.This is shown by the succession of the oscillation imposed by de gradation-aggradation, with an amplitude, for the analysed cases, of 2,5-3 m., which include the oscillations imposed by scour and fill.Of course, introducing these elements into the morphometric relations to calcula te channel dimensioning and regulation, may offer new reference points ofinterpretation of the evolutional tendencies on long time, aspect we shalllater recall into attention.1962196419661968197019721974'f!l6 1978 FIG. FIG. 3 Streambed dynamics of Moldova river at Tupilati cross section.

The problem of morphological channel stability
In the conditions of heavy dynamics in Romanian channels, for arrangements of hydrotechnic constructions, canals, irrigations, it is necessary to identify the conditions of morphological stability.The literature of channel engineering is concerned to establish the theoretical morphometric relations which should, as truely as possible, check channel behavior on long periods.
To this, one proceeded to statistically remake the hydrometric data collected from channel sections which showed no important modifications along many years (1971)(1972)(1973)(1974)(1975)(1976)(1977)(1978)(1979).Morphometric relation of the regime equation type have been worked out (LEOPOLD & MADDOCK, 1953), of semiempiric equation type (VELIKANOV, 1958), and morphometric relations that explain the hydraulic resistancy of morphologically stable channels.These morphometric relations were established both for the physiographical conditions characterizing the drainage basins of Up per Mure § (Tirnava Mica, Tirnava Mare and Mure § rivers) and for the generalized form minding the hydrometric data of other Romanian streams (Jiu, Sorne §, Birlad, Lower Mure §) and, comparatively for a number of 18 canals from the Sarday amuna-Ganga irrigation systems of India, the la test published by G UPT A et al. 1966(cf. BÁTUCÁ, 1979).

Backwater curve influence of the reservoir on the channel stability
In the wiew ofwholly arranging Romania's rivers (until1990) when every discharge will be controled through dams, we draw attention on a case of the influence of backwater curve of reservoirs on chann<Yl stability.The hydraulic geometry of the Bistrita river, 10 km upstream of the Izvoru Muntelui Reservoir, is defined by relations: FIG. 2Streambed dynamics of Jijia river at Dorohoi cross-section.
notations signify: g = the acceleration of gravity; J = hydraulic slope, and d = d 50 = the 50% diameter of alluvial particles from the channelbed.Morphometric equations of channel hydraulic resistency.To express in an analytical form the hydraulic resistency of channels, through a statistical rework of hidrometric data, we established the following morphometric equation types: n = 0.030 d 50 lió V m = 1 .30 hm 314 d(mm) for channels having beds of alluvial materials with d 50 = 0.2-0.6 mm; for channels having beds of alluvial materials with d 50 = 0.6-2 mm; characteristic for the rivers Tirnava Mica~ Tírnava Mare and Mure §, from the Upper Mure § drainage basin, in which the suplimentary simbols signify: n = the Manning rugosity coefficient J = slope in %0, s, = alluvial material density and water density respectively.

TABLE 1
Sorne features on cross sections analysed.