Title: River training vs. flood risk in the upper basin,

Author: Adam Łajczak

Citation style: Łajczak Adam. (2007). River training vs. flood risk in the upper Vistula basin, Poland. "Geographia Polonica" (2007, no. 2, s. 79-96).

RIVER TRAINING VS. FLOOD RISK IN THE UPPER VISTULA BASIN, POLAND

ADAM ŁAJCZAK Department of Earth Sciences, University of , Będzińska 60; 41-200 Sosnowiec, Poland E-mail: [email protected]

Abstract: This paper assesses the effect of river training in the 20th century on the evolution of flood risk in the middle and lower courses of certain Polish mountain and upland rivers, and in the lowland Carpathian foreland. The overall anthropogenic impact on the flood risk is a combi- nation of two contradictory trends: (a) the shortening of the floodplain inundation time (between the levees) as a result of the deepening of the trained channel; and (b) the increasing height of the flood water and frequency of flood culminations, a result of the flood wave transformation. The author, in his flood risk analysis, regards the former trend as the more influential. The highest levels of all types of flood risks were found along the valley reaches with unembanked channels that displayed a tendency to reduce both their depth and gradient. This type of reach occurs im- mediately downstream of embanked reaches with a deepened channel. The author also addresses ways to mitigate flood risk levels, taking into account limitations stemming from local land devel- opment and legal conservation status.

Key words: flood risk, flood, upper Vistula basin, river training.

INTRODUCTION modifies, not just the duration and extent of flooding, but also the pace of concentration Flood risk should be understood as the like- and speed of flood wave movement. lihood of economic losses being suffered One of the purposes of river training is within a floodplain as a result of flood- to reduce the flood risk by accelerating the ing by overbank culminations of a moving drainage of a submerged floodplain along flood wave. It depends on the duration of a deliberately shortened channel. River the overbank discharge and the vertical and training projects trigger processes of sys- horizontal extents of flooding within the tematic deepening along the shortened and embankments. Contemporary flood risk is steepened reaches, and shallowing along modified by human activity, and its level is reaches with a less steep gradient (Brookes a combination of the hydrological effects of 1990; Łajczak 1995a). This accelerates the processes of long duration, such as drainage flow velocity and, in embanked rivers, also basin deforestation, agricultural expansion, increases the water level amplitude. Along urbanisation and river training. This activity the reaches with fast deepening channels

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the duration with an overbank water level Vistula and Valleys, and on data sup- may tend to reduce. However, an increased plied by the Institute of Meteorology and concentration of flood waves and increased Water Management. water level amplitude in the upstream reach may increase the flood risk along those less steep reaches further downstream where the STUDY AREA embanked cross-section volumes are low (Punzet 1991; Wyżga 1993; Łajczak 1995a). In Poland there are two areas in which sum- It is possible to control floods on trained mertime or early springtime floods predomi- rivers effectively by means of river dams nate, and turn into catastrophic events every (Punzet 1959, 1973). This is however im- few years (Fig. 1). One covers the drainage possible on rivers utilised for navigation, basins of the mountain tributaries of the since this type of hydrotechnical structure is rivers Vistula and Odra and the foreland absent. courses of those rivers, while the other spans River training has been increasingly per- the Vistula’s and Odra’s upland and lowland ceived as a controversial activity and some- tributaries, including those located in fore- times also as a contradictory one that does land areas. Most of the upper Vistula river not produce the expected reduction in flood system features a predominance of summer risk (Kajak and Okruszko 1990; Andrews rain floods, both annually and in the long- and Burgess 1991; Finlayson 1991; Angel- term. On the River , a typical Western stam and Arnold 1993; Łajczak 2006a, b). Carpathian watercourse, the flood risk is re- A heightened flood risk resulting from river stricted to the summer season, while on the training has only been noted over the last River Nida, the longest upland tributary of 20 years (e.g. Cooper et al. 1987; Howard the upper Vistula, it is restricted to the early 1992; Kajak 1993). This can be seen, in par- springtime, and less often the summertime. ticular, along those reaches that are becom- The foreland course of the upper Vistula has ing shallower. A heightened ground water a hydrological regimen driven primarily by table in the valley, one of the effects of the its mountain tributaries. Currently, the flood channel shallowing downstream of the fast- risk along this course of the river is limited deepening channel reaches (Żelazo 1993), to the summer months, but the early spring further extends the duration and extent of is also added, downstream of the River excessive water content within the flood- confluence (Dynowska 1971; Ziemońska plain beyond the flood embankments. 1973; Punzet 1991). About a century ago, the beginning of hydrological records (on water levels and STUDY OBJECTIVE AND MATERIALS discharges) coincided with the first active and passive measures mitigating flood risk This paper looks at the change in the flood in the Upper Vistula valley. The records can risk in the upper Vistula drainage basin, as now be used to assess the evolution of flood- a result of river training during the 20th cen- risk patterns, whether caused by natural tury. The study focuses on three rivers run- or by human-induced processes. ning through mountains, foothills, uplands The attempts at river training olong the and lowland forelands (the Rivers Raba course of the River Vistula that form the and Nida, and the foreland course of the subject of this study started at the end of the Vistula). The paper is based on the litera- 19th c. and lasted throughout the 20th. They ture, the author’s own research in the upper changed the earlier trends in the channel

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Figure 1. The upper Vistula drainage basin on the map of Poland (A). River network against principal geomorphological units (B). a) limit of the upper River Vistula drainage basin, b) area dominated by summer rain floods, c) main rivers, d) Cracow (K) and Warsaw (W), e) river reaches (Vistula, Raba and Nida) analysed in the paper, f) water gauge at Zawichost (Z) and Puławy (P) measuring the outflow from the upper Vistula drainage basin, g) large dams. Principal geomorphological units in the basin: h) Carpathian Mts., i) Polish Uplands, j) Sub-Carpathian basins.

development of its tributaries, involving both (the Wisłoka, Wisłok and San), which often shallowing and broadening. The morphology feature bedrock channels along their moun- of the Carpathian river channels, which are tain courses in the Beskidy Mountains. The crucial for the supply of water and bedload erection of numerous rubble dams in the and suspended load to the Vistula, exhibit Carpathian Mts. reduced bedload transport regional differences (Klimek 1979). The along the upper courses of mountain riv- westernmost gravel-bedded tributaries down ers and streams, while river dams, the first to and including the River are more of which was built in the 1930s, have been susceptible to change under the influence intercepting all of the bedload and most of of river training than the eastern tributaries the suspended material. As a result, the pace

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of shallowing downstream of the dams has Foothills and the lowland Carpathian fore- been reduced. This has been further assist- land (Fig. 2). The newly constructed river ed by a reduction in the supply of bedload training along the course of the river has led from the channel banks after their rein- to a shorter, narrower and deeper channel. In forcement with stone bands. From the flood- the aftermath of the river training measures risk point of view, the greatest hydrological the channel deepened by more than three me- consequences have been engendered by the tres in its middle and lower course, a typical deepening and narrowing of the channels, value recorded on the lower courses of other a process started in the early 20th c. On the Carpathian tributaries of the River Vistula. River Vistula and the lower courses of its The channel also increased in compactness mountain tributaries this process was mostly through a lowering of the width to average driven by channel shortening (Punzet 1981; depth ratio. This change resulted in an in- Klimek 1987; Wyżga 1993; Łajczak 1995a; creased water flow velocity, particularly dur- Wyżga and Lach 2002). Along the upper ing flood events, and consequently in greater courses of the Carpathian rivers, this devel- concentrations of flood waves and increased opment trend is assisted by a growing affor- speeds of flood wave travel (Wyżga 1993). As estation of their drainage basins, since the a further consequence, the differences be- mid 20th century, and by the trend for field tween the maximum flood water levels record- tracks in the depopulated areas of the Bes- ed during the same events on the lower course kid Niski and Western Bieszczady ranges to of the Raba and on the middle course of the revegetate (Izmaiłow et al. 2003). river have been increasing since the second River training has also included flood em- half of the 20th century. (Fig. 3). The faster bankments erected along the foreland course flow in the deeper channel results in a higher of the Vistula, along the lower courses of its bankfull discharge at the expense of overbank Carpathian tributaries and also locally along discharge. The narrow embankment-to-em- their intra-mountain-basin courses, along the bankment space in the lower course of the riv- lowland tributaries and some of the upland er additionally forces maximum water levels tributaries (Hennig 1991). The insufficient to become ever higher, while shortening the embankment-to-embankment space that re- duration of the flood wave event and increas- sulted has reduced the zone liable to flooding ing the flood wave velocity. Such a flood wave by a factor that is often larger than 10 (on the profile has a considerable impact on the flood River Vistula). Subsequent effects included situation along the River Vistula. higher maximum water levels on the River The Raba is one of those Carpathian riv- Vistula, (up to twice as high along the Cra- ers wherein the assessment of the impact of cow reach), and a faster travelling flood wave river training on the evolution of the flood (Soja and Mrozek 1990; Punzet 1991). risk is far from easy and straightforward. On the one hand, engineering has brought a reduction in flood risks along the trained CHANGES IN FLOOD RISK DURING reaches in terms of flood duration and area THE 20TH C. affected. Indeed, the flood wave travel times and the number of overbank discharge days CARPATHIAN RIVERS USING THE EXAMPLE have been reduced, and the floodplain in- OF THE MIDDLE AND LOWER RABA undation time is down dramatically. On the The River Raba provides an example of other hand, the greater flood wave speeds a gravel-bedded Western Carpathian river may, in certain situations, shorten the draining the Beskidy Mts., the Carpathian time local communities have to mount an

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Figure 2. The River Raba drainage basin. a) main rivers, b) limit of the drainage basin, c) selected water gauges in the middle and lower courses of the river, G- Gdów, P- Proszówki, d) partly or fully trained reach of the river, e) artificial lake Dobczyce (ZD), f) deepening [m] of the Raba channel at selected water gauges following river training in the 20th century, in parenthesis the value related to the second half of the 20th century (according to Wyżga 1993). Geomorphological units in the basin: g) Beskidy Mts., h) Carpathian Foothills, i) Sub-Carpathian basins.

adequate response, and therefore be con- UPPLAND RIVERS EXEMPLIFIED BY THE sidered to increase the flood risk. Addition- MIDDLE AND LOWER NIDA ally, river training has increased flood risks The River Nida is an example of an upland along the untrained reaches downstream, as river with a sandy channel and a low gradient reflected by the increasing maximum flood in its middle and lower courses (0.2–0.5‰). wave levels in the lower course during the Along this stretch, the river meanders with largest events. This trend is observed despite a meander coefficient that reaches 2.0 local- a parallel trend to reduce the volume of the ly. Its floodplain is regularly submerged dur- flood wave, as measured above the bank- ing springtime floods, and less often during full stage. A further flood risk improvement summertime, and the water can stagnate for could be achieved via effective use of the Do- over three months. During the period 1950– bczyce Dam, especially in view of the mostly 1995, the middle course (between Brzegi and positive experience with dams as flood man- Pińczów) was shortened and partly equipped agement instruments on other Carpathian with flood embankments, and extensive wet- tributaries of the Vistula (see: Punzet 1959, lands were also drained within the flood- 1973, 1991). plain, especially along a long anastomosing

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Figure 3. Hydrograph of selected flood waves in the middle and lower River Raba courses between Gdów and Proszówki.

reach near Umianowice (Łajczak 2006b). at Brzegi (Fig. 5). Because of the slow trans- The lower course of the river (downstream port of sand, the river has been observed from Pińczów) has remained untrained, to become shallower downstream around with a natural channel and floodplain Pińczów, initially in the old channel but more (Fig. 4). The river training has been aimed recently (since 1970) also in the new, initially at accelerating the draining of flooded areas deep channel. This process varies in speed, and the drying out of the floodplain. From and depends on the intensity of the upstream the point of view of flood control, the result river training work. The zone subject to the has been mixed. shallowing process is moving, but has not The river has begun to deepen its short- yet reached the next water gauge at Wiślica. ened channel, as recorded by a water gauge The slow downcutting trend along the mouth

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Figure 4. River Nida drainage basin (A) and middle and lower courses of the river (B). a) main rivers, b) limit of the drainage basin, c) water gauges, d) partly trained river reach, e) formerly anastomosing reaches of the river currently partly or fully trained, f) floodplain extent. Geomorphological units in the basin: g) Polish Uplands, h) Sub-Carpathian basins.

reach of the Nida below Wiślica is a result it shrank to less than ten days and increased of a similar trend on the Vistula since the again during the last decade (to 30 days per beginning of its training in the late 19th cen- year). The main cause of the increase in the tury. On the deepened reach of the Nida, the flood risk near Pińczów (the largest town bankfull discharge volume has been grow- on the river) observed since the mid-1990s ing, as recorded by the water gauge at Brzegi. seems to be the continuing shallowing of the During the period 1939–1990, this produced river channel, as a result of misguided train- a trend whereby the number of overbank dis- ing and drainage projects, and only partly charge days was reduced from 10 to less than due to the greater frequency of large flood 2 days. The extension of the floodplain inun- events. The engineering measures have re- dation time observed along this reach since sulted in effective drainage of large marshy 1990 is independent of a gradual deepening areas adjacent to the anastomosing reaches. of the channel, and is caused by the increas- The flood risks near Pińczów could be partly ing frequency of large floods. Along the shal- mitigated by a revitalisation of the drained lowing channel the duration of the flood- wetlands and by returning their functions plain flooding has been growing into long (Łajczak 2006b). In the mouth reach of the periods that are independent of long-term river, downstream from Wiślica, the Vistula river discharge patterns. The inundation backwater effect generates a very high flood time near Pińczów had been growing until risk level, manifested by the long duration 1969 (reaching 80 days per year), after which of floodplain flooding (up to 100 days) and

KKsisiąążżkka1.indba1.indb 8855 22008-06-26008-06-26 110:56:310:56:31 86 Adam Łajczak water levels (C) at water gauge stations in Brzegi, Pińczów and Wiślica. water levels (C) at gauge stations in Brzegi, Pińczów 1 as well the change of overbank IN Figure 5. Minimum annual water levels Hmin during 1939–2003 (A) and change in the bankfull discharge Qbf (B),

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the highest water levels recorded along the of 4 m) and large scale deposition in the entire river. The fluctuation in the Vistula bank zones. Local level differences in the water level in the area reaches up to nine channel cross-section increased three-fold metres, and the backwater effect can go up compared to the early 20th century. Con- to Wiślica when large flood events are taking versely, there were also cases of the channel place on the main river. With its long reten- becoming shallower, i.e. in the Oświęcimska tion of flood water, the Nida river has seen Basin upstream of the and Soła a mixture of flood risk improvement and confluences and over a longer reach where deterioration, depending on the reach, as the river breaks through the Polish Uplands. a result of river training. The only significant The alternating pattern of deepened chan- changes in the risk levels include the flood- nel sections and sections becoming shal- plain inundation duration and territorial ex- lower is therefore a characteristic feature of tent. The river has no significant impact on the foreland River Vistula during the river- flood risks in the Vistula valley. training era (Fig. 6). Another characteristic Uniquely among major tributaries of the of the channel morphology’s impact on the upper Vistula, the Nida has a valley already discharge conditions is the continuing trend subject to several legal conservation statuses for the channel’s width to average depth (including as a Landscape Park, an ecologi- to decrease, as measured at water gauges cal corridor and a Natura 2000 area), which (Łajczak 1995a). As a result of an increased only allows environmentally sound methods speed of flow, caused by modifications to the of flood risk mitigation (Łajczak 2006a, b). cross- and longitudinal sections of the chan- These methods might include revitalisa- nel, the bankfull volumes continue to grow tion of the extensive wetlands, now mostly at various rates, as recorded by the gauges drained, and an expansion of the territory (Fig. 7). The bankfull volumes tend to dif- between the embankments to increase its fer ever more from the average medium-high volume. The earlier plans to erect a dam on discharge, to which they were similar prior the Nida upstream of Brzegi, and indeed to the first river training projects. During any training measures that would introduce the 20th c., this increase in the bankfull dis- large quantities of sand, should be seen as charge varied along the river course studied undesirable. and reached three-fold, depending on the scale of the cross-section change. THE FORELAND COURSE OF THE RIVER The combined average duration of the VISTULA flooding within the embanked floodplain The foreland course of the River Vistula is and the number of flood events vary greatly the most thoroughly investigated part of the along the stretch of river being investigated study area in terms of changes to the flood (Fig. 8). Both of these characteristics express risk caused by river training. The engineer- an increase in flood risk , and display a rela- ing projects, involving an approximately tionship wih the scale of the post-river train- 30% reduction in channel length, new stone ing cross-section evolution. They are highest spurs and bank protection, were designed in the reaches which have become shallower, to prepare the river for its role as the coun- and only slightly lower in the least deepen- try’s main waterway, and to protect adja- ing reaches. Conversely, the lowest values cent areas from flooding. During the 20th are observed along the most deepened chan- century, modifications to the cross-section nel reaches. As an example, the reach of the of the river channel included deepening by Vistula crossing the Polish Uplands gap has an average of two metres (and a maximum a frequency and combined duration of flood

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Figure 6. Change in channel depth following river training in the foreland River Vistula CH.L.CH. and average thickness of deposits in bank zones M.B.A. a) selected water gauges (1—Smolice, 2—Jagodniki, 3—Karsy, 4—Koło, 5—Zawichost). The confluences of the principal Carpathian tributaries are also indicated. Changes in the depth of channel until 1930, 1960 and 1990.

events up to 15 times greater than along the discharge fluctuations and was a result of Cracow reach of the river. channel deepening (Fig. 9). The trend is Since at least 1930, the foreland Vistula proportionally related to the deepening rate has displayed a trend whereby the overbank (Łajczak 1995a, 1999). However during the water levels have been shrinking. Until 1990, period 1931–1990, the duration of the over- this trend was unrelated to the long-term bank water levels along the most deepened

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Figure 7. Change to bankfull discharge Qbf compared to medium high discharge MHQ against minimum annual water levels Hmin at selected water gauges on the foreland River Vistula. Water gauges are numbered as in Fig. 6.

reaches of the channel displayed only a weak level duration after 1990, when a number decline, accompanied by generally low val- of large flood events occurred. During the

ues of the IN1 parameter, up to a maximum period 1931–1990, overbank discharge along of approximately ten days per year. This the most deepened Vistula channels oc- may mean that the decline in this parameter curred on average every second year, but along the most deepened reaches had already prolonged periods of up to ten years without started before 1930. Lack of data prevents such periods were also recorded. Further a similar analysis of the overbank water downstream, along less deepened reaches of

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Figure 8. Differentiation of duration of overbank IN1 water levels and number of flood events

with such water levels per year IN2, along the foreland course of the River Vistula. a) selected water gauges (1—Smolice,2—Jagodniki, 3—Szczucin, 4—Zawichost).

the river, the decline in the duration of flood- bank water levels along the foreland course ing on the floodplain was more marked. of the Vistula from 1931 to 1990 might sug- Downstream from the Dunajec confluence gest that such a trend may have prevailed

the IN1 parameter dropped from approxi- earlier on, i.e. in the late 19th century and mately 10–20 days per year during the years during the first three decades of the 20th 1931–1960 to less than ten days before 1990. century, when the pace of channel deep- Over the period 1931–1990, this section of ening was already advanced, such as near the river experienced overbank water levels Cracow. Downstream, where the deepen- nearly every year, which may mean that the ing process only started after the 1930s, the duration of the flooding of the embanked trend towards a rapid decline in the periods space is shortened more effectively during of floodplain flooding between the embank- the initial period of channel deepening fol- ments was only noted after 1930. Below the lowing river training than at an advanced confluence of the River San the trend to stage of change in the geometry of a trained a decline in the number of days with over- channel. Downstream of the River San bank water levels is not very advanced, and confluence, the Vistula channel is not sub- started only in the 1970–1980s because the ject to intensive deepening, and there are river training efforts along this stretch of the places where it may even be becoming shal- river starting later than elsewhere. lower. Overbank water levels were recorded Another result of the engineering every year of the study period—peaking at projects in the foreland course of the Vistula 50–70 days per annum during the 1931–mid is the restriction of flooding of the embanked 1960s period. Afterwards, the number of floodplain along the considerably deepened days with overbank water levels declined channel (> 2 m) to just the summer season. again to a maximum of 30 days annually in Where the deepening is only minor (< 1 m), the 1980s. or the channel is becoming shallower, this The lack of a clear-cut trend involving part of the floodplain may also be flooded a reduction in the number of days with over- during rapid thaws. Prior to river training,

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the entire course of the river floodplain ex- and Zawichost, and by a half between Smo- perienced flooding during both rapid thaw lice (the confluence of the River ) and floods and summer floods. The foreland- Sierosławice (the confluence of the River course engineering projects effectively Raba), the latter being the most deepened eliminated the early-springtime flood risk channel section. The accelerated move- along the most modified reaches alone. Dur- ment of the flood waves along the deepened ing summertime, floods have only become reaches of the foreland Vistula is consider- shorter as a result of the training measures, ably influenced by the greater concentra- except along the upland gap reach. tion of similarly-induced flood waves on the

Figure 9. Change of duration of overbank water levels IN1 at selected water gauges on the foreland course of the River Vistula during 1931–1990. Water gauges numbered as in Fig. 8.

Another effect of the training of the Vis- lower courses of the Carpathian tributaries tula foreland is an increased concentration (Punzet 1991; Wyżga 1993). The post-river of flood waves, as a result of the accelerated training development of the foreland Vis- flow observed across the entire spectrum of tula channel has produced a trend for the water levels (Punzet 1991; Łajczak 1995a). volume of the flood waves to be reduced as According to Punzet, during the 20th c., the a result of their greater concentration and Cracow reach displayed a trend to a short- higher peaks (Punzet 1991). This could be ened duration and increased height of the partly explained by the effect that the bank- flood wave, as well as to an increase in the full discharge has been growing at the ex- frequency of extraordinary flood peaks. pense of the overbank discharge following A combined effect of these changes has been the advent of the river training projects, an to cause the flood wave travel time to short- effect previously overlooked. The trend for en along the foreland course of the river, in- a declining duration of the overbank water cluding by one-third between Goczałkowice level periods, initiated at various dates in

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the 20th c., has been observed along the • a trend towards an increasing frequen- considerably deepened reaches, and is inde- cy of exceptional flood peaks; pendent of the long-term fluctuations of the • an increasing speed of large flood Vistula discharge (Łajczak 1995a). waves. The erection of flood embankments has reduced the flooding zone up to 50-fold along the foreland course. Events whereby DISCUSSION AND CONCLUSIONS the water tops the new embankments and spills over are very rare, and limited to days As demonstrated by reference to the exam- with extreme water levels. Combined with ple of the trained foreland course of the the causes mentioned above, this dramatic Vistula, the alternating pattern of reaches reduction of the flooding zone has led to featuring varying degrees of cross-section increased water level amplitudes along the modification translates into hydrological course of the Vistula we are examining, effects, especially with regard to flood risk. especially after 1920 (Osuch 1991; Punzet This situation is typical for the entire course 1991). They are currently double the value of the river (Łajczak 1995a). The reach recorded before the embankments, and may downstream of the River San confluence, reach nine metres (Soja and Mrozek 1990). where the deposition is at its most intensive, Just as in the tributaries, assessment of stands out as having the greatest flood risk the foreland course of the Vistula in terms along the river (Jędrysik and Rusak 1982). of the impact of river training on flood risk This should be seen as a consequence of the is complex. Taking into account the duration shallowing and broadening processes, along of the overbank stages and the extent of the the entire channel length, that began at least flooding zone, the duration and area aspects as far back as in the 17th century, driven by of the flood risk were reduced during the 20th the deforestation of the drainage basin. Only century. This is a result of faster flood wave since the 20th century should this be seen as travel, and of the fact that minor summertime a consequence of the river-training triggered floods and all the thaw floods remain within development of the upstream reach (above the bankfull stage. The number of days with the River San confluence). Along the en- overbank discharge was reduced, with the tire foreland Vistula, as well as in the lower exception of reaches without any significant courses of the Carpathian tributaries and in deepening effect or with a shallowing trend. the upland and lowland tributaries, the cur- Just as in the Carpathian rivers, the increased rent flood risk is critically determined by the flood wave speed may, , be regarded as an ad- effects of river training measures. I would ditional risk factor in certain circumstances. therefore confirm Punzet’s conclusion (1991) The third feature of flood risk, i.e. the height that river training as broadly understood, of peak river water levels, demonstrates along the course of the Upper River Vistula a growing flood risk along the entire chan- has a large impact on the process of flood nel, within the embankments only, during the wave formation in the river. 20th century. Only extremely high water level The anthropogenic aspects to the cur- events threaten to spill over and undermine rent evolution of flood risks in the foreland the embankments, which then need mainte- course of the Vistula and in the middle and nance. This is a result of: lower courses of its tributaries involve an • a trend for peak river water levels dur- overlap effect of two contradictory trends: ing subsequent large flood events to increase, (a) the falling risk level in view of the short- and for their durations to shorten; ening of floodplain inundation time and

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territory, and (b) the growing risk from the On the Nida near Pińczów, the inundation increasing height of the flood water. These times were extended more than fourfold dur- two trends are typical over most of the river ing the period 1939–1969. In 1970, the con- course where its channel has been deepened. struction of an artificial channel reduced that The former of the two trends (a) seems to time to just a few days per year, but a back- be more significant for this analysis of flood lash reaction returned it to ten times more. risks. Indeed the benefit of the shortening of While at Pińczów the embankments reduce the inundation time over the unembanked the flooded area to a very narrow strip, just area or the undeveloped embanked flood- two kilometres downstream from the town plain is greater than the losses incurred, they disappear for the rest of the river course because of the rare cases of flooding of in- all the way to its confluence with the Vistula, habited and agriculturally used areas, as exposing a floodplain up to three kilometres a result of breaches in the adequately high wide. The historically unprecedented extreme levees. This can be illustrated by the fore- flood wave recorded during the 1997 summer land course of the Vistula between the con- flood not only topped the embankments along fluences of the Dunajec and Wisłoka rivers, many reaches, but remained high throughout where—after the erection of up to six-metre- the lower course of the river, causing a high high embankments—the floodplain inunda- degree of damage. Along its partly embanked tion time was reduced 10-fold and the area upland gap reach, the Vistula has a broad 20-fold during the period 1930–1990. The floodplain of up to one kilometre. Unlike its coinciding increase in the water level ampli- long upstream reach, this section of the river tude to nine metres was a result of the simul- displayed no reduction of flooding time, but taneous channel deepening and maximum a slow opposite trend after 1930. The num- floodwave level increase (Punzet 1981, 1991; bers of days with flood effects varied across Jędrysik and Rusak 1982; Soja and Mrozek a broad range of 10–70. The currently higher 1990; Hennig 1991; Osuch 1991; Łajczak flood risks observed along this reach are also 1995a, 2006a). The areas beyond the levees partly a result of the heightened culminating remain unprotected from exceptionally high floodwaves developing in the upstream reach floodwaves, despite the vertical extension of with its deepening channel (Łajczak 1995a,b, the embankments after the great floods, and 2006a,b). other maintenance measures. When looking at various flood risk miti- The greatest flood risk levels along the gation options in the foreland course of the river course analysed, in all three respects, Vistula and the middle and lower courses of were recorded in the valley sections coincid- its tributaries it must by assumed that the cur- ing with the channel reaches that displayed rent channel development trends, initiated a shallowing process, lower gradients and or accelerated by the training projects, will only partial or no embankments. This type of continue in the long term (Łajczak 1995a, valley section is typically found at the end of 1999). These options tend to be much more long stretches where the channel has been sig- limited in the valley sections with shallowing nificantly shortened, narrowed and deepened channels than those with deepened channels. (its slope gradient additionally increased) as In the former case the risk could be mitigat- a result of training measures, which normally ed by halting the channel deepening proc- included long uninterrupted embankments ess, or by even less realistic measures, such on both sides of a narrow floodplain. An illus- as a broadening of the distance between the tration of this pattern is found using certain embankments or an increase in their height. water gauges on the Rivers Nida and Vistula. In the latter case, featuring long reaches of

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