e-mail: [email protected] · 2014-05-14 · modelling approaches undertaken to define the proper...

Contribution of hydrodynamic and limnological

modelling to the sanitation of Lake Bled

Mitja Rismal, Boris Kompare, Rudi Rajar

Faculty of Civil and Geodetic Engineering, University of Ljubljana,

Hajdrihova 28, 57-1001 Ljubljana, Slovenia, P.O.Box 3422

E-mail: [email protected]

Abstract

Lake Bled is an alpine pearl in the north-western part of Slovenia. In this centuryeutrophication has progressed rapidly, endangering this previously beautiful oligotrophic lakeand the tourist economy of Lake Bled region. From the 1950's several sanitation measureshave been proposed and undertaken. This paper presents basic facts about the lake andmodelling approaches undertaken to define the proper restoration measures. Several modelswere used, beginning with the simplest Vollenweider model, continuing with Imboden's twobox steady-state model, progressing to a dynamic three box model, then modelling ofcirculation by a 2D and later by a 3D hydrodynamic (HD) model. Together withmeasurements of tracer dispersion the two HD models showed the basic pattern of thecirculation and the mixing of inflowing water. Finally the whole problem was tackled with theaid of artificial intelligence tools (the latest approach by Kompare et al. [4] is shownelsewhere in these Proceedings). All the three eutrophication models shoed that at present wecannot expect any amelioration of the trophic state of the lake unless we drastically cut theinput of nutrients to the lake. The last two models, and the dynamic one in particular, showthe benefits and drawbacks of the introduction of artificial flushing of the lake with waterfrom a nearby river, and the positive effects of the outflow of hypolimnetic water through asyphon pipe. The needed decrease of point and non-point sources of pollution withphosphorus to achieve an economically feasible mesotrophic state of the lake is alsocalculated. The possible negative effects of the syphonic outflow on the environment wereforeseen and predicted to be negligible, which was also demonstrated after the construction ofthe syphon.

1 Introduction

Lake Bled is a typical alpine lake. In the present century it has severely sufferedfrom natural but above all from anthropogenic eutrophication. The inflows arevery small (the exchange time being about 3 years), the encompassingwatershed is urbanized, and there is marked flushing of nutrients fromagricultural land and the forested part of the watershed. Besides external

Transactions on Ecology and the Environment vol 14, © 1997 WIT Press, www.witpress.com, ISSN 1743-3541

126 Water Pollution

factors, accelerated eutrophication is caused by internal ones, i.e. themorphological characteristics of the lake. This relatively shallow lake with asmall hypolimnion is also protected from strong winds by a rim of hillsTherefore, most of the year the lake is strongly stratified, showing anoxicconditions at the bottom and also in a great part of the hypolimnion.Anthropogenic influences are the main causes of eutrophication. Urbanisationand intensive agriculture in the watershed have increased flushing of nutrientsfrom agricultural and urban land, compounded by outflows of cess-pits, leakageof sewers, and combined sewer overflows (CSO's) during heavy rainfalls.

In 1955 a commission was established for sanitation of the lake. It elaborateda priority action plan which contained the following measures:1. Sanitation of the Bled sewerage system All users should be connected to the

sewerage system, cess-pits should be abandoned, inadequate sewers shouldbe improved (present problems: exfiltration of sewerage into the lake andinfiltration of the lake water into the sewers).

2. The mean residence time of the lake should be shortened by artificial surfaceflushing of the lake. For this purpose the introduction of cca. 3.0 mVs offresh water from another watershed was proposed. Cold (cca 6 °C), oxygenrich, and relatively unpolluted water from the river Radovna was planned tobe conducted into the lake during the spring and autumn turnover, topromote better mixing and oxygen uptake in the hypolimnion.

3. Oxygenating of hypolimnetic water by pumping and spraying to the surfacewas rejected as it does not guarantee a permanent solution.

4. The construction of a hypolimnetic syphon was also rejected, although thismeasure could have completely removed the totally anaerobicmonimolimnetic layer at the bottom. It was the opinion of the commissionthat this solution does not remove the causes of the problem, but merely theconsequences - in contrast to the flushing of the lake with Radovna.

A diversion aqueduct for cca 3.0 mVs of fresh water from the Radovna river,(10-times the natural inflow to the lake), was first constructed. Due toconstraints (loss of energy) at a hydroelectric power plant on the Radovna riverdownstream, the foreseen water diversion was reduced later to some 300-5001/s. At the same time nothing was done to reduce the input of nutrients into thelake from the hinterland. For the reconstruction of (the already well developed)sewer system, more design work was needed and also alternative solutions wereproposed. This is the main reason that the reconstruction of sewers is still notcompleted.

The described measures, which were never fully realised, have not producedthe desired effects. In the beginning, the oxygen balance was improved by thegreater inflow of the Radovna, but after the reduction of inflow (due to theconstraints of the hydroelectric power plant) the anaerobic layer returned to itsoriginal position. These newly developed conditions required another analysis ofpossible restoration measures taking into account the changed conditions andnew knowledge in limnology.


Water Pollution 127

For this reason we proposed to tackle the abatement of the lakeeutrophication with modern modelling tools which enable cost- and time-effective simulations of different engineering measures in lake management.

2 Basic data on Lake Bled

The lake lies on the eastern edge of the Julian Alps and is surrounded by hills.The average elevation of the surface is 475.88 m a.s.l. Extensive hydro-geo-morphological measurements of the lake were first made by Sketelj & Rejic[16, 17]. Its main characteristics are summarized in Table 1.

parameter units valuesurface areavolumeoriginal watershedwatershed with Miscawatershed with Radovnamaximal depth of western poolmaximal depth of eastern poolmean depthdepth of epilimniondepth of hypolimnionannual mean inflow/outflowmean annual precipitation

km%106 m3km?knfkm2mmmmm1/smm

1.25.4 .8.

107.30.24 .17.6 -

14 -272/1614

43869879703208090109

330

Table 1. Hydromorphological characteristics of Lake Bled

The detention time of the lake is approx. 3 years. The volume of thehypolimnion is too small compared to the epilimnion, i.e. the hypolimnion isonly 25-48 % of the lake's total volume, which is not enough for it to maintainoxic conditions through the whole year.

The size of the contributing area has changed significantly several times.From the original watershed of 487 ha (147 ha lake, 170 ha urbanised andagricultural, and 170 ha forest) the diversion of the Misca, a stream passingnearby, has added an additional 410 ha, making the watershed grow to 897 ha.This diversion was made in the previous century for energy purposes (mills) In1964, another diversion was introduced as a restoration measure, this time fromthe river Radovna, which has increased the contributing area 12-times comparedto the former watershed (with the Misca), and 22-times compared to theoriginal natural watershed (see Fig. 1). The increase of watershed has causedalso increased nutrients' loads and thus acceleration of eutrophication processes.

The transparency varies significantly during the year. In March/April algalblooms aggregate mainly on the surface and transparency is of the order of 1.0 -


128 Water Pollution

2.5 m; it increases to cca 4.0 m in June, and falls to its minimum in August (0.5- 1.5 m). During autumn and winter it can be as high as 9.5 m [2, 20].

410 ha artificially addedwatershed by the diversionof M/SCA creek

diversion of RECICA into MISCA creek

340 ha natural contributingarea to the lake

Figure 1. Growth of the watershed of Lake Bled

contribution fromMisca riverZaka riverimmediate watershedsewer M /part of Bledlake surfaceRadovna river

min. load61171983794

max. load29064972026594

takenaccou290405814

15294

intont

total 236 kg P/year 830 kg P/year 648 kg P/year

Table 2. Estimation of phosphorus loads to the lake in


Water Pollution 129

It was shown ([6, 13, 14, 18]) that phosphorus is the nutrient controllingeutrophication. The ratio between N and P in Lake Bled is 80, far beyond

17, where phosphorus becomes the limiting nutrient. In recent decades theconcentration of total phosphorus P was mostly over 50 j g/1, i.e. the lake was

in a hypereutrophic state. A summary of measured and estimated phosphorusloads is given in Table 2.

3 Determination of circulation by measurements and

hydrodynamic modelling

The basic circulation in alpine lakes of this type is well known: in spring thermalstratification begins to form, which becomes stronger in summer. In late autumnthe cooling of the epilimnion causes destratification and subsequently theturnover of the lake water, which can last for the whole winter if there is no icecover. Through the whole year winds are relatively weak in the lake region andexperience, as well as several simulations, have shown that in summer the windscannot overcome the strong stratification and can never cause mixing to thebottom.

A more detailed knowledge of the hydrodynamic (HD) circulation helps todetermine: (a) the pattern of transport-dispersion of nutrients in the lake; (b) thecirculation of surface inflow water from the Radovna river and of thesyphonoutflow, which also helps to understand the efficiency of the two pipelinesystems, and (c) the optimal locations for measurements.A combined methodology of measurement and HD modelling was used to

determine the circulation. As both the measurements and the 2D modelling havealready been described by Rajar and Cetina [7], only a short description is givenbelow.

Measurements. In greatest part of the lake the circulation velocities are verysmall, below Icm/s (only at the surface can the wind cause local velocities up toabout 30 cm/s). It would be very difficult to measure the flow velocitiesdirectly. Therefore two measurement with tracers (Rhodamine B and Uranine)were carried out. The tracer was introduced into the inflow pipeline from theRadovna river, and the concentration of the tracer was measured over the wholelake for the next few days ([5]). The results were used as an indirect method ofverification of the hydrodynamic model.

Modelling. As the measurements have shown that the water flowing to thelake from the Radovna river is stabilised in a horizontal layer, 2D modelling wasinitially performed in 1986 [7]. From 1995 on research is going on with the goalof a complete 3D simulation of the circulation. A fully 3D, baroclinichydrodynamic model (described in Rajar and Cetina [8]) is being applied.

Description of the Circulation. Measurements with tracer were carried outwith two discharges of the Radovna inflow: Case A with 200 1/s, and Case Bwith 600 1/s. Both cases were carried out in June with strong stratification of thelake water and with negligible winds.


130 Water Pollution

Fig. 2 shows the results of measurements for Case A. The temperature of theinflowing water was 8-10 °C, and the temperature distribution of the lake waterin Fig. 2b makes clear that the Radovna water finds its equilibrium density in thedepths between 10 and 14 metres. The velocity of horizontal spreading of thetracer is about Icm/s during the first day, but it decreases to about 2 mm/s bythe fourth day. A vertical cross-section (Fig. 2b) shows that during thefollowing days there is almost no mixing of the Radovna water with the lakewater. The effect of this facility is indeed limited to the layer of the samedensity/temperature as the inflowing water. The mixing is probably strongerduring the winter and early spring circulation, when there is almost nostratification. But during that time of the year there is a turnover of the lakewater, and oxygenation of the bottom water is enabled even without the extrainflow of the Radovna.

Location of thefront at the time f0 SMOmg/m*

D O.S-Smg/m* —*- Flow direction

Figure 2: Results of tracer measurements in Lake Bled a) Spreading ofthe tracer; b) Vertical cross section

During Case B, with a discharge of 600 1/s, the mixing of the Radovna waterwith the lake water was somewhat stronger. The thickness of the layer with the


Water Pollution 131

tracer was about 8 metres. But another disadvantage of the surface inflowappeared: the jet hit the bottom causing some resuspension of nutrient-richsediments. Subsequently, this would enrich the lake waters with phosphorus andits compounds, causing increased primary production, with eventual algaeblooms and increased eutrophication. For this reason it was decided that theinflow discharge should always be kept below 500 1/s.

The measurements were not able to show the effectiveness of thesyphonoutflow directly, but a simple analysis of the phenomenon, based on themass conservation law, has shown that the combined effect of the surface inflowand the bottom outflow is positive: the water of the worst quality at the bottomof the lake is drained out of the lake, thus contributing to its sanitation.

Fig. 3 shows the simulated circulation for Case B (inflow 600 1/s) in thehorizontal layer at the depth of 10 to 11.5 m The outlets of both inflowpipelines are at a depth of 18 m, but the 3D model properly shows that theRadovna water stabilises at depths of about 10 to 18 m. A vortex above theoutlets is formed because the discharge through the eastern outlet is about 3times greater than through the western one.

The study on 3D hydrodynamic modelling is not yet fully completed. Thepresent results show too much dispersion in the vertical direction. It has beenfound that a finer numerical grid and a more accurate turbulence model shouldbe used. This will be carried out in the continuation of the research.

WEST

SCALELENGTH Q 250 500 rrVELOC. 0 0.05 O.'O^-

Figure 3: Velocities in the layer at the depth of 10 to 11.5m, simulated by 3DHD model (The outlets of the two inflow pipes are at a depth of 18 m.)

4 Elaboration of sanitation measures with the use of

limnological models

Because former remedial actions for restoration of water quality wereineffective, in 1979 the Institute of Sanitary Engineering (IZH) was given thetask of proposing variant sanitation measures for Lake Bled, to evaluate them in


132 Water Pollution

the sense of their contribution to water quality, and to elaborate a list of prioritytasks. In these studies ([9, 10]) several methods of lake sanitation were takeninto consideration, i.e.:- Reduction of nutrient input into the lake.- Surface flushing with water from the river Radovna.- Deep-water flushing via syphoning (outflow) of hypolimnetic water.- Destratification of the lake and aeration of the anaerobic layers with air jets

or with aerator shaft pumps.- Chemical precipitation of nutrients in the lake.From the financial point of view the last two methods are the least attractive -also they do not offer a permanent solution of the problem So they were notadvised as feasible, unless in an emergency. The first three methods were foundto be feasible, each by itself and in combination. So we focused our research onthese three methods [9, 10].

Each of the three methods has its advantages and disadvantages. It is notpossible to totally cut-off the import of nutrients into the lake, but only toreduce them to some extent. Surface and/or deep-water flushing with waterfrom the river Radovna has limited capacity. The only practical way toadequately evaluate the contribution and efficiency of the listed methods wasmathematical modelling of the whole ecosystem, i.e. the lake and thecontributing area.

In 1980 we used a simple Vollenweider model [19] as a first orientativeapproach. The practical value of this model is its simplicity and the need forvery few data. With this model it was not possible to evaluate the differencesbetween surface- and deep-water flushing; instead, the differences betweendifferent rates of flushing was explored, see Table 3.

Radovna

m /s0

0.2280.7281.2281.728

Total inflow

rrP/s0.2720.5001.0001.5002.000

Phosphorusload

from Radovnakg P/year

0107344581817

Permissible pto obtain folkoligotrophickg P/year

188288483662824

losphorus loadtwing category:

mesotrophickg P/year

3525409061 2411 545

Table 3. Permissible load with P f after Vollenw eider's model [19]

From this first insight into the behaviour of Lake Bled ecosystem it can be seenthat artificial flushing cannot improve the water quality. Therefore, it isnecessary to reduce the external load and maybe to combine it with artificialflushing of the lake. So we decided that a more sophisticated model is needed to


Water Pollution 133

show the differences between the various ways and rates of flushing, andexternal as well as internal loading. We chose the model from Imboden [3] andextended it (Rismal [10]) to model surface and below surface inflows andoutflows. This model is also among the simpler ones; we chose it intentionallyso as to avoid unnecessarily complex processes, which are difficult to modeleven if one has perfect data, which did not hold in our case. The model is two-box and contains four equations describing the processes of photosynthesis andrespiration of phyto- and zoo-plankton in the epilimnion and hypolimnion.

permissible

PtotgP/(m2y)

0.200.150.100.05

load with

kgP/year

29422014773

needed surface outflowin mVs byresuspenbottom

mg

0

2.2271.3430.442

sion frc

P / (m2 d

L 10

)m the

ay)

100

needed deep-wateroutflow i.e. via syphon inmVs byresuspenbottom

mg

0

1.4960.306

sion fro

P / (m^ d;

10

2.2951.1730.459

m the

iy)100

2.891

Table 4. Permissible load with P after Rismal's model [10]

Different flushing and resuspension rates of P from the sediment werecalculated with an extended model of Imboden, to evaluate the possiblebehaviour of the lake within the real limits of controlling mechanisms. Theresults of the model show clearly the regions of prevalence for surface- and fordeep-water flushing; see Table 4. N.B.: Values are only entered for feasiblecombinations, i.e. surface flushing is only effective with zero resuspension fromthe bottom (resuspension of 100 mg P/(nfday) was estimated to be the highestexpected value). From Table 4 it can be seen that deep-water flushing has agreat advantage over surface flushing, e.g. for obtaining the load of 220kgP/year four times more water is needed for surface flushing. Also surfacetemperature would be decreased by intensive surface flushing, which renders thelake less attractive for tourists. It can also be seen from the table that flushingalone does not significantly increase water quality in the lake. The reason forthis is the transport of nutrients along with the inflows. The only way toimprove water quality is then simultaneous use of deep-water flushing andreduction of the external load.

In the next step we decided to evaluate the predictions of the extendedImboden model with a more accurate and also time dependent (dynamic) model.In the years 1990-92 we developed a dynamic model according to Griffin andFerrara [1]. The model simulates time-dependent balances of oxygen and


134 Water Pollution

phosphorus in the lake in accordance with external and internal forcingfunctions.

The model could not be quite properly calibrated and verified because of aninsufficient data base. We had to use literature data for some parameters.Nevertheless, the model qualitatively correctly displays the expected behaviour.Therefore we took the results of the model to be decisive for the definitivechoice of the optimal sanitation procedure.

A series of simulations was conducted with variant combinations of locationsand rates of inflows and outflows over a period of several years (Rismal & Cvikl[15]). Here we would like to describe only the extreme cases, the worst and thebest:(1). The worst case is the situation before the construction of the syphon(before 1980). There is only the Misca river inflowing to the lake (no diversionof the Radovna river), and there is only surface outflow (the syphon is not inoperation). From Fig. 4 two peaks in organic phosphorus, due to algal growth,can be clearly seen in early spring and in autumn, and an extremely lowconcentration of oxygen in the whole hypolimnion in the autumn. Anoxicconditions are developed at the bottom over almost the whole year (in particularif we take into account that biologically anoxic conditions take place atconcentrations less than 1.0 mg Oi/l).(2). The best situation is when the only inflow (besides the immediatewatershed) into the lake is the deep inflow of the Radovna river (200 1/s) andthe syphon is working at full capacity, see Fig. 5. The model shows that thewater quality significantly improves already in two years; in five years a greatreduction of biomass concentration can be expected as well. This indicates thatprimary productivity as a measure of trophic state is decreasing. At the sametime the concentration of oxygen in the hypolimnion is increasing and theperiods of anoxia at the bottom are getting shorter. The model also shows thedrop in photo synthetic production of oxygen in epilimnion due to decrease ofbiomass. Oxic conditions at the bottom at the same time prevent resuspensionof phosphorus, thus drastically reduce internal load and speed-up the processesof the lake restoration.

5 Proposed and accomplished sanitation measures

According to the results of our first mathematical models of the lake, and basedon a solid background in limnology and related sciences, a priority list ofimportant conclusions and reasonable restoration measures was proposed [9,10, 11, 12], as follows:1. Successful and rational restoration of the lake (regarding operation,

investment and operational costs) is only possible by the use of adequatelysophisticated and calibrated mathematical models. All the data needed,


Water Pollution 135

(mg/m3)10.00

9.00

8.00

7.00

6.00

5.00

4.00

3.00

2.00

1.00

0.00

_Porg,Epi

-Porg.Hypo

days

1 61 121 181 241 301 361 421 481 541 601 661 721

a) Concentration of organic phosphorus

(mg/m3)

2Ou

8000

7000

6000

5000

3000-

2000

nSTHpi—O.Hypo0,d

days

1 61 121 181 241 301 361 421 481 541 601 661 721b) Concentration of oxygene

Fig. 4: Results of the dynamic model for the worst case

especially chemical and bio-chemical analyses, their sites and frequencies ofsampling etc., must be strictly subordinated to the needs of the model.

2. First step in the approach to sanitation of the lake is to state the limitingfactor of eutrophication. In our case it is phosphorus. This limiting factordetermines all other planned activities, i.e. the search for the main sources ofpollution, the postulation of feasible restoration techniques and theevaluation of possible short- and long-term effects.


Water Pollution

(mg/m3)10.00

9.00

8.00

7.00

6.00

5.00

4.00

3.00

2.00

1.00

0.00

Porg.Epi

Porg.Hypo

1 181 361 541 721 901 1081 1261 1441 1621 1801

a) Concentration of organic phosphorus d*Y*

(mg/m3)8000

7000

6000

5000

4000

3000

2000

1000

1 181 361 541 721 901 1081 1261 1441 1621 1801b) Concentration of oxygene days

Fig. 5: Results of the dynamic model for the best case (foreseen to beimplemented)

3. The total annual phosphorus load to the lake is soundly estimated to be 648kg, of which the Misca river alone brings almost one halt i.e. 290 kg Thepart which cannot be reduced, i.e. input on the surface and from theimmediate contributing watershed, is 250 kg. Modelling and other analysesshowed that the optimal priority tasks list is as follows:


Water Pollution 137

3.1 The largest pollutant, the Misca, can be relatively easily excluded bydiversion to its original course by passing the lake (and not flowing intothe lake). This is also not questionable because the Misca no longer servesas a power source for mills. The other major sources of nutrients cannotbe so easily managed. So the next feasible step is:

3.2 Construction of the hypolimnetic siphon, as the model showed itsprevalent advantage among the other possible sanitation measures.

3.3 Sanitation of sewers, which partly leak sewerage to the lake, and partlydrain water from the lake; connection of all urbanised areas to thesewerage system.

3.4 Restoration of combined sewer overflows (CSO's) to decrease thequantities of mixed sewerage and rainfall-runoff diverted to the lakeduring rainy periods down to an acceptable 8 kg Pt<>t for an averagely wetyear. For this purpose retention basins were designed (Rismal, [12]) toavoid overflows at rain intensities smaller than 40 l/(s*ha).

4. The results of the models and practical experience with functioning of thesyphon since 1980 have shown:4.1 Permanent flushing of concentrated nutrients and products of

decomposition via the syphon decreases the rate of aging of the lake andcan even to some extent shift the lake water from eutrophic tomesotrophic or even oligotrophic state.

4.2 The syphon positively changes the thermal balance of the lake, whereasthe introduction of cold water from the Radovna river into thehypolimnion along with surface flushing has the opposite effects.

4.3 Surface flushing with as much as 3.0 mVs of Radovna water did notflush out algae, as was expected. The residence time of approx. 3 monthswith this flushing is still far too long compared to the time of growth ofalgae of approx. 7-14 days.

4.4 The net positive effect of the Radovna river is limited to flows between0.2 and 0.3 nf/s, or slightly more if the Misca river is not flowing into thelake. Namely, greater inflows also mean a greater input of nutrients,which cannot be balanced out by increased flushing rate.

Unfortunately, for several reasons, not all of the proposed sanitation measureshave been accomplished. The syphon pipeline has been successfully constructed(first pipe in 1980, second pipe in 1982). The main pollutant, the Misca river, isstill flowing into the lake. The sewer system was renovated to some extent, butthe retention basins were not constructed as was proposed.

6 Conclusions

With several mathematical models of Lake Bled ecosystem and with asynthesised knowledge of limnology and sanitary engineering we succeeded inelaborating step by step (from simple to complex) a sound priority task plan forthe lake's sanitation. According to this plan, which proposed a combination ofsanitation measures to be the most optimal, only one measure was completely


implemented, i.e. the construction of a syphon to divert anaerobic and nutrient-rich hypolimnetic water. This single measure has given significant and easilyobserved results even in its first year of operation. It was concluded from theresults that since the beginning of the operation of the syphon (begun in 1980)the concentration of phosphorus has dropped by more than a factor of two. Thetransparency also increased and concentration of oxygen in the hypolimnionincreased too. In the past fifteen years blooms of Oscilatoria rubescens, whichis the sign of developed eutrophication, were seldom seen, which is a greatcontrast to their previous abundant appearance each year [2]. The effectivenessof the syphonis also evident to the naked eye of the inhabitants of Bled.

The predictions of models, based on feasible reductions of external loads ofphosphorus, have shown that our expectations were correct. Because of thelack of quality data, the dynamic model could not have been properly calibrated,so the quantitative predictions might not be completely valid; but the results ofthe syphonoperation over more than 15 years have given us confidence in themodel.

The potential negative effects of the syphon outlet on the environment(smell, toxic compounds of anaerobic digestion, eutrophication and oxygendepletion of the receiving river, etc.) were foreseen [11] to be negligible, whichhas also been proven through experience.

We hope that the other proposed sanitation measures will also beimplemented in the near future, which will make it possible for the lake to retainits mesotrophic status and maybe even to regain oligotrophic conditions.

7 References

1. Griffin, T.T. & Ferrara, KA. A Multicompartment Model of PhosphorusDynamics in Reservoirs, Water Res. Bull., 1984, pp. 777-788.

2. HMZ. Research on water quality in Slovenia, HydrometeorologicalInstitute of the Republic of Slovenia (HMZ). Research publications foryears 1976 to 1991 (in Slovenian).

3. Imboden, DM Phosphorus Model of Lake Eutrophication, Limnologyand Oceanography, March 1974, Vol. 19 (2).

4. Kompare, B , Dzeroski, S. & Karalic, A Identification of the Lake Bledecosystem with artificial intelligence tools M5 and FORS, Proceedings ofthe Water Pollution 97. Bled, Slovenia, 18-20 June, 1997.

5. Leibundgut, C, Moeri, Th., Peschel, H. StroemungsuntersuchungenMitt els Tracerversuchen im Bledsee, Report, Geographisches Institut derUniversitaet Bern, 1983.


Water Pollution 139

6. Loffler, H, Sampl, A. Stellungnahme zu den Restauration - MassnahmenBlejsko jezero, Osterreichische Akademie der Wissenschaften, 1982.

7. Rajar, R. and Cetina, M. Mathematical simualtion of two-dimensional lakecirculation, Proceedings, HYDROSOFT, Southampton, 1986, ElsevierPubl.

8. Rajar, R., and Cetina, M. Hydrodynamic models as a basis for waterquality modelling: An experience. Ecological Modelling, accepted forpublication, 1997.

9. Rismal, M. A study of the syphon for the sanitation of Lake Bled, report,FAGG - IZH, Ljubljana, 1979. (in Slovenian)

10. Rismal, M. An assessment of diverse sanitation methods for Lake Bled,Gradbeni vestnik, Ljubljana 1980 (29), 2-3, pp. 34-43. (in Slovenian)

11. Rismal, M. An assessment of negative influences of the syphon outflowfrom Lake Bled on the environment, Gradbeni vestnik, Ljubljana, 198la(30), pp. 51-54.

12. Rismal, M. The influence of the sewerage of Bled on the pollution of thelake with nutrients, Vodoprivreda 13, 73 (1981/5), 1981b, pp. 383-393.(in Croatian)

13. Rismal, M., An assessment of sanitation measures for Lake Bled and theobtained results, Vodoprivreda 14, 75-76 (1982/1-2), 1982a, pp. 9-14. (inCroatian)

14. Rismal, M., The use of limnological models for the analysis and mitigationof eutrophication in lakes and impoundments, Vodoprivreda 14, 78-79(1982/4-5), 1982b, pp. 391-393. (in Croatian)

15. Rismal, M. & Cvikl, M. Dynamic limnological model of Lake Bled,Report, University of Ljubljana, FAGG - IZH, Ljubljana, 1991.

16. Sketelj, J. in Rejic, M. Preliminary report on research on Lake Bled,Gradbeni vestnik 61-64, 1958-59. (in Slovenian)

17. Sketelj, J in Rejic, M. Physical, chemical and biological research onchanges in Lake Bled (study). Report on the integral monitoring duringDec. 14-16, 1966, FAGG - IZH, Ljubljana, 1967.

18. Stauffer, R. Summary of Discussions on Lake Bled, Report, Ms, 1982.19. Vollenweider, R.A. Advances in defining critical loading levels for

phosphorus in lake eutrophication. Mem. Inst. Ital. Idrobiol, 1976, 33:53-86. (in Italian)

20. Vrhovsek, V. Primary productivity of eutrophic systems (Lake Bled),Annual reports for 1976-1991. (in Slovenian)


e-mail: [email protected] · 2014-05-14 · modelling approaches undertaken to define the proper...

Documents