Morphometry of Lake Bachotek with respect to

Limnological Review 4 (2004) 241–248

Morphometry of Lake Bachotek with respect to changes in its water table positions Rajmund Skowron*, Ryszard DoroŜyński** * Hydrology and Water Economy Department, Nicholas Copernicus University, Institute of Geography, Fredry 6/8, 87–100 Toruń ** Remote Sensing and Cartography Laboratory, Nicholas Copernicus University, Institute of Geography, Gagarina 5, 87–100 Toruń

Abstract: Basing on the analysis of aerial pictures and bathymetric plans of the lake the authors have established the relation between the changes in its water table position and the morphometry of its basin. In order to substantiate these observations, they have determined the position of the water table above sea level and made a new bathymetric plan of the lake using the available cartographic materials and aerial pictures (2003). The range of absolute water levels oscillation in the lake in 1961– 2003 was 186 cm (Skowron, 2002). Building a weir on the lake outflow caused a decrease in the annual amplitude of water levels on average to 39 cm, which corresponds to a change in surface area by 2.7 ha and in volume by c. 2.5 %. Key words: water levels oscillation, changes in surface and in volume.

Introduction In recent years in the Polish hydrological literature bolder and bolder attempts at defining the changes in lake water storage can be observed. This fact results from an increasing interest in water containers as potential sources of drinking water supplies and as recreation and rest sites, being at the same time important components of the landscape and remaining under strong antropopressure. Lake water reserves in Poland are estimated in a very wide range from 4–30 km3, with 26 biggest lakes gathering as much as 30 % of overall reserves (Choiński, 1995). Chełmicki (2001) raises an interesting issue of determining the variability of lake exploitation reserves, understood as variability of the volume of water contained in the water table oscillation zone in containers. The issue has usually been treated only marginally, while elaborating on other topics. A comparative analysis of bathymetric plans from the turn of XIXth and XXth c. and from the 60s of XXth c. has shown that the rate of surface

decrease amounts to several percent, whereas the decrease in water volume is considerably bigger and reaches from a dozen or so to several dozen per cent. Choiński (2002) believes that lake disappearance should be considered not only in terms of its surface but also and most importantly of its water volume. The lowering of water level in Lake Gopło in the course of the last 230 years by 3.2 m is one of the numerous well documented examples. It has caused 3.5-fold change in the lake volume, whereas its surface has changed from 11.244 do 2.154.5 ha, which is more than fivefold. (DoroŜyński, Skowron, 2002). Another example of change concerns lake Bachotek, discussed in the present article. Skowron (2002) observes that raising its water by less than 0.5 m causes an increase in its surface by 8 ha and in its volume by 5.6 %. Drwal (2003) in his turn has calculated and published the changes in surface and in volume for Lake DruŜno at its various water table levels. Raising the position of its water table by as little as 0.3 m caused an increase in its surface by as much as 1.720 ha and in

242

Rajmund Skowron, Ryszard DoroŜyński

its volume only by 30 %. Another example is given by Pazur and Starkel (1989) for lake GościąŜ located in Płocka Valley. Undoubtedly water retention changes are very diversified in the Polish conditions as far as the annual amplitude is concerned. An example of such a case is given by Borowiak (2000) for 48 lakes located on the Polish Lowland. Although the method of determining the changes in volume by means of volume variability coefficient (VVC) does not undermine the surface changes in those containers, it nevertheless shows significant differences between them. The highest coefficient values are characteristic for shallow lakes characterised by large surfaces (Gardno – 104.6, Bukowo – 76.1 and Łebsko – 73.4), for which the mean annual amplitude of water levels is contained within the limits of 75 – 100 cm. The lowest values of the coefficient, on the other hand, refer to deep lakes (Wigry – 1.8, Hańcza – 2.2 and Osiek – 2.3). The more useful it seems, therefore, to determine the changes in lake volume triggered by water table annual oscillations. Not only do their values characterise lake’s hydrological activity, but also indicate changes in the particular water balance components and a possibility of water storage at the time of spring surges. Lakes, being natural regulators of surface water reserves, are characterised by seasonal, annual and long-term variability of the waters stored in them. The extent to which these changes do actually occur depends on numerous factors, among which the character of the reception basin (the amount of water and the hydrological regime of the river which most often regulates the outflow from the lake) as well as the climatic conditions play the most significant role. Water table level in lakes systematically undergoes oscillations, which can be of seasonal, annual, many-year or historical character. These changes result in the first place from lack of equilibrium between the input and output components of water balance. It must be strongly emphasized here that water levels oscillations in lakes result from the stimulation of the particular natural environment components, both in the lake medium and in its immediate maintenance base. Observations of water levels become most often source material providing basic information on the hydrological processes taking place in the whole lake system An indubitable (Lange,influence 1986). on shaping the changes taking place in the lake environment has been

wielded by man, especially in the course of the last millennium. Irrigation and melioration works have played a special role in this processes, and they were conducted at a large scale in XVIIIth and XIXth c. (Kaniecki, 1997). Lakes as an important link in the water circulation chain in the reception basin play an important role regulating this circulation. Hence the diversity of hydrological functions they fulfil and their individual character. The hydrological functions lakes fulfil in the system of dehydration are characterised by a marked diversity and uniqueness (Drwal, 1982). That character is best described by water regime, the most remarkable expression of which is the rhythm of water table level changeability. The character of the changeability of the annual water table distribution in Polish lakes has shown that its most commonly occurring type is the 2-period type with one spring surge (April – first half of May) and one low at the turn of September and October (Borowiak, 2000). The 3-period type, in which two periods of high water levels – one in December (a rise caused by falls) and the other in April (a rise caused by melting) – are divided by two lows, with the summer-fall low being markedly deeper, is characterised The following by a considerably indicators describing smaller frequency. water level changeability, volume changeability and water exchange in lakes should be mentioned: mean annual amplitude of water levels, lake retention index, vertical and horizontal exchange coefficients and volume change index (Borowiak, 2000). The majority of those parameters are commonly used and do not require discussion except for lake retention index (ILR) and volume change index (IV). Lake retention index expresses the proportion of the lake mean volume to the total surface area of the reception basin, whereas the volume change index expresses the proportional participation of the volume of the active layer of lake water, as determined by the range of water table oscillations (V∆H) in proportion to the lake mean volume (V).

Material and methods The basic material used in this study were topographic maps at a scale of 1: 10,000 and aerial photographs from the years 1969 and 1996. Besides, the bathymetric plan of the lake from 1958, elaborated on by IIF (Inland Institute of Fishing),

Morphometry of Lake Bachotek with respect to changes in its water table positions

as well as the results of depth plumbing performed by the present authors in November 2003 were exploited to analyse the lie of the lake. Black an white aerial photographs from May 26, 1969 at a scale of 1: 16,000 were used to make a map of the lake at a scale of 1:10,000. The shoreline contour was elaborated on using the optical processor LUC. The surface area was measured with a polar planimeter, and the length of the shoreline – with a stadiometer . Using the aerial photographs from August 12, 1996, which are coloured photographs at a scale of 1:26,000, a vectorial version of the map was made at a scale of 1: 10,000, on which the reach of rooted and floating vegetation belts in the littoral zone was determined. For this purpose a stereoplanigraph A7 with a digital result registration option was used, and next using an option of the GeoSet programme, lake’s basic morphometric values were defined.

243

Topographic maps at a scale of 1: 10.000 (configuration from 1965) were used only to establish the geometrical matrix in order to co-ordinate the aerial photographs. Basing on the lake’s bathymetric plan from 1958 the basic morphometric and bathymetric values, vertical volume distribution and the indicators of lake basin geometry were defined. Depth soundings of the lake were performed using the echo sounder Simrad IS-15 from a boat along the transverse profiles, whereas the location of points was determined using the GPS set Garmin III plus (Fig. 1). Altogether c. 1000 soundings were performed, which provided basis for making a bathymetric plan and calculating the basic morphometric and bathymetric parameters of the lake. The bathymetric plan prepared in a digital form using the program Surfer 7.

Fig. 1. Situation drawning of Lake Bachotek together with its shorline (aerial pictures, August 1996) and pistions of the points of bathy metric sounding (14 – 15 November 2003) with fragments (A and B) of the most considerable surface area changes of the lake

244


Taking into consideration a non-uniform character of the source materials used in this study, being a complex result of application of various methods and measurement instruments, the comparability of the final results in spite of the high degree of their coincidence should be regarded with a proper reserve.

Results Lake Bachotek is a postglacial container located in the Brodnickie Lake District. It is a flow container constituting the last link in the system of lakes dehydrated by the river Skarlanka (a tributary of the Drwęca). The surface tributary (the river Skarlanka) and an undergorungd tributary have the biggest participation in its water balance. A relatively small value of the water exchange index (α = 2.60) with the total water exchange taking place every 140 days, as well as a big water levels amplitude (102 cm) and big depth mean (Dav. = 7.2 m), makes it possible to include the lake in the group of passive containers as far as its hydrological system concerned. In the water levels distribution of Lake Bachotek in the many-year period of 1961–2003 two periods can be differentiated: the first one till August 1995 and the other one following that date. The moment of water raising with a weir on the Tama Brodzka, placed immediately on the outflow of the Skarlanka from the lake, is the division line for this distribution. Water levels in the period 1961–1995 were conditioned by the natural features of the Skarlanka’s reception basin and by the inflows of the River Drwęca (Skowron, 2002). The lowest water levels were noted from July to September and the highest most often from January to May. The highest water level was noted in January 1982 – 339 cm, and the lowest – 153 cm – in September 1965, which gave the absolute amplitude of 186 cm, with the aaverage being 102 cm. The average annual water level in the lake remained at 71.28 m above sea level and corresponded to the water level on the water glass of 233 cm (Tab. 1, Fig. 2). In order to eliminate negative consequences of water table oscillations, which lead to a baring of a wide belt of the littoral for a period of 4 – 5 months and also to unfavourable habitat changes, in August 1995 a weir was built on the outflow of

the lake in Tama Brodzka. The raising of waters on the lake on average by 48 cm caused a decrease of the water level annual amplitude from 102 do 39, thus stabilising the mean annual water level at 281 cm (71.76 m above sea level). Table 1. Surfaces and volumes of Lake Bachotek in relation to the level of water table position No. 1 2 3 4 5 6 7 8 9

Water level above sea level 70.48 70.90 71.09 71.28 71.55 71.76 71.85 71.94 72.34

Water states in cm 153 195 214 233 260 281 290 299 339

Area in ha 207.5 210.5 211.8 213.2 215.1 216.6 217.2 217.8 220.7

Volume in thousands of m3 14.516,4 15.394,2 15.795,4 16.199,2 16.777,4 17.230,7 17.425,9 17.621,9 18.498,8

Explanations: 1. The lowest water level in the lake registered in September 1965, 2. Water level assumed for the bathymetric plan acc. to IIF (1958), 3. Water level at the time of making an aerial picture in May 1969, 4. Mean water level in the lake before water raising (1961 – 1995), 5. Water level assumed when sounding the lake in November 2003, 6. Mean water level in the lake after water raising (1996 – 2003), 7. Water level at the time of taking an aerial picture in August 1996, 8. The highest water level in the lake after water raising (March 2002), 9. The highest water level in the lake registered in January 1982

Fig. 2. Course of typical (annual) water levels in Lake Bachotek in the period 1961–2003: 1 – highest levels (HW), 2 – mean levels (MW), 3 – lowest levels (LW)

The consequence of those changes was a permanent flooding of the part of the littoral bared during late summer and of the lowest situated territories, found mostly in the southern part of the lake (Fig. 3). Raising water table level caused in the first place changes in the course of the shoreline, and an increase in the lake surface area and in The its volume. changes in the morphometry of Lake Bachotek can be analysed along two lines. The first one is the course of the shoreline, the other – the changes in the lie of the bottom, which may, however, have resulted from a low precision of the earlier measurements and from the water raising.


The changes in the course of the shoreline occour most clearly in the southern part of the lake. The shoreline in some fragments has moved even by 130 m, thus increasing the surface by c. 8 ha. A remarkable element of water raisng is the fooding of meadows, which were so far submerged only at highest water levels in the period of spring. (Fig. 3).

Fig. 3. Situational drawing of a northern (A) and Southern (B) fragment of Lake Bachotek (based on aerial pictures from May 1969 and August 1996): 1 – shoreline, 2 – reach of rootated vegetation, 3 – reach of floating vegetation, 4 – routes, 5 – direction of the river Skarlanka’a flow

A comparison of morphometric descriptions of the lake at extreme water table positions for absolute oscillations – 186 cm, pointed to a high hydrological activity of the container, which resulted in baring, at low water levels (September – August) of a wide belt (25–30 m) of the littoral. In the period of occurrence of the highest levels, in turn, (May – April) the lowest trerritories – meadows – were flooded for a period of 4–5 months. Basing on the aerial pictures from the years 1969 and 1996, differences in surface areas in relation to the registered water levels were defined. Making use of the relation between water level growth and the lake surface area, the values

245

of the surface areas corresponding to the typical water levels were calculated (Tab. 1). The adequacy of the above assumptions is confirmed by little variability in the mean lake bottom inclination within the range of littoral zone and in the waterside belt above the water table (α = 0o 11’). The data exposed in Table 1 indicate that after building a weir the lake surface in the annual cycle changes from 207.5 ha to 220.7 ha, which corresponds to the absolute water levels amplitude – 186 cm (change in surface by 6.2 %). Lake surface at the average water level amounted to 213.2 ha (233 cm) before the water raising, whereas after it – to 216.6 ha (281 cm). Stabilisation of the average water levels at 281 cm has decreased the range of water levels oscillations to 39 cm. Such an amplitude causes changes in surface by 2.7 ha (1.2 %). The bathymetric plans of Lake Bachotek, which were used in this study, are separated from each other by the period of 45 years. In both soundings different measuring equipment and different methods of analysing the data were used. In spite of the differences in results, the material obtained constitutes a good basis for making the comparative The systems analysis. of isobaths in both plans are slightly different. The most visible differences occur in the shallow areas of the lake, which were formed after the water raising. A distinct change may be observed especially in the course of the 10 m isobath, which on the IIF plan from 1958 has the insular character in the central part of the lake. Moreover, a considerably larger surface area is demarcated by 12.5 m isobath (Fig. 4). As follows from Table 2, the most significant differences occur, however, in lake volume, which in 2003 was almost by 3 m m3 greater (11.9 %) in comparison to the previous plan. Considering the small difference in the water table levels (65 cm) between the two soundings, finally the value quoted by IIF in Olsztyn was accepted as the basis for calculations. An increase of the mean lake depth to 8.5 m, a decrease of the mean bottom inclination (by over 0o 30’) resulting from a bigger participation of the shallows as well as diminishing of the character of the lake basin indenture, expressed by a relative depth index and geometrical lake basin index , have also been established. Simulated growths of volume indicate an insignificant change of the overall water volume.

246


Before water raising a change of water volume by ca. 4 m m3, i.e. 24.6 %, corresponded to the absolute amplitude of water table oscillations (186 cm). In

turn, a change of volume by ca. 850,000 m3,. i.e. 2.5 %. corresponds to the average annual amplitude of water level (39 cm).

Fig. 4. Bathymetric plan of Lake Bachotek (A – acc to IIF – 1958, B – acc the present authors’own research – 2003) Table 2. Morphometric and bathymetric features of Lake Bachotek alculated on the basis of bathymetric plans of IIF(1958) and measurements of 2003 r. Source Indicator Area in ha Lenght of shoreline in m Maximum length in m Development of shoreline Water level above sea level Maximum width in m Mean width in m Volume in thousand m3 Maximum depth in m Mean depta in m Relative depth according to Halbfass Contents index Mean bottom Inclination Depth index Ratio of lake basin form according to Murawiejski Empirical depth ratio c according to Fiłatowa Area of islands in ha

Conclusions The problem of lake and of the changes triggered by water table oscillations in Polish literature is a relatively rarely discussed issue. The present work on Lake Bachotek is an attempt at looking at these processes from the perspective of surface area

IIF 1958 211.0 11.225 4.115 2,17 70.9 900 512 15.394,2 24.3 7.2 0.0167 0.072 3o 26’ 14.12 0.35 0.048 3.6

Bathymetric measurements 2003 215.1 11.900 4.930 2.29 71.55 982 436 18.308,0 26.7 8.5 0.0182 0.085 2o 52’ 19.50 0.32 0.061 3.2

changes, but also including the changes in volume. The analysis of the available materials and on-spot observations lets us draw the following conclusions: — The analysis of a diversified source material has shown that the most informative type of data are aerial photographs as far as the changes of the


lake ecosystem are concerned. In the present study black and white as well as coloured aerial pictures from the years 1969 and 1996 were used, and the IIF from 1958. An updated bathymetric plan, elaborated on using the graphic programme Surfer 7 complements the above material; — Up to the time when the water was raised in the lake (1995) its mean water level oscillations amounted to 102 cm, and they lead to considerable changes in the surface area (7.4 ha) and, more importantly, to the changes in volume (13.6 %); — As a consequence of water table raising on average by 48 cm a stabilisation of levels at 281 cm had place, which in turn resulted in an increase in surface area by 3.4 ha and in volume by over 1 m m3, i.e. by 6.,4 % occurred. A noticeable decrease of the mean annual amplitude of water level oscillations (39 cm), had also place, which has caused very slight changes in the surface area. (2.7 ha) and volume by 2,5 % (Tab. 1). Contemporary possibilities of lake depth sounding with the use of modern measurement equipment – echo sounder, GPS – and of analysing the data with the use of graphic computer programmes have created new challenges for limnologists. More than half a century has passed since the last series of lake bathymetric plans was prepared. In face of the identified changes of linear, areal, and cubic parameters, an urgent need of preparing updated bathimetric plans has been rescognised.

References

Streszczenie Jezioro Bachotek jest rynnowym i przepływowym zbiornikiem połoŜonym na Poj. Brodnickim, odwadnianym przez Skarlankę (prawy dopływ Drwęcy), na którym prowadzi się codzienne pomiary stanów wody (IMiGW). Średni stan wody w jeziorze kształtował się na poziomie 233 cm (71,28 m n.p.m.) i charakteryzował się duŜymi wahaniami stanów wody, średnio 102 cm (absolutne 186 cm). Ten naturalny rytm stanów wody powodował w okresie niŜówek odsłanianie się szerokiej strefy litoralu, niekorzystne zmiany siedliskowe oraz wlewa-

247

Borowiak D., 2000, ReŜimy wodne i funkcje hydrologiczne jezior NiŜu Polskiego, Badania Limnologiczne, 2, Gdańsk. Chełmicki W., 2001, Woda: Zasoby, degradacja, ochrona, Wydawnictwo Naukowe PWN, Warszawa. Choiński A., 1995, Zarys limnologii fizycznej Polski, Wydawnictwo Naukowe UAM, Poznań. DoroŜyński R., Skowron R., 2002, Changes of the basin of Lake Gopło caused by melioration work in the 18th and 19th centuries, In: Limnological Review, 2, Lublin, 93–102. Drwal J. 1982, Wykształcenie i organizacja sieci hydrograficznej jako podstawa oceny struktury odpływu na terenach młodoglacjalnych, Zesz. Naukowe UG, Rozprawy i Monografie, 33, Gdańsk. Drwal J., 2003, Rola naturalnych predyspozycji oraz działań człowieka w budowie hydrograficznego systemy w delcie Wisły, In: Szczypek T., Rzętała M., (eds.) Człowiek i woda, Polskie Towarzystwo Geograficzne – Oddział Katowicki, Sosnowiec, 31–37. Glazik R., Gierszewski P., 2001, Influence of groundwater intakes on water resources of the chosen lakes located within GostynińskoWłocławski Landscape Park, In: Marszelewski W., Skowron R., (edit.) Limnological Review, vol. Kaniecki 1/2001, A.,Toruń, 1997, Wpływ 95–101.XIX-wiecznych melioracji na zmiany poziomu wód, In: Choiński A., (ed.) Wpływ antropopresji na jeziora, Wyd. Domini, Poznań – Bydgoszcz, 67–71. Lange W., 1986, Fizyczno-limnologiczne uwarunkowania tolerancji systemów jeziornych Pomorza, Zeszyty Naukowe UG, Rozprawy i Monografie, Gdańsk. Pazur A., Starkel L., 1989, New approach to explanation of changes in the volume and water level of the GościąŜ Lake, Zesz. Nauk. Politechniki Śląskiej, Ser. Matematyka-Fizyka, 57, Geochronometria, 5, 29–44. Skowron R., 2002, Hydrological changes caused by water damming in the lower stretch of the Skarlanka river (as exemplified by Lake Bachotek), Limnological Review, 2, Lublin.

nie się do systemu jezior: Bachotek – StraŜym – Zbiczno – Ciche, zanieczyszczonych wód Drwęcy. Wybudowanie jazu piętrzącego, bezpośrednio na wypływie Skarlani z jeziora w 1995 roku, spowodowało spiętrzenie poziomu wody o 48 cm i stabilizację stanów wody na poziomie 281 cm (71,76 m n.p.m.) (tab. 1, ryc. 2), zalanie najniŜej połoŜonych fragmentów (ryc. 1 i 3) oraz zmniejszenie rocznej amplitudy do 39 cm (Skowron, 2002). Wymiernym efektem tych zabiegów były przede wszystkim zmiany morfometryczne, w tym szczególnie powierzchni i objętości. W opracowaniu wykorzystano czarno-białe i kolorowe zdjęcia lotnicze z 1969 i 1996 roku, które były pod-

248


stawą do określenia zmian przestrzennych, a takŜe plany batymetryczne Instytutu Rybactwa Śródlądowego z 1958 roku i 2003 roku wykonanego przez autorów dla potrzeb tego opracowania (ryc. 1 i 4). Wykorzystując autograf A7, który umoŜliwił rejestrację wyników w postaci numerycznej oraz programu komputerowego GeoSet i Surfer 7, określono podstawowe wielkości morfometryczne jeziora. W okresie przed spiętrzeniem, przy średnich wahaniach stanów wody (102 cm), następowała zmiana powierzchni o 7.4 ha i objętości o 13.6 %. Natomiast dla wahań absolutnych (186 cm) zmiany te wynosiły, od-

powiednio: 6.2 ha i 24.6 %. Spiętrzenie wody wyeliminowało występowanie niekorzystnych zjawisk w strefie litoralu oraz zmniejszyło roczną amplitudę stanów wody do 39 cm. W wyniku tych zabiegów zmiana powierzchni i objętości uległa wyraźnemu zmniejszeniu i wynosi odpowiednio: 2,7 ha i 2,5 %. W pracy przedstawiono takŜe aktualny plan batymetryczny jeziora (ryc. 4), na którym autorzy stwierdzają niewielkie, choć istotne róŜnice (tab. 2), sugerując jednocześnie potrzebę opracowania nowych planów batymetrycznych jezior w Polsce.